HP 200 Series Services And Applications page 258

Using Synchronous Pass-Through to Consolidate Synchronous Traffic
Synchronous Traffic Requirements
Synchronous Traffic Requirements
The type of synchronous traffic that may be conveyed using sync pass-
through is limited to HDLC and HDLC derivatives such as SDLC and LAPB.
The following three conditions must be met for sync pass-through to
function.
The synchronous port hardware recognizes synchronous traffic that
uses flags (hex 7E character) as the idle line character. This means
that flags are transmitted when the link hardware has nothing else to
send. Flags are also used to delimit the beginning and end of synchro-
nous frames.
The maximum allowable sync pass-through frame size is 1600 bytes.
Thus, the maximum allowable synchronous frame is 1600 bytes
minus the encapsulating protocol size. (The size of the encapsulating
protocol varies by link type but never exceeds 22 bytes.) Generally, a
frame size greater than the maximum is not a problem, since most
synchronous protocols do not use frame sizes greater than 1024
bytes.
Additionally, the synchronous traffic must use the CRC-CCITT frame
check sequence. This is given by the generator polynomial
x
Synchronous traffic that does not conform to the above three requirements
cannot be conveyed using sync pass-through. The most notable type of
traffic that cannot be conveyed by sync pass-through is IBM BISYNC.
3-64
16 + x 12 + x 5 + 1