The Transmission Medium – Twisted pair

In order to utilize the existing telephone system with digital technologies, it is necessary to understand a general overview of the structure of this system. There exist different types of transmission media to connect workstations and devices together such as twisted pair cable, coaxial cable, and optical fibre. The most common transmission medium for both analog and digital data is the twisted pair. It is used in the connection of telephones in the buildings, as well as local loops (also known as subscriber loops) connecting all of the phones in a limited geographic area to a central office (CO). Physically, a twisted pair consists of two insulated wires arranged in a regular spiral pattern. The wires are made of copper, which provides conductivity, or steel coated with copper to increase its strength. The twisted pair is most suitable for transmitting data within short distances, and can be used for point-to-point links between devices within the network. In forming an electrically balanced structure, the wires twisted in pairs minimize signal coupling from pair to pair and tend to balance stray noise signals. However, this "balance" is not perfect, the resulting noise and cross talk become the limiting factor in the transmission of data through twisted pairs.

Figure 1. Twisted pair

The international telephone system based on copper wires carrying analog voice data is known as the Public Switched Telephone Network (PSTN). The service carried by the PSTN, known as POTS uses twisted pairs for the transmission of full duplex analog voice. With reference to Shannon’s channel capacity theorem:

 

Where C is the channel capacity in bits per second (bps),

     B is the channel bandwidth in Hertz (Hz), and

                        S/N is the signal to noise ratio of the channel (Watt/Watt).

The theorem states that if the information rate R (bps) is less than C, then the probability of error could reach zero. Therefore, an increase in the bandwidth B of the transmission channel allows an increase in the data rate R across the channel. Typically, for POTS, available B = 3.5kHz, and S/N = 30dB, hence the theoretical upper limit on data rate for POTS is roughly 35Kbps.

Originally designed only for voice transmission, filters are placed in the POTS to limit the bandwidth at 4kHz, which filters out high frequency noise and allows for relatively good quality of voice transmission. These filters at the edge of the CO severely limit the achievable data rates. Other factors that limit the achievable data rates over twisted pairs include thermal noise, impedance impairments, attenuation, and propagation speed distortion.

The question arises as to what part do twisted pairs play in limiting the bit rate. In fact, without filters, copper access lines can pass frequencies into the MHz region, hence allowing for a corresponding increase in the achievable bit rate. This feature is precisely what ADSL focuses on.