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

ADDRESS CONTROL INFORMATION

FCS

FLAG

495-14783

Figure 3. Synchronous Data Format

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 polynomial x16 + x12 + x5 + 1), the CRC can be removed by the local modem and added to the DTE frame by the 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.

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November 1996

3980-A2-GZ40-20

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Paradyne 3980-A2-GZ40-20 manual Synchronous Data Format, SDC Algorithm, DTE and DCE Frames

3980-A2-GZ40-20 specifications

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