WIRELESS LAN

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IT 2402\MOBILE COMMUNITCATION \ U-2 \Page

CONTENTS

- Infrastructure and Ad Hoc networks

- IEEE 802.11 WLAN - Advantages, Disadvantages, Infrared Vs radio transmission

-System Architecture

-Protocol Architecture

-Physical layer - FHSS, DSSS, Infrared

-MAC layer - DFWMAC-DCF, RTS / CTS, PCF with polling

- MAC Management Synchronization, Registration, Handoff, Power

Management, Roaming, Security

- Wireless local loop

- IEEE 802.16 WiMAX

1. Infrastructure and Ad Hoc networks

1.1. Infrastructure based networks:

Provides Wireless Devices access to infrastructure network

Contains forwarding function, medium access control function etc.

Communication between wireless nodes and the access points but not directly between nodes.

Access points, with network in between, can connect several wireless networks to form a larger network.

The network is simpler as most of the network functionality is located at the Access Point and client remains quite simple.

Coordination is required for medium access to avoid collision.

Typical infrastructure based wireless network is shown below:

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If access point controls the medium access by polling, collision may be avoided and this may also result in guaranteeing minimum bandwidth for certain nodes.

This architecture has less flexibility in case of disaster, system collapses.

Mobile cellular network, satellite cellular phones, are example of this type.

1.2 Ad Hoc Network: Following diagram depicts the simple ad hoc network concept.

- Nodes within an ad hoc network can only communicate if they are radio distant close to each other.

The device complexity is higher as they have to implement, medium access mechanism, mechanism handle hidden terminal, priority mechanism to provide certain quality service etc.

Exhibits greatest flexibility.

There are other types of system that are mix of these two types. That is, network infrastructure is used for basic services such as authentication, control of medium access for data with associated quality service, management functions etc, but also provide direct communication between wireless nodes.

2. IEEE 802.11 WIRELESS LAN (WiFi)

2.1 .Introduction

IEEE 802. x specifies a number of standards, like Ethernet, token ring etc. Wireless is also clubbed along with these as the protocol structure is similar. In all these, only the physical layer and Data Link Control layer are different leaving all other upper layers same. The data link layer is divided into Logical Link Control Layer and Medium Access Control Layer as shown below:

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General functionality of Physical layer is to provide encoding / decoding of signals, synchronization and bit transmission and reception. Similarly the functionality of MAC layer is - on transmission, to assemble data into frame with address and error detection fields, on reception , to disassemble frame and perform address recognition and error detection. Govern access to the LAN transmission medium and to provide interface to higher layers and perform flow and error control.

IN addition to the above functionality, WLAN inlcude the support of power management, handling hidden nodes ability to operate world wide. To create world wide operability, the ISM bands at 900MHz and 2.4 MHz are selected in addition to IR transmission reception.

Advantages of Wireless LAN:

Flexibility: Within radio coverage, nodes can access each other as radio waves can penetrate even partition walls.

Planning: No prior planning is required for connectivity as long as devices follow standard convention

Design: Allows to design and develop mobile devices.

Robustness: wireless network can survive disaster eg earthquakes. If the devices survive, communication can still be established.

Disadvantages:

Quality of Service: Lower than the wired counterparts due to low bandwidth (1 10 Mbps), higher error rates due to interference ( 10 exp 4 rather than 10 exp 10 as in the case of wired network)

Cost: Wireless LAN adapters are costly compared to wired adapters.

Proprietary Solution: Due to slow standardization process, many solution are proprietary that limit the homogeneity of operation.

Restriction: Individual countries have their own radio spectral policies. This restricts the proliferation of the technology

Safety and Security: Wireless Radio waves may interfere with other devices. Eg; In a hospital radio waves may interfere with high tech equipment.

Competing Requirement:

Global Operation: For global operation many national and international frequencies have to be considered so that LAN devices can be carried across the globe.

Low Power : As the devices will operate with batteries, the design should take care of these facts.

License Free operation: Should be able to operate in the ISM band of frequencies so that no license need be applied for its operation.

Robust Transmission Technology: Must be capable of operating in difficult condition where in high interference is expected from other electrical devices.

Simplified Spontaneous Co Operation: Must be able to network after power up without much complication.

Easy to Use: Must be easy to use by a common man without complicated procedure.

Protection to Investment: Must be able operate with existing system without modification.

Safety and Security: Should incorporate safe operation in places like hospital and other critical areas like armament depot etc. NO user must be able to read personal data during transmission that is encryption mechanism should be integrated. Must not be possible to collect roaming profile of any user.

Transparency for application: Existing application must continue to run over wireless LAN, may be with higher delay and lower bandwidth.

There are two technologies based on which the WLAN are set up. One based on IR technology (around 900nm) and other based on radio transmission at 2.4 GHz. Brief description of the same is given below:

IR System

Wireless LAN Technology uses, infrared or radio transmission technologies. Infrared technology (900 nano meters)uses diffused light reflected at walls or directed light if a line of sight exists between sender and receiver. LEDs or Laser diodes are used as source for transmission while photodiodes are used as receivers. The main advantages of infrared are, these are very cheap and all mobile devices (PDA, Laptos, Mobile phones) are fitted with IrDA Infra red Data Association - interface (Data rate of 115 kbps for 1.0 version and 1.152 4 Mbps for IrDA 1.1 Version). The disadvantage, is its lack of penetration into walls and other obstacles. Works with LOS for high data rate.

Radio System:

Advantages of radio transmission are it can cover large areas, can penetrate walls furniture, trees. RF does not need LOS unless the frequencies are very high. Current Transmission rates possible are 10 mbps.

Disadvantage: As the radio waves can penetrate the walls, shielding is not very simple and can interfere with other electrical devices. Radio transmission is permitted in specific band only. Very limited ranges of license free bands are available worldwide.

2.2 System Architecture:

2.2.1 Infrastructure based :

Following is the figure of infrastructure based network.

Nodes known as stations are wirelessly connected to access points (AP) that are within the radio coverage. The radio coverage area of the Access Point is known as Base Service Set (BSS). The association between the station and a BSS is dynamic. Station may turn off, come within range and go out of range etc. Two or more BSS are connected via a Distribution System (DS). This process extends the reachability of the nodes in a BSS. This network is called Extended Service Set (ESS). The ESS appears a single logical LAN to the logical control level (LLC). The DS connects the BSS / ESS AP with a portal - which is implemented in a device such as a bridge or router that is part of a wired LAN - which forms the Interworking units to other LANs.

