Mobile Tutorial Answers

7/31/2019 Mobile Tutorial Answers

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SECTION 4

Answers for the Mobile Communications

Tutorial Questions


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MOBILE COMMUNICATIONS NETWORKS

Explain the difference between the terms fixed, portable, mobile, and wireless in

the context of telecommunications. Use some examples to illustrate your answer.

Fixed, portable and wireless are terms used in telecommunications to describe

devices. They mean the following:

Fixed: Used to describe something which is stationary in one position and does not

move. For example, a desktop computer is fixed.

Portable: Means you could use the device with you in different locations such as a

laptop.

Mobile: Used to describe users. Users could move to different places and use the same

as well as different devices.

Wireless: Anything without wires or cables.

How is user mobility achieved when using email?

Email, short for electronic mail is the most common form of asynchronous

communication where you do not need a common clock between the communicating

devices. For example, I can send a mail to my friend and its not necessary for him to

check it when Im sending it. He can check it whenever he wants to.

The process involved in sending and receiving email is the same all over the world, so

it makes it easy to access your account (or inbox) from any part of the world.

Different types of softwares are available to provide the interface with the user. For

example, we can use a web based program such as hotmail. We can log into our

account from any computer and from any part of the world.

Describe briefly how one could use mobile communication in the emergency

services.

Mobile communication has its special use in emergency services. For example:

Police: The police department is being allotted with a frequency which only they canuse to communicate with each other. No one else is allowed to use that frequency. So

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it helps the police to communicate with each other with the help of wireless phone in

cases of emergency such as a theft.

Hospitals: In cases of accidents or emergencies, the injured persons information can

be sent to the hospital immediately by a wireless device.

Taxis: The taxi service has been allocated a frequency for its own use. They use that

frequency to communicate with all the cars running for that particular service. For

example, the person in charge at the main office can use his talking device to

communicate with all the taxis running on that service and ask for any who is free.

Each and every taxi on that service can hear that message. Any available taxi responds

by his own device to the main office. The person in charge their can then direct him to

the required area of emergency. Everyone on that frequency can hear all the messagesas the main office cannot send messages to each and every taxi and ask whether its

available or not.

What is the difference between location aware services and follow-on services?

Location aware services are the services you could use in the different location. These

services usually use different devices. Follow-on services are the services follow you

all the time. These services usually use a same device.

Draw the basic reference model used to describe a simple network, and briefly

explain the function of its components.

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[1]

Application: Provides different services to the applications

Presentation: Converts the information

Session: Handles problems which are not communication issues

Transport: Provides end to end communication control.

Network: Routes the information in the network.

Data Link: Provides error control between adjacent nodes.

Physical: Connects the entity to the transmission media.

Which frequency bands are normally used for wireless communication? List the

band names, and their usage. Why is there a need for a regulator on thefrequency spectrum?

850-900 MHz

Infrared transmission is used for directed links. For example, connect laptops, PAD

(Personal digital assistant).

The need for a regulator on a frequency spectrum is required as it is easy to find

common, worldwide regulations and for worldwide coordination of

telecommunication.

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What is an isotropic antenna? Draw the radiation pattern of a simple dipole

antenna? What is wavelength?

The purpose of a transmitting antenna is to radiate electromagnetic waves into air. An

isotropic antenna has no preferred direction of radiation. It radiates uniformly in alldirections.

The radiation pattern of a simple dipole antenna is as follows:

[2]

Wavelength is the distance between two successive crusts or troughs of a wave.

Describe clearly all the factors that affect wireless signal propagation. Use

diagrams to illustrate your answers. Why does long term fading occur?

Factors which affect signal propagation are:

Shadowing:

Signals are sometimes unable to pass through large obstacles. The higher the

frequency of the radio signal, the more it behaves like light and small objects such asa simple wall, a truck or even trees may block the signals.

Reflection:

If an object is large compared to the wavelength of the signal, reflection occurs. This

happens, for example, where there are huge buildings or mountains. The signals are

then reflected back. This reflected signal is not as strong as the original signal as some

of the signal power might have been absorbed by the obstacle itself.

Scattering:

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If the size of the signal is approximately the same as the wavelength, the radio signal

maybe scattered. This basically means that the incoming signal is scattered into many

smaller signals.

Diffraction:

Diffraction is very much similar to scattering. Signals are deflected at an edge and

propagated into different directions. The resulting signal has varying strength and this

depends on the location of the receiver.

[2]

Varying distance to the sender or more remote obstacles cause long-term fading, it

makes slow changes in the average power received. Long-term fading is basically

distance related attenuation.

What is meant by the term intersymbol interference?

Interference with neighbor symbols, Inter Symbol Interference (ISI). The signal

reaches a receiver directly and phase shifted.

In two or three sentences, explain what multiplexing is.

Multiplexing is a way which describes how several users can share a medium at the

same time with minimal interference. The task of multiplexing is to assign ways to

each communication channel with a minimum of interference and a maximum

possible channel utilization.

Describe clearly the four types of multiplexing.

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Space division multiplexing: A separate sender for each communication channel with

a wide enough distance between senders so as to avoid interference. The channels are

mapped onto different spaces which separate the channels and prevent the interference

ranges from overlapping. The space between the interference ranges is called as a

guard space.

Frequency division multiplexing: Separation of the whole spectrum into smaller

frequency bands. A channel gets a certain band of the spectrum for the whole time.

Users can transmit using their allocated frequency continuously as every user is

assigned his own frequency so there is a very little chance of interference. Guard

spaces are needed to avoid frequency bands overlapping.

