The mechanisms for the transfer of the BSSAP messages on the E-interface is defined in TS 29.002. The operation of the handover procedures between MSCs and the use of the BSSMAP messages for those procedures is described in TS 23.009 and TS 29.010. In the same way as a SCCP signalling connection is used for the messages relating to one MS on the MSC-BSS interface a TCAP dialogue is used on the E-interface for messages relating to one MS. As no correspondence to the connectionless service on the MSC-BSS interface is used on the E-interface none of the global procedures (see TS 48.008, subclause 3.1) are applicable. The management of the terrestrial circuits and IP connections between the MSCs is outside the scope of the E-interface (see TS 23.009), therefore all procedures, messages and information elements relating to terrestrial circuits or IP connections are also excluded from the BSSMAP procedures and messages used on the E-interface.

The BSSAP data which on the MSC-BSS interface is contained in the user data field of the exchanged SCCP frames (see TS 48.006) is on the E-interface transferred as the contents of the signalling info in a BSS APDU parameter as described in TS 29.002. The BSSAP data consists of a BSSAP header and a DTAP or BSSMAP layer 3 message. The BSSAP header contains, as specified in TS 48.006, subclauses 6.3.1, 6.3.2 and 6.3.3, of a discrimination parameter, possibly a Data Link Connection Identification (DLCI) parameter, and a length indicator.

For the description in the present document, the MSC's functionality related to the handover between MSCs has been split into three logical parts, MSC-A, MSC-T and MSC-I. The different roles need not necessarily be performed by different MSCs. MSC-A is the call/connection controlling part of the MSC where the call/connection was originally established and the switching point for handover between MSCs. (This corresponds to MSC-A as defined in TS 23.009 and TS 29.002). The MSC that is the MSC-A will not be changed during the duration of a call/connection. MSC-T is the part relating to the transitory state during the handover for the MSC to which the MS is handed over when Basic handover or Subsequent handover (see TS 23.009) take place. (This corresponds, depending on the type of handover to MSC-A, MSC-B or MSC-B' in TS 23.009 and TS 29.002). MSC-I is the part of an MSC through which the MSC-A, via an E-interface (or an internal interface) is in contact with the MS. (This corresponds, depending on the type of handover to MSC-A, MSC-B or MSC-B' in TS 23.009 and TS 29.002). The MSC that is the MSC-A can also have the role of either the MSC-I or the MSC-T during a period of the call/connection. The following is applicable for the involved MSCs concerning the exchange of BSSAP data on an E-interface before and after a successful inter MSC handover:

1. At Basic handover, two MSCs are involved, one MSC being MSC-A and one being MSC-T. When this handover has been performed, the two MSCs interworking on the E-interface have the roles of MSC-A and MSC-I respectively, i.e. the MSC that is the MSC-T during the handover is now the MSC-I.
2. At Subsequent handover back to MSC-A, two MSCs are involved. The MSC having the role of MSC-A has also the role of MSC-T. The other MSC involved has the role of MSC-I. When this handover has been completed, there is no exchange of BSS data on the E-interface, i.e. the MSC being the MSC-I before and during the handover is now no longer taking part.
3. At subsequent handover to an MSC not being MSC-A, three MSCs are involved. The roles of these MSCs are MSC-A, MSC-I, and MSC-T respectively. When this handover has been performed, the two MSCs interworking on an E-interface have the roles of MSC-A and MSC-I respectively, i.e. the MSC that is the MSC-T during the handover is now the MSC-I and the MSC being MSC-I during the handover is now no longer taking part.

For the exchange of the DTAP messages (TS 48.008, subclause 2.2), the involved MSCs shall act according to the following:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

The Assignment procedure (TS 48.008, subclause 3.1.1) is applied on the E-interface with following conditions:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

The handling of terrestrial resources is not applicable.

At Basic Inter MSC Handover (TS 23.009) the Handover resource allocation procedure (TS 48.008, subclause 3.1.5.2) is applied on the E-interface with the following conditions:

• the MSC-A acts as the MSC;
• the MSC-T acts as the target BSS.

At Subsequent Inter MSC Handover (TS 23.009) the Handover resource allocation procedure (TS 48.008, subclause 3.1.5.2) is applied on the E-interface with the following conditions:

• the MSC-I acts as the MSC;
• the MSC-T acts as the target BSS;
• if the MSC that is the MSC-A is not also the MSC-T, then this MSC shall act as the target BSS towards the MSC-I and as the MSC towards the MSC-T.

The handling of terrestrial resources is not applicable.

For the Handover execution procedure (TS 48.008, subclause 3.1.5.3) the applicable parts on the E-interface are the transfer of HANDOVER DETECT, HANDOVER COMPLETE and HANDOVER FAILURE messages at inter MSC handover. For those parts, the involved MSCs shall act according to the following:

• the MSC that is the MSC-A, acts as the MSC;
• the MSC that is the MSC-I, if it is not also the MSC-A, acts as the serving BSS;
• the MSC that is the MSC-T, if it is not also the MSC-A, acts as the target BSS.

For the Internal handover indication (TS 48.008, subclauses 3.1.6 and 3.1.7), the involved MSCs shall act according to the following:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

MSC internal handovers (inter BSS and intra BSS handover) at the MSC-I are decided and executed autonomously by that MSC together with the connected BSSs. At completion of the handover execution the MSC-I initiates the internal handover indication procedure.

