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Where are T1 MUX usually located?

I'm just starting in the WAN arena and wonder where a data T1 multiplexer usually located? Our data T1 is terminated with a Cisco 1720 router with T1 WIC that has an integrated CSU. That line then goes through to an external DSU, then a Network Interface Unit and out to the Demarcation Point (a closet I have never seen). We don't have an external channel bank or T1 Mux that I can see among our customer premises equipment. So where is the T1 multiplexer located? Is the Cisco performing T1 MUX or another device? Thanks for your input.

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New Member

Re: Where are T1 MUX usually located?

First, it seems you should go find and inspect the demarc point closet. You should know *everything* between the demarc. Often this is the telco's "Smartjack", which is a small box with an RJ-45 connector and some LED indicators. You should know about everything in between the telco's Smartjack and your router.

Next, if your WIC is truly the WIC-1DSU-T1, it is both a CSU and a DSU. If it is a WIC-1T, then it is just another serial port, without any CSU/DSU capabilities. From your description, I would suspect that what you call the "external DSU" or the "Network Interface Unit" are most likely the multiplexer.

The next question is whether you are sure that there is, in fact, a multiplexer in the mix at all. A multiplexer takes multiple signals and overlays them onto one T1 (in this case) to send to the other location. Multiplexers would not be used if the T1 was totally used for data from/to the Cisco router. Multiplexers would be used if, for example, you were using a portion of the T1 for data from the Cisco router, a portion of it for voice to a PBX, and perhaps even a third device. If there is a multiplexer in the scene, it often handles the CSU/DSU duties as well. At any rate, if it is there, you should find another device "sharing" the T1 with the Cisco router.

Hopefully that gives you more to go through. If you are still unsure, post back with exact model numbers of the devices you see (including the WIC) and we'll try to chip in and help...

New Member

Re: Where are T1 MUX usually located?

The T1 is solely for data, dedicated link to another site. So it appears there is not multiplexer. The Router is a Cisco 1700, not sure what WIC but it says CSU on the back. The external DSU is an ADC Kentrox Satellite 651 T1 DSU. Cat 5 is run from the Cisco to the DSU. Then the what I was calling a network interface unit but you are calling a smart jack, it’s a small box out it come the pair wires for the T1. If it were multiplexed would the Cisco do it or the DSU?

Seperately, What would be involved in linking two routers with a T1 cross over cable. On router would be linked to the dedicated data T1. Would this work if one router was set to line for clocking and the other to internal?

Thanks for your input its really appreciated.

New Member

Re: Where are T1 MUX usually located?

You sure that is a Cat5 running from the Cisco to the Kentrox (DSU)? It's usually a V.35 cable, which would be a much larger cable, with a big rectangular connector on the end plugging into the Kentrox unit. A regular straight through Cat5 patch is commonly used between the Smartjack and the Kentrox, same exact cable used for ethernet patches. By the way, still not sure what type WIC you have, the Kentrox box is a CSU/DSU, phrased that way because both CSU and DSU functions are almost always integrated in the same box anymore. It would be interesting to know what the labels of the ports on the Kentrox say that have cables running to the Cisco and to the Smartjack.

If you wanted to split the T1 between the Cisco and, say, a PBX, you would purchase a new CSU/DSU that has multiple equipment interfaces (you have just one on the Kentrox, the V.35 port) for both ends of the link. Usually you want to use the same exact model on both ends. Within the CSU/DSU config, you assign certain channels for the port going to the Cisco, and probably the remainder of the channels for the PBX. A T1 (also generically called a DS1) has 24 DS0 channels, each 64k.

One more clarification on a Smartjack: They are owned by the telco, usually has the telco's circuit number (CKT#) labeled on it somewhere, and some of the LEDs are commonly labeled: PWR STAT LB LBK ALOS DLOS TLOS HLOS RLOS ESF B8ZS. Keep in mind that these are owned and installed by the telco and it is illegal to tamper or move them in any way. I also learned something too! Two of my Smartjacks are called Network Interface Units, NIUs! I never noticed that before!

This is how a T1 interface is normally connected:

Cisco<---v.35--->CSU/DSU<---Cat5--->Smartjack to Telco

Here's how you would connect two routers back to back to simulate a T1:

Cisco<---v.35--->CSU/DSU<---Cat5 Crossover--->CSU/DSU<---Cisco

If you have an actual CSU/DSU WIC installed with an RJ-45 port, you can do this:

Cisco<---Cat5 Crossover--->Cisco

You can buy a custom DB60 to DB60 cable to connect a Cisco serial port back to back to another Cisco serial port. That's real common to simulate a T1 too. Keep in mind that where I say "Cat5 Crossover" I mean a crossover for a T1, not an ethernet crossover. The pinouts are different.

That's alot of info! Hope it helps!

New Member

Re: Where are T1 MUX usually located?

Thanks for all the info, very informative. I'll check out the markings on the WIC, I'm sure you are correct on the v.35 cable.

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