Technical data

www.westermo.com Theoretical and general applications 111
Ethernet Address & Packets
All Ethernet hardware has an address that uniquely identifies each node in a network.
This address is programmed into the device by the manufacturer, for example, a net-
work adapter card. This can not be changed by the user or by software, which means
there is not (should not be) two network adapter cards with the same address. This
address is often refered to as the MAC Media Access Control Address.
The Ethernet packet contains the following information:
… Preamble. The preamble is a 64-bit (8 byte) field that contains a synchroniza-
tion pattern consisting of alternating ones and zeros and ending with two con-
secutive ones. After synchronization is established, the preamble is used to
locate the first bit of the packet. The preamble is generated by the LAN inter-
face card.
… Destination Address. The destination address field is a 48-bit (6 byte) field that
specifies the station or stations to which the packet should be sent. Each station
examines this field to determine whether it should accept the packet.
… Source Address. The source address field is a 48-bit (6 byte) field that contains
the unique address of the station that is transmitting the packet.
… Type field. The type field is 16-bit (2 byte) field that identifies the higher-level
protocol associated with the packet. It is interpreted at the data link level.
… Data Field. The data field contains 46 to 1500 bytes. Each octet (8-bit field)
contains any arbitrary sequence of values. The data field is the information
received from Layer 3 (Network Layer). The information, or packet, received
from Layer 3 is broken into frames of information of 46 to 1500 bytes by
Layer 2.
… CRC Field. The Cyclic Redundancy Check (CRC) field is a 32-bit error check-
ing field. The CRC is generated based on the destination address, type and data
fields.
Preamable Destination address Source address Type Data CRC
8 bytes 6 bytes 6 bytes 2 bytes 46 – 1500 bytes 4 bytes
B A C K