CAS vs PRI Voice Channels

T1 telephone trunk lines are popular with PBX and IP PBX phone systems. Most current equipment offers an interface connection so that you don’t need separate a CSU/DSU box to connect to the line. What you may not know is that there are two type of T1 telephone lines available. One is CAS and the other is PRI, also known as ISDN PRI.

T1 CAS is the original design for T1 lines used as telephone trunks. The line is channelized or divided into 24 separate segments of 8 bits each sampled at 8 Kbps. Multiply 8 bits by 8 Kbps and you get 64 Kbps, which is the capacity of one T1 channel or DS0. The designation DS0 stands for Digital Signal level 0, the basic building block of a T1 line.

What goes into a DS0? It’s one digitized telephone call. The process of converting from analog to digital is called PCM or Pulse Code Modulation. The 8 bits are enough to represent 256 different voice levels and provide excellent voice quality.

A collection of 24 channels of 8 bits each strung together end to end plus 1 extra bit defines a T1 frame. The extra bit is used to synchronize the equipment so that it knows where the frame starts. Hence, the designation “framing bit.”

Now, here’s where CAS comes in. Notice that each of the telephone channels is filled up with digitized voice. The framing bit is occupied in synchronizing the line. So, how do you communicate dialing and switching information? The dialing is handled by transmitting the DTMF dial-tones through the voice channels just like you would over an analog line. Knowing whether the channel is on-hook, off-hook, ringing, etc, needs to be handled by some extra bits that aren’t designed into the system. The way this is done is by “robbing” one bit from each channel periodically to create a signaling channel. This degrades the voice channels by such a small amount that it isn’t noticed.

The robbed bit signaling technique combined with DTMF dialing means that each channel is responsible for its own operation. This is what is called Channel Associated Signaling or CAS. Another name for this is in-band signaling. Each channel on a T1 line is equivalent to one analog telephone line. There are separate transmit and receive paths on the T1 so that you can talk and listen at the same time. This is full duplex operation.

Here’s something interesting. You don’t need a PBX system to use a T1 line to bring in multiple business phone lines. Instead, you buy a device called a Channel Bank that converts 24 phone lines to one T1 line. The channel bank handles all the interfacing and conversion back and forth between analog and digital protocols.

The other type of T1 telephone line that has become even more popular than T1 CAS is T1 PRI or ISDN PRI. ISDN is a set of digital standards that include a low speed and a high speed option (BRI and PRI). PRI stands for Primary Rate Interface. You might think that it is something completely different from T1, but actually PRI is just a reorganization of the T1 channels.

With ISDN PRI, 23 of the 24 available voice channels are kept exactly the same as T1 voice channels, but are called B channels or Bearer channels. The remaining channel is called a D channel or Delta channel, sometimes called a Data channel. With ISDN, all the control and signaling is assigned to the D channel. There are no robbed bits from the voice channel. What that does is speed up the dialing and switching, an important feature for high volume call centers. The other advantage gained by having a D channel is that it can be used to supply digital data, such as Caller ID. ISDN signaling is known as CCS or Clear Channel Signaling to distinguish it from CAS. This is also referred to as out of band signaling as opposed to in-band signaling.

The nice thing about T1 lines is that they have well established standards to support both voice and data services. If you need telephone trunk lines for an in-house PBX or IP PBX systems, chances are that some configuration of T1 line will do the job. You may be surprised by how competitive T1 prices are now. Check T1 and ISDN PRI prices online to compare with your other telephone service options.

Is HPBX Your Next PBX?

It’s almost back to the future, to coin a phrase. Companies nationwide are getting rid of their phone switching equipment and going back to service providers, what we used to call telephone companies. Only the phones and the telephone trunk lines remain. Is technology reversing itself or is there something new going on here?

Make no mistake about it, telephony today is different from the telephone service of yesteryear. But there are similarities, too. What companies have found is that building and maintaining little telephone companies in-house entails a lot more cost and bother than you might expect. If that’s true, though, why did they ever leave good old Ma Bell in the first place?

