The original Ethernet header (prior to IEEE 802.2 and 802.3) did not use an LLC header.
Instead, it used a type ?¬?eld in the Ethernet header to identify the Layer 3 protocol being
carried in the Ethernet frame.
MAC Sublayer
The MAC sublayer deals with physical media access. The IEEE 802.3 MAC speci?¬?cation
de?¬?nes MAC addresses, which uniquely identify multiple devices at the data link layer. The
MAC sublayer maintains a table of MAC addresses (physical addresses) of devices. To
participate on the network, each device must have a unique MAC address.
The Role of CSMA/CD in Ethernet
Ethernet signals are transmitted to every station connected to the LAN, using a special set
of rules to determine which station can ???talk??? at any particular time. This topic describes
that set of rules.
Ethernet LANs manage the signals on a network by CSMA/CD, which is an important
aspect of Ethernet. Figure 1-93 illustrates the CSMA/CD process.
In an Ethernet LAN, before transmitting, a computer ?¬?rst listens to the network media. If
the media is idle, the computer sends its data. After a transmission has been sent, the
computers on the network compete for the next available idle time to send another frame.
This competition for idle time means that no one station has an advantage over another on
the network.
Pages:
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173