wifi
Content
4.4 An introduction to WiFi
WiFi (from ‘Wireless Fidelity’) is used to connect devices together in one of two network configurations known as ‘ad hoc’ and ‘infrastructure’. We shall explain these terms shortly. (As a starting point, though, you could look up the terms ‘ad hoc’ and ‘infrastructure’ in your dictionary.) In wireless LANs, nodes are usually referred to as stations – probably because each communicating device acts as a radio station with transmitter and receiver. These functions, and the necessary control functions, are provided by a wireless network interface card (wireless NIC) in a similar way to wired networks.
4.5 WiFi network structure
A WiFi network can operate in one of two different modes: ad hoc mode or infrastructure mode In an ad hoc network, stations communicate with each other directly, without the need for any intermediary or central control. This means that when one WiFi device comes within range of another, a direct communication channel can be set up between them. This is known as peer-topeer communication. Additional devices can join the network, all communicating with each other in a broadcast fashion. (In this context, ‘broadcast’ means that a message sent by one node will arrive at every other node in the network, regardless of the destination address.) Figure 14 provides a diagram of a possible geographical layout of an ad hoc network. This figure is similar to the diagrams of network topologies you met in Section 3.3, in that it is a representation of a network layout viewed as though you were floating above it and looking down. In Figure 14, the dark circles represent WiFi stations. The concentric circles around them show that the communication channel is radio waves. In this case, the diagram indicates nondirectional antennas. However, since the focus of this diagram is the network layout, then the detail of whether directional or non-directional antennas are used isn't important.
Figure 14 Representation of a WiFi ad hoc network Long description An ad hoc network is likely to be temporary – for example, a network set up for a business meeting where people want to share information stored on portable devices like lap-top computers and personal digital assistants (PDAs). (If you looked up the term ‘ad hoc’ in your dictionary you probably found a definition something like ‘for a specific purpose, impromptu, not pre-planned’.) An ad hoc network is independent of, and isolated from, any other network. In infrastructure mode (Figure 15), stations communicate with each other via a wireless access point (AP) which also acts as a connector between a wired network and the wireless network. The access point is effectively a base station that controls the communication between the other stations. Access points form part of a wired network infrastructure and are not mobile. (If you looked up the word ‘infrastructure’ in your dictionary the definition given probably used terms like ‘underlying foundation, basic structure, substructure’.)
Figure 15 Representation of a WiFi infrastructure network
4.6 WiFi stations
A WiFi station determines whether it is in range of an AP by transmitting an enquiry, known as a probe request frame, and waiting for a response. If more than one AP responds, the station will choose to communicate with the one that has the strongest signal. A probe request frame initiates the WiFi connection and is an example of a management frame – a type of frame that does not carry any message data. Just like the nodes on an Ethernet network, each station must have a means of being uniquely identified by a MAC address. Every message data frame sent must contain the MAC address of the source, destination and access point, as well as other management data that enables the frames to be correctly sequenced and errors to be detected. Because all the stations in a WiFi network share the same communication channel, only one station at a time can be allowed to send data. So a station waits until it detects a period of inactivity and then uses a special protocol which prevents two or more stations sending data at exactly the same time. The exchanges involved in these protocols are another example of management data. The WiFi standards do not define any upper limit on the number of stations that can join a network, though some particular equipment manufacturers may specify a limit. (We've seen one which stipulates a maximum of 128 stations connected to any one AP.) However, as the number of communicating stations increases, the channel capacity available for each station decreases. A
point will eventually be reached when the network becomes too congested to provide an adequate service
4.7 WiFi data rates and operating range
Just as for Ethernet, developments in technology have increased the achievable data rates since the first WiFi standard was developed in 1997. At the time of writing, the latest WiFi standard to be published – IEEE 802.11g – defines a data rate of 54 Mbps.
Activity 17: exploratory
How do you think the rate for transmitting messages between stations is affected by:
the management information that is included with each frame; the protocols used to enable multiple stations to share the communication channel; multiple users on a WiFi network?
The practical message data rate that can be achieved in a wireless network is often described as its throughput. Even in ideal operating conditions, the throughput may be only 50 per cent to 75 per cent of the maximum data rate. For WiFi, throughput is generally about half the maximum data rate possible on the communication channel, giving about 30 Mbps for 802.11g networks, and this has to be shared between all the stations on the network. The achievable data rate reduces with distance from the AP (or in the case of an ad hoc network, with distance from other stations). Maximum data rates can be achieved only within about 30 m of an AP, tailing off at distances greater than this. For 802.11g networks the data rate drops to as low as 1 or 2 Mbps at 100 m. Physical barriers such as partitions and walls will further reduce the maximum rate possible at a given distance from the AP.
