network protocols handbook.pdf

172Protocols GuideWAN - Broadband Access ProtocolsProtocol NameLAP-D: ISDN Link Access Protocol-ChannelDProtocol DescriptionLAP -D is the Layer 2 protocol in the ISDN suite, almost identicalto the X.25 LAP-B protocol. The three logical digital communicationchannels of ISDN perform the following functions:• B-Channel - Carries user service information including:digital data, video, and voice.• D-Channel - Carries signals and data packets betweenthe user and the network• H-Channel - Performs the same function as B-Channels,but operates at rates exceeding DS-0 (64 Kbps).The Link Establishment process in ISDN performed by LAP-Dis as follows:1. The TE (Terminal Endpoint) and the Network initially exchangeReceive Ready (RR) frames, listening for someoneto initiate a connection2. The TE sends an Unnumbered Information (UI) framewith a SAPI of 63 (management procedure, query network)and TEI of 127 (broadcast)3. The Network assigns anavailable TEI(in the range 64-126)4. The TE sends a Set Asynchronous Balanced Mode(SABME) frame with a SAPI of 0 (call control, used toinitiate a SETUP) and a TEI of the value assigned by thenetwork5. The network responds with an Unnumbered Acknowledgement(UA), SAPI=0, TEI=assigned.The LAPD is defined in CCITT Q.920/921. LAPD works in theAsynchronous Balanced Mode (ABM). This mode is totally balanced(i.e., no master/slave relationship). Each station may initialize,supervise, recover from errors, and send frames at anytime. The protocol treats the DTE and DCE as equals.Protocol StructureThe format of a standard LAPD frame is as follows:FlagAddressfieldControl field Information FCS Flag• Flag - The value of the flag is always (0x7E).” BitStuffing” technique is used in order to ensure that thebit pattern of the frame delimiter flag does not appearin the data field of the frame.• Address field - The first two bytes of the frame afterthe header flag are known as the address field. Theformat of the address field is as follows:1 2 3 4 5 6 7 8SAPI (6 bits) C/R EA0TEI (7 bits)EA1• SAPI (Service access point identifier), 6-bits (seebelow)• C/R (Command/Response) bit indicates if theframe is a command or a response• EA0 (Address Extension) bit indicates whetherthis is the final octet of the address or not• TEI (Terminal Endpoint Identifier) 7-bit deviceidentifier (see below)• EA1 (Address Extension) bit, same as EA0• Control field - The field following the Address Fieldis called the Control Field and serves to identify thetype of the frame. In addition, it includes sequencenumbers, control features and error tracking accordingto frame type. The following are the SupervisoryFrame Types defined in LAPD:RRREJRNRInformation frame acknowledgement and indication to receivemore.Request for retransmission of all frames after a given sequencenumber.Indicates a state of temporary occupation of station (e.g.,window full).• Some Unnumbered Frame Types supported in LAPDare – DISC (Request disconnection), UA (Acknowledgementframe), DM (Response to DISC indicatingdisconnected mode), FRMR (Frame reject), SABM,SABME, UI and XID.• FCS -The Frame Check Sequence (FCS) enables ahigh level of physical error control by allowing the integrityof the transmitted frame data to be checked.The sequence is first calculated by the transmitterusing an algorithm based on the values of all the bitsin the frame. The receiver then performs the samecalculation on the received frame and compares itsvalue to the CRC.• Window size - LAPD supports an extended windowsize (modulo 128) where the number of possible outstandingframes for acknowledgement is raised from8 to 128. This extension is generally used for satellitetransmissions where the acknowledgement delayis significantly greater than the frame transmissiontime. The type of the link initialization frame determinesthe modulo of the session and an “E” is addedto the basic frame type name (e.g., SABM becomesSABME).