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10 BSSAP

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10.1 BSSAP Position................................................................................................3 10.2 BSSAP Tasks...................................................................................................4 10.3 Base Station Management Application Part (BSSMAP)....................................5 10.4 Direct Transfer Application Part (DTAP) (1/4)...................................................6 10.4 Direct Transfer Application Part (DTAP) (2/4)...................................................7 10.4 Direct Transfer Application Part (DTAP) (3/4)...................................................8 10.4 Direct Transfer Application Part (DTAP) (4/4)...................................................9 10.5 DTAP Content and RIL-3................................................................................10 10.6 BSSAP Message Format................................................................................11

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10.1 BSSAP Position

Let's have a look at another important SCCP user, the Base Station Application Part(BSSAP).

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10.2 BSSAP Tasks

The BSS Application Part was created for two special signaling tasks in GSMnetworks. It supports messages for MSC-BSC communication as well as directcommunication between the serving MSC and mobile station. The latter is namedDirect Transfer Application Part (DTAP) and passes transparently through the wholeBSS. The BSS Management Application Part (BSSMAP) contains signaling

information to be exchanged between MSC and BSC.

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10.3 Base Station Management Application Part (BSSMAP)

The BSS Management Application Part is responsible for Radio ResourceManagement, e.g. triggering the assignment of a radio channel by the BSC toestablish a mobile terminated call, handover control if a handover occurs betweenBTSs of two different BSCs and Cell Broadcast SMS delivery defined by theNetwork Maintenance Center. We should note that this particular content is no realsignaling information, its the core CB-SMS content.

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10.4 Direct Transfer Application Part (DTAP) (1/4)

DTAP is responsible for connection management between the mobile and the MSC. A very obvious example for a DTAP CC message is the alert which triggers a ringingtone within the MS.

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DTAP also ensures the correct delivery of supplementary services, e.g. the MSISDN number of the connected party.

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Another DTAP task is to cater for the correct delivery of point-to-point shortmessages. In GSM networks, the short message service uses signaling resourcesrather than traffic channels to convey the information. The originating mobiletransfers the SMS content using DTAP SMS signaling information.Between MSC and SMS-C the communication is based on MAP messages. On theother hand, the receiving side uses DTAP between the MSC and the terminatingmobile.

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Finally, DTAP messages contain mobility management information, e.g. the relevantparameters for the authentication process that always procedes a communication.Between the authentication center and the serving MSC these parameters areincluded within a MAP message. the MSC converts this into a DTAP MM messageto be forwarded to the mobile station.

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10.5 DTAP Content and RIL-3

We may have gained the impression that SS7 signaling messages are used withinthe whole GSM down to the mobile. This is not true. According to the specificrequirements of the air interface, it is the BSC that terminates the SS7 messagetransfer, extracts the DTAP content and creates an other type of signaling, named

Radio Interface Layer 3 signaling. RIL-3 provides similar functionalities to MTP but isproperly adapted to the needs of the GSM air interface.

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10.6 BSSAP Message Format

At the end of this chapter let's have a look at the typical BSSAP message format.Like TCAP or ISUP, BSSAP is a SCCP user. That means the two types of BSSAPmessages - BSS MAP and DTAP - are included within the corresponding part of theSCCP message.

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