Mobile Computing (ECS-087)PUT-Set BSolution

1. (a) What are the pros and cons of having different size cells for wireless networking? Advantages of cellular systems with small cells are the following:

1. Higher capacity: Implementing SDM allows frequency reuse. If one transmitter is far away from another, i.e., outside the interference range, it can reuse the same frequencies. As most mobile phone systems assign frequencies to certain users (or certain hopping patterns), this frequency is blocked for other users. But frequencies are a scarce resource and, the number of concurrent users per cell is very limited. Huge cells do not allow for more users. On the contrary, they are limited to less possible users per km. This is also the reason for using very small cells in cities where many more people use mobile phones.

2. Less transmission power: While power aspects are not a big problem for base stations, they are indeed problematic for mobile stations. A receiver far away from a base station would need much more transmit power than the current few Watts. But energy is a serious problem for mobile handheld devices.

3. Local interference only: Having long distances between sender and receiver results in even more interference problems. With small cells, mobile stations and base stations only have to deal with ‘local’ interference.

4. Robustness: Cellular systems are decentralized and so, more robust against the failure of single components. If one antenna fails, this only influences communication within a small area.

Small cells also have some disadvantages:1. Infrastructure needed: Cellular systems need a complex infrastructure to connect all

base stations. This includes many antennas, switches for call forwarding, location registers to find a mobile station etc, which makes the whole system quite expensive.

2. Handover needed: The mobile station has to perform a handover when changing from one cell to another. Depending on the cell size and the speed of movement, this can happen quite often.

3. Frequency planning: To avoid interference between transmitters using the same frequencies, frequencies have to be distributed carefully. On the one hand, interference should be avoided, on the other, only a limited number of frequencies are available.

(b) What do you mean by cell in cellular concept and why the cell of cellular architecture is hexagonal?

Adjacent circles cannot be overlaid upon a map without leaving gaps or creating overlapping regions.

When considering geometric shapes which cover an entire region without overlap and with equal area, there are three sensible choices: a square, an equilateral triangle, and a hexagon.

A cell must be designed to serve the weakest mobiles within the footprint, and these are typically located at the edge of the cell.

For a given distance between the centre of a polygon and its farthest perimeter points, the hexagon has the largest area of the three.

By using the hexagon geometry, the fewest number of cells can cover a geographic region

Closely approximate a circular radiation pattern which would occur for an omni-directional base station antenna and free space propagation.

Permit easy and manageable analysis of a cellular system.

(c) Explain the word “mobile computing” and also give any suitable live example with merit of mobile computing. Mobile Computing is an umbrella term used to describe technologies that enable people to access network services anyplace, anytime, and anywhere. Ubiquitous computing and nomadic computing are synonymous with mobile computing. Mobile computing as a generic term describing ability to use the technology to wirelessly connect to and use centrally located information and/or application software through the application of small, portable, and wireless computing and communication devices.

• Mobile computing represents a shift in the distributed systems paradigm. • The potential of decoupled and disconnected operation, location-dependent computation

and communication, and powerful portable computing devices gives rise to opportunities for new patterns of distributed computation that require a revised view of distributed systems.

• However, factors such as weak network connectivity, energy constraints, and mobility itself raise new concerns regarding the security, reliability, and even correctness of a

mobile computing system. • Mobile Computing requires: Wireless network to support outdoor mobility and

handoff from one network to the next at a pedestrian or vehicular speed.• The following models of computing in the mobile environment are currently being

researched and investigated: Client/Server Client/Proxy/server Disconnected Operation Mobile Agents The Thin Client Model

The mobile computing is used in different context with different names. Some of them are: Nomadic Computing: use of portable computing devices (such as laptop and handheld

computers) in conjunction with mobile communications technologies to enable users to access the Internet and data on their home or work computers from anywhere in the world.

Ubiquitous Computing = Nomadic Computing + Mobile Computing Pervasive Computing: is a technology that pervades the user’s environment by making

use of multiple independent information devices (both fixed and mobile, homogeneous or heterogeneous) interconnected seamlessly through wireless or wired computer communication networks which are aimed to provide a class of computing / sensory

/communication services to a class of users, preferably transparently and can provide personalized services while ensuring a fair degree of privacy / non-intrusiveness.

(d) Explain the GSM location updating signalling sequence with suitable diagram. 1. The MS sends a Location Update request to the VLR (new) via the BSC and MSC.2. The VLR (new) sends a Location Update message to the HLR serving the MS which

includes the address of the VLR (new) and IMSI of MS. This updating of the HLR is not required if the new LA is served by the same VLR as the old LA.

3. The service and security related data for the MS is downloaded to the new VLR.4. The MS is sent an acknowledgement of successful location update.5. The HLR requests the old VLR to delete data relating to the relocated MS.The signaling sequence is as follows:

(e) Describe the following multiple access protocols:

(i) TDMA (ii) FDMA (iii) CDMAFrequency Division Multiple Access (FDMA): Comprises all algorithms allocating frequencies to transmission channels according to the frequency division multiplexing (FDM). Allocation can either be fixed (as for radio stations or the general planning and regulation of frequencies) or dynamic (i.e., demand driven).Channels can be assigned to the same frequency at all times, i.e., pure FDMA, or change frequencies according to a certain pattern, i.e., FDMA combined with TDMA. The latter example is the common practice for many wireless systems to circumvent narrowband interference at certain frequencies, known as frequency hopping. Sender and receiver have to agree on a hopping pattern, otherwise the receiver could not tune to the right frequency. Hopping patterns are typically fixed, at least for a longer period. The fact that it is not possible to arbitrarily jump in the frequency space (i.e., the receiver must be able to tune to the right frequency) is one of the main differences between FDM schemes and TDM schemes.Time Division Multiple Access (TDMA): Compared to FDMA, offers a much more flexible scheme, which comprises all technologies that allocate certain time slots for communication, i.e., controlling TDM. Now tuning in to a certain frequency is not necessary, i.e., the receiver can stay at the same frequency the whole time. Using only one frequency, and thus very simple receivers and transmitters, many different algorithms exist to control medium access. As already mentioned, listening to different frequencies at the same time is quite difficult, but listening to

many channels separated in time at the same frequency is simple. Almost all MAC schemes for wired networks work according to this principle, e.g., Ethernet, Token Ring, ATM etc. Now synchronization between sender and receiver has to be achieved in the time domain. Again this can be done by using a fixed pattern similar to FDMA techniques, i.e., allocating a certain time slot for a channel, or by using a dynamic allocation scheme. Dynamic allocation schemes require an identification for each transmission as this is the case for typical wired MAC schemes (e.g., sender address) or the transmission has to be announced beforehand. MAC addresses are quite often used as identification. This enables a receiver in a broadcast medium to recognize if it really is the intended receiver of a message. Fixed schemes do not need identification, but are not

as flexible considering varying bandwidth requirements. Code Division Multiple Access (CDMA): Codes with certain characteristics can be applied to the transmission to enable the use of code division multiplexing (CDM). Code division multiple access (CDMA) systems use exactly these codes to separate different users in code space and to enable access to a shared medium without interference. The main problem is how to find “good” codes and how to separate the signal from noise generated by other signals and the environment.A code for a certain user should have a good autocorrelation and should be orthogonal to other codes. Orthogonal in code space has the same meaning as in standard space (i.e., the three dimensional space). Two vectors are called orthogonal if their inner product is 0, as is the case for the two vectors (2, 5, 0) and (0, 0, 17): (2, 5, 0)*(0, 0, 17) = 0 + 0 + 0 = 0. The Barker code (+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1), for example, has a good autocorrelation, i.e., the inner product with itself is large, the result is 11. This code is used for ISDN and IEEE 802.11.

(f) In context to cellular network, discuss the following: (i) Cluster (ii) Frequency Reuse (iii) Cell Splitting (iv) SectorizationCluster:

The total coverage area is divided into clusters. There can be no co-channel interference within a cluster. The number of cells in a cluster is called the cluster size. This number is denoted

by N. The N cells collectively use the complete set of available frequencies.

There are two possible models to create minimal interference: three cell cluster and seven cell cluster.

3-cell cluster 7-cell cluster

Frequency Reuse: Cellular radio systems rely on an intelligent allocation and reuse of channels throughout a coverage region. Each cellular base station is allocated a group of radio channels to be used within a small geographic area called a cell. Base stations in adjacent cells are assigned channel groups which contain completely different channels than neighboring cells. The base station antennas are designed to achieve the desired coverage within the particular cell. By limiting the coverage area within the boundaries of a cell, the same group of channel may be

used to cover different cells that are separated from one another by distances large enough to keep interference levels within tolerable limits. The design process of selecting and allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning.

Cell Splitting:

Cell splitting is the process of subdividing a congested cell into smaller cells (called microcells), each with its own base station and a corresponding reduction in antenna height and transmitter power. Splitting the cell reduces the cell size and thus more number of cells have to be used For the new cells to be smaller in size the transmit power of these cells must be reduced. More number of cells more number of clustersmore channels high capacity The new cell radius = old cell radius/2. It can be permanent and dynamic.

Large Cell is also called Macro cell, and similarly, Small as Micro and Smaller as Pico cell.

