LAPB state Diagram

LAPB (Link Access Protocol, Balanced) is a data link layer protocol in the X.25 protocol stack. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. Image File history File links Size of this preview: 655 × 600 pixel Image in higher resolution (784 × 718 pixel, file size: 69 KB, MIME type: image/jpeg) The state diagram of the LAPB protocol File links The following pages on the English Wikipedia link to this file (pages on other... Image File history File links Size of this preview: 655 × 600 pixel Image in higher resolution (784 × 718 pixel, file size: 69 KB, MIME type: image/jpeg) The state diagram of the LAPB protocol File links The following pages on the English Wikipedia link to this file (pages on other... The data link layer is layer two of the seven-layer OSI model as well as of the five-layer TCP/IP reference model. ... For other senses of this word, see protocol. ... X.25 is an ITU-T standard protocol suite for wide area networks using the phone or ISDN system as the networking hardware. ... It has been suggested that the section HDLC from the article Measuring_data_throughput be merged into this article or section. ...

Link Access Protocol, Balanced (LAPB) is used to manage communication and packet framing between data terminal equipment (DTE) and the data circuit-terminating equipment (DCE) devices in the X.25 protocol stack. LAPB is nothing but HDLC in Asynchronous Balanced Mode (ABM). LAPB sessions can be established by either the DTE or DCE. The station initiating the call is determined to be the primary, and the responding station is the secondary. DTE: Abbreviation for data terminal equipment. ... The three-letter acronym DCE may stand for: Data Circuit-Terminating Equipment (DCE), also Data Communications Equipment (DCE) and Data Carrier Equipment (DCE). ... X.25 is an ITU-T standard protocol suite for wide area networks using the phone or ISDN system as the networking hardware. ... Asynchronous Balanced Mode (ABM) means a communication mode of HDLC and derivative protocols, supporting peer-oriented point-to-point communications between two nodes, where either node can initiate transmission. ...

In LAPB, since there is no master/slave relationship, the sender uses the Poll bit to insist on an immediate response. In the response frame this same bit becomes the receivers Final bit. The receiver always turns on the Final bit in its response to a command from the sender with the Poll bit set. The P/F bit is generally used when either end becomes unsure about proper frame sequencing 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): Carries upper-layer information and some control information. I-frame functions include sequencing, flow control, and error detection and recovery. I-frames carry send and receive sequence numbers.

S-Frames (Supervisory Frames): Carries control information. S-frame functions include requesting and suspending transmissions, reporting on status, and acknowledging the receipt of I-frames. S-frames carry only receive sequence numbers.

U-Frames (Unnumbered Frames): carries control information. U-frame functions include link setup and disconnection, as well as error reporting. U-frames carry no sequence numbers

Protocol structure

The format of LAPB frame is as follows:

Image File history File links LAPB.jpg Licensing I, the creator of this work, hereby release it into the public domain. ...

Flag - The value of the flag is always (0x7E). In order to ensure that the bit pattern of the frame delimiter flag does not appear in the data field of the frame (and therefore cause frame misalignment), a technique known as Bit Stuffing is used by both the transmitter and the receiver.

Address field - In LAPB, this field has no meaning since the protocol works in a point to point mode and the DTE network address is represented in the layer 3 packets. This byte is therefore put to a different use; it separates the link commands from the responses and can have only two values: 0x01 and 0x03. 01 identifies frames containing commands from DTE to DCE and responses to these commands from DCE to DTE. 03 is used for frames containing commands from DCE to DTE and for responses from DTE to DCE. Therefore, one side must be configured as a Layer 2 DTE and the other as a Layer 2 DCE (you must not confuse this with the more familiar Layer 1 DCE and DTE designations).

Control field - it serves to identify the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type.

Modes of operation - LAPB works in the Asynchronous Balanced Mode (ABM). This mode is totally balanced (i.e., no master/slave relationship) and is signified by the SABM(E) frame. Each station may initialize, supervise, recover from errors, and send frames at any time. The DTE and DCE are treated as equals.

FCS - The Frame Check Sequence enables a high level of physical error control by allowing the integrity of the transmitted frame data to be checked.

Window size - LAPB supports an extended window size (modulo 128) where the number of possible outstanding frames for acknowledgement is raised from 8 to 128.

Related Protocols LAPB , ISDN , X.25, Frame Relay , SDLC , HDLC ISDN is also short for isosorbide dinitrate Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network system, designed to allow digital (as opposed to analog) transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds, than available with analog... X.25 is an ITU-T standard protocol suite for wide area networks using the phone or ISDN system as the networking hardware. ... In the context of computer networking, frame relay (also found written as frame-relay) consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... SDLC can refer to: Synchronous Data Link Control System Design Life Cycle (or System Development Life Cycle) This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... It has been suggested that the section HDLC from the article Measuring_data_throughput be merged into this article or section. ...

LAPB address

LAPB commands and responses

Sponsor Source

LAPB is defined by ISO (http://www.iso.org ).

External links

• http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/x25.htm
• http://www2.rad.com/networks/1994/hdlc/hdlc.htm

Bibliography

Computer Communications, by Chaim Zieglier, PhD. Brooklyn College Brooklyn College is a senior college of the City University of New York, located in Brooklyn, New York. ...