3-GSM Radio Channel

Slide 1Physical channels transfer bits from one network element to another
Logical Channels
Distinguished by the nature of carried information and the way to assemble bits into data units
Three types
one-to-many: synchronization signals from BTS to MSs in a cell
many-to-one: from MSs to the same BTS
H
a
d
d
i
i

.

MS MS
:
BROADCAST CHANNEL (BCH)
H
a
d
d
i
i
In GSM the logical channels can be divided into two types:
Dedicated Channels
Over the radio path, different type of signaling channels are used to
facilitate the discussions between the mobile station and the BTS, BSC
and MSC/VLR. All these signaling channels are called Dedicated
Control Channels.
Traffic channels are also Dedicated Channels as each channel is
dedicated to only one user to carry speech or data.
Common Channels
Common Channels are those which are used for broadcasting different information to mobile stations and setting up of signaling channels between the MSC/VLR and the mobile station.
GSM Channels
Traffic Channel, Full Rate (FR)
22.8kb/s 13kb/s
Traffic Channel, Half Rate (HR)
11.4kb/s 6.5kb/s
Traffic Channel, Enhanced Full Rate (EFR)
22.8kb/s 13kb/s . 2.4 / 4.8 / 9.6 kbit/s FR .
Traffic Channels (TCH)
Broadcast Channels…
BCH BTS User (One Point To Multi Point) Ts0 .
Broadcast Control Channel (BCCH)
D.L . :
C.I

Frequency correction channel (FCCH)
Cell .
scan FCCH D.L .
BCCH .
Carrier Synchronization Frequency data burst
Synchronization channel (SCH)
D.L
1- (BSIC) BASE STATION IDENTITY CODE
2- TDMA .

H
a
d
d
i
i
Common Control Channel…
Paging channel (PCH)
MS . BTS LA MS
Random access channel (RACH)
MS ( SDCCH ) . BTS . MS BTS ( MS-> BTS )
RACH BCCH .
H
a
d
d
i
i
Access Grant Channel (AGCH)
MS SDCCH TCH . BTS MS .
Cell Broadcast Channel (CBCH)
.
GSM2 MS 1359 .
H
a
d
d
i
i
Dedicated Control Channel…
Uplink/Downlink BTS MS
Stand-alone dedicated control channel (SDCCH)
MS BTS DL/UL .
1- CALL SETUP
2-
4- SMS
H
a
d
d
i
i
Slow Associated Control Channel (SACCH)
SDCCH TCH .
BTS HAND OVER .
POWER CONTROLTIME ALIGNMENT .
:
Down Link : TA
Up Link : .
H
a
d
d
i
i
Fast Associated Control Channel (FACCH)
TCH HAND OVER . .
20 HO (Stealing)
GSM 2 Paging .
H
a
d
d
i
i
H
a
d
d
i
i
‏Time Slots and Bursts
We have already seen that the technique used in air interface is Time Division Multiple Access (TDMA) where one frequency is shared by, at the most , eight users. Consider the example of a 2Mbit/s PCM signal which can carry 30 speech channels with each channel occupying 64Kbit/s. The speech signals from the mobile stations must be placed into a 2Mbit/s signal that connects the BTS and the BSC.
The aim is that each mobile sends its information at a precise time, so that when the information arrives at the Base Transceiver Station, it fits into the allocated time slot in the 2Mbit/s signal. Each Mobile Station must send a burst (a burst occupies one TDMA timeslot) of data at a different time to all the other Mobile Station in the same cell.
H
a
d
d
i
i
Burst
TIME SLOT . :
1-NORMAL BURST
2-ACCESS BURST
NORMAL BURST
RACH ,SCH ,FCCH .
SYNCHRONISATION BURST
BTS .
FREQUNCY CORRECTION BURST
BTC .
ACCESS
DUMMY BURST
BTS .
H
a
d
d
i
i
This burst is used to carry information on TCH and on the control channels
BCCH, PCH, AGCH, SDCCH, ASCCH and FACCH.
6-bits is used as a tail
2*57 bits is used actual information after encryption
2*1 bits : Stealing Flag
26 bits is used for synchronization and problem detection
GP : Guard period
TB
3
.
156.25 Bits Per Slot Duration 577 µs
TB
3
Burst
TDMA BTS(BSIC) .
TB
3
68.25 bits = 0.252 msec which is equivalent to 75.5 km
propagation delay.
and BTS is 37.75 km.
Based on other system parameters, the max cell size is 35 km in GSM.
HO Call set up TA
TB
3
Burst
TB
3
Air Interface
.
T C H 1
T C H 2
T C H 3
T C H 5
T C H 5
T C H 6
T C H 7
T C H 8
T C H 3
T C H 4
T C H 5
T C H 6
T C H 7
T C H 8
T C H 1
T C H 2
T C H 3
T C H 5
T C H 5
T C H 6
T C H 7
T C H 8
T C H 1
T C H 2
T C H 3
T C H 5
T C H 5
T C H 6
T C H 7
T C H 8
TCH are located in all TS's except of TSL 0
TS 16 is usually used for CCS7 signalling (in the Orange Network the CCS7 is configured in TSL 1! )
Downlink and Uplink
I = idle
FACCH is not allocated slots in the multiframe. It steals TCH slots when required.
26 TDMA frames = 120 ms
t
t
t
t
t
t
t
f
t
t
t
t
t
t
t
t
t
t
t
f
t
t
t
t
t
t
s
0
25
i
Channels :
0
25
0
25
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
t
s
i
i
s
New
With the half rate coding it is possible to maximise the spectrum efficiency. When compared with the use of full rate traffic channels, half rate coding almost doubles the amount of radio resources.
Full rate speech and data is coded and transferred by using 16 kbit/s channels in the BSS. With the half rate coding 8 kbit/s transmission can be used on the BSC-BTS interface. This is possible also on the BSC transcoder interface when the BSS (BTS, BSC, Transcoder) and mobile stations support the half rate. The low rate data services can be employed with the half rate traffic channels.
H
a
d
d
i
i
f = FCCH
b = BCCH
r = RACH 1 RACH slots : 1 channel request message of 1 Subscriber
i=idle
s = SCH/SACCH
c = CCCH 1 CCCH block : 1 paging message for 1- 4 user
t = SDCCH/4 in 1 multiframe 4 users
Downlink
f
s
b
b
b
b
c
f
c
f
c
s
c
c
c
c
c
c
c
c
f
c
f
s
t
t
t
t
t
t
t
t
f
f
t
t
t
t
t
t
t
t
f
s
f
s
s
s
s
s
s
s
s
s
1.
2.
3.
4.
i
t
t
t
t
r
r
s
f
s
s
s
s
s
s
r
r
r
r
r
r
r
s
f
r
r
r
r
r
r
r
r
r
r
f
r
r
r
r
t
r
t
t
t
r
f
t
t
t
r
t
r
t
t
t
t
0
50
Uplink
H
a
d
d
i
i
i
r
r
r
r
r
r
r
f
r
r
r
r
r
r
r
r
r
r
r
r
r
r
f
r
r
r
r
r
r
r
r
r
r
f
r
r
r
r
r
r
r
r
r
r
f
r
r
r
r
r
r
r
r
r
Uplink
r
0
50
H
a
d
d
i
i
s
1.
2.
3.
5.
6.
7.
8.
4.
i
i
i
s
s
s
s
s
s
s
f
s
s
s
s
t
t
t
t
t
t
s
f
t
t
t
t
t
t
t
t
t
t
f
t
t
t
t
t
t
t
t
t
t
f
t
t
t
t
s
t
s
s
t
Uplink
s
0
50
i
i
i
H
a
d
d
i
i
f = FCCH
b = BCCH
t = SDCCH/4
i=idle
s = SCH/SACCH
c = CCCH = PCH/AGCH in this examples has a 9 CCCH block
f
s
b
b
b
b
c
f
c
f
c
s
c
c
c
c
c
c
c
c
f
c
f
s
c
c
c
c
c
c
c
c
f
f
c
c
c
c
c
c
c
c
f
s
f
c
c
c
c
c
c
c
c
s
BCCH/CCCH
1
2
3
4
5
6
7
8
9
i
f
s
b
b
b
b
c
f
c
f
c
s
c
c
c
c
c
c
c
c
f
c
f
s
t
t
t
t
t
t
t
t
f
f
t
t
t
t
t
t
t
t
f
s
f
s
s
s
s
s
s
s
s
s
1
2
3
BCCH/SDCCH/4
i
51 TDMA frames = 235 ms
Paging and the immediate assignment processes use the same resources on the signalling channels, CCCHs. There can be different strategies in defining their priorities to escjh other.
PCH has in normal case priority to AGCH.
H
a
d
d
i
i

