GSM signaling

1. TWS Proprietary - www.tws -inc.com - (770) 752-7033
GSM Overview
GSM Overview
] GSM is based on a modular
architecture to allow for inter-
operability between
elements.
] All interfaces are specified to
allow different equipment
vendors.
] Billing standards are defined
to allow for roaming.
] Advanced network
management is provisioned
in the standards.
] Signaling protocols are
integrated and are
internationally standardized.
] The switch signaling is
based on the SS7 standards,
adding two new user parts:
MAP and BSSAP.
] The BSS signaling is based
on an ISDN protocol called
LAPD.

2. TWS Proprietary - www.tws -inc.com - (770) 752-7033
System Architecture
System Architecture
Mobile Station (MS)
Base
Station
Subsystem
(BSS)
Base
Station
Controller
BSC A-bis
Base
Transceiver
Station
(BTS)
Mobile
Switching
Center
(MSC)
Mobile
Switching
Center
(MSC)
Home
Location
Register
(HLR)
Visitor
Location
Register
(VLR)
Authent
Center
(AUC)
Equip. Id
Register
(EIR)
Public Switched
Telephone Network
(PSTN)
Air-interface

3. TWS Proprietary - www.tws -inc.com - (770) 752-7033
FDMA, TDMA and CDMA
FDMA, TDMA and CDMA
FDMA (such as AMPS) uses one
user per frequency or radio channel
GSM combines FDMA with
TDMA, giving 8 users sharing
each frequency channel
With CDMA, all users
share the same radio
channel, separate by
different code sequences

4. TWS Proprietary - www.tws -inc.com - (770) 752-7033
Base Station Timeslots
Base Station Timeslots
Base station radiates one BCCH frequency
and one TCH frequency
BCCH timeslot 0 carries GSM system
information, frequency and timing
BCCH timeslot 1 carries SDCCH control
channel for call set up
BCCH Frequency
Timeslot: 0 1 2 3 4 5 6 7
BCCH SDCCH TCH TCH TCH TCH TCH
TCH
TCH Frequency
Timeslot: 0 1 2 3 4 5 6 7
TCH TCH TCH TCH TCH TCH TCH TCH
Base station
fTCH
fBCCH

5. TWS Proprietary - www.tws -inc.com - (770) 752-7033
Timing Advance
Timing Advance
Base station
Mobile 1 on timeslot 3 Mobile 2 on timeslot 2
The transmission from mobiles further from the base
station will arrive later than those near the BTS. The
mobiles further away are sent a TIMING ADVANCE so
they will transmit earlier and all signals will be received at
the base station in their correct timeslot.
Timeslot 0 1 2 3 4 5 6 7
MS2 MS1

6. TWS Proprietary - www.tws -inc.com - (770) 752-7033
Mobile Timeslots
Mobile Timeslots
The mobile does not transmit and receive at the same time.
This means that it does not need a duplexor so will be smaller and cheaper.
The mobile receives the downlink information from the BTS on its timeslot.
It re-tunes, and waits two timeslots before powering up the transmitter.
The mobile transmits voice on the uplink and powers down.
The mobile then retunes its receiver to measure the signal strength of the neighbors.
Rx Rx
Measure
Tx Tx
f1
f1 = MS receiving frequency
f2 = Adjacent cell frequency
(MS monitoring)
f3 = MS transmitting frequency
(f1 minus 70 MHz)
Measure
f3
f2

7. TWS Proprietary - www.tws -inc.com - (770) 752-7033
Call scenario
Call scenario
Base station has two frequencies:
BCCH and TCH
Mobile 2 wants to set up a call
Mobile 4 is IDLE
Mobile 3 is on a call on TCH
timeslot 0
Mobile 1 has call on
BCCH traffic timeslot 2
Base station

8. TWS Proprietary - www.tws -inc.com - (770) 752-7033
Base Station: timeslot 0
Base Station: timeslot 0
BCCH frequency:
Downlink transmits system
information
Mobile 2 sends access
request on Random Access
Channel on uplink
Idle mobile 4
monitors BCCH
Information on
downlink
Mobile 3 on TCH timeslot 0
sends uplink voice channel
TCH timeslot 0 sends
downlink voice channel to
mobile 3
Mobile 1 on BCCH traffic
timeslot 2 waits
Base station

9. TWS Proprietary - www.tws -inc.com - (770) 752-7033
Base Station: timeslot 1
Base Station: timeslot 1
BCCH frequency:
Downlink: SDCCH call
set up to mobile 2
Mobile 2 sets up call on
SDCCH on uplink
Idle mobile 4 waits
Mobile 3 on traffic channel
timeslot 0 waits
Traffic channel timeslot 1 does
not transmit
Mobile on BCCH traffic
timeslot 2 waits
Base station

10. TWS Proprietary - www.tws -inc.com - (770) 752-7033
Base Station: timeslot 2
Base Station: timeslot 2
BCCH frequency:
Downlink voice on traffic
channel on timeslot 2
Mobile sends voice
on BCCH traffic
timeslot 2 on uplink
Idle mobile 4 waits
Mobile 3 on traffic channel
timeslot 2 waits
Traffic channel
timeslot 2 transmits
voice on downlink to
mobile 2
Mobile 2 transmits voice on
traffic channel timeslot 2
Base station

11. TWS Proprietary - www.tws -inc.com - (770) 752-7033
Base station Timeslots
Base station Timeslots
0 1 2 3 4 5 6 7
BCCH SDCCH Call mobile 1 Idle Idle Idle Idle Idle
BCCH set-up mobile 2 Call mobile 1 Idle Idle Idle Idle Idle
BCCH set-up mobile 2 Call mobile 1 Idle Idle Idle Idle Idle
BCCH set-up mobile 2 Call mobile 1 Idle Idle Idle Idle Idle
BCCH set-up mobile 2 Call mobile 1 Idle Idle Idle Idle Idle
BCCH SDCCH Call mobile 1 Idle Idle Idle Idle Idle
BCCH SDCCH Call mobile 1 Idle Idle Idle Idle Idle
BCCH SDCCH Call mobile 1 Idle Idle Idle Idle Idle
BCCH SDCCH Call mobile 1 Idle Idle Idle Idle Idle
BCCH SDCCH Call mobile 1 Idle Idle Idle Idle Idle
BCCH Frequency
TCH Frequency 0 1 2 3 4 5 6 7
Call mobile 3 Idle Idle Idle Idle Idle Idle Idle
Call mobile 3 Idle Idle Idle Idle Idle Idle Idle
Call mobile 3 Idle Idle Idle Idle Idle Idle Idle
Call mobile 3 Idle Idle Idle Idle Idle Idle Idle
Call mobile 3 Idle Idle Idle Idle Idle Idle Idle
Call mobile 3 Idle Call mobile 2 Idle Idle Idle Idle Idle
Call mobile 3 Idle Call mobile 2 Idle Idle Idle Idle Idle
Call mobile 3 Idle Call mobile 2 Idle Idle Idle Idle Idle
Call mobile 3 Idle Call mobile 2 Idle Idle Idle Idle Idle
Call mobile 3 Idle Call mobile 2 Idle Idle Idle Idle Idle
Time
Time