Intel 536EX manual Intel Confidential, Fax Class 1 AT Commands, T.30 HDLC Frame Format

Fax Class 1 AT Commands

When the modem transfers data to the DTE, the DTE must filter the data stream by removing all character pairs beginning with <DLE>. The DTE also must recognize <DLE><ETX> as the stream terminator. Upon detecting <DLE><DLE>, the DTE must reinsert a single <DLE> in its place. The modem buffers up to 512 bytes of fax data received from the remote modem if the DTE cannot read the data immediately (AT+FRM command).

For the AT+FTH and AT+FRH commands, data is sent and received using HDLC formatting. The format for HDLC framing is provided below. When sending the HDLC data, the modem automatically generates the flags and the frame checking sequence (Frame Check Sum). All the other fields are transmitted to or received from the DTE (that is, these fields are under the control of the DTE).

Figure 8. T.30 HDLC Frame Format

				Facsimile	Facsimile	Frame
		Address	Control			Checking
Flag	Flag			Control	Information		Flag
		Field	Field			Sequence
				Field	Field
						(FCS)

After receiving a frame correctly using the AT+FRH command (FCS is OK), the DCE returns an ‘OK’ message to the DTE. If the frame is received in error (FCS is not OK, carrier is lost, or data is lost due to data overflow), the DCE returns an ‘ERROR’ message to the DTE; the DTE should then discard the frame.

Note: If the telephone line is on-hook, the modem responds back to the DTE with an ‘ERROR’ message whenever any of the fax AT commands, except the +FCLASS command, are sent to the modem.

Figure 9. CLASS 1 DTE-Generated HDLC Frame Information (AT+FTH=<mod>)

Address

Field

Control

Field

Facsimile

Control

Field

Facsimile

Information

Field

Figure 10. CLASS 1 DTE Reception of HDLC Frame Information (AT+FRH=<mod>)

Address

Field

Control

Field

Facsimile

Control

Field

Facsimile

Information

Field

Frame

Checking

Sequence

(FCS)

Like the data modem mode, the DTE needs to issue an ATD string or ATA command to the modem to originate or answer a fax call. Unlike the data modem mode, auto-answering is not supported by the modem while in fax mode. It is recommended that S-register S0 be set to ‘0’ (that is, use ‘ATA’) whenever the modem is expected to receive a fax call.

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