Media Access in Ethernet
Caveats and References
The media access control protocol for Ethernet local
area networks is called CSMA/CD (Carrier Sense Media
Access/Collision Detection).
These notes and applet are intended to provide an intuitive
understanding of the key concepts of that protocol.
They are not intended to be a precise description of
CSMA/CD.
Moreover, I have chosen not to include some of the
details concerning how CSMA/CD is implemented in the
Ethernet specifications.
Some of the places where I have taken liberties follow.
-
I do not allow a station to start transmitting a
new frame until the last bit of its previous frame
has reached both ends of the link.
Real Ethernet might allow a new transmission by the
same station to start earlier.
-
I have never seen a description of Ethernet give a
special name for the time period (which I refer to as the
message-in-progress phase) between when a station finishes
transmitting a frame and the last bit of that frame
reaches both ends of the link.
-
The particular values for minimum frame size,
link bandwidth, and link length that work together
for the applet have no intentional correspondence
to the particular values found in any of the versions
of the Ethernet specification.
For example, the minimum frame size that works need
not be 512 bits.
-
There are several different specifications for Ethernet
that differ on the physical media used as well as
the link bandwidth.
The calculations determining the maximum link length
(called the "collision domain diameter") for a given
minimum frame size are more complex that I have indicated.
The following references (and the IEEE Ethernet standards)
provide more detailed and descriptions of CSMA/CD and
Ethernet.
- L. Cassel and R. Austing,
Computer Networks and Open Systems, An Applications Perspective,
Jones and Bartlett, 2000.
- J.F. Kurose and K.W. Ross,
Computer Networking: A Top-Down Approach Featuring
the Internet, Second Edition,
Addison Wesley, 2003.
- L.L. Peterson and B.S. Davie,
Computer Networks: A Systems Approach, Third Edition,
Morgan Kaufmann, San Francisco, CA, 2003.
Acknowledgements and Developer Information
Current Version: February 2004
Copyright (C) Mark A. Holliday. All rights reserved.