nted transport based on a virtual circuit. The Packet Concept If an organization has large amounts of data to send, then the data can be delivered to a packet assembler/disassembler (PAD). The PAD can be a soft- ware package or a piece of hardware outboard of the computer system. The PAD acts as the originating mail clerk in that the originating PAD receives the data and breaks it down into manageable pieces or packets. In the data communications arena, a packet can be a variable length of information, usually up to 128 bytes of data (one page in the example). Other imple- menters of X.25 services have created packets up to 512 bytes, but the aver- age is 128, as shown in Figure 7-1. The 128-byte capability is also referred to as a fast select. The packet-switching system can immediately route the packet to a distant end and pass data of up to 128 bytes (1,024 bits). Overhead The PAD then applies some overhead to the packet as follows: An opening flag that is made up of 8 bits of information. Using a standard high-level data link control (HDLC) framing format, the HDLC Frame Figure 7-1 Layer 2 OSI The X.25 packet. X.25 Packet Layer 3 OSI Opening Information Address CRC 16 Closing (Packet Data) Bits: 1024* * Variations can exist using 4,096 bits of data I Information S Supervisory U = Unnumbered 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 X.25, Internets, Intranets, and Extranets opening flag is a sequence of 8 bits that should not be construed as real data. A 16-bit address sequence that is a binary description of the endpoints (the from address). Control information consists of 8 bits of data describing the type of HDLC frame that is traversing the network (this is a notation on the enve- lope that describes the information inside). These can be supervisory, unnumbered, or information fields. To