wireless lans (wlans) provide connectivity within a limited geographic area are a necessity in a...

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