Training: Wireless network

This chapter could be included in the Ethernet networks connections. Nevertheless, as wireless connection is rather evolutionary, I chose to make specifics parts.

Wireless connections permit to connect various equipments ... without wire. Connection perhaps either of the hertzian type, or by infra-red light. The connections will infra red require that the transmitter and the receiver are on the same line (look at themselves), which is not always hollowing out. These connections were used (without much success) for the keyboards and the mice but are established in certain printers.

The wireless network connections have taken, for a few years, very an other direction, the simultaneous connection of several apparatuses between-them. They can be various printers, scanners and peripherals or even of networks. The difficulty of implementation holds of the zone of reception, related on the power of the transmitter, the detection of the receiver (from where a protocol defining this one clearly) and of the data security transmitted. This safety must hold account of the checking of the transmitted data but also of the encoding of the data. Nothing is used for to make safe a network if a simple hertzian receiver could pump all the data circulating on the network.

Currently, several types of networks "without wire" are on the market for current distributions. Connection Internet by satellite are seen in a chapter with share.

The solutions hertzian pose problems of environment that few manufacturers announce. There is to only walk in an industrial building (out of sheet) to realize that the environment poses which problems of connections GSM for example. The maximum distances provided by the manufacturers speak about discovered grounds, which is seldom the case in the dwellings or companies, even if it is possible to install external antennas in many cases. The environments disturbed by electromagnetic fields (electric machines of strong powers) pose the same problems as in traditional wiring networks. In many cases, it will be necessary to mix solutions with wiring network and microwave link.

11.2. Bluetooth

This type of wireless connection allows hard cover two apparatuses via a microwave link. These apparatuses can be numerical cameras, PDA, printers,.. Bluetooth exploits the frequency band of the 2,45 Ghz ISM (Industrial, Scientific & Medical) which is normally free of right for the majority of the countries. The number of distinct frequencies used is 79. You could thus use 79 different networks in the same part. The flow of connection is of maximum 1 Mb/s for distant peripherals of maximum 4 meters and 75 kb/s for higher distances. The maximum distance is 10 meters, but can reach in certain cases 100 meters. Indeed, Bluetooth technology defines 2 categories of powers radio frequency for the personal networks, the short beach (0 dBm) which authorizes distances up to 10 meters and the average beach (+ 20 dBm) which carries up to 100 meters. The radio connection supports at the same time the data transmission and vocal with a maximum speed of data of 72 kb/s, which is in practice the maximum rate.

Made safe, this connection is transparent only if the two apparatuses know each other. Each peripheral east receives a code with manufacture on six bytes : three first appointing the manufacturer and the three others the machine. Indeed, each apparatus bluetooth can be decontaminated for an automatic connection or be activated for only certain apparatuses. The peripherals thus use protection systems avoiding the transfer of unauthorized data. Nevertheless, safety is often decontaminated by defect and the hacking is thus possible to recover for example the data of the address book of a GSM or a PDA starting from another apparatus or to use the GSM of the neighbor for a connection Internet.

Within a network bluetooth, an apparatus is useful of Master and until 7 peripherals slaves which divide the band-width. It is possible in theory to make communicate until 10 groups of apparatuses, that is to say 80 apparatuses.

On the contrary connections IEEE 802.11, this type of connection is not dedicated for the connections networks (even if it is possible). It makes it possible for example to connect a PDA directly to Notebook or a GSM.

11.3. IEEE 802.11

Microwave link also using the frequency band of the 2,45 Ghz (ISM). The maximum flow is 2 Mb/s at a maximum distance of 100 meters. Specificities of this old standard of wireless networks go back to 1997. It is not currently used any more.

11.4. IEEE 802.11a

This standard operates in the frequency band 5-6 Ghz. The diagram of modulation used is "orthogonal frequency-division multiplexing" (OFDM). In this type of modulation, the signal is cut out and sent on several of different frequencies. This limits the interferences and makes possible of the speed transmissions of data going up to 54 Mb/s (either approximately 10 MB/s), but more generally the communications occur to 6 Mb/s, 12 Mb/s or 24 Mb/s.