Sectoring: As opposed to cell splitting, where D/R is kept constant while decreasing R, sectoring

keeps R untouched and reduces the D/R Capacity improvement is achieved by reducing the number of cells per cluster, thus

increasing frequency reuse

In this approach first SIR is improved using directional antennas The CCI may be decreased by replacing the single omni-directional antenna by several

directional antennas, each radiating within a specified sector The factor by which the co-channel interference is reduced depends on the amount of

sectoring used. A cell is normally partitioned into three 120º sectors or six 60º sectors as shown in Figure.

120 degree sectoring 60 degree sectoring

2. (a) Draw and define 802.11 protocol stack regarding the following points: (i) Physical Layer (ii) MAC Sublayer Protocol (iii) Frame Structure

1. Physical layer: IEEE 802.11 supports three different physical layers: one layer based on infra-red and

two layers based on radio transmission (primarily in the ISM band at 2.4 GHz, which is available worldwide).

All PHY variants include the provision of the clear channel assessment signal (CCA). This is needed for the MAC mechanisms controlling medium access and indicates if the medium is currently idle.

The PHY layer offers a service access point (SAP) with 1 or 2 Mbit/s transfer rate to the MAC layer (basic version of the standard).

The following are the three versions of a PHY layer defined in the standard. (i) Frequency hopping spread spectrum:Frequency hopping spread spectrum (FHSS) is a spread spectrum technique which allows for the coexistence of multiple networks in the same area by separating different networks using different hopping sequences. The selection of a particular channel is achieved by using a pseudo-random hopping pattern. The standard specifies Gaussian shaped FSK (frequency shift keying), GFSK, as modulation for the FHSS PHY. Figure shows a frame of the physical layer used with FHSS. The frame consists of two basic parts, the PLCP part (preamble and header) and the payload part. While the PLCP part is always transmitted at 1 Mbit/s, payload, i.e. MAC data, can use 1 or 2 Mbit/s. Additionally, MAC data is scrambled using the polynomial s(z) = z7 + z4 + 1 for DC blocking and whitening of the spectrum.

Figure- Format of an IEEE 802.11 PHY Frame using FHSS

The fields of the frame fulfill the following functions:

● Synchronization: The PLCP preamble starts with 80 bit synchronization, which is a 010101... bit pattern. This pattern is used for synchronization of potential receivers and signal detection by the CCA.● Start frame delimiter (SFD): The following 16 bits indicate the start of the frame and provide frame synchronization. The SFD pattern is 0000110010111101.● PLCP_PDU length word (PLW): This first field of the PLCP header indicates the length of the payload in bytes including the 32 bit CRC at the end of the payload. PLW can range between 0 and 4,095.● PLCP signalling field (PSF): This 4 bit field indicates the data rate of the payload following. All bits set to zero (0000) indicate the lowest data rate of 1 Mbit/s. The granularity is 500 kbit/s, thus 2 Mbit/s is indicated by 0010 and the maximum is 8.5 Mbit/s (1111). ● Header error check (HEC): Finally, the PLCP header is protected by a 16 bit checksum with the standard ITU-T generator polynomial G(x) = x16 + x12 + x5 + 1.

(ii) Direct sequence spread spectrum:Direct sequence spread spectrum (DSSS) is the alternative spread spectrum method separating by code and not by frequency. In the case of IEEE 802.11 DSSS, spreading is achieved using the 11-chip Barker sequence (+1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1). The key characteristics of this method are its robustness against interference and its insensitivity to multipath propagation (time delay spread). However, the implementation is more complex compared to FHSS. IEEE 802.11 DSSS PHY also uses the 2.4 GHz ISM band and offers both 1 and 2 Mbit/s data rates. The system uses differential binary phase shift keying (DBPSK) for 1 Mbit/s transmission and differential quadrature phase shift keying (DQPSK) for 2 Mbit/s as modulation schemes. All bits transmitted by the DSSS PHY are scrambled with the polynomial s(z) = z7 + z4 + 1 for DC blocking and whitening of the spectrum. Figure shows a frame of the physical layer using DSSS. The frame consists of two basic parts, the PLCP part (preamble and header) and the payload part. While the PLCP part is always transmitted at 1 Mbit/s, payload, i.e., MAC data, can use 1 or 2 Mbit/s.

Figure- Format of an IEEE 802.11 PHY Frame using DSSS

The fields of the frame have the following functions:● Synchronization: The first 128 bits are not only used for synchronization, but also gain setting, energy detection (for the CCA), and frequency offset compensation. The synchronization field only consists of scrambled 1 bits.● Start frame delimiter (SFD): This 16 bit field is used for synchronization at the beginning of a frame and consists of the pattern 1111001110100000.● Signal: Originally, only two values have been defined for this field to indicate the data rate of the payload. The value 0x0A indicates 1 Mbit/s (and thus DBPSK), 0x14 indicates 2 Mbit/s (and thus DQPSK). Other values have been reserved for future use, i.e., higher bit rates. ● Service: This field is reserved for future use; however, 0x00 indicates an IEEE 802.11 compliant frame.● Length: 16 bits are used in this case for length indication of the payload in microseconds.● Header error check (HEC): Signal, service, and length fields are protected by this checksum using the ITU-T CRC-16 standard polynomial.

(iii) Infra red:The PHY layer, which is based on infra red (IR) transmission, uses near visible light at 850–950 nm. The standard does not require a line-of-sight between sender and receiver, but should also

work with diffuse light. This allows for point-to-multipoint communication. The maximum range is about 10 m if no sunlight or heat sources interfere with the transmission. Typically, such a network will only work in buildings, e.g., classrooms, meeting rooms etc.

2. Medium Access Control Layer:MAC layer fulfills several functions such as:

Control medium access Offer support for roaming, authentication, and power conservation. Asynchronous data service Optional time-bounded service. While 802.11 only offers the asynchronous service in ad-hoc network mode, both service

types can be offered using an infrastructure-based network together with the access point coordinating medium access.

The asynchronous service supports broadcast and multi-cast packets, and packet exchange is based on a ‘best effort’ model, i.e., no delay bounds can be given for transmission.

The following three basic access mechanisms have been defined for IEEE 802.11: the mandatory basic method based on a version of CSMA/CA, an optional method avoiding the hidden terminal problem, and finally a contention-free polling method for time-bounded service. The first two methods are also summarized as distributed coordination function (DCF), the third method is called point coordination function (PCF). DCF only offers asynchronous service, while PCF offers both asynchronous and time-bounded service but needs an access point to control medium access and to avoid contention. The MAC mechanisms are also called distributed foundation wireless medium access control (DFWMAC).For all access methods, several parameters for controlling the waiting time before medium access are important. Figure shows the three different parameters that define the priorities of medium access. The values of the parameters depend on the PHY and are defined in relation to a slot time. Slot time is derived from the medium propagation delay, transmitter delay, and other PHY dependent parameters. Slot time is 50 μs for FHSS and 20 μs for DSSS.The medium, as shown, can be busy or idle (which is detected by the CCA). If the medium is busy this can be due to data frames or other control frames. During a contention phase several nodes try to access the medium.

Figure- Medium access and inter-frame spacing

Short inter-frame spacing (SIFS): The shortest waiting time for medium access (so the highest priority) is defined for short control messages, such as acknowledgements of data packets or polling responses. For DSSS SIFS is 10 μs and for FHSS it is 28 μs.

PCF inter-frame spacing (PIFS): A waiting time between DIFS and SIFS (and thus a medium priority) is used for a time-bounded service. An access point polling other nodes only has to wait PIFS for medium access. PIFS is defined as SIFS plus one slot time.

DCF inter-frame spacing (DIFS): This parameter denotes the longest waiting time and has the lowest priority for medium access. This waiting time is used for asynchronous data service within a contention period. DIFS is defined as SIFS plus two slot times.

MAC framesFigure shows the basic structure of an IEEE 802.11 MAC data frame together with the content of the frame control field. The fields in the figure refer to the following:● Frame control: The first 2 bytes serve several purposes. They contain several sub-fields.● Duration/ID: If the field value is less than 32,768, the duration field contains the value indicating the period of time in which the medium is occupied (in μs). This field is used for setting the NAV for the virtual reservation mechanism using RTS/CTS and during fragmentation. Certain values above 32,768 are reserved for identifiers.● Address 1 to 4: The four address fields contain standard IEEE 802 MAC addresses (48 bit each), as they are known from other 802.x LANs. The meaning of each address depends on the DS bits in the frame control field.● Sequence control: Due to the acknowledgement mechanism frames may be duplicated. Therefore a sequence number is used to filter duplicates.● Data: The MAC frame may contain arbitrary data (max. 2,312 byte), which is transferred transparently from a sender to the receiver(s).● Checksum (CRC): Finally, a 32 bit checksum is used to protect the frame as it is common practice in all 802.x networks.

The frame control field shown in Figure contains the following fields:● Protocol version: This 2 bit field indicates the current protocol version and is fixed to 0 by now. If major revisions to the standard make it incompatible with the current version, this value will be increased.● Type: The type field determines the function of a frame: management (=00), control (=01), or data (=10). The value 11 is reserved. Each type has several subtypes. ● Subtype: Example subtypes for management frames are: 0000 for association request, 1000 for beacon. RTS is a control frame with subtype 1011, CTS is coded as 1100. User data is transmitted as data frame with subtype 0000. .

