Dynamic Bandwidth Allocation
for T1 Lines
How to get the most efficient
use of your T1 bandwidth for voice and data
By: John Shepler
T1 lines are digital trunks that can carry
voice, data or both. The traditional T1 line is configured as
24 channels of 64 Kbytes each for a total of 1.5 Mbps bandwidth.
Each channel carries one digitized telephone call. Since the
channels are pre-configured, if you have only 12 telephone lines
in use, the other 12 are simply idle. In some cases, the line
is set up to be 23 telephone calls plus 1 data channel to carry
information such as Caller ID and signaling. This is called a
T1 PRI or Primary Rate Interface. The biggest application for
T1 voice lines is to support PBX telephone systems and call centers.
A T1 line can also be configured as one
big data pipe instead of 24 individual channels. Unchannelized
T1 lines are often used to provide dedicated Internet access
with 1.5 Mbps bandwidth. Another application is point to point
private data transmission between business locations.
Discrete T1 Services
If you need both telephone and Internet or data exchange service,
there are several ways to accomplish this. One is to buy separate
T1 lines, each set up for a particular purpose. A T1 voice line
would service the telephone system. A separate T1 data line would
provide Internet service for the business. In larger businesses,
it is possible that more than 24 phone conversations would be
going on concurrently. In that case, multiple T1 lines would
be used for the voice application. T1 data lines can be bonded
to incrementally increase bandwidth to 3, 4,5, 6, 9 or more Mbps.
Integrated T1 Services
Small and medium size businesses may not need a full T1 line
for telephone service and another full T1 line for Internet access.
Integrated T1 service combines voice and data on to a single
T1 connection. A static setup allocates a certain number of channels
to voice and the rest to data. A typical arrangement is 12 phone
lines plus 768 Kbps of Internet data over the remaining bandwidth.
Many businesses can get by with a single T1 line serving all
their telecom needs.
Dynamic T1 Lines
A newer and more flexible arrangement is Dynamic T1 line service.
A dynamic T1 line may be channelized, but the channels will be
assigned as needed. If there are no phone calls in progress,
the entire bandwidth is used for data. As soon as someone pick
up a handset, one channel of 64 Kbps is dedicated to that call
and data bandwidth is reduced by 64 Kbps.
Another technical approach gets rid of
the channelization completely and sends everything as data packets
over the T1 line. Internet data is already packetized, so the
telephone calls must be converted from analog or TDM (Time Division
Multiplexing) digital to VoIP packets at each end of the line.
The beauty of this approach is that bandwidth can be allocated
on the fly packet by packet, making optimum use of the available
bandwidth. The process is known as dynamic bandwidth allocation.
Ensuring Voice Quality
To ensure voice quality, the voice packets are given priority
over the data packets. Email and Web traffic, even data downloads,
aren't much affected by short delays, also called latency. A
data packet can wait for a voice packet. Voice is a continuous
stream and will become distorted or drop out if the latency delays
get too long. The other quality control comes from the dedicated
T1 line from customer to carrier. The carrier installs a IAD
or Integrated Access Device at the customer location to handle
the conversions, and telephone gateways and routers at the carrier
end. This arrangement can guarantee sufficient bandwidth to ensure
toll quality telephone calls.
Note that even though dynamic bandwidth
allocation is based on packet switched technology, you don't
necessarily need VoIP telephones to take advantage of this service.
The IAD provides analog phone lines or a T1 telephone interface
to your PBX or Key Telephone System, plus a WAN data port for
your Internet router. You can use the equipment you already have.
If you have already installed an enterprise VoIP phone system,
you can also take advantage of dynamic bandwidth allocation without
the need for conversions to TDM or analog standards.
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