IT Baseline Protection Manual - The Information Warfare Site

Threats Catalogue Deliberate Acts Remarks
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T 2.46 Exceeding the maximum allowed cable/bus
length or ring size
In accordance with the types of cable, topology and transmission protocols
involved, maximum cable and bus lengths, as well as maximum ring sizes for
networks have been stipulated in order to ensure the functions of the network
as defined by applicable standards. Excessively long cables and buses, as well
as excessively large rings, prolong signal transmission times beyond the limit
specified for the type of transmission protocol involved, thus reducing the
availability of the network segment or the communications bandwidth.
The phenomena which can occur depend on the type of the access control
method used:
- In the case of network segments which use the CSMA/CD (Carrier Sense
Multiple Access/Collision Detection) access method, all stations have the
same access rights to the medium, although it can only be used by one
station at a time. For this purpose, every station first checks whether the
medium is free for use (carrier sense). If so, the station starts the
transmission of data. If several stations carry out this procedure in a
parallel context (multiple access), a collision occurs and is recognised by
all sending stations (collision detection), whereupon the medium is
checked again and transmission is repeated.
If the maximum defined signal propagation delay is exceeded on the
medium, collisions might not be detected in the specified time interval
(collision detection). This means that one end appliance already started to
transfer data while another end appliance still assumes the transfer medium
to be free. In this case, so-called late collisions occur, thus corrupting the
affected data packet and, depending on the length of the data packet, the
medium may be blocked beyond reasonable limits. This can severely
impair the effective transmission bandwidth of the medium. Although
individual data packets might be discarded in this process, the network
access protocol normally prevents data from being lost. For example,
Ethernet and Fast Ethernet use the CSMA/CD communication protocol.
- Transmission techniques based on the token passing procedure use a
special data packet (named token) to determine which station may occupy
the medium. A station which receives this token occupies the medium and,
in accordance with the token passing procedure in use, passes the token on
to the next station. This ensures that the medium is only occupied by a
single station at one time.
Synchronous data transmission at a constant bit rate is a characteristic of
network segments using token passing procedures. When the medium is
busy, the relevant time intervals are used to transmit the data packets.
When the medium is free, these time intervals are used to forward the
token. If the maximum signal propagation time is exceeded, the constant
bit rate specified for the transmission protocol in use can no longer be
guaranteed, thus causing a break down of all communication. For example,
Token Ring and FDDI use the token passing procedure.
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IT-Baseline Protection Manual: Oktober 2000