IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Organisation Remarks
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Communications component
The communications components establish the connection between the server
and the public switched telephone network. Communications are processed
using the T.30 protocol. This protocol determines amongst other things the
connection setup, the exchange of originator fax numbers and transmission
and acknowledgement of the document. Transmissions using the Group 3
standard are generally effected at 9,600 bps and 14,400 bps. In addition, the
Modified Huffmann, Modified Read and Modified Modified compression
techniques are also in use. The Group 3 standard is the most widely used.
There is also a Group 4 standard; however, this requires ISDN. Transmission
speeds of 64 KB per second are possible with this standard. The Group 4
standard has not succeeded in becoming as well established in the last few
years, as appropriate stand-alone devices are relatively expensive. Moreover,
the Group 3 and Group 4 standards are mutually incompatible.
At the start of communication, both the transmission speed and the
compression method are negotiated between the devices. The highest speed
and the best possible compression procedure which are supported by both
devices are selected.
The following communications components can be considered when using a
fax server:
a) Fax modem
Fax modems are available very cheaply. However, they are not always
sufficiently tamper-resistant and moreover are not supported by all fax server
applications when operated continuously. Therefore they should be confined
to private usage and to individual workstations.
b) Passive ISDN cards
Passive ISDN cards are simple to build and hence inexpensive. The main
communication load is borne by the computer. This can be a problem where
heavy use is made of the fax server (e.g. when it is used for serial fax
transmissions). With passive ISDN cards, assuming that the recipient has an
appropriate device, transmission using the Group 4 standard is generally also
possible. If fax data has to be transmitted using the Group 3 standard, then the
data has to be converted accordingly. As in the case of fax modems, passive
ISDN cards should be used only on a single workstation or in the private
domain.
c) Active ISDN cards
Active ISDN cards, also known as ISDN controllers, have a processor of their
own. This means they can handle the ISDN protocol largely independently.
According to the specification of the Common ISDN API (CAPI), the fax data
must be transmitted to the ISDN card in Structured Fax File (SFF) format.
Conversion must take place on the fax server. Like modems, active ISDN
cards in the Group 3 standard only support transmission rates of 9,600 and
14,400 bps, using the Modified Huffmann compression code. A major
disadvantage both of fax modems and also of active and passive ISDN cards is
the fact that these can also be used for other purposes than fax transmission,
e.g. in modem operation or as a remote access component. However, for
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IT-Baseline Protection Manual: Oktober 2000