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Wireless and Mobile Networks
Lecture 3. Wireless LANs
Lecture objectives
• Understand basic concepts related to WLANs
• Understand the IEEE 802 architecture for Wireless LANS
• Understand the IEEE 802.11 architecture and its related services
• Understand the factors that increase risks of Wireless and Networks
• Understand the key elements, the role of different services, and phases of IEEE 802.11i
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BASIC CONCEPTS
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• Wireless LANs (WLANs)
– Indispensable adjunct to wired LANs
– Wireless devices use WLANs
• Astheironlysourceofconnectivity • Or to replace cellular coverage
• Simple WLAN configuration
– There is a backbone wired LAN
– User modules include workstations, servers, devices
– Control module (CM) interfaces to WLAN
• Providing bridge or router functionality
• May have control logic to regulate access
• May provide wireless connectivity to other wired networks
Introduction (1)
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Introduction (2)
• Multiple-cell wireless LAN
– Multiple CMs connected by a wired LAN
– Creates many issues for balancing cell loading and providing best connections for UMs
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Example Single-Cell Wireless LAN Configuration
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Example Multiple-Cell Wireless LAN Configuration
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Ad Hoc Networking
• Temporary peer-to-peer network set up to meet immediate need – Peer-to-peer, no centralized server
– Maybe a temporary network
– Wireless connectivity provided by WLAN or Bluetooth, ZigBee, etc.
• Example:
– Group of employees with laptops convene for a meeting; employees link computers in a temporary network for duration of meeting
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Ad Hoc Wireless LAN Configuration
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Wireless LAN motivations
• Cellular data offloading
– WLANs may provide higher data rates and more available capacity
– Cellular providers may encourage this to offload demand on their networks
• Sync/file transfer
– Avoid use of cables
• Internet access
• Multimediastreaming
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Wireless LAN Requirements
• Throughput
• Number of nodes
• Connection to backbone LAN
• Servicearea
• Battery power consumption
• Transmission robustness and security
• Collocated network operation
• License-freeoperation
• Handoff/roaming
• Dynamicconfiguration
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Wireless LAN physical layer
• Multi-cell arrangement
• Transmission Issues
– No licensing needed – Four microwave bands
• 902-928MHz
• 2.4-2.5GHz
• 5.725-5.875GHz
• 58-64 GHz (60-GHz mmWave bands) – Higher capacity
– Less competition
– More expensive equipment
– Spread spectrum (more on this topic on Lecture 6)
• DSSS CDMA or OFDM
• Over 1 Gbps possible with OFDM, channel bonding, and MIMO
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IEEE 802 ARCHITECTURE
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Protocol architecture (1)
• Developed by the IEEE 802.11 working group
• Uses layering of protocols
• LAN protocols focus on the lower layers of the OSI model
– Called the IEEE 802 reference model
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Protocol architecture (2)
• Functions of physical layer:
– Encoding/decoding of signals
– Preamble generation/removal (for synchronization) – Bit transmission/reception
– Includes specification of the transmission medium
• Sublayers
– Physical medium dependent sublayer (PMD)
• Transmitting and receiving user data through a wireless medium
– Physical layer convergence procedure (PLCP)
• Mapping 802.11 MAC layer protocol data units (MPDUs)
into a framing format
• Sending and receiving between stations using same PMD sublayer
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Protocol architecture (3)
• Functions of medium 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
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Separation of LLC and MAC
• The logic required to manage access to a shared-access medium is not found in traditional layer 2 data link control
• For the same LLC, several MAC options may be provided
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MAC Frame Format
• MACcontrol
– Contains MAC protocol information
• DestinationMACaddress
– Destination physical attachment point
• Source MAC address
– Source physical attachment point
• CRC
– Cyclic redundancy check
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Logical Link Control
• Characteristics of LLC not shared by other control protocols:
– Must support multi-access, shared-medium nature of the link
– Relieved of some details of link access by MAC layer
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LLC Services
• Unacknowledged connectionless service – No flow- and error-control mechanisms
– Data delivery not guaranteed
• Connection-modeservice
