ass1mpc with answer

8/11/2019 Ass1MPC With Answer

http://slidepdf.com/reader/full/ass1mpc-with-answer 1/27

SRI MUTHUKUMARAN INSTITUTE OF TECHNOLOGY

Department of Computer Science an En!ineerin!

A""e""ment Te"t I

I# Year #II Seme"ter

Su$ Name% Mo$i&e an 'er(a"i(e an Computin! Mar)"% *+

Su$ Coe% CS,-+, Time% ./0+ Hr"

'art 1 A

1. Define CDMA?

Code Division Multiple Access systems use codes with certain characteristic to separate different

users. To enable access to the shared medium without interference. The users use the same frequency and

time to transmit data. The main problem is to find good codes and to seprate this signal from noise. The

good code can be found by two characteristics 1.rthogonal !.Auto correlation.

!. "hat is polling?

#olling is a centrali$ed scheme with one master and several slave stations. The master can collectthe list of stations during the connection phase and can poll these slaves according to many schemes li%e

round robin& random access& reservation schemes etc.

'. "hat are the four types of handover available in ()M?

1. *ntra cell +andover

!. *nter cell *ntra ,)C +andover

'. *nter ,)C *ntra M)C handover

-. *nter M)C +andover

-. "hat is the information in )*M?

card type& serial no& list of subscribed services

#ersonal *dentity umber/#*0

#in nloc%ing 2ey/#20

An Authentication 2ey/2*0

3. "hat is multiple4ing?

Multiple4ing is transmitting multiple signals over a single communications line or computer channel. The two common multiple4ing techniques are 5DM& which separates signals by modulating the

data onto different carrier frequencies& and TDM& which separates signals by interleaving& bits one after

the other.

6. "hat is guard space?

(uard spaces are needed to avoid frequency band overlapping is also called channel interference.



7. "hat are the advantages and disadvantages of cellular systems?

The advantages of cellular systems are&

• +igher capacity

• 8ess transmission power

• 8ocal interface only

• 9obustness

The disadvantages of cellular systems are&

• *nfrastructure needed

• +andover needed

• 5requency planning

:. ;4pansion of (M)C& <89 and )()

(M)C = (ateway Mobile services switching center

<89 =<isitor 8ocation 9egister

)() > serving (#9) support node

. "hat are the advantages of "8A?

+igh fle4ibility

)imple Design

;asy planning

8ow>cost

1@. "hat are ad hoc networ%s?

Adhoc networ%s do not need any infrastructure to wor% and each node can communicate with

other nodes directly& so that the access point controlling medium is not necessary.

11. "hat is wireless communication?

"ireless communication is the transfer of information over a distance without the use of

electrical conductors or wires. The distance involved may be short& long

1!. "hat is meant by frequency reuse?

The concept of simultaneous use of same frequency channels at different cells that are sufficiently

placed at a distance from each other is %nown as frequency reuse.

1'. "hat is a signal?

)ignals are the physical representation of data. The users of a communication system can only

e4change data through the transmission of signals.

)ignals are functions of time and location.

)ignal parameters #arameters representing the value of data.

)ignal parameter of periodic signal #eriod T frequency f B 1T amplitude A



1-. "hat are the services provided by supplementary services?

ser identification

Call redirection

Call forwarding Closed user groups

Multiparty Communication

13. "hat is modulation?

Modulation is the process of varying one or more properties of a high>frequency periodic

waveform& called the carrier signal& with respect to a modulating signal /which typically contains

information to be transmitted0.

16. "hat is hopping sequence?

Transmitter and receiver stay on one of these channels 5DM and TDM. The pattern of channelusage is called the hopping sequence.

17. "hy is physical layer in *;;;:@!.11 subdivided? "hat are its sub layers?

The physical layer in *;;;:@!.11 is subdivided because a sub layer has to be dependent on the

upper layers /architecture dependant0 and the other has to be medium dependant. The two sub layers are

namely&

#hysical layer convergence protocol

#hysical medium dependant sub layer

1:. "hat are the benefits of using infrared for transmission?

*t is simple and e4tremely cheap senders and receivers.

*t defines higher data rates

The electrical devices do not interfere with the infrared transmission

1. "hat is *;;; :@!.11 standard?

The *;;; :@!.11 standard specifies the physical and medium access layer adapted to the special

requirements of wireless 8As. This standard offers the time bounded and asynchronous services. The data

rate of this standard is 3- Mbitss at 3 (+.

!@. "hat are the functions of MAC management?

)upports the association and re>association of a station to an access point and roaming between

different access points. *t maintains the MAC information base /M*,0. *t also controls the authentication

mechanisms& encryption and power management.



#art ,

1. ;4plain the system architecture of ()M

*/ E2p&ain t3e "4"tem arc3itecture of GSM/ 5NO# ,+.,6 5MAY ,+..6 5MAY ,+.,6

System architecture

A GSM system consists of three subsystems, the radio sub system (RSS),

the network and switching subsystem (NSS), and the operation subsystem

(OSS). Each subsystem will be discussed in more detail in the following sections.

Generally, a GSM customer only notices a very small fraction of the whole network –

the mobile stations (MS and some antenna masts of the base transceiver stations

(!"S.

