Management of the canal networks to packet switched AX.25 is done by the protocol an HDLC (the High-a level the Data Link the Control).
According to smartercomputing, an HDLC frame has the following structure consisting of six fields: FLAG | ADDRESS |CONTROL | PID + DATA | FCSFLAG | FLAG-this is a unique sequence of bits (01111110), which serves to define the boundaries of the frame.This sequence is reserved for this purpose and are taken elsewhere in the frame does not appear such a combination. ADDRESS-This field specifies the destination address. AX.25 uses a minimum of 14 and maximum of 70 bytes containing the address of the sender address of the recipient and where necessary to 8 call of repeaters. CONTROL-This is a byte, which determines the type of frame. AX.25 protocol in this field would contain the number of frames in one or two three-bit fields. PID – ID protocol. This is the first byte in the data field. DATA-This field contains the data to be transmitted. The existence of this field is optional. FCS – Shestnadesetbitovo field control.
LAPB (Link the Access Procedure, Bed-Channel) allows the parties DTE and DCE to make contact with each other. In the process of transmission of information LAPB control whether block data received in proper sequence and without error in the receiver.
Similarly, the minutes of the channel level, LAPB uses three types of formats for data blocks:
- The data block (Information (I) frame);
- Data blocks supervisor (Supervisory (S) frames);
- Unnumbered data units (Unnumbered (U) frames).
LAPD (Link Access Procedure, D-channel) is similar to the HDLC and LAPB. LAPD is used in the D-channel ones to provide flow and corresponding receiving control and signaling information. The shape of the block data LAPD lot like form HDLC; Also, as HLDC, LAPD uses blocks of data supervisor, Information and nepronumerovanite data blocks. LAPD protocol is formally defined in CCITT Q.920 and Q.921 SSITT.
LLC, (Logical Link the Control)-allows part of Datalink Layer operate independently of technology. Participate in the process not encapsulation. Process data from network protocols and IP packets and adds control information. Add DSAP (destination service access point) and SSAP (source service access point).