4.abis & a interfaces

8/10/2019 4.Abis & a Interfaces

1/22

Live on Top


2/22

Abis interface


3/22

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.


4/22

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.


5/22

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.


6/22

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.


7/22

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.


8/22

Example

(a) Star

configuration

(full rate)

(b) Line

connection

(four BTSs

with two TRX

each).


9/22

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


10/22

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.


11/22

GSM interfaces


12/22

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


13/22

A interface


14/22

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.


15/22

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.


16/22

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.


17/22

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


18/22

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.


19/22

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


20/22

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


21/22

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


22/22

A interface

4.abis & a interfaces

Documents