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.
- 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?
This topic was also discussed here.
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.