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Introduction
1-8 ASMi-450 Installation and Operation Manual
1.2 Operating Environment
This section describes the HDSL environment, with special emphasis on the
implementation used in the ASMi-450 to provide the background information
required for understanding the configuration parameters of the ASMi-450
system.
Transmission
Media
HDSL systems are intended to operate on the local subscriber plant, which
typically uses a mixture of unshielded twisted-wire pairs. Moreover, it also
necessary to tolerate bridges taps. Therefore, HDSL systems must operate
properly on this media. The only requirement is that the lines must not be
loaded. In addition, it is assumed that the nominal impedance of the loops is
135 ohms, and that the loops are balanced with respect to ground.
HDSL Line Signal
The line code on the HDSL loop is 2B1Q (2 Binary, 1 Quaternary). This is a
four-level pulse-amplitude modulation code without redundancy, under which
each pair of information bits is converted to a quaternary symbol, called quat
(bits can assume two signal levels, whereas quats have four levels).
The encoding rule of the 2B1Q code is as follows:
Binary Digits Quaternary Symbol
00 -3
01 -1
10 +3
11 +1
The levels of the quaternary signal are symmetrically located around the 0V,
and the nominal peak symbol level specified by the HDSL standards is 2.64V.
Figure 1-5 illustrates the 2B1Q encoding rule.
Figure 1-E. 2B1Q Encoding Rule
Due to the encoding of two bits into one symbol, the symbol rate on the HDSL
line is half the bit rate. Taking into consideration the highest payload rate
supported by the ASMi-450, 1152 kbps, and some overhead, the required line
data rate is 1168 kbps (equivalent to a symbol rate of 584 kbaud).
+3 (+2.64V)
+1 (+0.88V)
(0.00V)
-1 (-0.88V)
-3 (-2.64V)
Quats
Bits
-1
01
+3
10
+1
11
-3
00
-3
00
+1
11
+3
10
-3
00
-1
01
-1
01
+1
11
-1
01
-3
00
+3
10
+3
10
-1
01
+1
11
time