Wi-Fi (802.11 technology)
Contents
1 Wi-Fi 2 Applications of the fields, domains and products 3 Working Principle 4 Information Processes 5 Performance characteristics o 5.1 802.11a o 5.2 802.11b o 5.3 802.11g o 5.4 802.11n 6 Reference 7 Further Reading
Wi-Fi
Wi-Fi is a wireless local area networks (WLAN) technology [1], also known as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 technology (see figure1). Wi-Fi can connect users to content and communications over computer and mobile, portable stationary applications and basically providing a one hop unlimited access to a building’s Ethernet network.
Figure1: Logo of Wi-Fi
Applications of the fields, domains and products
Based on the feature of middle range cover (<300 meter), Wi-Fi is predominantly used in campuses and enterprise buildings. A wireless router (see figure 2), which integrates a Wireless Access Point, Ethernet switch and internal router firmware application, is usually used to connect a group of wireless devices to an adjacent wired LAN [2]. Wi-Fi certified and compliant devices are installed in not just in personal computer, but in equipment like PDA, home entertainment equipment, MP3 players, smart phones, printers, digital cameras, to name a few [3].
Figure 2: A Linksys wireless router
Working Principle
Basically, the working principle of Wi-Fi can be described as a computer's or other compatible device’s wireless adapter translates data into a radio signal (2.4 and 5 GHz radio bands [1]) and transmits it to a wireless router by using an antenna. When the wireless router receives the signal, it will decode it and send the data to the Internet by using a physical, wired Ethernet connection (see figure 3). The working process also works in reverse. When the router receives data from the Internet, it will translate the data into a radio signal and send it to the computer’s or other compatible device’s wireless adapter [4].
Figure 3: Working Principle of Wi-Fi
Information Processes
The wireless adapter used for Wi-Fi communication can transmit and receive radio waves, and it can convert 1s and 0s into radio waves and convert the radio waves back into 1s and 0s, which are just very similar to the working principle of radios used for walkie-talkies, cell phones and other devices [4].
Performance characteristics
Wi-Fi products are identified as 802.11, and are then further identified by a lower case letter that identifies which specific technology is in operation [3]. The main Wi-Fi standard generations using in the current wireless devices are listed below (also see table):
802.11a
802.11a, approved in 1999, transmits at 5 GHz unlicensed bands and can move up to 54 megabits of data per second. Orthogonal frequency-division multiplexing (OFDM), a efficient coding technique that splits radio signal into several sub-signals before they reach a receiver, is used to greatly reduce interference [4].
802.11b
802.11b, approved in 1999, defines complementary code keying (CCK) modulation to deliver raw data rates up to 11 megabits in the 2.4 GHz frequency band of the radio spectrum. Currently, when the faster standards become less expensive, the use of 802.11b is not as popular as before [1].
802.11g
802.11g, a backwards-compatible extension to the 802.11b standard in the 2.4 GHz band, was approved in 2003 [5]. But it is not as slow as 802.11b, because it can handle up to 54 megabits of data per second use the same OFDM coding as 802.11a [4].
802.11n
802.11n, approved in 2009 [6], is the newest standard that is widely available. This standard significantly improves speed and range. As can be seen from the table, it can handle up to 450 megabits of data per second.
Table: Wi-Fi Generations [3]
Wi-Fi Technology 802.11a 802.11b 802.11g
Frequency Band 5 GHz 2.4 GHz 2.4 GHz 2.4 GHz, 5 GHz, 2.4 or 5 GHz (selectable), or 2.4 and 5 GHz (concurrent)
Bandwidth or maximum data rate 54 Mbps 11 Mbps 54 Mbps
802.11n
450 Mbps
Besides the ones mentioned above, some other certification sets, defined by a set of features that relate to performance, exist as well. For example, 802.11i is defined for security enhancements, 802.11d, 802.11h and 802.11j are extended for regulatory enhancements and 802.11e provides quality of service (QoS) enhancements [5].
Reference
[1] Jim Geier (2007) 2007 Wireless Ntework Industry Report [2] http://en.wikipedia.org/wiki/Wifi [3] http://www.wi-fi.org/discover_and_learn.php [4] http://www.howstuffworks.com/wireless-network1.htm [5] Tropos Networks, Inc. (2007) 802.11 Technologies: Past, Present and Future [6] http://www.qwiki.com/q/#!/Wi-Fi
Further Reading
Mattern, F. (2004, November 2004) ‘Wireless future: ubiquitous computing’, Paper presented at the Wireless Congress, Munich, Germany. Vijay Kumar Garg (2007) Wireless communications and networking, Amsterdam; Boston: Elsevier Morgan Kaufmann (2007)
WiFi (from ‘Wireless Fidelity’) is used to connect devices together in one of two network configurations known as ‘ad hoc’ and ‘infrastructure’. We shall explain these terms shortly. (As a starting point, though, you could look up the terms ‘ad hoc’ and ‘infrastructure’ in your dictionary.) In wireless LANs, nodes are usually referred to as stations – probably because each communicating device acts as a radio station with transmitter and receiver. These functions, and the necessary control functions, are provided by a wireless network interface card (wireless NIC) in a similar way to wired networks.