The maximum distance between the central point (which function like Hub) and the stations east of 366 m to 6 Mbps in outside and of 91 m with 6 Mbps in interior. For short distances, it is faster than the 802.11B Wifi.

This standard is sometimes called Wifi5. It is used little in Europe but very established in the United States.

11.5. IEEE 802.11b - Wifi - IEEE 802.11 HR

Derived from the IEEE 802.11 (1999), this microwave link also uses the frequency band of the 2,4 Ghz. It is used as connection network via specific charts networks and a central apparatus called access point (Access Point) functioning as a hub (the total band-width is thus divided between the various PC. This connection allows a maximum capacity of 11 Mb/s on a ray of a hundred meters but the range strongly depends on the environment (walls or partitions...). The number of peripheral is limited to 10 by stations.

This solution is currently adapted for the wireless networks. Connection uses the 2 low layers of OSI model which are used for transport. Each PC, portable and peripheral included a chart network of the type WIFI with an antenna. A concentrator (HUB, switch or even router) is used as central point for the division or possibly for a connection towards a traditional concentrator.

The method of catch of line is of type CSMA/CA, identical to the Ethernet networks. A large difference all the same. When a transmitting station on a telegraphic connection Ethernet, it is with the listening of all the stations on the cable, which could not be the case in a microwave link. Indeed, the fact that 2 stations can be connected on the central node does not include that the stations can communicate directly between them if the distance is too important. For that, one uses the mechanism of "Virtual Carrier Sense". A station wanting to emit transmits a small package called RTS (Request To Send), which indicates the source, the destination and the duration of the transmission. The station answers, if it is free, by a package of control called CTS (Clear To Send) which includes same information of duration. All the stations which receive a RTS or a CTS start an indicator of Virtual Carrier Sense (called VOR - Network Allocation Vector) for a certain duration.

Practically all the component makers networks include such apparatuses in their catalogue.

The maximum distance in outside east of 503 m to 1 Mbps in outside and of 152 m in 1 Mbps in interior.

A router WIFI can be used of router or bridge. He generally uses 2 directional antennas. The networks adaptaters are specific, with an external antenna. Below the photograph of a router Wifi de D-Link.

11.6. Wireless network IEEE 802.11B+

The 802.11 B+ is derived from the 802.11 B. It uses the same frequency band but characteristic of in encoding specific since this one is done on 64, 128 or even 256 bits. For recall, current versions Internet To explore cryptent only on 128 bits. This system allows flows of 22 Mbps, that is to say the double of 802.11b.

It is completely compatible going down with the standard 802.11B. A peripheral 802.11B+ will thus accept connection with the peripherals 802.11B. On the other hand, this standard is not standardized. It is thus possible that apparatuses 802.11B+ different manufacturers are not compatible.

11.7. Wireless network 802.11 G

Even if standardization goes back to May 2003, some apparatuses left front. The first apparatuses really to the standard left at the beginning of July 2003. This standard wireless idem allows connections with 54 Mbps by using the frequency band of the 2,4 Ghz (that the 802.11 b). This use of the same zone of frequency should make it possible to mix access points 802.11 B and 802.11 B+ (in the same mark). The central point adapts its speed according to the connected peripheral, allowing to customers 802.11 B to connect itself.

11.8. Wireless 802.11G+ connection

This improvement of the 802.11G left the at the beginning of 2004 and double 802.11G connection speed to reach 108 Mb/s.

11.9. 802.11N

11.10. Infra-red connection.

This type of connection will disappear and replaced by connections hertzian seen higher. The first problem of this type of connection comes from its operating mode, the light. The connected apparatuses must be perfectly opposite one the other, which is not always easy. Moreover, many solutions were proposed. Even if the IrDA connection (installed in the HP990CXi printers for example) became more extensive than the other connections, this multitude of system to strongly reduced the fields of activity.

The infra-red connection IrDA allows a connection of 1 meters for a maximum speed of 16 Mb/s