One problem with business phone service versus what you have in your home is that there are lots and lots of phones. If you only have one or two lines, you probably have just one or two phones. It’s likely that each phone is tied to a particular phone line. Perhaps you have multi-line phones where each line has a switch or “key” so you can select it. Some small offices have only a single phone set one one line and use a second line to connect a FAX machine or credit card terminal.

So what’s the problem with having so many phones? It’s pretty likely that by the time you have a few dozen phones, your office facilities are so spread out that people can’t lean over to the next desk and have a conversation. They either have to get up and walk down the hall or make a phone call. If each phone has its own line, then those calls will go through the phone company and you’ll likely pay a charge. If you want to talk to someone in a branch office in another state, you’re surely going to pay long distance charges.

What companies really wanted was something of an intercom feature, where they could just dial a few digits and get to another phone in-house. To accomplish that, they bought PBX or Private Branch eXchange equipment. The PBX is the device that connects phones and also managed outside lines as needed. All the phones connect to the PBX and the PBX connects to the phone company, but only for calls leaving the premises.

The local telephone companies offer a service called Centrex (Central Office Exchange) to compete with the PBX. The switching equipment is kept at the central office, like always, but the system functioned more like a PBX. The Centrex system based on the individual copper pair from each phone to the CO is now being replaced by HPBX or Hosted PBX service.

What’s different about HPBX is that it is a VoIP phone system. Communication is over a WAN network connection called a SIP Trunk. This frees the user from being tied to a particular local phone company by those ubiquitous twisted copper pair. Now any service provider anywhere in the world can be your phone company as long as you can get a digital connection between their facilities and yours. What you need in-house is IP phones, also called SIP telephones, connected to your converged voice and data network. Your service provider hosts the PBX switching function with termination to the public switched telephone network as needed.

Are you ready for a phone system upgrade or merely concerned that what you are doing now costs more than it could? Get competitive quotes for hosted PBX telephone systems, some of which include new phones as part of the service fee.

VoIP Trunk Service Providers

Any business with more than a few outside phone lines knows that trunk lines are the way to go. The next question is what type of trunk service to get. You might think that with a VoIP telephone system, the obvious choice is SIP Trunking. But is it? Take a closer look at what’s available and then decide.

The idea of telephone trunking originated with the phone companies back when Ma Bell ran the industry. When phones were new, a couple of wires were all you needed to connect from your location to the local phone company. Then everybody wanted a phone and businesses started installing switchboards to handle all the lines they needed. You know those old-timey photos of phone line congestion in the cities. That’s what happens when you build out one line at a time.

The mass of subscriber lines was gotten under control using multi-pair cable, often buried. But long distance lines between switching offices were another matter. The more simultaneous calls you carried, the more wires were needed. That gets expensive fast when you’re talking about dozens or hundreds of miles. Wouldn’t it be nice if one pair of wires could carry multiple conversations?

That’s the origin of the trunk line. The early systems were set up like radio stations on the dial, except over wires. Each call modulated its own carrier frequency that was spaced far enough in the spectrum from adjacent carriers so that they wouldn’t interfere. This was analog carrier telephony. The switch from analog to digital technology came with the T1 line. It’s been around for at least half a century and is still going strong.

T1 telephone trunks are divided into 24 separate channels, each transporting one two-way telephone call. All the dialing and switching is done on these channels, just as it would be on analog phone lines. You can get devices called channel banks to convert T1 to and from 24 analog phone lines at your office. The T1 line itself is two twisted pair running from your building to the nearest phone company central office.

The advantage of T1 lines as telephone trunks is that save a lot of wire and can save cost, too. There is no economy of scale when you buy your phone lines one at a time. Soon you have a dozen or two at the price of a dozen or two times the cost of one line. If you convert all those lines into a T1 telephone line (trunk), you’ll generally save money even if you only use half the capacity of the T1. If you need dozens of lines for your call center, T1 can save you a bundle.

