20.9 Overload
Elements of the network have to be able to transmit the peak signal of the multichannel system load.
The system overload requirements are dimensioned such that the probability of the peak busy hour transmission signal exceeding the peak value of a sinusoid signal of power P is 0.01.
P
in dBmO is given by Equation 20.33U1(CCITT,
1985, Supplement
22) where t is the activity factor (see Table 20.6); Lq
is
the channel
loading factor (see Table 20.6); and n is the number of active
channels.
The value of n is derived from assuming that the probability n channels are active in an N channel system is given by a binomial distribution and that n is only exceeded with a probability of l% (i.e. the value of n is not exceeded 99% of the time) as in Equation 20.34. g is the standard deviation of loud/soft talkers (see Table 20.6) and N the system channel capacity.
Peq - has been calculated for some of the more important systems, as in Table 20.8.
20.9.1 Overload measurement.
The overload point is precisely defined according to one of two definitions, using harmonic/intermodulation products or using gain change.
20.9.1.1 Harmonic/intermodulation products
CCITT (1985) Recommendation G223, defines the overload as:
'The overload point or overload level of an amplifier is at that value of absolute power at the output at which the absolute power level of the third harmonic increases by 20dB when the input signal to the amplifier is increased by ldB.'
If the 3rd harmonic is outside the frequency band of interest then the 2A-B intermodulation product may be used instead of the 3rd harmonic by the definition:
'The overload point or overload level of an amplifier is 6dB higher than the absolute power level in dBm, at the output of the amplifier, of each of two sinusoid signals of equal amplitude and of frequency A and B respectively, when these absolute power levels are so adjusted that an increase in ldB of both of their separate levels at the input of the amplifier cause an increase, at the output of the amplifier, of 20dB in the intermodulation product of 2A-B.'
20.9.1.2 Gain change
From Equation 20.13 it can be seen the gain to the fundamental signal decreases by a factor given by expression 20.35 due to the non-linear characteristic.
This effect can be used to give a definition of overload such that:
'The overload is the output power in dBm longest proposed international link is 25000km (CCITT,1985, G.I 03) and is constructed of a number of sub hypothetical links of typically 2500km.
The advantage of this last method is that it is easy to perform and can be measured at any frequency in the band of interest.
20.10 Hypothetical reference system
Hypothetical reference systems (CCITT, 1985, G.212 — G.222 & G.229) have been constructed to define network building and how the various degradation factors are apportioned and planned. The longest proposed international link is 25000km (CCITT,1985, G.I 03) and is constructed of a number of sub hypothetical links of typically 2500km.