RADIO SUBSS!"#

As the name im#lies, the radio subsystem (RSS) com#rises all radio

s#eci$c entities, i.e., the mobi$e stations (#S) and the base station subsystem

(BSS). "he A interface is ty#ically based on circuit%switched &'M%) systems (*.)+

Mbit-s, carrying u# to ) + kbit-s connections, whereas the / interface uses the

Signalling System 0o. 1 (SS1 based on 2.*3 carrying management data to-from the

4SS.

● Base station subsystem (BSS)5 A GSM network com#rises many !SSs, each

controlled by a base station controller (!S'. "he !SS #erforms all functions

necessary to maintain radio connections to an MS, coding-decoding of voice, and

rate ada#tation to-from the wireless network #art. !esides a !S', the !SS contains

several !"Ss.



%unctiona$ architectureo& a 'S# system

● Base transceier station (B!S)5 A !"S com#rises all radio e6ui#ment, i.e.,

antennas, signal #rocessing, am#li$ers necessary for radio transmission. A !"S can

form a radio cell or, using sectori7ed antennas, several cells (see section *., and is

connected to MS via the Um inter&ace (8S90 : interface for mobile use, and to the

!S' via the Abis inter&ace. "he :m interface contains all the mechanisms

necessary for wireless transmission ("9MA, ;9MA etc. and will be discussed in

more detail below. "he Abis interface consists of < or + kbit-s connections. A GSMcell can measure between some <)) m and 3 km de#ending on the environment

(buildings, o#en s#ace, mountains etc. but also e=#ected tra>c.

● Base station contro$$er (BS)5 "he !S' basically manages the !"Ss. 8t reserves

radio fre6uencies, handles the handover from one !"S to another within the !SS,



and #erforms #aging of the MS. "he !S' also multi#le=es the radio channels onto

the $=ed network connections at the A interface. "able +.< gives an overview of the

tasks assigned to the !S' and !"S or of tasks in which these entities su##ort other

entities in the network.

● #obi$e station (#S)5

• The M) comprises all user equipment and software needed for communication with a

()M networ%. An M) consists of user independent hard> and software and of the

"u$"cri$er ientit4 mou&e 5SIM6& which stores all user>specific data that is relevant to

()M.' "hile an M) can be identified via the internationa& mo$i&e e7uipment ientit4

5IMEI6& a user can personali$e any M) using his or her )*M& i.e.& user>specific

mechanisms li%e charging and authentication are based on the )*M& not on the deviceitself. Device>specific mechanisms& e.g.& theft protection& use the device specific *M;*.

"ithout the )*M& only emergency calls are possible.

• The )*M card contains many identifiers and tables& such as card>type& serial number& a list

of subscribed services& a per"ona& ientit4 num$er 5'IN6& a 'IN un$&oc)in! )e4

5'UK6& an aut3entication )e4 Ki& and the internationa& mo$i&e "u$"cri$er ientit4

5IMSI6 /;T)*& 11c0.

• The #* is used to unloc% the M). sing the wrong #* three times will loc% the )*M. *n

such cases& the #2 is needed to unloc% the )*M. The M) stores dynamic information

while logged onto the ()M system& such as& e.g.& the cip3er )e4 Kc and the location

information consisting of a temporar4 mo$i&e "u$"cri$er ientit4 5TMSI6 and the

&ocation area ientification 5LAI6.

• Typical M)s for ()M @@ have a transmit power of up to ! "& whereas for ()M 1:@@ 1

" is enough due to the smaller cell si$e. Apart from the telephone interface& an M) can

also offer other types of interfaces to users with display& loudspea%er& microphone& and

programmable soft %eys.

• 5urther interfaces comprise computer modems& *rDA& or ,luetooth. Typical M)s& e.g.&

mobile phones& comprise many more vendor>specific functions and components& such as

cameras& fingerprint sensors& calendars& address boo%s& games& and *nternet browsers.

• #ersonal digital assistants /#DA0 with mobile phone functions are also available. The

reader should be aware that an M) could also be integrated into a car or be used for

location trac%ing of a container.

N"!*OR+ AND S*I!,IN' SUBSS!"#



• The EheartF of the ()M system is formed by the net8or) an "8itc3in! "u$"4"tem

5NSS6.

• The )) connects the wireless networ% with standard public networ%s& performs

handovers between different ,))s& comprises functions for worldwide locali$ation of

users and supports charging& accounting& and roaming of users between different providers in different countries.

"he 0SS consists of the following switches and databases5

● #obi$e serices switching center (#S)5

• M)Cs are high>performance digital *)D switches. They set up connections to other

M)Cs and to the ,)Cs via the A interface& and form the fi4ed bac%bone networ% of a

()M system. Typically& an M)C manages several ,)Cs in a geographical region.

• A !ate8a4 MSC 5GMSC6 has additional connections to other fi4ed networ%s& such as

'STN and ISDN. sing additional inter8or)in! function" 5I9F6& an M)C can also

connect to pu$&ic ata net8or)" 5'DN6 such as G.!3.

• An M)C handles all signaling needed for connection setup& connection release and

handover of connections to other M)Cs. The "tanar "i!na&in! "4"tem No/ : 5SS:6 is

used for this purpose. ))7 covers all aspects of control signaling for digital networ%s

/reliable routing and delivery of control messages& establishing and monitoring of calls0.

• 5eatures of ))7 are number portability& free phonetollcollectcredit calls& call

forwarding& three>way calling etc. An M)C also performs all functions needed for supplementary services such as call forwarding& multi>party calls& reverse charging etc.

