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.

Increasing the cable length not only prolongs signal propagation time but also increases the signal attenuation. If the cable length exceeds the maximum value specified in the applicable standard, the resulting signal attenuation could be high enough to prevent the system from distinguishing between the various signal levels as specified in the standard. In this case, communication can no longer be ensured along the entire length of the wires or optical fiber cables that are in use.