6 Use of the SCCP

08.063GPPRelease 1999Signalling transport mechanism Specification for the Base Station System - Mobile Services Switching Centre (BSS - MSC) interfaceTS

The MTP and the SCCP are used to support signalling messages between the MSC and the BSS. One user function of the SCCP, called BSS Application Part (BSSAP) is defined. In the case of point-to-point calls the BSSAP uses one signalling connection per active Mobile Station having one or more active transactions for the transfer of layer 3 messages. In the case of a voice group or broadcast call there is always one connection per cell involved in the call and one additional connection per BSS for the transmission of layer 3 messages. There is an additional connection for the speaker in a broadcast call or the first speaker in a voice group call up to the point at which the network decides to transfer them to a common channel. Additional connections may also be required for any mobile stations in the voice group or broadcast call which the network decides to place on a dedicated connection. The BSSAP user function is further subdivided into two separate functions:

– the Direct Transfer Application sub-Part (DTAP) is used to transfer messages between the MSC and the MS; the layer-3 information in these messages is not interpreted by the BSS. The descriptions of the layer 3 protocols for the MS-MSC information exchange are contained in the 04-series of 3GPP Technical Specifications;

– the BSS Management Application sub-Part (BSSMAP) supports other procedures between the MSC and the BSS related to the MS (resource management, handover control), or to a cell within the BSS, or to the whole BSS. The description of the layer 3 protocol for the BSSMAP information exchange is contained in 3GPP TS 08.08.

Both connectionless and connection-oriented procedures are used to support the BSSMAP. Rec. 3GPP TS 08.08 explains whether connection oriented or connectionless services should be used for each layer 3 procedure. Connection oriented procedures are used to support the DTAP. Section 6.4 deals with the use of connectionless services of the SCCP.

A distribution function located in BSSAP, which is reflected in the protocol specification by the layer 3 header defined in section 6.3, performs the discrimination between the data related to those two subparts, as illustrated in 3GPP TS 08.08 figure 1.

The error handling for the BSSAP header is specified in 3GPP TS 08.08.

This section describes the use of SCCP connections for MS transactions. Clause 6.1 describes the connection establishment procedures. Clause 6.2 describes the connection release procedures. Clause 6.3 describes the distribution between BSSMAP and DTAP messages and the data transfer over a SCCP connection. The structure of the user data field in the SCCP message is described in clauses 6.3 and 6.4 and in figure 3.

6.1 Connection establishment

A new SCCP connection is established when information related to the communication between an MS and the network on a dedicated radio resource has to be exchanged between BSS and MSC, and no such SCCP connection exists between the MSC and the BSS involved for the concerned mobile station. A new SCCP connection for each cell, an additional connection for each BSS, and optionally connections for particular participants in a voice group or broadcast call are established when a voice group or broadcast call is established. A new SCCP connection is also established in the case of an external handover between the cells of one BSS for a point-to-point call, or for participants in a voice group or broadcast call who are supported on a dedicated channel.

Various SCCP connection establishment cases have to be distinguished:

i) following an Access Request made by the MS on the Random Access Channel, a dedicated radio resource has been successfully allocated and a layer-2 connection has been established on the allocated resource. The SCCP connection establishment is then initiated by the BSS;

ii) the MSC decides to perform an external handover and a new dedicated radio resource has to be reserved in the new BSS. The SCCP connection establishment is then initiated by the MSC; (Note that the old BSS and the new BSS may be the same.)

iii) following a request for a voice group or broadcast call received at a MSC, SCCP connections are established between the MSC and BSS for each cell in the group call area and the MSC and BSS for each BSS in the group call area. This is initiated by the MSC. Note that a SCCP connection for the originator has already been established via case i);

iv) during a voice group or broadcast call the network may decide to place some participants on a dedicated channel and will perform SCCP connection establishment to support this channel. Such a decision will normally be based on responses received to notification messages in the case where the notification messages do not contain details of the location of the group or broadcast call channel.

The above cases are the only cases currently identified for SCCP connection establishment. Others may emerge in the future.

