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The Bfv API Functions
November 2009 52
Fax Infopkt Parameters
The T.30 protocol requires two communicating fax machines to agree
on several transmission parameters at the beginning of a facsimile
transmission. These transmission parameters include the bit rate,
scan time, coding scheme, and the basic document format
parameters – vertical resolution, page width, and page length. The
least capable fax machine determines the values of these
parameters; both fax machines adjust to the final values.
Two infopkts:
INFOPKT_T30_PARAMETERS
INFOPKT_DOCUMENT_PARAMETERS
affect the parameters that are negotiated during the T.30 protocol
handshaking procedure.
The INFOPKT_T30_PARAMETERS infopkt specifies the desired values
of the transmission parameters. Normally the default values are
used, but INFOPKT_T30_PARAMETERS is useful, for example, for
setting a lower bit rate.
The INFOPKT_DOCUMENT_PARAMETERS infopkt sets the document
related parameters: vertical resolution, length, and width (only the
vertical resolution is programmable). These parameters format the
fax data that is sent out on the phone line.
INFOPKT_DOCUMENT_PARAMETERS is required and specifies the
desired resolution. If it appears in the midst of an infopkt stream,
both machines might renegotiate to the new parameter values.
While document parameters define an entire fax document sent over
the telephone line, page parameters define an entire page only, and
strip parameters define horizontal strips of data within a page.
For ASCII data, there are page parameters and strip parameters.
For G3 data, there are strip parameters. Because page formatting
elements (top and bottom margins, etc.) are inherent in G3 data;
there are no separate page parameters.
The INFOPKT_PAGE_PARAMETERS infopkt defines the ASCII page
parameters, which apply only to pages. Its use is optional, and when
it is not included with ASCII data infopkts, the module uses the
default values (see Appendix E in Volume 6 of the Bfv API Reference
Manual for default values).