IEEE 802.11 Wireless LAN Standard

Introduction to Chapter 29

IEEE 802 Protocol Layers TCP/IP is the more popular protocol especially after it was incorporated it into UNIX (public, open source). TCP/IP is known today as the Internet Protocol. It is only defined through 4 layers.

MAC

LLC

Protocol Architecture Functions of physical (lowest) layer:

Encoding/decoding of signals Preamble generation/removal (for

synchronization) Bit transmission/reception Includes specification of the transmission

medium and topology (normally considered tobe below the physical layer but critical towireless LAN design)

Protocol Architecture Functions of media access control (MAC) layer:

On transmission, assemble data into a frame withaddress and error detection fields

On reception, disassemble frame and perform addressrecognition and error detection

Govern access to the LAN transmission medium Functions of logical link control (LLC) Layer:

Provide an interface to higher layers and perform flowand error control

TCP/IP

IEEE 802.11 Architecture (model) Distribution system (DS) – the network backbone Access point (AP) – a bridge or relay Basic service set (BSS)

Stations competing for access to shared wireless medium Isolated or connected to backbone DS through AP The entity in which the stations are within range of each other although

BSSs can easily overlap Extended service set (ESS)

Two or more BSS interconnected by DS usually a wired LAN 802.11~WiFi is a CSMA/CD protocol, contention based, 500 ft 802.16 or WiMAX (Worldwide Interoperability for Microwave Access), is a

long range system (MAN), known as Broadband WirelessAccess, a possible replacement for cell phones GSM/CDMA.Frequencies 2 – 66 GHz, uses SOFDMA (scalable OFDM) andbeginning to incorporate MIMO schemes, actuallycomplements WiFi (end devices with both capabilities)

802.11 Architecture Model

DS

ESS

IEEE 802.11 Services

Access Control

802.11 MAC and Physical Layer

The lower segment of the Layer 2 services (MAC)is made up of reliable data delivery, mediumaccess control and security.

The Physical Layer (Layer 1) where the electronsmove, consists of three physical media – DSSS(direct sequence), FHSS (frequency hopping) andInfrared in conjunction with the 802.11 standardsof today (802.11a/b/g/n/ac).

The Three Physical Media Defined by Original 802.11 Standard Direct-sequence spread spectrum

Operating in 2.4 GHz ISM band Data rates of 1 and 2 Mbps

Frequency-hopping spread spectrum Operating in 2.4 GHz ISM band Data rates of 1 and 2 Mbps

Infrared 1 and 2 Mbps Wavelength between 850 and 950 nm

Wi-Fi Infrastructure

Wi-Fi Infrastructure (continued)

Authentication – validate a stations identity Stations associate to an Access Point (AP) The AP is the normally the authenticator in a wireless

environment initiating the Extensible AuthenticationProtocol (EAP) for authentication.

The authenticator server is a entity that provides anauthentication service to an authenticator. When used(normally in an enterprise environment) this servertypically executes EAP methods for the authenticator(AP). When used in an 802.11 environment this is aRADIUS server configured by the network admin.

EAP (Extensible Authentication Protocol) Types

802.11i Wireless Security -Authentication and Encryption

802.11i – the security standard for 802.11 wireless LANsconsisting of 4 phases of discovery, authentication(802.1X) and encryption

IEEE 802.1x Authentication (port based network access control)

Dynamically varying encryption keys 802.1x wraps EAP (Extensible Authentication Protocol) into

Ethernet frames instead of using the point-to-point protocol (PPP) Most of major wireless LAN vendors offer proprietary versions

of dynamic key management using 802.1x as a deliverymechanism

In typical 802.1x implementations, the client can automaticallychange encryption keys as often as necessary to minimize thepossibility of eavesdroppers cracking the current key

The actual server doing the authentication, typically a RADIUSserver in an enterprise environment, is called the authenticationserver (AS). The device in between, such as a wireless accesspoint, is called the authenticator

802.1x requires a lot of management overhead but good security

Web Based Authentication

Typical Authentication Settings

Typical Radius Server Settings

Security with 802.11/11i and WPA (Wireless Protected Access) – Encryption

Encryption Protocols

Wireless Encryption Options Open – no security, easy access to user’s entire network and computer MAC Address – limit access to specific hardware MAC address (unique to

every piece of hardware) but data communications completely open WEP – secure but vulnerable, shared (secret) key assured authentication but

since it was a fixed key used in each transmission it was easy to break, thus out-of-date but part of legacy equipment requirements, master key of 40 or 104 bits

WPA or WPA-PSK – strong security, TKIP used for WPA and AES used withWPA-PSK. Setup requires a WPA Passphrase or Network Key along with theSSID (Service Set Identifier – a unique 32-character network name thatdifferentiates one wireless LAN from another, normally known or discovered).