● ,ome $ocation register (,-R)5

• The +89 is the most important database in a ()M system as it stores all user>relevant

information. This comprises static information& such as the mo$i&e "u$"cri$er ISDN

num$er 5MSISDN6& subscribed services /e.g.& call forwarding& roaming restrictions&

(#9)0& and the internationa& mo$i&e "u$"cri$er ientit4 5IMSI6.

•

Dynamic information is also needed& e.g.& the current &ocation area 5LA6 of the M)& themo$i&e "u$"cri$er roamin! num$er 5MSRN6& the current <89 and M)C. As soon as

an M) leaves its current 8A& the information in the +89 is updated.

• This information is necessary to locali$e a user in the worldwide ()M networ%. All these

user>specific information elements only e4ist once for each user in a single +89& which

also supports charging and accounting. The parameters will be e4plained in more detail in

section -.1.3. +89s can manage data for several million customers and contain highly



speciali$ed data bases which must fulfill certain real>time requirements to answer

requests within certain time>bounds.

● .isitor $ocation register (.-R)5

• The <89 associated to each M)C is a dynamic database which stores all important

information needed for the M) users currently in the 8A that is associated to the M)C

/e.g.& *M)*& M)*)D& +89 address0.

• *f a new M) comes into an 8A the <89 is responsible for& it copies all relevant

information for this user from the +89. This hierarchy of <89 and +89 avoids frequent

+89 updates and long>distance signaling of user information.

• The typical use of +89 and <89 for user locali$ation will be described in section -.1.3.

)ome <89s in e4istence& are capable of managing up to one million customers.

Operation subsystem

• The third part of a ()M system& the operation "u$"4"tem 5OSS6& contains the necessary

functions for networ% operation and maintenance.

• The )) possesses networ% entities of its own and accesses other entities via ))7

signaling. The following entities have been defined

● Operation and maintenance center (O#)5 "he /M' monitors and controls allother network entities via the / interface (SS1 with 2.*3. "y#ical /M' management

functions are tra>c monitoring, status re#orts of network entities, subscriber and

security management, or accounting and billing.

● Authentication centre (Au)5

• As the radio interface and mobile stations are particularly vulnerable& a separate AuC has

been defined to protect user identity and data transmission.

• The AuC contains the algorithms for authentication as well as the %eys for encryption andgenerates the values needed for user authentication in the +89.

• The AuC may& in fact& be situated in a special protected part of the +89.



● "/uipment identity register ("IR)5

• The ;*9 is a database for all *M;*s& i.e.& it stores all device identifications registered for

this networ%. As M)s are mobile& they can be easily stolen. "ith a valid )*M& anyone

could use the stolen M).

• The ;*9 has a blac%list of stolen /or loc%ed0 devices. *n theory an M) is useless as soon

as the owner has reported a theft. nfortunately& the blac%lists of different providers are

not usually synchroni$ed and the illegal use of a device in another operatorHs networ% is

possible /the reader may speculate as to why this is the case0. The ;*9 also contains a list

of valid *M;*s /white list0& and a list of malfunctioning devices /gray list0.

01 Discuss the protoco$ architecture o& 'S#1 (#A 0233)

4RO!OO-S

• The main interest lies in the m interface& as the other interfaces occur between entities

in a fi4ed networ%. La4er .& the physical layer& handles all raio>specific functions. This

includes the creation of bursts according to the five different formats& mu&tip&e2in! of

bursts into a TDMA frame& "4nc3roni;ation with the ,T)& detection of idle channels&

and measurement of the c3anne& 7ua&ity on the downlin%.

• The physical layer at m uses (M)2 for digital mou&ation and performs

encr4ption<ecr4ption of data& i.e.& encryption is not performed end>to>end& but only

between M) and ,)) over the air interface.

• )ynchroni$ation also includes the correction of the individual path delay between an M)

and the ,T). All M)s within a cell use the same ,T) and thus must be synchroni$ed to

this ,T). The ,T) generates the time>structure of frames& slots etc.

• A problematic aspect in this conte4t is the different round trip times /9TT0. An M) close

to the ,T) has a very short 9TT& whereas an M) '3 %m away already e4hibits an 9TT of

around @.!' ms.

• *f the M) far away used the slot structure with> out correction& large guard spaces would

be required& as @.!' ms are already -@ per cent of the @.377 ms available for each slot.

Therefore& the ,T) sends the current 9TT to the M)& which then adIusts its access timeso that all bursts reach the ,T) within their limits. This mechanism reduces the guard

space to only '@.3 μs or five per cent.

• AdIusting the access is controlled via the variable timin! a(ance& where a burst can be

shifted up to 6' bit times earlier& with each bit having a duration of '.6 μs /which results

in the @.!' ms needed0. As the variable timing advance cannot be e4tended a burst cannot

be shifted earlier than 6' bit times. This results in the '3 %m ma4imum distance between



an M) and a ,T). *t might be possible to receive the signals over longer distancesJ to

avoid collisions at the ,T)& access cannot be allowed.

4rotoco$ architecture &or signa$ing

• The main tas%s of the physical layer comprise c3anne& coin! and error

etection<correction& which is directly combined with the coding mechanisms. Channel

coding ma%es e4tensive use of different for8ar error correction 5FEC6 schemes.

