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1. BTS function
Modulation/De-modulation
Channel Coding/Decoding
Interleaving
Encryption/Ciphering
Frequency Hopping
TRAU Frame Formatting
BCCH management
Signal Strength Measurement for active connections
Idle Channel Measurements on free channels
To identify a cell uniquely across PLMNs, an identity called the Cell Global
Identity (CGI) is defined. CGI is obtained by the concatenation of LAI and the
CI.
Frequency Correction Channel: FCCH
One way channel operating in forward direction and using frequency correction
burst format
Bears information for Frequency Synchronization
142 all 0 bits in this burst causes GMSK modulator to deliver an unmodulated
carrier for the entire duration of the timeslot
Upon detecting this sine wave the MS can adjust its frequency reference
appropriately
Synchronization Channel (SCH)
Uses the synchronization burst format
Raw Data information for SCH is of 89 bits
64 bits are same for each cell and helps them to achieve timing synchronization.
6 bits are for the identification of BTS and mapped on Base Station Identity
Code (BSIC) = NCC (3 bit) + BCC (3 bit)
BSIC avoids ambiguity or interference which can arise when a MS can receive SCH
from two cells using the same BCCH frequency.
Network Colour Code (NCC)
Used to identity the BTS for which measurement is made.
Base-Station Colour Code (BCC)
Each 8 BCC value maps to a different Training Sequence.
Different training sequences allow for a better transmission in case of
interference
19 bits represent the TDMA frame number (reduced frame number)
Broadcast control channel (BCCH)
One way channel operating in the forward direction and using the normal burst
format
BCCH Occur in timeslot 0 of some specific carriers known as BCCH carriers
After locking on to the frequency and frame structure in the cell, MS needs some
more general information broadcast on the BCCH for call setup purposes
Cell Identity (CI)
Network Identity (LAI)
Control Channel structure
BCCH Frequencies of neighboring cells
GPRS Supported or not.
Paging Channel: PCH
One way channel operating in the forward direction and using the normal burst
format
Mobile subscribers are paged this channel for incoming calls or short messages
Every MS in a cell periodically listen to this channel
Uses same coding scheme as used for BCCH
Random Access Channel: RACH
One way channel operating in the reverse direction and using the access burst
format
When MS wants to initiate dialogue with network, this channel is used to send
request to network for a dedicated resource
The actual communication between the MS and the network will takes place later
on the dedicated channel.
If the request is not granted within a specific time period, the MS repeats the
2. request on RACH
Access Grant Channel: AGCH
AGCH is a one way channel operating in forward direction and using the normal
burst format
In response to requests from different MS on RACH, the network allocates a
specific dedicated signaling channel (SDCCH) against each request for further
communication.
The response to the request is sent on AGCH.
Uses same coding scheme as used for BCCH
Stand-alone Dedicated Control Channel: SDCCH
Two way channel using normal burst format
As per the allocation conveyed over the AGCH, both the MS & the BTS switch over
to the assigned SDCCH for a further communication
The Following tasks require the use of SDCCH
Location Updates
Call Setup
SMS
Uses the same coding scheme as used by BCCH
Slow Associated Control Channel: SACCH
Two way channel using normal burst format
SACCH is always associated with TCH or SDCCH
When associated with a TCH, the SACCH occurs in 12 or 25 frame of each 26-frame
multi frame
Each message comprises of 456 bits so 4 multi frames are required to transmit a
message
Since a 26-frame multiframe requires 120ms, a SAACH message over 4 multi-frame
requires 480ms.
Thus, power control that is linked to SAACH exchanges is hindered by the low
rates of SAACH exchange. (But then, SAACH was meant to be slow!)
Used to convey the periodic carrier-signal strength measurements to the network
While an MS is busy on a call over a traffic channel (TCH) or in communication
with MSC on the SDCCH, MS takes periodic carrier-signal strength measurements on
own base station & neighboring base stations.
Based on the analysis of measurements taken by BTS & the MS, the BSC conveys
information on timing advance & MS transmitter power control
Uses the same coding scheme as used by BCCH
Fast Associated Control Channel: FACCH
FACCH is a two way channel using normal burst format
FACCH can be associated with SDCCH or TCH
FACCH works on the principle of stealing
The burst of speech is replaced by FACCH signaling
FACCH is used to convey
Handover information
Uses the same coding scheme as used by BCCH
Full Rate Traffic Channel
This channel carries information at rate of 22.8 Kbps
Half Rate Traffic Channel
This channel carries information at rate of 11.4 Kbps
Enhanced Full Rate Speech
GMSK Adaptive Multi Rate (Half Rate and Full Rate)
Circuit Switched Data (Transparent): 600/1200, 2400, 4800, 9600, 14400.
