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Frame Reception
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.