Time division multiplexing: All sender use the same frequency but at different points

in time. A channel is given the whole bandwidth for a certain period of time. Again,

guard spaces which now represent time gaps, have to be used to separate the periodswhen the users are transmitting.

Code division multiplexing: All channels use the same frequency for transmission at

the same time but these are separated using different codes. Every channel has its own

unique code which is used to distinguish it from others.

Basically to summarize:

S.D.M division of physical space of a medium.

T.D.M allocation of time slot

F.D.M. Assigning different frequencies

C.D.M assigning code to each channel

What types of multiplexing are used by radio stations in transmitting channels?

Radio stations use SDM and FDM in transmitting channels.

Why does one need to convert digital signals to analogue signals when

transmitting data?

Digital modulation is required if digital data needs to be transmitted over a medium

that only allows for analog transmission. If we take the example of our voice, it

cannot be transmitted in the way we speak. It has to be converted to some form of

transmittable data. For that purpose we require digital modulation which is used to

convert digital data to an analogue one for transmission.

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What is a baseband signal?

A baseband signal can be gained when digital data is modulated by digital

modulation.

List and explain the three basic analogue modulation schemes.

Amplitude Modulation (AM):

It is a type of modulation in which the amplitude of the carrier wave is varied

according to some characteristic of the modulating signal. In case of an analog signal

to be sent, the amplitude of the radio wave is modulated to be directly proportional to

the value of the analog signal at that time. Amplitude modulation typically produces a

modulated output signal that has twice thebandwidth of the modulating signal, with a

significant power component at the original carrier frequency.

Frequency Modulation (FM):

Frequency Modulation (FM) is the encoding ofinformation in either analogue or

digital form into a carrier wave by variation of its instantaneous frequency in

accordance with an input signal. Frequency modulation requires a widerbandwidth

than amplitude modulation by an equivalent modulating signal, but this also makes

the signal more robust against interference. Frequency modulation is also more robust

against simple signal amplitude fading phenomena

Phase Modulation (PM):

Phase Modulation (PM) is a form of modulation in which the carrier waves phase is

caused to vary in accordance with the modulating signal. Phase modulation is a form

of angle modulation (because the angle of the sine wave carrier is changed by the

modulating wave). PM and FM are very similar in the sense that any attempt to shift

the frequency or phase is accomplished by a change in the other.

Describe briefly amplitude, frequency and phase shift keying.There are three ways in which the bandwidth of the channel carrier may be changed.

They are altering the amplitude, frequency and phase of the carrier sine wave. They

are amplitude-shift-keying (ASK), frequency-shift-keying (FSK) and phase-shift-

keying (PSK), respectively.

Amplitude Shift Keying (ASK): very simple and low bandwidth requirements, very

susceptible to interference. The bandwidth of the signal remains unchanged.

Frequency Shift Keying (FSK): It usually makes the use of two bits (1 or 0) using a

different frequency for them. The resulting can be regarded as the sum of two

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amplitude modulated signals of different carrier frequency. It also needs larger

bandwidth.

Phase Shift Keying (PSK): This modulation scheme uses shifts in phase of a signal to

represent data, for example, a phase shift of can change the logic 0 to a 1 andvice versa. This scheme is more complex and robust against interference.

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Explain what QAM is?

Quadrature Amplitude Modulation (QAM) is a combination of amplitude modulation

and phase shift keying. Its a method for encoding digital data in an analog signal in

which each combination of phase and amplitude represents one of sixteen four bit

patterns.

Why are spread spectrum techniques important in signal transmission? Give an

example of one technique that implements spread spectrum?

[3]

The problem of signal transmission is that frequency dependent fading can wipe out

narrow band signals for duration of the interference. Spread spectrum techniques can

solve this problem. In other words they can resist narrowband interference. In Code

Division Multiple Access systems all users transmit in the same bandwidth

simultaneously. Systems following this idea are spread spectrum systems. In thistransmission technique, the frequency spectrum of a data-signal is spread using a code

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(key) with that signal. As a result the bandwidth occupancy is much higher then

required.

The codes used for spreading are unique to every user. This is the reason that a

receiver which has knowledge about the code of the intended transmitter is capable of

selecting the desired signal. As the bandwidth requirements are high, less power

spectral density is required and in the channel the signal appears to be noise.

Use a block diagram to explain how a chipping sequence can be used to spread

the signal.

[2]

Direct sequence spread spectrum systems take a user bit stream and perform an XOR

with chipping sequence (a randomly generated data sequence) to spread the signal as

the figure shown above.

Briefly explain Frequency hopping spread spectrum technique with the help of a

simple diagram.

[4]

Frequency hopping spread spectrum technique splits the allocated bandwidth into

many channels of smaller bandwidth in the spectrum which divides into many

possible frequencies to which the data will be sent over. There exists a code which

determines at any particular moment in time what frequency it will transmit at,

hopping from frequency to frequency.

The time spent on any one channel with a certain frequency is known as dwell time.

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FHSS has two types; slow hopping and fast hopping. In slow hopping, the transmitter

uses one frequency for several bits of data. For fast hopping systems, the transmitter

changes frequency several times during the transmission of a single bit. A simple

example of FHSS is the Blue tooth technology. Blue tooth performs about 1600 hops

per second and uses 79 hop carriers equally spaced.

Explain how one can use just three frequencies to cover an entire area when

using cells in mobile communication? Why is it better to use small cells instead of

large cells in a cellular system? What are the negative aspects of this?

[2]

The cells are combined in clusters. No two adjoining cells can have the same

frequency, as shown in the figure above. So that is why one can use just three

frequencies to cover an entire area (implementing SDM).

