| Wireless networks can be divided into two broad | | | | by the government. Examples of such companies |
| segments: short-range and long-range. Short-range | | | | include many of the wide area network providers. |
| wireless pertains to networks that are confined to | | | | 2. Spread spectrum - By design, spread spectrum |
| a limited area. This applies to local area networks | | | | trades off bandwidth efficiency for reliability, |
| (LANs), such as corporate buildings, school | | | | integrity, and security. It consumes more |
| campuses, manufacturing plants or homes, as well | | | | bandwidth than narrow-band technology, but |
| as to personal area networks (PANs) where | | | | produces a signal that is louder and easier to |
| portable computers within close proximity to one | | | | detect by receivers that know the parameters of |
| another need to communicate. These networks | | | | the signal being broadcast. To everyone else, the |
| typically operate over unlicensed spectrum | | | | spread-spectrum signal looks like background |
| reserved for industrial, scientific, medical (ISM) | | | | noise. Two variations of spread-spectrum radio |
| usage. The available frequencies differ from | | | | exist: frequency-hopping and direct-sequence.a). |
| country to country. The most common | | | | Frequency-hopping spread spectrum (FHSS) - |
| frequency band is at 2.4 GHz, which is available | | | | FHSS uses a narrowband carrier that rapidly |
| across most of the globe. Other bands at 5 GHz | | | | cycles through frequencies. Both the sender and |
| and 40 GHz are also often used. The availability of | | | | receiver know the frequency pattern being used. |
| these frequencies allows users to operate | | | | The idea is that even if one frequency is blocked, |
| wireless networks without obtaining a license, and | | | | another should be available. If this is not the case, |
| without charge. | | | | then the data is re-sent. When properly |
| Long-range networks continue where LANs end. | | | | synchronized, the result is a single logical channel |
| Connectivity is typically provided by companies | | | | over which the information is transmitted. To |
| that sell the wireless connectivity as a service. | | | | everyone else, it appears as short bursts of noise. |
| These networks span large areas such as a | | | | The maximum data rate using FHSS is typically |
| metropolitan area, a state or province, or an | | | | around 1 Mbps.b). Direct-sequence spread |
| entire country. The goal of long-range networks is | | | | spectrum (DSSS) - DSSS spreads the signal |
| to provide wireless coverage globally. The most | | | | across a broad band of radio frequencies |
| common longrange network is wireless wide area | | | | simultaneously. Each bit transmitted has a |
| network (WWAN). When true global coverage is | | | | redundant bit pattern called a chip. The longer the |
| required, satellite networks are also available. | | | | chip, the more likely the original data can be |
| Many of the wireless technologies in the WPAN, | | | | recovered. Longer bits also require more |
| WLAN, and WWAN categories transmit | | | | bandwidth. To receivers not expecting the signal, |
| information using radio waves. For this to take | | | | DSSS appears as low-power broadband noise and |
| place, the data is superimposed onto the radio | | | | is rejected. DSSS requires more power than |
| wave, which is also known as the carrier wave, | | | | FHSS, but data rates can be increased to a |
| since it carries the data. This process is called | | | | maximum of 2 Mbps. |
| modulation. There are many modulation techniques | | | | 3. Orthogonal Frequency Division Multiplexing |
| available, all with certain advantages and | | | | (OFDM) - OFDM transmits data in a parallel |
| disadvantages in terms of efficiency and power | | | | method, as opposed to the hopping technique |
| requirements. The modulation techniques are as | | | | used by FHSS and the spreading technique used |
| follows: | | | | by DSSS. This protects it from interference since |
| 1. Narrowband technology - Narrowband radio | | | | the signal is being sent over parallel frequencies. |
| systems transmit and receive data on a specific | | | | OFDM has ultrahigh spectrum efficiency, meaning |
| radio frequency. The frequency band is kept as | | | | that more data can travel over a smaller amount |
| narrow as possible to allow the information to be | | | | of bandwidth. This makes it effective for |
| passed. Interference is avoided by coordinating | | | | high-data-rate transmissions. The drawbacks of |
| different users on different frequencies. The radio | | | | OFDM are that it is more difficult to implement |
| receiver filters out all signals except those on the | | | | than either FHSS or DSSS, and consumes greater |
| designated frequency. For a company to use | | | | amounts of power. |
| narrowband technology, it requires a license issued | | | | |