# Influencing minimum frame lengh

Feb 11th, 2007

Hi All,

THE QUESTION:

How the distance between stations and the number of repeaters influence the minimum frame lenght in an ethernet LAN?

The fundemental rule of Ethernet is that when one station is transmitting - all other stations in the same collision domain must be able see the transmission and recognize the medium as busy.

If you have a very short frame size it only takes a short amount of time to transmit that fr ame. If you have a very long distance between stations and a very small frame size, it is possible for the transmitting station to finish transmitting the frame before the far end station starts to receive the frame. The far end station might decide to transmit while the other frame is still on the way down the wire. The two frames would collide somewhere near the middle causing a late collision. Furthermore, since each station finished transmitting their frames before the collision occured, there would be no way for the stations to know for sure that the frames they transmitted where corrupted, and therefore would not attempt to retransmit the frame, resulting in lost data.

The moral of the story is that there is a direct relationship between minimum frame size and distance between stations. The shorter the distance between stations, the shorter the minimum frame size can be. The longer the distance between stations, the larger the minimum frame size should be.

Since the 802.3 IEEE commitee decided that 64 bytes was a desireable minimum frame size, this resulted in a calculation that provides us with the distance and repeater limitations we have today.

Hope this helps. Please rate this post ;)

Overall Rating: 4.5 (3 ratings)

## Replies

bhedlund Sun, 02/11/2007 - 19:29

The fundemental rule of Ethernet is that when one station is transmitting - all other stations in the same collision domain must be able see the transmission and recognize the medium as busy.

If you have a very short frame size it only takes a short amount of time to transmit that fr ame. If you have a very long distance between stations and a very small frame size, it is possible for the transmitting station to finish transmitting the frame before the far end station starts to receive the frame. The far end station might decide to transmit while the other frame is still on the way down the wire. The two frames would collide somewhere near the middle causing a late collision. Furthermore, since each station finished transmitting their frames before the collision occured, there would be no way for the stations to know for sure that the frames they transmitted where corrupted, and therefore would not attempt to retransmit the frame, resulting in lost data.

The moral of the story is that there is a direct relationship between minimum frame size and distance between stations. The shorter the distance between stations, the shorter the minimum frame size can be. The longer the distance between stations, the larger the minimum frame size should be.

Since the 802.3 IEEE commitee decided that 64 bytes was a desireable minimum frame size, this resulted in a calculation that provides us with the distance and repeater limitations we have today.

Hope this helps. Please rate this post ;)

albert.remo Sun, 02/11/2007 - 19:40

Hi,

Originally, Ethernet has a frame length of not more than 1518 thus we come up with an MTU of 1500. Some equipment now supports a frame size of more than 1500 to support various applications. I think distance does not directly influence this size instead it influences the reliability on how these frames are transported. For example, 10Base5 is designed to be reliable of up to 500m and exceeding that limit would cause some frames not to arrive to the destination completely and may encounter some loss or errors. Fiber connections can be reliable up to as far as 10Km without loss or errors. Repeaters amplifies the signal from the source going to the destination thus extending the distance that the frame can travel.

Some people may limit their MTU to a lower value so that smaller frames may traverse through the transmission link (minimizing the chance of losing large frames during transmission problems) but this will incurr more headers with the same amount of payload thus is not advisable.

To be able to have a reliable transmission, always follow the standards and try using reliable transport protocols. Check on TCP and UDP and various applications that uses these transport protocols.

Attached are some information on IEEE standards, hope these helps.

Regards,

Albert

Attachment:
microcisconet Sun, 02/11/2007 - 19:57

Hi Guys,

I'm most greatful for your contributions and mentorring.

Thanks alot

bhedlund Sun, 02/11/2007 - 19:58

Thanks,