Propagation Delay

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:

• speed of light in a vacuum = 300,000,000 metres/second
• speed of electricity in a copper cable = 200,000,000 metres/second
• (200,000,000 m/s) / (10,000,000 bits / s) = 20 metres per bit

We can further determine that a minimum size Ethernet frame consisting of 64 bytes or 512 bits will occupy 10,240 metres of cable.

The Relationship

The only time that an Ethernet controller can detect collisions on the wire is when it is in the transmit mode. When an Ethernet NIC has finished transmitting and switches to receive mode, the only thing it listens for is the 64 bit preamble that signals the start of a data frame. The minimum frame size in Ethernet is specified such that, based on the speed of propagation of electrical signals in copper media, an Ethernet card is guaranteed to remain in transmit mode and therefore detecting collisions long enough for a collision to propagate back to it from the farthest point on the wire from it.

Take, for example, a length of 10BASE5 thick Ethernet cabling exactly 500 meters long (the maximum that the spec allows) with two stations, Station A and Station B, attached to the farthest ends of it.

If Station A begins to transmit, it will have transmitted 25 bits by the time the signal reaches Station B, 500 meters away. If Station B begins to transmit at the last possible instant before Station A's signal reaches it, the collision will reach Station A 25 bit-times later (the time it takes for the signal on the wire to travel one bit-length -- 20 metres in copper cable). Station A will have transmitted only 50 bits when the collision reaches it -- nowhere near the 512 bit boundary for an early collision.

Upon closer examination, however, a peculiarity arises. If a normal collision happens before the 512 bit boundary, Station A would have to be over 5000 metres away from Station B before a late collision occurred. Examine the maths for yourself: 512 bits times 20 metres/bit = 10,240 metres. That's 256 bits or approximately 5000 metres for the signal to propagate from Station A to Station B and 5000 metres for the collision event to propagate back to and be detected by Station A. It seems like a late collision would never occur with a maximum cable length of only 500 meters. What is the reason for the overhead?

The reason for the overhead is twofold. First of all, while the maximum possible cable segment length in Ethernet is 500 metres, it is possible to extend that length with up to 4 repeaters before the IEEE 802.3 spec is violated. This means that the signal may have to travel through as much as 2500 metres of cable to reach Station B, or 5000 metres of cable round trip. The second and final reason for the overhead lies solely in the carefulness of Ethernet's inventors. Generally the spec is twice as strict as it needs to be, allowing ample room for errors.

Herein lies one of the greatest strengths and weaknesses of Ethernet. It is a strength in that if you need to, you can probably get away with violating the specs -- an extra length of cable here, an extra repeater there and your network continues to run normally. It is a weakness in that while you can get away with violating the specs, there is a very fine line between a network that is violating the specs and is running and a network that is violating the specs and is crippled by late collisions and you never know which extra bit of wire or extra repeater is going to cross the line.

Despite this dire warning, there are some general rules for violating specs:

• If your vendor tells you you can violate the spec and you're not mixing vendors, it's probably ok. If you mix vendors, obey the strictest vendor.
• If something is wrong with your network and you know that it violates the spec in places, those places should be the first ones you check. Try segmenting the network with a bridge and see which side of the bridge the problems are on.