Chances are that you won’t need a channel bank unless your phone system is completely analog. Most PBX systems have T1 ports already installed. Even the new IP PBX systems designed for VoIP telephones usually have T1 ports where you just plug in one or more T1 lines for your outside telephone service.

A closely related service that is even more popular than T1 telephone trunks is ISDN PRI. This service runs on a T1 line and is very similar to a T1 trunk. The difference is that only 23 channels are used for individual phone lines. The remaining channel is used for all the dialing, switching and data such as Caller ID. The switching is faster than with T1 telephony and most everyone wants Caller ID anyway. The port for this is called a PRI, ISDN PRI or T1 PRI an it is likely installed in your system now.

The other digital trunk option that is becoming popular thanks to VoIP is the SIP Trunk. SIP is the switching protocol used with IP telephones in a VoIP system. A SIP Trunk, which might also be carried on a T1 line, is not broken up into channels. Instead it is more like your local area network that transports Ethernet packets.

This is one of the big advantages of a SIP Trunk. It can connect directly to your network without having to convert from one protocol to another. In fact, just like your network carries both VoIP phone traffic and computer data traffic, a SIP Trunk can provide both telephone and broadband service from your service provider. This is the preferred arrangement, as it allows the provider to establish class of service controls to maintain voice quality regardless of other network traffic.

As you might suspect, an IP PBX system works well with a SIP Trunk. But SIP trunks can also be ordered with customer premises equipment that offers a T1 or ISDN PRI digital handoff or even analog phone lines.

Which type of telephone trunking service is right for your business? Compare prices, availability and options of T1, PRI and SIP Trunks to see what makes the best economic as well as technical sense for your particular situation.



Etching of phone lines in New York City circa 1890 courtesy of Wikimedia Commons.

What is a PRI in VoIP?

We tend to think of VoIP as a “new generation” phone service based on IP technology unrelated to the historical public switched telephone network. Well there’s some truth in that, but no matter what you implement you’ll still be dealing with the good old PSTN. The bigger your company or organization, the more “outside” phone lines you’ll need. You should take a close look at ISDN PRI to make the connection to all the phones that are not part of your enterprise VoIP telephone system.

What is a PRI? It stands for Primary Rate Interface. That’s telephony talk for a particular standard related to another telephone industry term, ISDN. ISDN is the Integrated Services Digital Network. There’s a history to the development of ISDN that relates to the once lofty ambition of replacing all analog home and business phone lines with digital versions. You may not have heard of this, as it fizzled when broadband Internet came along and usurped the data role intended for ISDN. Even so, the business version persists and is more popular now than ever. That’s what’s called PRI or ISDN PRI or even T1 PRI.

Why T1 PRI? It’s because ISDN PRI is physically transported on a T1 line. T1 is another telephone industry standard widely used in business for dedicated Internet and point to point data connections. T1 lines have the advantages of being provisioned on copper wires so you don’t need fiber optics installed. They can be extended to nearly any distance, are highly reliable and reasonably priced. The pricing has gotten really attractive lately due to all the competition in the industry. With all this going for it, T1 makes a great choice to carry PRI.

What’s different between a regular T1 line and a T1 PRI line? The line speeds and voltages are the same. What differentiates a PRI is how it is organized. Instead of a big chunk of bandwidth, ISDN PRI is set up as 23 channels that each represent one digital telephone line. A 24th channel is used for signaling and data such as Caller ID. It’s an upgrade to an older T1 telephone standard that was set up as 24 phone channels and no Caller ID.

The beauty of bringing in an ISDN PRI line or “trunk”, as it is called, is that you get 23 outside phone lines coming in on a single digital line. It’s a lot more compact and is almost always cheaper, usually a lot cheaper, than dealing with 24 individual analog business lines.

