THE SIGNALING PROTOCOL STACK

THE SIGNALING ATM ADAPTATION LAYER SAAL 121 arrived when the SSCOP receiver buffer was full, or it might have been invalid, in which case, the SSCOP discarded it. It will also be lost in the unlikely case where all of the ATM cells carrying the SD PDU are lost.

5.3.2 Primitives

In the OSI model, the functions of a layer n provide a set of services which can be used by the next higher layer n + 1. The function of a layer n are built on services it requires from the next lower layer n − 1. The services in each layer are provided through a set of primitives. Each primitive might have one or more parameters that convey information required to provide the service. There are four basic types of primitives: request, indication, response , and confirm. A request type primitive is passed from a layer n to a lower layer n − 1 to request a service to be initiated. An indication type primitive is passed from a layer n − 1 to a higher layer n to indicate an event or condition significant to layer n. A response type primitive is passed from a layer n to a lower layer n − 1 to complete a procedure previously invoked by an indication primitive. Finally, a confirm type primitive is used by a layer n − 1 to pass results from one or more previously invoked request primitives to an upper layer n. These primitive types are also used between a signaling protocol and SAAL see Figure 5.4. SAAL functions are accessed by a signaling protocol through the AAL-SAP, using the primitives: AAL-ESTABLISH, AAL-RELEASE, AAL-DATA, and AAL-UNIT-DATA. The AAL-ESTABLISH is issued by a signaling protocol to SAAL in order to request the establishment of a connection over the UNI to its peer protocol. This is necessary, in order for the two peer signaling protocol to exchange signaling messages. This is a reliable connection that is managed by the SSCOP as described above. The AAL-RELEASE primitive is a request by a signaling protocol to SAAL to termi- nate a connection established earlier on using the AAL-ESTABLISH primitive. The AAL-DATA primitive is used by a signaling protocol to request the transfer of a signaling message to its peer signaling protocol. Signaling messages have a specific structure, and will be discussed below in detail. Finally the AAL-UNIT-DATA is used to request a data transfer over an unreliable connection. An example of how these primitives are used to establish a new connection over the UNI between two peer signaling protocols is shown in Figure 5.5. The primitive AAL- ESTABLISH.request is used to request SAAL to establish a connection. In order to simplify the presentation, we do not present the signals exchanged between the SSCF and the SSCOP. In response to this request, SSCOP sends a BEGIN frame to its peer SSCOP. Signaling protocol SAAL Signaling protocol SAAL request confirm indication response ATM end-station ATM switch Figure 5.4 The four primitive types. 122 SIGNALING IN ATM NETWORKS AAL-EST. request SAAL SAP BEGIN BEGIN ACK AAL-EST. indication AAL-EST. confirm SAAL SAP SSCOP SSCOP AAL-EST. response Figure 5.5 Establishment of a connection between two peer signaling protocols. The peer SAAL generates an AAL-ESTABLISH.indication to the peer signaling protocol, and its SSCOP returns a BEGIN ACKNOWLEDGE frame, upon receipt of which, the SAAL issues a AAL-ESTABLISH.confirm to the signaling protocol. An example of how a connection over the UNI between two peer signaling pro- tocols is terminated is shown in Figure 5.6. The signaling protocol issues an AAL- RELEASE.request to SAAL, in response of which the SSCOP sends an END frame to its peer SSCOP. The peer SAAL sends an AAL-RELEASE.indication to the peer sig- naling protocol, and its SSCOP returns an END ACKNOWLEDGE frame, upon receipt of which the SAAL issues a AAL-RELEASE.confirm to the signaling protocol. An example of how a signaling protocol transfers messages to its peer protocol is shown in Figure 5.7. The signaling protocol transfers a message to SAAL in an AAL- DATA.request, which is then transferred by SSCOP in an SD frame. The SD frame is passed onto AAL 5, which encapsulates it and then breaks it up to 48 byte segments, each of which is transferred by an ATM cell. Figure 5.7 also shows the POLLSTAT frames exchanged between the two peer SSCOPs. The SD frame at the destination side is delivered to the peer signaling protocol using the AAL-DATA.indication primitive. AAL-REL. indication SSCOP SSCOP AAL-REL. response AAL-REL. request SAAL SAP END END ACK AAL-REL. confirm SAAL SAP Figure 5.6 Termination of a connection between two peer signaling protocols.