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Signamax
™
065-7921PoE 12-Port 10/100/1000BaseT/TX Web Smart PoE Switch
34
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 frameTooLong.
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.