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8/14/2019 Wireless Data Networks Wireless Data Networks
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Motivation
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Wireless Data Networks
Wireless WANs (Mobile Data Network) Voice-Oriented: GSM, CDMA
Data-Oriented: Mobitex, D-AMPS
Voice/Data Mixed: IMT-2000
Wireless LANS Data-Oriented: IEEE 802.11, Hyperlan
Wireless Local Loop (IEEE 802.16) Data/Voice-Oriented: WiMax
Wireless PANS (IEEE 802.15) Data/Voice-Oriented: Bluetooth
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Wireless Data Services
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Wireless Data NetworkCategories
Infrared (IR) LANs (1-10 Mbps) Direct beam and diffused
Spread spectrum LANs (1-54 Mbps) IEEE802.11, 802.15(PAN),Bluetooth
Narrowband microwave (100 Mbps) IEEE802.16 (WiMAX)
Applications: LAN Extension -WLAN Wireless Broadband Access Nomadic Access Ad hoc Networking
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Data vs. Voice
voice traffic changes into data traffic
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Wireless LAN Configuration
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IEEE802 Standard Series
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IEEE802 Standard Series 802.1: Higher level interface (HILI)
802.2: Logical Link Control 802.3: CSMA/CD Ethernet (10Mbps) 802.4: Token Bus 802.5: Token Ring 802.6: MAN 802.7: Broadband Technical Advisory Group 802.8: Fiber Optics Technical Advisory Group 802.9: Integrated Service LAN Interface 802.10: Standard for Interoperable LAN Security
802.11: Wireless LANs 802.12: Demand Priority 802.14: Cable TV Based Broadband Communication
Networks
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Challenges in WLANs
Multiple devices need to share the ether
channel efficiently Problems: interference, contention, access
control, channel quality varies over space andtime
Different service requirements Voice (real-time, reservation-based) Data (best effort, reliable deliver)
Different approaches and trade-offs Centralized vs. Distributed
Other challenges Mobility
Power conservation
Security considerations
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Challenges in WLANs
Intrinsic Problems Multi-path
Hidden terminals
Burst errors
Near-far: If two terminals at differentdistances from the receiver start transmission atthe same power simultaneously, the receiverwill get more power from the nearer transmitter.
The SNR of the farther transmitter may getbelow detection in some cases and as a result,the communication channel may be jammed.
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Random Access Methods
Fixed access techniques (FDMA,TDMA and
CDMA) are inefficient in transmittingbursty data! The random access techniques are used in
mobile data networks, which can bedivided into two groups:
ALOHA based access methods: The mobile terminals transmit their contention packet
without any coordination between them.
CSMA (Carrier-Sense Multiple Access): The mobile terminals senses the availability of the
channel before it transmits its packets.
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Pure ALOHA
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Slotted ALOHA
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Reservation Based ALOHA
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CSMA: Listen-before-talk
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CSMA/CA
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Performance of Random Access Methods
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Hidden Terminals
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History of IEEE802.11
1987: Started as 802.4L
1989: Moved to 802.11
1997: MAC & PHY for 1&2Mbps at2.4GHz
1999 (a, b): PHY for 11Mbps at 2.4GHz(3 Ch) and 54Mbps at 5GHz (12 ch)
2000 (c, d): Supplement to 802.1dbridges. Dynamic regulatory domain
update
Current (e, g, h, i, j, k, m, n, p, r, s)
IEEE 802 11 WLAN S d d
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IEEE 802.11 WLAN StandardActivities
802.11a: 5 GHz, 54 Mbps
802.11b: 2.4 GHz, 11 Mbps 802.11d: Multiple regulatory domains 802.11e: Quality of Service (QoS) 802.11f: Inter-Access Point Protocol (IAPP) 802.11g: 2.4 GHz, 54 Mbps
802.11h: Dynamic Frequency Selection (DFS) and Tran Power 802.11i: Security Ratified | WPAv2 Draft 9 802.11j: Japan 5 GHz Channels (4.9-5.1 GHz) 802.11k: Measurement 802.11m: Maintenance
802.11n: High Throughput 802.11p: Wireless Access for Vehicular Environment 802.11r: Public WLAN Fast Roaming 802.11s: Mesh Networking
O i f IEEE802 11
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Overview of IEEE802.11Architecture
