Exercise 1.1Z: ISDN Connection

From LNTwww

a telephone connection scenario

We consider the scenario shown in the picture:

A woman from Munich dials a number in Hamburg with her ISDN phone. However, she cannot reach the person she wants to talk to, so she leaves him a message on tape.

The distortion-free connection is fully described by

  • a damping factor  $\alpha$,
  • a term  $\tau$, and
  • the current signal-to-noise ratio (SNR)





Notes:  The task shall establish a relation between this real scenario and the functional units of a general message transmission system mentioned in  Theorieteil  .



Questions

1

Which of the statements are true regarding source and sender?

The news source is the caller. The source signal  ${q(t)}$  is the acoustic wave of her voice signal.
The unit designated "transmitter" contains, among other things, a signal converter and a modulator.
The transmit signal  $s(t)$  is analog.

2

Which of the statements are true regarding recipients and sinks?

The received signal  $r(t)$  is digital.
The news sink is the telephone set in Hamburg.
The news sink is the answering machine.
The following applies  $v(t) = \alpha \cdot q(t - \tau ) + {n(t)}$.
There is an ideal transmission system.


Solutions

(1)  The first two statements are correct:

  • Das Sprachsignal  ${q(t)}$  muss zunächst in ein elektrisches Signal gewandelt und anschließend für die Übertragung aufbereitet werden.
  • Bei ISDN ist das Sendesignal  ${s(t)}$  digital.


(2)  Richtig sind die Lösungsvorschläge 3 und 4:

  • Das Empfangssignal  ${r(t)}$  ist aufgrund des unvermeidbaren thermischen Rauschens stets analog.
  • Die Nachrichtensinke ist der Anrufbeantworter.
  • Bei einem idealen Übertragungssystem müsste  $v(t) = {q(t)}$  gelten.
  • Aufgrund des additiven Rauschterms  ${n(t)}$, der Dämpfung  $\alpha$  und der Laufzeit  $\tau$  gilt jedoch hier:
$$v(t) = \alpha \cdot q ( t - \tau) + n(t).$$
  • Es handelt sich nach unseren Definitionen um ein verzerrungsfreies System.