It is better to use small cells instead of larger ones as we get more space. It has several

other advantages such as it has higher capacity, uses less transmission power, has

local interference only, and robustness. Additional traffic appears as noise to other

users and if the noise level is too high, users drop out of cells.

What is medium access control?

Medium access control comprises all mechanisms that regulate user access to a

medium using SDM, TDM, FDM, or CDM. MAC is similar to the traffic regulation inthe highway. For example, several vehicles using the same street crossing in TDM,

requires rules to avoid collision, MAC is the rule which allows this to be done.

What is meant by hidden and exposed terminals?

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A B

C

[2]

Hidden terminals:

If A wants to send a packet of data to B, it does so. C cannot receive A as it cannot

sense it. To C, A is a hidden terminal. It is outside its radio range. At the same time, C

wants to send a message to B. It does not know that A is already transmitting to B as it

cannot sense the medium. As a result, there is a collision at B as both A and C wants

to transmit. But as A is not in the range of C, it cannot detect the collision, therefore

collision detection does not work either. Thus, A is hidden for C.

Exposed terminals:

If B sends something to A and C wants to transmit a data packet as well, the carrier

sensing signal makes C wait as B is already transmitting to A. But as C is out of range

for A, therefore it doesnt have to wait for B to get free. Thus, C is exposed to B.

Describe what the problem is with near and far terminals.

A B C

[2]

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[5]

In mobile signal propagation, the strength of the signal decreases proportionally to the

square of the distance. This is because every transmitter antenna or a base station has

a certain range as shown in the above figure. The signal of terminal B therefore

drowns out As signal and C cannot receive A as it is a far terminal for it.If C (for example) was an arbiter for sending rights, terminal B would drown out

terminal A already on the physical layer. Therefore, we do have problems with near

and far terminals concerning data transmission.

What is frequency division multiple access? Give an example of such a scheme?

The transmission of multiple separate signals through a shared medium by modulating

the separate signals into different frequency bands is known as frequency division

multiple accesses. In FDMA, each transmitter is assigned a distinct frequency channel

so that receivers can differentiate among them by tuning to the desired channel. A

very simple example is radio broadcast. Every radio station has its own frequency,

assigned distinctly to that station. It will send the signal in this transmission channel.

If we want to receive this signal, we should tune into this frequency and thus, we will

be able to hear all that is happening on that radio channel.

What is TDMA?

Time division multiple access is a method for sharing a medium by allowing severalusers to share the same frequency by dividing it into different time slots. The users

can transmit and use that channel in the time slot allotted to them. This allows

multiple users to share the same transmission medium whilst using only a part of the

bandwidth they require.

Describe fixed TDMA?

Fixed TDMA (Time Division Multiple Access) is the allocation of time slots for a

channel in a fixed pattern. This results in fixed delays but at the same reduces the

possibility of interference. Each channel knows exactly which time slot is reserved for

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it and can access it at that very moment. This is the crucial factor for fixed TDMA.

There is a fixed pattern for transmission, for example, the interval between any

transmissions is 15 milliseconds and the length of the time slot is 500 microseconds.

Therefore, each sender will transmit at an interval of 15 milliseconds for a period of

500 microseconds. This fixed pattern is repeated every 15 milliseconds and itguarantees access to the channel every 15 milliseconds, independent of any other

connections.

What is classical Aloha? Describe how it can be improved?

The classical Aloha scheme was invented by the University of Hawaii and was used

for wireless connection of several stations. This method does not co-ordinate with

different users when transmitting data; instead each station can access the medium

anytime they want. There is no central station to co-coordinating between the users

for transmission. In this system, if two or more users transmit at the same time, acollision occurs and the data is destroyed. It needs to be transmitted again.

The classical Aloha method has its problems. It can be improved by the introduction

of time slots, known as Slotted Aloha. All the stations are synchronized and

transmission of data can only begin at the start of a time slot as shown in the above

figure. This introduction of time slots for data transmission improves the throughput

of the aloha system by 50 percent.

Explain the principle of reservation in TDMA. Use simple diagrams to illustrate.

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[5]

[2]

Every frame consists of N mini-slots and x data-slots

Every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. ).kNx =

Other stations can send data in unused data-slots according to a round-robin sending

scheme (best-effort traffic).

Describe CDMA?

CDMA stands for Code Division Multiple Access; it uses codes to separate different

users enable access to a shared medium without interference. All users can send on the

same frequency, at the same time and can use the whole bandwidth of the channel.

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Each user who wishes to gain access to the transmission medium is allocated with a

unique random number. Users who have knowledge about the coded signal, or in

other words, know what the code of the transmitting signal is, can gain access only.

The receiver can tune into the signal if and translate it if it knows the random number.

Why are protocols important in mobile communications?

A protocol is a set of rules that governs how information is delivered and

communicated between multiple users. IP provides a connectionless, reliable,best-

effort packet delivery service. IP is anetwork layer protocol. IP is the standard that

defines the manner in which the network layers of two hosts interact. These hosts may

be on the same network or reside on physically distinct heterogeneous networks. IP

requires the location of any host connected to the Internet to be uniquely identified by

an assigned IP address.

Name the main elements of the GSM system architecture and describe their

functions.

The main elements of the GSM system architecture are as follows:

RSS (Radio Subsystem):

It covers all radio aspects of the GSM network such as mobile stations and base

stations subsystems.

