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80
Am79C978
Data Symbol RLL25 Encoding
The RLL25 code is the version of TM32 that was devel-
oped for the HomePNA PHY. It produces both the high-
est bit rate for a given value of ISBI and TIC size. In a
manner similar to run length limited disk coding, RLL25
encodes data bits in groups of varying sizes, specifi-
cally: 3, 4, 5, and 6 bits. Pulse positions are assigned
to the encoded bit groups in a manner, which causes
more data bits to be encoded in positions that are far-
ther apart. This keeps both the average and minimum
bit rates higher.
Data symbol RLL25 codes data by traversing a tree as
illustrated in Figure 42. Assuming that successive data
bits to be encoded are labeled A, B, C, D,
…
, etc. The
encoding process begins at the root node and pro-
ceeds as follows:
1. If the first bit (bit A) is a one, the next three bits (B,
C, and D) select which one of the eight positions 1-
8 is transmitted. The encoding process then contin-
ues at the root node.
2. If bit A is a zero and bit B is a one, the next three bits
(C, D, and E) select which one of the eight positions
9-16 is transmitted. The encoding process then
continues at the root node.
3. If bit A is a zero, bit B is a zero, and bit C is a one,
the next three bits (D, E, and F) select which one of
the eight positions 17-24 is transmitted. The encod-
ing process then continues at the root node.
4. Finally, if bits A, B, and C are all zeros, position 0 is
transmitted. The encoding process then continues
at the root node.
As a result, Symbol 0 encodes the 3-bit data pattern
000, positions 1-8 encode the 4-bit data pattern 1BCD,
positions 9-16 encode the 5-bit data pattern 01CDE,
and positions 17-24 encode the 6-bit data pattern
001DEF. If the data encoded is random, 50% of the po-
sitions used will be for 4-bit patterns, 25% will be for 5-
bit patterns, 12.5% will be for 6-bit patterns, and 12.5%
will be for 3-bit patterns.
Management Interfaces
The HomePNA PHY may be managed from either of
two interfaces (the managed parameters vary depend-
ing on the interface):
1. Remote Control-Word management commands
embedded in the HomePNA AID header on the wire
network.
2. Management messages from a local management
entity.
Figure 42.
RLL 25 Coding Tree
C
D
These select position 1 - 8
A
B
E
F
Awaiting coding and transmission
Encoded and
Start: Examine the next
bits to be encoded
C
D
1
B
These select position 9 - 16
C
D
0
1
E
These select position 17- 24
1
D
0
0
E
F
0
0
0
A =
B =
C =
1
1
1
1
0
0
0
Send symbol 1-8
Send symbol 9-16
Send symbol 17-24
Send symbol 0
Data stream from MAC controller
22206B-46