GMS/DCS BSS

BSS Signalling

Training Document

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No. of pages

47/TT

Edited by/Translator

3-Sep-96Miia Sjögren

Author

3-Sep-96Pasi Mehtonen

Approved by

3-Sep-96Philippe Esterman

Previous issue(x) approved

TABLE OF CONTENTS

1. Signalling in BSS..........................................................................................

2. OSI Model.....................................................................................................2.1. PEER ENTITIES....................................................................................

3. SIGNALLING LAYERS IN BSS.................................................................3.1. SIGNALLING LAYERS IN THE BSS..................................................3.2. LAYER 1 (PHYSICAL LAYER)...........................................................

3.3. LAYER 2 (LINK LAYER).....................................................................3.4. LAYER 3 (NETWORK LAYER)...........................................................

4. FUNCTIONS OF THE LAYERS IN THE BSS..........................................4.1. BTS MANAGEMENT LAYER, BTSM.................................................4.2. RADIO RESOURCES LAYER, RR.......................................................4.3. BSS APPLICATION LAYER, BSSAP..................................................4.4. MOBILITY MANAGEMENT LAYER, MM.........................................4.5. CALL CONTROL, CC...........................................................................

5. LAPD PROTOCOL.....................................................................................

LAPD PROTOCOL

2. Signalling in BSS

Signalling channels used in the BSS are:

Common Channel Signalling no.7, CCS7, between the MSC and the BSC

Link Access protocol on D-channel, LapD, between the BSC and the BTS. There are two types of signalling links used:

TRX Signalling between the BSC and the Transceiver, TRXSIG

OMU Signalling between the BSC and the OMU unit in the BTS, OMUSIG.

LAPD PROTOCOL

Link Access Protocol on D-channel modified, LapDm, between the BTS and the MS.

LAPD PROTOCOL

LAPD PROTOCOL

Fig. 1.1 Signalling in the BSS

LAPD PROTOCOL

3. OSI Model

GSM has adopted the lower three layers of the seven layer OSI model for data transfer. Use of this model allows the creation of an open system which is accessibile to other systems using a build according to the same model.

Before looking at the GSM implementation of this model, a short introdution to message passing is necessary.

LAPD PROTOCOL

LAPD PROTOCOL

Fig. 2.1 Seven layers in OSI Model

3.1. PEER ENTITIES

Entities of the same layer in different systems, peer entities, communicate with each other using Protocol Data Units (PDU) which consists of Protocol Control Information (PCI) and user information.

LAPD PROTOCOL

PDUs are transferred through the Service Access point (SAP) to the lower layer where they become known as Service Data Units (SDU). The lower layer adds its own PCI to this, to create the PDU for that layer. At the receiving end, each layer strips off the PCI to create an SDU which it passes to the layer above.

LAPD PROTOCOL

4. SIGNALLING LAYERS IN BSS

4.1. SIGNALLING LAYERS IN THE BSS

GSM has adopted only the lower three layers of the OSI seven layer model. The layers are Layer 1-Physical layer, Layer 2-Link layer and Layer 3-Network layer. The characteristics and GSM implementation of these is described below.

LAPD PROTOCOL

LAPD PROTOCOL

Fig. 1.1 Signalling layers in GSM / DCS 1800

BSSAP BSS Application PartBSSMAP BSS Management Application PartDTAP Direct Transfer Application PartMM Mobility ManagemertCM Connection ManagementCC Call ControlSS Supplementary ServicesSMS Short Message Services

LAPD PROTOCOL

SCCP Signalling Connection Control PartMTP Message Transfer PartRR Radio ResoursesBTSM BTS ManagementLapD Link Access Protocol on the D channelLapDm Link Access Protocol on the D channel modified

LAPD PROTOCOL

4.2. LAYER 1 (PHYSICAL LAYER)The physical layer is responsible for the error free transfer of physical data units. This layer defines the electrical and mechanical characteristics of the transmission path.

In Nokia GSM and DCS 1800, the BTS-MS physical layer is the radio path, and the BTS-BSC physical layer is a 2048kbps CEPT data stream on unbalanced 75W coaxial cable or 120W twisted copper pairs.

LAPD PROTOCOL

4.3. LAYER 2 (LINK LAYER)The function of the link layer is to transfer frames, error free, between two stations using the bit transfer connection created by the physical layer. The tasks of this layer include opening, maintaining, and closing a connection between two stations.

In GSM, the data link layer is implemented with the standard LapD (Link Access Protocol on D channel) protocol on the BTS-BSC link, and with a GSM specific LapDm protocol on the BTS-MS (radio interface) link.

LAPD PROTOCOL

4.4. LAYER 3 (NETWORK LAYER)The network layer isolates the higher levels from routing and connection tasks. It is responsible for getting the message to its destination through an arbitrary network topology.

In GSM, this layer is hierarchically divided into three sublayers; CM (Connection Management), MM (Mobility Management), and RR (Radio Resource) layers.