Figure- IEEE 802.11 MAC packet Structure

(b) List the entities of mobile IP and describe data transfer from a mobile node to a fixed node and vice-versa. Why and where is encapsulation needed?Entities and terminology used in mobile IP:● Mobile node (MN): A mobile node is an end-system or router that can change its point of attachment to the internet using mobile IP. The MN keeps its IP address and can continuously communicate with any other system in the internet as long as link-layer connectivity is given.● Correspondent node (CN): A peer with which a mobile node is communicating. The CN can be a fixed or mobile node.● Home network: The home network is the subnet the MN belongs to with respect to its IP address. No mobile IP support is needed within the home network.● Foreign network: The foreign network is the current subnet the MN visits and which is not the home network.● Foreign agent (FA): The FA can have the COA, acting as tunnel endpoint and forwarding packets to the MN. The FA can be the default router for the MN. FAs can also provide security services because they belong to the foreign network as opposed to the MN which is only visiting.

● Care-of address (COA): The COA defines the current location of the MN from an IP point of view. All IP packets sent to the MN are delivered to the COA, not directly to the IP address of the MN. Packet delivery toward the MN is done using a tunnel.There are two different possibilities for the location of the COA:

• Foreign agent COA: The COA could be located at the FA, i.e., the COA is an IP address of the FA. Many MN using the FA can share this COA as common COA.

• Co-located COA: The COA is co-located if the MN temporarily acquired an additional IP address which acts as COA.

● Home agent (HA): The HA provides several services for the MN and is located in the home network. The tunnel for packets toward the MN starts at the HA. The HA maintains a location registry, i.e., it is informed of the MN’s location by the current COA.

IP packet delivery: Figure illustrates packet delivery to and from the MN. A correspondent node CN wants to send an IP packet to the MN. CN does not need to know anything about the MN’s current location and sends the packet as usual to the IP address of MN (step 1). This means that CN sends an IP packet with MN as a destination address and CN as a source address. The internet, not having information on the current location of MN, routes the packet to the router responsible for the home network of MN.

Packet delivery to and from the mobile node

• The MN sends the packet as usual with its own fixed IP address as source and CN’s address as destination (step 4). The router with the FA acts as default router and forwards the packet in the same way as it would do for any other node in the foreign network.

• As long as CN is a fixed node the remainder is in the fixed internet as usual. If CN were also a mobile node residing in a foreign network, the same mechanisms as described in steps 1 through 3 would apply now in the other direction.

A tunnel establishes a virtual pipe for data packets between a tunnel entry and a tunnel endpoint. Packets entering a tunnel are forwarded inside the tunnel and leave the tunnel unchanged. Tunneling, i.e., sending a packet through a tunnel, is achieved by using encapsulation.Encapsulation is the mechanism of taking a packet consisting of packet header and data and putting it into the data part of a new packet. The reverse operation, taking a packet out of the data part of another packet, is called decapsulation. Encapsulation and decapsulation are the operations typically performed when a packet is transferred from a higher protocol layer to a lower layer or from a lower to a higher layer respectively. Here these functions are used within the same layer.The HA takes the original packet with the MN as destination, puts it into the data part of a new packet and sets the new IP header in such a way that the packet is routed to the

• The HA now intercepts the packet, knowing that MN is currently not in its home network. The packet is not forwarded into the subnet as usual, but encapsulated and tunnelled to the COA. A new header is put in front of the old IP header showing the COA as new destination and HA as source of the encapsulated packet (step 2).

• The foreign agent now decapsulates the packet, i.e., removes the additional header, and forwards the original packet with CN as source and MN as destination to the MN (step 3).

COA. The new header is also called the outer header for obvious reasons. Additionally, there is an inner header which can be identical to the original header as this is the case for IP-in-IP encapsulation, or the inner header can be computed during encapsulation.

(c) Sketch a neat diagram showing the Bluetooth protocol stack. State the functions of the following layers: (i) Radio layer (ii) Baseband layer (iii) L2CAP layer

The Bluetooth protocol stack can be divided into a core specification (Bluetooth, 2001a), which describes the protocols from physical layer to the data link control together with management functions, and profile specifications (Bluetooth, 2001b).On top of L2CAP is the cable replacement protocol RFCOMM that emulates a serial line interface following the EIA-232 (formerly RS-232) standards. This allows for a simple replacement of serial line cables and enables many legacy applications and protocols to run over Bluetooth. RFCOMM supports multiple serial ports over a single physical channel. The telephony control protocol specification – binary (TCS BIN) describes a bit-oriented protocol that defines call control signaling for the establishment of voice and data calls between Bluetooth devices. It also describes mobility and group management functions.

Radio Layer: This layer includes specification of the air interface, i.e., frequencies, modulation, and transmits power.Design Issues:

Bluetooth devices will be integrated into typical mobile devices and rely on battery power. This requires small, low power chips which can be built into handheld devices.

Worldwide operation also requires a frequency which is available worldwide. The combined use for data and voice transmission has to be reflected in the design, i.e.,

Bluetooth has to support multi-media data.Bluetooth uses the license-free frequency band at 2.4 GHz allowing for worldwide operation with some minor adaptations to national restrictions. A frequency-hopping/time-division duplex scheme is used for transmission, with a fast hopping rate of 1,600 hops per second. The time between two hops is called a slot, which is an interval of 625 μs. Each slot uses a different frequency. Bluetooth uses 79 hop carriers equally spaced with 1 MHz.

Transmitter characteristics: Each device is classified into 3 power classes, Power Class 1, 2 & 3.

Power Class 1: is designed for long range (~100m) devices, with a max output power of 100 mW and minimum is 1mW with mandatory power control.

Power Class 2: for ordinary range devices (~10m) devices, with a max output power of 2.5 mW, nominal power of 1 mW, and minimum power of 0.25mW with optional power control.

Power Class 3: for short range devices (~10cm) devices, with a max output power of 1 mW.

Baseband Layer:The baseband is the digital engine of a Bluetooth system. It is responsible for constructing and decoding packets, encoding and managing error correction, encrypting and decrypting for secure communications, calculating radio transmission frequency patterns, maintaining synchronization, controlling the radio, and all of the other low level details necessary to realize Bluetooth communications.The channel is represented by a pseudo-random hopping sequence hopping through the 79 or 23 RF channels. Two or more Bluetooth devices using the same channel form a piconet. The hopping sequence is unique for the piconet and is determined by the Bluetooth device address (BD_ADDR) of the master; the phase in the hopping sequence is determined by the Bluetooth clock of the master. The channel is divided into time slots where each slot corresponds to an RF hop frequency. Consecutive hops correspond to different RF hop frequencies.The data exchange takes place with every clock tick. The clock synchronization is with respect to that of the master. Transmission takes place by way of TIME DIVISION DUPLEXING (TDD). The channel is divided into time slots, each 625 μs in length. The time slots are numbered according to the Bluetooth clock of the piconet master. A TDD scheme is used where master and slave alternatively transmit. The master shall start its transmission in even-numbered time slots only, and the slave shall start its transmission in odd-numbered time slots only. The packet start shall be aligned with the slot start.

The Baseband handles three types of links: SCO (Synchronous Connection-Oriented): The SCO link is a symmetric point-to-point link between a master and a single slave in the piconet. The master maintains the SCO link by using reserved slots at regular intervals (circuit switched type). The SCO link mainly carries voice information. The master can support up to three simultaneous SCO links while slaves can support two or three SCO links. SCO packets are never retransmitted. SCO packets are used for 64 kB/s speech transmission. Polling-based (TDD) packet transmissions: In this link type one slot is of 0.625msec (max 1600 slots/sec) and master/slave slots (even-/odd-numbered slots). ACL (Asynchronous Connection-Less) link: The ACL link is a point-to-multipoint link between the master and all the slaves participating on the piconet. In the slots not reserved for the SCO links, the master can establish an ACL link on a per-slot basis to any slave, including the slave already engaged in an SCO link (packet switched type). Only a single ACL link can exist. For most ACL packets, packet retransmission is applied.Logical link control and adaptation protocol (L2CAP):Besides protocol multiplexing, flow specification, and group management, the L2CAP layer also provides segmentation and reassembly functions. Depending on the baseband capabilities, large packets have to be chopped into smaller segments. The L2CAP layer accepts up to 64 kbyte. The L2CAP is a data link control protocol on top of the baseband layer offering logical channels between Bluetooth devices with QoS properties. L2CAP is available for ACLs only. Audio applications using SCOs have to use the baseband layer directly. L2CAP provides three different types of logical channels that are transported via the ACL between master and slave:● Connectionless: These unidirectional channels are typically used for broadcasts from a master to its slave(s).

● Connection-oriented: Each channel of this type is bi-directional and supports QoS flow specifications for each direction. It defines average/peak data rate, maximum burst size, latency, and jitter.● Signaling: This third type of logical channel is used to exchanging signalling messages between L2CAP entities. Each channel can be identified by its channel identifier (CID). Signaling channels always use a CID value of 1, a CID value of 2 is reserved for connectionless channels. For connection-oriented channels a unique CID (>= 64) is dynamically assigned at each end of the channel to identify the connection (CIDs 3 to 63 are reserved).

