Signamax
™
065-7921PoE 12-Port 10/100/1000BaseT/TX Web Smart PoE Switch
35
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 VLAN ID, which are explained respectively 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 of 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.