• 5;C adds redundancy to user data& allowing for the detection and correction of selected

errors. The power of an 5;C scheme depends on the amount of redundancy& coding

algorithm and further interleaving of data to minimi$e the effects of burst errors. The5;C is also the reason why error detection and correction occurs in layer one and not in

layer two as in the *))* reference model.

• The ()M physical layer tries to correct errors& but it does not deliver erroneous data to

the higher layer. Different logical channels of ()M use different coding schemes with

different correction capabilities.

• )peech channels need additional coding of voice data after analog to digital conversion&

to achieve a data rate of !!.: %bits /using the 1' %bits from the voice codec plus

redundancy& C9C bits& and interleaving /(oodman& 170. As voice was assumed to be

the main service in ()M& the physical layer also contains special functions& such as (oice

acti(it4 etection 5#AD6& which transmits voice data only when there is a voice signal.

This mechanism helps to decrease interference as a channel might be silent

appro4imately 6@ per cent of the time /under the assumption that only one person spea%s

at the same time and some e4tra time is needed to switch between the spea%ers0.

• During periods of silence /e.g.& if a user needs time to thin% before tal%ing0& the physical

layer generates a comfort noi"e to fa%e a connection /complete silence would probably



confuse a user0& but no actual transmission ta%es place. The noise is even adapted to the

current bac%ground noise at the communication partnerHs location.

• All this interleaving of data for a channel to minimi$e interference due to burst errors and

the recurrence pattern of a logical channel generates a e&a4 for transmission. The delay

is about 6@ ms for a TC+5) and 1@@ ms for a TC+5.6 /within 1@@ ms signals in fi4ed

networ%s easily travel around the globe0. These times have to be added to the

transmission delay if communicating with an M) instead of a standard fi4ed station

/telephone& computer etc.0 and may influence the performance of any higher layer

protocols& e.g.& for computer data transmission /see chapter 0.

• )ignaling between entities in a ()M networ% requires higher layers. 5or this purpose& the

LA'Dm protocol has been defined at the m interface for &a4er t8o. 8A#Dm& as the

name already implies& has been derived from lin% access procedure for the D>channel

/LA'D0 in *)D systems& which is a version of +D8C /(oodman& 170& /+alsall&

160. 8A#Dm is a lightweight 8A#D because it does not need synchroni$ation flags or

chec%summing for error detection.• 8A#Dm offers reliable data transfer over connections& re>sequencing of data frames& and

flow control /;T)*& 1'b0& /;T)*& 1'c0. As there is no buffering between layer one

and two& 8A#D has to obey the frame structures& recurrence patterns etc. defined for the

m interface. 5urther services provided by 8A#Dm include segmentation and

reassembly of data and ac%nowledgedunac%nowledged data transfer.

• The networ% layer in ()M& &a4er t3ree& comprises several sublayers as 5igure -.7 shows.

The lowest sublayer is the raio re"ource mana!ement 5RR6. nly a part of this layer&

RR H& is implemented in the ,T)& the remainder is situated in the ,)C. The functions of

99H are supported by the ,)C via the =TS mana!ement 5=TSM6.

• The main tas%s of 99 are setup& maintenance& and release of radio channels. 99 also

directly accesses the physical layer for radio information and offers a reliable connection

to the ne4t higher layer.

• Mo$i&it4 mana!ement 5MM6 contains functions for registration& authentication&

identification& location updating& and the provision of a temporar4 mo$i&e "u$"cri$er

ientit4 5TMSI6 that replaces the internationa& mo$i&e "u$"cri$er ientit4 5IMSI6 and

which hides the real identity of an M) user over the air interface.

• "hile the *M)* identifies a user& the TM)* is valid only in the current location area of a

<89. MM offers a reliable connection to the ne4t higher layer. 5inally& the ca&&

mana!ement 5CM6 layer contains three entities ca&& contro& 5CC6& "3ort me""a!e

"er(ice 5SMS6& and "upp&ementar4 "er(ice 5SS6.

• )M) allows for message transfer using the control channels )DCC+ and )ACC+ /if no

signaling data is sent0& while )) offers the services described in section -.1.1.'. CC

provides a point>to>point connection between two terminals and is used by higher layers

for call establishment& call clearing and change of call parameters. This layer also



provides functions to send in>band tones& called ua& tone mu&tip&e fre7uenc4 5DTMF6&

over the ()M networ%.

• These tones are used& e.g.& for the remote control of answering machines or the entry of

#*s in electronic ban%ing and are& also used for dialing in traditional analog telephone

systems. These tones cannot be sent directly over the voice codec of a ()M M)& as the

codec would distort the tones.

• They are transferred as signals and then converted into tones in the fi4ed networ% part of

the ()M system. Additional protocols are used at the Abis and A interfaces /the internal

interfaces of a ()M system not presented here0. Data transmission at the physical layer

typically uses pu&"e coe mou&ation 5'CM6 systems.

• "hile #CM systems offer transparent 6- %bits channels& ()M also allows for the

submultiple4ing of four 16 %bits channels into a single 6- %bits channel /16 %bits are

enough for user data from an M)0.

• The physical layer at the A interface typically includes leased lines with !.@-: Mbits

capacity. 8A#D is used for layer two at Abis& ,T)M for ,T) management. Si!na&in!

