1. Global System for Mobile (GSM)
Chapter 11: Wireless Systems and Standards

2. GSM System Architecture
Fig. 2.3

3. Interfaces in GSM

4. GSM adopts TDMA/FDMA mode channel width: 200KHz
time
Concept:
channel is composed of a series of timeslots of periodicity. Different signal energies are distributed into different timeslots. The adjacent channel interference is restricted by connection choosing from time to time. So the useful signal is passed only in the specified timeslot.
User 3
User 2
User 1
Frequency
Adopts 采用
Timeslots 时隙
Concept 概念
GSM adopts TDMA/FDMA mode
channel width: 200KHz
each channel has 8 timeslots 4

5. GSM Timeslot and Frame structure
Frequency
200KHz
Interval 间隔
time BP
15/26ms
interval

6. Frequency Resource EGSM900 : up: 880~890MHz down: 925~935MHz
duplex interval: 45MHz
bandwidth: 25MHz，
frequency interval: 200KHz
EGSM900 :
up: 880~890MHz
down: 925~935MHz
duplex interval: 45MHz
bandwidth: 10MHz，
frequency interval: 200KHz
GSM1800 :
up: MHz down: MHz
duplex interval: 95MHz， working bandwidth: 75MHz，
frequency interval: 200KHz
GSM1900MHz:
up:1850~1910MHz
down:1930~1990MHz
duplex interval: 80MHz， working bandwidth: 60MHz，
frequency interval: 200KHz 6

7. Propagation Characteristic
Frequency Resource
Single Band Network
General Priority
Which one?
900MHz
High
1800MHz
Low
1900MHz
New Operator
First, then , then
Why?
Reason
Propagation Characteristic
For Operator
For Subscriber

8. Frequency Resource Single Band Network Single Band 900MHz 1800MHz
Dual Band
1900MHz
Triple Band
Coexist three kind of handset
The first kind support only
GSM1900 provided by motorola
In a sense, the network determines the handsets can be selected.
But nowadays, most handsets support dual band.

9. Propagation characteristic
Frequency Resource
Single Band Network
Cell coverage radius :
We know
Propagation characteristic
The higher the propagation frequency
The higher the propagation loss
The smaller the cell coverage radius.
900MHz
1800MHz
1900MHz 9

10. Timeslot and Frame structure
1 super high frame = 2048 super frame = TDMA frame
2045
2046
2047 3 2 1
2044 49 47 48 50 24 25
BCCH
1 super frame = 1326 TDMA frame（6.12s）
CCCH
SACCH/TCH
SDCH
FACCH
1 multiplex frame=26TDMA frames（120ms）
1 multiplex frame = 51 TDMA frame 1 24 25 1 49 50
1 TDMA frame=8 timeslot（120/26=4.615ms） 1 2 3 4 5 6 7

11. Physical Channel The physical channel adopts FDMA and TDMA techs.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
The physical channel adopts FDMA and TDMA techs.
On the time domain, a specified channel occupies the same timeslots in each TDMA frame, so it can be identified by the timeslot number and frame number. 11

12. Numbering Arrangement
International Mobile Subscriber Identification number (IMSI)
It identifies a unique international universal number of a mobile subscriber, which consists of MCC+MNC+MSIN.
1) MCC: country code, 460
2）MNC: network code, 00 or 01
3）MSIN: subscriber identification, H1H2H3H4 9XXXXXX,
H1H2H3H4: subscriber registering place
H1H2: assigned by the P&T Administrative Bureau (operator )to different provinces, to each province
H3H4: assigned by each province/city
the IMSI of user will be written into the SIM card by specific device and software and be stored into the HLR with other user information. 12

13. Mobile Subscriber ISDN Number（MSISDN）
Numbering Arrangement
Mobile Subscriber ISDN Number（MSISDN）
It is the subscriber number commonly used. China uses the TDMA independent numbering plan:
CC+NDC+ H1H2H3H4 +ABC
CC: country code, 86
NDC: network code, 135—139, 130
H1H2H3H4: HLR identification code ABCD: mobile subscriber number inside each HLR 13

14. Numbering Arrangement
International Mobile Equipment Identification code
(IMEI)
It will uniquely identify a mobile station. It is a decimal number of 15 digits. Its structure is:
TAC+FAC+SNR+SP
TAC=model ratification code, 6 digits
FAC=factory assembling code, 2 digits
SNR=sequence code, 6 digits
SP=reserved, 1 digit 14

