 be more specific, these break down as follows: 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 that 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-specific information follows the HDLC information. The packet information consists of the following information (this information is simi- lar to the to address and the designation of the routing that will be used, such as first class, book rate, and so on): General format identifier (GFI) Four bits of information that describe 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) Four bits that describe the grouping of channels. Because only 4 bits are available, only eight combinations are used. Logical channel number (LCN) An 8-bit description of the actual channel being used. The theoretical number of channels (ports) available is 2,048. Although the number of logical channels can be 2,048, most organizations implement significantly fewer ports or channels. The next overhead elements consist of the Packet Type Identifier (PTI), an 8-bit sequence that describes the type of packet being sent across the network. Six different packet types are used in an X.25-switching network. These packet types define what is expected of the devices across the network. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25, Internets, Intranets, and Extranets Chapter 7 The variable data field is now inserted. This is where the 128 bytes of information are contained in the packet. The 128-byte field is the standard implementation, but as mentioned, it can