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Tech Geek · Guest

My question is regarding the transmission of multiple T1 signals on
Category-5E cables bundled together. The reason I have this question is
one of my data center customers decided to extend (48) T1 lines from
the Telco room to IVR equipment over (48) Cat5E cables. Then terminate
on standard 48-port RJ-45 patch panel. The intent is to run each T1 on
an individual Cat5E cable (ie. Tx & Rx within each cable). My cusomter
claims that there would be no intereference between all these T1 lines
running together in the cable tray. The distance is over 100 feet.

Normally, the way I design T1 cable runs is to either use 56-pair ABAM
and terminate on a DSX panel; or substitute the ABAM with Category-3
and terminate each Cat-3 bundle one for Tx and one for Rx.

I was taught by an SBC engineer once that running more than 6-7 T1s in
individual Category-5E/6 cables next to each other will cause
interference or cross-talk. Is this true from your knowledge and
experience??

James Knott · Guest

Tech Geek wrote:

rimrock · Joined: 24 Aug 2005 Posts: 2 Location: Portland, OR

There is a difference between designing a cable solution for a specific installation where you know what you are dealing with, and designing a solution that will always work. (proper Engineering design)

One of the most critical parameters in what you can "get away with" in designing cable for T1 transport is the difference in signal level between the various T1s. A T1 signal is reduced in strength as it passes through the cable. At its generation, it is typically 6 Volts peak to peak. This is called 0dbDSX0 (commonly refered to as 0db or dsx0). It is the reference level used at DSX cross connect points.

In a CO, at a T1 DSX, there can be literally 1000s of T1s running through cross connect trays unshielded. This only works because they are all at the same signal level. There is no significant Near End Cross Talk (NEXT) generated.

This "same level" phenomenon is why you can bundle multiple transmit signals or multiple receive signals into a common Unshielded Twisted Pair (UTP - ie: Cat5) cable or in a shelded bundle of a T-Screened cable (group shielded bundles in a common cable) without worrying about NEXT.

However, it is not uncommon for received signals on a normal (repeatered) T1 line to be significantly below the level they were transmitted at. (T1 signal degrades at a rate of 4db to 6db per 1000 cable ft depending on wire gauge and splices, etc) At the telco demark (point of hand off of a T1 line at a customer's location) the signal will be between dsx0 and -16 db DSX0 by engineering standard. (-16 db is 16 db below the nominal transmit level and is approximately 1V ptp)

Another place where 0dbDSX0 signals are common is if the telco delivers the T1 by DSL or M13 multiplexer (fiber optics). These devices output at or near dsx0 level and if your T1 is delivered by one of these methods, the signals will be at full strength at the demark.

Generally, if the signal levels are within 7.5 db of one another through out a cable length, NEXT is not significant, and you can "get away with" running UTP for T1. However, if a receive signal at -10 db and a transmit signal at 0 db are run through a UTP for even relatively short distances, you can expect to get enough cross talk to cause bipolar violations (errors). This is especially true if multiple T1s are run through the same bundle. The lower strength signals will all be corrupted.

In summary, if you know that the signals are at the same levels and your distances are not too long (less than 500 ft) you can use UTP and not worrry about Cross Talk. Just be sure to physically separate UTP cables carrying signals of different levels.

However, if you know that the signals are not the same level, or you don't know what the signal levels are or how the T1 lines are engineered, you can design a solution that will always work. This is why the telco standards for wiring were developed. They don't know what they will run into at each installation because they have T1s coming into an office and cannot always plan for the characteritics of each one. By using individually shielded twisted pair wire, or T-Screened cable with all of the transmit signals at the same level in one bundle and the receive signals at the same level in a separate bundle NEXT can be avoided.

Otherwise you are left with "sometimes it works...."
_________________
Cliff Liles

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