# Minimum package length vs maximum cable length

## Learning goals

1. understand that the cable length has an influence to transfer rate
2. understand that speed of light is responsible for the connection between cable length and transfer rate
3. be able to calculate the maximum cable length for a given transfer rate
4. understand that the cable length is part of the Ethernet protocol

## Script

Why Ethernet frame has to have a certain minimum length

The reason is that Ethernet is a non deterministic protocol. It means that any device on the network can start transferring data at any given time. The collision may happen. And what you want to achieve is that when you send data you want to occupy the entire shared medium. Imagine that you transfer 1 bit of data. Let’s assume that you have 100MBit/s Ethernet card. It means that you can send 100 million bits per second. So for sending 1 bit you need 10 nano seconds of time.

Transferring bit over the cable means that you put electromagnetic wave on the cable. The data spreads at the speed of light (the speed of light is approximately 300 million m/s). So we can multiply (speed of light*10ns). And what we see is 100MBit/s Ethernet device within 1 clock cycle the signal would have traveled something like 3 meters. That means that if you only put 1 bit on the cable and you want to occupy the entire network and medium than this medium should be shorter than 3 meters. But this doesn’t make sense.

Cables have to be 100 meters and longer. And the inventors of Ethernet said that let’s just have an Ethernet frame that has to have a certain minimum length. The data section of the Ethernet frame has to at least 46 bytes. So even if you want to send only 1 bit of data you still have to have payload of at least 46 bytes. Additionally there is 18 bytes for Header (14) and Checksum (4). So this makes 64 bytes=512 bits which is an amount of clock cycles that you occupy the wire when you start transferring Ethernet. If we 512 clock cycles * 3meters then we realize that can achieve a distance of 1500 m.

In real Ethernet the distance that will be supported is a little bit shorter to make sure that everything works fine. But we really want to occupy the cable all the time when you’re sending. You could have achieved this in various ways:
- You could have made the cable shorter
- You could have made the packages longer
- You can also change the clock frequency

But one thing you cannot change is the speed of light --Jane Kruch (discusscontribs) 10:41, 29 October 2013 (UTC)

## Quiz

1 How much time passes by in a 100 Mbit/s ethernet device per clock cycle?

 10 nano seconds 10^-8 seconds

2 How much time passes by in 10 MBit/s ethernet device per clock cycle?

 1 nano second 10 nano seconds 100 nano seconds

3 How far can a Bit travel approximately on a 10 Mbit/s ethernet device during one clock cycle?

 1 meter 3 meter 30 meter 100 meter