"4"tem No/ : 5SS:6 is used for signaling between an M)C and a ,)C. This protocol also

transfers all management information between M)Cs& +89& <89s& AuC& ;*9& and MC.

An M)C can also control a ,)) via a =SS app&ication part 5=SSA'6.

'. ;4plain (#9) architecture

Introduction

• The !enera& pac)et raio "er(ice 5G'RS6 provides pac%et mode transfer for

applications that e4hibit traffic patterns such as frequent transmission of small volumes/e.g.& typical web requests0 or infrequent transmissions of small or medium volumes /e.g.&

typical web responses0 according to the requirement specification /;T)*& 1:a0.

• Compared to e4isting data transfer services& (#9) should use the e4isting networ%

resources more efficiently for pac%et mode applications& and should provide a selection of Ko) parameters for the service requesters.

• (#9) should also allow for broadcast& multicast& and unicast service. The overall goal in

this conte4t is the provision of a more efficient and& thus& cheaper pac%et transfer servicefor typical internet applications that usually rely solely on pac%et transfer.

• etwor% providers typically support this model by charging on volume and not on

connection time as is usual for traditional ()M data services and for +)C)D.

• The main benefit for users of (#9) is the Lalways onH characteristic = no connection has

to be set up prior to data transfer. Clearly& (#9) was driven by the tremendous success of

the pac%et>oriented internet& and by the new traffic models and applications.



• The main concepts of (#9) are as follows /;T)*& 1:b0. 5or the new (#9) radio

channels& the ()M system can allocate between one and eight time slots within a TDMA

frame. Time slots are not allocated in a fi4ed& pre>determined manner but on demand. Alltime slots can be shared by the active usersJ up> and downlin% are allocated separately.

Architecture

• The G'RS arc3itecture introduces two new networ% elements& which are called G'RS

"upport noe" 5GSN6 and are in fact routers. All ()s are integrated into the standard

()M architecture& and many new interfaces have been defined /see 5igure -.160. The

!ate8a4 G'RS "upport noe 5GGSN6 is the interwor%ing unit between the (#9)

networ% and e4ternal pac)et ata net8or)" 5'DN6.

• This node contains routing information for (#9) users& performs address conversion&

and tunnels data to a user via encapsulation. The (() is connected to e4ternal networ%s

/e.g.& *# or G.!30 via the (i interface and transfers pac%ets to the )() via an *#>based

(#9) bac%bone networ% /(n interface0.

• The other new element is the "er(in! G'RS "upport noe 5SGSN6 which supports the

M) via the (b interface. The )()& for e4ample& requests user addresses from the

G'RS re!i"ter 5GR6& %eeps trac% of the individual M)sH location& is responsible for

collecting billing information /e.g.& counting bytes0& and performs several securityfunctions such as access control.

• The )() is connected to a ,)C via frame relay and is basically on the same hierarchy

level as an M)C. The (9& which is typically a part of the +89& stores all (#9)>relevant

data. (()s and )()s can be compared with home and foreign agents& respectively& ina mobile *# networ% /see chapter :0.

• #ac%et data is transmitted from a #D& via the (() and )() directly to the ,)) and

finally to the M). The M)C& which is responsible for data transport in the traditionalcircuit>switched ()M& is only used for signaling in the (#9) scenario.

• Additional interfaces to further networ% elements and other #8Ms can be found in ;T)*

/1:b0. ,efore sending any data over the (#9) networ%& an M) must attach to it&

following the procedures of the mo$i&it4 mana!ement. The attachment procedureincludes assigning a temporal identifier& called a temporar4 &o!ica& &in) ientit4

5TLLI6& and a cip3erin! )e4 "e7uence num$er 5CKSN6 for data encryption.

•5or each M)& a G'RS conte2t is set up and stored in the M) and in the corresponding)(). This conte4t comprises the status of the M) /which can be ready& idle& or standbyJ

;T)*& 1:b0& the C2)& a flag indicating if compression is used& and routing data

/T88*& the routing area 9A& a cell identifier& and a pac%et data channel& #DC+&identifier0. ,esides attaching and detaching& mobility management also comprises

functions for authentication& location management& and ciphering /here& the scope of

ciphering lies between M) and )()& which is more than in standard ()M0. *n i&e

mode an M) is not reachable and all conte4t is deleted.



'4RS architecture re&erence mode$

• *n the "tan$4 state only movement across routing areas is updated to the )() but not

changes of the cell. #ermanent updating would waste battery power& no updating wouldrequire system>wide paging.

• The update procedure in standby mode is a compromise. nly in the rea4 state every

movement of the M) is indicated to the )(). 5igure -.17 shows the protocolarchitecture of the transmission plane for (#9). Architectures for the signaling planes

can be found in ;T)* /1:b0.

• All data within the (#9) bac%bone& i.e.& between the ()s& is transferred using the

G'RS tunne&&in! protoco& 5GT'6. (T# can use two different transport protocols& either the reliable TC' /needed for reliable transfer of G.!3 pac%ets0 or the non>reliable UD'

/used for *# pac%ets0.

• The networ% protocol for the (#9) bac%bone is I' /using any lower layers0. To adapt to

the different characteristics of the underlying networ%s& the "u$net8or) epenentcon(er!ence protoco& 5SNDC'6 is used between an )() and the M).

