Active TopicsActive Topics  Display List of Forum MembersMemberlist  Search The ForumSearch  HelpHelp
  RegisterRegister  LoginLogin
Ask Professor William
 Industrial Ethernet University Forum : Ask Professor William
Subject Topic: Minimum cable distance for UTP Post ReplyPost New Topic
Author
Message << Prev Topic | Next Topic >>
jonesnco
Newbie
Newbie


Joined: 03 September 2007
Location: United States
Posts: 1
Posted: 03 September 2007 at 9:24pm | IP Logged Quote jonesnco

I have been trying to find specific answers related to the minimum cable length required for 802.3 Ethernet, Fast Ethernet and Gigabit Ethernet when using UTP.  I have found non-IEEE sources stating Fast Ethernet has a 2.5m minimum cable length when using Cat5 (or better) UTP.  I have been unable to find anything on Gigabit Ethernet requirements in this regard.  I realize this is somewhat of an esoteric topic but I would assume you have probably stumbled across it in some IEEE 802.3 standards documents at some time and might remember.

Thanks. 

Dan

Back to Top View jonesnco's Profile Search for other posts by jonesnco
 
Professor W.
Admin Group
Admin Group
Avatar
Professor

Joined: 13 November 2003
Location: United States
Posts: 7
Posted: 07 September 2007 at 12:50pm | IP Logged Quote Professor W.

Various networking technologies specify that stations must be separated by some minimum distance. Occasionally a customer is surprised about the lack of a minimum length requirement for their CAT 5 cable that connects one Industrial Ethernet device to another.

Network cable minimum length specifications arise because of an impedance mismatch between elements in the signal path. When electrical energy is transmitted through a cable, some of it is not absorbed by the receiving station and is instead "reflected" back to the source. This reflected energy disturbs the purity of the signal -- and if the effect is of sufficient magnitude, the signal can be degraded to the extent that its information is corrupted or even useless. If source and destination transceiver impedances match the impedance of the cable itself, there is no reflected energy and signal energy is delivered completely.

Reflected signal is always an issue in a bus topology because only the end devices have transceiver impedance values that match the impedance of the cable. Mid-bus device transceivers must have higher impedance values to avoid "loading" the cable (preventing proper signal delivery to devices further along the bus). Thus, a mid-bus device transceiver presents an impedance mismatch to the cable and inevitably creates some degree of reflected energy. To keep the reflected energy to acceptable levels, a bus protocol will specify a minimum length of cable that separates two bus devices.

Industrial Ethernet uses a star topology in which devices exist only at each end of a cable. With this topology, both transceiver impedances match the cable impedance and no appreciable signal reflection occurs. This lack of signal reflection is true whether the devices are separated by 100 m or 100 mm. Therefore, there is no need to specify a minimum cable length.
Back to Top View Professor W.'s Profile Search for other posts by Professor W. Visit Professor W.'s Homepage
 

If you wish to post a reply to this topic you must first login
If you are not already registered you must first register

  Post ReplyPost New Topic
Printable version Printable version

Forum Jump
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot delete your posts in this forum
You cannot edit your posts in this forum
You cannot create polls in this forum
You cannot vote in polls in this forum

Powered by Web Wiz Forums version 7.9
Copyright ©2001-2004 Web Wiz Guide

This page was generated in 0.1563 seconds.