PRI Phone Line Connections

Traditional analog business phone lines are useful in very small retail and office locations and any business where just a few outside lines are all you need. Once you accumulate more than half a dozen, it’s time to look at a telephone trunk like option like ISDN PRI.

ISDN PRI stands for Integrated Services Digital Network Primary Rate Interface. Now you know why they abbreviate it. ISDN is a set of standards for how to transmit phone calls digitally on ordinary telephone wire. PRI is the larger of two major options. The other, BRI or Basic Rate Interface, provides two telephone lines, one telephone and one 64 Kbps data line, or a 128 Kbps data line. It’s mostly used for low speed data connections or to transmit high quality audio for broadcast and production.

PRI is the business telephone trunk service that is highly popular for use with PBX phone systems. PRI is transmitted on a T1 line using two twisted copper pair. Chances are you are already familiar with T1 lines for dedicated Internet access at 1.5 Mbps. ISDN PRI, also called T1 PRI, uses the same T1 line technology. In fact, T1 lines were originally designed to transport telephone conversations in bulk between telco switching offices.

What do you get with ISDN PRI service? A single trunk can support up to 23 separate phone conversations, each in its own time division multiplexed channel. Think of these as the equivalent of 23 outside lines. Each line can be set up for local, long distance, incoming only, outgoing only, toll free or some combination. There are actually 24 channels on the T1 line. The last one is reserved for switching signals to support the other 23 plus data such as Caller ID.

ISDN PRI lines almost always plug into an interface card made especially for this purpose. There’s at least one in most PBX phone systems. Some have four or more ISDN PRI connectors so that you can add dozens of outside lines if your operation is large enough to need them. Call centers are big users of ISDN PRI, as are large business offices.

PRI pricing has been getting better over the years, following the declining cost of T1 lines. A few years ago, PRI became more cost effective when you exceeded 10 or 12 outside phone lines. Today, that number is more like 6 or 10 lines, depending on where you are located.

Other than analog phone lines, the direct competitor to ISDN PRI is SIP Trunking. PRI is a traditional telephone technology that connects your PBX system to the public switched telephone network. SIP Trunking is a packet based technology originally designed to support enterprise VoIP telephone systems. SIP Trunks connect directly to converged networks for enterprise VoIP support and can supply both phone and Internet service for smaller companies. SIP Trunks can also be ordered with analog, IP or PRI handoffs to connect to any PBX telephone system.

What type of telephone trunk line is best for your particular situation? Consider the interfaces you have available and get competitive prices and availability for business telephone lines and trunks to make a good decision.

What Is a DS3 Circuit Or T3 Line?

Companies that have outgrown their T1 lines often upgrade to DS3 bandwidth. Let’s take a look at what DS3 has to offer, how it relates to T3 lines, and recent competition from Ethernet over Copper services.

DS3 stands for Digital Signal level 3. The specification for it is found in the T-carrier standards developed by Bell Labs for the telephone industry. The way it works is that DS0 is the smallest unit with a bandwidth of 64 Kbps. That’s just the right size to transport one digitized telephone call using PCM (Pulse Code Modulation), the original digital phone standard. You combine 24 DS0s to create a DS1 that runs on a T1 line. DS3 is the equivalent of 28 DS1s or 672 DS0s.

You’ll often hear DS3 referred to as T3, like DS1 is called T1. The fine line of difference between DS3 and T3 is that DS3 refers to the actual structure of the data stream and T3 refers to the physical transmission layer. Most of the time, they are treated as equivalent. The reason that the digital signal has its own definition is that you can multiplex DS3 as well as DS1 to make much larger bandwidth services. At the far end, you can then demultiplex the signals to recover the original DS3 or DS1.

Combine the 672 voice channels plus the bits needed for synchronization and line maintenance and you have a bandwidth of 44.736 Mbps. That’s commonly referred to as 45 Mbps. You can order DS3 as a telephone trunk line if you have a high capacity phone systems like a PBX serving a major corporation. You can also order DS3 as a data line with one big chunk of bandwidth, namely 45 Mbps.

If you order a T3 line or DS3 service, it will be delivered to you on a pair of 75 ohm coaxial cables using BNC connectors. Look at a DS3 router card and you’ll see the connectors, one for receive and one for transmit. Some cards support more than one DS3 connection so you’ll see multiple sets of connectors.

The coaxial cable used to connect your DS3 router can be no more than 450 feet in length (only 225 feet if using small diameter coax). That’s fine for connecting to the telco demarc in your building, but how does the DS3 get to the central office?

The actual provisioning of DS3 services can be via fixed wireless transmission or SONET fiber optic service. DS3 can be easily encapsulated into a SONET STS-1, as they are both telco standards designed to be compatible. An OC-3 service can carry 3 STS-1s for 3 DS3s.

What if you are not in range for fixed wireless and there’s no fiber in the area? Are you stuck with the 1.5 Mbps T1 bandwidth? No, not really. T1 lines can be bonded to create larger bandwidths. This works well up to 10 or 12 Mbps. Bridging the gap between bonded T1 and DS3 is Ethernet over Copper. This technology also uses multiple copper pair instead of fiber strands, but can deliver much higher bandwidth than T1 technology.

Ethernet over Copper (EoC) bandwidth starts at typically 2 Mbps and goes up to 200 Mbps in some areas. This not only gives you a way to grow your bandwidth incrementally beyond T1, but may delay fiber installation indefinitely. The one limitation of EoC is that it is distance limited. How far you are from the office where your copper bundle is terminated determines the bandwidth that is possible. That office also needs to have EoC equipment available.

Have you outgrown your T1 line service or have an immediate business need for bandwidth in the 45 Mbps range? If so get pricing and availability of DS3 bandwidth services, including Ethernet over Copper if available.



Photo of BNC connector courtesy of Wikimedia Commons.