Table of Contents
Advertisement
Synchronous Data Format
The SDC feature supports the High-level Data Link Control (HDLC) and Synchronous Data Link Control
(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 3).
FLAG
A 3800Plus 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 this local modem, since the whole frame
need not be buffered.
The SyncDTE 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
16
polynomial x
+ x
remote modem.
SDC Algorithm
The compression algorithm is based on V.42bis British Telecom Lempel ZIV (BLTZ).
DTE and DCE Frames
Data exchanged between the local and remote DCEs is according to the LAPM protocol. The structure of a
LAPM frame (DCE frame) is identical to the HDLC frame. The data from the DTE is compressed into the
information field of the DCE frame. After compression, one DTE frame is segmented into one or several DCE
frames.
12
ADDRESS
CONTROL
Figure 3. Synchronous Data Format
12
5
+ x
+ 1), the CRC can be removed by the local modem and added to the DTE frame by the
November 1996
INFORMATION
FCS
FLAG
495-14783
3980-A2-GZ40-20
Advertisement
Table of Contents