15. Mobile Subscriber Roaming Number (MSRN)
Numbering Arrangement
Mobile Subscriber Roaming Number (MSRN)
The MSRN is temporarily distributed to the subscriber by the VLR according to the request by the HLR when this subscriber is called. The MSRN is released and can be assigned to other subscriber later.
　CC + NDC M1M2M3 + ABC
　CC: country code, 86
　NDC: mobile network code, 135—139, 130
　M1M2: same as the H2H3 of MSISDN
　ABC: 15

16. Numbering Arrangement
Temporarily Mobile Subscriber Identification Number
(TMSI)
To insure the IMSI security, the VLR will assign a unique TMSI number for the accessed subscriber. It is used locally only and is a 4-byte TMSI number BCD code. 16

17. ARFCN Absolute Radio Frequency Channel Number
MHz band for subscriber-to-base transmission(reverse link)
MHz band for base-to-subscriber transmission(forward link)
ARFCN denotes a forward and reverse channel pair which is separated in frequency by 45 MHz and each channel is time shared between as many as eight subscribers using TDMA

18. GSM uses paired radio channels
UPLINK
DOWNLINK
890MHz
915MHz
935MHz
960MHz
124
124

19. GSM - TDMA/FDMA higher GSM frame structures GSM TDMA frame 1 2 3 4 5 6
MHz
124 channels (200 kHz)
downlink
frequency
MHz
124 channels (200 kHz)
uplink
higher GSM frame structures
time
GSM TDMA frame 1 2 3 4 5 6 7 8
4.615 ms
Because of natural and man-made electromagnetic interference, the encoded speech or data signal transmitted over the radio interface must be protected from errors. GSM uses convolutional encoding and
block interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below.
Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality than
others. The bits are thus divided into three classes:
Class Ia 50 bits - most sensitive to bit errors
Class Ib 132 bits - moderately sensitive to bit errors
Class II 78 bits - least sensitive to bit errors
Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightly
attenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of
constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remaining
Class II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.
To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks
are transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.
Recall that each time-slot burst is transmitted at a gross bit rate of kbps. This digital signal is modulated onto the analog carrier frequency using Gaussian-filtered Minimum Shift Keying (GMSK).
GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter is related to
power consumption, which should be minimized for the mobile station.
GSM time-slot (normal burst)
guard
space
guard
space
tail
user data
Training S
3 bits
57 bits
26 bits 1 3
546.5 µs
577 µs 19

20. GSM Air Interface Specification
Fig. 2.8

21. LOGICAL CHANNELS TRAFFIC SIGNALLING FULL RATE Bm 22.8 Kb/S HALF RATE
Lm Kb/S
BROADCAST
COMMON CONTROL
DEDICATED CONTROL
FCCH
SCH
BCCH
RACH
PCH
AGCH
FCCH -- FREQUENCY CORRECTION CHANNEL
SCH SYNCHRONISATION CHANNEL
BCCH -- BROADCAST CONTROL CHANNEL
PCH PAGING CHANNEL
RACH -- RANDOM ACCESS CHANNEL
AGCH -- ACCESS GRANTED CHANNEL
SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL
SACCH -- SLOW ASSOCIATED CONTROL CHANNEL
FACCH -- FAST ASSOCIATED CONTROL CHANNEL
SDCCH
SACCH
FACCH
DOWN LINK ONLY
BOTH UP &
DOWNLINKS
UPLINK ONLY