Circuit Switched Data (Non-transparent): 9600, 14400.
Group 3 Fax: 2400, 4800, 9600, 14400.
CS 1 to 4
MCS 1 to 9
High-Speed Circuit Switched Data (HSCSD) for 9.6/14.4 kbps
Enhanced Circuit Switched Data 28.8/32.0/43.2 Kbps per TS
Generally two configurations are mainly used
Separate SDCCH: FCCH + SCH + BCCH + CCCH
Addresses a channel configuration in which no SDCCH are available on TS 0.
In this case SDCCH sub channels are defined on TS 1
Rest of the TS are used by Traffic channels
Combined SDCCH: FCCH + SCH + BCCH + CCCH + SDCCH/4
3. Addresses a channel configuration in which all control channels are assigned to
TS 0
In this case TS1 is also available for Traffic channels
The downlink direction of TS 0 of the BCCH-TRX is used by various channels.
FCCH
SCH
BCCH
Four SDCCH sub channels (optional);
CCCH
This use is possible because the logical channels can time-share TS 0 in
different TDMA frames of 51 frame Multi frame
Multiplexing of FCCH + SCH + BCCH + CCCH on TS 0 of radio frequency C0 (51 Frame
Multiframe)
Cycle of 51 TDMA frame (0-50), The structure is repeated after IDLE frame
It contains one block of 4 frames for BCCH and 9 Blocks of 4 frames for CCCH
(AGCH/PCH)
Referred to as OMUSIG channel
There is one OMUSIG channel per BTS (typically 16 to 64Kbps).
BSC controls operations of the whole BTS through this channel.
There is O&M SW running on BTS that handles all command on O&M channel.
OMUSIG uses LAPD as link layer protocol.
Typical operations include:
Initialization
Configuration
SW Download
Alarm handling and Fault reporting
Block and Reset
Loop Test and other test operations
Referred to as TRXSIG channel
There is one TRXSIG channel per TRX (typically 16 Kbps).
All mobile signalling (including RR signalling) is carried over TRXSIG channel.
There is RR SW running on each TRX that handles all command on TRX signaling
channel.
TRXSIG uses LAPD as link layer protocol.
Typical operations is explained in subsequent slides.
Apart from the OMUSIG and TRXSIG channels, there are traffic channels for each
Timeslot of a TRX.
Thus, if there are N TRX, then there are 8N traffic channels, each of 16Kbps
link speed.
A host of traffic types can be carried, each with different channel coding. The
following are some of the basic traffic channel types:
Full Rate
Half Rate Speech
The traffic channels are carried over either TRAU frame format or GPRS Layer 1
frame format.
These frame formats define how BTS and BSC exchange stream of information at
layer 1.
For example, while 13Kbps is required for carrying user traffic on Abis, the
TRAU frame is allocated 3Kbps. Thus, there is bandwidth for signaling traffic.
Note: There is no LAPD for traffic channels. LAPD provides reliable delivery and
is not feasible for user traffic.
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At L3, Abis Interface defines a set of sub-procedures (48.058)
At L2, the LAPD protocol is followed based on Q.931(48.056)
At L1, the E1 PCM structure is followed
The TRXSIG Layer 3 Signaling has four basic set of procedures:
Common Channel Management
Dedicated Channel Management
4. Radio Link Management
TRX Management
Refer 3GPP 48.058 for details
Common Channel Management includes procedures for enabling mobile to obtain
dedicated channels (I.e. SDCCH and TCH) for communication. Till such state is
reached, the communication takes place through RACH and AGCH.
The procedures belonging to this category include:
System Information Broadcast (over BCCH)
Paging (over PCH)
Initial Channel Request (via RACH)
Immediate Assignment (over AGCH)
Note: The logical channels shown in brackets do not apply for Abis. It
only indicates for example that Paging message received over Abis is sent over
PCH channel or random access received over RACH is relayed over Abis.
Dedicated Channel Management includes procedures for managing the state of
dedicated channels (I.e. SDCCH and TCH).
The procedures belonging to this category include:
Channel Activation/release
Handover support
Mode modify
Power Control
Measurement Reporting
Ciphering Control
Radio Link Management procedures relay the actions on air interface to BSC over
Abis or receive instructions from BSC to trigger actions over air interface.
