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.

IP T1 Line Service

T1 line service has been available for decades. It originated as a proprietary telephone company technology designed as a trunk for transporting multiple phone calls between switching centers. Later, T1 was made available for businesses to support their PBX phone systems. When the Internet came along, T1 lines were pressed into service as dedicated Internet connections. But now we live in an IP world. Can T1 also act as an IP connection?

Yes, it can. That may seem counterintuitive to those who understand the origins and technology behind T1 service. The T-Carrier specifications, of which T1 is the most popular line service, describe a system organized as multiplexed channels 64 Kbps wide. That channel size was optimized to transport a single independent digitized telephone call using PCM (Pulse Code Modulation). Clearly, T1 is oriented toward the switched circuit based public telephone network.

That’s true, but the important thing to remember about digital network technologies is that a bit is a bit. It’s either true or false, one or zero, regardless of the particular physical layer or higher level protocols involved. That means that a time division multiplexing technology like T1 can be used to transport a packet oriented technology like IP or Ethernet with a little protocol conversion.

That’s essentially what’s being done when T1 lines are used for data transmission. It’s been a long time since anything but Ethernet has been the standard for local area networks. Ethernet is an IP technology, based on packet switching. Those packets can be queued up and loaded into T1 frames for transmission through T1 lines and even multiplexed into T3 and SONET frames for longer haul transmission. At the far end, the packets are offloaded and sent on their way through an IP edge router connected to the T1 CPE (Customer Premises Equipment.)

If you want to connect your Ethernet networks over metropolitan and wide area networks, one technology you’ll be interested in is called Ethernet over T1 or EoT1. This is also called EoDS1 for Ethernet over DS1.

DS1 is simply the data service or protocol that runs on T1 lines. The reason there are two designations is that T1 describes a physical line consisting of two twisted pair copper telco loops running at 1.5 Mbps. The DS1 is all the bits formated for transmission just before loading into the actual T1 line. You can take that same DS1 and feed it into an add/drop multiplexer and transport it along with many others on a T3 circuit or a fiber optic service like OC-3. At the other end, the DS1 can be demultiplexed from the other services on the same line and delivered separately.

In practice T1 and DS1 refer to the same thing to the actual user. What’s important is that Carrier Ethernet service can be delivered on a T1 line, albeit bandwidth limited to 1.5 Mbps. Who would want service that slow? Lots of companies need connectivity between locations, but don’t require high speeds. They may be using the service for retail credit card verification, communication with corporate headquarters, or to upload files to their web server. The big advantage they have with T1 lines is that they are almost universally available. If you can get business phone service, chances are that you can also get a T1 line installed. They are connected over the same telco wiring.

The competing service to Ethernet over T1 is Ethernet over Copper. That may seem like a fine line of distinction, but it is important. Ethernet over Copper (EoC) also uses twisted pair copper but a different modulation scheme that offers higher bandwidths but with distance limitations. If you are within a mile or two of the central office that has Ethernet over Copper equipment, you may be able to get 10 Mbps, 20 Mbps or even higher bandwidths from EoC. Some installations are transporting in excess of 100 Mbps over multiple copper pair.

Ethernet over T1 bandwidth can also be improved by adding more copper pair. The process of combining the bandwidths of several T1 lines into one larger bandwidth is called bonding. EoT1 bonding can deliver up to 10 or 12 Mbps to both metropolitan and rural areas, providing the equipment is available.

Are you interested in an IP connection for your computer or converged voice and data networks? If so, get pricing and availability for Ethernet over T1 or Copper now to see how much bandwidth is available for your business locations.