– Logical connection set up between two users – Flow- and error-control provided
• Acknowledged connectionless service – Cross between previous two
– Datagrams acknowledged – No prior logical setup
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IEEE 802.11 ARCHITECTURE AND SERVICES
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IEEE 802.11
• Started in 1990
– MAC and physical medium specifications
• Wi-Fi Alliance
– Industry consortium
– Creates test suites to certify interoperability of products • May identify a subset of the standard for certification
– Concerned with a range of market areas for WLANs
• IEEE 802.11 has an ever expanding list of standards
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IEEE 802.11 Architecture
• Distribution system (DS)
• Access point (AP)
• Basic service set (BSS)
– Stations competing for access to
shared wireless medium
– Isolated or connected to backbone DS through AP
• Extended service set (ESS)
– Two or more basic service sets interconnected by DS
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Distribution of Messages Within a DS
• Distribution service
– Used to exchange MAC frames from station in one BSS to station in another BSS
• Integrationservice
– Transfer of data between station on IEEE 802.11 LAN and station on integrated IEEE 802.x LAN
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Transition Types Based On Mobility
• No transition
– Stationary or moves only within BSS
• BSStransition
– Station moving from one BSS to another BSS in same ESS
• ESS transition
– Station moving from BSS in one ESS to BSS within another ESS
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Association-Related Services
• Association
– Establishes initial association between station and AP
• Reassociation
– Enables transfer of association from one AP to another, allowing station to move
from one BSS to another • Disassociation
– Association termination notice from station or AP
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IEEE 802.11 MEDIUM ACCESS CONTROL
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IEEE 802.11 Medium Access Control
• MAC layer covers three functional areas: – Reliable data delivery
– Access control – Security
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Reliable Data Delivery
• More efficient to deal with errors at the MAC level than higher layer (such as TCP)
• Frame exchange protocol
– Source station transmits data
– Destination responds with acknowledgment (ACK) – If source doesn‘t receive ACK, it retransmits frame
• Four frame exchange
– Source issues request to send (RTS)
– Destination responds with clear to send (CTS) – Source transmits data
– Destination responds with ACK
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Access control
• Centralized and decentralized mechanisms together – Distributed foundation wireless MAC (DFWMAC)
• Distributed coordination function (DCF) – Decentralized
• Point coordination function (PCF) – Centralized
• Both are available to the LLC layer
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Distributed coordination function
• Decentralized
• Carrier sense multiple access with collision avoidance (CSMA/CA) – Listen to the medium
– If idle, then transmit
– If not, wait a random time
• If busy again, expand the mean waiting time, randomly wait, and try again.
– Binary exponential backoff describes this procedure
• The backoff is the waiting process
• Mean random waiting times get exponentially larger – By a factor of 2 each time, hence the term binary.
– This process responds to heavy loads
• Since nodes do not know the loads of other nodes trying to send.
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IEEE 802.11 PHYSICAL LAYER
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• IEEE 802.11b
– DSSS (Lecture 6)
IEEE 802.11a and IEEE 802.11b
– Provides data rates of 5.5 and 11 Mbps
– Complementary code keying (CCK) and packet binary convolution coding (PBCC) modulation schemes
– First standard to make Wi-Fi become popular
• IEEE 802.11a
– Makes use of 5-GHz band
– Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps
– OFDM (Lecture 6)
– Subcarrier modulated using BPSK, QPSK, 16-QAM or 64-QAM (Lecture 5)
– Never became popular, but its formats and channel schemes are used for later releases of 802.11
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IEEE 802.11g
• Extended rates up to 54 Mbps in 2.4-GHz band
• Continued and extended PBCC from 802.11b that used DSSS
– Rates up to 33 Mbps
• Also used OFDM for rates up to 54 Mbps
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IEEE 802.11n
• Operates in both 2.4-GHz and 5-GHz bands
• MIMO (Lecture 4)
– Multiple parallel streams (up to 4 × 4), beamforming, or diversity
• Radio transmission schemes
– Channel bonding to combine two 20 MHz channels
• Can only use 20 MHz channels if other nodes are active
– Shorter 400 ns guard band (11% increase in data rate) – Higher coding rate of 5/6 (11% increase)
– 150 Mbps per 40 MHz, 600 Mbps for 4 parallel streams
• MAC enhancements
– Frame aggregation
