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User Manual
Publication date: January, 2005
Revision A1
33
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 recei ved.
For a received frame, the MAC will check:
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.
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.
If the frame is too long. If yes, throws it away and reports frameTooLong.
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.
If the length/type is valid. If not, discards the packet and reports lengthError.
If all five procedures above are ok, then the MAC treats the frame as good and
de-assembles the frame.