WPA2 and WPA2-PSK – very strong security (CCMP), combines bothTKIP + AES, requires a WPA Passphrase and SSID

Wireless Client Security Separation – dissallows associated wireless clients tocommunicate with each other (normally turned off but intended for hotspots andpublic access situations)

IEEE 802.11a (the enterprise wireless)

5-GHz band with data rates of 6, 9, 12, 18, 24, 36, 48, 54 Mbps Uses orthogonal frequency division multiplexing (OFDM) Subcarrier modulated using BPSK, QPSK, 16-QAM or 64-QAM Equipment was more expensive that consumer equipment for 802.11b 802.11a on 5 GHz is not interoperable with 802.11 b/g that operate on 2.4

Ghz although dual-band capable equipment is becoming more commonfor the consumer market.

5 GHz band is less crowded than 2.4 GHz (thus less degradation due toconflicts, interference, etc) but physically has less range since it isabsorbed more readily by walls and other solid objects in the LOS path

OFDM has fundamental propagation advantages in a high multipathenvironment while the higher frequencies enable smaller antennas withhigher gain which counteract the disadvantage of a higher frequency.

The increased number of usable channels (at least in the US) and the nearabsence of other interfering systems (microwave ovens, cordless phones,baby monitors) give 802.11a significant aggregate bandwidth andreliability advantages over 802.11b/g (you get what you pay for)

802.11 b/g/n IEEE 802.11b

Provides data rates of 5.5 and 11 Mbps at 2.4 GHz, a very crowded band Complementary code keying (CCK) modulation scheme Suffers interference from other products operating in the 2.4 GHz band

microwave ovens, Bluetooth devices, baby monitors & cordless telephones IEEE 802.11g

2.4 GHz, up to 54 Mbps, OFDM same as 802.11a Still has the interference problems of the 2.4 GHz band .11g and .11b can operate simultaneously but with an .11b user in the cell

the wireless network will degrade the .11g performance (AP must dotranslation for .11b) but still much faster than .11b alone. It is a myth thatthe entire network downmodes to .11b

Dual-band, or dual-mode Access Points and Network Interface Cards(NICs) that can automatically handle a and b/g are now common in all themarkets, and very close in price to b/g only devices

IEEE 802.11n and 802.11ac are the latest IEEE WiFi standards

802.11n Signal Processing (MIMO)

802.11n Spatial Multiplexing

802.11n Channel Bonding

802.11n Terms Wi-Fi Alliance – Organization that certifies 802.11a/b/g/n

products for operability, signified by the logo

Green Field Mode – eliminates support for 802.11a/b/gdevices when only 802.11n devices are present

MIMO – Multiple In, Multiple Out

MIMO Power Save Mode – conserves power consumptionby making use of multiple antennas and radios only whenneeded.

802.11n Relative Rate & Range

Wireless Range Considerations

Wireless Range Factors

802.11n Lessons Learned .11n has realized better rate versus range Backward compatible with 802.11 a/b/g stations

Mixed Mode (normal default for legacy compatibility) Legacy Mode – AP behaves like 802.11 a/g device with

improved performance but disabling .11n operation 802.11n Mode - .11n stations only, avoids air time

consumption from legacy devices (802.11b) Tools – monitoring, diagnosis, compliance

Needed to solve tough interference problems Key Design Parameters: site surveys, device placement,

security and wired network

802.11n Lessons Learned Live site surveys the only way to determine true

coverage 802.11n signal propagation more dependent on the

environment than 802.11a/b/g 802.11n has 8X more bandwidth at 5 GHz but

propagation characteristics are very different from2.4 GHz band thus one must perform site surveys inboth bands; at a minimum survey at 5 GHz

Although .11n has greater signal propagation than802.11a/b/g, distant stations and too many stationsper AP will lower performance

Security, Network Design Don’t use TKIP or especially WEP

Use WPA2/AES – anything else is a compromise onsecurity and performance

.11n operates 6-8X faster so encryption performancebecomes more important for APs

Wired networks and the switch/cabling infrastructuremust support Gigabit Ethernet to take full advantage of802.11n’s performance

Might need to re-evaluate the increased traffic load on thecore network with the performance aspects of 802.11n

RF Considerations .11n is optimized for 5 GHz and 802.11b devices on 2.4

GHz kill performance. 5 GHz is the key. Move to 5 GHz as much as possible, force users by turning

2.4 GHz radio power down and leaving 5 GHz at maximum Better to force 802.11 a/g/n in the network configurations

since probably not many .11b devices around any more Performance can vary greatly between NIC brands,

probably because of early pre-ratification implementationof 802.11n

Perform live testing of products and environment Note that many .11n options are still to come so flexible

APs (radios) are a key consideration

IEEE 802.11ac WiFi Standard Operates only on 5 GHz 1st generation 1.3 GBPS up to 6.9 GBPS later Increased channel width – from 40 MHz maximum in 802.11n to 80 MHz

in 802.11ac with 160 MHz in 2nd generation 802.11ac Higher speed modulation (higher order)

64 QAM in 801.11n to 256 QAM with 802.11ac

Increased spatial streams 3 spatial streams in 1st generation 4 spatial steams in 2nd generation Up to 8 in the future

Multi-user MIMO Support for multiple clients simultaneously communicating on the same channel

instead of just one at a time

Emphasis on capacity not coverage (APs w/dual CPUs, Cellular Interference Avoidance, RF optimized)

Will require gigabit Ethernet (backhaul) wired network infrastructure