Do you have a network with different topolopies like Token Ring and Ethernet? You wonder why the pervormace is different, when go accross the topology? It's quite amazing how fast you can copy files from Token Ring to Ethernet but how slow it could be if you do it the other way round. The reason is the different MTU (Maximum Transfer Unit) you get in your network. Each media type has a maximum frame size that cannot be exceeded. The link layer is responsible for discovering this MTU and reporting it to the protocols above. NDIS drivers may be queried for the local MTU by the protocol stack. Knowledge of the MTU for an interface is used by upper layer protocols such as TCP, which optimizes packet sizes for each medium automatically. If a network interface card (NIC) driver such as an ATM driver uses LAN emulation mode, it may report that it has an MTU higher than what is expected for that media type. For instance, it may emulate Ethernet but report an MTU of 9180 bytes. Windows NT accepts and uses the MTU size reported by the adapter even when it exceeds the normal MTU for a given media type. Below is a list of Default MTU size for different media.

Network	MTU(Bytes)
16 Mbit/Sec Token Ring	17914
4 Mbits/Sec Token Ring	4464
FDDI	4352
Ethernet	1500
IEEE 802.3/802.2	1492
X.25	576

HOWTO MTU under NT 4.0