22. Channel Type-Summary 14.4Kbit/s FR TCH (TCH/F14.4)
Data CH
4.8Kbit/s FR TCH (TCH/F4.8)
TCH
4.8Kbit/s HR TCH (TCH/H4.8)
FR Voice Traffic Channel (TCH/FS)
Voice CH
Enhanced FR Traffic Channel (TCH/EFR)
HR Traffic Channel (TCH/HS)
channel
FCCH (down)
SCH (down)
BCH
BCCH (down)
GSM系统中，信道分成逻辑信道和物理信道。时隙是基本的物理信道，一个载频包含8个物理信道。物理信道支撑着逻辑信道。逻辑信道按其功能分为业务信道（TCH）和控制信道（CCH）。
业务信道携载编码语音或用户数据，它有全速率业务信道（TCH/F）和半速率业务信道（TCH/H）之分：
1． 话音业务信道 TCH/F：全速率话音业务信道，总速率为22.8kb/s TCH/H：半速率话音业务信道，总速率为11.4kb/s
2． 数据业务信道 TCH/F9.6：9.6kb/s全速率数据业务信道 TCH/F4.8：4.8kb/s全速率数据业务信道 TCH/H4.8：4.8kb/s半速率数据业务信道 TCH/F2.4：≤2.4kb/s全速率数据业务信道 TCH/H2.4：≤2.4kb/s半速率数据业务信道
控制信道用于携载信令或同步数据，包括三类控制信道：广播信道、公共控制信道和专用控制信道。
1． 广播信道BCH 广播信道是一点对多点的单向下行控制信道，即基站到移动台单向传输，用于向MS广播各类消息，分为三种信道：
（1） FCCH：频率校准信道，该信道携载有用于MS频率纠正的信息。
（2） SCH：同步信道，携载MS帧同步和基站收发信台（BTS）识别信息。
（3） BCCH：广播控制信道，用于发送小区信息。在每个基站收发信台中总有一个收发信机含有这个信道，以向该小区中所有移动台广播系统消息。
2． 公共控制信道CCCH
公共控制信道是一点对多点的双向控制信道，为网络中MS所共用，它包括三种信道：
（1） PCH：寻呼信道，用于BTS寻呼MS（下行信道）。
（2） RACH：随机接入信道，用于MS随机提出入网申请，即请求分配专用控制信道（上行信道）。
（3） AGCH：准予接入信道，用于BTS对MS的随机接入请求作出应答，即分配一专用控制信道或直接分配一个TCH（下行信道）。
专用控制信道
专用控制信道是点对点的双向控制信道，使用时BTS将其分配给MS，进行BTS与MS之间点对点的传输。
（1） SDCCH：独立专用控制信道，用于传送信道分配等信息。SDCCH可分为以下几种：
 SDCCH/8：独立专用控制信道
 SDCCH/4：与BCCH/CCCH相组合的独立专用控制信道
（2） SACCH：慢速随路控制信道，与一条业务信道或一条SDCCH联合使用，用来传送用户信息期间某些特定信息，例如：功率和帧调整控制信息、测量数据等。该信道可分为以下几种：
 SACCH/TF：与TCH/F随路的慢速随路控制信道
 SACCH/TH：与TCH/H随路的慢速随路控制信道
 SACCH/C4：与SDCCH/4随路的慢速随路控制信道
 SACCH/C8：与SDCCH/8随路的慢速随路控制信道
（3） FACCH：快速随路控制信道，与一条业务信道联合使用，携带与SDCCH同样的信号，但只在没有分配SDCCH的情况下才分配FACCH，通过业务信道借取的帧（称为“偷帧”）来实现接续，传送如“越区切换”等指令。FACCH可分为以下几种：
FACCH/F：全速率快速随路控制信道
FACCH/H：半速率快速随路控制信道
通常TCH/F与SACCH总是成对分配的，TCH/F和SACCH的组合用TACH/F来表示。
RACH (up)
CCH
CCCH
AGCH (down)
PCH (down)
SDCCH (both)
DCCH
FACCH (both)
SACCH (both)

23. Traffic and Control channels
Traffic channels carry digitally encoded user speech or user data and have identical functions and formats on both the forward and reverse link.
Control channels carry signalling and synchronizing commands between the base station and mobile station.
Certain types of Control channels are defined just for forward or reverse.

24. Speech dedicated control channel frame and multiframe
Fig. 2.11

25. GSM Traffic Channels - TCH
GSM Traffic Channels may be either full-rate or half-rate and may carry either digitized speech or user data.
When transmitted as full-rate, user data is contained within one TS per frame.
When transmitted as half-rate, user data is mapped onto the same time slot, but is sent in alternate frames. That is two half-rate channel users would share the same time slot, but would alternately transmit during every other frame.

26. GSM TCH Continue..
In the GSM standard, TCH data may not be sent in TS 0 within a TDMA frame on certain ARFCNs which serve as the broadcast station for each cell.
TCH data are broken up every thirteenth frame by either slow associated control channel (SACCH) data, or the idle frames.
The 26th frame contains idle bit for the case when full-rate TCHs are used, and contains SACCH data when half-rate TCHs are used.
Full-TCH 22.8kbps, Half-TCH 11.4kbps.

27. The Control Channel Multiframe

28. Broadcast Channels - BCHs
Broadcast channel operates on the forward link of a specific ARFCN within each cell, and transmits data only in the first time slot (TS 0) of certain GSM frames.
The other seven timeslots are available for TCH and DCCH data, or are filled by dummy bursts. Further more, all eight timeslots on all other ARFCNs within the cell are available for TCH and DCCH data.
BCH provides synchronization for all mobiles within the cell and is occasionally monitored by mobiles in neighboring cells so that received power and MAHO decisions may be made by out-of-cell users.

29. Broadcast Control Channel - BCCH
Broadcast control channel (BCCH) is a base to mobile channel which provides general information about the network, the cell in which the mobile is currently located and the adjacent cells
BCCH also broadcast a list of channels that are currently in use within the cell.
Frame 2 to 5 in a control multiframe (4 out of every 51 frames) contain BCCH data.
Frame TS 0 may contains BCCH for specific frames, while for other specific frames TS 0 may contains FCCH or CCCHs.

