The Network Layer Protocol Architecture

www.pragsoft.com Chapter 11: Integrated Services Digital Network 153 Figure 11.131 Sample circuit-switched call scenario. Connect Acknowledge Information Call Proceeding Release Complete Setup Setup Acknowledge ISDN phone calling SS7 ISDN network Disconnect Release Alerting Alerting Setup called ISDN phone Connect Connect User Information User Information User Information User Information Disconnect Release Complete Release Data exchange Conversation Subscriber dials destination Ring tone Ring tone stops Subscriber picks up Phone rings Subscriber hangs up ISUP I.451 I.451 There are other messages which are not covered by our scenario. Calls can be temporarily suspended and later resumed using the Suspend and Resume set of messages. The Register, Facility, and Cancel set of messages are used to manage facilities. ISDN supports a variety of facilities, such as: reverse charging, call completion after busy, provision of charging information, X.25 flow control negotiation, X.25 fast select. As indicated in Figure 11.131, I.451 handles the signaling between TE and the local exchange, while the ISUP ISDN User Part was explained in Chapter 10 handles the common channel signaling in between exchanges within the network. For clarity, the ISUP signals are not shown here. I.451 and ISUP interwork to provide end-to-end signaling to the users. 154 Communication Networks Copyright © 2005 PragSoft Figure 11.132 ISDN network layer message structure. Field Description Protocol Discriminator Denotes the message protocol. Call Reference Length Length of the reference flag + value. Call Reference Flag Denotes the initiating D channel end. Call Reference Value Uniquely identifies the B channel call. Message Type Denotes the D channel message type. E sample 1 Denotes a fixed-length element. L fixed-length Element ID Uniquely identifies the element. E element Element Contents Element information contents. M zero or more A total of zero or more elements are E elements permitted. N sample Denotes a variable-length element. T variable-length Element ID Uniquely identifies the element. S element Contents Length Length of the element contents. Element Contents Element information contents. Figure 11.132 shows the general structure of the I.451 network layer messages just described. Each message consists of four parts. The Protocol Discriminator identifies the network level protocol to which the message belongs. This will either denote I.451 or X.25. The Call Reference denotes the B channel call to which this message applies. It consists of a Length field, a Flag, and a Value. The Flag denotes the call-initiating end of the D channel. The Message Type denotes one of the messages listed in Figure 11.130. The Elements of which there may be zero or more, depending on the Message Type are used for representing the message parameters. Elements may be fixed or variable length. A fixed length element consists of a 1 bit, followed by an Element ID, and Element Contents total of one octet. A variable length element consists of a 0 bit followed by an Element ID, a Content Length, and Element Contents total of at least three octets.

11.3. Frame Relay

Frame relay is a major ISDN data link protocol for end-to-end connections over the B channel. Whereas LAP-B supports packet-switched connections, frame relay is designed to support circuit-switched connections. Frame relay is an evolution of an earlier protocol, V.120, which supports multiple logical connections over the same B channel cicuit. Frame relay provides similar capabilities but removes the restriction that all the multiplexed logical connections over a B channel should be between the same two end-users. We will look at V.120 first and then discuss frame relay. www.pragsoft.com Chapter 11: Integrated Services Digital Network 155

11.3.1. V.120

V.120 is a popular protocol implemented by ISDN terminal adapters. It enables a TE2 to communicate with a TE1 or another TE2 connected to a TA over a B channel circuit see Figure 11.133. Figure 11.133 V.120-based connections. TA ST ST R R TE2 TE2 TE1 TE2 ST ISDN Network R V.120 TA V.120 NT NT NT V.120 supports asynchronous as well as synchronous transmissions. Synchronous transmission may be either as raw data or HDLC-based. Multiple logical channels between two end-users can be multiplexed onto the same B channel. Rate adaptation is provided for devices operating at less than 64 kbps. A sliding window protocol is used for flow control. Except for the address field and the data field, the V.120 frame structure is very similar to LAP-D and HDLC. The address field contains a Logical Link Identifier LLI for the identification of multiple logical connections over the same B channel circuit. Certain LLI values are assigned to specific purposes. For example, 0 denotes in-channel signaling and 256 means no multiplexing. As well as carrying user data, the data field may conatin one or two header octets. The first octet is called the terminal adaptation header and consists of a set of bit flags for controling a number of adaptation functions, including: segmentation of HDLC frames, error control, HDLC idle condition, and extension of the header with additional control information. The second octet, the control state information octet, is optional and also consists of a set of bit flags. The V.120 procedure involves the following steps. First, a B channel circuit is established using the Q.931 call control protocol over the D channel. The resulting circuit may be used in connectionless mode by using an LLI value of 256 for the frames. For a connection-oriented mode, however, additional steps are necessary to manage multiplexed logical links. Four messages are provided for this purpose: • Setup may be issued by either side to request a logical link to be established. 156 Communication Networks Copyright © 2005 PragSoft • Connect may be issued in response to a setup message in order to accept the connection. • Release may be issued by either side to release an existing connection. • Release Complete may be issued in response to a setup message to deciline a connection, or in response to a release message to confirm the release. These messages may be exchanged either over the D channel using Q.931 messages or over the B channel using V.120 frames with LLI set to 0. When the B channel circuit is no longer needed, it may be terminated by either side using the Q.931 call control protocol on the D channel.

11.3.2. Frame Relay