TNETX4080GGP datasheet(24/64 Pages) TI1

24 / 64 page

TNETX4080

ThunderSWITCH II™ 8-PORT 10-/100-MBIT/S ETHERNET™ SWITCH

SPWS050A – AUGUST 1998 – REVISED NOVEMBER 1998

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

frame format on the NM port (continued)

To provide a CRC word, which includes the header, the NM port generates a new CRC word as the frame is

being read out. It simultaneously checks the existing CRC in the frame and, if an error is found, ensures that

the final byte of the newly generated CRC is corrupted to contain an error, too. The CRC word is deliberately

corrupted if the header parity protection (described in the following) indicates an error in the header. In either

case, the pfe bit also is set to 1 after the final byte of the frame has been read from NMData.

If the frame was received on a port other than the NM port, then the crctype bit is set, depending on whether

or not an IEEE Std 802.1Q tag header was inserted into the frame during ingress.

–

If crctype = 1, a header was inserted.

–

If crctype = 0, a header was not inserted (crctype also is 0 if the frame VLAN ID was 0x000 and was

replaced by the port VLANID (PVID) from the PortxQTag register).

In an IEEE Std 802.1D-compliant application, the header simply can be removed from the frame to produce

a headerless frame with a correct CRC word.

–

All other bits in the byte are reserved and are 0.

The second TPID byte output contains:

D Odd-parity protection bits for the other three bytes in the tag header

D Bit 5 protects the first byte of the TPID field (i.e., the one containing crctype and source port number).

D Bit 6 protects the first byte of the VLAN ID field.

D Bit 7 protects the second byte of the VLAN ID field.

D All other bits in the byte are reserved and are 0.

TPID (Tag Protocol Identifier)

TCI (Tag Control Information)

Destination

Address

Source

Address

TPID

TCI

Length/Type

FCS

(CRC-32)

Data

Reserved

2nd

TCI

Byte

Odd Parity Bits

Reserved

Source

Port

Byte 1

Byte 2

CRC

Type

802.1Q header

1st

TCI

Byte

1st

TPID

Byte

10 0 0 0

0 00 0 0 00 Priority cfi

VLAN ID

6 Bytes

2 Bytes

46–1517 Bytes

2 Bytes

4 Bytes

7 6 5 4 3 2 1 0 7 6 5 4 3 21 0

7 6

4 3 21 07 6 5 4 3 2 1 0

Figure 2. NM Frame Format

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