RAD Data comm ASMI-450 operation manual Operating Environment, Transmission Media Hdsl Line Signal

Introduction

1.2 Operating Environment

Transmission Media

HDSL Line Signal

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.

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.

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.

+3	(+2.64V)
+1	(+0.88V)
	(0.00V)	time

-1 (-0.88V)

-3 (-2.64V)

Quats	-1	+3	+1	-3	-3	+1	+3	-3	-1	-1	+1	-1	-3	+3	+3	-1	+1
Bits	01	10	11	00	00	11	10	00	01	01	11	01	00	10	10	01	11

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).

1-8 ASMi-450 Installation and Operation Manual