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Draytek VigorSwitch G1280 Manual

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VigorSwitch G1280 User’s Guide
107
the receiver of the target device begins receiving the bit stream, and looks for the PRE
(Preamble) pattern and Start-of-Frame Delimiter (SFD) that indicates the next bit is the
starting point of the MAC frame until all bit of the frame is received.
For a received frame, the MAC will check:
1. If it is less than one slotTime in length, i.e. short packet, and if yes, it will be discarded
by MAC because, by definition, the valid frame must be longer than the slotTime. If the
length of the frame is less than one slotTime, it means there may be a collision happened
somewhere or an interface malfunctioned in the LAN. When detecting the case, the MAC
drops the packet and goes back to the ready state.
2. If the DA of the received frame exactly matches the physical address that the receiving
MAC owns or the multicast address designated to recognize. If not, discards it and the
MAC passes the frame to its client and goes back to the ready state.
3. If the frame is too long. If yes, throws it away and reports frame Too Long.
4. If the FCS of the received frame is valid. If not, for 10M and 100M Ethernet, discards the
frame. For Gigabit Ethernet or higher speed Ethernet, MAC has to check one more field,
i.e. extra bit field, if FCS is invalid. If there is any extra bits existed, which must meet the
specification of IEEE802.3. When both FCS and extra bits are valid, the received frame
will be accepted, otherwise discards the received frame and reports frameCheckError if
no extra bits appended or alignmentError if extra bits appended.
5. If the length/type is valid. If not, discards the packet and reports lengthError.
6. If all five procedures above are ok, then the MAC treats the frame as good and
de-assembles the frame.
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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
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
… 113


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