Sun Microsystems Sun PCI High Speed Quad Port Serial Interface Adapter 819-1207-11 - Manual (Page 56)

36 Sun PCI High Speed Quad Port Serial Interface Adapter User’s Guide • May 2010
inaudible—amount of distortion in the voice signal. Digital data streams 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. Obviously, 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
exceedingly expensive, this waste of bandwidth can be unacceptable. There are
alternatives.
Bipolar with 8-Zero Substitution
One 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 by the acronym B8ZS, which stands for Bipolar with 8-Zero
Substitution.
All CEPT lines (the European equivalent of T1) mandate the use of a variant of B8ZS
that holds the density requirement down 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
will not solve the ones-density problem in the short term.
HDLC Zero Insertion Algorithm
An alternative to B8ZS—an alternative used by the SunHSI/P 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/P, 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, you can say that in any
set of seven bits, at least one bit will be a zero.