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The Q.2931 signaling protocol, described in this chapter, is used exclusively over the UNI between a user and its ingress switch. It was standardized by ITU-T and it was subsequently modified by the ATM Forum. Q.2931 is based on Q.931, a protocol that is part of the Digital Subscriber Signaling System No.1 (DSS1), which was defined by ITU-T for signaling between a Narrowband ISDN (N-ISDN) user and its local exchange (see Section 12.1.5).
5.2 THE SIGNALING PROTOCOL STACK
The signaling protocol stack is shown in Figure 5.2. It is analogous to the ATM protocol stack shown in Figure 3.5 (see also Section 3.3). The ATM protocol stack shows the protocol layers used for the transfer of data, whereas the stack in Figure 5.2 shows the
Figure 5.2 The signaling protocol stack.
THE SIGNALING ATM ADAPTATION LAYER (SAAL)
protocol layers used for setting-up an SVC. As we can see, a signaling protocol (such as Q.2931, Q.2971, and PNNI) is an application that runs on top of SAAL. Below SAAL, we have the familiar ATM layer and the physical layer. The signaling protocol stack is often referred to as the control plane, as opposed to the data plane that refers to the ATM protocol stack.
5.3 THE SIGNALING ATM ADAPTATION LAYER (SAAL)
SAAL consists of an SSCS, which is composed of the service-specific coordination function (SSCF) and the service-specific connection oriented protocol (SSCOP). (See Figure 5.3.) The common part of the convergence sublayer is AAL 5 (see Section 3.7.3). The SSCF maps the services required by the signaling protocol to the services provided by SSCOP. The SSCOP is a protocol designed to provide a reliable connection over the UNI to its peer SSCOP. This connection is used by a signaling protocol to exchange signaling messages with its peer protocol over the UNI in a reliable manner.
Recall that in the data plane there is neither error control nor flow control between an end device and its ingress switch, or between any two adjacent ATM switches. In the signaling plane, however, it is not desirable to lose or to deliver erroneously signaling messages, as this might significantly impact network performance. In view of this, the SSCOP was developed to assure a reliable transfer of messages between peer signaling protocols.
5.3.1 The SSCOP
In the OSI model, the data link layer provides error/loss recovery and flow control on each hop using the HDLC go-back-n and selective-reject ARQ scheme. In the case where the transmission link is reliable, the selective-reject ARQ scheme turns out to be more effective than the go-back-n scheme since only the lost/erroneous PDU is retransmitted. These ARQ schemes were devised for networks where the transmission speeds and propagation
Figure 5.3 The signaling AAL (SAAL).
SIGNALING IN ATM NETWORKS
delays were much slower in comparison with modern networks. High speed networks, such as ATM networks, are characterized by high bandwidth-delay product. That is, the product of the transmission speed of an ATM link times the round trip propagation delay is high. Network designers became aware that the traditional selective-reject scheme was inefficient for networks with high bandwidth-delay product, since it only retransmitted one lost/erroneous PDU at a time. Several new lightweight protocols with a novel selection transmission scheme were proposed and were shown to have an improved throughput over traditional protocols. In these protocols, there is a periodic exchange of the full state of the communicating entities, decisions about received packets are communicated to the transmitter for entire blocks of SD PDUs, and the processing of the protocol is parallelized.
SSCOP was standardized in 1994; it incorporates many design principles of these lightweight protocols. It provides most of the services provided by LAP-D in ISDN and
message transfer part (MTP) level 2 in signaling system no. 7 (SS7).
The SSCOP PDUs
SSCOPís main function is to establish and release a connection to a peer SSCOP, and to maintain an assured transfer of data over the connection. This is done using the protocol data units (PDU) described in Table 5.1. The PDUs are grouped together according to their function. The first column gives the various functions, the second column gives the name of each SSCOP PDU and its abbreviation, and the last column provides a brief description of each PDU.
The BEGIN (BGN), BEGIN ACKNOWLEDGMENT (BGAK), and BEGIN REJECT (BGREJ) PDUs are used to establish an SSCOP connection between two peer entities. The BGN PDU is used to request the establishment of an SSCOP connection between two peer entities. BGN PDU requests the clearing of the peer SSCOPís transmitter and receiver buffers, and the initialization of the peer SSCOPís transmitter and receiver state variables. The BGAK PDU is used to acknowledge the acceptance of a connection request from the peer, and the BGREJ PDU is used to reject the connection request from the peer SSCOP.
The END PDU is used to release an SSCOP connection between two peers, and the ENDAK PDUs is used to confirm the release of an SSCOP connection.