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For
information on ATM testing 
The 25 Mbps interface operates at 25.6 Mbps
over twisted pair cables, compliant with ATM Forum UNI specifications.
The interface has 4B/5B NRZI coding with RJ48 electrical connectors.
The pin assignments of the standard 25 Mbps
UTP cable are as follows:
| Pin |
Straight Cable |
Cross Cable |
| 1 |
Rx+ (tip) |
Tx+ (tip ) |
| 2 |
Rx- (ring) |
Tx- (ring ) |
| 7 |
Tx+ (tip) |
Rx+ (tip ) |
| 8 |
Tx- (ring) |
Rx- (ring ) |
Pins 3, 4, 5 and 6 are not used.
The pin assignments of older IBM cables are
as follows:
| Pin |
Straight Cable |
Cross Cable |
| 3 |
Rx+ (tip)
|
Tx+ (tip)
|
| 4 |
Tx+ (tip)
|
Rx+ (tip)
|
| 5 |
Tx- (ring)
|
Rx- (ring)
|
| 6 |
Rx- (ring)
|
Tx- (ring)
|
Pin 1, 2, 7 and 8 are not used.
For further information about 25 Mbps, refer
to the ATM Forum Physical Interface Specification for 25.6 Mbps
over Twisted Pair Cable, June 1995 (af-phy-0040.00).
HEC generation
ATM HEC generation is used with the addition of COSET.
Cell scrambling
All 53 bytes of the ATM cell are scrambled prior to transmission;
however, scrambling may be disabled. The scrambling polynomial
is x10 + x7 + 1 and is XORed with the
incoming data every 4 clock periods.
Cell delineation
Cell delineation is performed using the 4B5B codes. X_X marks
the start of cells with the scrambler reset; X_4 marks the start
of cells with no scrambler reset.
8 KHz timing signal
A sync event timing marker is sent at
the next octet boundary after an incoming sync signal is detected.
The sync event has priority over all line activity.
Line coding
Line coding has the capability to
be disabled. All 53 bytes of the cell are encoded prior to transmission.
Commands are passed through the 4B5B codes as follows:
| X_X |
Start of cell with scrambler
reset. |
| X_4 |
Start of cell with no scrambler
reset. |
| X_8 |
Sync event. |
Reception of any other command is an error
and can cause cell discard.
Physical medium connectors
The 25 Mbps interface supports category 3, 4, 5 UTP cables.
A 100 Ohm resistive load is required.
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