• n top of )DC# and (T#& user pac%et data is tunneled from the M) to the (() and

vice versa. To achieve a high reliability of pac%et transfer between )() and M)& aspecial 88C is used& which comprises A9K and 5;C mechanisms for #T# /and later

#TM0 services.



'4RS transmission p$ane protoco$ re&erence mode$

• A $a"e "tation "u$"4"tem G'RS protoco& 5=SSG'6 is used to convey routing and Ko)>

related information between the ,)) and )(). ,))(# does not perform error correction and wor%s on top of a frame re&a4 5FR6 networ%.

• 5inally& radio lin% dependent protocols are needed to transfer data over the m interface.

The raio &in) protoco& 5RLC6 provides a reliable lin%& while the MAC controls access

with signaling procedures for the radio channel and the mapping of 88C frames onto the

()M physical channels.

• The raio interface at m needed for (#9) does not require fundamental changes

compared to standard ()M /,rasche& 170& /;T)*& 1:d0. +owever& several new

logical channels and their mapping onto physical resources have been defined. 5or e4ample& one M) can allocate up to eight pac)et ata traffic c3anne&" 5'DTCH"6.

• Capacity can be allocated on demand and shared between circuit>switched channels and

(#9). This allocation can be done dynamically with load supervision or alternatively&

capacity can be pre>allocated. A very important factor for any application wor%ing end>

to>end is that it does not LnoticeH any details from the ()M(#9)>related infrastructure.The application uses& e.g.& TC# on top of *#& *# pac%ets are tunneled to the (()& which

forwards them into the #D.

• All #Ds forward their pac%ets for a (#9) user to the (()& the (() as%s the

current )() for tunnel parameters& and forwards the pac%ets via )() to the M).

Although M)s using (#9) may be considered as part of the internet& one should %now

that operators typically perform an address translation in the (() using AT. All M)s



are assigned private *# addresses which are then translated into global addresses at the

(().

• The advantage of this approach is the inherent protection of M)s from attac%s /the

subscriber typically has to pay for traffic even if it originates from an attac%0 = private

addresses are not routed through the internet so it is not possible to reach an M) from the

internet. This is also a disadvantage if an M) wants to offer a service using a fi4ed&globally visible *# address. This is difficult with *#v- and AT and it will be interesting

to see how *#v6 is used for this purpose /while still protecting the M)s from outside

attac%s as air traffic is e4pensive0.

App$ication

• Communication"% ;>mail& fa4& unified messaging and intranet*nternet access& etc.

• #a&ue>ae "er(ice"% *nformation services and games& etc.

•

E>commerce% 9etail& tic%et purchasing& ban%ing and financial trading& etc.

• Location>$a"e app&ication"% avigation& traffic conditions& airlinerail schedules and

location finder& etc.

• #ertica& app&ication"% 5reight delivery& fleet management and sales>force automation.

• A(erti"in!% Advertising may be location sensitive. 5or e4ample& a user entering a mall

can receive advertisements specific to the stores in that mall.

-. "rite short note on 9outing and Mobility Management?

3. +ow secur ity provided in ()M?

GSM o?ers several security services using con$dential information stored in

the Au' and in the individual S8M (which is #lugged into an arbitrary MS. "he S8M

stores #ersonal, secret data and is #rotected with a &80 against unauthori7ed use.

(;or e=am#le, the secret key @i used for authentication and encry#tion #rocedures is

stored in the S8M. "he security services o?ered by GSM are e=#lained below5

● Access contro$ and authentication5 "he $rst ste# includes the authentication

of a valid user for the S8M. "he user needs a secret &80 to access the S8M. "he ne=t

ste# is the subscriber authentication (see ;igure +.<). "his ste# is based on a

challenge%res#onse scheme as #resented in section +.<.1.<.



● on6dentia$ity5 All user%related data is encry#ted. After authentication, !"S and

MS a##ly encry#tion to voice, data, and signaling. "his con$dentiality e=ists only

between MS and !"S, but it does not e=ist end%to%end or within the whole $=ed

GSM-tele#hone network.

●

Anonymity5 "o #rovide user anonymity, all data is encry#ted before transmission,and user identi$ers (which would reveal an identity are not used over the air.

8nstead, GSM transmits a tem#orary identi$er ("MS8, which is newly assigned by

the B4 after each location u#date. Additionally, the B4 can change the "MS8 at

any time.

"hree algorithms have been s#eci$ed to #rovide security services in GSM.

A$gorithm A7 is used for authentication, A8 for encryption, and A9 forthe generation o& a cipher key. 8n the GSM standard only algorithm A3 was

#ublicly available, whereas A and A were secret, but standardi7ed with o#en

interfaces. !oth A and A are no longer secret, but were #ublished on the internet

in <CC. "his demonstrates that security by obscurity does not really work. As it

turned out, the algorithms are not very strong. Dowever, network #roviders can use

stronger algorithms for authentication – or users can a##ly stronger end%to%end

encry#tion. Algorithms A and A (or their re#lacements are located on the S8M

and in the Au' and can be #ro#rietary. /nly A3 which is im#lemented in the devices

has to be identical for all #roviders.

