LAN ACCESS PROTOCOLS
If the length of the first frame is less than the minimum of 520 octets, an extension field
is added to maintain the frame at minimum length. Subsequent frames in a frame-burst
sequence do not need extension fields, and a burst may continue as long as the burst
limit has not been reached. The burst mode is not used in the 10-Mbps and 100-Mbps
Another improvement measure that was optional was to adopt a full-duplex transmis-
sion capability over point-to-point links. In Ref. 12, it has been pointed out that full-duplex
operation is much simpler than half-duplex because it involved no media contention, no
collisions, no need to schedule retransmissions, and no need for extension bits on the end
of short frames. It effectively doubles the bit rate capacity of the link allowing full-rate,
simultaneous, two-way transmission.
Flow control must also be incorporated on an Ethernet full-duplex link. On a point-
to-point link we might typically find on one end of the link perhaps a file server and on
the other side a network switch port. Suppose the receiving node encounters congestion.
It will request the sending node to stop transmitting frames by sending a "pause frame"
which is valid for a short time period.
VLAN2 tagging is another important option available to the IT/system engineer.
VLAN tagged frame is a basic data frame that has had a 4-octet VLAN header inserted
between the SA and length/type fields. It consists of two adjacent fields and indicates
that the frame is a VLAN frame. The first field is a 2-octet type value indicating that
the frame is a VLAN frame. The second field, also 2 octets long, contains a priority
value from 0 to 7, with 7 being the highest priority. It also has a VLAN ID to identify
the particular VLAN over which the frame is to be sent. Three advantages accrue not
previously available to Ethernet users.
1. It provides a means to expedite time-critical network traffic by setting priorities for
2. It simplifies network management by making adds, moves, and changes easier
3. It allows stations to be assigned to logical groups, to communicate across multiple
LANs as though they were a single LAN. Here we find bridges and switches that
filter destination addresses and forward VLAN frames only to ports of the VLAN
to which the traffic is destined.
There are three versions of the 100-Mbps Ethernet and all use UTP (unshielded
twisted pair) cable: 100Base-TX, 100Base-T4, and 100Base-T2. These designations (e.g.,
100Base-TX) are made up of three parts:
100 indicates the transmission rate or 100 Mbps.
"Base" means the transmission is baseband, where the raw electrical signal is applied
directly to the medium. In fact, all Ethernet transmission today is baseband. Broad-
band RF transmission is now obsolete in this application.
TX, T2, T4 indicates twisted pair or fiber-optic cable. T2, two twisted pair, T4, four
Each of the three types of Ethernet uses different encoding and a different set of media-
dependent sublayers. Table 11.1 gives an overview of the physical layer interface for the
three versions compared to their 10-Mbps counterpart.
VLAN stands for virtual LAN.