Just what is a telephone trunk? It’s a collection of telephone lines, all bundled together. We tend to think of landlines as individual connections. One analog landline is a single pair of small plastic coated copper wires twisted together in a spiral. This isn’t just to keep related wires in pairs to form a complete circuit. The twisted form causes induced magnetic fields to cancel out over distance. This reduces the amount of hum, noise and crosstalk from other telephone lines.
Companies that have a few outside lines often have what’s called a key telephone system. The “key” refers to the line select buttons on the phones themselves. It’s up to the user to note which lines are being used by observing an indicator light for that line button. Unlit line button are free lines that you can use to call out. Each line is an individual analog phone line. They generally come-in through cables of 25 pairs or more called binder groups. You might think of this large cable as an analog trunk line.
The impetus for digital trunk lines is the opportunity to use less copper wiring to transport the same number of phone calls. Stretching miles of binder cable from your location to the telephone company central office (CO) costs a goodly sum. That’s nothing compared to the cost of running long distance lines dozens or hundreds of miles. The first digital trunk was the T1 line. It uses multiplexing to create a stream of 24 channels in 24 serial timeslots. Each channel represents one telephone line. The T1 trunk itself uses 1 or 2 pair of wires versus 24 pair for an equivalent analog trunk.
A T1 telephone trunk can be connected to a channel bank that converts between the digital line and analog telephones. This allows companies with key telephone systems to lease a T1 line at lower cost than 24 separate analog phone lines. A more popular use for this many lines is to connect to an in-house PBX or Private Branch Exchange. The PBX system automatically manages all the outside lines and offers 3 or 4 digit dialing for in-house calls.
An upgrade to the T1 telephone line is the ISDN PRI trunk. The two standards are very similar. ISDN PRI stands for Integrated Services Digital Network Primary Rate Interface. This is quite a mouthful, but is part of an industry standard that has become very popular for business telephony. The difference between T1 telephone and ISDN PRI trunks is that ISDN reserves one of the channels for switching and data. ISDN PRI offers 23 phone lines plus Caller ID, ANI, and much faster switching. Both T1 and PRI interfaces are common on modern PBX phone system designs. They are either a standard features or can be added with a plug-in card.
What’s the advantage of T1 telephone or ISDN PRI to the business user? They cost less than ordering 23 or 24 separate analog phone lines. The person in the office taking a call on their desk phone can’t tell how the phone lines come into the back room. The manager paying the monthly phone bill sure can.
There’s an even newer form of digital telephone trunk available now called the SIP Trunk. SIP refers to the signaling system used by VoIP telephones. It is computer network based rather than telephone industry based. This allows voice and data to be combined or “converged.” Once you have a converged network, you no longer need separate LAN and telephone wiring. Everything in on the LAN.
What the SIP Trunk does is connect your LAN to your phone service provider. This is a long haul private point to point connection so your “phone company” can be located anywhere. It doesn’t matter where those twisted pairs in the ground go. On the network, one location is as close as another.
The other advantage of SIP Trunking is that it can transport both telephone calls and broadband Internet service at the same time without one interfering with the other. Telephone calls have priority in this system so that voice quality is preserved regardless of what is happening on the Internet. Your calls never actually go on the Internet. The simply share a trunk that transports both.
This offers a terrific advantage for smaller companies. Most every business now uses both telephone and Internet service, so why not have them both provided on a single line instead of two? This maximizes the efficiently of the trunk line and saves money for the business user.
ISDN PRI is still wildly popular as a trunk line for PBX systems. Newer IP PBX systems will support both ISDN PRI and SIP Trunking. Why make the change? Compare prices and features to see if SIP Trunks can save you significant money. Smaller companies that can’t possibly imagine using 23 outside phone lines will likely find that SIP Trunking can be scaled down to the number of lines they really need and that’s all they have to pay for. The option of having broadband delivered on your SIP Trunk is also a plus for smaller companies that don’t need the whole trunk for telephone lines.
A potential game changer is the rise of hosted PBX systems or PBX in the cloud. Hosted services connect via SIP Trunk and there is no PBX equipment or phone lines at your location. You pay per user by the month. This offers tremendous flexibility, as you can always order up more “seats” when business increases and you need them.
Are you interested in new business telephone service or are at the point where your current phone system is out of capacity or too costly to maintain? Don’t lock in your decision on how to proceed until you check the latest prices and features of ISDN PRI, SIP Trunking and Hosted PBX services.