Table 12. Access ID Symbol Pulse Positions and
Encoding
The following criteria must be met to guarantee reliable
collision detection:
At least one HomePNA station of a colliding group
must always detect a collision when the delay between
the beginning of its transmitted packet and the begin-
ning of the received colliding packet is between -1.5 µs
and +1.5 µs.
Pulse
Position
TICs from Beginning of AID
Symbol
Bit Encoding
1
2
3
4
66
86
00
01
10
11
In general, any received pulse at a HomePNA station
that does not conform to the pulse position require-
ments of AID symbols 0 through 7 shall indicate a col-
lision on the wire. When a transmitting station senses a
collision, it emits a JAM signal to alert all other stations
to the collision. The following conditions signify a COL-
LISION event:
106
126
The next two AID symbols (5 and 6) encode four bits of
control word information. The MSB is encoded in AID
Symbol 5. Control word messages are described fur-
ther in the Management Interfaces section.
1. A HomePNA station receives an AID that does not
match the one being sent.
AID Transmit Timing
The transmitter encodes the Access ID in a pulse posi-
tion in each 128 TIC interval. Each AID symbol interval
must have only one pulse. Pulse transmission must
start in only one of the four possible positions (mea-
sured from the beginning of the Access ID symbol) de-
fined in Table 12.
2. A HomePNA station receives a pulse outside the
AID_GUARD INTERVAL in AID intervals 0 to 7.
3. A HomePNA station receives a pulse inside the
SILENT_INTERVAL (AID symbol 7).
As in all cases, pulses received during a blanking inter-
val are ignored.
AID Receive Timing
The receiver allows for jitter by establishing a window
around each legal pulse position. This window is -2 +1
TICS wide on either side of the position. A pulse that ar-
rives outside of the legal AID positions is considered a
COLLISION event.
Passive stations (stations not actively transmitting dur-
ing the collision) cannot reliably detect collisions.
Therefore, once a collision is detected by a transmitting
station, the station must inform the rest of the stations
of the collision with a JAM pattern described below.
Only a transmitting station emits a JAM signal.
Collisions
Once a collision is detected, the COLLISION signal to
the MAC interface is asserted and is not reset until the
MAC deactivates the TXEN signal.
A Collision is detected only during Access ID and silent
intervals (AID symbols 0 through 7). In general during
a collision, a transmitting station will read back an AID
value that does not match its own and recognizes the
event as a collision and alerts other stations with a JAM
signal. Non-transmitting stations may also detect some
collisions by interpreting received non-conforming AID
pulses as collisions.
JAM Signal
A JAM pattern consists of 1 pulse every 32 TICs and
continues until at least the end of the AID intervals.
After the AID interval, the JAM pattern will continue
until TXEN from the MAC is deactivated.
With two transmitters colliding, each transmitter nor-
mally blanks its receive input immediately after trans-
mitting (and simultaneously receiving) a pulse.
Therefore, only when a transmitting station receives
pulses in a position earlier than the position it transmit-
ted will it recognize it as a pulse transmitted by another
station and signal a collision.
ACCESS ID Values
The access ID values for slave stations are picked by
each individual station randomly from the set of AID
slave numbers described in the management section.
During operation, each HomePNA station monitors
HomePNA frames received on the wire. If it detects an-
other HomePNA station using the same AID, it will se-
lect a new random AID.
For this reason, guaranteed collision detection is pos-
sible only as long as the spacing between successive
possible pulse positions in an AID symbol (20 TICs or
2.3 µs) is greater than the round trip delay between the
colliding nodes. At approximately 1.5 ns propagation
delay per foot, the maximum distance between two
HomePNA units must not be greater than 500 feet for
collision detection purposes (1.5 µs round trip delay
plus margin).
Silence Interval (AID symbol 7)
The Access ID symbols are followed by a fixed silence
interval of 129 TICs. The receive blanking interval is
the same as that of the AID symbols (1 through 6).
Any pulses detected in the silence interval are consid-
ered a COLLISION event for transmitting stations and
are handled as described in the Collisions section.
78
Am79C978
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