Signamax
™
065-7840C-WS 24-Port 10/100/1000BaseT/TX WebSmart Switch
Publication date: June, 2011
Revision 1.0
31
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 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 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 which 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, the MAC discards the frame. For Gigabit Ethernet or higher
speed Ethernet, the MAC has to check one more field, i.e. the extra bit
field, to determine if the FCS is invalid, or if there are any extra bits
existing, which must meet the specification of IEEE 802.3. When both
the 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 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.
What if VLAN tagging is applied?
VLAN tagging is a 4-byte long data immediately following the MAC source
address. When tagged VLAN is applied, the Ethernet frame structure will have a
little change shown as follows.
Only two fields, VLAN ID and Tag control information, are different in
comparison with the basic Ethernet frame. The rest fields are the same.
The first two bytes is VLAN type ID with the value of 0x8100 indicating the
Summary of Contents for 065-7940C-WS
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