Frame FormatsΒΆ
One fundamental part of the Ethernet specification is the arrangement of the data transfer format. When transferring data via Ethernet, the actual user data are preceded by a so-called preamble (which is among other things used to synchronize the receiver stations) as well as the hardware source and target address and a type length field. A checksum follows after the user data. All information mentioned above together constitute an Ethernet frame. During the development of the Ethernet, different types of frames arose. The following figure shows the structure of an Ethernet 802.3 frame.
Structure of an Ethernet 802.3 frame
It has to be observed that the transferred user data do not inevitably contain only useful information. When transmitting data using a protocol above Ethernet (refer to 1.2.5), each protocol layer passed prior to the actual transmission supplements the original user data by its specific frame or header, so that the maximum number of actual user data is smaller, depending on the used protocols.
MAC Address
Each Ethernet terminal device that has the MAC layer functions implemented (refer to 1.2.5 Ethernet and TCP/IP) has a world-wide unique hardware and MAC address. In this 6 bytes address, the two most significant bits of the first byte have specific functions. The most significant bit is also called the I/G bit (Individual/Group bit) and indicates whether it is an individual world-wide unique address (unicast address, I/G bit = 0) or a group address (I/G bit = 1). The second most significant bit is called the G/L bit and indicates whether it is a globally or a locally administered MAC address. A GAA (Globally Administered Address, G/L bit = 0) is an address which is fixed programmed by the device manufacturer and has to be unique all over the world. An LAA (Locally Administered Address, G/L bit = 1) can be a MAC address which has been changed afterwards for the use within a network. For this, it has to be observed that a MAC address has to be unique within a network.
The first 3 bytes of a MAC address are the manufacturer-related address part. Using this value, the manufacturer of an Ethernet chip can be determined. Each manufacturer of Ethernet components has one or several pre-defined address ranges assigned he can use for his products. 3COM, for example, uses among others the MAC address range 02-60-8C-xx-xx-xx.
For Ethernet, the MAC address is represented in a canonical form. This representation starts with the least significant bit (LSB) and ends with the most significant bit (MSB) of a byte. The following figure shows a global unicast address of the manufacturer 3COM.
Canonical representation 02-60-8C-00-00-01
Binary representation 01000000-00000110-00110001-00000000-00000000-10000000