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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 bits of the frame are 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 any extra bits exist, they must meet the
specification of IEEE802.3. When both FCS and extra bits are valid, the received frame
will be accepted, otherwise it 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, it 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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