About T1
If you need a T1 provider, T1
solutions is the place that provides the best T1 pricing, T1 line pricing,
and the best Internet Service. The T1
rates are either combined for transmission via faster circuits, or demultiplexed
into 64 kilobit per second circuits. This is for distribution to individual
subscribers. T1 signals can be transported on unshielded twisted pair telephone
lines. The transmitted signal consists of pips of a few hundred nanoseconds
width, each inverted with respect to the one preceding. When it comes to
the sending end, the signal is 1 volt. This is received greater than 0.01
volts. It thus requires repeaters about every 6000 feet. The information
is contained in the timing of the signals, not the polarity. A T1 circuit
requires two twisted pair lines, one for each direction.
New equipment uses the two lines at half the T1 rate and in full-duplex
mode. The sent and received signals separate at each end by components called
a "hybrid". Although this technique requires sophisticated equipment,
an advantage is that half the sent and half the received information is
mixed on any one line, making low-tech wiretaps less a threat. When a long
sequence of bits in the transmitted information would cause no pip to be
sent, bit stuffing is used so the receiving apparatus will not lose track
of the sending clock.
The rate of 1.544 Mbit/s is the rate in which the signals are transmitted.
It was achieved is as follows. Given that the highest frequency at which
voice communications occurs is at 4000 Hz, the required
digital sampling rate is 8000 Hz Since each T1 frame contains 1 byte of
voice data for each of the 24 channels, that system needs then 8000 frames
per second to maintain those 24 simultaneous voice channels. Because each
frame of a T1 is 193 bits in length (24 channels X 8 bits per channel +
1 framing bit = 193 bits), 8000 frames per second is multiplied by 193 bits
to yield a transfer rate of 1.544 Mbit/s (8000 X 193 = 1544000).
T1 used Alternate Mark Inversion (AMI) to reduce bandwidth and eliminate
the DC component of the signal. Each pulse had the opposite polarity of
the previous one, resulting in a three level signal which however only carried
binary data. Similar British 23 channel systems at 1.536 Mbaud in the 1970s
were equipped with ternary repeaters, in anticipation of using a 3B2T or
4B3T code to increase the number of voice channels in future, but in the
1980s the systems were merely replaced with European standard ones. American
T-carriers could only work in
AMI mode.
