Synchronous Data Format; Sdc Algorithm - Paradyne COMSPHERE 3920Plus Technical Reference Manual

COMSPHERE 392xPlus Modems
SYNCHRONOUS
DTE

Synchronous Data Format

The SDC feature supports the HDLC and SDLC type protocols for synchronous DTE data. An
HDLC frame is composed of an information field containing the data to transmit, plus an overhead
(address, control, and FCS fields) used by the DTE protocol. Flags opening and closing the HDLC
frame serve as frame delimiters (Figure H-3).
FLAG
A 392xPlus modem with the SDC feature supports DTE frame sizes up to 4300 bytes. This
conforms to the X.25 recommendation which defines a maximum size of data packets as equal to
4096 bytes.
If the modem has the SDC Delay Min configuration option set to Rx_Clock, there is no limitation
on the DTE frame size for the DTE frames transmitted from the remote modem to the local
modem, since the whole frame need not be buffered.
The Sync DTE CRC configuration option determines whether the CRC of the DTE frame (the FCS
field) is transmitted. If the CRC type is known to be the 16-bit CRC for HDLC frames as defined
by ITU-T (generator polynomial x
and added to the DTE frame by the remote modem.

SDC Algorithm

The compression algorithm is based on V.42bis British Telecom Lempel ZIV (BTLZ).
H-2
DATA
COMPRESSION
FUNCTION
INTERCHANGE
CIRCUITS
FUNCTION
Figure H-2. Synchronous Data Compression
ADDRESS CONTROL
Figure H-3. Synchronous Data Format
16
+ x
October 1998
DCE
ERROR
CORRECTION
FUNCTION
SIGNAL
CONVERTER
CONTROL
FUNCTION
INFORMATION FCS
12 5
+ x + 1), the CRC can be removed by the local modem
LINE
495-14785
FLAG
495-14783
3920-A2-GH30-40