2.2.2 Adhoc based:

IEEE 802.11 also allows to build Ad Hoc Networks as shown in the following figure:

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IN this case a BSS comprises a group of stations that use the same frequency. There is connectivity between the station within a BSS but not to nodes of other BSS. However if the radio frequency do not overlap, there can be more number of BSS in the same geographical area Also 802.11 does not specify any special nodes that support routing, forwarding of data or exchange of topology information.

2.3 Protocol Architecture: It is intended that IEEE 802.11 protocol architecture fits seamlessly into the other 802.x standards. Following figure depicts the 802.11 integrated to Ethernet, that is 802.3 protocol via abridge.

As it can be seen, higher layers look the same for wired as well as wireless nodes. Also, applications should not notice any difference apart from the lower bandwidth and higher access time from the wireless LAN. The upper part of the Data Link Control Layer - the LLC covers the differences of the medium access control layers needed for different media (IR, Radio- FHSS, DSSS CCK).

The PHYSICAL layer is subdivided into a PLCP Physical Layer Convergence Protocol and the PMD Physical Medium dependent Sub layer. As shown below:

The basic task of MAC layer comprises of medium access, fragmentation of user data and encryption.

The PLCP sub layer provides a carrier sense signal called Clear Channel Assessment(CCA) and provides a common PHY Service Access Point (SAP)

PMD sub layer handles modulation and encoding / decoding of signals.

The MAC Management layer supports, association and re association of a station to an access point and roaming between different access points. It also controls, authentication mechanism , encryption, synchronization of a station with regard to an access point and power management.

MAC Management also maintains MAC Management Information Base.

PHY Management includes channel tuning and PHY MIB maintenance.

Station management interacts with both the management layers and is responsible for additional layer functions such as - control of bridging and interaction with the distribution system in case of access point.

2.3.1 Physical Layer:

IEEE 802. 11 supports different type of physical layer. One layer based on Infra red and two layers based on the radio transmission (primarily in the ISM bands at 2.4 GHz). These were introduced in the year 1997. In addition , two more variants with higher data rates were introduced in the year 1999. These were IEEE 802.11 a and 802.11b . The details are shown below:

App PHY variants include the provision of the clear channel assessment (CCA). The purpose of this signal is to provide medium access mechanism by indicating if the medium is busy or idle. The transmission technology determines exactly how this signal is obtained.

Also, the physical layers offers a service access point (SAP) with a 1 or 2 Mbps transfer rate to the MAC layer.

Following chart gives the details of the PHYSICAL Layer specifications:

Direct Sequence Spread Spectrum:

In this up to seven channels, each with a bandwidth of 5 MHz can a be used Number of channel available in each country depends on the spectrum allocated by that country. The encoding schemes that is used is DBPSK for the 1 Mbps and DQPSK for 2 Mbps rate. DSSS makes use of the chipping code or pseudo noise sequence, to spread the data rate and hence the bandwidth of the signal. For IEEE 802.11, a 11 bit barker sequence is used. Maximum transmit power is limited to 100 mw EIRP(Equivalent Isotropically Radiated Power) All bits are transmitted by the DSSS PHY layer are scrambled with the polynomial s(z) =

z7 + z4 + 1.

Following figures shows a frame of the physical layer using DSSS:

The frame consists of two parts, the PLCP part (preamble and header) and the payload part. PLCP part is always transmitted at 1 Mbps where as the pay load, that is the MAC data can use either 1 Mbps or 2 Mbps. The details of the field is given below:

Synchronisation : The first 128 bits are for synchronization, gain setting, energy detection (for the CCA) and frequency offset compensation. These are scrambled 1 bits.

Start Frame Delimiter (SFD) : This 16 bit files is used for synchronization at the beginning of a frame and consists of pattern 1111001110100000.

Signal : As on now only two values have been specified . 0x 0A indicates 1 Mbps and Ox 14 indicates 2 Mbps. Other values have been reserved for future use.

Service : This fields is reserved for futures use. 0x 00 indicates an IEEE 802.11 compliant frame.

Length: 16 bits are used for length indication.

Header Error Check : Signal, service and length fields are protected by this checksum using CRC 16 polynomial.

Frequency Hopping Spread Spectrum:

Frequency hopping spread spectrum technique allows for coexistence of multiple networks in the same area by separating different networks using different hoping sequences. The standard defines 79 hopping channels for NA and 23 hopping channels for Japan (each with a bandwidth of 1 MHz in the 2.4 MHz ISM band.) For modulation, FHSS schemes uses two level Gaussian FSK for the 1 Mbps system and for four level GFSK for 2 Mbps system. In this bits one and zero are encoded as deviations from the current carrier frequency. In case of four level, four different deviations form the carrier frequency. While sending and receiving is mandatory at 1 Mbps for all devices, operation at 2 Mbps is optional. Following diagram shows the frame of the physical layer used with FHSS.

In this also there two parts - PLCP part and Payload part. Functions of various fields are described below:

Synchronisation: The PLCP preamble starts with 80 bit synchronization which is 010101 .. bit pattern. This is used for synchronization and for CCA.

Start Frame Delimiter (SFD): These 16 bits indicate that start of the frame and this provide frame synchronization. The SFD pattern is 0000110010111101

PLCP _PDU Length Word (PLW): This fields indicates the length of the payload in bytes including the 32 bit CRC at the end of the payload. PLW can range between 0 4095.

PLCP Signaling Files (PSF) : This four bit field indicates thje data rate of the payload following 1 or 2 Mbps.

Header error check : The PLCP header is protected by a 16 bit checksum with the standard ITU_ T generator Polynomial .

Infrared:

The IEEE 802.11 infrared scheme is omni directional rather than point to point. A range of up to 20 m is possible. The modulation scheme for the 1 Mbps data rate is known as 16 PPM (Pulse Position Modulation). In this scheme group of 4 data bits is mapped into one of the 16 PPM symbols, each symbol is a string of 16 bits. Each 16 bit string consists of fifteen 0 and one binary 1. For the 2 Mbps data rate, each group of 2 data bits is mapped into one of four 4 bit sequences. Each sequence consists of a three 0 and one binary 1. The actual transmission uses an intensity modulation schemes in which the presence of a signal corresponds to a binary 1 and the absence of a signal corresponds to a binary 0.

IEEE 802.11 a:

This makes use of 5 GHz band. In this orthogonal frequency division multiplexing (OFDM) is used. This is also called multi carrier modulation. Uses multiple carrier signals at different frequencies, sending some of the bits on each channel. This is similar to FDM. But in the case of OFDM, all of the sub channels are dedicated to a single data source. The possible data rates are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. The systems uses up to 52 sub carriers that are modulated using BPSK, QPSK 16 QAM or 64 QAM, depending on the data rate required.

