Advanced Server Configuration: Modems that Lie about their Capabilities
Modems that Lie about their Capabilities for HylaFAX.Class 2 and 2.0 fax modems all too frequently have bugs in their firmware. Sometimes it is possible to workaround firmware problems by restricting a modems’ capabilities. To do this the Class2DCCQueryCmd configuration parameter may be set in the per-modem configuration file. By default this parameter holds the command to send the modem to query its capabilities. However if the query string has a leading “! then no command is sent to the modem and instead the remainder of the string is treated as the response returned by the modem. Thus to avoid using a particular modem capability all you need to do is setup a capabilities string that has a restricted set of modem capabilities.
Capabilities are returned using a syntax specified in the Class 2/2.0 specification:
(vr),(br),(wd),(ln),(df),(ec),(bf),(st)
where,
- vr
- indicates which vertical resolutions are supported,
- br
- indicates which signaling rates are supported,
- wd
- indicates which page widths are supported,
- ln
- indicates which page lengths are supported,
- df
- indicates which data formats are supported,
- ec
- indicates whether or not the optional Error Correction Mode (ECM) is supported,
- bf
- indicates whether or not the optional Binary File Transfer (BFT) protocol is supported, and
- st
- indicates which minimum scan times are supported.
Each parameter is specified as single value, range of values (e.g. 0-3), or list of values (e.g. 0,1,2). The values are given in the Class 2/2.0 specification and in the following table (note that this table does not list values defined in the latest ITU T.class2 specification):
| Param | Value | Description | Param | Value | Description |
|---|---|---|---|---|---|
| vr | 0 | 98 lines/inch | df | 0 | 1-D Modified Huffman |
| 1 | 196 lines/inch | 1 | 2-D Modified Huffman | ||
| br | 0 | 2400 bits/sec | 2 | 2-D Uncompressed Mode | |
| 1 | 4800 bits/sec | 3 | 2-D Modified Modified Read | ||
| 2 | 7200 bits/sec | ec | 0 | disable ECM | |
| 3 | 9600 bits/sec | 1 | enable T.30 Annex A, ECM | ||
| 4 | 12200 bits/sec | 2 | enable T.30 Annex C, half duplex | ||
| 5 | 14400 bits/sec | 3 | enable T.30 Annex C, full duplex | ||
| wd | 0 | 1728 pixels in 215 mm | bf | 0 | disable file transfer modes |
| 1 | 2048 pixels in 255 mm | 1 | select BFT, T.434 | ||
| 2 | 2432 pixels in 303 mm | st | 0 | scan time/line: 0 ms/0 ms | |
| 3 | 1216 pixels in 151 mm | 1 | scan time/line: 5 ms/5 ms | ||
| 4 | 864 pixels in 107 mm | 2 | scan time/line: 10 ms/5 ms | ||
| ln | 0 | A4, 297 mm | 3 | scan time/line: 10 ms/10 ms | |
| 1 | B4, 364 mm | 4 | scan time/line: 20 ms/10 ms | ||
| 2 | unlimited length | 5 | scan time/line: 20 ms/20 ms | ||
| 6 | scan time/line: 40 ms/20 ms | ||||
| 7 | scan time/line: 40 ms/40 ms |
An example use of this mechanism is found in the prototype configuration file for the AT&T DataPort modem. The rev C01.66.10 firmware for the DataPort returns an invalid string for the AT+FDCC=? query command so the prototype configuration file supplies a valid one instead:
Class2DCCQueryCmd: "!(0,1),(0-5),(0-4),(0-2),(0),(0),(0),(0-7)"
Beware of specifying that a modem has capabilities that it does not; HylaFAX is likely to try and make use of them!