User`s guide

Signamax 065-7931 16-Port 10/100/1000BaseT/TX WebSmart Switch

Frame Reception

In essence, the frame reception is the same in both half duplex and full

duplex operations, except that full-duplex operation uses two buffers to transmit and

receive the frames 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 the 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

that has happened somewhere or an interface malfunctioned in

the LAN. When detecting this, 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, the MAC 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 10 M and 100

M Ethernet, the MAC discards the frame. For Gigabit Ethernet or

higher speed Ethernet, the MAC has to check one more field, i.e.

extra bit field, if the FCS is invalid. The MAC also checks if there

are any extra bits existing, which must meet the specification of

IEEE 802.3. When both FCS and extra bits are valid, the

received frame will be accepted; otherwise, the MAC discards the

received frame and reports frameCheckError if no extra bits were

appended or alignmentError if extra bits were appended.

5. If the length/type is valid. If not, the MAC 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.