Wireless Communications and Networks

Wireless Communications and Networks

IEEE 802.11 Wireless LAN Standard

Introduction to

Chapter 29

TCP/IP is the more popular protocol especially after it

IEEE 802 Protocol Layers

LLC

MAC

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 to be below the physical layer but critical to wireless LAN design)

Protocol Architecture

Functions of media access control (MAC) layer:

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

On reception, disassemble frame and perform address recognition 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 flow and 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 carrier-sense multiple access/collision detection

802.16

or

WiMAX

(Worldwide Interoperability for Microwave Access), is a long range system (MAN), known as Broadband Wireless

Access, a possible replacement for cell phones GSM/CDMA.

Frequencies 2 – 66 GHz, uses SOFDMA

(scalable OFDM) and beginning to incorporate MIMO schemes, actually complements WiFi

(end devices with both capabilities)

802.11 Architecture Model

ESS

DS

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, medium access control and security.

The Physical Layer (Layer 1) where the electrons move, consists of three physical media – DSSS

(direct sequence), FHSS (frequency hopping) and

Infrared in conjunction with the 802.11 standards of 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 Authentication

Protocol (EAP) for authentication.

The authenticator server is a entity that provides an authentication service to an authenticator. When used

(normally in an enterprise environment) this server typically executes EAP methods for the authenticator

(AP). When used in an 802.11 environment this is a

RADIUS 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 LANs consisting 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 delivery mechanism

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

The actual server doing the authentication, typically a RADIUS server in an enterprise environment, is called the authentication

server (AS)

. The device in between, such as a wireless access point, 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 outof-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 with

WPA-PSK. Setup requires a WPA Passphrase or Network Key along with the

SSID (Service Set Identifier – a unique 32-character network name that differentiates one wireless LAN from another, normally known or discovered).

WPA2 and WPA2-PSK – very strong security (CCMP), combines both

TKIP + AES, requires a WPA Passphrase and SSID

Wireless Client Security Separation – dissallows associated wireless clients to communicate with each other (normally turned off but intended for hotspots and public 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 common for the consumer market.

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

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

The increased number of usable channels (at least in the US) and the near absence of other interfering systems (microwave ovens, cordless phones, baby monitors) give 802.11a significant aggregate bandwidth and reliability 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 do translation for .11b) but still much faster than .11b alone. It is a myth that the 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 the markets, 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/g devices when only 802.11n devices are present

MIMO – Multiple In, Multiple Out

MIMO Power Save Mode – conserves power consumption by making use of multiple antennas and radios only when needed.

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 from

2.4 GHz band thus one must perform site surveys in both bands; at a minimum survey at 5 GHz

Although .11n has greater signal propagation than

802.11a/b/g, distant stations and too many stations per AP will lower performance

Security, Network Design

Don’t use TKIP or especially WEP

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

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

Wired networks and the switch/cabling infrastructure must support Gigabit Ethernet to take full advantage of

802.11n’s performance

Might need to re-evaluate the increased traffic load on the core 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 implementation of 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

1 st 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 2 nd 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 1 st generation

4 spatial steams in 2 nd 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

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