Control I/S/ or U : Control information consists of 8 bits of data describing the type of HDLC frame.
• Information (I). Used to transfer data across the link at a rate determined by the receiver and with error detection and correction.
• Supervisory (S). Used to determine the ready state of the devices receiver is ready (RR), receiver is not ready (RNR) or reject (REJ).
• Unnumbered (U). Used to dictate parameters, such as set modes, disconnect and so on.
Packet information : The packet information consists of GFI, LGN and LCN.
• General format identifier (GFI). 4 bit of information that describes how the data in the packet is being used, from/to an end user, from/to a device controlling the end user device and so on.
• Logical channel group number (LGN). 4 bits of information that describe the grouping of channels.
• Logical channel number (LCN). 8 bit of information of the actual channel being used. Theoretical number of logical channels available is 2048. But most organizations implement significantly fewer ports or channels.
Packet type identifier (PTI). It is an 8 bit sequence that describers the type of packet being sent across the network. Six types of packets are used in X-25 switching network. They are call request, call accept, clear request, interrupt request, reset request and restart request.
Variable data field. 128 byte field is the standard implementation. But it can be as much as 512 bytes.
Cyclic Redundancy Check (CRC). The 16 bit sequence is used for error detection and/or correction.
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