Ethernet is the de facto standard for local area networks
(LAN). Figure 1 shows the ethernet frame structure according to
IEEE 802.3. Every frame starts with a 7 byte preamble, having the
bit sequence 101010. The manchester coding of this bit sequence
generates a 5.6 us lasting 10MHz oscillation. This is used to
enable the receiver to synchronize. The following frame delimiter
10101011 marks the beginning of the frame. The frame contains a
destination address and a source address. The standard mentions 2
byte and 6 byte address schemes, but the 10 Mbit/sec ethernet
version uses the 6 byte (48 bit) address scheme only. The MSB
(bit 47) of the address is used for multicast and broadcast
services, bit 46 is used to distinguish between local and global
addresses. Local addresses are assigned by the network operator
and have no meaning outside that particular network in question.
Global addresses are given by IEEE in order to ensure that they
are unique in the whole world. The address range of the usable
48-2=46 bits provides about 7x10^13 unique addresses. Every
ethernet station in the world can address every other ethernet
station with its unique ethernet address. The routing of ethernet
frames between subnetworks is done by network layer protocol. In
the TCP/IP protocol world, the address
resolution protocol (ARP) does the mapping between IP and
ethernet address. In the OSI world, the
ethernet address may be part of the NSAP. The data field of the frame may in theory be of length zero.
In order to avoid unnoticeable frame collisions on a LAN, the
padding field ensures that a valid ethernet frame has a length of
at least 64 bytes, counted from the destination address field to
the checksum field. The last field of the ethernet frame contains
the 32 bit checksum for the cyclic redundancy error detection
algorithm. 7 1 2 or 6 2 or 6 2 0-1500 0-46 4 Preamble Frame Destination Source Data Data Pad Check Figure 1 - Ethernet Frame Structure
according to IEEE 802.3 For more information on ethernet
check this site.
Ethernet
delimiter
address
address
length
sum