American Fibertek SM24p-PoE Manuel d'instructions télécharger pdf (Page 38)

User Manual

Rev.A1

1-Mar-11

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, 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 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.

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 a 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

received frame is tagged VLAN and the next two bytes are Tag Control Information

1 2 ... 33 34 35 36 37 38 39 40 41 42 43 ... 124 125

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American Fibertek SM24p-PoE Manuel d'instructions Page 38

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