Exercise 1.1Z: ISDN Connection

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

an attenuation coefficient $\alpha$,
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 communications system mentioned in the theory section .

Questions

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 transmitted signal ${s(t)}$ is 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.
However, due to the additive noise term ${n(t)}$, the attenuation $\alpha$ and the delay time $\tau$ the following applies here:

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

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

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

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