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

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 BSS5. 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

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

GSM Inter-MSC handoff

1. MS sends signal measurements to serving BSS

2. Serving BSS sends handoff request to MSC3. 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 TMSI5. Target VLR returns TMSI6. Target MSC reserves a trunk to target BSS7. 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

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 timeslot14. Voice connection is switched to target BSS, which

responds when handoff is complete15. Target MSC notifies serving MSC16. Old network resources are released

Geometric Representation Cells are commonly represented by

hexagons.

Why hexagon? How about circle? How about square, or triangle?

Hexagonal Cells

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.

Example K=7

Channel Reuse

Coordinate System Use (i,j) to

denote a particular cell.

Example: Cell A is

represented by (2,1).

Distance Formula

RK

RjijiD

3

)(3 22

22 jijiK where

Reuse factor

R D

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)

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

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?

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

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

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

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

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…

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)