What's The Difference Between
PBX T1 PRI and VoIP T1 Service?
T1 Lines can be configured for
either legacy PBX systems or the newer IP PBX systems.
By: John Shepler
Digital business telephone systems can
be based on one of two important technology standards. These
are TDM or Time Division Multiplexing vs VoIP or Voice over Internet
Protocol. Both are digital standards, as opposed to the analog
single line or Key telephone systems found in very small phone
systems. Generally, TDM is associated with PBX or Private Branch
Exchange systems traditionally found in medium and larger size
businesses. VoIP is the newer technology thought to need completely
different equipment and more likely chosen for brand new installations.
But there is more of an overlap in these technologies than you
Advantages of PBX Phone Systems
Legacy PBX phone systems are often designed with very specialized
hardware and programming chosen for high reliability. The phone
sets may be either analog or digital and are individually connected
to the PBX system on their own wiring network. The advantage
of having a PBX system is that it is your own little phone company
within the company. Users can call each other using just 3 or
4 digits and there are no line or per minute charges. It's only
when someone makes a call outside the company that you incur
local or long distance charges. Those outside calls are made
using a group of shared phone lines on a first come, first served
PBX Phone Line Connections
The outside phone lines are most often supplied to the PBX system
using a T1 PRI digital line service. T1 PRI bundles up to 23
local or long distance lines together into one digital line service.
There are actually 24 channels, with one reserved for signaling
and data such as Caller ID. PRI is also known as primary rate
interface. What's important about T1 PRI is that all of the channels
are completely separate just like analog phone lines. If a channel
isn't being used, it just sits there idling until needed. This
channelization prevents crosstalk of one call into another and
degradation of voice quality or dropped calls from signal interference.
T1 PRI is the highest call quality you can get by virtue of it's
strict design characteristics.
VoIP for Cost Savings
If T1 PRI is the gold standard, then why have anything else?
The reason is cost savings. Remember that unused T1 PRI channels
do nothing although you are paying for the entire line by the
month. The other cost that might be saved is having a separate
telephone network with its own wiring plus an incompatible computer
network. Combining the two networks is called convergence. The
slower speed telephone network is converted to a format compatible
with the higher speed computer network so all of the signals
can travel together on one set of wired. That format is called
Ethernet or IP for Internet Protocol. As you might guess, IP
is also the standard used on the actual Internet.
A phone system designed to run on an IP
network is called VoIP for Voice over IP. Instead of having separate
phone lines or channels, the digitized voice is loaded into the
same type of data packets used for transmitting computer data.
In a sense, VoIP packets are computer data as far as IP networks
are concerned. It is up to the network operator, usually the
company's IT department, to ensure that both voice and data have
the bandwidth needed to maintain quality of service.
From Standard to IP Telephony
The conversion from regular telephone signals to VoIP can be
made by an IP PBX system or can be done right inside the phone
set, often called an IP phone or SIP phone. SIP is a particular
protocol used by many but not all IP PBX systems.
Notice that in a VoIP phone system the
PBX has become an IP PBX. Some systems use other terminology
such as voice gateway or soft switch. When VoIP calls connect
to standard telephones, those called need to be terminated to
the PSTN or Public Switched Telephone Network. The termination
can be analog phone lines or even T1 PRI digital phone lines
connected to electronic interface cards that plug into the IP
PBX. Two IP PBX systems are connected together using an unchannelized
Yes, You Can Mix Systems
Just to mix things up a little further, legacy PBX systems can
also be converged onto computer networks by adding a IP conversion
card to the PBX system. The resulting IP datastream can be conveyed
on a unchannelized T1 line across long distances or connected
to a LAN router or switch. Again, once converted from TDM to
IP format, voice quality is no longer automatic but must be engineering
into the local or wide area network.
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