LAP-D: ISDN Link Access Protocol- Channel D

LAP -D is the Layer 2 protocol in the ISDN suite, almost identical to the X.25 LAP-B protocol. The three logical digital communication channels 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 between the 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-D is as follows:

1. The TE (Terminal Endpoint) and the Network initially exchange Receive Ready (RR) frames, listening for someone to initiate a connection
2. The TE sends an Unnumbered Information (UI) frame with 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 to initiate a SETUP) and a TEI of the value assigned by the network
5. The network responds with an Unnumbered Acknowledgement (UA), SAPI=0, TEI=assigned.

The LAPD is defined in CCITT Q.920/921. LAPD works in the Asynchronous 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 any time. The protocol treats the DTE and DCE as equals. 

Protocol Structure - LAP-D: ISDN Link Access Protocol- Channel D

The format of a standard LAPD frame is as follows:

• Flag - The value of the flag is always (0x7E).” Bit Stuffing” technique is used in order to ensure that the bit pattern of the frame delimiter flag does not appear in the data field of the frame.

• Address field - The first two bytes of the frame after the header flag is known as the address field. The format of the address field is as follows:

• SAPI(Service access point identifier), 6-bits (see below)
• C/R(Command/Response) bit indicates if the frame is a command or a response
• EA0(Address Extension) bit indicates whether this is the final octet of the address or not
• TEI(Terminal Endpoint Identifier) 7-bit device identifier (see below)
• EA1(Address Extension) bit, same as EA0
• Control field - The field following the Address Field is called the Control Field and serves to identify the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type. The following are the Supervisory Frame Types defined in LAPD:

Some Unnumbered Frame Types supported in LAPD are – DISC (Request disconnection), UA (Acknowledgement frame), DM (Response to DISC indicating disconnected mode), FRMR (Frame reject), SABM, SABME, UI and XID.

FCS -The Frame Check Sequence (FCS) enables a high level of physical error control by allowing the integrity of the transmitted frame data to be checked. The sequence is first calculated by the transmitter using an algorithm based on the values of all the bits in the frame. The receiver then performs the same calculation on the received frame and compares its value to the CRC.

Window size - LAPD supports an extended window size (modulo 128) where the number of possible outstanding frames for acknowledgement is raised from 8 to 128. This extension is generally used for satellite transmissions where the acknowledgement delay is significantly greater than the frame transmission times. The type of the link initialization frame determines the modulo of the session and an "E" is added to the basic frame type name (e.g., SABM becomes SABME).

The LAP-D protocol is based on standards developed by the ITU Q-series documents .

http://www.nationalisdncouncil.com/isdnassistance : The National ISDN Council (NIC) specification
http://www.catcouncil.org/isdn/index : The Council for Access Technologies (CAT formerly NIC)specification and documents.