IEEE 802.11b:

It is an extension of the IEEE 802.11 DS-SS scheme, providing rates of 5.5 and 11 Mb[s. The chipping rate is 11 MHz, which is the sme as the original scheme, thereby occupying the same bandwidth. To achieve higher data rate in the same bandwidth at the same chipping rate, a modulation scheme known as Complementary Code Keying (CCK) is used.

In CCK, input data are treated in blocks of 8 bits at a rate of 1.375 MHz ( 8 bits / symbol * 1.375 MHz = 11 Mbps). Six of these bits are mapped into one of the 64 codes sequences based on the use of 8 x 8 Walsh matrix. The out put of the mapping and two additional bits forms the input to a QPSK modulator

2.3.2 MEDIUM ACCESS CONTROL:

a) MAC Mechanisms:

Three methods defined in IEEE 802.11 are known as Distribution Foundation Wireless MAC.

Basic version based on CSMA/CA (Broadcast method)

Optional method to avoid hidden terminal problem. RTS / CTS (Unicast method)

Contention free polling method for time bounded service. PCF (Point Coordinated Function)

First two are summarized as Distributed Coordinated Function (DCF) the third method is called Point Coordinated Function. (PCF)

MAC mechanism is also called DFWMAC (Distributed Foundation Wireless Medium Access Control).

For all access methods, several parameters are defined that control the waiting time for the nodes. The duration is defined as multiples of slot time. Slot time is derived from medium propagation delay, transmitter delay and other PHY layer dependent parameters (50 micro sec for FHSS & 20 Micro sec for DSSS)

Three different IFS (Inter Frame Space) parameters define the priority of the medium. These are DIFS (DCF IFS) , PIFS(PCF IFS) and SIFS (Short IFS).

Short IFS (SIFS)

Shortest IFS, Highest priority, (DSSS = 10 Micro sec and FHSS = 28 Micro sec)

Used for Acknowledgement, CTS and poll Response which are all immediate response actions

Point Coordination Function IFS (PIFS)

Mid-length IFS

Used by centralized controller in PCF scheme when using polls (SIFS + 1 slot time)

Distributed coordination function IFS (DIFS)

Longest IFS

Used as minimum delay of asynchronous frames contending for access

Used for all ordinary asynchronous traffic

i)Basic Method DFWMAC:

Basic and mandatory access method based on CSMA /CA. If the medium is idle for a certain duration of DIFS(ascertained with CCA), a station accesses the medium immediately. (short delay light load). If medium is busy, nodes enter contention phase by choosing Radom Backoff time. After this period, if the medium is still busy, it has to wait once again fir DIFS. To provide fairness, the backoff timer is stopped, and continued from remained duration in the next cycle. Backoff timer values increased exponentially from 0-7,15,31,63,127,255 depending load being light or heavy.

Station 3,1,are ready to transmit. Station 3 waits for DIFS and with channel being free, starts transmission. Station 2 defer transmission and waits for DIFS after channel busy period. By this time station2 and station 5 become ready to transmit. Station 1,2,5 wait for DIFS period after busy period and generate backoff time. Station 2 having shortest backoff time, accesses the medium first and station 1 and 5 enter the contention phase in the cycle with the residual back off timer. By this time station 4 joins and generates backoff time equal to that of station 5. After DIFS station 2,4,5 stars counting backoff timer and as station4, 5 have same back off timer, the transmission clashes. Once again in the next cycle, station1 completes the backoff timer successfully and occupies the medium. Station 4,5 enter the contention phase once again.

In case of Unicast transmission, receiver answers directly with ACK after SIFS period which has the shortest duration. This ACK ensures correct reception (correct checksum at the receiver). This is important in the error prone environment. If no ACK is received after SIFS, the sender resends the data after going the through contention cycle.

ii))Contention Based RTS/CTS to avoid hidden node problem

Hidden node problem arises in case of WLAN, when two nodes that are not within the radio range of each other try to communicate with a node that is within radio range of each of them. To avoid collision at the receiver, RTS CTS based contention mechanism is used.

In this, after waiting for DIFS plus random backoff time, the sender can issue a RTS control packet. RTS is not given higher priority. RTS will contain the address of the receiver and the duration of data. All station receiving this information will set the Net Allocation Vector- NAV in accordance with duration field . NAV is the earliest time when the station can access the medium. The receiver receiving the RTS, sends CTS after SIFS. The CTS control packet contains duration field and the address of the node from which it will receive the data. All nodes listening to this transmission will set their NAV till the acknowledgement is received. As all the nodes closer to both transmitter and receiver are set to NAV, this will avoid the hidden node problem.

iii)PCF Access Mechanism

PCF (Point Coordination Function) mechanism sits over DCF to support contention free time transmission operation. (As the operation is complicated, most manufacture have not opted for it). PCF operation is available only for infrastructure network. AP playing the point of coordinator, stops all other terminals and polls other stations in semi periodic manner as shown below

Contention based methods do not guarantee minimum bandwidth to all nodes. The medium access is not fair. In order to provide equal opportunity to all nodes, IEEE 802.11 specifies an optional methods called PCF. PCF provides time bounded service. Ad Hoc network cannot use this provision. Point Coordinator splits the access time - super frame period - into contention free period and contention based period. In the contention free period, the point coordinator (Access Point) waits for PCF IFS period (instead of DIFS) and access the medium and starts the polling with each station. This is shown in the following figure.

Access Point sends data1 to first station. Station 1 sends ACK after SIFS. After SIFS duration, PC sends data to station2. In case there is no data to be sent by the AP to a node, the AP waits for a duration equal to SIFS and polls the next station as shown below. This process continues, thus providing equal time to all the station. At the end, it generates CFend (Contention free period End) control pulse at which time, contention based medium access takes place. At the end of contention based period, the super frame repeats itself.

This ensures mini duration for each node in the system to access the Access Point.

2.3.3 MAC FRAME FORMAT:

The frame transmission in case of WLAN can be categorised into three types: Data, Management and Control. These are indicated in the MAC frame. Following figure shows the MAC frame format.

This is a general format : (a) in octets while (b) in bits

Overall frame structure is shown in the(a). Details are as follows:

FC : Frame Control indicates the type of frame : Data/Management / control

D/I : Duration / connection ID: When used as a duration field, indicates the time in microseconds the channel will be used by the source. In some cases, it may indicate connection identifier.

Addresses: These are Source/ Destination/ Sender and Receiver addresses.

SC: Sequence Control: Four bits are used for numbering fragmentation of a message while the 12 bits are used sequence number sent between a transmitter and a receiver.

Frame Body: These are MAC SDU limited to a length of 2312 bytes.

Frame Check Sequence: a 32 bit Cyclical Redundancy Check.

Details of the frame control fields are shown in part (b) of the above figure.:

Protocol Version: 802.11 version. Currently it is set to 0

Type: Identifies the frame as control, management or data frame.