Figure shows the three packet types belonging to the three logical channel types. The length field indicates the length of the payload (plus PSM for connectionless PDUs). The CID has the multiplexing/demultiplexing function.

3. (a) What do you understand by clustering? How clustering is used for mobile wireless networks? Discuss. Clustering: Node mobility results in frequent failure and activation of links, causing a routing algorithm reaction to topology changes and hence increasing network control traffic. Ensuring effective routing and QoS support while considering the relevant bandwidth and powerconstraints remains a great challenge. Given that MANETs may comprise a large number of MNs, a hierarchical structure will scale better. Hence, one promising approach to address routing problems in mobile environments is to build hierarchies among the nodes, such that the network topology can be abstracted. This process is commonly referred to as clustering and the substructures that are collapsed in higher levels are called clusters. In clustering procedure, a representative of each subdomain (cluster) is ‘elected’ as a cluster head (CH) and a node which serves as intermediate for inter-cluster communication is called gateway. Remaining members are called ordinary nodes. The boundaries of a cluster are defined by the transmission area of its CH. With an underlying cluster structure, non-ordinary nodes play the role of dominant forwarding nodes as shown in Figure. Cluster architectures do not necessarily include a CH in every cluster. CHs hold routing and topology information, relaxing ordinary MHs from such requirement; however, they represent network bottleneck points. In clusters without CHs, every MH has to store and exchange more topology information, yet, that eliminates the bottleneck of CHs. There are two approaches for cluster formation, active clustering and passive clustering. In active clustering,MHs cooperate to elect CHs by periodically exchanging information, regardless of data transmission. On the other hand, passive clustering suspends clustering procedure until data

traffic commences. It exploits on-going traffic to propagate “cluster-related information” (e.g., the state of a node in a cluster, the IP address of the node) and collects neighbor information through promiscuous packet receptions.

Figure- Cluster heads, gateways and ordinary nodes in mobile ad hoc network clustering

Adaptive Clustering:Personal communications and mobile computing require a wireless network infrastructure whichis fast deployable, possibly multihop, and capable of multimedia service support. The first infrastructure of this type was the Packet Radio Network (PRNET). PRNET was totally asynchronous and was based on a completely distributed architecture.One of the features of adaptive clustering is multihopping, i.e. the ability of the radios to relay packets from one to another without the use of base stations. Most of the nomadic computing applications today are based on a single hop radio connection to the wired network (Internet or ATM). Figure- shows the cellular model commonly used in the wireless networks. A, B, C, and D are fixed base stations connected by a wired backbone. Nodes 1 through 8 are mobile nodes. A mobile node is only one hop away from a base station. Communications between two mobile nodes must be through fixed base stations and the wired backbone.

Figure- Conventional cellular networks (single-hop)

Figure-1 A multihop situation occurs when base station B fails

This solution can be extended to multihop networks by creating clusters of radios, in such a way that access can be controlled and bandwidth can be allocated in each cluster. The notion of cluster has been used also in earlier Packet Radio nets, but mainly for hierarchical routing rather than for resource allocation.

Passive clustering eliminates major control overhead of active clustering, still, it implies larger setup latency which might be important for time critical applications; this latency is experienced whenever data traffic exchange commences. On the other hand, in active clustering scheme, the MANET is flooded by control messages, even while data traffic is not exchanged thereby consuming valuable bandwidth and batterypower resources.

If a base station fails, a mobile node may not be able to access the wired network in a single hop. For example, in Figure-1, if base station B fails, node 4 must access base stations A or C through node 2 or node 5 which act as wireless multihop repeaters.

The Multicluster Architecture:

A major challenge in multihop, multimedia networks is the ability to account for resources so that bandwidth reservations (in a deterministic or statistical sense) can be placed on them. In cellular (single hop) networks such accountability is made easy by the fact that allstations learn of each other’s requirements, either directly, or through a control station (e.g. base station in cellular systems).

Most hierarchical clustering architectures for mobile radio networks are based on the concept of clusterhead. The clusterhead acts as a local coordinator of transmissions within the cluster. It differs from the base station concept in current cellular systems, in that it does not have special hardware and in fact is dynamically selected among the set of stations. However, it does extra work with respect to ordinary stations, and therefore it may become the bottleneck of the cluster. To overcome these difficulties, in our approach we eliminate the requirement for a clusterhead altogether and adopt a fully distributed approach for cluster formation and intracluster communications.

(b) What was the motivation for designing the CODA system? Discuss CODA file system in detail. The CODA File System: Coda is a descendant of version 2 of the Andrew File System (AFS), which was also developed at Carnegie Mellon University (CMU) in the 1990s, and inherits many of its architectural features from AFS. It is now integrated with a number of popular UNIX-based operating systems such as Linux. Coda has features of high scalability, disconnected operation, replication, naming and security.AFS was designed to support the entire CMU community, which implied that approximately 10,000 workstations would need to have access to the system. To meet this requirement, AFS nodes are partitioned into two groups. One group consists of a relatively small number of dedicated Vice file servers, which are centrally administered. The other group consists of a very much larger collection of Virtue (Client) workstations that give users and processes access to the file system, as shown in Figure-2.

Figure-2 The overall organization of AFS

Virtue: Coda ClientsThe internal architecture of a Virtue workstation is shown in Figure-3. The important issue is that Venus runs as a user-level process. Again, there is a separate Virtual File System (VFS) layer that intercepts all calls from client applications, and forwards these calls either to the local file system or to Venus, as shown in Figure-3. This organization with VFS is the same as in NFS. Venus, in turn, communicates with Vice file servers using a user-level RPC system. The RPC system is constructed on top of UDP datagrams and provides at-most-once semantics. There are three different server-side processes. The great majority of the work is done by the actual Vice file servers, which are responsible for maintaining a local collection of files. Like Venus, a file server runs as a user-level process. In addition, trusted Vice machines are allowed to run an authentication server, which we discuss in detail later. Finally, update processes are used to keep meta-information on the file system consistent at each Vice server.

Coda follows the same organization as AFS. Every Virtue workstation hosts a user-level process called Venus, whose role is similar to that of an NFS client. A Venus process is responsible for providing access to the files that are maintained by the Vice file servers. In Coda, Venus is also responsible for allowing the client to continue operation even if access to the file servers is (temporarily) impossible. This additional role is a major difference with the approach followed in NFS.

Figure-3 The internal organization of a Virtue workstation (Client)

Processes in CODA:Coda maintains a clear distinction between client and server processes. Clients are represented by Venus processes; servers appear as Vice processes. Both type of processes are internally organized as a collection of concurrent threads. Threads in Coda are non-preemptive and operate entirely in user space.Naming in CODA:In CODA file system, Files are grouped into units referred to as volumes. A volume is similar toa UNIX disk partition (i.e., an actual file system), but generally has a much smaller granularity. It corresponds to a partial subtree in the shared name space as maintained by the Vice servers. Usually a volume corresponds to a collection of files associated with a user. Like disk partitions, volumes can be mounted.Volumes are important for two reasons. First, they form the basic unit by which the entire name space is constructed. This construction takes place by mounting volumes at mount points. A mount point in Coda is a leaf node of a volume that refers to the root node of another volume. The second reason why volumes are important is that they form the unit for server-side replication.File Identifiers:Considering that the collection of shared files may be replicated and distributed across multiple Vice servers, it becomes important to uniquely identify each file in such a way that it can be tracked to its physical location, while at the same time maintaining replication and location transparency.Each file in Coda is contained in exactly one volume. As we mentioned above, a volume may be replicated across several servers. For this reason, Coda makes a distinction between logical and physical volumes. A logical volume represents a possibly replicated physical volume, and has an associated Replicated Volume Identifier (RVID). An RVID is a location and replication-independent volume identifier. Multiple replicas may be associated with the same RVID. Each physical volume has its own Volume Identifier (VID), which identifies a specific replica in a location independent way.

Unlike NFS, Coda provides a globally shared name space that is maintained by the Vice servers. Clients have access to this name space by means of a special subdirectory in their local name space, such as /afs. Whenever a client looks up a name in this subdirectory, Venus ensures that the appropriate part of the shared name space is mounted locally.

Figure-4 The implementation and resolution of a Coda file identifier

The second part of a file identifier consists of a 64-bit file handle that uniquely identifies the file within a volume. In reality, it corresponds to the identification of an index node as represented within VFS. Such a vnode as it is called, is similar to the notion of an inode in UNIX systems.

(c) Discuss the impact of mobile computing on following aspects of data management: (i) Transactions (ii) Data Dissemination (iii) Query Processing (iv) Caching

Transaction Processing: Transactions models for mobile environments are different than those used in centralized or distributed databases in the following ways:• Computation and communication have to be supported by stationary hosts.• The transactions are prolonged due to the mobility of both data and users, and due to frequent disconnections.• The models should support and handle concurrency, recovery, disconnection and mutual consistency of the replicated data objects.• As mobile hosts move from one cell to another, the states of transaction and accessed data objects, and the location information also change.• Computations might have to be split into sets of operations executed on mobile and stationary hosts.

