TRANSMISSION FACTORS IN LONG-DISTANCE TELEPHONY
North American practice called for a minimum of 4-dB loss on all four-wire circuits
to ensure against singing. CCITT recommends 10 dB for minimum loss on the national
network (Ref. 5, p. 3).
The modern digital network with its A/D (analog-to-digital) circuits in PCM channel
banks provides signal isolation, analog-to-digital, and digital-to-analog. As a result, the
entire loss scenario has changed. This new loss plan for digital networks is described in
Introduction to Transmission-Loss Engineering
One major aspect of transmission system design for a telephone network is to establish
a transmission-loss plan. Such a plan, when implemented, is formulated to accomplish
1. Control singing (stability).
2. Keep echo levels within limits tolerable to the subscriber.
3. Provide an acceptable overall loudness rating to the subscriber.
From preceding discussions we have much of the basic background necessary to develop
a transmission-loss plan. We know the following:
A certain minimum loss must be maintained in four-wire circuits to ensure against
Up to a certain limit of round-trip delay, echo may be controlled by adding loss (i.e.,
inserting attenuators, sometimes called pads).
It is desirable to limit these losses as much as possible, to improve the loudness
rating of a connection.
National transmission plans vary considerably. Obviously the length of a circuit is impor-
tant, as well as the velocity of propagation of the transmission media involved.
Velocity of Propagation. A signal takes a finite amount of time to traverse from
point A to point B over a specific transmission medium. In free space, radio signals
travel at 3
m/sec or 186,000 mi/sec; fiber-optic light guide, about 2
or about 125,000 mi/sec; on heavily loaded wire-pair cable, about 0
.22 × 10
or 14,000 mi/sec; and 19-gauge nonloaded wire-pair cable, about 0
.8 × 10
50,000 mi/sec. So we see that the velocity of propagation is very dependent on the types
of transmission media being employed to carry a signal.
Distances covered by network connectivities are in hundreds or thousands of miles
(or kilometers). It is thus of interest to convert velocities of propagation to miles or
kilometers per millisecond. Let's use a typical value for carrier (multiplex) systems of
105,000 mi/sec or 105 mi/msec (169 km/msec).
First let's consider a country of small geographic area such as Belgium, which could
have a very simple transmission-loss plan. Assume that the 4-dB minimum loss for singing
is inserted in all four-wire circuits. Based on Figure 8.19, a 4-dB loss will allow up to
4 msec of round-trip delay. By simple arithmetic, we see that a 4-dB loss on all four-
wire circuits will make echo tolerable for all circuits extending 210 mi (338 km) (i.e.,
× 105). This could be an application of a fixed-loss type transmission plan. In the case
of small countries or telephone companies covering a rather small geographic expanse,
the minimum loss to control singing controls echo as well for the entire system.