For the Release due to BSS generated reasons procedure (TS 48.008, subclause 3.1.9.2) the involved MSCs shall act according to the following:

• the MSC-I acts as the BSS;
• no further action is taken by the MSC-A.

Additionally, at Basic Inter MSC Handover and at Subsequent Inter MSC Handover (TS 23.009), if the MSC that is the MSC-A is not also the MSC-T, the Release due to BSS generated reasons procedure (TS 48.008, subclause 3.1.9.2) is applied on the E-interface with the following conditions:

• the MSC-T acts as the BSS;
• no further action is taken by the MSC-A.

For the Classmark handling (TS 48.008, subclause 3.1.13), the involved MSCs shall act according to the following:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

For the Cipher mode control (TS 48.008, subclause 3.1.14), the involved MSCs shall act according to the following:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

For the Trace invocation (TS 48.008, subclause 3.1.11), the involved MSCs shall act according to the following:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

Additionally, at Basic Inter MSC Handover and at Subsequent Inter MSC Handover (TS 23.009), if the MSC that is the MSC-A is not also the MSC-T, the Trace invocation (TS 48.008, subclause 3.1.11) is applied on the E-interface with the following conditions:

• the MSC-A acts as the MSC;
• the MSC-T acts as the target BSS.

For the Queuing Indication (TS 48.008, subclause 3.1.17), the involved MSCs shall act according to the following:

• at Assignment and at Basic Inter MSC handover:
  • the MSC-A acts as the MSC;
  • the MSC-I acts as the BSS.
• at Subsequent Inter MSC handover:
  • the MSC-I acts as the MSC;
  • the MSC-T acts as the BSS;
  • if the MSC that is the MSC-A is not also the MSC-T, then this MSC acts as the target BSS towards the MSC-I and as the MSC towards the MSC-T.

For the Data Link Control SAPI not Equal to "0" (TS 48.008, subclause 3.1.18), the involved MSCs shall act according to the following:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

For the Location Acquisition procedure (TS 48.008, subclause 3.1.28), the involved MSCs shall act according to the following:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

For the LSA handling (TS 48.008, subclause 3.1.27), the involved MSCs shall act according to the following:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

For the Common Id (TS 48.008), the involved MSCs shall act according to the following:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

In the Service Change procedure for network-initiated SCUDIF (see TS 23.172), the MSC-I sends the CHANNEL MODIFY REQUEST message (see TS 48.008) to the MSC-A. The request was originally sent by the RNC using RAB MODIFY REQUEST to trigger execution of the alternative RAB configuration (see TS 25.413). For the CHANNEL MODIFY REQUEST message, the involved MSCs shall act according to the following:

• the MSC-A acts as the MSC;
• the MSC-I acts as the BSS.

For the applicable BSSMAP messages transferred on the E-interface the following exceptions to the descriptions in TS 48.008, subclause 3.2.1 are valid: Assignment request message:

• excluded information elements:
  • circuit identity code;
  • AoIP Transport Layer Address (MGW);
  • Call Identifier;
  • Codec List (MSC Preferred).
• if received, the information element shall be treated as an information element with an unrecognisable identifier.

Assignment complete message:

• excluded information element:
  • circuit pool;
  • circuit identity code;
  • AoIP Transport Layer Address (BSS);
  • Speech Codec (Chosen);
  • Codec List (BSS Supported).
• if received, the information element shall be treated as an information element with an unrecognisable identifier.

Assignment failure message:

• excluded information elements:
  • circuit pool;
  • circuit pool list;
  • Codec List (BSS Supported).
• if received, the information element shall be treated as an information element with an unrecognisable identifier.

Handover request message:

• excluded information element:
  • circuit identity code;
  • AoIP Transport Layer Address (MGW);
  • Call Identifier;
  • Codec List (MSC Preferred).
• if received, the information element shall be treated as an information element with an unrecognisable identifier.

Handover request acknowledge message:

• excluded information element:
  • circuit pool;
  • circuit identity code;
  • AoIP Transport Layer Address (BSS);
  • Speech Codec (Chosen);
  • Codec List (BSS Supported).
• if received, the information element shall be treated as an information element with an unrecognisable identifier.

Handover failure message:

• excluded information elements:
  • circuit pool;
  • circuit pool list;
  • Codec List (BSS Supported).
• if received, the information element shall be treated as an information element with an unrecognisable identifier.

Handover performed message:

• excluded information elements:
  • Speech Codec (Chosen);
  • Codec List (BSS Supported).
• if received, the information element shall be treated as an information element with an unrecognisable identifier.

For the applicable BSSMAP information elements transferred on the E-interface the following exceptions to the description in TS 48.008, subclause 3.2.2.5 are valid: Cause information element:

• excluded causes:
  • call control;
  • CCCH overload;
  • handover successful;
  • requested terrestrial resource unavailable;
  • terrestrial circuit already allocated;
  • circuit pool mismatch;
  • switch circuit pool;
  • Call Identifier already allocated.
  The corresponding cause values shall be considered as reserved for national use.

Cell identifier information element:

• excluded format:
  • Cell Identity.
  The corresponding cell identification discriminator value shall be considered as reserved.