What is the purpose of Modems?

The word modem is an acronym for Modulator-Demodulator. Basically, a modem is used for transmitting and receiving data over a communication channel, such as twisted-pair telephone lines, coaxial cables, and optical fibres. Currently the purpose of a modem is to convert a computer’s data stream to analog format so that it can be transmitted over the analog telephone line.

At the source, modulation techniques are used to convert digital data (0’s and 1’s) into analog form for transmission across the channel. At the destination, the received analog signal is converted to digital data via demodulation. This is a simplified explanation of how a modem works, and there are other issues that require attention; such as channel impairments, encryption, error detection/correction, data compression, modulation, handshake negotiation, and echo cancellation. These features will be discussed a bit later.

 

Why do we need modems which can achieve better than 28.8 kbps?

At the present, most analog modems used at home offer modest bit rate transmission of data across the telephone line, at 28.8kbps, 33.6kbps, or 56kbps. However, there are increasingly greater demands for modems that can offer reliable transmission of data at multi-megabit speeds.

In general, high-speed modems are required for high-quality and high-speed data transfer, to eliminate time delay, and to provide real-time service and communication. Currently, there are many applications that require fast transmission rates, in particular, the downstream rate. Many applications and services may be offered commercially in the future with the help of high-speed modems. Possible high-speed applications include:

• Internet access: faster downloads; enjoyable and efficient use of graphics and audio intensive applications (e.g. Java script, Applet, Shockwave), multimedia applications, interactive 3-dimensional applications; interactive games.
  

• Computer Networking: LAN (Local Area Network), WAN (Wide Area Network), intranets, extranets.
  
• Entertainment: music-on-demand, video-on-demand (VoD), movies-on-demand, MPEG movies (1.5 Mbps), interactive TV, HDTV (High Definition Television 24 Mbps).
  
• Home Shopping online shopping facilities, virtual garage sales, and virtual auctions.
  
• Remote/Distance Learning: virtual classrooms, virtual laboratories, and library of online text, audio, and video publications.
  
• Medical Purposes: immediate access to patients’ records in hospitals or health care facilities, transmission of medical images.
  
• Remote Professional Services: legal services, health care, and real estate.
  
• Telecommuting: video conferencing, interactive meetings and collaboration.

High-speed modems will facilitate the growth and availability of many applications and services in the commercial market. The growth of technology makes these high-speed modems more accessible to the general public at affordable prices.

The following table shows the various modem standards, and their corresponding data rates.

 

Table 1. Comparison of ITU standards Source: [9]