T1 Digital Cross
Specialized equipment to connect
T1 lines together to reroute signals and groom traffic.
By: John Shepler
If your organization is large enough to use more
than one T1 line for voice or data transmission, you may need
a way to switch traffic between lines or within the individual
channels or timeslots of your channelized T1 lines. The device
that does this is called a DCS or Digital Cross-Connect System.
They are also sometimes called DACS or Digital Access Cross-Connect
Systems after a term coined by AT&T for their equipment that
does this function.
The DCS is a sophisticated circuit switch.
It can switch entire T1 lines to other T1 lines, like you might
do manually with patch panel called a DSX. One application is
to change the destination of audio or video streams to broadcast
stations or satellite uplinks. Another use is to reroute traffic
in the event of a line outage on a particular path.
More That Just a T1 Line Switch
But the DCS can also reach inside the T1 lines to access each
DS0 channel of the 24 TDM (Time Division Multiplexing) channels
per T1. You could conceivably connect the data coming from channel
2 of one T1 line into channel 12 of another T1 line. That's useful
when each 64 Kbps DS0 is unique, such as individual telephone
calls from a cellular base station or a branch office PBX system.
In these cases, rearranging the channels is less important than
combining channels from multiple fractional T1 lines into a single
full T1 line.
Cellular phone systems often use digital cross connect systems
to combine telephone calls from multiple base stations into full
T1 lines for transmission to the mobile switching center and
on to the public switched telephone network. This is called backhaul
aggregation or T1 traffic grooming. If there is too much traffic
for a single T1 line to backhaul, multiple T1 lines can be connected
to the DCS or a M13 multiplexer can aggregate traffic up to the
Branch office PBX telephone system may
also face the situation of needing only fractional T1 service.
At the home office, a DCS can combine phone channels to fill
entire T1 trunk lines that go out to the telephone carrier.
T1 / E1 Conversion
T1 lines are used in the North America and Japan, but elsewhere
in the world the standard is E1. E1 retains the 64 Kbps DS0 channels
but combines 32 of these into a 2 Mbps line speed instead of
the 24 channels carried by a T1 line. The voice codec protocol
is also different. T1 uses Mu-law. E1 uses A-law. A T1/E1 DCS
can interface between T1 and E1 lines by performing the necessary
voice and signaling conversions and assigning channels so that
no traffic is lost.
Fiber Optic Cross Connects
Fiber optic networks have their own version of the digital cross-connect
system called the optical cross-connect or OXC. There are two
ways to implement one of these. The OEO or Optical Electrical
Optical approach converts the optical signalsinto electrical
signals, performs the cross connect function in the electrical
domain, and then reconverts the electrical signals to optical.
An alternate approach is called a transparent OXC or photonic
cross-connect. Individual wavelengths or entire fiber beams are
switched using optical components only, so the signal stays as
a fiber optic light beam through the OXC.
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