|This page was nominated for deletion on 5 January 2010 (UTC). The result of the discussion was keep.|
|WikiProject Computing / Networking||(Rated Start-class, Low-importance)|
This article seems a lot like
- Answers.com is a wikipedia mirror. The bottom of the page identifies the material as coming from Wikipedia. -- Whpq (talk) 14:25, 12 January 2010 (UTC)
The following uncited rationale for the 5-4-3 rul has been supplied (twice) by user:220.127.116.11
- A repeater normally listens for the 0101 preamble and then locks onto the bit stream. Once locked on it would then repeat each bit out the other port(s). However some number of bits would be lost at the start while the repeater was locking onto the bit stream. As the frame propagated through each repeater the preamble would get shorter and shorter. Too many bits lost meant that NIC cards may not have enough preamble bits to lock on and the entire frame would be missed.
- As each manufacturer of repeaters (hubs) did slightly different implementations each repeater operated differently. Each different repeater would lose more or less bits while locking on. Some would lose 1 or 2 while others could lose as many as 5 or 6 bits. You could create a network with more repeaters if you knew how each repeater operated. However this information was not always easy to obtain and difficult for users to calculate. Hence the 5-4-3 rules was created. It was easy to understand and worked in all cases. In a lab at DEC they knew how many bits their repeaters would lose and knowing this were able to create an 11 segment, 10 repeater, 3 active segment (11-10-3) network that was completely viable.
Clause 18.104.22.168 of IEEE 802.3-2008 describes repeater requirements and doesn't appear to allow shortening of the preamble. Here's another ref that supports a requirement to transmit a full preamble: . A couple other refs are less clear and may support the claims above: , .
I believe the primary purpose of the rule is to insure collision detection works properly. ~KvnG 15:35, 2 June 2013 (UTC)
- The preamble shortening by repeaters does happen but usually it's not an issue and therefore it gets ignored, the collision detection being a far more important point.
- The preamble actually is longer than necessarily required – different hardware needs different number of cycles to sync – and any repeater will lose the cycles it required to sync and can't make them up since the frame is following right behind. After some research I've found a source for this in the 802.3 standard and will add it to the page in a minute. Zac67 (talk) 16:32, 2 June 2013 (UTC)
This page may or may not be right in the case of coaxial and FOIRL links, but isn't at all close for 10baseT.