Authentication

!efore a subscriber can use any service from the GSM network, he or she

must be authenticated. Authentication is based on the S8M, which stores the

indiidua$ authentication key +i, the user identi6cation I#SI, and the

algorithm used for authentication A7. Authentication uses a challenge%res#onse

method5 the access control A' generates a random number RAND as challenge,

and the S8M within the MS answers with SR"S (signed res#onse as res#onse (see;igure +.<+. "he Au' #erforms the basic generation of random values 4A09, signed

res#onses S4ES, and ci#her keys @c for each 8MS8, and then forwards this

information to the DB4. "he current B4 re6uests the a##ro#riate values for 4A09,

S4ES, and @c from the DB4.



;or authentication, the B4 sends the random value 4A09 to the S8M. !oth

sides, network and subscriber module, #erform the same o#eration with 4A09 and

the key @i, called A. "he MS sends back the S4ES generated by the S8M the B4

can now com#are both values. 8f they are the same, the B4 acce#ts the subscriber,

otherwise the subscriber is reFected.

"ncryption

"o ensure #rivacy, all messages containing user%related information are

encry#ted in GSM over the air interface. After authentication, MS and !SS can start

using encry#tion by a##lying the ci#her key @c (the #recise location of security

functions for encry#tion, !"S and-or !S' are vendor de#endent. @c is generated

using the individual key @i and a random value by a##lying the algorithm A. 0otethat the S8M in the MS and the network both calculate the same @c based on the

random value 4A09. "he key @c itself is not transmitted over the air interface.

MS and !"S can now encry#t and decry#t data using the algorithm A3 and

the ci#her key @c. As ;igure +.<3 shows, @c should be a + bit key – which is not



very strong, but is at least a good #rotection against sim#le eavesdro##ing.

Dowever, the #ublication of A and A on the internet showed that in certain

im#lementations <) of the + bits are always set to ), so that the real length of the

key is thus only 3+ conse6uently, the encry#tion is much weaker.



6. ;4plain the advantages& disadvantages and Design goals of "ireless 8A

*IR"-"SS -AN

"he global goal of BA0s is to re#lace o>ce cabling, to enable tether less access to

the internet and to introduce a higher He=ibility for ad%hoc communication in, e.g.,

grou# meetings.

AD.AN!A'"S

● %$e:ibi$ity5 ithin radio coverage, nodes can communicate without further

restriction. 4adio waves can #enetrate walls, senders and receivers can be #laced.

Sometimes wiring is di>cult if $rewalls se#arate buildings. &enetration of a $rewall

is only #ermitted at certain #oints to #revent $re from s#reading too fast.

● 4$anning5 /nly wireless ad%hoc networks allow for communication without

#revious #lanning, any wired network needs wiring #lans. As long as devices follow

the same standard, they can communicate. ;or wired networks, additional cabling

with the right #lugs and #robably inter working units have to be #rovided.

● Design5 ireless networks allow for the design of small, inde#endent devices

which can for e=am#le be #ut into a #ocket. 'ables not only restrict users but also

designers of small &9As, note#ads etc. ireless senders and receivers can be

hidden in historic buildings, i.e., current networking technology can be introduced

without being visible.

● Robustness5 ireless networks can survive disasters, e.g., earth6uakes or users

#ulling a #lug. 8f the wireless devices survive, #eo#le can still communicate.

0etworks re6uiring a wired infrastructure will usually breakdown com#letely.



● ost5 After #roviding wireless access to the infrastructure via an access #oint for

the $rst user, adding additional users to a wireless network will not increase the

cost.

DISAD.AN!A'"S5

● ;ua$ity o& serice5 BA0s ty#ically o?er lower 6uality than their wired counter

#arts. "he main reasons for this are the lower bandwidth due to limitations in radio

transmission (e.g., only <–<) Mbit-s user data rate instead of <))–<,))) Mbit-s,

higher error rates due to interference (e.g., <)–+instead of <)–<* for $ber o#tics,

and higher delay-delay variation due to e=tensive error correction and detection

mechanisms.

● 4roprietary so$utions5 9ue to slow standardi7ation #rocedures, many

com#anies have come u# with #ro#rietary solutions o?ering standardi7ed

functionality #lus many enhanced features. Dowever, these additional features only

work in a homogeneous environment.

● Restrictions5 All wireless #roducts have to com#ly with national regulations.

Several government and non%government institutions worldwide regulate the

o#eration and restrict fre6uencies to minimi7e interference.

'onse6uently, it takes a very long time to establish global solutions. BA0s are

limited to low%#ower senders and certain license%free fre6uency bands, which are

not the same worldwide.

● Sa&ety and security5 :sing radio waves for data transmission might interfere

with other high%tech e6ui#ment in, e.g., hos#itals. S#ecial #recautions have to be

taken to #revent safety ha7ards.

D"SI'N 'OA-S



● '$oba$ operation5 "he infrastructure of wireless A0s, BA0 e6ui#ment may be

carried from one country into another – the o#eration should still be legal in this

case.

● -ow power5 "he BA0 design should take this into account and im#lement s#ecial

#ower%saving modes and #ower management functions.

● -icense<&ree operation5 BA0 o#erators do not want to a##ly for a s#ecial license

to be able to use the #roduct. "he e6ui#ment must o#erate in a license%free band,

such as the *.+ GD7 8SM band.

● Robust transmission techno$ogy5 'om#ared to their wired counter#arts,

BA0s o#erate under di>cult conditions. 8f they use radio transmission, many other

electrical devices can interfere with them. BA0 transceivers cannot be adFustedfor #erfect transmission in a standard o>ce or #roduction environment.

