At the Data Link layer the frame structure is nearly identical for all speeds of Ethernet (10/100/1,000/10,000 Mbps). However at the physical layer almost all versions of Ethernet are substantially different from one another with each speed having a distinct set of architecture design rules.

In the version of Ethernet that was developed by Digital, Intel and Xerox (DIX) prior to the adoption of the IEEE 802.3 version of Ethernet, the Preamble and Start Frame Delimiter (SFD) were combined into a single field, though the binary pattern was identical. The field labeled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version, as both uses of the field were common throughout industry.

The Ethernet II Type field is incorporated into the current 802.3 frame definition. The receiving node must determine which higher-layer protocol is present in an incoming frame by examining the Length/Type field. If the two-octet value is equal to or greater than 0x600 (hexadecimal), then the frame is interpreted according to the Ethernet II type code indicated.

Another frame definition appears when 1000 Mbps Ethernet is used in a half duplex transmission. The transmission must be a minimum of 512 octets in length to meet the timing requirements for collision detection, unlike the minimum of 64 octets (512 bits) required for slower speeds of Ethernet. If the actual frame transmitted is less than 512 octets then special extension symbols are appended following the actual frame to stretch the transmission out long enough to equal the minimum of 512 octets. These extension symbols are counted as part of the frame for the purpose of meeting the timing requirements for collision detection, but are not counted against the minimum frame size since they are discarded just like the Preamble and SFD. The extension is inserted after the FCS field and not included in the FCS calculations.

Web Links Working with Ethernet Frames http://howto.lycos.com/lycos/step/ 1,,5+30+34556+34588+34592,00.html