VigorSwitch G1280 User’s Guide
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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.
The maximum length of the extension is equal to the quantity (slotTime - minFrameSize). The
MAC continues to monitor the medium for collisions while it is transmitting extension bits,
and it will treat any collision that occurs after the threshold (slotTime) as a late collision.
