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Chapter 14Wireless LANs
Wireless LAN Applications
Example
Single Cell
Wireless LAN
Configuration
Example
Multiple Cell
Wireless LAN
Configuration
Wireless LAN Requirements
In addition to requirements typical of any LAN, a WLAN also has the following requirements:
ThroughputNumber of nodesConnection to backbone LANService areaBattery power consumptionTransmission robustness and securityCollocated network operationLicense-free operationHandoff/roamingDynamic configuration
Wireless LAN TechnologyWLANs are categorized according to the transmission technique that is usedAll current WLAN products fall into one of the following categories:
Table 14.1 Key IEEE 802.11 Task Groups
Table 14.2 IEEE 802.11 Terminology
Wireless Fidelity (Wi-Fi) Alliance
Wireless Ethernet Compatibility Alliance (WECA)Industry consortium formed in 1999 to meet the concern of whether products from different vendors will successfully interoperateSubsequently renamed the Wi-Fi (Wireless Fidelity) AllianceCreates a test suite to certify interoperability for 802.11 products
Term used for certified products is Wi-FiConcerned with a range of market areas for WLANs, including enterprise, home, and hot spots
IEEE 802.11 Architecture
Distribution of Messages Within a Distribution System (DS)
The two services involved with the distribution of messages with a DS are distribution and integration
Association-Related ServicesTransition types, based on mobility:
No transitionBSS transitionESS transition
The DS needs to know the identity of the AP to which the message should be delivered in order for that message to reach the destination station
To meet this requirement a station must maintain an association with the AP within its current BSS
Reliable Data Delivery
A WLAN using the IEEE 802.11 physical and MAC layers is subject to considerable unreliability
Even with error correction codes a number of MAC frames may not successfully be received
IEEE 802.11 includes a frame exchange protocolWhen a station receives a data frame from another station it returns an acknowledgment (ACK) frame to the source stationThis exchange is treated as an atomic unit not to be interrupted by a transmission from any other stationIf the source does not receive an ACK within a short period of time the source retransmits the frame
Four-Frame Exchange
To further enhance reliability a four-frame exchange may be used
A source first issues a Request to Send (RTS) frame to the destinationThe destination then responds with a Clear to Send (CTS)The source transmits the data frame and the destination responds with an ACKThe RTS alerts all stations that are within reception range of the source that an exchange is under wayThe CTS alerts all stations that are within reception range of the destination that an exchange is under wayThe RTS/CTS portion of the exchange is a required function of the MAC but may be disabled
Access Control
IEEE 802.11Protocol
Architecture
IEEE 802.11 Physical LayerIssued in five stages:
IEEE 802.11Includes the MAC layer and three physical layer specifications, two in the 2.4-GHz band (ISM) and one in the infrared, all operating at 1 and 2 Mbps
IEEE 802.11aOperates in the 5-GHz band at data rates up to 54 Mbps
IEEE 802.11bOperates in the 2.4-GHz band at 5.5 and 11 Mbps
IEEE 802.11gOperates in the 2.4-GHz band at data rates up to 54 Mbps
IEEE 802.11nOperates in either the 2.4-GHz band or the 5-GHz band with data rates in the hundreds of Gbps
Table 14.3 IEEE 802.11 Physical Layer Standards
Original IEEE 802.11
Three physical media are defined in the original 802.11 standard:
Direct-sequence spread spectrum (DSSS) operating in the 2.4-GHz ISM band, at data rates of 1 and 2 MbpsFrequency-hopping spread spectrum (FHSS) operating in the 2.4-GHz ISM band, at data rates of 1 and 2 MbpsInfrared at 1 and 2 Mbps, operating at a wavelength between 850 and 950 nm
IEEE 802.11b
IEEE 802.11a
IEEE 802.11g
Table 14.4 Estimated Distance (m) Versus Data Rate
IEEE 802.11nThis standard is defined to operate in both the 2.4-GHz and the 5-GHz bands and can therefore be made upwardly compatible with either 802.11a or 802.11b/gEmbodies changes in three general areas:
Multiple-Input-Multiple-Output (MIMO)
Average Throughput per User
Gigabit WLANsIEEE 802.11ac
Next step up for the old 802.11a Wi-Fi standardIs in the 5-GHz band, but provides wider channels for higher data throughputUses a channel width of either 40 MHz or 80 MHz to deliver dataMay also make use of MU-MIMO (multiuser multiple-input multiple-output)
IEEE 802.11adOperates in the 60-GHz band and is expected to deliver data rates of up to 6 GbpsDownside of this millimeter band Wi-Fi is that its range will be in feet rather than yards
Li-Fi
IEEE 802.11 Security Considerations
Access and privacy services:
Wireless LAN Security Standards
Wired Equivalent Privacy (WEP)Defined by 802.11 for privacyContained major weaknesses
Wi-Fi Protected Access (WPA)A set of security mechanisms that eliminates most 802.11 security issuesWas based on the current state of the 802.11i standardAs 802.11i evolves, WPA will evolve to maintain compatibility
Summary Wireless LAN applications
Wireless LAN requirements
Wireless LAN technology
Wi-Fi architecture and services IEEE 802.11 architecture
IEEE 802.11 services
Gigabit WLANs Gigabit Wi-Fi
Li-Fi
Chapter 14: Wireless LANs
IEEE 802.11 medium access control
IEEE 802.11 physical layer
IEEE 802.11 security considerations
Access and privacy services
Wireless LAN security standards