● Simp$i6ed spontaneous cooperation5 "o be useful in #ractice, BA0s should

not re6uire com#licated setu# routines but should o#erate s#ontaneously after

#ower%u#.

● "asy to use5 8n contrast to huge and com#le= wireless A0s, wireless BA0s are

made for sim#le use. "hey should not re6uire com#le= management, but rather

work on a #lug%and%#lay basis.

● 4rotection o& inestment5 A lot of money has already been invested into wired

BA0s. "he new BA0s should #rotect this investment by being intero#erable with

the e=isting networks.

● !ransparency &or app$ications5 E=isting a##lications should continue to run

over BA0s, the only di?erence being higher delay and lower bandwidth.

7. ;4plain the )ystem Architecture of *;;; :@!.11 )TADA9D)

"he 8EEE standard )*.<< (8EEE, <CCC s#eci$es the most famous family of

BA0s in which many #roducts are available. As the standardIs number indicates,



this standard belongs to the grou# of )*.= BA0 standards, e.g., )*. Ethernet or

)*.3 "oken 4ing.

"his means that the standard s#eci$es the #hysical and medium access layer

ada#ted to the s#ecial re6uirements of wireless BA0s, but o?ers the same interface

as the others to higher layers to maintain intero#erability. "he #rimary goal of the

standard was the s#eci$cation of a sim#le and robust BA0 which o?ers time%

bounded and asynchronous services. "he MA' layer should be able to o#erate with

multi#le #hysical layers, each of which e=hibits a di?erent medium sense and

transmission characteristic.

SS!"# AR,I!"!UR"

ireless networks can e=hibit two di?erent basic system architectures are

• *nfrastructure>based

• Ad>hoc.

;igure shows the com#onents of an infrastructure and a wireless #art as

s#eci$ed for 8EEE )*.<<. Several nodes, called stations (S!Ai), are connected to

access points (A4). Stations are terminals with access mechanisms to the wireless

medium and radio contact to the A&.

Stations and the A& which are within the same radio coverage forma basic

serice set (BSSi). "he e=am#le shows two !SSs – !SS< and !SS* – which are

connected via Distribution system1

A distribution system connects several !SSs via the A& to form a single

network and thereby e=tends the wireless coverage area. "his network is now called

an e:tended serice set ("SS) and has its own identi$er, the ESS89. "he ESS89 is

the JnameI of a network and is used to se#arate di?erent networks. ithout

knowing the ESS89 it should not be #ossible to #artici#ate in the BA0.



"he

"he distribution system connects the wireless networks via the A&s with a

porta$, which forms the inter working unit to other BA0s. "he architecture of the

distribution system is not s#eci$ed further in 8EEE)*.<<. 8t could consist of bridged

8EEE BA0s, wireless links, or any other networks.

Dowever, distribution system serices are de$ned in the standard

Stations can select an A& and associate with it. "he A&s su##ort roaming thedistribution system handles data transfer between the di?erent A&s.

A&s #rovide synchroni7ation within a !SS, su##ort #ower management, and

can control medium access to su##ort time%bounded service

8n addition to infrastructure%based networks, 8EEE )*.<< allows the building

of ad%hoc networks between stations, thus forming one or more inde#endent !SSs

(8!SS as shown in ;igure 1.+. 8n this case, an 8!SS com#rises a grou# of stations

using the same radio fre6uency. Stations S"A<, S"A*, and S"A are in 8!SS<,S"A+and S"A3 in 8!SS*. "his means for e=am#le that S"A can communicate directly

with S"A* but not with S"A3.



4RO!OO- AR,I!"!UR"

;igure shows the most common scenario5 an 8EEE )*.<< wireless BA0

connected to a switched 8EEE )*. Ethernet via a bridge.

A##lications should not notice any di?erence a#art from the lower bandwidth

and #erha#s higher access time from the wireless BA0. "he BA0 behaves like a

slow wired BA0. 'onse6uently, the higher layers (a##lication, "'&, 8& look the same

for wireless nodes as for wired nodes.

"he u##er #art of the data link control layer, the logical link control (BB',

covers the di?erences of the medium access control layers needed for the di?erent

media.

"he 8EEE )*.<< standard only covers the #hysical layer 4, and medium

access layer #A like the other )*.= BA0s do.

"he #hysical layer is subdivided into the physica$ $ayer conergence

protoco$ (4-4) and the physica$ medium dependent sub layer 4#D (see

;igure 1.. "he basic tasks of the MA' layer com#rise medium access,

fragmentation of user data, and encry#tion.



"he &B'& sub layer #rovides a carrier sense signal, called clear channel

assessment (''A, and #rovides a common &DK service access #oint (SA&

inde#endent of the transmission technology.

;inally, the &M9 sub layer handles modulation and encoding-decoding of

signals. "he &DK layer (com#rising &M9 and &B'& and the MA' layer will be

e=#lained in more detail in the following sections.

"he #A management su##orts the association and re%association of a

station to an access #oint and roaming between di?erent access #oints. 8t also

controls authentication mechanisms, encry#tion, synchroni7ation of a station with

regard to an access #oint, and #ower management to save battery #ower. MA'

management also maintains the MA' management information base (M8!.

"he main tasks of the 4, management include channel tuning and

&DKM8! maintenance. ;inally, station management interacts with both

management layers and is res#onsible for additional higher layer functions.

ass1mpc with answer

Documents