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SIP Trunking vs ISDN PRI Phone Lines
A closer look at telephone line options for business PBX phone systems.

By: John Shepler

Business telephone lines have gone through several transformations. What started out as analog landlines evolved into digital T1 CAS trunks, then ISDN PRI, and finally SIP packet based network connections. Amazingly, all of these technologies are still available. We’ll take a closer look into the two most popular configurations used in multi-line PBX business phone systems. 

The Primary Rate Interface
Find SIP and SIP telephone trunks for PBX plus other network services here. ISDN PRI is short for the mouthful of an expression that is Integrated Services Digital Network Primary Rate Interface. Often the shorthand PRI or ISDN PRI is used for obvious reasons.

ISDN PRI is a standard telecom service that is based on the earlier T1 line specification. T1 got its start as a means of consolidating 24 pairs of twisted wire phone lines into 2 pair to save the expense of long distance cabling and overloading of telephone poles. This was accomplished by digitally multiplexing the 24 separate channels so that they were all transported together on a single 1.5 Mbps digital signal. An additional advantage of going digital was the reduction of noise and crosstalk on long distance phone calls.

A major benefit of the T1 specification is that existing phone company infrastructure can be reused, including wire landlines and microwave relays. High frequency digital signals degrade over distance fairly quickly, so regenerative repeaters were added to the lines to build the signal back up. With this arrangement, T1 could be offered over long distances to serve virtually all of the customers who were using the old analog phone lines. 

One issue with T1 was that if each of the 24 channels carried only the audio telephone conversations, other wiring would have to be provided for signaling. A better solution was to multiplex signal bits into the data stream by robbing a bit here and a bit there in each of the channels. This degraded the audio very slightly and allowed the T1 CAS (Channel Associated Signaling) system to be self-contained. 

Additional features, such as caller ID, were introduced by the phone companies and desired for business phones and especially call centers. This is where PRI comes in. The same basic T1 line organization was kept, except that only 23 channels were assigned as individual telephone lines. One complete channel was assigned to handle all of the signaling, data and other background services. 

Most PBX systems today have interface cards where you directly plug-in a ISDN PRI cable and have 23 phone lines (or as many as you have ordered) at your command. If you need more than 23 lines, you plug in additional PRIs to add more channels.

Is there a Secondary Rate Interface?
No, but there is a related standard called BRI or Basic Rate Interface. The phone companies thought this would catch on as a two phone line system or higher speed dial-up modem service. By the time it was heavily deployed, computer communications had moved on to broadband and BRI never caught on. It has been popular for radio station broadcasts and remote announcing, such as voice over artists, because it can carry high quality audio signals. Recently, this service has been replaced by broadband Internet and is very difficult to find anymore. 

What is SIP and How Does It Compare to PRI?
SIP is Session Initiation Protocol. It is a technology competitor to PRI. SIP is really a standard for transmitting VoIP or Voice or Internet Protocol phone calls over networks typically shared with other computing devices. 

As you probably suspect, SIP is a packet based protocol versus the channel based protocol used by ISDN and T1. In channelized systems, a complete voice channel is assigned to a phone call for as long as the call is in progress. If no one is talking, the line just sits there idle. There is no sharing of facilities. In VoIP telephony, the conversation is broken up into small packets that are routed on the network to their destination. Many conversations can be on a network at the same time along with file transfers, print jobs and everything else digital. If there is a pause in one conversation, no packets are sent to the network for that channel only. 

As you can see, VoIP is more efficient at sharing the network facilities. It comes at a cost though. Sometimes everybody wants to talk or send data at the same time. Unless the network is carefully designed, it can become flooded with too many packets to handle and things back up. This network congestion causes delays in transfer that breaks up phone conversations. Any packets lost will also distort the conversation. 

Network Considerations for SIP
Careful network engineering is necessary to ensure that the sensitive voice packets are transferred without delay or corruption. Usually, these data streams are given a priority using Class of Service mechanisms within the network. 

A network line dedicated to SIP is called a SIP trunk. This is the equivalent of an ISDN PRI line. Any digital line capable of carrying Ethernet packets can be a dedicated SIP trunk, although T1 lines, DS3 bandwidth and Carrier Ethernet services are often used for SIP. By giving the SIP phone conversations their own line, there is no interference from other data packets. SIP trunks are most often used to connect business and call center PBX systems directly to phone service providers. Like PRI, most PBX systems have network interface cards that plug directly into a SIP trunk.

SIP vs PRI
One protocol isn’t necessarily more appropriate than the other. It depends on the application. Most PBX systems can be set up to connect to SIP trunks or PRI trunks regardless of whether the phone sets themselves are analog handsets or VoIP phones. SIP trunks are particularly useful with newer Unified Communications or Unified Communications as a Service (UCaaS) systems that integrate computers, desk sets video conferencing and mobile phones into a single interoperable system. In these systems everything is in packet format and running on a single network. 

Which is Right For You?
The best choice in telephone trunks is the one that works best with the system you have or the system you desire, and at the lowest cost for the performance you need. Prices of both PRI and SIP have fallen dramatically over the last few years, so you owe it to yourself to see what telephone trunk line options are available for your business needs.

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