Wright, Michael and Mukul Patel. 2000. Scientific American – How Things Work Today. London: Marshall Publishing Ltd.
The world is currently in the middle of a communications revolution as dramatic as the Industrial Revolution that created our modern society two centuries ago. The growth of telecommunications systems, the arrival of the Internet, and the proliferation of computers in every aspect of our lives are transforming both industrial and knowledge-based economies round the world. Higher disposable incomes and increased leisure time are also fuelling demand for luxury electronic goods and new forms of entertainment. Since the British inventor Alexander Graham Bell patented the telephone in 1877, telephony has become the most important form of distance communication. The telephone network is now truly global, with submarine cables and communications satellites linking every continent. This globe-spanning network handles phone calls, fax transmissions, and internet traffic. The long-distance “backbones” of the telephone network are high capacity optical-fiber cables. Lower-capacity copper cables connect individual phones to the network. The analog signal from an ordinary phone is sampled 4,000 times per second and converted into an 8-bit digital signal. Several conversations can then be transmitted simultaneously down the same cable, using a technique known as multiplexing, which increases the capacity of the network. Routing calls through the telephone network is done automatically be telephone exchanges. In near future, the telephone network may also carry video and music channels, interactive televisions, videophone calls.
Analog signal – Analog electric signals from ordinary phones are carried by copper cable to the local exchange. Base station – The base station receiving the strongest signal from a cell phone routes the call to the cell phone exchange. Cell – The cell phone network is divided into hexagonal cells, each with a base station in the middle. Cell phone exchange – Calls are routed to the main exchange or direct to a base station by the cell phone exchange. Cell phone network – Base stations send digital information to the cell phone exchange over optical-fiber or copper cable. Communication satellite – Orbiting satellites are used to route calls between places not linked by a cable. Digital signal – Digital information is multiplexed, allowing multiple signals to be transmitted simultaneously. Fax machine – Fax transmissions are sent over the telephone network. Line-of-sight microwave link – Digitized call from local exchanges are often routed to the main exchange via terrestrial microwave links. Local exchange – The local exchange digitizes calls for long-distance transmission. Long-distance or international connection – Optical fibers are used to transmit long-distance calls, many optical fiber cables are laid on the seabed. Main exchange – The main exchange handles communications between ordinary phones and the cell phone network and routes long-distance and international calls. Microwaves – Digital information is sent from cell phones to base stations using microwave frequencies. Moving cell phone – Mobility is the prime asset of the cell phone. Optical fibers – Transmitting digital information by light pulses enables many calls to be sent down one fiber simultaneously. Satellite uplink – Encrypted digital information is sent to satellites using microwave frequencies. Seamless reconnection – As the cell phone moves from one cell to another, the call is rerouted from one base station to the next, without the break in the conversation. Urban cell – Cell are smaller in urban areas, giving the network greater capacity. Weakening Signal – As the cell phone moves farther away from the base station, the signal weakens. -----
Tanenbaum, Andrew S. and Maareten van Steen. 2002. Distributed Systems Principles and Paradigms. New Jersey: Prentice-Hall, Inc.
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