Sub Type: Further identifies various function under each type. Details of Type and Sub type are shown below:

1. Management Type (00)

0000 / 0001 : Station requesting association with a AP (BSS) and APs response

0010 / 0011 : Re association request and response. Sent by a station when moves out of a BSS and moves into a different BSS.

0100 / 0101 : Probe Request and Response Used to obtain information from another station or AP

1000 : Beacon

1001 : Announcement Traffic Indication Map : ATIM: A station making an announcement to other station that it has buffered data to transmit and its power low.

Disassociation: Used by a station to terminate Association.

1011 / 1100 : Authentication and De authentication. Indicates using secure communication.

2. Control Type : 01

1010 : Power Save Poll: Request the AP or other station to transmit any buffered data as the station is in power saving mode.

1011 / 1100 : RTS and CTS;

1101 : ACK

1110 / 1111 : CF End / CF end with ACK.: Announces the end of contention free period (PCF) / ACK the CF end.

3. Data Frames : 10:

0000 / 0001 : Data / Data with CF ACK. Simple data transfer / Sends ACK for previously received data along with Data

0010 / 0011 : Data + CF Poll : Used by PCF to deliver data a mobile station and also to request that the mobile station send a data frame that it may have buffered.

0100 / 0101 : No data / CF ACK no Data indicates that the frame carries no data, polls or ACK, but used to carry power management bit int the frame fields to the AP to indicate the AP that is changing to low power operation mode.

0110 / 0111 : CF Poll (no data)/ CF Poll ACK (no data) : These fames with no data.

2.3.4 MAC Management Sub Layer:

This sub layer handles various management functions such as Synchronization, Registration, Handoff, Power management, roaming and security. These are explained briefly:

a) Synchronization:

To synchronize all stations, IEEE 802.11 specifies a TCF (Time Synchronization Function). Synchronized clocks are needed for power management, for coordinated PCF and (Super frame, CF and Contention based period synchronization) and for synchronization of FHSS hopping sequence. A beacon is transmitted periodically which contains the following information:

time stamp (For Synchronization)

BSS ID

Traffic Indication Map (TIM)

Power Management

Roaming

i)Infrastructure Based Network

Beacon transmission is not periodic. Its transmission is deferred, if the medium is busy. As it can be seen in the following figure, second beacon is deferred till the medium is free. In Infrastructure based network, access point performs the sync function. All other nodes, adjust their local timer to the time stamp.

ii)Ad Hoc Network

In case of ad hoc network, beacon is transmitted whichever station access the medium first using random backoff algorithm. If the medium is busy at the scheduled interval, its transmission is deferred by the node. Later, once again each node contends for access to the medium and which ever node access the medium first, it sends the beacon. This can be seen in the following figure. In the first case, station access the medium first and transmits the beacon. However, at the scheduled repetition, the medium becomes busy and as the medium becomes free, all nodes contend for access and this time node 2 access the medium first and transmits the beacon frame. All other nodes synchronize with the second node now.

b) Registration: . The beacon is a management frame that is transmitted quasi periodically by the AP to establish the timing synchronisation function (TSF). It contains information such as the BSS ID, Time stamp (for synchronisation), power management , and roaming. Received Signal Strength (RSS) measurements are made on the beacon message. Any node receiving the beacon, if it decides to register with the AP, will a Association Request frame. The AP receiving the same, will send back Association Response frame back. With this , the registration will complete and the node will form the part of the AP. Only on registration, the distribution system will know to which BSS an MS is attached.

c) Hand Off: There are three types of mobility in WLAN .

No transition : Implies the MS is within or is moving within a BSA.

BSS Transition: Indicates that the MS moves from one BSS to another in the same ESS.

ESS Transition: This is the movement of MS from one BSS to another BSS that is part of new ESS. In this case connection may break unless it has higher layer IP connection.

The handoff procedure is a WLAN is shown in the following figure:

The AP broadcast a beacon signal periodically (typically once in 100 ms). An MS that scans the beacon signal and associates itself with the AP with the strongest beacon. The beacon contains information corresponding to the AP such as Time Stamp, beacon interval, capability, ESS Id and traffic indication Map (TIM). The MS uses this information in the beacon to distinguish between different APs.

The MS keeps tack of the RSS of the beacon of the AP with which is it is associated, and when the RSS becomes weak, it starts to scan for stronger beacons from the neighbouring APs. The scanning process can be either active or passive. In passive scanning, the MS simply listens to available beacons. In active scanning, the MS sends a probe request to a targeted set of APs that are capable of receiving its probe. Each AP that receives the probes responds with a probe response that contains the same information that is available in the beacon except for the TIM. The probe response thus serves the MS to select the AP with strongest beacon and sends re-association requests to the new AP. In response, the new AP sends re association which contains information of MS and that of old AP. In Response the new AP sends the re association response that has the information about the supported bit rates, station ID and so on needed for communication. The old AP is not informed by the MS about the change of location. The hand off is intimated by using IAPP (Inter Access point protocol) standard that intimates the old AP about handoff through wired network.

d) Power Management:

Power Management is carried out when the station is in awake position during reception of inbound data and sleep position during idle period. Throughput is traded for battery life. Longer off period, low throughput and vice versa.

States of a station: sleep and awake and buffering of data in senders. Sleeping station wakes periodically and stays awake for certain duration. If it detects that it has to receive data, it keeps awake. Waking up at right time requires TSF(Timing Synchronization Function.

A receiver node knows when to transmit data but does not know when it receive data or when to wake up.

System should be synchronized to sleep and wake up transparently. (Known to other nodes)

Longer off period saves battery but throughput is reduced and vice versa.

i)Infrastructure Based Network

AP buffers data for sleeping nodes and sends TIM (Traffic Indication Map) which contains list of address for which uni-cast data are buffered in AP. AP sends multicast or broadcast data periodically at each DTIM (Delivery Traffic Indication Map) DTIM is multiple time of TIM.

Once the nodes are synchronized, the AP sends Traffic indication map (TIM), periodically in which it transmits the address of the nodes for which it has the data buffered. Any node that has a data to receive will continue to wake up till the time the data is transmitted by AP and received by the node. At the end, the node will go to sleep mode. In case of multi cast and broad cast of data for a number of nodes or to all nodes, the DTIM is transmitted by the AP at regular intervals which is multiples of TIM interval as shown in the above figure.

ii) Ad-hoc Network: In case of Ad Hoc Networks, Traffic Indication map is announced by the node that has a data to transmit. The node (address) to which this TIM is applicable, sends a Acknowledgement TIM. On receipt of this, the node transmits data and receives acknowledgment from the receiver. As there is no central agency like AP to control it, it is more complicated. In this there is collision of ATIMs are possible.

e) Roaming:

No or bad connection? Then perform:

i) Scanning

scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer

Scanning involves the active search for a BSS. IEEE 802.11 differentiates between passive and active scanning.

