Role of protocols.

Protocols are rules and procedures for communicating. The term "protocol" is used in a variety of contexts. For example, diplomats from one country adhere to rules of protocol designed to help them interact smoothly with diplomats from other countries. Rules of protocol apply in the same way in the computer environment. When several computers are networked, the rules and technical procedures governing their communication and interaction are called protocols.

Keep three points in mind when you think about protocols in a network environment:

There are many protocols. While each protocol facilitates basic communications, each has different purposes and accomplishes different tasks. Each protocol has its own advantages and restrictions.

Protocols can also work together in a protocol stack, or suite. Just as a network incorporates functions at every layer of the OSI reference model, different protocols also work together at different levels in a single protocol stack. The levels in the protocol stack "map," or correspond, to the layers of the OSI reference model. For instance, the TCP/IP protocol's application layer maps to the OSI reference model's presentation layer. Taken together, the protocols describe the entire stack's functions and capabilities.

Cellular systems and standards.

A cellular network or mobile network is a wireless network distributed over land areas called cells, each served by at least one fixed-location transceiver, known as acell site or base station. In a cellular network, each cell uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed bandwidth within each cell.

When joined together these cells provide radio coverage over a wide geographic area. This enables a large number of portable transceivers (e.g., mobile phones,pagers, etc.) to communicate with each other and with fixed transceivers and telephones anywhere in the network, via base stations, even if some of the transceivers are moving through more than one cell during transmission.

Cellular networks offer a number of desirable features:

More capacity than a single large transmitter, since the same frequency can be used for multiple links as long as they are in different cells

Mobile devices use less power than with a single transmitter or satellite since the cell towers are closer

Larger coverage area than a single terrestrial transmitter, since additional cell towers can be added indefinitely and are not limited by the horizon

Major telecommunications providers have deployed voice and data cellular networks over most of the inhabited land area of the Earth.

The Global Positioning System (gps)

The Global Positioning System (GPS) is a U.S.-owned utility that provides users with positioning, navigation, and timing (PNT) services. This system consists of three segments:

1) The space segment constellation consists of 24 satellites, that transmit one-way signals that give the current GPS satellite position and time. 24 Space Vehicles (SVs) are distributed equally among six circular orbital planes. The orbital planes are centered on the Earth, not rotating with respect to the distant stars. Orbiting at an altitude of approximately 20,200 kilometers; orbital radius of 26,600 km, each SV makes two complete orbits each sidereal day.

2) The control segment consists of worldwide monitor and control stations that maintain the satellites in their proper orbits through occasional command maneuvers, and adjust the satellite clocks. It tracks the GPS satellites, uploads updated navigational data, and maintains health and status of the satellite constellation.

3) The user segment consists of the GPS receiver equipment, which receives the signals from the GPS satellites and calculates the user's three-dimensional position and time. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock. Receivers typically have between twelve and twenty channels.