1. Wireless communication and
mobile Programming

2. Overview of GSM
 GSM Stands for global system for mobile
Communications
 GSM is based on set of standards formulated in early 1980.
 GSM was developed by ETSI(European
Telecommunications Standards Institute)
 GSM offers data services with maximum bit rate 14.4 kb/s
 GSM uses combination of FDMA ,TDMA

3. FDMA/TDMA

4.  Uplink/Downlink of 25MHz
 890 -915 MHz for Up link
 935 - 960 MHz for Down link
 Combination of frequency division and time
division multiplexing
 FDMA
 124 channels of 200 kHz
 TDMA
 Burst
Air Interface: MS to BTS

5. Number of channels in GSM
 Freq. Carrier: 200 kHz
 TDMA: 8 time slots per freq carrier
 No. of carriers = 25 MHz / 200 kHz = 125
 Max no. of user channels = 125 * 8 = 1000
 Considering guard bands = 124 * 8 = 992 channels

7. Cellular Concept

8. Cellular Concept
 Base stations (BS): implement space division multiplex
 Each BS covers a certain transmission area (cell)
 Each BS is allocated a portion of the total number of
channels available
 Cluster: group of nearby BSs that together use all
available channels
 Mobile stations communicate only via the base station,
using FDMA, TDMA, CDMA…

9. Advantage of FDMA
 Simple algorithmically, and from a hardware standpoint
 Fairly efficient when the number of stations is small and
the traffic is uniformly constant
 No need for network timing
 Capacity increase can be obtained by reducing the
information bit rate and using efficient digital code

10. Advantage of TDMA
 Flexible bit rate
 No frequency guard band required
 No need for precise narrowband filters
 Easy for mobile or base stations to initiate and execute
hands off
 Extended battery life

11. GSM System Hieararchy

12. Architecture of GSM

13. Mobile Station (MS)
 MS consists of following two components
 Mobile Equipment (ME)
 Mobile Subscriber Identity Module (SIM)
 Removable plastic card
 Stores Network Specific Data such as list of
carrier frequencies and current Location Area ID
(LAI).
 Stores International Mobile Subscriber Identity
(IMSI) + ISDN
 Stores Personal Identification Number (PIN) &
Authentication Keys.
 Also stores short messages, charging
information, telephone book etc.

14. Base Transceiver Station (BTS)
 A Base Station Transceiver (BTS) is a radio transceivers
station that communicates with the mobile stations. Its
backend is connected to the BSC
 One per cell
 Consists of high speed transmitter and receiver
 Function of BTS
 Provides two channels
Signalling and Data Channel

15. Base Station Controller (BSC)
 Controls multiple BTS
 A Base Station Controller (BSC) is a high-capacity switch
with radio communication and mobility control
capabilities.
 The functions of a BSC include radio channel
allocation, location update, handover, timing
advance, power control and paging.
 Time and frequency synchronization signals to BTSs
 Time Delay Measurement and notification of an MS to
BTS
 Power Management of BTS and MS

16.  The mobile switching center (MSC) performs the
telephony switching function.
 Switching node of a PLMN (Public Land Mobile
Network)
 Allocation of radio resource (RR)
 Handoff
 Mobility of subscribers
 Location registration of subscriber
 There can be several MSCs in a PLMN
Mobile Switching Centre (MSC)

17.  Connects mobile network to a fixed network
 Entry point to a PLMN
 Usually one per PLMN
 Request routing information from the HLR and routes the
connection to the local MSC
Gateway MSC (GMSC)

18. HLR/VLR
 HLR - Home Location Register
 Contains semi-permanent subscriber information
 For all users registered with the network, HLR keeps user profile
 MSCs exchange information with HLR
 When MS registers with a new GMSC, the HLR sends the user
profile to the new MSC
 VLR - Visitor Location Register
 Contains temporary info about mobile subscribers that are
currently located in the MSC service area but whose HLR are
elsewhere
 Copies relevant information for new users (of this HLR or of
foreign HLR) from the HLR
 VLR is responsible for a group of location areas, typically
associated with an MSC

19. AuC/EIR
 AuC: Authentication Center
 The authentication center (AUC) provides authentication
and encryption parameters that verify the user's identity
and ensure the confidentiality of each call.
 The AUC protects network operators from different types
of fraud found in today's cellular world.
 The GSM has standard encryption and authentication
algorithm which are used to dynamically compute
challenge keys and encryptions keys for a call.