BSS MSC

CR {SSN=BSSAP, a1, BSSMAP message}
——————————————->

CC {a1,a2, BSSMAP or DTAP message or no user data}
<——————————————

or
CREF{a2, DTAP message or no user data
<——————————————

a1 = source local reference,
a2 = destination local reference

CC Connection Confirm

CR Connection Request

CREF Connection Refused

Figure 1/08.06: Set-up of SCCP connections on the first BSS/MSC interface

BSS MSC

CR {SSN=BSSAP, a1, BSSMAP message or no user data}
<—————————————–

CC {a1, a2, BSSMAP message or no user data}
——————————————>

or
CREF{a2, BSSMAP message or no user data}
——————————————>

a1 = source local reference,
a2 = destination local reference

CC Connection Confirm

CR Connection Request

CREF Connection Refused

Figure 2/08.06: Set-up of SCCP connections on a new BSS/MSC (handover) interface
or for a voice group or broadcast call initiation

6.1.1 Establishment procedure in case i)

In this case, the connection establishment is performed at the reception by the BSS of the first layer-3 message from the MS (piggybacked on the SABM frame). This message (LOCATION UPDATING REQUEST, CM-SERVICE REQUEST, CM REESTABLISHMENT REQUEST, IMSI DETACH, PAGING RESPONSE, or IMMEDIATE SETUP) which contains the identity of the MS is transferred to the MSC together with a cell identification, in a BSSMAP message (COMPLETE L3 INFORMATION) included in the user data field of the SCCP Connection Request message (see figure 1).

After the reception of the Connection Request message, the MSC may check, based on the received identity, whether another association already exists for the same Mobile Subscriber. Two options among others are described hereafter:

– after the reception of the Connection Request message, the MSC sends a Connection Confirm message and checks based on the received identity, whether another connection already exists for the same Mobile Subscriber. If another connection exists for the same Mobile Subscriber, the resources assigned for this previous connection are released after the identity of the Mobile Subscriber using the new connection has been successfully checked, e.g. by authentication or by ciphering procedure;

– if such an association exists, the connection establishment is refused by sending a Connection Refused message;

NOTE: The first option allows the new establishments and the reestablishments.

– when the SCCP connection is to be established, a Connection Confirm message is sent back to the BSS. This message may optionally contain a BSSMAP or DTAP message in the user data field.

If the connection establishment is refused for any reason, a SCCP Connection Refused message is sent back to the BSS. This message may optionally contain, in the user data field, a DTAP message which is forwarded to the MS.

The procedures in case of connection establishment failure are specified in 3GPP TS 08.08.

6.1.2 Establishment procedure in case ii)

In this case, the connection establishment is undertaken by the MSC as soon as the MSC decides to perform an external handover to a new cell for a point-to-point call or for participants in a voice group or broadcast call who are supported on a dedicated channel.

A Connection Request message is sent to the BSS. The user data field of this message may contain the BSSMAP HANDOVER REQUEST message (see figure 2). It is preferable to transfer the layer 3 messages in the user data field of the Connection Request in order to complete the establishment of the relation between the radio channel requested and the SCCP connection as soon as possible.

When receiving the Connection Request message, containing the BSSMAP HANDOVER REQUEST message, the BSS allocates the necessary resources for the requested handover. A Connection Confirm message is also returned to the MSC and may contain the BSSMAP HANDOVER REQUEST ACKNOWLEDGEMENT or QUEUEING INDICATION message in the user data field.

If the handover resource allocation fails (see TS 3GPP TS 08.08) before the SCCP connection is established then the SCCP Connection Refused message may contain the BSSMAP HANDOVER FAILURE message in the user data field.

The procedures in case of connection establishment failure are specified in 3GPP TS 08.08.

6.1.3 Establishment procedure in case iii)

In this case connection establishment is undertaken by the MSC on the reception of a voice group or broadcast call initiation request.

At the reception of the voice group or broadcast call establishment request message, the MSC will determine that a voice group or broadcast call is required and retrieve the required information concerning, inter alia, the affected cells. SCCP connections are then established by the MSC to the BSS for each of these cells. A separate connection is established by the MSC to each affected BSS.

A Connection Request message for each cell in the call and for each BSS in the call is sent to the BSS. The user data field of this message may contain the VGCS/VBS SETUP or VGCS/VBS ASSIGNMENT REQUEST message (see figure 2). It is preferable to transfer the layer 3 messages in the user data field of the Connection Request in order to complete the establishment of the relation between the radio channel requested and the SCCP connection as soon as possible.