NSS (Network and Switching Subsystem):

This is perhaps the most important element of the GSM network as it is responsible

for connecting the wireless network to the standard public networks. It performs

handovers between different base stations, and provides global functions such as

roaming and charging. It consists of mobile switching centers, home location registers

and visitor location registers.

OSS (Operation Subsystem):

This layer of the GSM is mainly responsible for the operation and management of the

network. This subsystem has some of its own entities and can access others as well

such as Operation and maintenance center (OMC), Authentication center (AuC) and

equipment identity register (EIR).

What multiplexing schemes are used in GSM?

A GSM network makes use of Frequency and Time division multiplexing.

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What are the main functions of the Base station controller?

A Base station controller is usually responsible for the control of many Base

Transceiver station (BTS). A BSS has many BTS under its control. It basically

handles allocation of channels, receives measurements from cell phones and manageshandovers from BTS to BTS. One main function of the BSC is to act as a functional

unit to permit a common data path to handle more data sources than there are channels

on that path. Therefore, a GSM network is often designed to have many BSCs

distributed into separate regions near the BTS.

What are the components of the radio subsystem of GSM?

The components of the radio subsystem of GSM are as follows:

Base Station Subsystem (BSS)Base Transceiver Station (BTS)

Base Station Controller (BSC)

Mobile Stations (MS)

Describe the key elements of the Network and Switching Subsystem of GSM.

The key elements of the Network and Switching Subsystem (NSS) of GSM are:

Mobile Services Switching Center (MSC):

Controls all connections via a separated network to and from a mobile terminal within

the domain of the MSC. Several BSC can belong to a MSC

Home Location Register:

Central master database containing user data, permanent and semi-permanent data of

all subscribers assigned to the HLR.

Visitor Location Register:

Local database for a subset of user data, including data about all users currently in the

domain of the VLR

What is purpose of the Home Location Register? Where does it reside?

A Home Location Register (HLR) is a database that contains semi-permanent mobile

subscriber information for a wireless carriers' entire subscriber base. HLR subscriber

information includes the International Mobile Subscriber Identity (The IMSI is a

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unique non-dial able number allocated to each mobile subscriber in the GSM system

that identifies the user and his or her subscription within the GSM network), service

subscription information, location information (the identity of the currently serving

Visitor Location Register (VLR) to enable the routing of mobile-terminated calls),

service restrictions and supplementary services information. It resides in the networkand switching subsystem of the GSM network.

Why a Visitor Location is Register useful?

The Visitor Location Register (VLR) contains all user data required for call handling

and mobility management for mobile users currently located in the area controlled by

the VLR. The VLR is a database that contains temporary information about

subscribers which is needed by the MSC in order to service visiting users to that

particular area. When a mobile station roams into a new MSC area, the visitor

location register connected to that mobile switching center will request data about the

mobile station from the home location register, reducing the need for interrogation of

the home location register. When a mobile user roams away from his home location

and into a remote location, messages are used to obtain information about the

subscriber from the HLR, and to create a temporary record for the subscriber in the

VLR. There is usually one VLR per MSC.

List and describe the main components of the Operation Subsystem of GSM?

The main components of the Operation Subsystem (OSS) of GSM are:

Authentication Center (AuC):

The GSM network as a whole is very vulnerable to the outside world; therefore an

Authentication Centre (AuC) is needed for the protection of user identity and data

transmission. The AuC contains the algorithms as well as codes for encrypting data.

Equipment Identity Register (EIR):

The Equipment Identity Registers (EIR) is a storage place where all the registration

takes place such as registration of users, mobile stations and so on. It has all the userdata stored. The EIR can also block all stolen devices, thus the SIM becomes useless.

Operation and Maintenance Center (OMC):

The Operation and Management Center (OMC) controls and monitors all network

entities. Some typical management functions of an OMC are traffic monitoring, or

accounting and billing.

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Describe clearly with the help of a diagram, the steps taken to make a mobile

terminated call.

1: calling a GSM subscriber 3: Signal call setup to HLR2: Forwarding call to the GMSC 4,5: request MSRN from VLR

6: Forward responsible MSC to GMSC 8, 9: get current status of MS

10, 11: paging of MS 12, 13: MS answers

14, 15: security checks 16, 17: connection setup.

7: forward call to current MSC

[2]

Describe clearly with the help of a diagram, the steps taken to make a mobile

originated call.

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1, 2: connection request

3, 4: security check

5-8: check resources (free circuit)

9-10: set up call

[2]

Describe the different types of handover that occur in GSM. Use a diagram to

help.

[2]

When a mobile station moves, it may move out of the range of the base station to

which it is joined or connected. When this occurs, a change of connection known as

handover may occur. If the mobile has a currently active connection, the network

provides a transparent transition to the new base station, and maintains theconnection. This is only possible if capacity is available on the new base station.

There are four different types of handover in the GSM system, which involve

transferring a call between

channels (time slots) in the same cell,

cells (Base Transceiver Stations) under the control of the same Base Station

Controller (BSC),

cells under the control of different BSCs, but belonging to the same Mobile

services Switching Centre (MSC), and

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Cells under the control of different MSCs.

The first two types of handover involve only one Base Station Controller (BSC). To

save bandwidth, they are managed by the BSC without involving the Mobile service

Switching Center (MSC), though it informs the MSC when the handover is done. The

last two types of handovers are handled by the MSCs involved.

What are the reasons for implementing handover in GSM?

Handover is an important process in GSM transmission. As single cells do not cover a

whole service area as it is very difficult to do so, thus handover becomes necessary.

Sometimes in the case of intra-cell handover, the frequency needs to be changed in

order to make the transmission possible.

