Introduction
LAP-B: Link Access Process Balance
(LAP-B: LINK Access Procedure Balanced for X.25)
Link Access Process Balance (LAP-B) is a data link layer protocol, which is responsible for managing the organization of communication and data packet frames between DTE devices and DCE devices in X.25.
LAP-B share the same frame format, frame type, and field functions as SDLC and HDLC, but the latter is different from the LAP-B is limited by the ABM transmission mode and is only applicable to combination stations. . The LAP-B circuit can be established by DTE or DCE. Start the call to the station called the primary station, the other station of the response is called the secondary station. Also different from the P / F bit of Lap-b.
In LAP-B, since there is no main slave relationship, the sender uses the POLL bit to request immediate response. In the response frame, this bit is turned into the final ratio of the receiving end. The receiving end always opens the Final bit to respond to commands from the sender Poll bit. Since the confirmation response may be lost and cause any end that the frame cannot be ensured whether the frame is properly sorted, the P / F ratio is used, and the reference point is required.
LAP-B frame type
LAP-B frame type:
• Information frame (I-frame) transmits high-level protocol information and some control information, mainly Function is sorted, control traffic, error monitoring, and recovery, which carries sends and receives serial numbers.
· Monitor frame (S-frame) transmit control information, the main function is to request and suspend transmission, report status information and confirmation reception I-frame, which only carries the reception number.
· Non-digital frame (U-frame) Carry control information, the main function is to establish and terminate links and report errors, which do not carry the serial number.
protocol structure
LAP-B frame format is as follows:
1 byte 1 byte 1-2 Bytes variable 2 bytes 1 byte
Flag Address Field Control Field Information FCS flag
· Flag - This field value is 0x7e. To ensure that the bit pattern of the frame separator is not appearing in the frame of the frame, the Bit Stuffing technology is usually utilized at the sender and the recipient.
· Address Field - In LAP-B, all Address Field does not make practical significance because the protocol works in point-point mode. The DTE network address is described in the third layer packet.
· Control Field - Identify the frame type. In addition, according to the frame type division, this field also includes serial number, control characteristics, and error tracking.
· MODES OF OPERATION - LAP-B is working in asynchronous balancing mode (ABM). This mode is completely balanced (that is, there is no master / slave relationship) and it is represented by the SABM (E) frame format. Each station is likely to initialize, supervise management, error recovery, and sending frames. DTE and DCE will treat it for the same.
• FCS - Frame Quality Sequence (FCS) field allows high-level physical error control to be verified by license transmission frame data.
• Window Size - LAPB supports the extension window size (analog to 128), confirming the size of the frame may be extended from 8 to 128.
protocol structure: LAPD, ISDN, X.25, Frame Relay, HDLC, SDLC