Alcatel-Lucent 1643 AMS Applications And Planning Manual page 254

SHDSL Overview
Payload mapping
The 1643 Access Multiplexer AM / 1643 Access Multiplexer Small AMS LTU supports
the "unaligned" E1 mapping according to ITU-T Rec. G991.2 and the TU-12 mapping.
The two mapping schemes are described in the following.
E1 mapping acc. to ITU-T Rec. G.991.2/Annex E.5, E.6 and E.7
The SHDSL standards define E1 over SHDSL mapping to use the parameters n = 32
and i = 0, i.e. there is no Z-bit channel and 32 bytes per SHDSL payload subblock,
sufficient to hold 125 ms of E1 traffic. In the "unaligned" mode there is no further
relationship between the position of the E1 bits and the SHDSL payload octets: An E1
frame can start anywhere, even in the middle of an octet position.
This mode is implemented asynchronously, i.e. the SHDSL bit-stuffing mechanism is
used to accommodate the frequency differences between the SHDSL carrier, frequency
locked to the LTU NE clock, and the E1 signal of which the timing is set by the NE
that created this signal. In asynchronous mode each SHDSL frame has either four or
zero stuff bits present. The long term average is such that the clock differences are
exactly compensated.
TU-12 mapping
The TU-12 frame has a length of 144 octets or 500 µs. The octets are numbered
according to the figure below:
TU-12 frame
Z-bit
Sub-Block 4j 3 -
j = 1, 2, .., 12
Each TU-12 frame is mapped into four subsequent subblocks with n = 36, using
positions B1 through B36 in each sub-block. The first TU-12 frame uses subblocks 1
through 4, the second uses subblocks 5 through 8, all the way up to the 12th TU-12
frame in subblocks 45 through 48. The TU-12 mode operates synchronously, i.e. the
SHDSL stuffing mechanism is not used and the SHDSL line clock and TU-12 payload
clock are the same. They are both derived from the TG function in the LTU in the
downstream direction. Since the NTU is supposed to operate in loop-timed mode, the
same is true for the upstream signal. SRUs operate in through-timed mode, so the
synchronisation keeps untouched.
....................................................................................................................................................................................................................................
12-8
µ (2304 kbit/s)
144 bytes or 500 s
Z-bit
Sub-Block 4j 2 -
Alcatel-Lucent - Proprietary
See notice on first page
Z-bit
Sub-Block 4j 1 -
SHDSL frame structure
Z-bit
Sub-Block 4j
365-312-801R7.2
Issue 3, May 2007