Cable Length Restriction¶
Cable Length Restrictions
For the maximum possible cable lengths within an Ethernet network various factors have to be taken into account. So, for twisted pair cables (TP cables) are used as transmission medium for 10 Mbit/s Ethernet (10Base-T) as well as for 100 Mbit/s (Fast) Ethernet (100Base-TX). For a transmission rate of 10 Mbit/s, cables of at least category 3 (IEA/TIA 568-A-5 Cat3) or class C (according to European standards) are allowed. For Fast Ethernet with a transmission rate of 100 Mbit/s, cables of category 5 (Cat5) or class D or higher have to be used. The maximum length of a segment which is the maximum distance between two network components is restricted to 100 m due to the electric properties of the cable.
Furthermore, the length restriction for one collision domain has to be observed. A collision domain is the area within a network which can be affected by a possibly occurring collision (i.e. the area the collision can propagate over). This, however, only applies if the components operate in half duplex mode since the CSMA/CD access method is only used in this mode. If the components operate in full duplex mode, no collisions can occur. Reliable operation of the collision detection method is important which means that it has to be able to detect possible collisions even for the smallest possible frame size of 64 bytes (512 bits). But this is only guaranteed if the first bit of the frame arrives at the most distant subscriber within the collision domain before the last bit has left the transmitting station. Furthermore, the collision must be able to propagate to both directions within the same time. Therefore, the maximum distance between two ends must not be longer than the distance corresponding to the half signal propagation time of 512 bits. Thus, the resulting maximum possible length of the collision domain is 2000 m for a transmission rate of 10 Mbit/s and 200 m for 100 Mbit/s. In addition, the bit delay times caused by the passed network components have also to be considered.
The following table shows the specified properties of the respective cable types per 100 m.
| Parameter | 10Base-T [10 MHz] | 100Base-TX [100 MHz] |
|---|---|---|
| Attenuation [dB / 100m] | 10.7 | 23.2 |
| NEXT [dB / 100m] | 23 | 24 |
| ACR [dB / 100m] | N/A | 4 |
| Return loss [dB / 100m] | 18 | 10 |
| Wave impedance [Ohms] | 100 | 100 |
| Category | 3 or higher | 5 |
| Class | C or higher | D or higher |