Analog and Digital Business Telephone Lines
Comparing analog POTS lines with ISDN PRI and SIP Trunking.
By: John Shepler
Business telephone lines, once available in only a single analog version, have proliferated over the years. You can still get the legacy analog subscriber loop. You can also get a variety of digital lines and trunks that may offer cost and performance advantages.
Plain Old Telephone Service
The basic business telephone line is little changed from its invention over a century ago. It consists of a single small gauge twisted pair copper wires that carry all the necessary signals. This is an analog telephone line. Analog phone service is also known as POTS for Plain Old Telephone Service. Many businesses have multi-line phones, but all they do is connect to multiple POTS lines. One to four lines is typical of a small office phone system. If there are lighted pushbuttons for each outside line, this may be called a “key” telephone system.
POTS telephone lines generally include local and long distance calling, features such as Caller ID, 3 way calling, and perhaps a toll free number. One POTS line may be connected full time to an office FAX machine. Some companies that have digital telephone systems may still keep a POTS line for the FAX machine, as not all digital services support FAX.
The simplest digital phone line is a single VoIP or broadband phone service that uses the Internet as a substitute for the twisted pair analog phone line. The cost savings realized is due to the fact that most businesses need broadband Internet access as well as telephone service. Using the Internet to connect the phone to the service provider avoids the charges for a separate telephone line.
Limitations of Internet Phone Service
The main limitation to broadband phone service is that the Internet was never designed to support high quality two-way real time voice or video services. It is critical to have enough bandwidth to support all the voice and data traffic on the broadband connection and to give voice packets priority. When bandwidth becomes restricted, voice quality starts to get garbled and the call may even be dropped. Another factor is latency or time delay between source and destination. The longer the latency, the more the phone starts to act like a two-way radio where only one person can talk at a time. Latency is seldom, if ever, a factor on analog lines or carefully engineered private networks.
Value of Dedicated Lines
Enterprise VoIP systems, consisting of many telephone sets connected to a converged voice and data LAN, avoid the limitations of the Internet by using dedicated circuits transport calls between internal phones and to the connection point or termination with the Public Switched Telephone Network. That termination may be within the company, where the connection is to multiple POTS lines or a digital trunk line. It may also be at a service provider connected to the enterprise by a converged voice and data line called a SIP Trunk.
What's a Trunk?
A “trunk” line is simply a bundling of multiple telephone lines in one cable. That may be a fat cable with many analog copper pair or it can be a digital trunk line with few wires that transport many telephone calls in channels or packet streams.
How ISDN PRI Works
The most popular digital trunk line is called ISDN PRI. This is also called T1 PRI because it is carried on a T1 digital line. What a PRI digital trunk gives you are up to 23 outside telephone lines plus a dedicated channel for switching signals and data such as Caller ID. Some PBX telephone systems have provisions to connect to two or more PRI trunks. This is especially true for call centers and large corporate office buildings. Note that each of the business lines in the ISDN PRI trunk can be configured as local, long distance, inbound only, outbound only, toll free or some combination of these per customer requirements.
SIP Trunking is New
The newest business telephone trunk is called SIP Trunking. SIP is the control and signaling protocol for VoIP telephone systems. One SIP trunk can carry dozens of phone calls, even more than a PRI trunk. The other way SIP trunks can be configured is for both voice and data on the same line. This is especially valuable for companies that have converged networks shared by both computers and telephones. The SIP services provider brings in both broadband Internet access plus business telephone lines on the same SIP trunk.
What type of service will work best for your company? There may be a range of options to choose from. Get prices and availability for business telephone service now, so you have up to date information to make an informed purchasing decision.
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