VLAN tagging is a MAC option that provides three important
capabilities not previously available to Ethernet network users and network
managers:
� Provides
a means to expedite time-critical network traffic by setting transmission
priorities for outgoing frames.
�Allows
stations to be assigned to logical groups, to communicate across multiple LANs
as though they were on a single LAN. Bridges and switches filter destination
addresses and forward VLAN frames only to ports that serve the VLAN to which the
traffic belongs.
�Simplifies
network management and makes adds, moves, and changes easier to administer.
A VLAN-tagged frame is simply a basic MAC data frame that
has had a 4-byte VLAN header inserted between the SA and Length/Type fields, as
shown in Figure 7-11.
Figure 7-11 VLAN-Tagged Frames Are Identified When the
MAC Finds the LAN Type Value in the Normal Length/Type Field Location
The VLAN header consists of two fields:
�A
reserved 2-byte type value, indicating that the frame is a VLAN frame
�A
two-byte Tag-Control field that contains both the transmission priority (0 to 7,
where 7 is the highest) and a VLAN ID that identifies the particular VLAN over
which the frame is to be sent
The receiving MAC reads the reserved type value, which is
located in the normal Length/Type field position, and interprets the received
frame as a VLAN frame. Then the following occurs:
�If
the MAC is installed in a switch port, the frame is forwarded according to its
priority level to all ports that are associated with the indicated VLAN
identifier.
�If
the MAC is installed in an end station, it removes the 4-byte VLAN header and
processes the frame in the same manner as a basic data frame.
VLAN tagging requires that all network nodes involved with
a VLAN group be equipped with the VLAN option.
The Ethernet Physical Layers
Because Ethernet devices implement only the bottom two
layers of the OSI protocol stack, they are typically implemented as network
interface cards (NICs) that plug into the host device's motherboard. The
different NICs are identified by a three-part product name that is based on the
physical layer attributes.
The naming convention is a concatenation of three terms
indicating the transmission rate, the transmission method, and the media
type/signal encoding. For example, consider this:
�10Base-T
= 10 Mbps, baseband, over two twisted-pair cables
�100Base-T2
= 100 Mbps, baseband, over two twisted-pair cables
�100Base-T4
= 100 Mbps, baseband, over four-twisted pair cables
�1000Base-LX
= 100 Mbps, baseband, long wavelength over optical fiber cable
A question sometimes arises as to why the middle term
always seems to be "Base." Early versions of the protocol also allowed for
broadband transmission (for example, 10Broad), but broadband implementations
were not successful in the marketplace. All current Ethernet implementations use
baseband transmission.
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