Understanding the network.pdf - Back to Home

The PSTN was originally designed for voice transport, which was easily achievable
using a single full-duplex transmission pair. For the actual transmission to take
place, however, it must have a dedicated transmission path for the duration of the
call. It is possible to have multiple parties share the same transmission path, but it
becomes difficult to keep straight who you are talking to, and it is not very private.
This need for each transmission path to have an actual physical transport channel is
not a very efficient use of bandwidth. It also presents significant scaling problems
for communications systems because it is just not practical for every connection to
have its own discrete wire path to transmit.
FDM
One way of addressing this problem is to send multiple signals over the same wire
pair using a technique called Frequency Division Multiplexing (FDM). At one time,
FDM was used to transmit multiple analog signals over CO interconnection lines or
trunk lines. FDM works like this: A single copper trunk loop has a bandwidth 96KHz.
A basic voice transmission only requires 3.1KHz of bandwidth for transmission. FDM
separates the available bandwidth into 4KHz "channels" (3.1KHz for transmission
and 900Hz channel separation). This permits the transmission of 24 channels over
one trunk loop. FDM transmission uses modulators to merge the different local loop
transmissions into specific passbands (the frequency ranges of a specific channel on
the medium), and then all the channels are transmitted as a single signal.
Demodulators are then used on the remote CO's switch to separate the different
channels back into their discrete forms for local loop transmission.
Although FDM works well for voice transport, it is not suited to data, mainly because
voice transmission can be "dirty," but data transmission must be "clean." To
transmit any analog signal, it needs to be amplified and sent across a transmission
medium. When an analog signal is transported over a copper wire, it adds noise.
When the analog signal is passed through the transport segment from amplifier to
amplifier, any noise it picks up along the way is added to the signal and amplified.
So the longer the transmission distance, and the more times the signal is passed
between amplifiers, the worse the signal will be when it reaches its destination. With
audio transmission systems, it is possible to filter noise, but with data transmission
systems, you cannot filter without degrading the signal, which results in corrupted
data.
Digital Carrier Systems
Although FDM makes more efficient use of the transport medium's available
bandwidth, it has some practical limitations in terms of signal quality and distance.
Therefore, in major metropolitan areas, interCO trunks were originally deployed
using discrete wire pairs. By the 1960s, a real need arose for a CO-to-CO