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The NOTIFY message is sent by the user or the network to indicate information pertaining to a call. The STATUS message is sent by the user or the network in response to a STATUS ENQUIRY message. Finally, the STATUS ENQUIRY message is sent by the user or the network to solicit a STATUS message from the peer Q.2931 protocol.
a. Call establishment
The steps involved in establishing a call are shown in Figure 5.13. The calling user initiates the procedure for establishing a new call by sending a SETUP message to its ingress ATM switch across the UNI. The ingress switch sends a CALL PROCEEDING message to the calling user if it determines that it can accommodate the new call. (If it cannot accommodate the new call, it rejects it by responding with a RELEASE COMPLETE message.) The ingress switch calculates a route to the destination end device over which the signaling messages are transferred. The same route is used to set up a connection over which the data will flow. It then forwards the SETUP message to the next switch on the route. The switch verifies that it can accommodate the new connection, and the forwards the SETUP message to the next switch, and so on, until it reaches the end device of the called user. The PNNI protocol is used to progress the SETUP message across the network.
SIGNALING IN ATM NETWORKS
Figure 5.13 Call establishment.
If the called user can accept the call it responds with CALL PROCEEDING, ALERTING, or CONNECT message. (Otherwise, it sends a RELEASE COMPLETE message.) Upon receiving an indication from the network that the call has been accepted, the ingress switch sends a CONNECT message to the calling user, who responds with a CONNECT ACKNOWLEDGMENT.
b. Call clearing
Call clearing is initiated when the user sends a RELEASE message. When the network receives the RELEASE message, it initiates procedures for clearing the connection to the remote user. Once the connection has been disconnected, the network sends a RELEASE COMPLETE message to the user, and releases both the call reference value and the connection identifier. Upon receipt of RELEASE COMPLETE message the user releases the connection identifier and the call reference value.
1. Why does the HDLC selective-reject ARQ does not work well in a network with a high bandwidth-delay product?
2. What are the basic differences between the error recovery scheme in the SSCOP and the more traditional ARQ schemes, such as go-back-n and selective reject?
3. Describe the sequence of primitives issued to set up a connection between two peer signaling protocols.
4. What is the purpose of the call reference flag in the signaling message?
5. In which information element, the calling user indicates its traffic parameters?
6. In which information elements the calling user indicates the QoS parameters?
7. Trace the sequence of the signaling messages issued to set up a connection.
The Multi-Protocol Label Switching (MPLS) Architecture
The multi-protocol label switching (MPLS) scheme is based on Cisco’s tag switching, which in turn was inspired by the IP switching scheme, an approach to switching IP packets over ATM proposed by Ipsilon Networks (Ipsilon was later on purchased by Nokia). MPLS was standardized by IETF, and it introduces a connection-oriented structure into the otherwise connectionless IP network. MPLS circumvents the CPU-intensive table look-up in the forwarding routing table necessary to determine the next hop router of an IP packet. Also, it can be used to introduce QoS in the IP network. Interestingly enough, since the introduction of tag switching, and subsequently of MPLS, several CPU-efficient algorithms for carrying out table look-ups in the forwarding routing table were developed. The importance of MPLS, however, was by no means diminished since it is regarded as a solution for introducing QoS into the IP networks.
MPLS requires a set of procedures for the reliable distribution of label bindings. MPLS does not require that a single label distribution protocol is used. In view of this, various schemes have been proposed for the distribution of labels, of which the label distribution protocol (LDP) and the resource reservation protocol - traffic engineering (RSVP-TE) are the most popular.
In this chapter, we describe the basic features of the MPLS architecture. The label distribution protocols LDP and its extension CR-LDP, and RSVP and its extension RSVP-TE are presented in the following chapter. MPLS has been extended to generalized MPLS (GMPLS), which is described in Section 9.5. Before we proceed to describe the MPLS architecture, we review some basic concepts of IP networks in the following section. This section can be skipped by the knowledgeable reader.
6.1 THE INTERNET PROTOCOL (IP): A PRIMER
IP is part of the TCP/IP suite of protocols used in the Internet. TCP corresponds to the transport layer of the OSI model, and IP corresponds to the network layer of the OSI model. In this section, we describe the current version of IP, known as IP version 4 (IPv4).