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參數(shù)資料
| 型號： | DM9801A |
| 文件頁數(shù)： | 45/60頁 |
| 文件大?。?/td> | 605K |
| 代理商： | DM9801A |
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DM9801A
1M Home Phoneline Network Physical Layer Single Chip Transceiver
Final
Version: DM9801A-DS-F01
May 30, 2001
45
Sync Transmit Timing
The Sync interval (AID symbol 0) delineates the beginning
of a frame and is composed of a SYNC_START pulse
followed by a SYNC_END pulse after a fixed silence
interval as shown in figure 22 above. Timing for this (AID
symbol 0) starts (TIC = 0) at the beginning of the
SYNC_START pulse. The SYNC_END pulse starts at
TIC = 126.
At TIC = 129, this AID symbol 0 ends and the next AID
symbol begins with the symbol timing reference reset to
TIC = 0. No information bits are coded in the SYNC (AID
symbol 0 interval).
Sync Receive Timing
As soon as the SYNC_START pulse is detected, the
receiver disables (Blanks) further detection until
AID_END_BLANK (located at TIC = 61) after which
detection is re-enabled for the next received pulse. The
receiver allows for jitter by establishing a window around
each legal pulse position. This window
AID_GUARD_INTERVAL is 2 TICS wide on either side of
the position.
A SYNC_END pulse which arrives outside
AID_GUARD_INTERVAL of the legal TIC = 126 is
considered a NOISE_EVENT that may be used in setting
the adaptive Squelch level, aborts the packet, and sets the
receiver in search of a new SYNC_START pulse and
SYNC interval. If it is a transmitting station, the Collision
event is asserted.
The SYNC interval is followed by six Access ID symbols
(symbols 1 through6). Transmit timing is shown in figure
22 and receive timing in figure 23(above). Data is encoded
in the relative position of each pulse with respect to the
previous one. A pulse may occur at one and only one of
the four possible positions within the AID symbol yielding
two bits of data coded per AID symbol.
The decoded bits from the AID symbols 1 to 4 produce 8
bits of Access ID which are used to identify individual
stations and to detect collisions. The MSB is encoded in
AID symbol 1 and is the leftmost bit in Table 2.
Pulse
Position
Tics From Beginning
of AID Symbol
Bit Encoding
1
66
00
2
86
01
3
106
10
4
126
11
Access ID Symbol Pulse Position and Encoding
Table 2
The next two AID symbols (5 and 6) encode four bits of
control information. The MSB is encoded in AID Symbol 5.
Collisions
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 alerting other stations with a Jam signal. Non-
transmitting stations may also detect non-conforming AID
pulses as collisions.
With two transmitters colliding, each transmitter normally
blanks its receive input immediately after transmitting and
simultaneously receiving a pulse. Therefore, only when a
transmitting station receives pulses in a position earlier
than the position it transmitted will it recognize it as a pulse
transmitted by another station and signal a collision.
For this reason guaranteed collision is possible only as
long as the spacing between successive possible pulse
positions in an AID symbol (20 TICS or 2.3
μ
s) 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 stations must not be
greater than 500 feet for collision detection purposes
(1.5
μ
s round trip delay plus margin).
The following criteria must be met to guarantee reliable
collision detection:
At least one station of a colliding group must always detect
a collision when the delay between the beginning of its
transmitted packed and the beginning of the received
colliding packed is between -1.5
μ
s and +1.5
μ
s
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