Data Dissemination:In broadcast systems, there is no traditional network stack. Data is transmitted in the form of buckets. Buckets are also called data blocks or frames. Practically, buckets reside on top of the wireless Medium Access Control (MAC) protocol. In broadcast systems, mobile clients must wait until the server broadcasts the required information. Therefore, client waiting time is determined by the overall length of broadcast data, which is usually referred to as broadcast cycle. Clients must keep listening to the broadcast channel until the arrival of required information.The concept of selective tuning is introduced for reducing power consumption. By using selective tuning, mobile clients stay in doze mode most of the time and turn into active mode only when the requested information is expected to arrive. Indexing techniques are used to implement

The approach followed in Coda is to assign each file a 96-bit file identifier. A file identifier consists of two parts as shown in Figure-4. The first part is the 32-bit RVID of the logical volume that the file is part of.To locate a file, a client first passes the RVID of a file identifier to a volume replication database, which returns the list of VIDs associated with that RVID. Given a VID, a client can then look up the server that is currently hosting the particular replica of the logical volume. This lookup is done by passing the VID to a volume location database which returns the current location of that specific physical volume.

selective tuning in wireless environments. Indices are broadcast together with data to help mobile clients locate the required information. In most systems, buckets are classified into index and data buckets.

Query Processing:Query optimization techniques have to consider the effects of mobility. Query processing in mobile environments can be divided into queries that involve only the content of the database, and location based queries. Mobility has several effects on the ACID properties. Location data may involve location based queries or location aware queries. Due to fast changing location data, queries may be answered in an approximate way. Another major issue is querying the broadcast data on the air, but under the premise of transactional data. Other typical issues are finding the best execution plan for a query that involves data broadcast on different channels, and defining the organization of the broadcast data so that the consumed energy is minimized.

Caching:Cache management plays an important role in mobile computing because of its potential to alleviate the performance and availability limitations during weak connections and disconnections. It can reduce contention on limited bandwidth networks. This improves query response time and supports disconnected or weakly connected operations. If a mobile user has cached a portion of the shared data, different levels of cache consistency may be requested. In a strongly connected mode, the user may want the current values of the database items belonging to its cache. During weak connections, the user may require weak consistency when the cached copy is a quasi-copy of the database items. Each type of connection may have a different degree of cache consistency associated with it, namely weak connection corresponds to weaker level of consistency. Cache consistency is severely hampered by both disconnections and mobility, since a server may be unaware of the current locations and connection status of clients.

4. (a) What is the mobile agent? Describe the agent server architecture with suitable diagram. Also list the security threats to a mobile agent system.

Mobile Agent:A mobile agent consists of the program code and the program execution state (the current values of variables, next instruction to be executed, etc.). Initially a mobile agent resides on a computer called the home machine. The agent is then dispatched to execute on a remote computer called a mobile agent host (a mobile agent host is also called mobile agent platform or mobile agent server). When a mobile agent is dispatched the entire code of the mobile agent and the execution state of the mobile agent is transferred to the host. The host provides a suitable execution environment for the mobile agent to execute. The mobile agent uses resources (CPU, memory, etc.) of the host to perform its task. After completing its task on the host, the mobile agent migrates to another computer. Since the state information is also transferred to the host, mobile agents can resume the execution of the code from where they left off in the previous host instead of having to restart execution from the beginning. This continues until the mobile agent returns to its home machine after completing execution on the last machine in its itinerary.

– Mobile agents are migrating processes associated with an itinerary dynamic code and state deployment

– Implement the agents of the previous architectures as mobile agents, E.g., server-side agents can relocate during handoff client-side agent dynamically move on and off the client

– Implement the communication using mobile agents: clients submit/receive mobile agents to/from the server

Mobile agents1. Map directly to real life situations2. Need a generic execution environment

3. Can work in both modes push pull

4. Can work off-line5. Provide local interactions 6. Provide multi-hop solutions7. Program that can migrate from system to system within a network environment8. Performs some processing at each host9. Agent decides when and where to move next

How does it move? Save state Transport saved state to next system Resume execution of saved state

Classification basis Type of migration and code shipping mechanism Agent tracking and directory service Resource access control mechanism Communication

Local, global, communicating partners

Typical Mobile Agent Framework:

A mobile agent contains the following 3 components: Code - the program (in a suitable language) that defines the agent's behavior. State - the agent's internal variables etc., which enable it to resume its activities after

moving to another host.

DataBase

Desktop

Server

SystemResources

Laptop

User Application Legacy Software

Execution EnvironmentMobile Agent

Service AgentApplication

Agent MigrationLocal Communication

Global Communication

Attributes - information describing the agent, its origin and owner, its movement history, resource requirements, authentication keys etc. Part of this may be accessible to the agent itself, but the agent must not be able to modify the attributes

Levels of Mobility:Weak Mobility

When moving a mobile agent carries code + data state Data State - global or instance variable On moving, execution has to start from the beginning

Strong Mobility When moving a mobile agent carries

code + data state + execution state Data State - global or instance variable Execution State – local variables and threads On moving, execution can continue from the point it stopped on the

previous hostClassification of Security Threats: Threats in mobile computing systems can be broadly classified as: threats emanating from an agent attacking an agent platform, an agent platform attacking an agent, an agent attacking another agent on the agent platform, and other entities attacking the agent platform.

Platform-to-agent: This category represents the class of threats where hosts compromise the

agents. The set of threats include masquerading, denial of service, eavesdropping, and alteration. These attacks are most difficult to detect and prevent, since the host has full control of the agents’ code and data.

Masquerading: An agent platform can masquerade as another agent platform in an attempt to deceive a mobile agent as to its true destination. As an example, a mobile agent entrusted with the task of finding the “lowest price” of a commodity by visiting various virtual shops, can be fooled by a malicious masquerading platform, by making it believe that all other shops have quoted a higher price. Thus, the masquerading platform can harm both the visiting agent and other agent platforms.

Denial of Service: A malicious agent platform may ignore service requests, introduce unacceptable delays during the execution of time critical tasks or even terminate the agent without notification. Agents on other platforms waiting for the results of a non-responsive agent can become deadlocked. An agent can also become live-locked if more work is continuously generated for the agent by the malicious platform.

Eavesdropping: The classical threat of eavesdropping in electronic communication is more serious in mobile agent systems because an agent platform can, not only monitor communications, but also every instruction executed by the agent, all unencrypted or public data it brings to the platform and all data generated on the platform. An agent may be exposing proprietary algorithms, trade secrets or other sensitive information. Even if the platform is unable to automatically extract the secret information, it may be able to infer the meaning from the types of services requested.

Alteration: Alteration includes modification of data, state and code. Modification cannot be prevented but it should be possible for another

agent or platform to detect unauthorized modifications. Modification is typically prevented by using digital signatures. But digital signatures are useful only for signing code and static data. The original author can digitally sign the agent’s code and read–only data. Signatures cannot be used to detect malicious modifications to dynamic data modified at different hosts. A host can change the data generated by other hosts in the agent’s itinerary. Such changes, if not immediately detected, will be impossible to track down after the agent has visited other platforms and undergone countless changes in state and data. Check–pointing and rollback will be extremely difficult because an agent’s final state and data might be dependent on the behavior of countless autonomous agents whose behavior cannot be recreated.

Agent-to-platform: This category represents the set of threats in which agents exploit security

weaknesses of an agent platform or launches attacks against an agent platform. Often, an agent platform is required to execute programs from potentially untrusted sources. This means that we can no longer say, “don’t download and run untrusted programs”, any more. These threats include masquerading, denial of service and unauthorized access.

Masquerading: An agent can take the identity of another agent to gain unauthorized access to resources or to shift the blame for any actions for which it does not want to be held accountable. The trust of an agent or the owner of an agent can be destroyed by a masquerading agent.

Denial of Service: Mobile agents can launch a denial of service attack by consuming excessive amounts of the platform’s computing resources. There is also the possibility of a program consuming excessive resources due to bugs in the program. Practices to help mitigate these risks like configuration management, design reviews and testing are not immediately applicable to mobile code systems because the mobile computing paradigm requires a platform to accept and execute an agent whose code may have been developed outside its organization and has not been subject to any a priori review.

Unauthorized access:Access control mechanisms are used to prevent unauthorized users from accessing resources. Resource allocation should be done in accordance with the platform’s security policy. Authentication is used to identify an agent by the agent platform. How to authenticate and trust an agent which might have visited many untrusted hosts in its itinerary is a serious issue.

Agent-to-Agent: This category represents the set of attacks in which agents exploit security

weaknesses of other agents or launch attacks against other agents. These threats also include masquerading, denial of service, unauthorized access and repudiation.

Masquerading: An agent can pose as a platform offering some services to another agent. For example, an agent can pose as a “virtual shop” offering various goods and services, and fool another agent into revealing credit card numbers. As usual, masquerading harms both the agent that is deceived and whose identity has been assumed.

