atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services T1's, CSU's and DSU's T1, PRI question

Hey guys-
We have been talking about T1, PRI, and BRI in my Business Data Communications and Networking class and I have some questions.
I was always told that a T1 line is 24 channels of 64K. Doing the math this only adds up to 1.536 Mbps, however we always say a T1 is 1.544 mbps. This leaves an extra 8 bits. Is this for signaling?
Along the same lines, a PRI uses 23 channels of 64K for data plus 1 64K channel for signaling. Again, what is the extra 8 bits for? I understand that on a BRI you have two 64K channel and then 16K for signaling.
Hoping someone can enlighten me with the answers
Thanks!
Jeff

Your math is right, Jeff.

With T1s (and PRIs), those eight extra bits are often referred to as "overhead" bits, and are used to perform network management functions such as CRC error checking, etc.

Overhead bits and signaling are two completely different things.

On standard T1s, the signaling is handled WITHIN each of the twenty-four 64K channels (also referred to as "CAS"/pronounced "Kass" for "Channel Associated Signaling"). With PRI, the signaling for all twenty-three "bearer" (or "B") channels is handled by the PRI's individual "D" channel, which handles both signaling and control. This "D-channel signaling" used by PRIs is also referred to as "FAS" or "Facility Associated Signaling".

So T1s use "CAS" signaling, PRIs use "FAS" signaling, and they both use overhead bits for error checking.

Don't forget that the terminology varies depending on where you find yourself..

It sounds like a cool class. :thumb:

You will also run across RBS for Robbed Bit Signalling. It's synonymous with CAS as mike explained. Although I think RBS is a more specific term used for FXO/FXS channels and CAS is a more general/global term. I don't ever recall E&M/Wink-Start circuits being referred to as RBS.

And on a side-note, D4/AMI has 56K per channel, with the 8K being stolen for framing (7-bits per frame with 1-bit reserved for signalling). This is why you don't have PRI on a D4/AMI T1 as computers are based on 8-bits to a byte. D4/AMI audio is transmitted as A-Law as a result.

One day when you are really bored you can read up on PRI, NFAS, SS7, and all the lovely mess that they inspire.

SS7 is the really interesting one. All kinds of madness that goes on in that protocol. I prefer to think of it as carrier-grade VoIP.

James,
I really like that comparison of SS7 to "Carrier-grade VoIP". That's a good one!

Jeff,
With James' mention of the D4/AMI T1s with 56K channels, these D4/AMI T1 beasts do exist in many places, and here locally, they're called "1344" T1s.

And to throw another wrinkle out there a T-1 is a T-1 only differece is the signaling mode. The only change we made testing a T-1 was if it were D4/AMI we'd change it to ESF/B8ZS other wise you're not testing the whole T-1. AT&T had built in monitors on the lines, across the network it was always ESF at the end point it could be converted to D4. I have tried to explain to many a T-1 "tech" when they say why do I see 5 errors here and only 2 here. The reason is because you're not looking at the full bit stream when testing or monitoring D4. Usually they'd just get a glazed look and walk away.

Yeah. The funny thing is that Verizon down here will build a whole new circuit just to change you from D4/AMI to ESF/B8ZS. I used to know how to switch the smart jack but I haven't had to use it in a few years. Most of what i'm doing is more and more SIP based.

I did some research online and found a few other things. According to Wikipedia:
A more common understanding of how the rate of 1.544 Mbit/s was achieved is as follows. (This explanation glosses over T1 voice communications, and deals mainly with the numbers involved.) Given that the highest voice frequency which the telephone system transmits is 4,000 Hz, the required digital sampling rate is 8,000 Hz (see Nyquist rate). Since each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs then 8,000 frames per second to maintain those 24 simultaneous voice channels. Because each frame of a T1 is 193 bits in length (24 channels X 8 bits per channel + 1 framing bit = 193 bits), 8,000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s (8,000 X 193 = 1,544,000).
I have never heard any of this before...

hi jeff,

the extra 8 bits in your original post was actually 8,000 bits. this is the extra bit in each frame multiplied by the frame rate of 8000 per second.

techinny

Hey Jeff… Everybody else has done a great job of explaining all this but while your on this subject the other piece that will help make all the “magic” less “magical” with voice channels on T1 is PCM. PCM Hope that don’t make your head spin too bad but it will help make this part “the required digital sampling rate is 8,000 Hz” make sense.

Jeff -

You've pretty much got it. If you want/need to know more there is a halfway decent book I used to use in class called "The guide to T-1 Networking. How to buy, install and use T-1 from the desktop to DS3 " by William Flanagan. Check on Amazon and you can find some used copies for as cheap as $0.01 (plus shipping).

I don't think the book's been revised in years (I think the last edition was '98) but the basics are still the same and it was pretty good at explaining it all.

Sam