Why is this even a question? It’s because the century old public telephone system has been based on analog transmissions over twisted pair copper wiring. Many smaller companies still have multiple analog business lines connected directly to their phones or to Key Telephone systems or in-house PBX systems. Others who are big enough to need at least a half-dozen outside lines have moved to digital transmission via ISDN PRI, also called T1 PRI.
So, if PRI is digital then what’s the issue with VoIP?
Well, there’s digital and there’s digital. PRI telephone trunks are actually digital replications of the old analog lines. Each PRI consists of 23 channels that each carry one digitized phone conversation. One other channel is used for switching and data, like Caller ID. Those 23 PRI “B” channels are strung together end to end for transmission. Each channel is 8 bits wide and sampled 8,000 times a second. At the far end, the channels are separated and converted back to analog phone signals.
Note that nowhere here is any mention of packets. You know that computer networks all communicate using standardized packets of information. Each packet has both address and data bits. This allows switches and routers to get them from wherever they originate to wherever they are intended.
TDM (Time Division Multiplexed) lines, such as PRI, use channels instead of packets. The switching function is provided by the central office switch. When a phone call is in process, there is a direct electrical connection between the two callers. That connection is set up at the beginning of the call and torn down when the call is over.
If you are thinking that PRI service isn’t really compatible with VoIP, you’d be partially right. It’s really a conversion process between your office phone system that might be VoIP and the Public Switched Telephone Network (PSTN) that is still analog and TDM based. What VoIP really needs is a telephone line that is compatible with packet switched technology.
The first obvious option is to use the Internet to connect you with your VoIP phone service provider. After all, the Internet is readily available, relatively inexpensive to access, and packet based. Remember that Internet Protocol (IP) is the basis of VoIP.
What gets missed is that there is a big difference in the technology that includes Internet Protocol and the actual performance of the public Internet itself. One issue is that the Internet is open to anyone to do pretty much anything they want on it. There is no traffic manager ensuring that each user gets adequate resources for their particular application. There is a pool of resources and it’s first packet come, first served. Your voice packets may be competing with users backing up their files to the cloud, accessing ecommerce websites or downloading HD videos. If there is enough low latency bandwidth for everyone, then no problem. If the pipes get clogged up like rush hour on the Interstate, then you get what you get.
That can be a big problem for sensitive packet streams such as VoIP calls. Network congestion can cause dropped packets that distort the voice, delay some packets so they arrive out of sequence and are discarded, or impose a noticeable delay that makes two-way conversations difficult.
You can improve your chances of getting decent VoIP voice quality by using dedicated instead of shared bandwidth for your connection. That means Ethernet over Copper or T1 lines instead of cable broadband or DSL. You can also impose QoS (Quality of Service) through your own router so that at least your VoIP conversations aren’t competing with computer file transfers in your own office.
A better idea, although a bit pricier, is to use a dedicated private line called a SIP trunk between your network and your service provider. QoS is maintained by the service provider from end to end. As long as you have enough bandwidth to support all the calls you want to make, everything should work beautifully all the time. SIP trunks can be set up to provide you with both telephone service and broadband Internet access. This is done by dividing up the trunk so that phone calls and Internet are kept completely separate.
Are you considering a change to your business phone system because you are dissatisfied with how it works or are looking to add features or reduce costs? Investigate enterprise VoIP and hosted PBX solutions with SIP Trunks as a better approach for your company.