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Live on Top
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Abis interface
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Abis interface
1. Physical layer
Layer 1 of the Abis interface is a physical link which receives data from and
transmits data to the transport layer based on the bottom layer driver of the
hardware. Abis interface physical layer adopts the PCM link with the working rate at 2048
kbit/s to provide 32 channels at 64kbit/s.
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Abis interface
2. Data link layer
The data link layer of Abis uses LAPD protocol. It utilizes the service on
the physical layer, and provides connection-oriented or connectionless
services for layer 3. LAPD addresses TRX through TEI, and uses different logical links for
message transfer:
RSL: Radio Signaling Link used to support traffic managementprocedures, one for each TRX.
OML: Operation & Maintenance Link used to support network
management procedures, one for each SITE. L2ML: L2 management link, for transferring the management
messages at L2.
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Abis interface
Functions of LAPD includes:
Establishes one or several data links on the D channel.
Implements sequence control to keep the order of the frames that pass the
data link connections.
Checks the transmission errors, format errors and operation errors in the
data link connections.
Makes recovery based on the detected transmission errors, format errorsand operation errors.
Notifies the management layer entities of the unrecoverable errors.
Flow control.
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Abis interface
3. The BTSM (BTS Management)
RR (Radio Resource Management) messages are mapped onto the
BSSAP (BSS Application Part) in BSC.
In BTS, most of RR messages are handled as transparent messages.
However, some of them have to be interpreted and executed by BTS (for
example, cipher, random access, paging and assignment), these
messages are processed by the BTSM (BTS Management) entities inBSC and BTS.
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Abis interface
BSC and BTS do not interpret CM (Connection Management) and MM (MobilityManagement) messages.
These messages are transferred over the A-interface by DTAP (Direct Transfer
Application Part). At the Abis interface, DTAP messages are transferred as transparent messages.
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Example
(a) Star
configuration
(full rate)
(b) Line
connection
(four BTSs
with two TRX
each).
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Example for Abis interface
Common Star
Logic
One Site with 6
TRXs
T S N o . 0 1 2 3 4 5 6 7
0 S y n c h r o n o u s
1 T 0 C 0 T 0 C 1 T 0 C 2 T 0 C 32 T 0 C 4 T 0 C 5 T 0 C 6 T 0 C 7
3 R S L 04 T 1 C 0 T 1 C 1 T 1 C 2 T 1 C 3
5 T 1 C 4 T 1 C 5 T 1 C 6 T 1 C 76 R S L 1
7 T 2 C 0 T 2 C 1 T 2 C 2 T 2 C 3
8 T 2 C 4 T 2 C 5 T 2 C 6 T 2 C 79 R S L 2
1 0 T 3 C 0 T 3 C 1 T 3 C 2 T 4 C 3
1 1 T 3 C 4 T 3 C 5 T 3 C 6 T 4 C 71 2 R S L 3
1 3 T 4 C 0 T 4 C 1 T 4 C 2 T 4 C 3
1 4 T 4 C 4 T 4 C 5 T 4 C 6 T 4 C 7
1 5 R S L 4
1 6 T 5 C 0 T 5 C 1 T 5 C 2 T 5 C 31 7 T 5 C 4 T 5 C 5 T 5 C 6 T 5 C 7
1 8 R S L 5
1 9 I d l e
2 0
2 1
2 2
2 3
2 4
2 5
2 6
2 7
2 8
2 9
3 0 I d l e
3 1 O M L
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GSM interfaces
GSM offers standard external interfaces including:
Um interface between MS and BSS,
A interface between BSS and MSC.
The interface protocols and interface procedures strictly follow the ETSI
specifications.
But the interfaces
Abis interface between each BTS and BSC.
between BSS and OMC
Are internal interfaces, and are related to specific equipment from different
manufacturers.
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GSM interfaces
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GSM interfaces
MS: Mobile Station CM: Connection Management
MM: Mobility Management
MTP: Message Transfer Part (MTP) BTSM: Base Transceiver Station Site Management
RR: Radio Resource Management
BSSMAP: Base Station Subsystem Management Application Part
DTAP: Direct Transfer Application Part
SCCP: Signaling Connection Control Part
LAPD: Link Access Procedure on the D channel
LAPDm: Link Access Procedure on the Dm channel
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A interface
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A interface
1. Physical layer
The physical layer of the A-interface is 120-ohm symmetrical twisted pair
or 75-ohm
coaxial cable whose rate is 2 Mbit/s.
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A interface
2. Message Transfer Part (MTP)
The main function of MTP is to ensure reliable signaling message
transfer in the signaling network.
MTP protocols are defined in ITU-T Q.701~710 Recommendations. MTP comprises three functional levels:
1. signaling data link function,
2. signaling link function
3. signaling network function.
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A interface
1. signaling data link function:
Signaling data link (layer 1) is the channel used for signaling transmission
The data rate is 64kbit/s
its important features is that the signaling link is transparent, i.e. the datatransferred on it cannot be changed
2. signaling link function
Signaling link function (layer 2) regulates the functions and procedures tosend the signaling to the data link,
The purpose of layer 2 is to guarantee error-free transmission of
message codes in the case that there exist bit errors on layer 1.
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A interface
3. signaling network function.
By controlling the route and performance of the signaling network,
signaling network function (level 3) guarantees that signaling information
can be reliably transferred to the user part, whether the signaling networkis in normal state or not.
Signaling network functions include:
1. signaling message processing: sends signaling messages to thecorresponding signaling links or user parts.
2. signaling network management: re-construct the signaling network
and to keep and recover the normal transfer ability of the signalingunit when the signaling network fails
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A interface
3. Signaling Connection & Control Part (SCCP)
Network layer provides connectionless services and connection-oriented
services.
SCCP has routing and network management functions. The routing function of SCCP: is to perform addressing as per the
address information such as DPC, SSN,
The network management function of SCCP: is to implementmanagement of signaling point state and subsystem state, switchover of
active/standby subsystem, broadcasting of status messages and testing
of subsystem state.
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A interface
BSSAP
It represents the GSM-specific user signaling on the A interface.
It is two parts, the DTAP and the BSSMAP The BSSMAP includes all messages exchanged between the BSC and
the MSC that are actually processed by the BSC. Examples are
PAGING, HND_CMD, and the RESET message.
The DTAP comprises all messages exchanged between a subsystem of
the NSS and the MS. The messages are transparent for the BSS.
Ai t f
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A interface
1. BSSMAP
the BSSMAP comprises all messages exchanged as RR messages
between the MSC and the BSC, as well as messages used for control
tasks between the BSC and the MSC. BSSMAP messages can be divided into connectionless and connection-
oriented messages.
The connectionless messages consist of Block/Unblock,Resource, Reset, etc.
The connection-oriented messages include Assignment,
Handover, Clear and Cipher messages.
Ai t f
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A interface
2. DTAP messages The DTAP messages can be divided into Mobile Management (MM) messages
and Call Control (CC) messages.
The MM messages consist of messages related to authentication, CM servicerequest, identification request, IMSI detach, location update, MM state, TMSI re-
allocation, etc.
The (CC) call control messages consist of alerting, call proceeding, connection,
setup, modification, release, disconnection, notification, state query, DTMFstartup messages, etc.
Ai t f
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A interface