1. GSM handoffs
 Intra-BSS: if old and new BTSs are
attached to same base station
 MSC is not involved
 Intra-MSC: if old and new BTSs are
attached to different base stations but
within same MSC
 Inter-MSC: if MSCs are changed

2. GSM Intra-MSC handoff
1. Mobile station monitors signal quality and
determines handoff is required, sends signal
measurements to serving BSS
2. Serving BSS sends handoff request to MSC with
ranked list of qualified target BSSs
3. MSC determines that best candidate BSS is under
its control
4. MSC reserves a trunk to target BSS
5. Target BSS selects and reserves radio channels for
new connection, sends Ack to MSC
6. MSC notifies serving BSS to begin handoff,
including new radio channel assignment

3. GSM Intra-MSC handoff
7. Serving BSS forwards new radio channel assignment
to mobile station
8. Mobile station retunes to new radio channel, notifies
target BSS on new channel
9. Target BSS notifies MSC that handoff is detected
10. Target BSS and mobile station exchange messages to
synchronize transmission in proper timeslot
11. MSC switches voice connection to target BSS, which
responds when handoff is complete
12. MSC notifies serving BSS to release old radio traffic
channel

4. GSM Inter-MSC handoff
1. MS sends signal measurements to serving BSS
2. Serving BSS sends handoff request to MSC
3. Serving MSC determines that best candidate BSS
is under control of a target MSC and calls target
MSC
4. Target MSC notifies its VLR to assign a TMSI
5. Target VLR returns TMSI
6. Target MSC reserves a trunk to target BSS
7. Target BSS selects and reserves radio channels
for new connection, sends Ack to target MSC
8. Target MSC notifies serving MSC that it is ready
for handoff

5. GSM Inter-MSC handoff
9. Serving MSC notifies serving BSS to begin handoff,
including new radio channel assignment
10. Serving BSS forwards new radio channel assignment
to mobile station
11. Mobile station retunes to new radio channel, notifies
target BSS on new channel
12. Target BSS notifies target MSC that handoff is
detected
13. Target BSS and mobile station synchronize timeslot
14. Voice connection is switched to target BSS, which
responds when handoff is complete
15. Target MSC notifies serving MSC
16. Old network resources are released

6. Geometric Representation
 Cells are commonly represented by
hexagons.
 Why hexagon?
 How about circle?
 How about square, or triangle?

7. Hexagonal Cells

8. Channel Reuse
 The total number of channels are
divided into K groups.
 K is called reuse factor or cluster size.
 Each cell is assigned one of the groups.
 The same group can be reused by two
different cells provided that they are
sufficiently far apart.

9. Example K=7

10. Channel Reuse

11. Coordinate System
 Use (i,j) to denote a
particular cell.
 Example:
 Cell A is
represented by
(2,1).

12. Distance Formula
D = 3(i + ij + j ) R
2 2
= 3K R
R
where
D
K = i + ij + j
2 2
Reuse factor

13. Air Interface: MS to BTS
 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
 Modulation used
 Gaussian Minimum Shift Keying (GMSK)

14. 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

15. Frequency Reusage
 If a mobile company got the bandwidth of
12MHz with the guardband of 5 KHz and the
seperation band of 10 KHz
 How many channels will be available for
communication in the above scenario ?
 How many channels will be available if we have a
cluster of K = 19 in a BSC of 380 cells?
 How many channels will be available if we reuse
frequency at level one?

16. Outgoing call setup
 User keys in the number and presses send
 Mobile transmits request on uplink signaling channel
 If network can process the call, BS sends a channel
allocation message
 Network proceeds to setup the connection
 Network activity:
 MSC determines current location of target mobile
using HLR, VLR and by communicating with other
MSCs
 Source MSC initiates a call setup message to MSC
covering target area

17. Incoming call setup
 Target MSC initiates a paging message
 BSs forward the paging message on
downlink channel in coverage area
 If mobile is on (monitoring the signaling
channel), it responds to BS
 BS sends a channel allocation message and
informs MSC
 Network activity:
 Network completes the two halves of the
connection

18. LAI
 Location Area Identifier of an LA of a PLMN (A
Public Land Mobile Network is a generic name for all mobile wireless networks
that use land based radio transmitters or base stations.)
 Based on international ISDN numbering plan
 Country Code (CC): 2,3+ decimal digits
 Mobile Network Code (MNC): 2,3 decimal digits

Location Area Code (LAC) : maximum 5 decimal digits,
or maximum twice 8 bits, coded in hexadecimal
 Is broadcast regularly by the BTS on broadcast
channel

19. Cell Identifier (CI)
 Within LA, individual cells are uniquely
identified with Cell Identifier (CI).
 It is maximum 2*8 bits
 LAI + CI = Global Cell Identity

20. 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…

21. Example: Incoming Call Setup
MS ↓ BSS/MSC ------ Paging request (PCH)
MS ↑ BSS/MSC ------ Channel request (RACH)
MS ↓ BSS/MSC ------ Immediate Assignment (AGCH)
MS ↑ BSS/MSC ------ Paging Response (SDCCH)
MS ↓ BSS/MSC ------ Authentication Request (SDCCH)
MS ↑ BSS/MSC ------ Authentication Response (SDCCH)
MS ↓ BSS/MSC ------ Cipher Mode Command (SDCCH)
MS ↑ BSS/MSC ------ Cipher Mode Compl. (SDCCH)
MS ↓ BSS/MSC ------ Setup (SDCCH)
MS ↑ BSS/MSC ------ Call Confirmation (SDCCH)
MS ↓ BSS/MSC ------ Assignment Command (SDCCH)
MS ↑ BSS/MSC ------ Assignment Compl. (FACCH)
MS ↑ BSS/MSC ------ Alert (FACCH)
MS ↑ BSS/MSC ------ Connect (FACCH)
MS ↓ BSS/MSC ------ Connect Acknowledge (FACCH)
MS BSS/MSC ------ Data (TCH)