Denial of Service:

It is possible for a malicious agent to launch a denial of service attack against other agents. For example, repeatedly sending messages to another agent will cause undue burden on the message handling routines of the recipient. If the agent is charged based on its utilization of CPU resources, this could lead to a potential monetary loss to the victim.

Unauthorized access:If the access control mechanism of the platform is poor, then an agent can directly invoke the public methods of other agents, or modify its code and data. Eavesdropping by agents is also an issue.

(b) Discuss the schemes of mobile transaction management. The traditional transaction models do not have the ability to deal with other challenging requirements of mobile transactions, such as supporting the mobility of transactions and coping with disconnections. Consequently, there are many advanced transaction models that have been developed to particularly support mobile transactions. The following are the mobile transaction models that have the ability to efficiently support mobile transactions.

1. Clustering Model: Clustering offers a replication scheme to mobile environments where mobile clients suffer of disconnection variations. It assumes a fully distributed system and is designed to maintain database consistency. The database is dynamically divided into clusters, each one groups together semantically related or closely located data. A cluster may be distributed over several strongly connected hosts. When a MH is disconnected it becomes a cluster by itself. For every object two copies are maintained, one of them (strict version) must be globally consistent, and the other (weak version) can tolerate some degree of global inconsistency but must be locally consistent. MTs are either strict or weak. Weak transactions access only weak versions whereas strict ones access strict versions.Strict transactions are executed when hosts are strongly connected and weak transactions when MHs are weakly connected or disconnected. Two kinds of operations are introduced: weak reads and weak writes. Strict transactions contain standard reads and writes (strict operations), whereas weak transactions contain weak operations. At reconnection a synchronization process (executed on the database server) leads the database to a global consistent state. Distributed transactions can be executed only inside a cluster as strict transactions. MHs may participate but only in connected mode. In disconnected mode MHs execute only weak transactions.

2. Two-tier replication Model: Two-tier replication is a lazy replication mechanism which considers both transaction and replication approaches for mobile environments where MHs are occasionally connected. A master version for each data and several replicated versions (copies) exist. Two types of transactions are supported: base and tentative transactions.Base transactions access master versions (lazy-master replication scheme) whereas tentative transactions access tentative versions (local copies). Tentative transactions may perform updates on the MH in a disconnected mode. When the connection is established, tentative transactions are re-executed as base transactions (coordinated by the current BS) to reach global consistency.Results of this re-execution may have defined acceptance criteria which allow results to be different. Transaction re-executions allow local updates to persist.

3. HiCoMo Model: High Commit Mobile (HiCoMo) is a mobile transaction model devoted to decision making applications. Its goal is to allow updates during disconnections on aggregate data stored on MHs. There exist base tables on FHs from which aggregate tables are obtained. They represent a summary or statistics (e.g. average, summation, minimum, maximum) that is stored on MHs. Similar to Clustering and Two-tier replication two transaction types are considered: HiCoMo and base transactions.

HiCoMo transactions are executed on aggregate tables during MH disconnections. Base transactions reflect the modifications made by HiCoMo transactions on base tables. Thus, at reconnection, a HiCoMo transaction is transformed into base transactions – one per base table accessed during the generation of aggregate tables. To obtain a high rate of successful executions only commutative operations – addition and subtraction- are considered for HiCoMo transactions and an error margin is tolerated between HiCoMo and base transaction re-executions.

4. IOT Model: Coda file system uses an optimistic replication scheme where only write/write conflicts are taken into account. Isolation-Only Transaction (IOT) extends Coda addressing read/write conflicts with a transaction service. In IOT, transactions are a sequence of file access operations. Transactions are classified into two categories (similar to Clustering, Two-tier replication and HiCoMo): first class whose execution does not contain any partitioned file accesses (i.e. the client maintains a server connection for every file accessed) and second class which are executed under disconnections. First class transactions commit immediately after being executed, whereas second class ones go to a pending state and wait for validation. When reconnection becomes possible, second class transactions are validated according to the desired consistency criteria i.e. local serializability, global serializability, global certifiability. If validation is successful, results are integrated and committed. Otherwise, transactions enter the resolution state. Resolution may be automatic(re-execution, application specific, abortion) or manual (notification to users). (c) Give any example of general authentication and privacy procedure for D-AMPS and also sketch the diagram suitable to it. D-AMPS uses a challenge response procedure based on a private key for authentication and privacy. However, the authentication and privacy algorithm, the nature of the private key, and the procedures for generating and transporting the authentication results for verification are different from those used in the GSM. A general description of the procedure specified in IS-41 is summarized here.

At the time of subscription the MS is programmed with information specific to the subscriber or the terminal, such as mobile identification number (MIN) and electronic serial number (ESN), as well as the cellular authentication and voice encryption (CAVE) algorithm. Since the D-AMPS currently does not utilize a subscriber identity module (as in the case of GSM), the private key (called the A key) is provided to the subscriber through a secure means (e.g., through registered mail). The subscriber then uses the terminal's keypad to enter the 64-bit A key into the MS, and its correct entry is verified by the security software within the MS. The A key also resides in the HLR/AC in the subscriber's home network.

Once the subscriber-specific data, the CAVE algorithm, and the A key have been successfully programmed into the MS, the HLR/AC asks the MS to generate the secret shared data (SSD) by sending a RANDSSD (random number for SSD generation) parameter to the MS. This may take place when the MS makes the initial registration request. The MS utilizes RANDSSD, the A key, and the ESN as input to the CAVE algorithm to generate the SSD. This SSD is then used for generating authentication results and cryptographic keys. The SSD is also resident at the HLR/AC in the home network. The SSD for the specific MS may be changed at the discretion of the service provider (e.g., at fixed intervals or when fraudulent use of the terminal is suspected).

To allow the visited network to authenticate a roaming subscriber autonomously, the SSD is passed from the HLR/AC of the home network to the serving VLR, along with the subscriber-specific data (MIN/ESN). This transfer takes place when the subscriber first roams into the new network and invokes reregistration/location update procedures. If the

roaming agreement (between the home and the visited networks) does not include sharing of SSD, authentication response may be verified and crypto keys generated at the HLR/AC and sent to the VLR. However, it is more efficient if SSD is shared.

In current implementations, the VLR broadcasts a global challenge at frequent intervals in the form of a 32-bit random number (RAND), which can be used by any terminal served by the VLR to generate necessary authentication result. The use of the global challenge (as opposed to a unique challenge to individual terminals) enables the MS to respond to the challenge and send the result as part of a service request (e.g., call setup), thereby eliminating additional messaging on the radio channel. The authentication result (an 18-bit response called AUTHR) and the ciphering key at the MS are generated by activating the CAVE algorithm, using RAND (global challenge), SSD and MIN/ESN.

When the call setup request message (containing the AUTHR) from the MS is received by the MSC/VLR, the VLR also generates its own AUTHR using RAND, SSD, and MIN/ESN for the MS and compares it with AUTHR from the MS. If the two match, the MS is authenticated and the start ciphering command (with transfer of ciphering key to the base station) is issued.

An unauthorized interception of the SSD during its transport from the HLR to the VLR may result in the impersonation of a user and fraudulent use of the network. In D-AMPS a call count is used to prevent such fraud as well as general MS duplication or cloning. The call count is incremented in the MS upon a request from the network—generallyduring a call. The network also maintains the count. If multiple mobile stations are sharing an identity, the network will notice the discrepancy and take necessary action (e.g., changing the SSD).

During network access and other times (at the discretion of the network), a unique challenge may be used to verify the authenticity of the terminal/user. However, the unique challenge/response procedure does not generate cryptographic keys. Authentication for registration and call termination use ESN and MIN, whereas call originations use ESN and a subset of the dialed digits, the latter replacing the MIN as input to the CAVE algorithm.

The general authentication and privacy procedure used in D-AMPS is illustrated in Figure-5, which assumes shared SSD. Confidentiality for the users in terms of their identity and location is an important security aspect in mobile networks, where the user identity may be transported over an unencrypted radio channel during initial registration or call setup.

Figure-5 General authentication and privacy procedure for D-AMPS 5. (a) Name the main differences between Adhoc network and other network. What advantages do Adhoc networks offer? Explain in detail by giving suitable example.A MANET can be defined as a system of autonomous mobile nodes that communicate over wireless links without any preinstalled infrastructure. In MANETs communication between nodes is done through the wireless medium. Because nodes are mobile and may join or leave the network, MANETs have a dynamic topology. Nodes that are in transmission range of each other are called neighbors. Neighbors can send directly to each other. However, when a node needs to send data to another non-neighboring node, the data is routed through a sequence of multiple hops, with intermediate nodes acting as routers.

Self-Configuring and Self-Healing Processes:

Each node identifies the nodes that are available for communications, based on signal strength, which is mainly related to distance, but is also affected by obstructions or interference. Some nodes may be beyond range; others may be detectable but have insufficient signal strength for reliable communications. Once the available nodes are identified, this information is communicated to other nodes, along with information about the desired destination. Using the lists of available connections, the network configuration algorithm selects a particular routing for each user to its destination. This process requires system operating software to have good decision-making algorithms, based on practical criteria for signal strength, path reliability over time, and network configuration patterns. Over time, or even near-continuously, the network will change. Users may come and go, nodes may be in motion, or changes in the electromagnetic environment may alter the propagation between nodes. As these changes take place, the network will update its configuration and identify new paths from users to destinations

In the GSM, this threat is addressed by assigning a temporary, local identity (TMSI) for use across the radio channel. No such mechanism is currently available in DAMPS, though with the recent adoption of IMSI for D-AMPS (IS-41,-Revision D), this potential security threat will be minimized.