30. Frequency correction channel (FCCH)
FCCH is a special data burst which occupies TS 0 for the very first GSM frame (frame 0) and is repeated every ten frames within a control channel multi-frame.
FCCH allows each subscriber unit to synchronize its internal frequency standard (local oscillator) to the exact frequency of the base station.

31. Synchronization channel (SCH)
SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is used to identify the serving base station while allowing each mobile to frame synchronize with the base station.
The frame number (FN), which ranges from 0 to 2,715,647, is sent with base station identity code (BSIC) during the SCH burst.
The SCH is transmitted once every ten frames within the control channel multiframe.

32. Common Control Channels - CCHs
On the broadcast (BCH) ARFCN, the CCH occupy TS 0 of every GSM frame that is not otherwise used by the BCH or the Idle frame.
CCCH are the most commonly used control channels and are used to page specific subscriber, assign signaling channels to specific users, and receive mobile request for service.
Three types named: PCH, RACH and AGCH

33. Paging channel (PCH)
PCH provides paging signals from the base station to all mobiles in the cell, and notifies a specific mobile of an incoming call which originates from the PSTN.
PCH transmits the IMSI of the target subscriber, along with a request for acknowledgment from mobile unit on the RACH.
PCH may be used to provide cell broadcast ASCII text messages to all subscribers.

34. Random access channel (RACH)
RACH is a reverse link channel used by a subscriber unit to acknowledge a page from the PCH, and is also used by mobiles to originate a call.
All mobiles must request access or respond to PCH alert within TS 0 of a GSM frame.
In establishing service, the GSM base station must respond to the RACH transmission by allocating a channel and assigning a stand-alone dedicated control channel (SDCCH) for signaling during a call.
This connection is confirm by BS over the AGCH.

35. Access grant channel (AGCH)
AGCH is used by the base station to provide forward link communication to the mobile, and carries data which instructs the mobile to operate in a particular physical (time slot and ARFCN) with a particular dedicated control channel.
AGCH is used by the base station to respond to a RACH sent by a mobile station in a previous CCCH frame.

36. Dedicated Control Channels - DCCHs
Stand-alone dedicated control channel (SDCCH) is a bi-directional channel allocated to a specific mobile for exchange of location update information and call set up information.
SDCCH ensures that the MS and the BS remain connected while BS and MSC verify the subscriber unit and allocate resources to the mobile.

37. Slow associated control channel (SACCH)
SACCH is a bi-directional channel used for exchanging control information between base and a mobile during the progress of a call set up procedure. The SACCH is associated with a particular traffic channel or stand alone dedicated control channel.
On the forward link, the SACCH is used to send slow but regular changing control information to the mobile, such as transmit power level instructions and specific timing advance instructions for each user on the ARFCN.
Reserve SACCH link carries information about the received signal strength and quality of TCH.

38. Fast associated control channel (FACCH)
Fast associated control channel (FACCH) is a bi-directional channel which is used for exchange of time critical information between mobile and base station during the progress of a call. The FACCH transmits control information by stealing capacity from the associated TCH.
The stealing is done by setting two special bits, called stealing bits, in a TCH forward channel burst. If the stealing bits are set, the time slot is known to contain FACCH data, not a TCH, for that frame.

39. Location update from the mobile
Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH request for location updating
SDCCH authenticate
SDCCH authenticate response
SDCCH switch to cipher mode
SDCCH cipher mode acknowledge
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
SDCCH allocate TMSI
SDCCH acknowledge new TMSI
SDCCH switch idle update mode 39

40. Call establishment from a mobile
Mobile looks for BCCH after switching on
RACH send channel request
AGCH receive SDCCH
SDCCH send call establishment request
SDCCH do the authentication and TMSI allocation
SDCCH send the setup message and desired number
SDCCH require traffic channel assignment
FACCH switch to traffic channel and send ack (steal bits)
FACCH receive alert signal ringing sound
FACCH receive connect message
FACCH acknowledge connect message and use TCH
TCH conversation continues

41. Call establishment to a mobile
Mobile looks for BCCH after switching on
Mobile receives paging message on PCH
Generate Channel Request on RACH
Receive signaling channel SDCCH on AGCH
Answer paging message on SDCCH
Receive authentication request on SDCCH
Authenticate on SDCCH
Receive setup message on SDCCH
Receive traffic channel assignment on SDCCH
FACCH switch to traffic channel and send ack (steal bits)
Receive alert signal and generate ringing on FACCH
Receive connect message on FACCH
FACCH acknowledge connect message and switch to TCH 41

42. Thank you