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:

• The speech signal  ${q(t)}$  must first be converted into an electrical signal and then prepared for transmission.
• For ISDN the transmit signal is  ${s(t)}$  digital.

(2)  Correct are the solutions 3 and 4:

• The received signal  ${r(t)}$  is always analog due to the unavoidable thermal noise.
• The message sink is the answering machine
• In an ideal transmission system  $v(t) = {q(t)}$  should apply.
• Due to the additive noise term  ${n(t)}$, the damping  $\alpha$  and the running time  $\tau$  but applies here:

$$v(t) = \alpha \cdot q ( t - \tau) + n(t).$$

• By our definitions, this is a distortion-free system.