Explain clearly how authentication is achieved in mobile phones.

[2]

A radio medium can be accessed by anyone, authentication of users to prove that they

are who they claim to be, is a very important element of a mobile network.

Authentication involves two functional entities, the SIM card in the mobile, and theAuthentication Centre (AuC). Each subscriber is given a secret key, one copy of

which is stored in the SIM card and the other in the Authentication Centre. During

authentication, the AuC generates a random number that it sends to the mobile. Both

the mobile and the AuC then use the random number, in conjunction with the

subscriber's secret key and a ciphering algorithm, to generate a number that is sent

back to the AuC. If the number sent by the mobile is the same as the one calculated

by the AuC, the subscriber is authenticated.

Explain how encryption is achieved in mobile phones.

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Encryption is a method of encoding data so that it cannot be read by a third party.

There are several levels of encryption, and mobile phones can support different levels,

to match what the network is using. After authentication has been done as described

above, using the same random number combined with the SIM cards private key,

both the mobile and the AUC generate a cipher key using algorithm. This key is notpassed over the air, but is used by both network and mobile to encrypt each packet of

data. Combining the cipher key and the frame number they produce a 114 bit

sequence which is XOR'd with the first two 57 bit data blocks when they are

transmitted

What is GPRS?

GPRS, standing for General Packet Radio Service, is a way of passing data over a

mobile phone network. Until GPRS, the standard method of surfing the net, getting

emails or, most commonly, using WAP on a mobile phone was done using the circuit-switched method - dialing up, staying online, then logging off.

GPRS allows us to be always logged on for data connection for GSM mobile phones,

allowing for faster browsing, web surfing and email.

As well as being faster than the older dialup method, the main advantage is that you

no longer pay for your online time per-minute, you pay for the amount of data you

transfer. As an example, the O2 online Leisure tariff charges 3 per megabyte used,

which is enough to send or receive up to 400 emails of 100 words, or download up to

1000 Mobile Internet (WAP) pages. Another bonus of GPRS is that sessions can be

suspended when an incoming voice call is detected, so you don't lose calls .

What is TETRA?

TETRA stands for terrestrial trunked radio systems. These systems use many different

radio carriers but only assign a specific carrier to a certain user for a short period of

time according to demand. It is currently being used by the emergency services such

as the police force, taxi services, hospitals, fleet managements, and rescue teams and

so on.

What is the use of TETRA?

It offers interfaces to public networks, i.e., voice and data services. It is another

system for private use like GSM.

Distinguish between packet switched and circuit switched transmission?

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Packet Switching:

Data packets are broken down into smaller packets before they are actually sent. Each

data packet is transmitted individually across the following channel and can followany route to the destination. Thus, each packet has header information about the

source, destination and packet numbering etc. At the receiver end, the packets are re-

assembled into the original message.

Circuit Switching:

Data packets are transmitted quickly, in a sequence at a constant rate. It is a point-to-

point transmission technique in which a dedicated connection should be established

before any data transmission can take place, for example, a telephone call. Once the

transmission is over, the circuit is free for other uses.

Difference between Packet Switching and Circuit Switching:

The difference between the two is that in packet switching techniques, the

communication channels are not dedicated to passing messages from the source to the

destination. In packet switching, different messages and even different packets can

pass through different routes. There is a dead time between the source and destination

at some point in time, and that is when the channel can be used by other users.

Circuit Switching is ideal when data has to be transmitted quickly in order. Therefore,

when transmitting real time data, such as audio and video, circuit switching will be

used. Packet Switching is more efficient and robust for data that can withstand delays

in transmission, such as e-mail messages, and Web pages.

Explain what is meant by the term Mobile IP?

A protocol is set of standard rules used to communicate between several users.

Internet Protocol (IP) is a network layer protocol and is the standard that defines the

way in which the network of two hosts interacts. These hosts maybe on the same

network or physically different ones. IP provides a connectionless packet delivery

service. Mobile IP is a type of protocol designed to enable mobile computers to stay

connected to the internet regardless of their location and without changing their IP

address. Mobile IP is a standard protocol that makes mobility transparent to

applications and higher level protocols like the Transmission Control Protocol (TCP).

It solves the problem by allowing each mobile node to have two IP addresses and

maintaining the binding between the two addresses. One of the IP addresses is the

permanent home address that is assigned at the home network and is used for

identification purposes. The other is a temporary care-of address that represents the

current location of the host provided by the foreign network. It has no geographical

limitations and no physical connection is required. There are no changes required tothe present IP address and it supports security as well.

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What is meant by the terms foreign network and home network in the context of

mobile IP?

The home network is the subnet the Mobile Node (MN) belongs to with respect to itsIP address. No mobile IP support is needed within the home network. It is the network

or virtual network which matches the subnet address of the mobile node.

The foreign network is the current subnet the MN visits. It is a router on a mobile

node's visited network which provides routing services to the mobile node while

registered.

Describe how an IP packet is delivered from a correspondent node to a mobile

node, which is not in its home network.

Figure 1 Mobile IP Components and Relationships

[6]

Computer Node (CN) sends an IP packet and the Internet routes the packet to the

router responsible for the home network of Mobile Node (MN). Then the Home

Agent (HA) knowing that the MN is currently not in its home network intercepts the

packet. The packet is encapsulated and tunneled to the Care Of Address (COA). A

new header is put in front of the old IP header showing the COA as new destinationand HA as source of the encapsulated packet. Then the Foreign Agent (FA)

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.

What is encapsulation and tunneling?