When receiving the Connection Request message, containing the VGCS/VBS SETUP or VGCS/VBS ASSIGNMENT REQUEST message, the BSS allocates the necessary resources for the requested call. A Connection Confirm message is also returned to the MSC and may contain the VGCS/VBS SETUP ACK, VGCS/VBS ASSIGNMENT RESULT or VGCS/VBS QUEUEING INDICATION message in the user data field.

If the resource allocation fails (see 3GPP TS 08.08) before the SCCP connection is established then the SCCP Connection Refused message may contain the VGCS/VBS SETUP REFUSE or VGCS/VBS ASSIGNMENT FAILURE message in the user data field.

The procedures in case of connection establishment failure are specified in 3GPP TS 08.08.

6.1.4 Establishment procedure in case iv)

In this case, the connection establishment may be performed at the request of the BSS or MSC. It will normally be as a result of a response to a notification message for a voice group or broadcast call which is received by the BSS.

At the reception of the Connection Request message, the MSC may check, based on the received identity, whether another association already exists for the same MS. If it is not a CM REESTABLISHMENT and such an association exists, the connection establishment is refused. If such an association exists but the new SCCP CR is for a CM REESTABLISHMENT then the old SCCP connection shall be released. When the SCCP connection is to be established, a Connection Confirm message is sent back to the BSS. This message may optionally contain a BSSMAP or DTAP message in the user data field.

If the connection establishment is refused for any reason, a SCCP Connection Refused message is sent back to the BSS. This message may optionally contain, in the user data field, a DTAP message which is forwarded to the MS.

The procedures in case of connection establishment failure are specified in 3GPP TS 08.08.

6.2 Connection release

This procedure is always initiated at the MSC side.

A connection is released when the MSC realizes that a given signalling connection is no longer required. That may occur, in normal cases:

– when a BSSAP release procedure is terminated;

– when a handover resource allocation procedure has failed and a signalling connection was established.

The MSC sends a SCCP released message. This message shall not contain any user data field.

Abnormal cases: a connection failure may be detected by the connection supervision service provided by SCCP. The procedures in that case are specified in 3GPP TS 08.08.

6.3 Transfer of DTAP and BSSMAP data

The DTAP and BSSMAP Layer 3 messages between the MSC and the BSS are contained in the user data field of the exchanged SCCP frames. This field is optional for the Connection Request (CR) (except for BSS originated connections, see clause 6.1); Connection Confirm (CC) and Connection Refused (CREF). The use of this field in such frames in the various establishment cases, which allows reduction n in delay and improves efficiency, is described in clause 6.1. The user data field is a mandatory parameter of the Data frames (DT); the user data field always contains either a DTAP or a BSSMAP message.

6.3.1 Distribution function

6.3.1.1 CCITT Recommendation

The distribution of messages between the BSSMAP and DTAP functions and the distribution/multiplexing of DTAP messages to/from the various radio link layer 2 access points are performed in an intermediate layer of protocol between SCCP and Layer 3 later referred as the distribution sublayer.

The protocol for this sublayer simply consists of the management of a one or two octet Distribution Data Unit. Each SCCP User Data field necessarily contains such a distribution Data Unit as a header, followed by the length indicator and the actual Layer 3 BSSMAP or DTAP message.

6.3.1.2 ANSI Specification

The distribution of messages between the BSSMAP and DTAP functions and the distribution/multiplexing of DTAP messages to/from the various radio link layer 2 access points are performed by a function of BSSAP referred to as a distribution function. The distribution of messages is performed based on a distribution data unit.

The distribution data unit includes a Discrimination Parameter (DP) field, which is coded on one octet. One bit (i.e. least significant bit) of the octet referred as a bit D indicates whether it is a DTAP (value D=1) or a BSSMAP (value D=0) message. The other bits of the octet can be used to separate message groups for different air interfaces (Figure below).

If a single radio system supports one air interface, the A-interface includes only one message group and no separation between message groups is needed. The case is different when radio system supports multiple air interfaces, and the A-interface includes several message groups. In that case, there must be a mechanism which facilitates the selection of the right message group according to the used air interface.

Figure 2a: Distribution of message groups according to examples of the air interface types

6.3.2 Transfer of DTAP messages

The DTAP function is in charge of transferring layer 3 messages from the MS (resp from the MSC) to the MSC (resp to the MS) without any analysis of the message contents. The interworking between the layer 2 protocol on the radio side and signalling system 7 at the landside is based on the use of individual SCCP connections for each MS and on the distribution function.