MS FCCH .
SCCH BTS .
BCCH .
.
/ACCESS FOR REGISTRATION
MS RACH .
SDCCH .
AGCCH MS .
SDCCH .
SACCH .
MS BCCH PCH .
among the control channels
FCCH
the control list.
Camp on BCCH and
Mobile looks for BCCH after switching on
RACH send channel request
SDCCH authenticate response
When a mobile station is first switched on it is necessary to read the BCCH in order to determine its orientation within the network.
The mobile must first synchronize in frequency and then in time. The FCCH, SCH and BCCH are all transmitted on the same carrier frequency which has a higher power density than any of the other channels in a cell because steps are taken to ensure that it is transmitted information at all times. The mobile scans around the available frequencies, picks the strongest and then selects the FCCH. Fc+67.7kHz
H
a
d
d
i
i
/PAGING
PCH .
MS RACH .
AGCH MS .
MS SDCCH .
SACCH .
MS .
/ACCSS FOR CALL
MS RACH .
AGCH SDCCH .
.

RACH send channel request
SDCCH require traffic channel assignment
SDCCH send call establishment request
SDCCH send the setup message and desired number
FACCH switch to traffic channel and send ack (steal bits)
FACCH receive alert signal ringing sound
FACCH acknowledge connect message and use TCH
TCH conversation continues
Receive signaling channel SDCCH on AGCH
Receive alert signal and generate ringing on FACCH
Receive authentication request on SDCCH
Generate Channel Request on RACH
Answer paging message on SDCCH
Authenticate on SDCCH
FACCH acknowledge connect message and switch to TCH
Receive connect message on FACCH
Receive traffic channel assignment on SDCCH
Mobile receives paging message on PCH
FACCH switch to traffic channel and send ack (steal bits)
Channel Allocation
RACH SDCCH
AGCH SDCCH Authentication Call set up
.
TCH , SDCCH .
MS FACCH .

Documents

3-GSM Radio Channel