Figure-6 Example of Mobile Adhoc Network

Main characteristics of MANET: Bandwidth constrained, variable capacity links: Wireless links have significantly lower

capacity than their hardwired counterparts. Also, due to multiple access interference conditions, fading, and noise etc. The wireless links have low throughput.

Energy constrained operation: All or Some of the nodes in a MANET may rely on batteries. In this scenario, the most important system design criteria for optimization may be energy conservation.

Limited physical security: Mobile wireless networks are generally more prone to physical security threats than are fixed- cable nets. The increased possibility of denial-of-service, spoofing and eavesdropping attacks should be carefully considered. Existing link security techniques are often applied within wireless networks to reduce security threats. As a benefit, the decentralized nature of network control in MANET provides additional robustness against the single points of failure of more centralized approaches.

Autonomous and infrastructure less: Network is self-organizing and is independent of any fixed infrastructure or centralized control. The operation mode of each node is distributed peer-to-peer capable of acting as an independent router as well as generating independent data.

Security Threats: There are higher chances of physical security threats like spoofing, eavesdropping and denial of service (DoS) in wireless networks as compared to wired networks. We can have individual random mobility, group mobility, motion along preplanned routes, etc. The mobility model can have major impact on the selection of a routing scheme and can thus influence performance.

Multihopping: a multihop network is a network where the path from source to destination traverses several other nodes. Ad hoc nets often exhibit multiple hops for obstacle negotiation, spectrum reuse, and energy conservation. Battlefield covert operations also favor a sequence of short hops to reduce detection by the enemy.

Scalability: in some applications (e.g., large environmental sensor fabrics, battlefield deployments, urban vehicle grids, etc) the ad hoc network can grow to several thousand nodes. For wireless “infrastructure” networks scalability is simply handled by a hierarchical construction. The limited mobility of infrastructure networks can also be easily handled using Mobile IP or local handoff techniques.

Connection to the Internet: as earlier discussed, there is merit in extending the infrastructure wireless networks opportunistically with ad hoc appendices. For instance, the reach of a domestic wireless LAN can be extended as needed (to the garage, the car parked in the street, the neighbour’s home, etc) with portable routers.

Advantages of Ad Hoc Networks:

Main characteristics of MANET:

Mobility: the fact that nodes can be rapidly repositioned in ad hoc networks. Rapid deployment in areas with no infrastructure often implies that the users must explore an area and perhaps form teams/swarms that in turn coordinate among themselves to create a taskforce or a mission.

The principal advantages of an ad hoc network include the following: Independence from central network administration Self-configuring, nodes are also routers Self-healing through continuous re-configuration Scalable: accommodates the addition of more nodes Flexible: similar to being able to access the Internet from many different locations Fast installation: the level of flexibility for setting up AHN’s is high, since t hey do

no t r equ i r e any p r ev ious i n s t a l l a t i on o r i n f r a s t ruc t u r e and , t hus , t hey c an be brought up and torn down in very short time.

Dynamic topologies : nodes can arbitrarily move around the network and can disappear temporally from the AHN, so the network topology graph can be continuously changing at undetermined speed.

Fault tolerance: owi ng to t he l i m i t a t i ons o f t he r a d io i n t e r f a ces a nd the dynamic topology, AHN’s support connection failures, because routing and transmission control protocols are designed to manage these situations.

Connectivity: the use of centralized points or gateways is not necessary for  the communication within the AHN, due to the collaboration between nodes in the task of delivering packets.

Mobility: the wireless mobile nodes can move at the same time in different directions. Although the routing algorithms deal with this issue, the performances imu l a t i ons s how tha t t he r e i s a t h r e s ho ld l eve l o f node mob i l i t y s uch tha t p ro toco l operation begins to fail.

Cost: AHN’s could be more economical in some cases as they eliminate fixed infrastructure costs and reduce power consumption at mobile nodes.

Spectrum reuse possibility: owing to short communication links (node-to-node instead of node to a central base station), radio emission levels could be kept at low level. This increases spectrum reuse possibility or possibility of using unlicensed bands.

(b) Describe any two of the following: (i) DSDV (ii) DSR (iii) TORADestination Sequenced Distance Vector Routing(DSDV):

It is Proactive routing - based on Bellman – Ford. Packets are transmitted according to the routing table. Each node maintains routing table with entry for each node in the network<dest_addr, dest_seqn#, next-hop, hop_count, install_time>.

- Each node maintains its own sequence number which is Updated at each change in neighborhood information. Used for freedom from loops. Used to distinguish stale routes from new ones.

DSDV: Routing Update:

- Each node periodically transmits updates to keep table consistency Includes its own sequence number #, route table updates

<dest_addr, dest_seq#, hop-count>- Nodes also send routing table updates for important link changes (i.e. link breaks)- When two routes to a destination received from two different neighbors

Choose the one with the greatest destination sequence number. If equal, choose the smallest hop-count.

- When a node finds that a route is broken, it increments the sequence number of the route and advertises it with infinite metric.

- When X receives information from Y about a route to Z. Let destination sequence number for Z at X be S(X), S(Y) is sent from Y

If S(X) > S(Y), then X ignores the routing information received from Y. If S(X) = S(Y), and cost of going through Y is smaller than the route known to

X, then X sets Y as the next hop to Z. If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated to

equal S(Y).

Stale Entries:

Stale entries are defined to be entries that have not been updated the last few update periods.Stale entries are deleted at the same time when routing updates are applied to the routing table. Any route using that host as a next hop is deleted, included the route indicating that host as the actual destination.

DSDV: Full Dump/Incremental Updates-Routing table updates create lots of control traffic. DSDV addresses this problem by using two types of routing update packets.1. Full Dumps: • Packets carry all routing table information (Several NPDUs). • Packets are transmitted relatively infrequently.2. Incremental Updates: • Packets carry only information changed since last full dump. • Fits within one network protocol data unit (NPDU). • When updates can no longer fit in one NPDU, send full dump.

DSDV Protocol:

We consider a collection of mobile computers (nodes) which may be far from any base station. The computers (nodes) exchange control messages to establish multi-hop paths in the same way as the Distributed Bellman-Ford algorithm. These multi-hop paths are used for exchanging messages among the computers (nodes). Packets are transmitted between the nodes using routing tables stored at each node. Each routing table lists all available destinations and the number of hops to each destination. For each destination, a node knows which of its neighbours leads to the shortest path to the destination. We need to maintain the consistency of the routing tables in a dynamically varying topology. Each node periodically transmits updates. This is done by each node when significant new information is available. We do not assume any clock synchronization among the mobile nodes.

The route-update messages indicate which nodes are accessible from each node and the number of hops to reach them. We consider the hop-count as the distance between two nodes. However, the DSDV protocol can be modified for other metrics as well. A neighbour in turn checks the best route from its own table and forwards the message to its appropriate neighbour. The routing progresses this way. There are two issues in this protocol :

X Y Z

How to maintain the local routing tables. How to collect enough information for maintaining the local routing tables.

Maintaining Local Routing Table:

- We will first assume that each node has all the necessary information for maintaining its own routing table.

- This means that each node knows the complete network as a graph. The information needed is the list of nodes, the edges between the nodes and the cost of each edge.

- Edge costs may involve : distance (number of hops), data rate, price, congestion or delay.

- We will assume that the edge cost is 1 if two nodes are within the transmission range of each other.

- The DSDV protocol can also be modified for other edge costs.

Route Advertisements:

The DSDV protocol requires each mobile node to advertise its own routing table to all of its current neighbours. Since the nodes are mobile, the entries can change dynamically over time. The route advertisements should be made whenever there is any change in the neighbourhood or periodically. Each mobile node agrees to forward route advertising messages from other mobile nodes. This forwarding is necessary to send the advertisement messages all over the network. In other words, route advertisement messages help mobile nodes to get an overall picture of the topology of the network.

Route Table Entry Structure:

- The route advertisement broadcast by each mobile node has the following information for each new route :

The destination’s address The number of hops to the destination The sequence number of the information received from that

destination. This is the original sequence number assigned by the destination.

An Example of Route Update:

N2

N3

N11

N4 N5

At the start, each node gets route updates only from its neighbour.

For N4, the distances to the other nodes are : N5=1, N3=1, N2= ∞, N1 = ∞

All nodes broadcast with a sequence number 1.

After this, nodes forward messages that they have received earlier.

The message that N2 sent to N3 is now forwarded by N3.

For N4, the distances are now : N5=1, N3=1, N2=2, N1= ∞

All messages have sequence number 1.

Finally, after second round of forwarding, n4 gets the following distances :

n5=1, n3=1, n2=2, n1=3

Suppose n5 has moved to its new location. Also, n5 receives a new message from n1 with a sequence number 2. This message is forwarded by n5 to n4. Two distances to n1 in n4

Distance 3 with seqence number 1, and Distance 2 with sequence number 2

Since the latter message has a more recent sequence number, n4 will update the distance to n1 as 2.