Generally encapsulation means to include one thing into another so that the included

thing is not apparent. Encapsulation is the inclusion of one data structure within

another structure so that the first data structure is hidden for the time being.

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Tunneling is the transmission of data intended for use only within a private network

through a public network in such a way that the routingnodes in the public network is

unaware that the transmission is part of a private network. Tunneling is generally

done by encapsulating the private network data and protocol information within the

public network transmission units so that the private network protocol informationappears to the public network as data.

How does a mobile node know that is has moved into a foreign network?

Home Agents (HA) and Foreign Agents (FA) sends advertisement messages into their

physical subnets. A Mobile Node (MN) listens to these messages and detects, if it is in

the home or a foreign network. Foreign agents have a security association with the

home agent.

Having received a care-of-address from a foreign agent, how does a mobile noderegister with the home agent?

Once a mobile node has been given a care of address, it needs to register itself with

the home agent. The purpose of registration is that the home agent should be aware of

the current location of the mobile node for the correct transfer of information. A

mobile node sends a registration request containing the care of address to the foreign

agent. The foreign agent will check for the validity of the registration request and if it

is valid will forward it to the home agent. The Home Agent checks the validity of the

registration request, which includes authentication of the Mobile Node. If the

registration request is valid, the Home Agent creates a mobility binding (an

association of the Mobile Node with its care-of address), a tunnel to the care-of

address, and a routing entry for forwarding packets to the home address through the

tunnel.

The Home Agent then sends a registration reply to the Mobile Node through the

Foreign Agent (if the registration request was received via the Foreign Agent). The

home agent can send the request back directly to the mobile node as well. The reply

basically depends on how the registration request was sent to the home agent.

Why are firewalls a problem for mobile IP?

The firewall imposes restrictions on packets entering of leaving the private network.

Packets are not allowed through unless they confirm to a filtering specification, or

unless there is a negotiation involving some sort of authentication. Because of this,

the firewall must be explicitly targeted as the destination node by outside packets

seeking to enter the private network.

What are ad-hoc networks?

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Ad-hoc networks are a collection of mobile nodes which can communicate over radio

and do not need any pre-installed communication infrastructure. Communication can

be performed if two nodes are close enough to exchange data. Any mobile host can

become a router and pass information to the destination. Therefore, there is not need

for routers in ad-hoc networks.This type of network must be able to start by itself as its need can arise anywhere any

time. The network topology will also change dynamically as the mobile hosts move

around within the network and so the routing protocol must be flexible enough to

ensure that data gets routed correctly and efficiently.

Give some examples in which ad-hoc networks would be beneficial.

Ad-hoc networks can be of significance in:

1) In a conference room during meetings where the participants can shareinformation.

2) In a lecture, where students can share information with the professor.

3) In an airport maybe, where the staff wants to share pieces of information with

them.

4) In an emergency rescue mission among the rescue workers to coordinate the

effort. For example, after an earthquake or flood knocks the entire

infrastructure down.

5) In battle among soldiers to coordinate defense or offense.

How does mobile IP differ from Internet IP?

Mobile IP enables mobility in the Internet without changing existing wired systems.

Internet IP is fixed.

Explain why TCP used in fixed networks is not suitable for the mobile transport

layer.

The mechanisms within TCP have been designed for situations that are completely

different from those in mobile networks.

In traditional TCP, why does the sender reduce the data rate by half when it

notices a missing acknowledgement?

In traditional Transmission Control Protocol (TCP), data information is sent from the

sender to the receiver using packet switching. Each time a packet of data is sent, the

receiver sends an acknowledgement to the sender that it has got hold of the data.

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The sender tends to increase the data rate each time after getting an acknowledgment.

This increase is usually an exponential increase. In real time systems, many users are

transmitting data on the same channel, so an increase in data rate can exceed the

capacity of the channel. This gives rise to congestion problems. As soon as a sender

receives a missing acknowledgement, it reduces its transmission rate to half because itrecognizes the fact that congestion might have occurred.

Explain briefly with the help of a diagram, how indirect TCP works?

[2]

Indirect TCP as it is called, does not make any changes to the host connected to the

fixed network (millions of users use this system). It splits up the connection at, for

example, the foreign agent into two TCP connections, so we do not need the long end-

to-end connection anymore. Indirect TCP hosts in the fixed part of the network and

therefore, does not notice the characteristics of the wireless part

Briefly outline the key advantages and disadvantages of indirect TCP?

Advantages:

Indirect Transmission Control Protocol (Indirect TCP) is very simple to control asthere are no changes required to make in the fixed networks to the basic TCP model.

All the optimizations made for the basic model can be used in I-TCP. The failures

which come up in the wireless link do not propagate into the fixed network. A mobile

hop is used only between the foreign agent and the mobile host, therefore a very fast

transmission rate is possible

Disadvantages:

As there is no direct connection between the senders and the receivers, they are

connected via the foreign agents; an acknowledgment sent to the sender for

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transmission of data does not necessarily mean that the receiver has actually got hold

of the data packet. There might be some hidden delays possible within the foreign

agents when they are transacting with the data to a new foreign agent.

How does snooping TCP work?

wired Internet

buffering of

local retransmissionst

ent

st

correspondent

hoforeign

ag

mobile

ho

snooping of

[2]

The basic idea behind a snooping TCP is the same as that of a TCP, the only

difference being that an addition of a foreign agent. When a sender transmits data to

the receiver, it receives an acknowledgment for a successful transmission. Not always

will the transmission be successful. There is always data lost on the communication

link from both sides. Here the foreign agent comes into affect. It immediately re-

transmits data from itself to the mobile host or to the correspondent host, i.e. in either

way. The foreign agent actually keeps the data which the sender wants to transmit to

the receiver, so that whenever there is a transmission failure, it can send the data on

behalf of the sender. The foreign agent therefore snoops the packet flow and

recognizes acknowledgements in both directions.

