mobile station structure

7/31/2019 Mobile Station Structure

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Mobile station structure:

TE Terminal network independent

MT Mobile termination performs all network specific tasks, medium access, coding etc.

GSM-PLMN GSM Public land mobile network

ISDN Integrated services digital network

PSTN public switched telephone network

S interface for data transmission

R interface also defined for some terminals, all of them are defined in ISDN

Salient features of gsm

Roaming

Better transmission quality

Security

Higher capacity

Device independence

Gsm channels

DCS 1800 digital cellular system , 1710-1785MHz uplink, 1805-1880M downlink

GSM 900 890-915 MHZ uplink, 935-960M downlink

PCS 1900 personal communication service 1850-1910MHz uplink,1930-1990MHZ downlink

Some terminologies

GSM global system for mobile communication.

VLR visitor location register is updated as you move from one cell to another , it saves your location

HLR home locaton register which saves the information of our services activated.

EIR equipment identity register saves the imei number

ISDN is the network like PTCL.

AUC register saves the authentication information like our sim validity and balance information.

When a gsm subscriber asks for call.

hlr always saves the informations about the current location of user, the vlr which is responsible for call

setups updates hlr for location changes, and asks hlr for user data needed for call setup.

How to locate a gsm subscriber

to locate the ms, several numbers are needed for example, mobile station international isdn number,

the only important number for user of gsm is phone number, note that it is the sim no associated with

the ms.


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handover may be due to load balancing. the MSC, BSC may decide that the traffic in one cell is too high

so as to shift some MS to another cell.

typical radius for a cell is 35 km in country side and 100m in cities because in cities, the reflections from

the buildings and other objects may fake the signal. For handover decisions, average value of signals is

observed rather than the instantaneous value, a threshold is set to avoid ping pong effect, too high a

threshold may cause call drop and too low a threshold may cause too many handovers.

types of handover

intra cell handover:

within the cell, narrow band interference could make transmission at certain frequencies difficult. the

bsc then decides to shift the carrier at some other frequency.

inter cell intra bsc handover;

the mobile station moves from one cell to another, but stays within the control of same bsc, the bsc

then performs handover, assigns new radio channel in the new cell and leaves the old one.

inter bsc intra msc

as bsc can control only limited no of cells, gsm may have to perform handover between two cells

controlled by different bsc then handover is controlled by msc.

inter msc

a handover may be required between the two cells of different msc, then both the msc perform

handover together.

interleaving means to map the adjacent bits to non adjacent carrier frequencies for mitigation of

frequency fading .

gsm operations:

speech

digitizing and source coding

channel coding

interleaving

burst formatting

ciphering

modulation

radio channel


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encryption:

cipher key is used for enabling the encryption. Once authentication is done, bss and ms communicate ,

the data should be in encrypted form. both the ms and bss calculate the same kc based on the random

value.After generating kc, the encryption of data starts, and this data is transmitted over air.

Data transmission channels in gsm

data transmission channels in gsm:

tch f4.8

tch f9.6

tch f14.2

the low data rates are because of error correction bits . this low data rate is enough for sms and mms

but not enough for internet and multimedia applications.

hscsd high speed circuit switched data

bundling of several time slots for achieving higher aiur air interface user rate 57.5 kbits.s using 4 slots

each of 14.7 kbits/s.

disadvantage

The disadvantage of this topology of data transmission is that gsm requires n times signalling during

handover, call setup and release. Each channel is treated separately. The probability of checking and

service degradation during handover increases.n

mobile station structure

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