Responding to Topology Changes: Some of the links in a mobile network may be broken when the nodes move. A broken link is described by a distance ∞. When a link to a next hop is broken, any route through that next hop is given a distance ∞. This is considered as a major change in the routing table and immediately broadcast.

N11

N2

N3

N4

N5

1. Link between B and D breaks2. Node B notices break and

- Updates hopcount for D & E to infinity- Increments seq# for D & E

B’s Route Table

Node B sends update with new route information

B’s Route Table

Advantages of DSDV:

- DSDV is an efficient protocol for route discovery. Whenever a route to a new destination is required, it already exists at the source.

- Hence, latency for route discovery is very low.

- DSDV also guarantees loop-free paths.

Disadvantages:

- However, DSDV needs to send a lot of control messages. These messages are important for maintaining the network topology at each node.

- This may generate high volume of traffic for high-density and highly mobile networks.

- Special care should be taken to reduce the number of control messages.

Dynamic Source Routing:

Advantage- Routes maintained only between nodes who need to communicate. This reduces overhead

of route maintenance.- Route caching can further reduce route discovery overhead.- A single route discovery may yield many routes to the destination, due to intermediate

nodes replying from local caches.

Dynamic Source Routing:

Disadvantages- Packet header size grows with route length due to source routing.- Flood of route requests may potentially reach all nodes in the network.- Care must be taken to avoid collisions between route requests propagated by neighboring

nodes insertion of random delays before forwarding RREQ

- Increased contention if too many route replies come back due to nodes replying using their local cache

Route Reply Storm problem- An intermediate node may send Route Reply using a stale cached route, thus polluting

other caches.- This problem can be eased if some mechanism to purge (potentially) invalid cached

routes is incorporated.- Some proposals for cache invalidation are

Static timeouts. Adaptive timeouts based on link stability.

Temporarily Ordered routing Algorithm (TORA):

The 3 Routes of TORA:

Route Creation: Establishing a set of directed links from the source to destination. Route Maintenance: Changes in topology cause routes to be reestablished. Route Erasure: Upon partition detection routes are removed.

Three Control Packets:

Query (QRY) flooded through network to establish routes. Update (UPD) propagates back if route exists and re-orient route structure Clear (CLR) flooded through network to erase invalid routes.

Route Creation:

A node will create a route if it has no downstream neighbors to the destination. It will set Route Required (RR) flag and broadcast a QRY packet. The QRY packet contains the destination node. An UPD packet will be used to reply.

Upon Receiving a QRY

IF RR flag is set, does not forward and discard QRY packet.

IF RR flag not set and has no downstream neighbors, it sets RR and rebroadcasts QRY.

If node has at least 1 downstream neighbor and the height for that link is NULL (-,-,-,-,i) it sets its height to the minimum of its neighbors, increments its d value and broadcasts an update.

If the node has a downstream neighbor and its height is non-Null and the RR flag is set. It will discard the UPD packet.

If the RR flag is not set it will send an update packet.

Upon Receiving an UPD

If the reflection bit of the neighbors height is not set and its route required flag is set it sets its height for the destination to that of its neighbors but increments d by one. It then deletes the RR flag and sends an UPD message to the neighbors, so they may route through it.

If the neighbors route is not valid or the RR flag was unset, the node only updates the entry of the neighbors node in its table.

Route Maintenance:

Maintenance Cases: 1

1 Generate: The node has lost its last downstream link due to a failure. The node defines a new "reference level", so it sets oid (originator id) to its node id and t to the time of the failure. This is done only if the node has upstream neighbors. If not it sets its height to NULL.

Maintenance Cases: 2

2 Propagate: The node has no more downstream link due to a link reversal following the receipt of an update packet and the reference levels (t,oid,r) of its neighbors are not equal. The node then propagates the references level of its highest neighbor and sets the offset to a value which is lower (-1) than the offset of all its neighbors with the maximum level.

Maintenance Cases: 3

3 Reflect: The node has lost its downstream links due to a link reversal following the receipt of an update packet and the reference heights of the neighbors of the node are equal with the reflection bit not set. The node then reflects back the reference height by setting the reflection bit. It's d value is set to 0.

Maintenance Cases: 4

4 Detect: The node has lost its downstream links due to a link reversal following the receipt of an update packet and the reference heights of the neighbors of the node are equal with the reflection bit set. This means that the node has detected a partition and begins the route erasure procedure. The height values are set to NULL.

Maintenance Cases: 5

5 Generate: The node has lost its last downstream link due to a link reversal following the receipt of an update packet and the reference heights of all the neighbors are equal with the reflection bit set and the oid of the neighbors heights isn't the node's id. The node then sets t to the time of the link failure and sets oid to its own id. The d value is set to 0. This means that the link failure required no reaction. The node experienced a link failure between the time it propagated a higher reference (from someone else) and the time this level got reflected from a place further away in the network. Because the node didn't define the new reference level itself this is not necessarily an indication of a partitioning of the network. So the node simply defines a new higher reference level with the time of the link failure.

Route Erasure:

When a node has detected a partition it sets its height and the heights of all its neighbors for the destination in its table to NULL and it issues a CLR packet. The CLR packet consists of the reflected reference level (t,oid,1) and the destination id.

If a node receives a CLR packet and the reference level matches its own reference level it sets all heights of the neighbors and its own for the destination to NULL and broadcasts the CLR packet. If the reference level doesn't match its own it just sets the heights of the neighbors its table matching the reflected reference level to NULL and updates their link status (->undirected).

Advantages: That of an on-demand routing protocol – create a DAG only when necessary. Multiple paths created. Good in dense networks.

Disadvantages:

Same as on-demand routing protocols. Not much used since DSR and AODV outperform TORA. Not scalable by any means.

(c) Distinguish among proactive, reactive and hybrid routing protocols of MANET. Explain GSR protocol.

Hybrid Routing:- Proactive for neighborhood, Reactive for far away (Zone Routing Protocol).- Proactive for long distance, Reactive for neighborhood (Safari Protocol).- Attempts to strike balance between the two.

Global State Routing:

- Global State Routing (GSR) is based on Link State (LS) routing and Distance Vector routing.

- In the LS routing method, each node floods the link state information directly into the whole network (global flooding) once a link change between itself and its neighbors is detected. A node gets to know the whole topology by obtaining link information. LS routing works well in static topology networks. If links change quickly at high mobility, frequent global flooding will lead to huge control overhead (large amount of small packets).

- The knowledge of full network topology as LS routing should be maintained, but the inefficient flooding mechanism has to be avoided.

- Unlike LS, GSR does not flood link state packets. Instead, every node maintains its link state table based up-to-date ( LS information received from neighboring nodes) It will periodically exchange its LS information with its neighbors only (no global flooding). This means that GSR is MAC (medium access control) layer efficient as it keeps the overhead of control message low. GSR still finds accurate and optimal paths. GSR could be described as being based on LS routing, which has the advantage of routing accuracy, and the dissemination method used in DBF(Distributed Bellman-Ford), to avoid inefficient flooding like in LS routing.

Each node maintains:

A neighbor list:

- Containing the list of nodes adjacent to the node (hop=1)

A topology table:

- Containing the link state information reported by a destination and a timestampindicating the time at which this has been reported.

A next hop table:

- Containing the next hop to which the packets for this destination have to be forwarded

A distance table:

- Containing the shortest distance to each destination node- Initially, each node learns about its neighbors by examining each received packet

and thus builds up its neighbor list. Each node updates link state information in its topology table by receiving link state messages from its neighbors. LS packets with larger sequence numbers replace the older ones with smaller sequence numbers. So every node learns the entire network topology.The entire topology map (link state table) is exchanged periodically with neighbors only, meaning that there is no global flooding. Then each node computes the shortest paths itself using the newly rebuild topology map, based on Dijkstra’s algorithm.

- In summary this means that based on the link state vectors, nodes maintain a global knowledge of the network topology and take their routing decisions locally.

- The following section will describe some performance measurements under different circumstances (simulated) and compare GSR with both protocols it is partly based on, namely LS and DBF.

Routing Inaccuracy:

- GSR is less accurate than LS as it updates routing information only every 3 time slots, but it still outperforms DBF. Link State reacts fastest to topology changes.

Control Overhead:

- The control overhead of GSR and DBF remains constant regardless of mobility as the routing information is exchanged periodically with adjacent neighbors only. LS has maximum overhead since it is event triggered. This validates that LS is not suitable for high mobility environment.

Mobility Impact:

- As described previously, the impact of mobility to routing inaccuracy is independent of the impact to control overhead. Overall, higher mobility causes higher inaccuracy for all 3 protocols.

Update Interval:

- The routing inaccuracy and overhead may be improved or degraded with change in update interval for GSR and DBF.

Radio Range:

- A higher transmission range means that one will get a larger connectivity degree but also a larger control packet size for GSR and LS. More nodes can be reached in one hop without requiring routing decision at a higher transmission range. Spatial reuse is less efficient when transmission range is high and routing inaccuracy (routing error rate) decreases for higher transmission ranges.