4.5 WiFi network structure
A WiFi network can operate in one of two different modes: ad hoc mode or infrastructure mode In an ad hoc network, stations communicate with each other directly, without the need for any intermediary or central control. This means that when one WiFi device comes within range of another, a direct communication channel can be set up between them. This is known as peer-topeer communication. Additional devices can join the network, all communicating with each other in a broadcast fashion. (In this context, ‘broadcast’ means that a message sent by one node will arrive at every other node in the network, regardless of the destination address.) Figure 14 provides a diagram of a possible geographical layout of an ad hoc network. This figure is similar to the diagrams of network topologies you met in Section 3.3, in that it is a representation of a network layout viewed as though you were floating above it and looking down. In Figure 14, the dark circles represent WiFi stations. The concentric circles around them show that the communication channel is radio waves. In this case, the diagram indicates nondirectional antennas. However, since the focus of this diagram is the network layout, then the detail of whether directional or non-directional antennas are used isn't important.
Figure 14 Representation of a WiFi ad hoc network Long description An ad hoc network is likely to be temporary – for example, a network set up for a business meeting where people want to share information stored on portable devices like lap-top computers and personal digital assistants (PDAs). (If you looked up the term ‘ad hoc’ in your dictionary you probably found a definition something like ‘for a specific purpose, impromptu, not pre-planned’.) An ad hoc network is independent of, and isolated from, any other network. In infrastructure mode (Figure 15), stations communicate with each other via a wireless access point (AP) which also acts as a connector between a wired network and the wireless network. The access point is effectively a base station that controls the communication between the other stations. Access points form part of a wired network infrastructure and are not mobile. (If you looked up the word ‘infrastructure’ in your dictionary the definition given probably used terms like ‘underlying foundation, basic structure, substructure’.)
Figure 15 Representation of a WiFi infrastructure network
4.6 WiFi stations
A WiFi station determines whether it is in range of an AP by transmitting an enquiry, known as a probe request frame, and waiting for a response. If more than one AP responds, the station will choose to communicate with the one that has the strongest signal. A probe request frame initiates the WiFi connection and is an example of a management frame – a type of frame that does not carry any message data. Just like the nodes on an Ethernet network, each station must have a means of being uniquely identified by a MAC address. Every message data frame sent must contain the MAC address of the source, destination and access point, as well as other management data that enables the frames to be correctly sequenced and errors to be detected. Because all the stations in a WiFi network share the same communication channel, only one station at a time can be allowed to send data. So a station waits until it detects a period of inactivity and then uses a special protocol which prevents two or more stations sending data at exactly the same time. The exchanges involved in these protocols are another example of management data. The WiFi standards do not define any upper limit on the number of stations that can join a network, though some particular equipment manufacturers may specify a limit. (We've seen one which stipulates a maximum of 128 stations connected to any one AP.) However, as the number of communicating stations increases, the channel capacity available for each station decreases. A
point will eventually be reached when the network becomes too congested to provide an adequate service
4.7 WiFi data rates and operating range
Just as for Ethernet, developments in technology have increased the achievable data rates since the first WiFi standard was developed in 1997. At the time of writing, the latest WiFi standard to be published – IEEE 802.11g – defines a data rate of 54 Mbps.
Activity 17: exploratory
How do you think the rate for transmitting messages between stations is affected by:
the management information that is included with each frame; the protocols used to enable multiple stations to share the communication channel; multiple users on a WiFi network?
The practical message data rate that can be achieved in a wireless network is often described as its throughput. Even in ideal operating conditions, the throughput may be only 50 per cent to 75 per cent of the maximum data rate. For WiFi, throughput is generally about half the maximum data rate possible on the communication channel, giving about 30 Mbps for 802.11g networks, and this has to be shared between all the stations on the network. The achievable data rate reduces with distance from the AP (or in the case of an ad hoc network, with distance from other stations). Maximum data rates can be achieved only within about 30 m of an AP, tailing off at distances greater than this. For 802.11g networks the data rate drops to as low as 1 or 2 Mbps at 100 m. Physical barriers such as partitions and walls will further reduce the maximum rate possible at a given distance from the AP.