The structure of the user data field is given in figure 3. The user data field contains a distribution data unit, a length indicator, and the actual layer 3 message.

The Distribution Data Unit consists of two parameters: the Discrimination parameter and the Data Link Connection Identification (DLCI) parameter.

The Discrimination parameter, which is set to the "Transparent" value, is coded on one octet, as follows:

8

7

6

5

4

3

2

1

0

0

0

0

0

0

0

D

The discrimination bit D is set to the "Transparent" value 1.

The DLCI parameter is used for MSC to BSS messages to indicate the type of data link connection to be used over the radio interface. In the direction BSS to MSC the DLCI parameter is used to indicate the type of originating data link connection over the radio interface. The DLCI parameter is coded in one octet, as follows:

8

7

6

5

4

3

2

1

C2

C1

0

0

0

S3

S2

S1

C2 C1 represents the control channel identification

C2=0; C1=0 indicates that the control channel is not further specified;

C2=1; C1=0 represents the FACCH or the SDCCH;

C2=1; C1=1 represents the SACCH;

other values are reserved.

S3 S2 S1 represents the SAPI value used on the radio link, which coding is specified in 3GPP TS 04.06.

Bits 4, 5 and 6 are spare.

The length indicator is coded in one octet, and is the binary representation of the number of octets of the subsequent layer 3 message parameter.

6.3.3 Transfer of BSSMAP messages

The transfer of BSSMAP messages over a SCCP connection allows the BSSMAP functions in both the MSC and the BSS to identify to which particular Mobile Station association the exchanged message (e.g. assign, handover request, etc..) applies.

The structure of the user data field is given in figure 3. The user data field contains a distribution data unit, a length indicator, and the actual layer 3 message.

The Distribution Data Unit only consists of the Discrimination parameter, which is set to the "Not Transparent" value.

This parameter is coded on one octet, as follows:

8

7

6

5

4

3

2

1

0

0

0

0

0

0

0

D

The discrimination bit D is set to the "Not Transparent" value 0.

The length indicator is coded in one octet, and is the binary representation of the number of octets of the subsequent layer 3 message parameter.

The coding of the BSSMAP layer 3 messages is specified in 3GPP TS 08.08.

6.4 Connectionless services

Some BSSMAP procedures described in 3GPP TS 08.08 use the connectionless services of the SCCP.

The structure of the user data field of the unit data message (UDT) is given in figure 3. The user data field contains a distribution data unit, a length indicator, and the actual layer 3 message.

The Distribution Data Unit only consists of the Discrimination parameter, which is set to the "Not Transparent" value.

6.4.1 Discrimination parameter (CCITT Recommendation)

This parameter is coded on one octet, as follows:

8

7

6

5

4

3

2

1

0

0

0

0

0

0

0

D

The discrimination bit D is set to the "Not Transparent" value 0.

The length indicator is coded in one octet, and is the binary representation of the number of octets of the subsequent layer 3 message parameter.

The coding of the BSSMAP layer 3 messages is specified in 3GPP TS 08.08.

6.4.2 Discrimination parameter (ANSI Specification)

This parameter is coded on one octet, as follows:

8

7

6

5

4

3

2

1

X

x

X

x

x

x

x

D

The discrimination bit D is set to the "BSSMAP" value 0.

The bits indicated with X values denote to air interface message groups as shown in the table "Coding Of The Discrimination Parameter for PCS 1900" (see clause 6.3.2.2).

The length indicator is coded in one octet, and is the binary representation of the number of octets of the subsequent layer 3 message parameter.

6.4.3 User Data Field Structure

DTAP BSSMAP
+—————–+ +—————–+
1 | DISCRIMINATION | 1 | DISCRIMINATION |
+—————–+ +—————–+
+—————–+                          
2 | DLCI |                          
+—————–+                          
——————Distribution Data Unit—————-
+—————–+ +—————–+
3 | LENGTH IND L | 2 | LENGTH IND L |
+—————–+ +—————–+
———————-Length Indicator——————
+—————–+ +—————–+
4 | LAYER 3 | 3 | LAYER 3 |
+—————–+ +—————–+
: :
: :
+—————–+ +—————–+
L+3 | MESSAGE | L+2 | MESSAGE |
+—————–+ +—————–+
———————-Layer 3 message——————-

Figure 3/08.06: Structure of the User Data Field