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Prezentace wifi_Vančišin Jakub

Y04A2Z - Anglický jazyk 2-1

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the format and structure of those signals. There are for example 802.11a standard, 802.11g standard and 802.11b standard, which is currently widest used.

802.11b networks operate in a special band of radio frequencies around 2.4 GHz, which is in most parts of the world reserved for unlicensed radio services. Unlicensed means that anybody using equipment that complies with the technical requirements can send and receive radio signals on these frequencies without a need for a radio license. These frequencies are slightly different from one part of the world to another. You can see the main frequency assignments in table 1 in the enclosures. Any other country in the world uses one of these frequencies. In North America, Wi – Fi devices use 11 channels. Many other countries (for example in Europe) have authorized 13 channels, Japan uses 14 channels and France only 4. All over the world, the set of channel numbers is the same, so channel number 9 in New York uses exactly the same frequency as channel 9 in Tokyo or Paris. You can see the list of channels used in different parts of the world in table 2.

So, as I have mentioned, the principle of Wi–Fi lies in sending data from transmitter to receiver in the form of radio signals. If the receiver is not available, for example it’s receiving data from another transmitter at the moment, other data addressed to this receiver are sent to the a switching center that holds them until a link to the destination becomes available again.

However, before the data are transmitted, they must be slightly modified. The message that the transmitter wants to send and that consists of a long stream of ones and zeroes, is divided into a specific number of smaller parts called packets. Packets of more than one message can travel together at the same circuit, combine with packets containing other messages as they travel between switching centers and reassemble themselves into the original message at the destination.

Each data packet must also contain some more information that is added by the transmitter. This information includes for example the address of the packet’s destination, the sequence order of this packet relative to other packets in the original transmission, and so on. Some of this information instructs the switching center where to forward each packet and other information tells the destination device how to reassemble the data in the packet back into the original message. Each time the packet is sent for example from a laptop computer in a wireless network through an office LAN connected through the internet to some distant computer connected to another LAN, there can be a lot of other information added and then remov

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Vloženo: 26.04.2009

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