Passive scanning - listening into the medium to find other networks, i.e., receiving the beacon of another network issued by access point.

Active scanning - sending a probe on each channel and waiting for a response. Beacon and probe responses contain the information necessary to join the new BSS.

ii)Reassociation Request

station sends a request to one or several AP(s)

iii)Reassociation Response

success: AP has answered, station can now participate

failure: continue scanning

iv)AP accepts Reassociation Request

signal the new station to the distribution system

the distribution system updates its data base (i.e., location information)

typically, the distribution system now informs the old AP so it can release resources

f)Security:

IEEE 802.11 provides both privacy and authentication mechanism. The mechanism is known as Wired Equivalent Privacy (WEP). To provide privacy and data integrity, WEP uses an encryption algorithm based on the RC4 algorithm. Following figure shows the encryption process.

Above figure shows the encryption process. The integrity algorithm is the 32 bit CRC that is appended to the end of MAC frame. For encryption process, a 40 bit secret key is shared by two participants in the exchange. An Initialisation Vector I(IV ) is I concatenated to the secret key. The resulting block form the seed that is input to the pseudorandom number generator (PRNG) defined in RC4. The PRNG generates a bit sequence of the same length as that of MAC frame plus its CRC. A bit by bit exclusive OR between the MAC frame and the PRNG sequence produces the cipher text. The IV is changed periodically (as often as every transmission). Every time the IV is changed, the PRNG sequence is changed, which provides protection against eavesdropper.

At the receiving end, the receiver retrieves the IV from the data block and concatenates this with the shared secret key to generate the same key sequence used by sender. This key sequence is then XORed with the incoming block to recover the plaintext. This technique makes use of the property A(B (B = A. Finally the receiver compares the incoming CRC with the CRC calculated at the receiver to validate integrity.

Authentication:

There are two types of authentication : Open System and Shared Key.

Open system authentication simply provides a way for two parties to agree to exchange data and provides no security benefits. IN this one party sends a MAC control frame, known as authentication frame to other party. The frame indicates that this is an open system authentication type. The other party responds with its own authentication frame and the process is complete.

Shared Key Authentication: This requires that the two parties share a secret key not shared by any other party. This key is used to assure that both sides are authenticated to each other. The procedure is as follows:

a. A sends a MAC authentication frame with an authentication algorithm identification of Shared Key and with station identifier that identifies the sending station

b. B responds with an authentication frame that includes a 128 octet challenge text which is generated using WEP PRNG.

c. A transmits an authentication frame that includes the challenge text just received from B. The entire frame is encrypted using WEP.

d. B Receives the encrypted frame and decrypts it using WEP and the secret key shared with A. If decryption is successful (matching CRC), then B compares the inkling challenge text with the challenge text that it sent in the second message. B then sends an authentication message to A with a status code indicating success or failure.

3.Wireless Local Loop

3.1 Introduction

Wired technologies respond to reliable, high-speed access by residential, business, and government subscribers requirements For example, ISDN, xDSL, cable modems etc. However, increasing interest shown in competing wireless technologies for subscriber access known as WLL or Fixed Wireless Access. Initially they were considered for providing quick telephone connection to residents, office etc based on quick deployment of WLL technology.

Wireless local loop (WLL)

Narrowband offers a replacement for existing telephony services (MMDS- Multichannel Multipoint Distribution Services)

Broadband provides high-speed two-way voice and data service(LMDS- Local Multipoint Distribution Services)

Some of the advantages of WLL are listed below:

Cost wireless systems are less expensive due to cost of cable installation thats avoided

Installation time WLL systems can be installed in a small fraction of the time required for a new wired system

Selective installation radio units installed for subscribers who want service at a given time

With a wired system, cable is laid out in anticipation of serving every subscriber in a given area

3.2 Wireless Local Loop Technologies

*Satellite-Based Systems

Provide Telephony services for rural communities and isolated areas

*Cellular-Based Systems

Provide high-power, wide-range, median subscriber density and median circuit-quality WLL services

Offers both mobility and fixed wireless access via the same platform as cellular

*Low-tier PCS or Microcellular-Based Systems

Provide low-power, narrow-range, high subscriber density and high circuit-quality WLL services

*Fixed Wireless Access (FWA) Systems

Proprietary radio systems

Disadvantage of the cellular approach

*Limitation on toll-quality voice and signaling transparency

Disadvantage of low-tier PCS and microcellular approaches

*Narrow radio coverage range

FWA addresses these issues

3.3 WLL Configuration:

A simple configuration of WLL is shown below: WLL antennas are mounted on the top a tallest building and they are connected through medium to receiver antennas that are mounted on top of residential buildings, private and public offices as shown below.

WLL with DECT:

3.4 Wireless Local Loop Architecture

Wireless Access Network Unit (WANU)

Components

BTSs or Radio Ports (RPs)

A Radio Port Control Unit (RPCU)

Access Manager (AM) and HLR

Functions provided by WANU

Authentication, Air Interface Privacy

Over-the-Air Registration of Subscriber Units

Radio Resource Management

InterworkingFunction (IWF)

Operation and Maintenance (OAM)

Routing, Billing, and Switching

Protocol conversion and transcoding of voice and data

Wireless Access Subscriber Unit (WASU)

Functions provided by WASU

Air Interface UWLLtoward the network

A traditional interface TWLLtoward the subscriber

This Interfaces include

Protocol conversion

Transcoding

Authentication Function

OAM

Signaling Functions

Modem Function to support voice-band data

Switching Function (SF)

3.5 Propagation Considerations for WLL

Most high-speed WLL schemes use millimeter wave frequencies (10 GHz to about 300 GHz) as there are wide unused frequency bands available above 25 GHz band. At these high frequencies, wide channel bandwidths can be used, providing high data rates. Also, high frequencies, small size transceivers and adaptive antenna arrays can be used

Millimeter wave systems have some undesirable propagation characteristics. These are listed below:

Free space loss increases with the square of the frequency; losses are much higher in millimeter wave range

Above 10 GHz, attenuation effects due to rainfall and atmospheric or gaseous absorption are large

Multipath losses can be quite high

Because of these limitations, WLL serves cells of limited radius. Obstructions including foliage must be avoided along or near the line of sight. Rainfall and humidity limit the range and availability of WLL system.

