Information Technology Reference
In-Depth Information
26.3 Ethernet - media access control (MAC) layer
When sending data the MAC layer takes the information from the LLC link layer. Figure
26.3 shows the IEEE 802.3 frame format. It contains 2 or 6 bytes for the source and destina-
tion addresses (16 or 48 bits each), 4 bytes for the CRC (32 bits) and 2 bytes for the LLC
length (16 bits). The LLC part may be up to 1500 bytes long. The preamble and delay com-
ponents define the start and end of the frame. The initial preamble and start delimiter are, in
total, 8 bytes long and the delay component is a minimum of 96 bytes long.
A seven-byte preamble precedes the Ethernet 802.3 frame. Each byte of the preamble has
a fixed binary pattern of
10101010
and each node on the network uses it to synchronise
their clock and transmission timings. It also informs nodes that a frame is to be sent and for
them to check the destination address in the frame.
The end of the frame there is a 96-bit delay period, which provides the minimum delay
between two frames. This slot time delay allows for the worst-case network propagation de-
lay. The start delimiter field (SDF) is a single byte (or octet) of
10101011
. It follows the
preamble and identifies that there is a valid frame being transmitted. Most Ethernet systems
use a 48-bit MAC address for the sending and receiving node. Each Ethernet node has a
unique MAC address, which is normally defined as hexadecimal digits, such as:
4C - 31 - 22 - 10 - F1 - 32 or
4C31 : 2210: F132.
A 48-bit address field allows 2
48
different addresses (or approximately 281 474 976 710 000
different addresses). The LLC length field defines whether the frame contains information or
it can be used to define the number of bytes in the logical link field. The logical link field can
contain up to 1500 bytes of information and has a minimum of 46 bytes; its format is given
in Figure 26.3. If the information is greater than this upper limit then multiple frames are
sent. Also, if the field is less than the lower limit then it is padded with extra redundant bits.
The 32-bit frame check sequence (or FCS) is an error detection scheme. It is used to de-
termine transmission errors and is often referred to as a cyclic redundancy check (CRC) or
simply as a checksum.
7 bytes
1 byte
6 bytes
6 bytes
2 bytes
4 bytes
96 bits
Start
delimiter
Destination
address
Source
address
Preamble
Length
FCS
Delay
10101011
10101...0101010
46 to 1500 bytes
Data field (Logical Link Control)
Figure 26.3
IEEE 802.3 frame format
If the transmission rate is 10Mbps, the time for one bit to be transmitted will be:
