IT Baseline Protection Manual - The Information Warfare Site

Safeguard Catalogue - Communications Remarks
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FDDI
The FDDI (Fiber Distributed Data Interface) standard was defined in 1989 by
ANSI and is based - like Token-Ring - on the token passing technique.
However, FDDI additionally makes use of early token release, which forwards
the token to the next station immediately after the last data packet has been
send. This reduces the idle times in the ring and helps achieve a higher
bandwidth.
FDDI uses optical fibre cables as the transmission medium, and provides a
transmission rate of 100 Mbit/s. Due to its high throughput, FDDI is ideal for
use in the backbone areas. Additional advantages include the high fault
tolerance resulting from the double-ring topology, and the electromagnetic
stability arising from the use of optical fibre cables. As opposed to Ethernet,
FDDI is also suitable for performance-dependent multimedia applications,
because it ensures a maximum delay time.
If both rings are used for data transfer, a transmission rate of as much as 200
Mbit/s is achievable; the advantage of the high fault tolerance is eliminated in
this case however, because if one of the rings malfunctions, it is no longer
possible to switch over automatically to the other one.
FDDI components are more expensive than Ethernet components offering a
similar functionality; for this reason, the benefits derived from the use of
FDDI should always be compared with the costs it generates.
FDDI can also be operated on copper cables, in which case it is termed CDDI
(Copper Distributed Data Interface).
ATM
ATM stands for Asynchronous Transfer Mode, and involves a transmission
technique which is very suitable for use in the backbone area of a network,
and which can also supply real-time services in this area.
In ATM, information of all types is transferred in packets of a fixed length,
termed cells. The information can consist of any required data, including
video and audio data. The standard length of the packets allows the ATM
switches to process the cells almost entirely through the use of hardware
components, thus achieving a higher throughput. This results in calculable
delays during the transfer of any type of information, so that separate
bandwidths can be guaranteed for individual applications. ATM is therefore a
very suitable technology for multimedia applications, as it guarantees a
computable real-time response and, thus, Quality of Service (QoS). This
means that the required bandwidths can be allocated statically or dynamically
to every connected device.
Transmission as such takes place on the basis of virtual links. No fixed
channels are activated between communicating stations; instead, the cells are
transferred through the network via routes determined shortly before the cells
were generated. This achieves typical transmission rates of roughly 25 MBit/s,
155 MBit/s and 622 MBit/s.
ATM components are still very expensive though; so to safeguard
investments, efforts should therefore be made to integrate ATM components
with the other technologies already existing in the network. However, ATM
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IT-Baseline Protection Manual: Oktober 2000