A–24
Fig. A-21 Attenuation Characteristics of Subscriber Lines
Number of links
The “number of links” is the number of times that modulation (multiplex)
is performed between exchanges in the telephone hierarchy at or above the
EO stage. You can also think of this as the number of transit trunks
between exchanges that a telephone conversation passes through.
(3) Group delay distortion
It takes a certain amount of time to transmit voice or data signals from a
transmitting terminal to a receiving terminal over a telephone line. This is
called “delay”. Distortion occurs in the waveform if the delay is inconsis-
tent within the required frequency band. This is called “group delay distor-
tion”.
Group delay distortion occurs due to multiplexing that is performed at
exchange in the telephone hierarchy at or above the EO stage.
Group delay distortion does not cause any problems during telephone con-
versations as the sensitivity of the human ear is not so sharp. However, in
G3 facsimile transmission, this prevents image signals from being received
properly and errors occur frequently just like attenuation distortion.
Fig. A-22 Group Delay Characteristics between EOs
0
14
12
10
8
6
4
2
0
With hump
1.0
2.0
3.0
3.4
Without
hump
Atten
uation
(dB)
Frequency (kHz)
18
16
14
12
10
8
6
4
2
0
1000
2000
3000
7
5
3
1
5 link
7 link
3 link
1 link
Group
dela
y
time
(ms)
Frequency (Hz)