What are the advantages and disadvantages of snooping TCP?

Advantages:

As the data sent by the sender to the receiver is held by the foreign agent (as it goesthrough it), if there are any transmission failures, the sender does not need to send that

particular data again as this can be done by the foreign agent. Therefore, this helps in

saving time and the sender can send some more fresh pieces of data in the mean time.

This is because for the sender to spend time on transmitting the same data again

which wasnt received by the receiver, the foreign agent does this on behalf of the

sender.

Disadvantages:

Snooping TCP does not actually isolate the wireless link and it might sometimes

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become useless depending on the encryption schemes. As data is transferred via a

foreign agent, it might actually take longer to send it to the receiver by this method

rather than directly.

What functions does the transport layer represent?

This layer is used in the reference model to establish an end-to-end connection. It has

the following functions:

Flow and congestion control, quality of service

Local retransmissions and acknowledgements

Content filtering, compression, picture downscaling

What is the difference between the transport layer and the network layer?

The transport layer is used in the reference model to establish an end-to-end

connection. Network layer is responsible for routing packets through a network of

establishing a connection between two entities over many other intermediate systems.

Explain why TCP is needed?

Transmission Control Protocol (TCP) is responsible for verifying the correct delivery

of data from client to server. Data can be lost in the intermediate network as there are

many users wanting access to the network. The capacity of the network may exceed

during these data transmissions by multiple users. Congestion occurs as a result. TCP

controls the data transmission flow and rate. As soon as it detects congestion, it

immediately reduces the data transmission rate of the users to half in order to

overcome the problem of congestion. It has made the use of networks fair and equally

distributed as every user has the chance to transmit. TCP further adds support to

detect errors or lost data and to trigger retransmission until the data is correctly and

completely received.

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SECTION 5

Answers for the OPNET EXPERIMENT

Questions

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If you will install a LAN on the first floor of the office building and you will be

asked to evaluate whether you can support the addition of few intelligence

workstations on the second floor by extending the LAN service to the second

floor. What are your main recommendations to establish this process? What is

the current demand on the server from the first floor LAN and will it be able to

handle the load of the second network on the second floor? What is the current

total delay across the first floor LAN and what will happen once the second

network is placed on the second floor?

If an extension has to be made to an existing Local Area Network (LAN), a new

topology similar to the one on the first floor has to be modeled. It is critical that we

make sure that the extension to the present network will not cause the network as a

whole to fail. This verification can be achieved by testing and simulating the new

network separately before actual implementation. Through simulating the extension in

OPNET, the actual results can be seen before it is implemented. From those results

and data collected, it can be seen how the network on the second floor will behave

before it can actually be put into implementation.

The current demand on the server for the first floor is about 5602 bits per second and

the current total delay is about 0.41 milliseconds. When the network was expanded to

the second floor, the load changed to 10678 bits per second. It was observed that the

delay of the network after extension did not change significantly and was 0.40

milliseconds.

What are the first three requirements to start building the small internetworks?

The first three requirements to start building a small internetwork are as follows:

a) A server

b) Router

c) Switch.

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Draw a diagram of a small LAN networks for a two floor office and state the

function of each element in the diagram.

Node:

A network object that can send and receive data

Link:

A communication medium that connects nodes to one another. Links can represent

electrical or fibre optical cables.

3Com switch:

Used to connect both the networks together.

Number of workstations connected to the server: 30

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Sm_Int_server:

The main server for this network to which all workstations are connected.

Sm_Application_Config:

Configurations for the applications

Sm_Profile_Config:

Profile configurations.

What are your conclusions if the Ethernet delay of a network is approximatelysame with the network expansion by one or two nodes?

If the Ethernet delay of a network is approximately the same with the expansion by

one or two nodes, this suggests that the network is stable and consistent. It does not

overload because it is able to adjust more nodes within itself.

Why the average queuing delay in M/M/1 queue does appear to have a large

change early? Does this stabilize after a long period of time?

The performance of an M/M/1 queuing system depends on the following parameters:

Packet arrival rate

Packet size

Service capacity

If the total effect of the average packet arrival rate and the average packet size

exceeds the service capacity, the queue size will grow indefinitely as it cannot hold

data which is beyond its capacity.

When performing this experiment and simulating the results, we basically have to

make sure the queue reaches steady state for a specific arrival rate, packet size, and

service capability.

When the simulations are performed, the average queuing delay appears to have a

large change early. This is because at the start of the simulation, the numbers of

packets collected are small. So there arent many samples to take the average of. The

system is sensitive to small number of data samples. As more time passes, the queue

system stabilizes as more and more samples processed by the server and collected at

the destination.

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If the mean queuing delay is around 15 seconds, what is the mean service rate if

the mean interarrival time is 1.0 second, mean service requirement 9000b/p and

service capacity is 9600b/s?

mean service requirement; =/1 9000 b/p

mean service capacity ; C = 9600 b/s

mean interarrival time = 1.0 seconds

mean queuing delay = 15.0 seconds

mean service rate ; =C [((1/9000)*9600)] = 1.067 packets/second

What is the time-average queue size if the mean arrival rate, mean service rate

requirement and service capacity are 1p/s, 9000b/s and 9600b/s respectively?

