Businesses that only need a handful of telephone lines usually start off with a few analog POTS (Plain Old Telephone Service) lines and add more, one at a time, as needed. At some point you start to wonder if a digital trunk makes more sense than a dozen separate POTS lines. You are likely right.
Digital trunks are a technology to transport multiple phone calls to a subscriber using fewer wires. Each analog line requires a separate pair of wires. A trunk may use one or two pair to carry 23 or 24 phone calls.
How is this accomplished? The most popular technique is digitizing each phone call and then multiplexing all those bits onto a serial data stream. The line is synchronized at both ends so that individual time slots can be separated into channels. Each channel holds one concurrent call. This technology was standardized by Bell Labs in the T-Carrier system. The most popular of the T-Carriers is T1, although T3 is still used but generally called DS3.
The basic T1 telephone line consists of 24 identical channels of 8 bits each sampled 8,000 per second. That adds up to a line speed of approximately 1.5 Mbps. T1 lines were the first deployed as digital telephone trunks. PABX systems often have T1 interface cards and connectors so you can just plug in a T1 line and have up to 24 outside lines available. Larger systems have multiple line interfaces for 48 or more outside telephone lines.
T1 telephone is still highly popular, but is generally deployed in the form of ISDN PRI. PRI means Primary Rate Interface which uses a channelized T1 line for transport. ISDN is another standard called Integrated Services Digital Network. ISDN uses 23 of the T1 time slots for telephone lines but reserves one channel for data and switching. You give up the capability of one telephone line but gain much faster call switching plus Caller ID and ANI (Automatic Number Identification) for the other 23 phone lines.
ISDN PRI is readily available, less expensive than ever before and compatible with most PABX systems. With the capability of handling up to 23 outside lines, it has the capacity to handle the needs of most small to medium size offices and contact centers. By adding additional interfaces, a PABX system can handle 46, 69, 92 or over 100 outside telephone lines.
The competition for ISDN PRI is a newer service called SIP Trunking. SIP is the switching system used for VoIP telephone systems. A SIP Trunk is basically a packet network line with the bandwidth to handle multiple simultaneous calls and perhaps data transfers as well. Voice packets are given priority on a SIP Trunk to prevent call distortion and dropping of phone calls. A T1 line can be used as a SIP Trunk as well as for ISDN PRI, depending on how it is connected at each end.
Many PABX systems can now directly connect to SIP Trunks as well as T1 telephone lines and ISDN PRI. Some are IP PBX systems that use IP telephones as well as standard analog desk phones. What threatens all of these trunks now is not a new design of PABX but dumping the PABX completely in favor of hosted telephone services.
Hosted voice or telephone in the cloud moves the PABX from your office to a cloud service provider hundreds or thousands of miles away. You swap our your phones with IP telephones and a SIP Trunk. At the far end, the service provider does all of your in-house and outside line switching. It’s the provider who contracts for ISDN PRI trunks or other connection to the public telephone system. You simply pay per phone per month for local and long distance service plus in-house call switching. Often that is one flat fee per “seat.”
Which type of telephone system and trunking arrangement is right for your company? Get competitive quotes for POTS, T1 telephone, ISDN PRI, SIP Trunking and Hosted Voice to help with your trade study and selection.