Introduction to Data Communications
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34. Basic Frame Structure

The Generic Packet X is used as an introduction to Synchronous Data Transmission. As we explore more standards and protocols, we find that we can expand the frame structure (packet) into better defined sections that will allow easier understanding of different frame types (Ethernet, Token Ring, SDLC, HDLC, Frame Relay, ATM, Cell Relay, etc...). It also will provide a point of reference.

Basic Frame Structure

34a. Preamble: Starting Delimiter/Alert Burst/Start of Header

At the beginning of each frame (packet), there will be a sequence of octets (8 bit words), called the Preamble. The Preamble is used to:

Inform the receiving station that a new packet is arriving
Synchronize the receive clock with the transmitted clock

The Preamble is a series of octets with a specific bit pattern that is used only by the Preamble.

Names used by other protocols for the Preamble are: Starting Delimiter, Alert Burst and Start of Header. All perform the same 2 basic functions.

34b. Address Field(s): Source and/or Destination

The Address Field consists of a Source Address and/or a Destination Address. The Source and Destination Addresses are hexadecimal numbers that identify the sender - Source and receiver - Destination. The Network Addresses reside in either the Network Interface Card's firmware or can be either assigned during the initialization of the NIC.

The purpose of the Source Address is to identify to the network who is sending data. The purpose of the Destination Address is to identify to the network who should be receiving the data.

Under some protocols, there may not be both Source and Destination Addresses. Only one address may be present.

Introduction to Data Communications
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