network protocols handbook.pdf

192Protocols GuideWAN - Other WAN ProtocolsProtocol NameLAPB: Link Access Procedure,BalancedProtocol DescriptionLink Access Procedure, Balanced (LAPB) is a data link layerprotocol used to manage communication and packet framingbetween data terminal equipment (DTE) and the data circuitterminatingequipment (DCE) devices in the X.25 protocol stack.LAPB, a bit-oriented protocol derived from HDLC, is actually theHDLC in BAC (Balanced Asynchronous Class) mode. LAPBmakes sure that frames are error free and properly sequenced.LAPB shares the same frame format, frame types, and fieldfunctions as SDLC and HDLC. Unlike either of these, however,LAPB is restricted to the Asynchronous Balanced Mode (ABM)transfer mode and is appropriate only for combined stations.Also, LAPB circuits can be established by either the DTE orDCE. The station initiating the call is determined to be the primary,and the responding station the secondary. Finally, LAPBuse of the P/F bit is somewhat different from that of the otherprotocols.In LAPB, since there is no master/slave relationship, the senderuses the Poll bit to insist on an immediate response. In the responseframe this same bit becomes the receivers Final bit. Thereceiver always turns on the Final bit in its response to a commandfrom the sender with the Poll bit set. The P/F bit is generallyused when either end becomes unsure about proper framesequencing because of a possible missing acknowledgement,and it is necessary to re-establish a point of reference.LAPB’s Frame Types:• I-Frames (Information frames): Carry upper-layer informationand some control information. I-frame functionsinclude sequencing, flow control, and error detection andrecovery. I-frames carry send and receive sequencenumbers.• S-Frames (Supervisory Frames): Carry control information.S-frame functions include requesting and suspendingtransmissions, reporting on status, and acknowledgingthe receipt of I-frames. S-frames carry only receivesequence numbers.• U-Frames (Unnumbered Frames): Carry control information.U-frame functions include link setup and disconnection,as well as error reporting. U-frames carry nosequence numbers.Protocol StructureThe format of an LAPB frame is as follows:1 byte 1 byte 1-2 bytes variable 2 byte 1 byteFlagAddressfieldControl field Information FCS Flag• Flag - The value of the flag is always (0x7E). In orderto ensure that the bit pattern of the frame delimiterflag does not appear in the data field of theframe (and therefore cause frame misalignment), atechnique known as Bit Stuffing is used by both thetransmitter and the receiver.• Address field - In LAPB, the address field has nomeaning since the protocol works in a point to pointmode and the DTE network address is representedin the layer 3 packets.• Control field – Serves to identify the type of the frame.In addition, it includes sequence numbers, controlfeatures and error tracking according to the frametype.• Modes of operation - LAPB works in the AsynchronousBalanced Mode (ABM). This mode is totallybalanced (i.e., no master/slave relationship) and issignified by the SABM(E) frame. Each station mayinitialize, supervise, recover from errors, and sendframes at any time. The DTE and DCE are treated asequals.• FCS - The Frame Check Sequence enables a highlevel of physical error control by allowing the integrityof the transmitted frame data to be checked.• Window size - LAPB supports an extended windowsize (modulo 128) where the number of possible outstandingframes for acknowledgement is raised from8 to 128.Related protocolsLAPD, ISDN, X.25, Frame Relay, HDLC, SDLCSponsor SourceLAPB is defined by ISO (http://www.iso.org).Referencehttp://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/x25.htmX.25http://www2.rad.com/networks/1994/hdlc/hdlc.htmHigh Level Data Link Control