– Block acks (one ACK to cover multiple packets)
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• Up to 6.937 Gbps
• 5-GHz only operation
• Upto8×8MIMO(Lecture4)
• Up to 160 MHz (8 × 20 MHz channels) – Special RTS/CTS to check for legacy devices
• Up to 256 QAM (Lecture 5)
• Multiuser MIMO (Lecture 4)
– Simultaneous beams to multiple stations – Advancedchannelmeasurements
• Larger frame size
• A-MDPU is required
• “Wave 1” products up to 1.3 Gbps
• “Wave 2” products use 160 MHz channels and four spatial streams
802.11ac
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• WiGig
• Upto7Gbps
– Replacement of wires for video to TVs and projectors
• Uses 60-GHz bands
– Called millimeter waves (mmWave)
– Fewer devices operate in these bands – Higher free space loss
– Poor penetration of objects
– Likely only useful in a single room
• Adaptive beamforming and high gain directional antennas – Can even find reflections when direct path is obstructed
• Four modulation and coding schemes
• Personal BSS so devices can talk directly
802.11ad
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IEEE 802.11 physical layer standards
2009
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WLAN SECURITY
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WLAN Security
• Factors to higher security risk in wireless networks (compared to wired networks)
– Channel: broadcasted messages are more susceptible to interception and jamming
– Mobility: mobility increases security risk
– Resources: mobile devices have limited memory and processing power to
counter threats
– Accessibility: some wireless devices may operate unattended, increasing the risk of physical attacks
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• Three points of attack – Client
– Access Point
– Wireless medium
WLAN Security
• Original Wired Equivalent Privacy (WEP) was much too weak
– 802.11i provided stronger Wi-Fi Protected Access (WPA)
– Robust Security Network (RSN), or WPA2, is the final 802.11i standard
• 802.11iservices
– Authentication through an authentication server – Access control
– Encryption for privacy with message integrity
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Elements of IEEE 802.11i
(WPA2)
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802.11i Services
• Authentication: a protocol to define an exchange between a user and
an authentication Server (AS)
– Mutual authentication
– Temporary key exchange over a wireless link
• Access control: enforcing of the use of authentication function, routing, facilitating key exchange
• Privacy with message integrity: MAC-level data are encrypted along a message integrity code
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IEEE 802.11i Phases of Operation
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Discovery Phase
• Purpose: STA and AP to recognize each other, agree on security
capabilities, and establish association for future secure communication • Threeexchanges
– Network and security capability discovery: STA discovery the existence of the network (monitoring Bacon frames from APs or probing channels)
– Open System Authentication: AP and STA exchange identifiers. Exists for backwards compatibility with existing 802.11 hardware
– Association: AP and STA agree on a set of security capabilities, or refuse association if unable to agree
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Authentication Phase
• Purpose: mutual authentication between STA and AS
– Allow only authorized stations to use the network
– Provide STA with assurance that it is communicating with a legitimate network – Relies on EAP (Extensible Authentication Protocol)
• Before AS authenticate STA, only control and authentications messages pass between them (AP blocks other traffic)
– Once STA is authenticated and keys exchanged, traffic is unblocked
• Phases:
– Connect to AS: STA sends request to AP for connection with AS
– EAP exchange: STA and AS authenticate to each other – Secure key delivery: AS generates key and sends to STA
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Key Management Phase
• In this phase, a variety of cryptographic keys are generated and distributed to STAs
– Pairwise keys: communication between STA and AP – Group keys: multicast communication
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Protected Data Transfer Phase
• Two schemes used to provide message integrity and data confidentiality:
– Temporal Key Integrity Protocol (TKIP): developed to require only software update on devices implementing WEP
– Counter Mode-CBC MAC Protocol (CCMP): requires newer devices equipped with specific hardware to support this scheme
• Secure data transfer occurs only between STA and AP. Security is not provided end-to-end
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Sources for this lecture
Cory Beard, William Stallings. Wireless Communication Networks and Systems, 1st edition. Pearson Higher Education, 2016
(Chapter 11)
All material copyright 2016
Cory Beard and William Stallings, All rights reserved
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