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Draytek VigorSwitch G1240 Manual

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VigorSwitch G1240 User’s Guide
27
Bits 11-0
VID (VLAN Identifier)
0x000: Null VID. No VID is present and only user
priority is present.
0x001: Default VID
0xFFF: Reserved
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.
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.
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The switch is a layer 2 Ethernet Switch equipped with 24 Fast Ethernet ports and 2 optional
modules which support Gigabit Ethernet or 100M Ethernet. Each port on it is an
independent LAN segment and thus has 26 LAN segments and 26 collision domains,
contrast to the traditional shared Ethernet HUB in which all ports share the same media and
use the same collision domain and thus limit the bandwidth utilization. With switch’s
separated collision domain, it can extend the LAN diameter farther than the shared HUB
does and highly improve the efficiency of the traffic transmission.
Due to the architecture, the switch can provide full-duplex operation to double the
bandwidth per port and many other features, such as VLAN, bandwidth aggregation and so
on, not able to be supported in a shared hub.
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As per the description in the section of “What’s the Ethernet”, Ethernet utilizes CSMA/CD
to arbitrate who can transmit data to the station(s) attached in the LAN. When more than
one station transmits data within the same slot time, the signals will collide, referred to as
… 33 …


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