Understanding Ethernet Cable Length Limitations

Network Cable Propagation Delay

Typical propagation delay for category 5e UTP is a bit less than 5 nS per meter (worst case allowed is 5.7 nS/m)

when hosts A and B are at opposite ends of the network. Suppose host A begins transmitting a frame at time t. It takes it one link latency (let’s denote the latency as d) for the frame to reach host B. Thus, the first bit of A’s frame arrives at B at time t+d. Suppose an instant before host A’s frame arrives (i.e., B still sees an idle line), host B begins to transmit its own frame. B’s frame will immediately collide with A’s frame, and this collision will be detected by host B. Host B will send the 32-bit jamming sequence, as described above. (B’s frame will be a runt.) Unfortunately, host A will not know that the collision occurred until B’s frame reaches it, which will happen one link latency later, at time t+2×d. Host A must continue to transmit until this time in order to detect the collision. In other words, host A must transmit for 2×d to be sure that it detects all possible collisions. Considering that a maximally configured Ethernet is 2500 m long, and that there may be up to four repeaters between any two hosts, the round-trip delay has been determined to be 51.2 μs, which on a 10-Mbps Ethernet corresponds to 512 bits. The other way to look at this situation is that we need to limit the Ethernet’s maximum latency to a fairly small value (e.g., 51.2 μs) for the access algorithm to work; hence, an Ethernet’s maximum length must be something on the order of 2500 m.

On a given Ethernet composed of multiple segments connected with repeaters, all of the stations are involved in the same collision domain. The collision algorithm is limited to 1024 distinct backoff times. Therefore, the maximum number of stations allowed in the standard for a multi-segment LAN linked with repeaters is 1024. However, that doesn’t limit your site to 1024 stations, because Ethernets can be connected together with packet switching devices such as switching hubs or routers.