Fresnel Zone

Unlike the case for mobile communication, in case of WLL, direct line of sight between the transmitter and receiver antenna must be free of any obstruction At these high frequencies, radio signals are lost due to any obstruction. Understanding of Fresnels zone gives importance of obstruction free line of sight between transmitter and receiver antenna. It s based on the theory that any small element of space in the path of an EM wave may be considered the sources of secondary wavelet and radiation can be built up of super position of all these wavelets. On the basis of this theory, it can be shown that objects lying within a series of concentric circles around the direct line of sight between two trans receivers have constructive or destructive effects on communication. These are called Fresnels Zone. Out of this, those that fall within the first circle, that is the first Fresnel Zone, have the most serious effects.

Consider a point along the path between a transmitter and receiver, that is a distance S from the transmitter and a distance D from the receiver, with the total distance along the path equal to S + D . The radius of the first Fresnel zone at the point is

Where R, S, D are in the same units and is the wavelength of the signal along the path. For convenience, it can be restated as follows:

Where is R is expressed in meters and the two distance are in kilometer and the frequency in gigahertz.

For S & D = 10 KM and for f= 2.5 GHz, Rm=17.3 M

S & D = 10 KM and for f= 28 GHz, Rm=5.17 M

S & D = 1 KM and for f= 28 GHz, Rm=3.1 M

If there is no obstruction within 0.6 times the radius of the first Fresnels zone, at any point between the two trans-receivers, then attenuation due to obstruction is negligible. Also, height of the two antenna must be such that at no point along the path at which the ground is within 0.6 times the radius of the first Fresnels zone.

Atmospheric Absorption

Radio waves at frequencies above 10 GHz are subject to molecular absorption. Peak of water vapor absorption take s place at 22 GHz and peak of oxygen absorption is near 60 GHz. There are favorable frequency slots that are useful for WLL communication. These are given below:

From 28 GHz to 42 GHz

From 75 GHz to 95 GHz

These details can be seen in the figure shown below:

These effects are different for different temperature, relative humidity and atmospheric pressure.

Effect of Rain :

Radio waves at high frequencies are severely attenuated due to rain. Presence of raindrops can severely degrade the reliability and performance of communication links (We can observe this phenomenon in our DTH operated TV sets. As they operate at above 10 GHz, these signals are subjected to severe absorption during rainy period and signals are lost). The effect of rain depends on drop shape, drop size, rain rate, and frequency.

Estimated attenuation due to rain:

Where A = attenuation (dB/km); R = rain rate (mm/hr) ; a and b depend on drop sizes and frequency

Effects of Vegetation :

Trees near subscriber sites can lead to multipath fading. Multipath effects from the tree canopy are diffraction and scattering. Measurements in orchards found considerable attenuation values when the foliage is within 60% of the first Fresnel zone. Multipath effects highly variable due to wind

4. WiMAX

4.1 WiMAX - What is WiMAX ?

WiMAX would operate similar to WiFi but at higher speeds over greater distances and for a greater number of users. WiMAX was formed in April 2001, in anticipation of the publication of the original 10-66 GHz IEEE 802.16 specifications. WiMAX is to 802.16 as the WiFi Alliance is to 802.11.

WiMAX is:

Acronym for Worldwide Interoperability for Microwave Access.

Based on Wireless MAN technology.

A wireless technology optimized for the delivery of IP centric services over a wide area.

A scalable wireless platform for constructing alternative and complementary broadband networks.

A certification that denotes interoperability of equipment built to the IEEE 802.16 or compatible standard. The IEEE 802.16 Working Group develops standards that address two types of usage models:

A fixed usage model (IEEE 802.16-2004).

A portable usage model (IEEE 802.16e).

4.2 WiMax Speed and Range:

WiMAX is expected to offer initially up to about 40 Mbps capacity per wireless channel for both fixed and portable applications, depending on the particular technical configuration chosen, enough to support hundreds of businesses with T-1 speed connectivity and thousands of residences with DSL speed connectivity. WiMAX can support voice and video as well as Internet data.WiMAX could potentially be deployed in a variety of spectrum bands: 2.3GHz, 2.5GHz, 3.5GHz, and 5.8GHz

4.3WiMAX - Salient Features

WiMAX is a wireless broadband solution that offers a rich set of features with a lot of flexibility in terms of deployment options and potential service offerings. Some of the more salient features that deserve highlighting are as follows:

Two Type of Services:

WiMAX can provide two forms of wireless service:

Non-line-of-sight: service is a WiFi sort of service. Here a small antenna on your computer connects to the WiMAX tower. In this mode, WiMAX uses a lower frequency range -- 2 GHz to 11 GHz (similar to WiFi).

Line-of-sight: service, where a fixed dish antenna points straight at the WiMAX tower from a rooftop or pole. The line-of-sight connection is stronger and more stable, so it's able to send a lot of data with fewer errors. Line-of-sight transmissions use higher frequencies, with ranges reaching a possible 66 GHz.

OFDM-based physical layer:

The WiMAX physical layer (PHY) is based on orthogonal frequency division multiplexing, a scheme that offers good resistance to multipath, and allows WiMAX to operate in NLOS conditions.

Very high peak data rates:

WiMAX is capable of supporting very high peak data rates. In fact, the peak PHY data rate can be as high as 74Mbps when operating using a 20MHz wide spectrum.More typically, using a 10MHz spectrum operating using TDD scheme with a 3:1 downlink-to-uplink ratio, the peak PHY data rate is about 25Mbps and 6.7Mbps for the downlink and the uplink, respectively.

Scalable bandwidth and data rate support:

WiMAX has a scalable physical-layer architecture that allows for the data rate to scale easily with available channel bandwidth.For example, a WiMAX system may use 128, 512, or 1,048-bit FFTs (fast fourier transforms) based on whether the channel bandwidth is 1.25MHz, 5MHz, or 10MHz, respectively. This scaling may be done dynamically to support user roaming across different networks that may have different bandwidth allocations.

Adaptive modulation and coding (AMC):

WiMAX supports a number of modulation and forward error correction (FEC) coding schemes and allows the scheme to be changed on a per user and per frame basis, based on channel conditions.AMC is an effective mechanism to maximize throughput in a time-varying channel.

Link-layer retransmissions:

WiMAX supports automatic retransmission requests (ARQ) at the link layer for connections that require enhanced reliability. ARQ-enabled connections require each transmitted packet to be acknowledged by the receiver; unacknowledged packets are assumed to be lost and are retransmitted.

Support for TDD and FDD:

IEEE 802.16-2004 and IEEE 802.16e-2005 supports both time division duplexing and frequency division duplexing, as well as a half-duplex FDD, which allows for a low-cost system implementation.

WiMAX uses OFDM:

Mobile WiMAX uses Orthogonal frequency division multiple access (OFDM) as a multiple-access technique, whereby different users can be allocated different subsets of the OFDM tones.

