Ask Professor Phil: Why is There No Agreement About Ethernet Frames?

Dear Professor Phil,

Since the AV world has gone a little crazy over Ethernet and IP, I decided to try to learn more about each by using books and web sources. To my surprise, it seems as though there is not uniform agreement about what is and what is not included in an Ethernet frame. For example, some sources talk about a preamble; others discuss tags. However, IP descriptions seem to be uniform. Am I confused?
—Girish, Greensboro, NC


Let’s start with what was in the original frame when Ethernet was first invented. Then, we’ll look at variations that were introduced to overcome some of the limitations of the original design.

The original DIX Ethernet, which preceded any formal standard, appeared in around 1978. DIX referred to the three companies that sponsored its development: DEC (Digital Equipment Company), Intel, and Xerox. They wanted a packet type network technology that would be simple but still be able to carry data at megabit speeds. The fastest existing standard technology operated at 56kb/sec. What resulted was a simple frame with fields that addressed the following points:

1) Which station is sending the frame

2) Which station should receive the frame

3) What type of payload does it contain

4) The payload

5) A code used to insure that the frame was transmitted without errors

The IEEE created a series of committees to study proposals such as the one from the DIX group. Committee 802.3 specified the electrical, mechanical, physical and functional aspects of Ethernet and ratified it as a standard. For nearly a decade, engineers saw no need to make major modifications to the standard. IP and TCP, which were often used along with Ethernet, seemed to supply the other necessary attributes. These were providing for:

1) Priority delivery

2) Segmentation of the network stations

3) Retransmission of lost data

4) Proper flow control

Yet, network engineers gradually wanted some way to incorporate some of these attributes into the network Ethernet switches. This would eliminate the need for more complex routers and gateways. So, over time, capabilities were added at the Ethernet level by modifying the original frame format. For example, tags (additional fields) were added to create:

• VLAN segmentation (802.1q)

• Priority delivery (802.1p)

• Route establishment (SPB - 802.1aq)

• Timing (AVB - 802.1as)

  • You also mentioned the preamble. This is a synchronization pattern that contains no real data. Some books include it; others do not.

Phil Hippensteel, PhD, is an instructor at Penn State Harrisburg & a regular contributor to AV Technology Magazine. Send your tech questions to pjh15[at]