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Ab Abis interface
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.
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 management procedures, 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.
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 errors and operation errors. – Notifies the management layer entities of the unrecoverable errors. – Flow control.
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 in BSC and BTS.
Abis interface • BSC and BTS do not interpret CM (Connection Management) and MM (Mobility Management) 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.
Example (a) Star configuration (full rate)
(b) Line connection (four BTSs with two TRX each).
Example for Abis interface • Common Star Logic • One Site with 6 TRXs
T S N o . 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
0
1
T 0C 0 T 0C 4 T 1C 0 T 1C 4 T 2C 0 T 2C 4 T 3C 0 T 3C 4 T 4C 0 T 4C 4 T 5C 0 T 5C 4
2
3 S y n c h ro n T 0C 1 T 0C 5 R S L 0 T 1C 1 T 1C 5 R S L 1 T 2C 1 T 2C 5 R S L 2 T 3C 1 T 3C 5 R S L 3 T 4C 1 T 4C 5 R S L 4 T 5C 1 T 5C 5 R S L 5 Id l e
Id l e O M L
4 5 o u s T 0C 2 T 0C 6
6
7
T 0C 3 T 0C 7
T 1C 2 T 1C 6
T 1C 3 T 1C 7
T 2C 2 T 2C 6
T 2C 3 T 2C 7
T 3C 2 T 3C 6
T 4C 3 T 4C 7
T 4C 2 T 4C 6
T 4C 3 T 4C 7
T 5C 2 T 5C 6
T 5C 3 T 5C 7
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.
GSM interfaces
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
A interface
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.
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.
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 data transferred on it cannot be changed 2. signaling link function – Signaling link function (layer 2) regulates the functions and procedures to send 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.
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 network is in normal state or not. – Signaling network functions include:
1. signaling message processing: sends signaling messages to the corresponding 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 signaling unit when the signaling network fails
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 implement management of signaling point state and subsystem state, switchover of active/standby subsystem, broadcasting of status messages and testing of subsystem state.
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.
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 connectionoriented messages.
• The connectionless messages consist of Block/Unblock, Resource, Reset, etc. • The connection-oriented messages include Assignment, Handover, Clear and Cipher messages.
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 service request, identification request, IMSI detach, location update, MM state, TMSI reallocation, etc. – The (CC) call control messages consist of alerting, call proceeding, connection, setup, modification, release, disconnection, notification, state query, DTMF startup messages, etc.
A interface
