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What if a VLAN tagging is applied?
VLAN tagging is a 4-byte long data immediately following the MAC source
address. When tagged VLAN is applied, the Ethernet frame structure will have a
little change shown as follows.
Only two fields, VLAN ID and Tag control information are different in
comparison with the basic Ethernet frame. The rest fields are the same.
The first two bytes is VLAN type ID with the value of 0x8100 indicating the
received frame is tagged VLAN and the next two bytes are Tag Control Information
(TCI) used to provide user priority and Both VLAN ID and TCI will be explained in
the following table.
Bits 15-13
User Priority 7-0, 0 is lowest priority
Bit 12
CFI (Canonical Format Indicator)
1: RIF field is present in the tag header
0: No RIF field is present
Bits 11-0
VID (VLAN Identifier)
0x000: Null VID. No VID is present and only user
priority is present.
0x001: Default VID
0xFFF: Reserved
Table 3-5
Note: RIF is used in Token Ring network to provide source routing and comprises
two fields, Routing Control and Route Descriptor.
When MAC parses the received frame and finds a reserved special value
0x8100 at the location of the Length/Type field of the normal non-VLAN frame, it will
interpret the received frame as a tagged VLAN frame. If this happens in a switch,
the MAC will forward it, according to its priority and egress rule, to all the ports that
is associated with that VID. If it happens in a network interface card, MAC will
deprive off the tag header and process it in the same way as a basic normal frame.
For a VLAN-enabled LAN, all involved devices must be equipped with VLAN
optional function.
At operating speeds above 100 Mbps, the slotTime employed at slower
speeds is inadequate to accommodate network topologies of the desired physical
extent. Carrier Extension provides a means by which the slotTime can be increased
to a sufficient value for the desired topologies, without increasing the minFrameSize
parameter, as this would have deleterious effects. Nondata bits, referred to as
extension bits, are appended to frames that are less than slotTime bits in length so
that the resulting transmission is at least one slotTime in duration. Carrier Extension
can be performed only if the underlying physical layer is capable of sending and
receiving symbols that are readily distinguished from data symbols, as is the case in
most physical layers that use a block encoding/decoding scheme.