The Data Link Layer

150 Communication Networks Copyright © 2005 PragSoft Figure 11.129 LAP-D command and response frames. Frame Type Command Response Description Information I I Used for exchange of user data. Supervisory RR RR Receive Ready. Station is ready to receive frames and acknowledged receipt of earlier frames. RNR RNR Receive Not Ready. Station is unable to receive frames, but acknowledged receipt of earlier frames. REJ REJ Reject. Rejects a frame according to the Go-Back- N scheme. Unnumbered SABM Set Asynchronous Balanced Mode. SABME Set Asynchronous Balanced Mode Extended. UI Unnumbered Information. Used for unacknowledged mode of information transfer. UA Unnumbered Acknowledgment. Unsequenced acknowledgment frame. DISC Disconnect. Forces a slave station into disconnect mode. DM Disconnect Mode. Used by a slave to indicate it is in disconnect mode. XID XID Exchange identification information. FRMR Frame Reject of a protocol-violating frame. The acknowledged service involves the exchanging of all three types of frames between the network and a TE. First, a link is requested using the SABM or SABME command, which may be accepted by a UA response or rejected by a DM response. Then, the two ends may exchange information frames using the I command. The supervisory frames are used during this phase for error control using the Go-Back-N scheme and flow control using the sliding window protocol. Finally, the connection may be terminated by a DISC command. FRMR is used to reject a frame which violates the protocol e.g., incorrect frame length or invalid sequence numbers. This results in the connection being aborted. The XID command is used for exchanging identification information e.g., negotiating a new set of values for LAP-D parameters. The unacknowledged service provides basic exchange of user information using the UI command, which are received unacknowledged. No error control or flow control is provided, although CRC-based error detection is still performed and incorrect frames are simply discarded. Though LAP-D is based on LAP-B, some important differences exist. Firstly, unlike LAP-B which is designed for point-to-point connection, LAP-D can support multiple DEs connected via a multidrop line. Secondly, LAP-D supports UI frames, while LAP-B only supports sequenced information frames. Lastly, LAP-D uses a different addressing scheme. www.pragsoft.com Chapter 11: Integrated Services Digital Network 151

11.2.3. The Network Layer

The ISDN network layer is defined by CCITT recommendations I.450 and I.451. It supports circuit and well as packet switched connections between a TE and the network, and end-to-end. Figure 11.130 summarizes the ISDN network layer messages exchanged between TEs and the network over the D channel. Figure 11.131 illustrates the use of these messages in a typical scenario which involves various D channel signals for handling a B channel circuit-switched call between two subscribers with ISDN telephones. The scenario proceeds as follows. The calling subscriber picks up the receiver and dials the called subscriber’s number. The calling subscriber’s telephone accumulates the digits address and uses them as a parameter in a Setup message which it sends to the network. This message is conveyed by the network to the called subscriber’s telephone using SS7 signaling. At the same time, the network sends a Setup Acknowledge message to the calling telephone and asks for further information, which the latter provides using an Information message, and the network confirms, using a Call Proceeding message, that the call is being established. Figure 11.130 ISDN network layer messages. Message Purpose Setup Used by calling TE to convey to the called TE a request for call establishment. Setup Acknowledge Used by the network to acknowledge the connect request from the calling TE. Alerting Used by called TE to convey to calling TE that it has started alerting the user. Call Proceeding Used by the network to inform the calling TE that the call is being established. Connect Used by called TE to convey to calling TE that it has accepted the call. Connect Acknowledge Used by network to acknowledge the receiving of the connect message from the called TE. Detach Used for releasing the B channel without disposing of the call reference information. Detach Acknowledge Acknowledges the releasing of the B channel in response to a Detach message. Disconnect Used to indicate the completion of the call so that the channel and call reference may be released. Release Used to indicate that the channel and call reference have been released. Release Complete Used to indicate the acceptance of a Release request and therefore the releasing of the channel and call reference. Suspend Used by a TE to request from the network the suspension of an existing call. 152 Communication Networks Copyright © 2005 PragSoft Suspend Acknowledge Used by the network to acknowledge that the call has been suspended. Suspend Reject Used by the network to indicate that the call suspend request has been rejected. Resume Used by a TE to request the resuming of a previously suspended call. Resume Acknowledge Used by the network to acknowledge that the suspended call has been resumed. Resume Reject Used by the network to indicate that the call resume request has been rejected. User Information Used by a TE to exchange user information with another TE. Register Used by a TE to register itself for the use of a long-term network facility. Register Acknowledge Used by the network to acknowledge that the TE has been registered with the requested long-term facility. Register Reject Used by the network to reject a long-term facility registration requested by a TE. Facility Used by a TE to request the activation of a network facility. Facility Acknowledge Used by the network to acknowledge that the requested facility has been activated. Facility Reject Used by the network to reject a facility activation requested by a TE. Cancel Used by a TE to request the cancellation of a network facility. Cancel Acknowledge Used by the network to acknowledge that the requested facility has been canceled. Cancel Reject Used by the network to reject a facility cancellation requested by a TE. Information Used for providing additional information. Status Used during a call to report call-related conditions, including unexpected messages. Congestion Control Used for the flow control of messages exchanged between TEs. When the called phone starts ringing, it conveys an Alerting message via the network to the calling phone, which in response produces a ring tone to indicate that the destination phone is ringing. When the called subscriber picks up the receiver, its phone conveys a Connect message via the network to the calling phone. As a result, the ring tone stops and a B channel circuit is established for exchanging digitized speech. During this time, other information may also be conveyed over the D channel e.g., using User Information messages. The call is terminated when one of the two subscribers hangs up its phone. This causes a Disconnect message being conveyed to the other phone via the network. The network responds with a Release message, which the phone confirms with a Release Confirm. On the other side, the phone responds with a Release message, which the network confirms with a Release Confirm. Hence the call is completed and the B and the D channel are released. 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.