Chapter 1 - Introduction HCD-E1
Installation & Operation Manual
1-20 Functional Description 01/01/01 08:07
HDSL Environment
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 is also
necessary to tolerate bridged taps. HDSL systems properly operate 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 pairs is 135Ω,
and that the pairs are balanced with respect to ground.
Because of the variance in the transmission characteristics of the lines, HDSL
systems must compensate for the differential transmission delay between the
lines being used by a given system.
HDSL Line Signal
The line code on the HDSL line 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 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 1-6 illustrates the 2B1Q encoding rule.
+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
Figure 1-10 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 a payload rate of
2048 kbps and framing overhead, for two-loop operation the required
per-loop data rate is 1168 kbps (equivalent to a symbol rate of 584 kbaud).