Max length of 100BaseT Full Duplex conn with CAT5e

[bcarrero]

Hi,

everybody knows that the segment max length for a 100BaseT connection is +/- 100 meters.

Is this because of the risk of collisions due to the delay introduced by the copper cable? (Making it compulsory for half duplex connections)

Does the length limitation applies to FullDuplex mode? If this is so, is it because of the degradation of the signal when it travels through the wire?

Thanks in advance,

B

[jasyoung]

The limit isn't due to attenuation or degredation, but timing. If a cable is longer than 100 meters, then under some circumstances the CSMA/CD (Carrier Sense Multiple Access with Carrier Detection) algorithm can break down.

Electrical signals travel at finite speed, so if it takes too long for a signal to get from here to there, it's possible to get into a situation where stations collide after the first 64 bytes of a frame have already gone out. This is known as a late collision. This is a protocol violation, and it's bad because it's quite possible that the sending station will not notice that a collision has taken place and won't try to retransmit the frame - resulting in frame loss.

CSMA/CD doesn't apply to full duplex mode. To answer your question, it's possible to "cheat" the standard length limitation to some extent especially when running full duplex. I do not know of any guidance as to "how much". Are you going to get 110 meters? Most likely. Are you going to get 500 meters? No. Cable quality and number of splices and patches, etc, come into play here.

[scottmac · Accepted solution]

The 100 meter specification does indeed apply to factors such as attenuation and crosstalk (usually expressed as the ACR - Attenuation to Crosstalk Ratio).

If you lose enough signal through loss/attenuation on the receives side of the link, the crosstalk from the same port transmitting can interfere with the received signal ... especially in full duplex situations.

To be within spec, a category-rated UTP span will do 100 meters. Better quality cabling / premium quality cabling may give you more headroom, but even so, just like nearly everything else in the industry, intentionally violating a specification can come back to bite you later.

Also be aware that the spec applies as follows:

(------------------) (----------------------) (-------------------)

5M stranded 90 M Solid 5M Stranded

Jumper "In-Wall" Jumper

Stranded cable has MUCH greater loss than soid conductor and is NOT suitable for the long segment of the span.

All components should be at the same rating (Cat5, 5e, 6) - including the clear plastic plugs, modular inserts, panels -EVERYTHING- that the conducts the signal .

The timing factors mentioned above do apply somewhat to UTP, but are typically associated with coax-based or repeater/hub (common bus) media.

The "Library" section of anixter.com has some good information.

Good Luck

Scott