The Radio Resource layer, RR, produces services for the Mobility Management layer. The functions of this layer include establishing,

LAPD PROTOCOL

maintaining, and releasing physical connections (i.e. RF traffic and control channels). The main part of this function is located in the BSC, although some of the functions are performed by the BTS.

The Mobility Management layer, MM, controls the registration of subscriber equipment in the network and identification of users. This function is located in the MSC.

The Connection Management layer, CM is the highest layer in the GSM implementation of the OSI model. This is indicated by its location (top of the protocol stack in both MSC and MS). This layer is basically the interface to the user in the GSM system.

LAPD PROTOCOL

The connection management layer is subdivided into three segments; Call Control segment, CC, which is responsible for establishing, maintaining and releasing calls, Suplementary Services segment, SS, and Short Message Service segment, SMS.

LAPD PROTOCOL

5. FUNCTIONS OF THE LAYERS IN THE BSS

5.1. BTS MANAGEMENT LAYER, BTSM

The BTSM (Base Transceiver Station Management) layer is responsible for controlling the BTS. Messages from the Radio Resources (RR) layer are sent transparently to the MS in a BTSM Data Request Message, and in the reverse direction, the L3 information is sent in a BTSM Data Indication message.

LAPD PROTOCOL

There are a total of 41 valid messages associated with the BTSM layer. They are listed below in numerical order according to the hexadecimal value of the message type identifier:

01 Data Request 21 Channel Activation02 Data Indication 22 Channel Activation ACK03 Error Indication 23 Channel Activation NACK04 Establish Request 24 Connection Fail05 Establish Confirmation 25 Deactivate SACCH06 Establish Indication 26 Encryption Command07 Release Request 27 Handover Detect08 Release Confirmation 28 Measurement Result

LAPD PROTOCOL

09 Release Indication 29 Mode Modify Request0a Unit Data Request 2a Mode Modify ACK0b Unit Data Indication 2b Mode Modify NACK11 BCCH Information 2c Physical Context Request12 CCCH Load Indication 2d Physical Context Confirmation13 Channel Required 2e RF Channel Release14 Delete Indication 2f MS Power Control15 Paging Command 30 BS Power Control16 Immediate Assignment

Command31 Preprocess Configure

17 SMS Broadcast Request 32 Preprocessed Measurement Result

19 RF Resource Indication 33 RF Channel Release ACK

LAPD PROTOCOL

1a SACCH Filling

1b Overload

1c Error Report

Table1. BTSM Messages

LAPD PROTOCOL

5.2. RADIO RESOURCES LAYER, RR

The RR layer is responsible for the management and allocation of radio resources. The messages from this layer are sent transparently through the BTS to the corresponding layer in the MS. However when these messages appear on the Abis link, they may be embedded in lower layer structures.

There are a total of twenty eight valid messages on the RR layer. These are listed below in numerical order of the message type identifier.

LAPD PROTOCOL

0a Partial Release 27 Paging Response0d Channel Release 28 Handover Failure0f Partial Release Complete 29 Assignment Complete10 Channel Mode Modify 2b Handover Command12 RR Status 2c Handover Complete14 Frequency Redefinition 2e Assignment Command15 Measurement Report 2f Assignment Failure16 Classmark Change 32 Cipher Mode Complete17 Channel Mode Modify ACK 35 Cipher Mode Command19 System Information 1 39 Immediate Assignment

Extended1a System Information 2 3a Immediate Assignment Reject1b System Information 3 3b Additional Assignment

LAPD PROTOCOL

1c System Information 4 3f Immediate Assignment1d System Information 5

1e System Information 6

Table 2. Messages on RR Layer

LAPD PROTOCOL

5.3. BSS APPLICATION LAYER, BSSAP

The only messages from this layer (and the CM & MM layers which are the ones above the BSSAP layer) that will be seen on the Abis link are BTS

transparent messages destined for the MS. The BSSAP layer is divided into two parts: BSSMAP (BSS Management Application Part) which carries the MSC - BSS communication, and DTAP (Data Transfer Application Part) which carries the BSS transparent messages from the MM and CM layers.

LAPD PROTOCOL

Messages from the DTAP layer will be discussed in the CM and MM sections. There are a total of thirty-eight valid messages on the BSSMAP layer, the majority of which will not be seen on the Abis link since they are used only for

MSC-BSC communication, or are altered by the BSC before going to the BTS and MS.

The SCCP layer does not generate any messages which can be seen on the Abis link.