The interface is also generally very easy. Most PBX and IP PBX telephone systems either come with ISDN PRI capability or offer it through a plug-in module you can buy. All the circuitry to convert between 23 individual phone lines and the PRI digital trunk are implement on that circuit board. You simply plug-in one or more ISDN PRI lines and you have outside phone lines and a lot of them.

But why would you connect a T1 PRI to a VoIP phone system rather than keep it 100% VoIP? The simple explanation is that the rest of the world isn’t serviced by VoIP or at least the same VoIP provider. The only thing universal is the PSTN, where you can dial any number and get connected to any telephone in the world. Your enterprise VoIP phone system can save you a bundle on internal calls that stay on your own network. You can also buy a SIP Trunk that connects you to a telephone service provider completely in IP or Internet Protocol. But, guess what happens at that service provider when you want to call an outside number? That’s right. You’ll be switched through to the ISDN PRI lines that connect them to the public phone network.

Which is the best deal for your company when it comes to analog (POTS) phone lines, ISDN PRI or SIP trunking? Why not let an expert consultant get you competitive quotes for these various options so you make an informed decision? The service is fast, easy and free.

Why Are VoIP and SIP Trunking So Popular?

If your old PBX system is getting long in the tooth and it’s time to start considering a replacement, you’ll do well to look into replacing that PBX with an IP PBX and those phone lines with a SIP Trunk.

Why are these new technologies catching on so fast when traditional telecom equipment and analog or ISDN phone lines work just fine? The answer is new features combined with cost savings while maintaining call quality.

You might be getting a headache right now, thinking about the capital investment and installation expense required to change out all those phone sets that are still getting the job done. Breathe easy. You don’t have to swap out any of your old phones unless you want to.

There are good reasons why you might like a new VoIP SIP phone. For one thing, they plug right into your computer network and don’t need separate telephone wiring of their own. You manage them like any other device on the network. Since a SIP phone can plug into any network jack just like a PC, it’s much easier to manage moves, adds and changes. You want to move to a different office? No problem. Just take your phone with you and plug it in to the network jack in the new office. The phone still has its own MAC address that identifies it as yours, so it doesn’t matter where on the network it sits.

But what about the old phones you want to keep? Just connect their phone wires to your new IP PBX. You’ll need to order interface cards with FSX ports to make the connections, but the IP PBX doesn’t mind a bit. You can even have a mix of SIP phones connected to your LAN and analog phones connected to FSX ports. The IP PBX will treat them all as individual telephones. Of course, the SIP phones may have extra features like displays or the ability to coordinate with computer terminals, but they’re still telephones.

By the way, you don’t necessarily need to have a PBX system on your premises anymore. VoIP has opened the door to hosted PBX, where the call switching hardware is located at an offsite supplier who provides telephone services at an attractive price. It’s a way to avoid the often hefty capital investment required for a new in-house IP PBX system.

VoIP telephony has become popular in business environments with lots of phones, primarily because of the ability to converge the LAN network to support voice as well as data. That makes it possible to keep the phone system truly off the public network for all locations that are reached by the corporate LAN. It’s only when you need to talk to outside parties that you need public phone lines. Or do you?

Yes, you do. But you don’t have to provide those lines yourself. What a SIP trunk does is extend your internal network to a service provider who then connects to the public phone system. Economy of scale generally means that telephone calls over a SIP trunking service are less expensive that terminating them yourself to analog phone lines or ISDN PRI trunks connected to the phone company. SIP trunking services also generally provide dedicated Internet service over the same SIP trunk. After all, it is just a network extension. As long as quality of service is maintained, you can get your telephone and Internet services from the same provider over a single SIP trunk and realize a cost savings.

Are you interested in exploring the expanded performance and cost savings available with VoIP and SIP Trunking services? You may be surprised at what you can get for much less than you are paying now. Enter a quick Enterprise VoIP quote request now with a brief description of your needs and you’ll get a fast response and personalized service.