WhichVoIP Blog

The previous blog explained why we have 8 bits in a DS0, here we’ll explain how we get these 8 bits.

When  your voice is digitally sampled the amplitude or how loud you are speaking is captured, this happens 8000 times a second. However this digital sample is not what is sent. For a T1 line an algorithm called u-law is used to compand the data. This is a logarithmic type of algorithm that has the effect of giving greater resolution to lower value samples i.e. when you are talking quietly, and less resolution to higher amplitude samples. This intuitively makes sense, if someone is shouting then you can afford to loose a little differentiation between sounds but when someone is whispering you want to hear or accentuate the different levels so you can make out what they say. Typically when your voice is sampled, an Analog to Digital converter that has a 13 bit output is used, the u-law algorithm as well as performing this logarithmic compression has the effect of reducing the voice sample to the 8 bits that is sent.

Up until now we have confined our discussions to voice T1’s. Next time we’ll take a look at how data is transmitted via a T1 line.

WhichVoIP team

http://www.whichvoip.com

How did the 8KHz frame rate and the 8 bits per DS0 come about?

I’ll take the easy one first - Why 8 bits per DS0? For digital or telecom engineers 8 or any multiple of 2 is a perfect number. It’s very easy in digital logic to count in multiples of two thus it is very easy to extract these DS0’s, generate the frequencies and clocks required to receive and transmit at these rates and multiplex these T1 streams into T3 streams or higher rates. The trickier question is how do we get our voice to be transferred from one point to another in only 8 bits every 125us(this is the equivalent time of an 8KHz frame rate).

There is a theorem called Shannons theorem or sometimes referred to as Nyquist-Shannons theorem that drives the fundametal basics of all digital communications.  First remember that T1’s were invented long before the advent of the internet or VoIP and they were designed with the specific goal of transferring voice in a digital format. The result of Shannons theorem is that to sample an analog signal(your voice in this case), convert it to a digital format and then reconstitute it as an analog signal(your voice at  the other end) that has lost no or little information(the person you are talking to can understand you) the original analog signal must be sampled at a rate of at least twice the highest frequency you want to transmit.  The typical human voice has a frequency range of about 50Hz to 3500Hz. So if you want to sample that voice and digitize it you have to sample it at a rate of at least 7000Hz. As in all good designs there has to be some design margin, often referred to as a guard band in communication theory, a rate of 8000Hz was chosen. The frame rate of 8KHz became the standard.

Now we know why 8 bits is used in a DS0 and the rate it must be sent at. In the next blog we’ll go into a little more detail on how the 8 bits are generated and what they represent.

WhichVoIP Team

http://www.whichvoip.com

If you have ever searched for Business VoIP or have been serious about using any business telecom service then you will have come across the term T1. Over the course of the next few weeks we’ll endeavour to explain what a T1 is and delve down into the details of it for you techies out there.

So what is a T1? Well a T1 really defines 4 things.

1. The physical medium used.
2. The bandwidth or clock rate.
3. The type of signaling and encoding used.
4. The format or frame the data is transferred in. 

With a T1 there are two physical mediums. One is 100 Ohm shielded twisted pair and the other is 75 Ohm Coaxial cable. Typically inside patch panels the 100 Ohm twisted pair is used as it is cheap and flexible but for transmission to the outside world a balun is used to match to 75 Ohm Coaxial cable which is typically used to connect to the ISP or PBX.

The bandwidth of a T1 is 1.544Mbits/sec that’s for one direction, one of the advantages of a T1 is that you get this dedicated bandwidth in both the uplink and downlink directions.

T1 uses a signal encoding called B8ZS which stands for Binary 8-zero Substitution - quite a mouthful. Without going into too much detail just now, this type of encoding allows data and a clock to be sent across the same cable – vital for synchronous data transfer.

Finally the format – Each T1 is split into 24 channels referred to DS0’s. Each channel is 8 bits wide and the frame is sent on a 8KHz boundary. The data capacity is therefore 24×8x8000 = 1.536Mbits/sec. Didn’t I say earlier that a T1 was 1.544Mbits/sec? Well there is something called a framing bit, this is 1 bit every frame that a receiver on the other end can lock onto to ensure that whatever equipment is there can extract the correct DS0’s in the correct order. This framing bit makes for an extra 8000 bits/sec giving a grand total of 1.544Mbits/sec.

T1’s are essential a for any business that employs more than about ten people at the one site. They are very reliable and give you a guaranteed bandwidth.

If you are starting a business or you already own a  business and are planning on expanding, fill out this form for a quote on how much a T1 is – you maybe pleasantly surprised.

Next week or when we have time we’ll explain how the 8KHz frame rate came about and how we get 8 bits in a DS0.

WhichVoIP team

http://www.whichvoip.com