What causes the maximum frame lenght and in consequence the MTU of the different network technologies

The different Maximum Transmission Units - MTU - (on Layer 3) are caused by the different maximum frame sizes (on Layer 2) of the differnt network technologies.

Some MTUs:

Ethernet: 1500 Bytes
PPPoE: 1492 Bytes
WLAN (802.11): 2304 Bytes
ISDN: 576 Bytes
...

It is clear that each network technology needs a minimum frame size to enable the collision detection working within the specified maximum network segment distances.

But what is the reason for the given MTUs and the maximum frame sizes?

Update 1

This topic was also discussed here.

Neverland

Posted 2017-01-11T19:50:18.460

Reputation: 111

tl;dr: Design Decissions. PPPoE is the odd one out, because its MTU is a direct result of Ethernet’s MTU. – Daniel B – 2017-01-12T06:30:05.947

But who specified (and why) what e.g. the MTU of Ethernet is 1500 Bytes and not 2000 Bytes or 5000 Bytes or whatever? – Neverland – 2017-01-12T07:21:40.700

The guys who designed it. Wikipedia has a section on the history of Ethernet. I’m not sure you’ll find some justification. Justification is not required in specs.

– Daniel B – 2017-01-12T07:38:17.177

Answers

There tend to be different answers for different link layers. For Ethernet, it was about being long enough to use the medium efficiently yet not so long as to hog the medium and increase latency for other traffic. Also, to not require excessive buffer resources (RAM was expensive back then). ISDN's MTU was probably based on similar considerations.

For 802.11, someone thought 2048 was a nice round number, and padded it some to leave room for link-layer encryption. However, no one ever uses it that way, because most people want to get their traffic onto an Ethernet LAN, so it's easier and more efficient to just use 1500 as the MTU.

Spiff

Posted 2017-01-11T19:50:18.460

Reputation: 84 656

Do any equations exist which prove that e.g. a MTU of 1500 Bytes with Ethernet utilizes a transmisson medium used of average size better than another MTU? – Neverland – 2017-01-12T07:24:00.810