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UNDERSTANDING AIRTIME PROTOCOLS THE PACKET REVISITED A packet is the information unit formed when a message is partitioned into more manageable sections for transmission and recovery. Most landline packets have three logical subsections created when control information is added to user data (.igure 13-1). Packets are not required to travel only on packet switched networks. They function quite nicely on circuit switched systems as well often a source of confusion for persons first encountering adaptive packet assembly in Microcom s protocols sent over circuit switched facilities. . | The Wireless Data Haadbaak Fourth Edition. James F. DeRose Copyright 1999 John Wiley Sons Inc. ISBNs 0-471-31651-2 Hardback 0-471-22458-8 Electronic IV SOME PRIMITIVE TECHNICAL CONSIDERATIONS The Wireless Data Haadbaak Fourth Edition. James F. DeRose Copyright 1999 John Wiley Sons Inc. ISBNs 0-471-31651-2 Hardback 0-471-22458-8 Electronic 13 UNDERSTANDING AIRTIME PROTOCOLS 13.1 THE PACKET REVISITED A packet is the information unit formed when a message is partitioned into more manageable sections for transmission and recovery. Most landline packets have three logical subsections created when control information is added to user data Figure 13-1 . Packets are not required to travel only on packet switched networks. They function quite nicely on circuit switched systems as well often a source of confusion for persons first encountering adaptive packet assembly in Microcom s protocols sent over circuit switched facilities. Since February 1997 GTE Wireless has offered CS-CDPD1 circuit switched CDPD which enables CDPD users to transmit packet data over existing circuit switched networks. 2 To grasp the principle of the packet the most common form of landline implementation will be discussed first. It is possible to transmit landline packets over radio links as is routinely done in data over cellular. As bit rates rise however error-checking codes become less satisfactory and are usually augmented with error correction codes. These differences will be explored second. 13.1.1 Message Segmentation The Flag When the user message is partitioned into packets a reserved separation character called the flag marks both the beginning and the end of the packet Figure 13-2 . The most common flag implementation found in such diverse protocols as SDLC HDLC MNP and CDPD is the 8-bit sequence 01111110. In these protocols the flag is one of three reserved patterns. The flag can also be manipulated in hardware by a technique known as bit stuffing which permits data transparency. These .
