802.11 Systems

IEEE 802.11 Wireless LAN Standard
Introduction to
Chapter 29
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.
IEEE 802 Protocol Layers
LLC
MAC
Protocol Architecture
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Functions of physical (lowest) layer:
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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
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Functions of media access control (MAC) layer:
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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:
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Provide an interface to higher layers and perform flow
and error control
TCP/IP
IEEE 802.11 Architecture (model)
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Distribution system (DS) – the network backbone
Access point (AP) – a bridge or relay
Basic service set (BSS)
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Extended service set (ESS)
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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
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
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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
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Direct-sequence spread spectrum
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Frequency-hopping spread spectrum
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Operating in 2.4 GHz ISM band
Data rates of 1 and 2 Mbps
Operating in 2.4 GHz ISM band
Data rates of 1 and 2 Mbps
Infrared
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1 and 2 Mbps
Wavelength between 850 and 950 nm
Wi-Fi Infrastructure
Wi-Fi Infrastructure
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(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
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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)
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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
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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
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(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
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IEEE 802.11b
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IEEE 802.11g
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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
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
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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
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.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
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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
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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
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.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
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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)
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Increased spatial streams
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Support for multiple clients simultaneously communicating on the same channel
instead of just one at a time
Emphasis on capacity not coverage
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3 spatial streams in 1st generation
4 spatial steams in 2nd generation
Up to 8 in the future
Multi-user MIMO
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64 QAM in 801.11n to 256 QAM with 802.11ac
(APs w/dual CPUs, Cellular Interference Avoidance, RF optimized)
Will require gigabit Ethernet (backhaul) wired network infrastructure