This is a report I am doing for my college.. I thought it might be useful for people to read, I also thought you may have some constructive criticism for me..

I have already posted one of these but this is new and improved..


This report describes the difference between “analogue” and “digital” signalling that may be used over computer terminals such as the PSTN or IDSN. This report explains the function of a “MODEM” in the exchange of information between connected terminals. It also details bandwidth and bit rate information with regard to MODEM specifications.


“A digital system is one that uses discrete values (often electrical voltages), especially those representable as binary numbers, or non-numeric symbols such as letters or icons, for input, processing, transmission, storage, or display, rather than a continuous spectrum of values (i.e., as in an analogue system).”

An example of a digital network is ISDN (Integrated Services Digital Network). ISDN was developed to send voice, video and data over standard copper phone lines. It works by using three data channels, two ‘B’ channels (64kbps each) and one D’ channel (16Kbps / 64kbps). The two ‘B’ channels are used for transmitting the voice, video or data signals, while the ‘D’ channel is used for signalling between the ISDN switches on the network and in the site.


“An analogue signal is any variable signal continuous in both time and amplitude. It differs from a digital signal in that small fluctuations in the signal are meaningful.”

An example of an analogue system is the PSTN (Public Switched Telephone Network). Originally analogue, but now mostly digital, the PSTN is the international network of copper cables that carries voice data. Generally the PSTN is limited to around 52kbps. It is generally governed by the ITU (International Telecommunications Union formerly known as the CCITT), which among other things standardises MODEM’s internationally to ensure compatibility, which I will talk about later in the report. ISDN was developed to work on this system in a digital format to allow greater amounts of data to be transmitted over the existing copper wire PSTN. PSTN is also known as POTS, ‘Plain Old Telephone System’. PSTN was first introduced around 1878, when the first telephone networks were being constructed in North America.

There are several key differences between analogue and digital signals in terms of data communication over systems such as PSTN or IDSN.

The most basic and obvious difference is that analogue signals can be any value (Indiscrete), within a defined range whereas digital signals can only take a limited number (Discrete) of values within a defined range.

If you look at the voltage for an analogue signal, you can see that it varies anywhere between the defined voltage range, whereas if you look at the voltage range for a digital signal, it is either on or off, the range is either maximum voltage or none at all, i.e. there are only two acceptable signals.

Digital signals are not all simple as illustrated above. An example of a complex digital signal is what is known as a ‘carrier modulation scheme’. This type of scheme is used to increase the capacity of data that may be transferred through chosen communications systems. Carrier signals have three physical properties, termed as modules, which are, frequency, amplitude and phase. Through the manipulation of these properties, data capacity is increased. An example of a scheme such as this would be QAM (Quadrature Amplitude Modulation). An example of QAM in use is Telewest, which uses QAM 64. The way QAM 64 works is by modulating the amplitude of the two carrier waves, the amplitude and the phase. QAM 64 has 64 points (See the constellation diagram in Figure 2). Each point represents 6 bits. The transmission rate is capable of sending 80 kbps. There is only one problem with carrier modulation, the more points or more complex the scheme, the more room there is of error or ‘noise’ as the signal gets harder for the demodulator to differentiate between because the points are getting closer and closer together (fig 2) meaning that the signal is becoming more and more like an analogue signal, thus bringing similar problems to analogue signals, i.e. misinterpretation of data. So basically, the more complex the modulation technique used, the more chance of noise

The main disadvantage to using analogue transmission, as I was just saying is ‘data confusion.’ This phenomenon may occur if the signal strength (voltage) was weakened or increased during a transmission in some way for example. Data confusion could arise due to the facts that for analogue, voltages do not have to be specific unlike digital, resulting in data sent to be interpreted incorrectly. The original data sent would be corrupted by the voltage surge, a phenomenon impossible for digital signal. Any increase or decrease in voltage would have no impact on digital data. Analogue signal can also be easily distorted by other electrical signals, which will also cause data to be interpreted incorrectly.

The problem with data transfer is that computers can’t understand analogue signals. The reason why, is that computers work in digital signals, for a computer to understand an analogue signal, it need to be transformed into a digital signal first.

Conversion between analogue and digital

As computers work with digital signals only, over certain systems, such as ISDN, the signal needs to be converted between analogue and digital. This happens as the data signals travel over standard telephone lines (Usually over PSTN). A hardware device is required to convert between the two signals; we call this device a ‘modem’. A modem or modulator-demodulator is the name of the piece of hardware that is used to convert between the two signals. When transmitting the signal, the modem ‘modulates’ the signal, when receiving it ‘demodulates’ the signal.

Modems were first developed in the 1950’s, the US air defence needed to transmit data across their existing telephone lines. The US had developed the first modems in the 1950’s and they were putting them to use! It was only in 1962 that the first commercial modem was released; it was released by a company called ”American Telephone & Telegraph Company” (AT&T). This first commercial modem allowed full-duplex transmission.

What is duplex transmission?

Full Duplex is a term used to describe the format of data transmission; a full-duplex transmission means that data is sent from both directions at the same time, much like a road with traffic moving in both directions. Other duplex formats include: Half Duplex, where the data being sent goes in both directions, but only one way at a time.

“A duplex communication system is a system composed of two connected parties or devices which can communicate with one another in both directions.”

The first MODEM commercially released was the Bell 103 and boasted data rates up to 300 bits per second! Shortly after the Bell 103, and then came the Bell 212, which reached speeds of 1200 bits per second. It also used a method of modulation called phase-shift keying (PSK). This was a step up from the frequency-shift keying (FSK) method that the Bell 103 used.

An organisation previously known as the “Comité Consultatif International Téléphonique et Télégraphique”. This translated to English means “International Consultative Committee on Telecommunications and Telegraphy”. This is now known as the ITU or the “International Telecommunications Union”. It is an international organisation, which developed standards of modems to ensure compatibility. Prior to the standards developed by this organisation, companies like the American Telephone and Telegraph Company set their own standards, but were not universally adhered unlike the current V numbers.

V numbers are defined and are universally accepted today as the process through which transfer between analogue and digital occurs. What follows is a description of what the V classifications are. Although not the most recent in the series, the most common modem in use today is the V.34bis. The first standard to be internationally recognised was the V.22bis

Below is a list of the V number standards and their specifications:

Provides 1200 bits per second at 600 baud (state changes per second)

The first true world standard, it allows 2400 bits per second at 600 baud

Provides 4800 and 9600 bits per second at 2400 baud

Provides 14,400 bits per second or fallback to 12,000, 9600, 7200, and 4800 bits per second

Provides 19,200 bits per second or fallback to 12,000, 9600, 7200, and 4800 bits per second; can operate at higher data rates with compression; was not a CCITT/ITU standard

Provides 28,800 bits per second or fallback to 24,000 and 19,200 bits per second and backwards capability with V.32 and V.32bis

Provides up to 33,600 bits per second or fallback to 31,200 or V.34 transfer rates

The trunk interface between a network access device and a packet network at data rates greater than 19.2 Kbps. V.35 may use the bandwidths of several telephone circuits as a group. There are V.35 Gender Changers and Adapters.

Same transfer rate as V.32, V.32bis, and other standards but with better error correction and therefore more reliable

Provides up to 56,000 bits per second downstream (but in practice somewhat less). Derived from the x2 technology of 3Com (US Robotics) and Rockwell's K56flex technology


This report has considered the main differences between analogue and digital signals in respect of data communication.

I have concluded that Analogue transmission of data is not as fast or reliable as digital data.

This is the reason for example that we are converting out TV purely to digital which will allow us to view more channels with more quality.