Introduction

You may know that the minimum frame size in an Ethernet network is 64 bytes or 512 bits, including the 32 bit CRC. You may also know that the maximum length of an Ethernet cable segment is 500 meters for 10BASE5 thick cabling and 185 meters for 10BASE2 thin cabling. It is, however, a much less well known fact that these two specifications are directly related. In this essay, we will discuss the relationship between minimum frame size and maximum cable length.

Propagation Delay

Before we discuss frame size and cable length, an understanding of signal propagation in copper media is necessary. Electrical signals in a copper wire travel at approximately 2/3 the speed of light. This is referred to as the propagation speed of the signal. Since we know that Ethernet operates at 10Mbps or 10,000,000 bits per second, we can determine that the length of wire that one bit occupies is approximately equal to 20 metres or 60 feet via the following maths:

Introduction

Again, we are going to make use of a step-by-step example in order to fully understand how and why late collisions occur.

Late Collisions

Late collisions, on the other hand, are not normal and are usually the result of out of spec. cabling or a malfunctioning adapter. A late collision is defined as any collision that occurs after 512 bits of the frame have been transmitted.

In this discussion we will refer to the same network described in the discussion of early collisions, but with one modification: In this network, the network administrator has violated the maximum cable length (500 meters for 10BASE5 thick ethernet, 185 meters for 10BASE2 thin ethernet) by either adding too many repeaters in between Stations A and B or by laying too much wire between them.

Introduction

We are going to have a look at a step-by-step early collision example to help understand what it's all about.

Early Collisions

In this example, we will refer to an imaginary Ethernet network consisting of Stations A and B and any number of other stations. The status of the network is such that the wire is idle (nobody is talking) and 9.6 microseconds have passed since anybody last talked on the wire.

An early collision is any collision that occurs before 512 bits of the frame have been put onto the wire.

Introduction

The word "Collision" shouldn't be any new news to people who work with networks everyday. If it is thought, don't worrie, that's why you are here.

A collision is an event that happens on an Ethernet network when two stations simultaneously "talk" on the wire. Collisions are a normal part of life in an Ethernet network and under most circumstances should not be considered a problem.

Introduction

CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. It refers to the means of media access, or deciding "who gets to talk" in an Ethernet network.

A more elegant term for "who gets to talk" is to refer to the "media access method", which, in this case, would be "CSMA/CD".