DATA COMMUNICATIONS
© Copyright Brian Brown, 1995-2000. All rights reserved.

Part 3b: Cables and Network Communication Systems

Open Wire | Twisted pair | Coax | Microwave | Satellite | Fiber Optic | Cellular Telephone
Satellite Cellular Telephone | Pagers | Summary | References

Fiber Optic Cable
This is cable made from fine strands of silica (glass), coated with a plastic sheath. The signals are converted to light pulses using a laser. Each fiber optic strand can support thousands of speech channels and multiple TV channels simultaneously. It is used mainly for long haul links and intercontinental links.

A strand of silica glass fiber (thinner than a human hair), is coated with a reflective surface. When light (provided by a laser) is shown into the strand, it travels along the fiber strand (the reflective layer prevents it from escaping).

Fiber optic cable is used for long haul telecommunications links, high speed data communications links for computers, and information services to homes (e.g., PAY TV).

Fiber optic cable has the advantages of

• high capacity
• immune to interference
• can go long distances

Its disadvantages are,

• costly
• difficult to join

Cellular Telephone

Also known as mobile phones, cellular telephones are a recent technology designed for mobile users who need to make telephone calls from various locations. The telephone is portable and carried with the user. A cellular phone uses radio frequencies to talk to a nearby cell site (a site that handles cellular calls). The cellular phone regularly communicates with the nearest cell site to inform the network that it is connected.

A Cell Site is a circular geographical area that handles cellular phones within its defined physical space. Larger coverage is obtained by overlapping cell sites to form a cellular network. As a user moves location from one cell site to another, the call is transferred to the nearest cell site responsible for that physical area.

Each cell site is linked back to a master site that provides an interconnection to the regular telephone network. Calls handled by each cell site are relayed back to the master cell site which then relays it to the telephone network. Frequencies can be reused by other cell sites making for efficient sharing of facilities. Many calls can be handled by one frequency (especially where digital phones are used)

Cellular telephone suits large geographical areas, including remote sites. It is cheaper to install that traditional copper cable, and is making large inroads into countries with high dense populations that cannot upgrade their existing infrastructures (such as pacific/asia).

Satellite Cellular Telephone
This is cellular phones using low earth orbiting (LEO) satellites. There are currently two systems in place, the Iridium and Teledesic satellite networks. The advantage of a satellite cellular network over a land based one is much wider geographical coverage, especially in mountainous terrain and at sea. One of the problems of the land based cellular coverage is the amount of cell sites required, and their accurate positioning to avoid blind spots where calls cannot be made.

The Teledesic Network
The Teledesic network is another constellation of low earth orbiting satellites, 288 (plus spares) in total. Its major sponsor is Motorola. Its purpose is to provide high speed data access to services such as the Internet, video conferencing and high quality voice and data connections.

© www.teledesic.com

The Teledesic network will support millions of simultaneous users, with speeds up to 64Mbps on the downlink path and up to 2Mbps on the uplink path.

Pagers
Pagers are small hand held devices that allow one way communication between two parties. Paging was first developed by Charles Neergard in 1949. Neergard working as a radio engineer, and was annoyed with the way in which doctors in the hospital he was at were continually called over the loudspeaker system. His proposed paging system allowed a much quieter method of informing people that a message was waiting for them.

A ground based radio transmitter sends out a constant stream of messages on a particular frequency. Pagers, which are essentially radio receivers, monitor this stream of messages sent by the transmitter. Each pager has a built-in address code (sometimes called a cap code). When an incoming message is detected that has the pagers built-in address associated with it, the pager decodes and displays the message.

There are 4 main types of pagers

Summary
Technology has happened at a fast pace. Since modern communications has begun, early forms of media quickly become outdated and are replaced with newer more modern methods.

For example, in New Zealand, we have seen the Public Telephone Switched network evolve from the early use of coaxial cable to interconnect main centers, then its replacement with the use of microwave stations, and now, today, the heavy use of fiber optic cable as a preferred means of interconnecting main centers together.

Each media is suited to different purposes and each has their place. In the early days of telephones, the telephone companies wired each persons home with a twisted pair cable. What limitations do you think such a connection now poses? In addition, as people become more and more involved in the technological revolution, what additional services might they want to access from their home (one such example is the Internet) and how do you think the existing telephone connection limits the provision of those additional services?

Make a list of some additional services you think people might be interested in and the benefits of having access to such services from their home might bring.

Further Internet based references

ftp://mirror.lcs.mit.edu/telecom-archives/index.html Telecom Archives
http://www.att.com/attlabs/chrono/ Cronological History of telecommunications, AT&T

http://www.radiodesign.com/cellwrks.htm Radio Design Group Inc. How cellular telephones work

http://www.exmachina.com/history.htm Rose, D.S. A very brief history of paging
http://www.radiodesign.com/pgrwrks.htm Radio Design Group Inc. How pagers work

http://www.teledesic.com/ The Teledesic satellite network

© Copyright B Brown. 1995-2000. All rights reserved.