Service arrival rate: 1 p/s

Service capacity: 9600 b/s

Mean service requirement: 9000 b/s

=)(

1 C

=1067.1

1

= 14.9 seconds

The queue model requires means of generating, queuing and serving packets,

what is the module that all these functions can be done with?

All these functions stated above in the question can be done with the node module.

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Draw a diagram of the M/M/1 queue system and state briefly the operation of

each module?

The M/M/1 queue has a Poisson distribution of packets. Data packets are generated

and sent to the infinite buffer. When a packet reaches the buffer, it is processed by a

server and sent to its destination.

The queuemodule is used to represent the infinite buffer and also the server.

: mean packet arrival rate

1/: mean packet size

C: service capacity

The M/M/1 queue model requires a way of generating, queuing, and serving packets,

all of which can be done with existing node modules.

If you have a finite buffer on the packets arrived on the process module of a

node, what are your recommendations on the quality of services?

If we have a finite buffer on the process module, the quality of service will drop

drastically. This is due to the fact that the buffer will be able to handle a certain

amount of data packets. When the data packets exceed the capacity of the finite

buffer, the buffer will not be able to process them and send them to the server. This

has a negative impact as the server sends the data packets to their destinations. If the

buffer is not collecting the data packets as it cannot because of limited capacity, the

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data packets wont reach their destination and get destroyed. This reduces the

percentage of successful transmitted data.

What are the three main nodes used in the Mobile Radio Network? Give a short

list of the modules used in each node?

The three main nodes used in the Mobile Radio Network are:

a) Transmitter node: A transmitter node transmits data packets in all directions. It

consists of a packet generator module, a radio transmitter module, and an

antenna module.

b) Receiver node: A receiver node measures the quality of signal emitted from

the stationary nodes. It consists of an antenna module, a radio receiver module,

a sink processor module, and an additional processor module that works with

the directional antenna.c) Mobile jammer node: The jammer nodes create noise. The jammers trajectory

takes it in and out of the radio range of the receiver node, increasing and

decreasing interference at the receiver.

The transmitter and the receiver nodes are also known as the stationary nodes.

Distinguish the variations of throughput of the mobile network using isotropic

antenna pattern and directional antenna?

An antenna is a device that radiates radio signals. An isotropic antenna is one which

radiates equally in all directions. The number of packets received initially using the

isotropic antenna increases. As the radiations are equally spread in all directions, a

large number of packets are received. But one thing which has to be kept in mind is

that as this antenna radiates in all directions, it is more vulnerable to picking up

interference. This is what happens and the throughput decreases with the passage of

time as more and more interference is picked up.

A directional antenna is used to transmit in one certain direction or angle. Initially the

throughput is slow to start with. As it is a directional antenna, the number of datasamples transmitted are low. With the passage of time, the throughput increases as

data is transmitted in a certain direction and less interference is picked up. The

process is more effective as less unwanted signal is picked up.

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Illustrate the main elements of a Generic transmitter and receiver node models

for Aloha and CSMA?

A simple Alohasystem can be designed using a simple source generator and a bus

transmitter. But we design a more general model so that it can be used later for theCSMA model as well.

The transmitter node consists of a generator, processor and a bus transmitter. The

generic transmitter node must generate packets, process them, and send them on to the

bus. This can be done by using a source processor to generate packets, another

processor to perform any necessary operations, and a bus transmitter that transmits the

packets on the bus link. Bus transmitters also have internal queuing capability they

will issue all submitted packets onto the bus in a first come first serve order.

The transmitter process only has to receive packets from the generator and send themon to the transmitter. The Aloha transmitter process has only one unforced

state: waiting for the arrival of a packet from the generator. The transmitter cannot

collect statistics.

The generic receiver node basically consists of a processor and a bus receiver. The

generic receiver node monitors the movement of packets across the bus. The receiver

does not require a generator because it simply checks the packets moving across the

bus.

The receiver node is also responsible for handling received packets for collecting

statistics.

Comment on the variations of the throughput of both Aloha and CSMA against

the channel traffic?

The CSMA protocol achieves a maximum channel throughput of about 0.5 while the

Aloha protocol achieves a maximum channel throughput of about 0.185. The

throughput difference is 0.315. There are a few reasons for this. It transmits data

packets randomly without any control. The Aloha system does not coordinate mediumaccess. It is a random scheme without a central station controlling the other

workstations and coordinating between them. If any two stations transmit at the same

time, which they usually do, a collision occurs which subsequently reduces the

throughput as the data needs to be sent again.

Theoretical analyses have shown that a pure Aloha system has a channel throughput S

as a function of channel traffic G given by S = Ge-2G. This relationship gives a

maximum channel throughput of Smax = 1/2e 0.18.

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At low traffic levels, collisions hardly ever occur. At high traffic levels, the channel is

overloaded and excessive collisions prevent packets from being successfully received.

Thus, the maximum throughput is achieved near G = 0.5 and is close to the expected

value of 0.18.

The Carrier Sensing Multiple Access (CSMA) on the other hand, has a little bit of

improvement to the Aloha system. It uses the sensing technique before accessing the

medium. Sensing or listening is seeing whether the medium is free for transmission or

not and then actually transmitting. Sensing the medium and accessing only when the

carrier is idle decreases the probability of a collision. Therefore, the throughput of

CSMA is slightly better.

The CSMA protocol seems to be better than the Aloha protocol at all channel traffic

loads. The theoretical channel throughput S as a function of channel traffic G in a

CSMA channel (with negligible propagation delay) is given by S=G (1+G)e-G/(G+e-G).This formula predicts a maximum throughput of approximately 0.5 at channel traffic

of approximately 1.0.

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