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Distance limitations

I hate to ask this question but it will settle a bet with me and a friend of mine.

My friend told me that the distance limitiations for fast ethernet is 600m when using a switch instead of a hub over Cat5 copper cabling. (He said he was reading straight from a Network Associated Sniffer book...hehe) I told him that switch or hub does not make a difference here...its the fast ethernet protocol itself which makes a difference.....and the limitation is 100m when running over copper.

Could someone valididate that I am correct.

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Re: Distance limitations

I'm with you on this bet. I think that nearly everyone else in the connector-crimping, switch-configuring, access-control-list-writing network engineering world would side with you, too.

"The 100BASE-TX media system is designed to allow segments of up to 100 meters in length when using data grade unshielded twisted-pair wire that has a characteristic impedance of 100 ohms and meets the EIA/TIA Category Five wire specifications. Segments of 100BASE-TX are limited to a maximum of 100 meters to ensure that the round trip timing specifications are met. This is in contrast with the 10BASE-T media system, where the maximum segment length for the 10-Mbps link is mostly limited by signal strength.

"For example, if you use high quality twisted-pair cable in a 10BASE-T segment, it's possible to reach segment lengths of approximately 150 meters with success. This is not true in the Fast Ethernet system, where the segment length for twisted-pair segments is set at a maximum of 100 meters for signal timing reasons. The EIA/TIA cabling standard recommends a segment length of 90 meters between the wire termination equipment in the wiring closet, and the wall plate in the office. This provides 10 meters of cable allowance to accommodate patch cables at each end of the link, signal losses in intermediate wire terminations on the link, etc."

from the Quick Reference Guides to 100 Mbps Fast Ethernet by Charles Spurgeon (this is the guy who also wrote the O'Reilly book "Ethernet: The Definitive Guide")

http://www.ethermanage.com/ethernet/100quickref/ch10qr_4.html#HEADING3

Some other useful links for learning about Ethernet (including Fast Ethernet and Gigabit Ethernet) without actually plowing through the specifications themselves:

http://www.ethermanage.com/ethernet/ethernet.html

http://www.ethermanage.com/ethernet/quickref.html

http://www.ethermanage.com/ethernet/edg/edg.html

Your friend might very well have read that it is possible to push Fast Ethernet to 600m using a switch, over Category 5 cabling. However, that kind of of long-distance cable run would be non-compliant with both the IEEE 802.3u 100BASE-TX specifications, and the ANSI/TIA/EIA-568A (1995) Category 5 cabling guidelines. Sure, it may work. But anyone who chooses to actually rely on such an excessively long link in a production network seriously needs to have his/her head examined.

I have lost count of the number of network problems I have troubleshot where excessive cable length was the source of the problem, or was at least a major contributing factor. People too cheap to put mid-span repeaters or switches in; or too cheap to create multiple wiring closets/intermediate distribution frames on a floor, and link them back to a central one over fiber. Or people just plain cutting corners or being lazy about the cabling installation. If you want the network to work properly, you have to build it right.

They write standards and specifications for a reason.

Any Fast Ethernet link distance over 100m should be fiber. Multimode fiber (MMF) is good to 2km in full duplex, 412m in half duplex; single mode fiber (SMF) can go even further.

Hope this helps.

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