Thursday, November 16, 2006

How Does A T1 PRI Work?

Here's a brief description of the difference between T1 voice vs PRI.....

"PRI or Primary Rate Interface is a switched service deliver OVER a T1 connection . If someone were to say “I want to order a PRI to location X.” What is being requested is a connection to a trunk side module capable of ISDN protocol to a Telco switch delivered to location X via a DS1 rate (T1) circuit.

Now if someone were to say “I want to order a T1 to that SAME location X.” … well… That wouldn’t be enough info… A T1 from where to where? A T1 for what? By just asking for a T1, nothing is understood or implied about where or what the circuit would be used for. A T1 can be used to truck data at the rate of 1.544 Mbps from one location to another… With channel banks and appropriate DS0 level cards, a T1 can be used to truck up to 24 separate and distinct DS0 signals (analog data, analog voice, or Digital Data Service) from one location to another… A T1 can be used to connect one location to an ISP Internet Edge device to connect a customer to the World Wide Web… In other words, a T1 is a multi use pipe…"

With a point to point T1 you can use it for voice and/or data depending on the equipment you have at each end. They are basically just providing the "Pipe" and it is up to you what it is used for.

The thing with T1's and PRI's is it all depends on what equipment is connected to it on each end.

As quoted above, a T1 is just a 24 channel circuit that can be used for multiple things. A PRI is a protocol that uses the 24th channel to control what the other 23 channels are used for.

In a nutshell - a T1 has 24 channels using 56K for data and 8K for signalling. A PRI has 24 channels and uses 23 for data and the 24th for signalling. Also you can break down the channels on a T1 for different type of service. On the PRI all 23 channels have to be the same.

The features... or lack of... totally depend on where the circuit is going and what it is connected to on the other end.

Keeping it on simple terms...

PRI is a diffrent protocol to be used with voice services over the T1 line. Where as a t1 line can also be used for voice but without using the PRI protocol and probably using diffrent equipment at the other end to get the dial tone....

Of course....there are a lot more technical details dealing with ISDN lines (PRI's/BRI's) but you've got the basic idea.

To request quotes for PRI and/or T1 voice service.... comparing providers available in your area....use this free service: T1 Voice and PRI Rate Quotes


Blogger James Knott said...

I would also differentiate between a "T1" and a "DS1". I've long considered a DS1 to be an unformatted 1.544 (1.536 usable) Mb/s channel that can support a T1 (24 DS0 channels), among other things.

5:09 PM  
Blogger carrierone said...

A DS1 could be T1, E1, or J1. J1 is similar to T1 in Japan, and E1 is 32 channels (30 usable) from the European standard which is in use everywhere but north America, Japan, and the islands near America. An E1 is 2.048Mb/s and an E1 can be called a DS1.

This article does not address what it should, and that is the difference between a RBS (Robbed Bit Signalling) Voice T1 and a PRI or SS#7 T1.

RBS T1s come in a variety of flavors with the most common being E&M Wink Start and Loop start, for trunks and lines respectively. Either 2 or 4 bits are robbed depending on if the line coding is AMI/D4 or B8ZS/ESF used for line seizure and hangup. The advantage to E&M is to eliminate glare with busy calls going in and out don't collide and that the signalling works the same way for calls going either way. The drawback to it is that it only supports caller ID number because it comes in as touch tones not FSK burst like on Loop or Ground start. Ground start is typically something you see on payphones and is rare now. Wink start means you get a wink flash on the A/B bits after seizing the line so you know its ok to send digits and make a call, versus a dialtone which requires DSP power to process sound. The biggest disadvantages of E&M is you can't set a caller ID number (ANI) outbound without using Adtran style DTMF tones or old style MF which is rarely supported on CPE, and you can't get caller ID name service. The second disadvantage on it versus PRI is you can not reject a call. You must answer and then hangup.

Loop start is identical in the way it works as to a local POTS line. Its very simple, you've got onhook and offhook. Ringer cadence is also pulsed out on the A and B bits. Normally you see a dialtone and ordinary DTMF digits sent out for calling, and a FSK burst for incoming caller ID if subscribed. CLECs used to provision this a lot and then put in a channel bank. In a Class 5 switch like a DMS or 5ESS they provision lines with rotary groups and the whole 9 yards like you see on a ordinary line group on POTS.

PRI ISDN is a subset of SS#7 ISUP. They removed the dangerous features from SS#7 and simplified it for customer use. The D channel carries call setup and teardown information much the way that SIP does this for RTP calls on VoIP. It supports a variety of features including voice and data calls, whereby no one does data calls anymore. It only takes a few milliseconds to get a call ringing versus getting a dialtone or wink and outpulsing digits at a minimum of 250ms each which can take about 3 seconds of extra time. The Caller ID number can be set going outwards and CID name and number can come in, all of this on a data channel. You also get busy, intercepts, disconnected number messages back instead of voice tones (unless translations did it differently).

The nice thing about Loop start is you can get centrex type features from the LEC such as call transfer or three way which normally you would have to take up more trunks to do yourself. PRI has a B channel transfer feature but nobody supports it really.

I think PRI and RBS is going to be around for a while in Class 5 space until VoIP providers quit charging over $1 per DID because some organizations have large contiguous blocks and pay under $0.20 per. Its pretty much dead in the long distance Class 4 world though as every major carrier is using VoIP natively and they tend to charge fees for the D channel or in other cases the extra taxes and fees on toll calls make it expensive to use. There are also a lot of idiotic big-name "SIP phone systems" out there that have SIP phones but require T1 or E1 trunks. Shoretel is infamous for doing this as they require a SIP carrier to pay THEM to do an interop.

11:22 PM  

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