SHDSL frame structure
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Purpose
This chapter explains the SHDSL frame structure.
General structure and overhead
An SHDSL line can transport a payload of max. 2312 kbit/s. The overhead of the
SHDSL signal adds, nominally, 8 kbit/s to the payload rate which makes the overall
maximum bitrate 2320 kbit/s.
The SHDSL signal is organized in frames with a nominal length of 6 ms, where each
frame contains 4 payload blocks of k = 12 × (i +8×n)bits each and, overall,
nominally 48 overhead bits. But this number can also be 46 or 50 to allow for positive
and negative stuff operations, to align the payload to the SHDSL line clock in case of
asynchronous operation. The payload bits associated with the parameter i are often
called “Z-bits”.
The length of an SHDSL frame, expressed in bits, depends on the payload rate, so on
the parameters n and i. The structure is always the same; the payload is divided in four
equal blocks separated and surrounded by overhead bits as shown in the figure below.
The SHDSL frame structure and the overhead bits are defined in ITU-T Rec. G.991.2.
16 bits OH k bits Payload Block 1 10 bits OH 10 bits OH
10 bits OH 0-4 bits OH
k bits Payload Block 2 k bits Payload Block 3
k bits Payload Block 4
6 ms or (4 * k + 48 +/
–
2) bits
bits 1 ~ 16
bits k+17 ~ k+26
bits 3k+37 ~ 3k+46 bits 4k+47 ~ 4k+50
bits 2k+27 ~ 2k+36
Sub-Block 5
Sub-Block 6
Sub-Block 7
Sub-Block 8
Sub-Block 9
Sub-Block 10
Sub-Block 11
Sub-Block 12
Sub-Block 1
Sub-Block 2
Sub-Block 3
Sub-Block 4
k
s
payload bits
k
s
= i + 8 * n
Payload Sub-blocks are transmitted
in numerical order (1 through 48)
Z
1
Z
2
Z
i
Z bits (i )
B
1
B
2
B
n
“Bearer” bytes (n * 8)
SHDSL Overview
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365-312-801R7.2
Issue 3, May 2007
Alcatel-Lucent - Proprietary
See notice on first page
12-7
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