Introduction to Data Communications
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4. Data Communications

Data Communications is the transfer of data or information between a source and a receiver. The source transmits the data and the receiver receives it. The actual generation of the information is not part of Data Communications nor is the resulting action of the information at the receiver. Data Communication is interested in the transfer of data, the method of transfer and the preservation of the data during the transfer process.

In Local Area Networks, we are interested in "connectivity", connecting computers together to share resources. Even though the computers can have different disk operating systems, languages, cabling and locations, they still can communicate to one another and share resources.

The purpose of Data Communications is to provide the rules and regulations that allow computers with different disk operating systems, languages, cabling and locations to share resources. The rules and regulations are called protocols and standards in Data Communications.

4a. Why Telecommunications?

What does networking have to do with telephones?

Telephones and networking work hand in hand. The telecommunications industry has been gradually integrating with the computer industry and the computer industry has been gradually integrating with the telecommunications industry. The common goal is to join distantly located Local Area Networks into Metropolitan and Wide Area Networks (MANs and WANs). How do you connect Local Area Networks across a city or country? You use a telecommunication carrier either a local carrier or long distance carrier.

The Internet is a good example of the collabaration between computers and the telephony industry. The Internet is a collection of computer networks connected by telecommunication systems. Most people don't realize it but the telecommunications industry is the backbone for the Internet. Pretty much all data communications that leaves a LAN utilizes a telecommunication service.

At the end of 1996, there was a major milestone where more data was communicated in North America's telecommunications system than voice. Currently, in 2005, the majority of communication on telephony circuits is data and the minority is voice. This doesn't mean that the voice traffic is reducing in volume, it means that the data communicaions side has added a whole new dimension to telephonyy. In addition, there is even more voice communications now due to the explosion of use of cellular phones.

There are two main methods of sending data over telephone services: voice channels and data channels.

4b. Voice Channels

First thing that comes to mind when discussing telephone systems, is the phone at home. The dialup phone is called the Plain Old Telephone Set or System and uses the acronym POTS. A voice conversation using a POTS line uses the most basic of telephony services: the Voice Channel. This initially doesn't seem to have much to do with Networks!

In data communications, we use voice channels for modem communication to connect to BBSs (Bulletin Board Services) or to connect to the Internet. We also use voice channels to connect LANs using remote access. Due to the bandwidth limits on the Voice Channel, the data transfer rate is relatively slow.

Voice Channel: Dial-up connection through a modem using standard telephone lines. The current transfer rate is close to 56 kbps using a V.90 modem. Typical Voice Channel communication rates are: 300, 1200, 2400, 9600, 14.4k, 19.2k, 28.8k, 33.6k and 56 kbps (bits per second).

4c. Data Channels

Data channels are dedicated lines for communicating digitized voice and data. There are many different protocols (rules and standards) for data channels. Some common data channel services are T-1 lines, Frame Relay, X.25 and ADSL.

Data Channels are special communications channels provided by the "common carriers" such as Telus, Sprint, Bell Canada, etc.. for transferring digital data. Data Channels are also called "Leased Lines". They are "directly" connected and you don't have to dial a connection number. The connections are up and running 24 hours per day. They appear as if there were a wire running directly between the source and destination. Typical transfer rates for data communication are: 56 k, 128k, 1.544 M, 2.08 M, 45M and 155 Mbps.

Common carriers charge for data connections by the following methods or combinations of there of

  1. the amount of data transferred (megabytes per month)
  2. the transfer rate (bits per second)
  3. the amount of use (time per month)
  4. the quality of service
  5. the latency of transmission

Introduction to Data Communications
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Copyright Eugene Blanchard Jan 1998, January 2007