Uses CSMA/CD MAC, RTS/CTS with
optional PCF supporting all threePHYs
Supports three PHY layers DSSS FHSS DFIR
Supports WLAN with wiredinfrastructure as well as independentad-hoc WLANs
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Terminologies
Access Point (AP)
Provides access to distribution services via the wirelessmedium
Basic Service Area The coverage area of one access point
Basic Service Set (BSS) A set of stations controlled by one access point
Distribution System (DS) The fixed (wired) infrastructure used to connect a set of BSS to
create an extended service set (ESS)
Portal(s) The logical point(s) at which non-802.11 packets enter an ESS
MAC Protocol Data Unit (MPDU) Packets that describe protocol
MAC Service Data Unit (MSDU) Packets that describe service
A hit t /R f
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Architecture/ReferenceModel
IEEE802 11 Protocols in
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IEEE802.11 Protocols inContext
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IEEE 802.11 Protocol Layers
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Sub-layer Responsibilities
LLC: Provide an interface to higher layers
and performs flow and error control MAC Sublayer: access mechanism, data
format MAC Management: roaming in ESS,
power management, and security. PLCP: carrier sensing assessment, forming
packets for PHYs PMD: modulation and coding
PHY Layer Management: channel tuning Station Management: interacts with MAC
and PHY
D t il d Vi f P t l
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Detailed View of ProtocolArchitecture
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Physical Layer in 802.11
Three options Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum (DSSS)
Diffused Infra Red (DFIR) not widely used.
Note, same MAC layer but all 802.11,802.11 a and 802.11 b all are incompatibleat the physical layer!
IEEE 802 11a and IEEE
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IEEE 802.11a and IEEE802.11b
IEEE 802.11a
Makes use of 5-GHz band Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps Uses orthogonal frequency division multiplexing
(OFDM) Sub-carrier modulated using BPSK, QPSK, 16-QAM or
64-QAM
IEEE 802.11b Provides data rates of 5.5 and 11 Mbps Complementary code keying (CCK) modulation
scheme based on DSSS
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IEEE 802.11g
IEEE 802.11g Up to 54 or 108 Mpbs at 2.4GHz Mandatory backward compatibility with IEEE802.11b
Mandatory OFDM and CCK
Optional CCK- OFDM and PBCC (Packet Binary
Convolutional Code)
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MAC Layer Functionality
MAC Sublayer Format of messages (data and control) Access control/mechanisms
1. Contention Mode For access to the channel by multiple
contending devices
1. Contention-free schemes RTS (Ready To-Send)/CTS (Clear-To-Send)
tackle the hidden terminal problem DATA and PCF for time bounded access
MAC Management Roaming support in the ESS, power
management and security
MAC Timing: Basic Access
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MAC Timing: Basic AccessMethod
For CSMA/CA, we have inter-frame spacing IFS.
As per the priority level of the packet, the IFS isdivided into three categories DCF-IFS (DIFS): used for contention, lowest priority,
longest delay. PCF-IFS (PIFS): for medium priority and medium
delay. Short-IFS (SIFS) used for high priority such as ACKs,
CTS, etc. has the lowest duration time and delay.
Distributed Coordination
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Distributed CoordinationFunction (DCF)
Use CSMA/CA Algorithm based on Inter-frameSpace (IFS):
If the medium is idle, the station waits to see if themedium is idle for a time equal to IFS. If so, it maytransmit immediately.
If the medium is busy, the station defers transmissionand continues to monitor the medium.
Once the current transmission is over, the station delaysanother IFS. If the medium remains idle for this period,
then it backs off the random amount of time and againsenses the medium. If the medium is still idle, it maytransmit. During the back-off time, if the mediumbecomes busy, the back-off timer is halted and resumeswhen the medium is idle.
CSMA/CA with ACK in
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CSMA/CA with ACK inan Infrastructure Network
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RTS /CTS Mechanism
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Wired Equivalent Privacy
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Wired Equivalent Privacy
WiFi Protected Access (WPA) /
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WiFi Protected Access (WPA) /IEEE802.11i