The procedures belonging to this category include:
Request/Indication for Link Establishment
Request/Indication for Link Release
Transparent data transfer from MS to BSC
Transparent data transfer from BSC to MS
TRX Management procedures include few other procedures related to measurement
handling.
The procedures belonging to this category include:
Interference on idle channels
Note: Measurement of interference on idle channels is different from
measurements for dedicated channels. The latter form part of dedicated channel
handling procedures.
Some form of flow control
Error control
LAPD frame format is specified by ITU Q.920 and Q.921.
3GPP TS 48.056 adapts this to provide signaling on Abis interface.
LAPD provides reliable data link layer for information transfer.
Option is available to send relatively less important messages in un-
acknowledged mode.
The mobile sees various protocol layers:
Layer 1: GSM Physical layer with BTS (already covered)
Layer 2: LAPDm layer with BTS
Layer 3a: RR layer with BSC
Layer 3b: MM/CC layer with MSC
Based on LAPD protocol, but adapted for mobile environment.
That is why it is referred to as modified LAPD or simply LAPDm.
The modifications from LAPD are as follows:
There is no Frame Check
This functionality is provided by channel coding/interleaving
There is no start and end flag
Frame delineation is done by use of burst
The first set of messages (I.e. SABME) can carry layer 3 data to save air
interface resources.
LAPDm carries ‘maximum‘ message size of 18 to 23bytes.
LAPDm is defined in 3GPP TS 04.05 and 04.06
There are different types of LAPDm format:
A-format: A frame in the A-format generally can be sent on any DCCH in both
5. directions, uplink and downlink. The A-format frame is sent as a fill frame when
no payload is available on an active connection.
B-format: The B-format is used on the Air-interface to transport the actual
signaling data; hence, every DCCH and every ACCH use this format. If the
information to be transmitted requires less space, this space has to be filled
with fill-in octets.
Bbis format: Most simple one is the Bbis in which there is no header/trailer
(just the information field). This is used for BCCH, PCH, and AGCH. For this,
addressing is not necessary, since these are CCCHs, in which addressing is not
required. In contrast to the DCCH, the CCCH transports only point-to-multipoint
messages.
Note: For CCCH, the LAPDm is effectively short-circuited (I.e. no LAPDm).
Note: For traffic channels, there is no LAPDm.
Important LAPDm parameters:
SAPI:
0 for RR, MM and CC
3 for SMS and SS
Um Protocols: L3 protocol
At L3, the protocols residing on Um interface are:
Radio Resource Protocol
Mobility Management Protocol
Call Control
Supplementary Service
Short Message Service
The message type is identified by the Protocol Discriminator.
L3 Um Protocols: Radio Resource Management
The RR protocol is specified in 04.18
The messages are processed in the BSS or even in the MSC.
The RR protocol is required for managing physical and logical channels of GSM
Air interface
Important procedures are:
Channel Request Procedures
System Information Broadcast Procedures
Paging Procedures
Cipher mode Handling
Handover Related Procedures
Measurement Reports
Some of these procedures are explained in Procedure section.
L3 Um Protocols: Mobility Management
The MM protocol is specified in 24.008
The MM messages are processed between MS and MSC/VLR.
The MM protocol is required for managing terminal mobility, temporary identity
management, authentication, etc.
Important procedures are:
IMSI Attach/Location Updated
IMSI Detach
Authentication Request/Response
Identity Request
TMSI Re-allocation
Service Request
L3 Um Protocols: Call Control
The CC protocol is specified in 24.008
The CC messages are processed between MS and MSC/VLR.
The CC protocol is required for managing calls.
Important procedures are:
Setup
Call Proceeding
Alerting
Connect
Release
6. Release Complete
L3 Um Protocols: SS and SMS
The SS and SMS related L3 protocol is specified in 24.010/011
These messages are processed between MS and MSC/VLR.
Important procedures are:
Register/Facility for SS procedure
Similarly, there are procedures for sending/receiving SMS
A Interface Protocol: BSSAP
Used between MSC and BSS
The BSS Application Part (BSSAP) is split into two sub application parts, these
are:
BSS Management Application Part (BSSMAP)
Supports the procedures between MSC and BSS for call handling and resource
management.
For e.g. Paging, Reset, etc.
Direct Transfer Application Part (DTAP).
This includes the MM and CC messages as discussed earlier.
These messages are transparent to the BSS
Defined in 3GPP TS 08.08
A Interface Protocol: BSSmAP
Used between MSC and BSS
Defined in 3GPP TS 08.08
Important procedures include
Assignment
Handover
Reset
Cipher Mode
Others