DS3 and T3
If you are interested in a DS3 line, you should familiarize yourself
with what it stands for and how it relates to T3. DS3 stands for Digital
Signal level 3. That represents the procedure that runs on the physical
T3 line. DS3 is a TDM or Time Division Multiplexed synchronous format
is how it is described. The system runs at 44.736 Mbps, which we usually
call 45 Mbps for short. The transmission rate of 44.736 Mbps is not some
arbitrary number. It has to be that speed so that DS1 signals that are
carried on T1 lines can also be carried on T3 lines.
T3 and T1 lines were designed as part of family of digital network services for
the telephone companies called the T-Carrier system. This is not exactly new
technology. The first T1 lines went into service in the late 1950s. What is new
is the availability and reasonable pricing of T3 and DS3 services to non-telecom
carrier businesses.
If you have a large organization, perhaps you want that 45 Mbps as your Internet
service connection. If you need a lot more bandwidth than the 1.5 Mbps delivered
by a T1 line, it might be time to step up to DS3 service delivered on a T3 line.
In addition, there are two types of T3
circuits. The oldest is channelized T3
which is used to transport digitized telephone calls. Each channel is 64 Kbps
wide and carries one phone call. A T1 line can carry 24 of these DS0 channels.
A T3 line carries 672 of them or 28 DS1s. A device called an M13 Multiplexer
combines the 28 DS1s and gets them aligned in the proper time slots to make a
DS3, which can then be carried on a T3 line. The other type of T3 format is unchannelized.
The physical connection from your equipment to the phone line Network Interface
Unit consists of two 75 ohm coaxial cables with BNC connectors. That's different
from a T1 line which gives you an RJ-48 jack at the NIU for use with unshielded
twisted pair cable. One of the T3 coaxial cables is transmit, the other is receive.
T3 is a full duplex circuit, meaning that transmit and receive are independent
and both run at the 45 Mbps rate. If you don't need all those little channels
to support phone calls or modem connections, unchannelized T3 gives you one big
data pipe with a payload rate of 44.210 Mbps. This is similar to an unchannelized
T1 line but with a much higher bandwidth.
The T3 line might stay as coaxial cable outside, but it is more likely to be
carried on a fiber optic cable between your location and the carrier's office.
It will likely be multiplexed onto a larger fiber optic carrier, even an OC48
or OC192, for long haul transmission.
Three to nine Mbps is your lowest cost
option, which may well be to bond several T1 lines together to get the equivalent
of a single larger circuit. Above that, fractional DS3 may be the better option
until you need a full T3 or multiple T3 lines. Once you get to needing a 100
Mbps connection to carry your network traffic, OC3 or 100 Mbps Ethernet service
on fiber optic cable should work for you.