Flexible and dynamic per user resource allocation:

Both uplink and downlink resource allocation are controlled by a scheduler in the base station. Capacity is shared among multiple users on a demand basis, using a burst TDM scheme.

Support for advanced antenna techniques:

The WiMAX solution has a number of hooks built into the physical-layer design, which allows for the use of multiple-antenna techniques, such as beamforming, space-time coding, and spatial multiplexing.

Quality-of-service support:

The WiMAX MAC layer has a connection-oriented architecture that is designed to support a variety of applications, including voice and multimedia services.

WiMAX system offers support for constant bit rate, variable bit rate, real-time, and non-real-time traffic flows, in addition to best-effort data traffic.

WiMAX MAC is designed to support a large number of users, with multiple connections per terminal, each with its own QoS requirement.

Robust security:

WiMAX supports strong encryption, using Advanced Encryption Standard (AES), and has a robust privacy and key-management protocol.

The system also offers a very flexible authentication architecture based on Extensible Authentication Protocol (EAP), which allows for a variety of user credentials, including username/password, digital certificates, and smart cards.

Support for mobility:

The mobile WiMAX variant of the system has mechanisms to support secure seamless handovers for delay-tolerant full-mobility applications, such as VoIP.

IP-based architecture:

The WiMAX Forum has defined a reference network architecture that is based on an all-IP platform. All end-to-end services are delivered over an IP architecture relying on IP-based protocols for end-to-end transport, QoS, session management, security, and mobility.

4.4 WiMAX - Reference Network Model

4.4.1 WiMAX - Building Blocks

A WiMAX system consists of two major parts:

A WiMAX base station.

A WiMAX receiver.

WiMAX Base Station:

A WiMAX base station consists of indoor electronics and a WiMAX tower similar in concept to a cell-phone tower. A WiMAX base station can provide coverage to a very large area up to a radius of 6 miles. Any wireless device within the coverage area would be able to access the Internet.The WiMAX base stations would use the MAC layer defined in the standard, a common interface that makes the networks interoperable and would allocate uplink and downlink bandwidth to subscribers according to their needs, on an essentially real-time basis.

Each base station provides wireless coverage over an area called a cell. Theoretically, the maximum radius of a cell is 50 km or 30 miles however, practical considerations limit it to about 10 km or 6 miles.

WiMAX Receiver:

A WiMAX receiver may have a separate antenna or could be a stand-alone box or a PCMCIA card sitting in your laptop or computer or any other device. This is also referred as customer premise equipment (CPE).WiMAX base station is similar to accessing a wireless access point in a WiFi network, but the coverage is greater.

Backhaul:

A WiMAX tower station can connect directly to the Internet using a high-bandwidth, wired connection (for example, a T3 line). It can also connect to another WiMAX tower using a line-of-sight microwave link.Backhaul refers both to the connection from the access point back to the base station and to the connection from the base station to the core network.It is possible to connect several base stations to one another using high-speed backhaul microwave links. This would also allow for roaming by a WiMAX subscriber from one base station coverage area to another, similar to the roaming enabled by cell phones.

4.4.2 System Reference Architecture

IEEE802.16 provides a communication path between a subscriber site which may either be a single subscriber device or a network of the subscribers premises (eg LAN, PBX, IP based network) and a core network, eg. Public Telephone network and the internet.

The IEEE 802.16e-2005 standard provides the air interface for WiMAX but does not define the full end-to-end WiMAX network. The WiMAX Forum's Network Working Group (NWG) is responsible for developing the end-to-end network requirements, architecture, and protocols for WiMAX, using IEEE 802.16e-2005 as the air interface. The WiMAX NWG has developed a network reference model to serve as an architecture framework for WiMAX deployments and to ensure interoperability among various WiMAX equipment and operators.

The network reference model envisions a unified network architecture for supporting fixed, nomadic, and mobile deployments and is based on an IP service model. Below is simplified illustration of an IP-based WiMAX network architecture. The overall network may be logically divided into three parts:

Mobile Stations (MS) used by the end user to access the network.

The access service network (ASN), which comprises one or more base stations and one or more ASN gateways that form the radio access network at the edge.

Connectivity service network (CSN), which provides IP connectivity and all the IP core network functions.

The network reference model developed by the WiMAX Forum NWG defines a number of functional entities and interfaces between those entities. Fig below shows some of the more important functional entities.

Base station (BS): The BS is responsible for providing the air interface to the MS. Additional functions that may be part of the BS are micromobility management functions, such as handoff triggering and tunnel establishment, radio resource management, QoS policy enforcement, traffic classification, DHCP (Dynamic Host Control Protocol) proxy, key management, session management, and multicast group management.

Access service network gateway (ASN-GW): The ASN gateway typically acts as a layer 2 traffic aggregation point within an ASN. Additional functions that may be part of the ASN gateway include intra-ASN location management and paging, radio resource management, and admission control, caching of subscriber profiles, and encryption keys, AAA client functionality, establishment, and management of mobility tunnel with base stations, QoS and policy enforcement, foreign agent functionality for mobile IP, and routing to the selected CSN.

Connectivity service network (CSN): The CSN provides connectivity to the Internet, ASP, other public networks, and corporate networks. The CSN is owned by the NSP and includes AAA servers that support authentication for the devices, users, and specific services. The CSN also provides per user policy management of QoS and security. The CSN is also responsible for IP address management, support for roaming between different NSPs, location management between ASNs, and mobility and roaming between ASNs.

The WiMAX architecture framework allows for the flexible decomposition and/or combination of functional entities when building the physical entities. For example, the ASN may be decomposed into base station transceivers (BST), base station controllers (BSC), and an ASNGW analogous to the GSM model of BTS, BSC, and Serving GPRS Support Node (SGSN).

4.5 IEEE 802.16 Protocol Architecture

Protocol Architecture

Physical and transmission layer functions:

Encoding/decoding of signals

Preamble generation/removal (Synchronization)

Bit transmission/reception

Medium access control layer functions:

On transmission, assemble data into a frame with address and error detection fields

On reception, disassemble frame, and perform address recognition and error detection

Govern access to the wireless transmission medium

Convergence layer functions:

Encapsulate PDU framing of upper layers into native 802.16 MAC/PHY frames

Map upper layers addresses into 802.16 addresses

Translate upper layer QoS parameters into native 802.16 MAC format

Adapt time dependencies of upper layer traffic into equivalent MAC service

4.5.1 WiMAX - Physical Layer

The WiMAX physical layer is based on orthogonal frequency division multiplexing. OFDM is the transmission scheme of choice to enable high-speed data, video, and multimedia communications and is used by a variety of commercial broadband systems, including DSL, Wi-Fi, Digital Video Broadcast-Handheld (DVB-H), and MediaFLO, besides WiMAX.

