ATM ADDRESSING Connection Oriented Network

128 SIGNALING IN ATM NETWORKS • Broadband high-layer IE, broadband low-layer IE : Used to check compatibility by the called user. • Broadband repeat indicator IE : Indicates how repeated IEs are to be interpreted. • Call state : Describes the current status of the call. • Called party number IE, and called party subaddress IE : Identify the called user. • Calling party number IE, and calling party subaddress IE : Identify the calling user. • Cause IE : Describes the reason for generating certain messages and indicates the loca- tion of the cause originator. • Connection identifier IE : Identifies the VPIVCI allocated to the connection at the UNI. • End-to-end transit delay IE : Indicates the maximum acceptable transit delay and the cumulative transit delay to be expected for the connection. • Extended QoS parameters IE : Specifies the acceptable values and the cumulative values of some of the QoS parameters. • Transit network selection IE : Identifies a transit network that the call can cross. An ATM end device or an ATM switch might not be able to process every information element included in a signaling message. In this case, the ATM equipment simply uses only the information elements that it needs, and it ignores the rest of them.

5.7.2 Q.2931 Messages

The Q.2931 messages can be grouped into the following three categories: call estab- lishment, call clearing, and miscellaneous. The messages for each category are given in Table 5.2. Each Q.2931 message uses the signaling message format described in Section 5.6, with the protocol discriminator set to 00001001, and contains a set of infor- mation elements. Below, we describe the function of each message, and then we show how they are used to establish and clear a call. • ALERTING : This message is sent by the called user to the network and by the network to the calling user to indicate that “called user alerting” has been initiated. Called user Table 5.2 Q.2931 messages. Message category Message Call establishment messages ALERTING CALL PROCEEDING CONNECT CONNECT ACKNOWLEDGEMENT SETUP Call clearing messages RELEASE RELEASE COMPLETE Miscellaneous messages NOTIFY STATUS STATUS ENQUIRY THE SIGNALING PROTOCOL Q.2931 129 alerting is used for calls that require human interface, such as voice. The information element used in this message is the connection identifier. • CALL PROCEEDING : The message is sent by the called user to the network or by the network to the calling user, to indicate that the requested establishment of the call has been initiated and no more call information is accepted. The information element used in this message is the connection identifier. • CONNECT : The message is sent by the called user to the network, or by the network to the calling user to indicate that the called user has accepted the call. The following information elements are used in this message: AAL parameter, broadband low-layer, connection identifier, and end-to-end transit delay. • CONNECT ACKNOWLEDGEMENT : This message is sent by the network to the called user to indicate that the user has been awarded the call. It is also sent by the calling user to the network to allow symmetrical call control procedures. • RELEASE : This message is sent by the user to request the network to clear an end-to- end connection. It is also sent by the network to indicate that an end-to-end connection is cleared. It also indicates that the receiving equipment is to release the connection identifier, and to prepare to release the call reference value after sending RELEASE COMPLETE. The cause information element is carried in this message. • RELEASE COMPLETE : This message is sent by the calling user or the network to indicate that the equipment sending the message has released its call reference value and, if appropriate, the connection identifier. The cause information element is carried in this message. • SETUP : This message is sent by the calling user to the network and by the network to the called user to initiate the establishment of a new call. The following information elements are used: AAL parameter, ATM traffic descriptor, broadband bearer capability, broadband high-layer, broadband low-layer, called party number, called party subad- dress, calling party number, calling party subaddress, connection identifier, end-to-end transit delay, extended QoS parameters, and transit network selection. 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. 130 SIGNALING IN ATM NETWORKS ATM network SETUP CALL PROC SETUP CONNECT CONNECT ACK Calling user Called user CONNECT Figure 5.13 Call establishment. If the called user can accept the call it responds with CALL PROCEEDING, ALERT- ING, 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. PROBLEMS 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 signal- ing 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. 6 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 – t raffic 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 TCPIP 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. IP provides a connectionless service using packet switching with datagrams. Packets in a connectionless network, such as the IP network, are referred to as datagrams. An IP host can transmit datagrams to a destination IP host without having to set up a connection to the destination, as in the case of X.25, frame relay, and ATM networks. IP datagrams Connection-oriented Networks Harry Perros  2005 John Wiley Sons, Ltd ISBN: 0-470-02163-2