ISDN PRI - 1.544 Mbps or 1.536 Mbps ?

• posted

"The Primary Rate Interface consists of 23 B-channels and one 64 kbit/s D-channel using a T1 line. Thus, a Primary Rate Interface user on a T1 line can have up to 1.544 Mbit/s service."

If you do the math:

23Bchannels x 64 + 1D channel x64 = 1536 kbps or 1.536Mbit/s. Even if you divide 1536 : 1024 still doesn't give 1.544 Mbps ....
• posted

A T1 line has 8000 frames each second of 192 data bits. When channelized for voice or ISDN PRI, you have 8 bits for each of the 24 channels in each frame. Each frame also has a framing bit. So

193x8000=1.544mb/s, with 8000bps being overhead. Not unlike a 9600bps async link really being able to transmit 7680bps of "real" data (assuming N-8-1 format).
• posted

I do not know where you got that, but even so, PRI _uses_ T1 line, and every book out there says that PRI consists of 23 B-channels _64kbps_each_ and 1 D channel also _64kbps.

• posted

Hi,

24 * 64kbps + 8kbps management channel used by telco

Hubert

• posted

Right, but what's your question? Just like I said, each T1 frame of

192 data bits (of which there are 8000 each second), carries eight bits from each of the 24 (23B+1D) 64kbps channels in a PRI (assuming the T1 is carrying a PRI, of course). And each frame has a 193rd bit for framing, which is the source of the 8000bps you were missing.

The 193x8000 frame format is inherent in a T1. You can channelize that many different ways, but with voice-like circuits, the sub channels are invariably bit sliced across the frame in such a way that no jitter, burstyness, or lag is introduced. Remember that basic voice service is 8000 eight bit samples per second, and that you'll find line/frame formats all over the (telco) network synchronized in some fashion to the 8000 samples-per-second rate for the precise reason that you can carry that data rate without jitter, burstyness, or lag.

• posted

Thank you!

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.