OFDM is an elegant and efficient scheme for high data rate transmission in a non-line-of-sight or multipath radio environment.

Adaptive Modulation and Coding in WiMAX:

WiMAX supports a variety of modulation and coding schemes and allows for the scheme to change on a burst-by-burst basis per link, depending on channel conditions. Using the channel quality feedback indicator, the mobile can provide the base station with feedback on the downlink channel quality. For the uplink, the base station can estimate the channel quality, based on the received signal quality.

Following is a list of the various modulation and coding schemes supported by WiMAX.

Downlink

Uplink

Modulation

BPSK, QPSK, 16 QAM, 64 QAM; BPSK optional for OFDMA-PHY

BPSK, QPSK, 16 QAM; 64 QAM optional

Coding

Mandatory: convolutional codes at rate 1/2, 2/3, 3/4, 5/6

Optional: convolutional turbo codes at rate 1/2, 2/3, 3/4, 5/6; repetition codes at rate 1/2, 1/3, 1/6, LDPC, RS-Codes for OFDM-PHY

Mandatory: convolutional codes at rate 1/2, 2/3, 3/4, 5/6

Optional: convolutional turbo codes at rate 1/2, 2/3, 3/4, 5/6; repetition codes at rate 1/2, 1/3, 1/6, LDPC

PHY-Layer Data Rates:

Because the physical layer of WiMAX is quite flexible, data rate performance varies based on the operating parameters. Parameters that have a significant impact on the physical-layer data rate are channel bandwidth and the modulation and coding scheme used. Other parameters, such as number of subchannels, OFDM guard time, and oversampling rate, also have an impact.

Following is the PHY-layer data rate at various channel bandwidths, as well as modulation and coding schemes.

WiMAX - OFDM Basics

OFDM belongs to a family of transmission schemes called multicarrier modulation, which is based on the idea of dividing a given high-bit-rate data stream into several parallel lower bit-rate streams and modulating each stream on separate carriers, often called subcarriers or tones. Multicarrier modulation schemes eliminate or minimize intersymbol interference (ISI) by making the symbol time large enough so that the channel-induced delays delay spread being a good measure of this in wireless channels are an insignificant (typically, < 10 percent) fraction of the symbol duration.Therefore, in high-data-rate systems in which the symbol duration is small, being inversely proportional to the data rate splitting the data stream into many parallel streams increases the symbol duration of each stream such that the delay spread is only a small fraction of the symbol duration.

OFDM is a spectrally efficient version of multicarrier modulation, where the subcarriers are selected such that they are all orthogonal to one another over the symbol duration, thereby avoiding the need to have nonoverlapping subcarrier channels to eliminate intercarrier interference.In order to completely eliminate ISI, guard intervals are used between OFDM symbols. By making the guard interval larger than the expected multipath delay spread, ISI can be completely eliminated. Adding a guard interval, however, implies power wastage and a decrease in bandwidth efficiency.

4.5.2 WiMAX - MAC Layer

The IEEE 802.16 MAC was designed for point-to-multipoint broadband wireless access applications. The primary task of the WiMAX MAC layer is to provide an interface between the higher transport layers and the physical layer.The MAC layer takes packets from the upper layer, these packets are called MAC service data units (MSDUs) and organizes them into MAC protocol data units (MPDUs) for transmission over the air. For received transmissions, the MAC layer does the reverse.The IEEE 802.16-2004 and IEEE 802.16e-2005 MAC design includes a convergence sublayer that can interface with a variety of higher-layer protocols, such as ATM TDM Voice, Ethernet, IP, and any unknown future protocol.The 802.16 MAC is designed for point-to-multipoint (PMP) applications and is based on collision sense multiple access with collision avoidance (CSMA/CA).

The MAC incorporates several features suitable for a broad range of applications at different mobility rates, such as the following:

Privacy key management (PKM) for MAC layer security. PKM version 2 incorporates support for extensible authentication protocol (EAP).

Broadcast and multicast support.

Manageability primitives.

High-speed handover and mobility management primitives.

Three power management levels, normal operation, sleep, and idle.

Header suppression, packing and fragmentation for efficient use of spectrum.

Five service classes, unsolicited grant service (UGS), real-time polling service (rtPS), non-real-time polling service (nrtPS), best effort (BE), and Extended real-time variable rate (ERT-VR) service.

These features combined with the inherent benefits of scalable OFDMA make 802.16 suitable for high-speed data and bursty or isochronous IP multimedia applications.Support for QoS is a fundamental part of the WiMAX MAC-layer design. WiMAX borrows some of the basic ideas behind its QoS design from the DOCSIS cable modem standard.Strong QoS control is achieved by using a connection-oriented MAC architecture, where all downlink and uplink connections are controlled by the serving BS.WiMAX also defines a concept of a service flow. A service flow is a unidirectional flow of packets with a particular set of QoS parameters and is identified by a service flow identifier (SFID).

4.6 Summary:

WiMAX is based on a very flexible and robust air interface defined by the IEEE 802.16 group.

WiMAX is similar to the wireless standard known as Wi-Fi, but on a much larger scale and at faster speeds.

The WiMAX physical layer is based on OFDM, which is an elegant and effective technique for overcoming multipath distortion.

The physical layer supports several advanced techniques for increasing the reliability of the link layer. These techniques include powerful error correction coding, including turbo coding and LDPC, hybrid-ARQ, and antenna arrays.

WiMAX supports a number of advanced signal-processing techniques to improve overall system capacity. These techniques include adaptive modulation and coding, spatial multiplexing, and multiuser diversity.

WiMAX has a very flexible MAC layer that can accommodate a variety of traffic types, including voice, video, and multimedia, and provide strong QoS.

Robust security functions, such as strong encryption and mutual authentication, are built into the WiMAX standard.

WiMAX defines a flexible all-IP-based network architecture that allows for the exploitation of all the benefits of IP.

WiMAX offers very high spectral efficiency, particularly when using higher-order MIMO solutions

4.7 WiMAX and Wi-Fi Comparison

Feature

WiMax(802.16a)

Wi-Fi(802.11b)

Wi-Fi(802.11a/g)

PrimaryApplication

Broadband WirelessAccess

Wireless LAN

Wireless LAN

Frequency Band

Licensed/Unlicensed2 G to 11 GHz

2.4 GHz ISM

2.4 GHz ISM (g)5 GHz U-NII (a)

ChannelBandwidth

Adjustable1.25 M to 20 MHz

25 MHz

20 MHz

Half/Full Duplex

Full

Half

Half

Radio Technology

OFDM(256-channels)

Direct SequenceSpread Spectrum

OFDM(64-channels)

BandwidthEfficiency