A communications signal which carries a purely digital wrapper signal and a method
and system for generating it and extracting overhead information therefrom. The
wrapper signal can be received by a high-performance format-specific receiver at
the end of the network as part of the overall payload, but can also be detected
by a low-bandwidth payload-bit-rate-insensitive receiver at an intermediate node.
This is achieved by using alternating payload and wrapper segments and providing
special digital coding on the wrapper segments. Specifically, each wrapper segment
consists of a contiguity of signal level sequences, each of which is a multi-bit
symbol that encodes a bit in the overhead bit stream. Each of the symbols is thus
a signal level sequence having one of two possible transition patterns, with the
appropriate symbol being chosen depending on whether the overhead bit is a logic
"zero" or a logic "one". The two symbol patterns which represent logic "zero" and
logic "one" each contain enough transitions to meet synchronization and automatic
gain control requirements associated with various network equipment, while containing
a sufficiently distinct average signal level (or pulse density) to allow a receiver
to associate the resultant short-term D.C. offset with the correct wrapper symbol.