A Made IT project
|Some Electrical Characteristics for Tn|
|Space||0 Vdc +/- 0.30 Vdc|
|Pulse width||648 ns +/- 15 ns|
|Encoding||AMI (bipolar) and B8ZS|
|Speed||T1: 1544 kbps +/- 50 ppm|
T1c: 3152 kbps +/- 50 ppm
T2: 6312 kbps +/- 50 ppm
T3: 44736 kbps +/- 50 ppm
T4: 274760 kbps +/- 50 ppm
The T1 (trunk) digital lines were developed to replace the analog
lines that interconnected the local telephone offices (CO, central
office) in the USA. The CO's were connected using 4-wire lines, and
frequency division multiplexing to get enough bandwidth to handle
enough calls between local offices. Each call was allocated 3kHz of
bandwidth, 0-3, 3-6, 6-9, etc. on the trunk line. Because of the
variation with time and temperature of the characteristics of the
discrete components used, these lines needed a lot of maintenance and
still were not free of cross talk due to overlapping bands. Every
1830m/ 6000ft an amplifier was needed that also amplified noise. T1 was
decided to sample a 4kHz bandswidth at an 8k (2x highest frequency)
sample rate taking 8 bit samples. That resulted in a 64kbps digital
signal. Combining 24 channels results in 1.536Mbps. Adding a framing
bit gives the 1.544Mbps that T1 is known for. The old analog amplifiers
were replaced by T1 repeaters or digital regenerators. The digital
signal was regenerated not amplified, thus discarding any noise that
was picked up.
The T1 trunk lines are changed over now to carry the ISDN network signals. The 2B+D Basic Rate Interface uses 64k for each B-channel plus 16k for the D-channel, the 23B+D Primary Rate Interface uses 64k for each B-channel plus 64k for the D-channel, which equals 1.544 Mbps, T1 bandwidth. The PRI in Europe uses 31B+D because the 2.048Mbps E1 lines there allow for 32 64k channels.
How they invented the speeds
T1 and up is the terminology for digital, two-way transmission of voice, data, or video over a single highspeed circuit. The transmission rate is based on the bandwidth for one voice channel in digital form. This channel is called DS-0 and consists of 64 kbps of bandwidth.
By Time Division Multiplexing 24 DS-0 channels, T1 is formed. But there is more. To separate the different channels a framing bit is used. For framing 8000 bps are used.
T1 gives you 24 analog voice channels plus the framing rate. This makes the T1 speed: 24 x 64000 + 8000 = 1.544 Mbps. For the other T-versions an equal equasion is used ending everything up to the specs given in the table below.
|Carrier||Signal Level||# of T1 signals||# of Voice Channels||Speed|
The DS-1 interface
There are several different possible interfaces for T1. For copper the DS-1 interface can transmit signals 1828 meter (6000 feet) over 24 AWG twisted pair cable. The actual connectors are a Sub-D15 female or a RJ45 female.
|Sub-D15 Female||RJ45 Female|
|2 and 7||7 and 8||Ground|
For a receiving multiplexer to recognize what is what in the incoming bitstream some smart things are done. First of all from every channel only 8 bits at a time are send. Each of those 8 bits is called a time slot, so T1 consists of 24 time slots in the frame. The entire frame is thus 192 (24 x 8) bits and 1 control bit (the framing bit) which makes up 193 bits in total. At a repeat speed of 8000 times per second you get the T1 rate of 1.544 Mbps.
The modern T1 lines use one of two framing schemes: SuperFrame (SF) or Extended SuperFrame (ESF).
|S-bits||Bit use in each
channel time slot