T1 is a digital service made available to the public in
1983, with the break up of AT&T. Prior to 1983 T1 was only used internally by the
phone companies to inter-connect Central Offices (CO's).
T1 operates at a baseband bit rate of 1.544 Mbps with 1.536 Mbps being available for
payload data and the remaining 8 Kbps reserved for framing. Typically the T1 is comprised
of 24 DS0's, 64 Kbps each. The T1 is transmitted as a Time Domain Multiplexed group of
DS0's, sometimes referred to as channels or timeslots.
T1 operates over special low capacitance shielded twisted pair cabling, although in some
instances standard twisted pair cabling can be used if precautions are taken to avoid
crosstalk. The phone company will normally terminate the T1 with an 8 pin RJ type of jack,
called an RJ-48C, it looks like a large phone jack. Due to the high bandwidth (frequency)
of T1, repeaters are placed along the line at about every 6000 feet.
These repeaters compensate for line loss and signal distortion. The signal itself is a
"return-to-zero", bipolar pulse stream. Bipolar means that the pulses deviate
from the "zero" line both positively and negatively. Typically the pulses
alternate in polarity, this is done for a couple of reasons; to maintain a zero Volt DC
offset and to serve as a form of error detection. Should a piece of equipment receive two
consecutive pulses of the same polarity (a bipolar violation), it is an indication of one
or more bit errors. This alternating pulse standard is known as Alternate Mark Inversion
(AMI).
The 8 Kbps framing format has two common standards that are available today, the newest
standard is called Extended Super Frame (ESF) and the preceding one is D4, sometimes
referred to as Super Frame (SF). Both of these standards still are commonly used today.
While both D4 and ESF offer ESF offers a few advantages over D4 in the form of additional
error information (CRC6) and it provides a 4 Kbps channel, called a Facility Data Link
(FDL), outside the 1.536 Mbps payload capacity. The FDL gives the phone company the
ability to non-intrusively interrogate the customers terminating equipment for error
statistics and line performance information to aid in predicting and preventing line
outages and maintain the highest possible quality of service. D4 on the other hand
provides for only framing synchronization. ESF has become the norm for new installation
throughout the majority of North America.
Due to the synchronous operation of T1 and its utilization of a return to zero, bipolar
format, T1 has a "ones density" requirement. This requirement stipulates a 12
1/2 % minimum ones to zeros ratio. Another words, on average, there must be at least a
single "1" for every eight "0"s. Since this is not always practical, a
couple of solutions have been adopted for T1. The first is called "bit
stuffing", every eighth (the LSB of each DS0) is stuffed with a "1", this
of course makes the overall bandwidth a multiple of 56Kbps. The second method is called
"Bipolar Eight Zero Substitution" or B8ZS, here every series of eight
consecutive zeros is substituted with four "1"s and four "0"s and two
intentional bipolar violations. This technique requires the T1 line to be configured for
B8ZS so as not to correct the intentional bipolar violations.
A typical T1 line between two locations is comprised of two twisted pair lines, one for
transmit and one for receive, running from the customer site to the central office. These
pairs are usually carefully selected pairs within standard multi-pair cables already
installed, repeaters are placed along the cable in required locations and powered by a
simplex current (60 ma) between the two pairs. When the T1 enters the central office it is
usually multiplexed up into a T3 or higher for transport to the distant location. At the
other end, the serving central office demultiplexes the T1 out of the higher bandwidth
service and routes it to a twisted pair cable to the customer site.
A Fractional T1 is nothing more than a T1 as previously
described, but with only some of the DS0s being used. The cost savings comes from the
phone company's ability to allow different customers to use separate portions of the same
T1. Another words as an example, customer "A" would use the first half of a T1
(DS0 #1 - #12) and customer "B" would use the second half (DS0 #13 - #24).
Fractional T1 lines "look" the same as full T1 lines in the sense of termination
and overall bit rate, still 1.544 Mbps.
The "path" from the customer site to the point-of-presence (POP) of the carrier
is a full T1. However, the customer is restricted in how many DS0s can be used to carry
the data, the remaining DS0s are stuffed with all "1"s. At the POP the DS0s
carrying the customer's data are multiplexed with other customers DS0s onto a full T1 and
then multiplexed up to T3s and up for transport to the other location.
At the other end all the required demultiplexing is done to transport the customer's DS0s
to the remote location via a full T1 with the unused DS0s once again stuffed with all
"1"s. At each end the terminating equipment is configured to ignore the unused
DS0s and "drop" the all "1"s and only pass on the DS0s carrying the
customer's data.