Telecommunication operators' fixed networks were originally constructed predominantly from copper transmission media carrying voice-band signals in the region 300 Hz to 3.4 kHz. In developed countries, the core or backbone of the network that interconnects the switching centres is now mainly optical fibre. However, the access portion of the network that connects switches to customers (the so called 'last mile') is still dominated by twisted copper pairs. Optical access network technologies have been developed and proven, however the sheer inertia of the installed copper base means that it could take many years to migrate the access networks from copper to fibre.
The cables that are normally used in the access networks are unshielded twisted pairs of differing gauges, which were originally specified and designed to carry analog telephony signals within a frequency band of 300 Hz to 3.4 kHz. By Shannon's Theorem, we know that without noise or impairments even a bandlimited access channel has infinite information carrying capacity. In reality, however, there are many sources of noise which collectively determine the information carrying capacity. In a well-designed digital transmission system, the effects of internal system noise are small.
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