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Draytek VigorSwitch G1240 Manual

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VigorSwitch G1240 User’s Guide
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In essence, the frame reception is the same in both operations of half duplex and full
duplex, except that full-duplex operation uses two buffers to transmit and receive the frame
independently. The receiving node always “listens” if there is traffic running over the
medium when it is not receiving a frame. When a frame destined for the target device
comes, 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
… 32 …


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