Wi-Fi (802.11 technology)
Contents
1 Wi-Fi 2 Applications of the fields, domains and products 3 Working Principle 4 Information Processes 5 Performance characteristics o 5.1 802.11a o 5.2 802.11b o 5.3 802.11g o 5.4 802.11n 6 Reference 7 Further Reading
Wi-Fi
Wi-Fi is a wireless local area networks (WLAN) technology [1], also known as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 technology (see figure1). Wi-Fi can connect users to content and communications over computer and mobile, portable stationary applications and basically providing a one hop unlimited access to a building’s Ethernet network.
Figure1: Logo of Wi-Fi
Applications of the fields, domains and products
Based on the feature of middle range cover (<300 meter), Wi-Fi is predominantly used in campuses and enterprise buildings. A wireless router (see figure 2), which integrates a Wireless Access Point, Ethernet switch and internal router firmware application, is usually used to connect a group of wireless devices to an adjacent wired LAN [2]. Wi-Fi certified and compliant devices are installed in not just in personal computer, but in equipment like PDA, home entertainment equipment, MP3 players, smart phones, printers, digital cameras, to name a few [3].
Figure 2: A Linksys wireless router
Working Principle
Basically, the working principle of Wi-Fi can be described as a computer's or other compatible device’s wireless adapter translates data into a radio signal (2.4 and 5 GHz radio bands [1]) and transmits it to a wireless router by using an antenna. When the wireless router receives the signal, it will decode it and send the data to the Internet by using a physical, wired Ethernet connection (see figure 3). The working process also works in reverse. When the router receives data from the Internet, it will translate the data into a radio signal and send it to the computer’s or other compatible device’s wireless adapter [4].
Figure 3: Working Principle of Wi-Fi
Information Processes
The wireless adapter used for Wi-Fi communication can transmit and receive radio waves, and it can convert 1s and 0s into radio waves and convert the radio waves back into 1s and 0s, which are just very similar to the working principle of radios used for walkie-talkies, cell phones and other devices [4].
Performance characteristics
Wi-Fi products are identified as 802.11, and are then further identified by a lower case letter that identifies which specific technology is in operation [3]. The main Wi-Fi standard generations using in the current wireless devices are listed below (also see table):
802.11a
802.11a, approved in 1999, transmits at 5 GHz unlicensed bands and can move up to 54 megabits of data per second. Orthogonal frequency-division multiplexing (OFDM), a efficient coding technique that splits radio signal into several sub-signals before they reach a receiver, is used to greatly reduce interference [4].
802.11b
802.11b, approved in 1999, defines complementary code keying (CCK) modulation to deliver raw data rates up to 11 megabits in the 2.4 GHz frequency band of the radio spectrum. Currently, when the faster standards become less expensive, the use of 802.11b is not as popular as before [1].
802.11g
802.11g, a backwards-compatible extension to the 802.11b standard in the 2.4 GHz band, was approved in 2003 [5]. But it is not as slow as 802.11b, because it can handle up to 54 megabits of data per second use the same OFDM coding as 802.11a [4].
802.11n
802.11n, approved in 2009 [6], is the newest standard that is widely available. This standard significantly improves speed and range. As can be seen from the table, it can handle up to 450 megabits of data per second.
Table: Wi-Fi Generations [3]
Wi-Fi Technology 802.11a 802.11b 802.11g
Frequency Band 5 GHz 2.4 GHz 2.4 GHz 2.4 GHz, 5 GHz, 2.4 or 5 GHz (selectable), or 2.4 and 5 GHz (concurrent)
Bandwidth or maximum data rate 54 Mbps 11 Mbps 54 Mbps
802.11n
450 Mbps
Besides the ones mentioned above, some other certification sets, defined by a set of features that relate to performance, exist as well. For example, 802.11i is defined for security enhancements, 802.11d, 802.11h and 802.11j are extended for regulatory enhancements and 802.11e provides quality of service (QoS) enhancements [5].
Reference
[1] Jim Geier (2007) 2007 Wireless Ntework Industry Report [2] http://en.wikipedia.org/wiki/Wifi [3] http://www.wi-fi.org/discover_and_learn.php [4] http://www.howstuffworks.com/wireless-network1.htm [5] Tropos Networks, Inc. (2007) 802.11 Technologies: Past, Present and Future [6] http://www.qwiki.com/q/#!/Wi-Fi
Further Reading
Mattern, F. (2004, November 2004) ‘Wireless future: ubiquitous computing’, Paper presented at the Wireless Congress, Munich, Germany. Vijay Kumar Garg (2007) Wireless communications and networking, Amsterdam; Boston: Elsevier Morgan Kaufmann (2007)
