How many simultaneous voice communication on T1 ?

[sarahr202]

Hi Every body!

1)Imagine two central offices connected by T1. How many Simultaneous voice calls can be supported on this T1 line?

My understanding is since t1 has 24 dso, and each dso is 64kbits, If PCAM is used at central office, then 12 voice simultaneous calls can be supported.

2)By the way how central offices connect to each other in pstn? do they use T3 or higher digital line?

3)At central office, Do we use only pcam to convert analog signal from local loop ?

Thanks a lot and have a good day!

[scottmac · Accepted solution]

1) Using traditional methods (TDM) 24, one per channel. More recently, many carriers use VoIP so the number varies with the CODEC, but is many times more than 24.

2) COs are generally connected with anything from a DS3 up to the OC192 (~192Gig) or OC768 (~40Gig). Many carriers use an "Inter-Exchange Carrier" (IXC)for long distance ("Inter-LATA")trunking.

3) For traditional telephony, something like a D4/D5 channel bank or similar. There are some other Subscriber Loop systems, D4/D5 is still pretty common.

Of course, VoIP services are changing the face of the old loop and moving it toward an all digital domain.

Good Luck

Scott

[lamav · Accepted solution]

Sarah:

There is another configuration that Scott left out, and that is the case in which the T1 is terminated on a statistical multiplexer, instead of a legacy T1 D4 channel bank.

With statistical multiplexing, the T1 is oversubscribed to accomodate idle times for certain channels.

Typically, the T1 signal is created when the multiplexer takes an 8-bit sample from each of the 24 channels and then adds a framing bit at the end. That is considered to be one frame of 193 bits. The multiplexer will scan each channel 8,000 times per second (8,000 frames), which would yield 1.544 Mbps.

On the receiving end, the T1 bit stream is de-multiplexed in the same methodical manner - 8 bits at a time, one channel at a time, 8,000 times a second.

The inefficiency arises when one of those channels is idle. To ensure that the receiving end will have 8-bits per channel, keepalive or filler bits are generated by the channel bank/multiplexer on the idle channel.

With statistical multiplexing, a bit-map is created that shows which channel at any given time is sending real traffic. When a channel is deemed idle, the stat mux will allow other users to occupy the bandwidth of that channel.

In this way, you may have 32 users, or more, intelligently and dynamically sharing the T1 line.

As a footnote to all this, the most popular statistical mux of the 90s, which is when this technology became widely deployed, was the IDNX 20/70/90.

HTH

Victor