In a cellular network, there are $10$ frequency channel pairs in cell A$.$ Each

frequency channel is divided into $8$ time slots, with $1$ of $8$ time slots reserved for

short messaging services. Each of the remaining $7$ time slots supports one voice

channel. How many subscribers can be engaged in a mobile telephone call at any

given time in cell A$?$ D

A$.10$

B$.30$

C$.50$

D$.70$

E$.80$

$30.$ The downlink in a cellular network refers to: B

A$.$ From PSTN to fixed line telephone.

B$.$ From base station to handset.

C$.$ From handset to base station.

D$.$ From handset to fixed line telephone.

E$.$ From one handset to another handset.

$31.$ Guard time is applied to TDMA in order to: C

A$.$ Prevent signals from adjacent time slots from colliding due to poor bandpass

filter.

B$.$ Prevent signals from adjacent time slots from colliding due to wrong carrier

frequency used.

C$.$ Prevent signals from adjacent time slots from colliding due to signal

propagation delay.

D$.$ Prevent signals from adjacent time slots from colliding due to back groud

noise.

E$.$ Prevent signals from adjacent time slots from colliding due to wrong PN code

used.