T1 to T1 lab question

Unanswered Question
Feb 14th, 2007

Quick question... I have the following for my lab:

3620 (1 T1 DSU/CSU)

2501 (2 ser, 1 eth)

2 x 3810's (1 3810V w/MFT-T1-RF's, APM-FXS-RF's)

2524 w/ all WIC's (4-wire 56/64-kbps DSU/CSU, Fractional T1/T1 DSU/CSU, serial x 2)

I'm using my 3620 as an MGCP gateway for VoIP/Security studies using IOS 12.3(21) IP/FW/IDS/etc...

I wanted to tinker more with the WIC's using frame-relay but am unsure if I can configure say a T1 WIC as if I provisioned it for a company, piped into another T1 WIC. Maybe a crossover?

E.g.: 3620(T1 WIC) --> rollover --> 2524(T1 WIC) or vice versa then using preconfigured configurations for VoIP.

I'm a bit lost on how to do something similar. Bottom line, I want to use either a 3810 or the 2524 as if I were a customer getting T1 service (frame-relay) into their router, and that it is getting this traffic from another router using that other router's T1 interface. Is this possible? I tried searching some of my books (ICND, BCMSN, BSCN, BCRAN, CCIE Large Scale IP, etc.) and have not come across anything even close to what what I would like to do. Any thoughts?

Maybe an ethernet to T1 connection? Any thoughts? Thanks in advance

I have this problem too.
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ellis_b Sun, 02/18/2007 - 12:04

if you are going from T1 RJ45 to T1 RJ45, then you need to make a T1 x-over cable. connectors 1 and 2 should go to 4 and 5 and vice versa. (4/5 to 1/2).

if you are trying to back to back DB60 ports, then you will need a DB60 serial x-over cable (dont try to make this one, just buy it for $10-$20)


Brad Ellis

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victor_djono1 Thu, 02/22/2007 - 23:17

Hi Brad,

May I know if T1 RJ45 x-over cable is the same regular RJ45 x-over cable? if not, how is it different?



huongvx Thu, 03/08/2007 - 04:12

Hi Victor,

You need to differentiate Ethernet and ISDN pin used.

Ethernet uses pin number 1,2,3,6

ISDN (most of isdn devices) uses pin number 1,2,4,5.



scottmac Sun, 03/11/2007 - 11:15

A T1 crossover uses pins 1&2 going to pins 4&5 (in order).

A DDS (56K) uses

1&2 --> 7&8

7&8 --> 1&2

A standard Ethernet / Fast Ethernet / Gig Ethernet crossover is

1&2 --> 3&6

3&6 --> 1&2

4&5 --> 7&8

7&8 --> 4&5

ISDN uses the same layout as a POTS phone

BRI (needs an ISDN switch to work, crossover won't work)

Line 1 / Pair 1 is 4&5

Line 2 / Pair 2 is 3&6

Line 3 / Pair 3 is 1&2

Line 4 / Pair 4 is 7&8


Same as T1, but you need an ISDN switch for signaling

Good Luck


scottmac Sun, 03/25/2007 - 12:33

Perhaps for an S/T; the U (the carrier-delivered pair to the NT1) can be any pair at the entrance, then rigged with an adapter cable or broken out to a biscuit block to Pair 1 (4&5).

My ISDN was delivered on line 2/Pair 2 (3&6). Pair two was broken out and fed to the NT1 as Pair 1 (4&5).

Good Point though, thanks for the clarification.

Good Luck


swmorris Sun, 03/25/2007 - 15:59

Your telco is stranger than most then! Even in USOC wiring, that's "line 2" position.

The IEEE specs for U and S/T are fairly specific about what wire pairs do what, and I don't recall there being any mention of delivery on pair 2.

Being an ex-cabling guy, I pay a lot of attention to those things usually, although in the myriad of other minutia, it's always possible for a slip!

But you're right, nothing a good cable change can't fix. I'd feel pretty comfortable in saying that would be a custom cable job there however! Just my two cents! :)



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