Exercise 1.1Z: ISDN Connection

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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

Solution

(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.