01 Assignment Request 25 SAPI n reject02 Assignment Complete 30 Reset

LAPD PROTOCOL

03 Assignment Failure 31 Reset ACK10 Handover Request 32 Overload11 Handover Required 33 Trace Invocation12 Handover Request ACK 34 Reset Circuit13 Handover Command 35 Reset Circuit ACK14 Handover Complete 40 Block16 Handover Failure 41 Block ACK17 Handover Performed 42 Unblock18 Handover Candidate Enquiry 43 Unblock ACK19 Handover Candidate Response 50 Resourse Request1a Handover Required Reject 51 Resourse Indication1b Handover Detect 52 Paging20 Clear Command 53 Ciphering Mode Command

LAPD PROTOCOL

21 Clear Complete 54 Classmark Update22 Clear Request 55 Ciphering Mode Complete23 SAPI n Clear Command 56 Queuing Indication24 SAPI n Clear Complete 57 Complete L3 Information

Table 3. BSSMAP Messages

LAPD PROTOCOL

5.4. MOBILITY MANAGEMENT LAYER, MM

The two ends of the MM Layer are located in the MS and MSC. This layer is responsible for the registration of subscriber equipment in the network and for identification of users. Among other things, this layer handles the Location Update, Authentication, and TMSI Reallocation procedures.

There are a total of sixteen valid messages on this layer.

01 IMSI Detach Indication 19 Identity response

LAPD PROTOCOL

02 Location Update Accept 1a TMSI Reallocation Command04 Location Update Reject 1b TMSI Reallocation Complete08 Location Update Request 21 CM Service Accept11 Authentication Reject 22 CM Service Reject12 Authentication Request 24 CM Service Request14 Authentication Response 28 CM Re-Establish Request18 Identity request 31 MM Status

Table4. MobilityManagementLayerMessages

LAPD PROTOCOL

5.5. CALL CONTROL, CC

The final layer to produce messages on the Abis link during normal call setup

is the Call Control (CC) Layer which is a sublayer of the Connection Management (CM) layer. This layer is responsible for establishing, maintaining, and clearing calls. The other two sublayers in the CM layer are associated with Suplementary Services (SS) and Short Message Service (SMS) functions.

There are a total of twenty-four valid messages on the Call Control Layer.

LAPD PROTOCOL

01 Alerting 25 Disconnect02 Call Proceeding 2a Release Complete03 Progress 2d Release05 Setup 31 Stop DTMF07 Connect 32 Stop DTMF ACK08 Call Confirmed 34 Status Enquiry0e Emergency Setup 35 Start DTMF0f Connect ACK 36 Start DTMF ACK10 User Information 37 Start DTMF Reject13 Modify Reject 39 Congestion Control17 Modify 3d Status1f Modify Complete 3e Notify

LAPD PROTOCOL

Table 5. Call Control Layer Messages

LAPD PROTOCOL

6. LAPD PROTOCOL

LapD is a protocol which is used for the signalling in the Nokia BSS between the BSC and the BTS. There are two different types of the LapD signalling links: OMUSIG and TRXSIG. OMUSIG is used for down-loading the BTS software from the BSC to the BTS and for monitoring purposes. TRXSIG is used for the BSS signalling

LAPD PROTOCOL

LapD messages are basically a framed collection of fixed length parameter fields used to convey data between two stations. There are three formats of LapD frames: information, supervisory, and unnumbered. A total of ten different frame types are used in GSM, the main ones are explained below:

SABME

This is normally the first frame to be sent when establishing a LapD connection. It places the receiving end in multi-frame acknowledge mode. Previously unacknowledged frames are discarded after receiving SABME. The DISC frame is used to terminate multi-frame acknowledge mode.

LAPD PROTOCOL

Unnumbered Ack (UA)

After receiving SABME or DISC, the receiver will respond with a UA frame. This indicates to the transmitter that the frame has been received.

Information (I)

This frame type is used to transfer information that is to be acknowledged by the receiving end. It can also contain acknowledgement for previously received frames.

LAPD PROTOCOL

Receiver Ready (RR)

The receiver ready frame indicates that the receiver is ready to receive an Information (I) frame, and to acknowledge previously received I frames.

Unnumbered Info (UI)

The unnumbered information frame is used for sending information which does not need to be acknowledged by the receiving end. An example of this is the broadcast of system messages on the LapDm link.

LAPD PROTOCOL

The LapDm must be capable of supporting two Service Access Points, one for ordinary messages (SAPI = 0), and one for short messages (SAPI =3).

LAPD PROTOCOL

SAPI & TEI

Each layer uses the services offered by the lower layers to perform its tasks and correspondingly offeres its own services to the higher layers. The services are sent from one layer to another via a Service Access Point (SAP). The Service Access Point Identifier (SAPI) is a six bit field which identifies the addressed receiving entity within a unit accessing the data link. Both ends of the data link (BTS and BSC) use the same SAPI value. GSM has specified the following use of SAPI values:

SAPI = 0 Radio Signalling

LAPD PROTOCOL

SAPI = 3 Short Message Service

SAPI = 62 Operation & Maintenance

The Terminal Endpoint Identifier (TEI) is a seven bit field that identifies the addressed unit accessing a data link. In the Nokia GSM implementation, since every D channel is a logically separate channel (and physically separate by the time they reach the LapD controllers in the FU), it is not necessary to have a different TEI for each BSC-FU link, however, for reasons of simplicity, it has been decided to use TEI = logical FU number.

LAPD PROTOCOL