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contains consecutive ones and zeros, however, so timing is needed to determine
how long a signal lasts, so that consecutive ones and zeros can be counted correctly.
The T-carrier system transports data synchronously, and a timing scheme is
essential for proper data delivery.
To maintain proper timing, the CSU counts the one bits in the datastream. It uses a
bipolar signaling mechanism that uses a 3.3 VDC current stream. Binary ones are
represented by either +/- 3.3 volts, and a binary zero is an absence of voltage flux.
There are two bipolar coding schemes used with T1 circuits: Alternate Mark
Inversion (AMI) and Bipolar 8 Zero Substitution (B8ZS).
AMI coding represents a binary one (called a mark) with alternate voltage pulses
between zeros (called spaces) which are represented by an absence of a pulse. This
is to say that if the first binary 1 is +3 volts, the next one will be –3 volts, regardless
of how many zeros are in between, hence the name alternate mark inversion. If the
ones are not represented by the correct polarity, a condition known as a bipolar
violation results.
As mentioned earlier, the T1 circuit timing is maintained by counting the ones in the
datastream. If the datastream does not have enough binary ones, the timing circuit
in the CSU cannot derive reliable timing. To avoid this problem, the AT&T
Compatibility Bulletin 119 specifies that a certain percentage of the bitstream traffic
must be binary ones. This is commonly referred to as "ones density." The ones
density specification requires that "no more than 15 contiguous zeros will present in
a transport frame" and "there shall be at least 'N'ones in every 8 (N+1) bits for an
average ones density of 12.5%."
To maintain compliance with the ones density rule, the CSU checks the datastream
to ensure that the bitstream is in compliance. If it is not, the CSU inserts "extra"
binary ones into the bitstream. Needless to say, this does affect how the data is
interpreted at the receiving end. Because of this, AMI encoding is not well-suited for
data transmissions, which cannot tolerate changes to the bitstream.
B8ZS
To transmit data, the capability to send any data pattern, including an indefinite
period of all zeros, is an essential requirement. Because of the highly variable
nature of data, a "clear channel" T-carrier circuit is required. To provide clear
channel T-carrier service, B8ZS encoding is used. B8ZS uses alternating polarities
to represent marks, the same way AMI does. When the MUX recognizes eight
consecutive zeros in the bitstream, however, it inserts a bipolar violation in bit
positions 4 and 7. This violation is recognized by the receiving MUX as a substitution
code, and the zeros are interpreted properly. The only trick with using B8ZS is that
all the equipment in the data path must recognize the B8ZS violation.