20.  is accessed by HLR to authenticate a user for service
 Contains authentication and encryption keys for
subscribers
 EIR: Equipment Identity Register
 The equipment identity register (EIR) is a database that
contains information about the identity of mobile
equipment that prevents calls from stolen, unauthorized, or
defective mobile stations

21. Registration Process

22. Call Establishment

23. Example: Outgoing call setup
 User keys in the number and presses send
 Mobile transmits Set Up message on uplink signaling channel
(RACH) to the MSC
 MSC requests HLR/VLR to get subscriber parameters
necessary for handling the call.
 VLR/HLR sends Complete Call msg to the MSC
 MSC sends an Assignment message to the BSS and asks it to
assign TCH for the MS
 BSS allocates a radio channel (TCH) and sends an Assignment
message to MS over SDCCH
 MS tunes to the radio channel (TCH) and sends an Assignment
Complete message to the BSS.
 BSS deallocates SDCCH. Now voice path is established
between MS and MSC
 MSC completes the PSTN side of the signaling.

24. Mobility management
 Why mobility and location management is required?
Very nature of mobile communications implies that the
MS is constantly changing its location, a need for tracking
mobile and restructuring existing connection as its moves,
mobility management and location management handle the
operation require for this purpose.

25. Location management
 Location management involve tracking of the location of
the MS as its moves for delivery of voice and data service.
 Basic operation:- Location update, paging, location
information Dissemination
 Location updates are the message sent by the MS
regarding its changing points of access to the fix network
 Updates may have varying frequency
 Types of Location update:-

26.  Location registration:-
Location registration takes place when MS is turned on
 Network send TMSI and LAI to MS store in SIM
 Generic Location update perform when LAI is different
 Chanel request message sent to target MSC/VLR include
TMSI,LAI

27. Generic location update procedure

28.  Periodic:-periodic location update is carried out when
network does not receive any location update request from
MS in specified time
 Issue:-
 Unnecessary cost in terms of both usage of spectrum and
network resource if location updates are too frequent
 Large area to be paged if location updates are few.
 Location update algorithms :- Static and Dynamic
 Static:- In static location updates the topology of cellular
network decides when the location updates need to be
initiated

29.  Each group of cell assign LAI
 BS broad cast LAI periodically
 Problem:-ping-pong effect
 Solution:-distance based, timer based
 Dynamic:- in dynamic location update mobility of users
and call pattern is used in initiating the location update
 Dynamic location update is state based and user profile
based
 State based decision are made based on current state
information

30.  state information include time elapsed, distance
travel, no of LA crossed
 Profile based maintain sequential list of LA

32.  Paging scheme:-Paging is broadcasting message in cell or
group of cell to bring out response from MS
 Blanket paging:-paging the MS in all cell of LA
simultaneously
 Location information Dissemination:- Every MS
associated with home network and home database

33. Handover
 Handoff management required to handle on going
connection when mobile terminal moves from coverage of
one point of access to another
 Maintaining the traffic connection with a moving user
when crossing cell boundaries
 Occurs when the quality or the strength of the radio signal
falls below certain parameters (signal quality reason)
 Occurs when the traffic capacity of a cell has reached its
maximum or is approaching (traffic reason)

34. Relative Signal Strength

35. Relative Signal Strength with Threshold

36.  For a high threshold (e.g., Th1), this scheme performs the
same as the relative signal strength scheme
 If the threshold is set quite low (e.g., Th3), the mobile may
move far into the new cell
 Threshold should not be used alone because its
effectiveness depends on prior knowledge of the crossover
signal strength between the current and the candidate base
stations

37. Relative Signal Strength with Threshold and
Hysteresis

38.  Handover occurs only if the current signal level
drops below a threshold, and
 the target base station is stronger than the current
one by a hysteresis margin H

40. Hard handover

41.  “Break before make”
 Connection is released before making the new connection
 Causes a short cut in the connection
 The terminal is linked to no more than one base station at
any given time
 Primarily used in FDMA and TDMA, where different
frequency ranges are used in adjacent cells

42. Soft handover

43.  “Make before break”
 New connection is established before the old connection is
released, avoiding a cut in the connection during handover
 After the successful handover, the old connection is
released
 Used in CDMA, where adjacent cells use the same
frequency range

44. Control over Handover
 Network-controlled Handover (NCHO)
 Network measures the transmission quality via base
stations and decides when handover should be executed
 Handover process (including data transmission, channel
and network switching) takes 100-200ms
 Mobile-assisted Handover (MAHO)
 Mobile terminal continuously measures signal strength
from serving and neighboring base stations and sends the
recorded values to the serving base station
 On the basis of these values, the network decides when
handover should take place

45.  Handover time between handover decision and execution
is approximately 1 second
 Mobile-controlled Handover (MCHO)
 Mobile terminal is completely in control of the handover
process, i.e., it measures signal strength and decides on
handover
 Very short reaction time (on the order of 0.1 seconds)