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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, 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. The DA of received frame should match the physical address of
the receiving MAC or the address of the to be recognized
designated multicast. If not, the frame will be discarded and the
MAC will pass the frame to its client and go back to ready state.
3. If the frame is too long; if yes, the frame will be thrown away and
a Too Long frame will be reported.
4. please note that this kind of sentence is fragment and not
grammatically correct. 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, to
see if FCS is invalid. to see if FCS is invalid.To meet the
specification of IEEE 802.3, the MAC will check to see if there
any extra bits exist. When both FCS and extra bits are valid, the
received frame will be accepted; otherwise the received frame
will be discarded and a frameCheckError will be reported if no
extra bit appended, or an alignmentError will be reported if extra
bits appended.
5. If the length/type is valid; if not, the packet will be discarded and
a lengthError will be reported.
6. If all above five procedures are processed without any error, the
MAC will treat the frame as a good one and start to disassemble
the frame.