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36 Sun PCI High Speed Quad Port Serial Interface Adapter User’s Guide • December 2005
between two computers are another matter, since the corruption of even one data bit
causes a packet to be rejected. Note that in a typical data packet it is quite easy to
produce bit patterns that violate the ones-density requirement. A random file could
easily contain a sequence of bytes that would produce 16 or more consecutive zero
bits if transmitted serially.
There are many different schemes for circumventing the ones-density requirement.
The most common technique simply reserves every eighth bit of the signal for a
density bit and forces this bit to be a one. These bits are not available for data
transmission, which means that 12.5 percent of the bandwidth of the T1 line is
wasted. When you consider that the lease cost for a coast-to-coast T1 line can be
expensive, this waste of bandwidth can be unacceptable. There are alternatives.
Bipolar with 8-Zero Substitution
One alternative of them uses a special code that transmission equipment can
generate when using the AMI signalling scheme. This special code depends on the
fact that two successive one bits that are represented by pulses of the same polarity
result in a signal known as a
Bipolar Violation . A CSU can be designed so that it will
automatically replace any string of eight consecutive zeros with a special code
pattern that contains two of Bipolar Violations. A compatible, receiving CSU
recognizes this special code and converts it back to a pattern of eight zeros. This
technique is known as B8ZS (Bipolar with 8-Zero Substitution).
All CEPT lines (the European equivalent of T1) mandate the use of a variant of B8ZS
that reduces the density requirement to no more than three consecutive zeros.
However, telephone companies in North America have been slow to adopt B8ZS,
because it would entail a significant capital investment. Therefore, the B8ZS solution
cannot solve the ones-density problem in the short term.
HDLC Zero Insertion Algorithm
An alternative to B8ZS, an alternative used by the SunHSI product, makes use of the
fact that the HDLC framing rules specify that any data stream that contains five or
more consecutive one bits requires that the transmitting end insert a zero bit after
the fifth one bit. This guarantees that the HDLC flag pattern
01111110
(hex 7E)
does not occur randomly inside a frame. The receiving end must automatically
discard the zero bit that follows a pattern of five consecutive ones. So, HDLC
framing, which is used by SunHSI/U, guarantees that, except for the flag pattern, in
any set of six bits at least one bit will be a zero. If you include the flag pattern in any
set of seven bits, at least one bit will be a zero.