T 2.46 Exceeding the maximum allowed cable/bus length or ring size

Depending on the cable type, the topology, and the transmission methods, the corresponding standards provide for maximum cable and/or bus lengths and maximum ring sizes in order to guarantee the functionality of the network according to this standard. Excessive cable, bus, or ring lengths prolong the signal runtimes beyond the extent designed for the corresponding transmission method, and so the availability of the respective network segment or the communication bandwidth is reduced.

The occurring phenomena depend on the access method:

• With the network segments that use the access method CSMA/CD (Carrier Sense Multiple Access/Collision Detection), all terminal devices access with equal rights although the medium can only be used exclusively by one terminal device in each case. For this, every terminal device initially checks whether the medium is available for use (carrier sense). If this is the case, the corresponding terminal device starts the transmission. If this happens simultaneously on several terminal devices (multiple access), a collision occurs, which is detected by the sending terminal devices (collision detection) and results in a repetition of the check as to whether the medium is available, including subsequent repeated transmission.
  
  If the maximum defined signal runtime on the medium is exceeded, collisions may not be detected in the designed interval (collision detection). This means that a terminal device has already begun transmitting data, while another terminal device still considers the transmission medium to be available. In this case, so-called late collisions occur, rendering the corresponding data packet useless and blocking the medium excessively depending on the length of the data packet. The transmission bandwidth available on the medium may be limited significantly as a result. Normally, a loss of information does not occur due to the protection of the network access protocol despite the loss of individual data packets. For example, Ethernet or Fast Ethernet use the CSMA/CD transmission method.
• Transmission methods based on the token passing procedure use a specific data packet (the so-called token) in order to define the terminal device which is allowed to occupy the medium. If a terminal device captures the token, it takes control of the medium and passes the token to the next terminal device depending on the implemented token passing procedure. This guarantees that only a single device has control over the medium at any one time.
  
  Synchronous data transmission at a constant bit rate is essential for network segments operating a token passing procedure. If the medium is not available, the corresponding intervals for the different bits are used for transmitting the data packets; once the medium is available, the intervals are used for passing the token. If the maximum admissible signal runtime is exceeded, the constant bit rate designed for the corresponding transmission method cannot be complied with any more and so communication fails. For example, token ring or FDDI are based on the token passing procedure.

Along with a prolongation of the signal runtime, longer cables increase dampening. If the cable lengths exceed the corresponding standard, the dampening caused by the cable may reach levels that make any differentiation of the different signal levels according to the definitions in the standard impossible. In this case, communication using the corresponding wires or glass fibres cannot be guaranteed along their entire length.