CENG531_LCN_6_WLANs(1).pdf

Lecture 6: Wireless Local Area Networks (WLANs) Chapter 6 Dr. Abdulmalik Alwarafy CENG 531: Wireless Communications and Sensor Networks Lecture 6 Objectives • Understand WLANs in terms of: o Components & characteristics o Standards & architecture o Different operating modes & protocol operation...

Lecture 6: Wireless Local Area Networks (WLANs) Chapter 6 Dr. Abdulmalik Alwarafy CENG 531: Wireless Communications and Sensor Networks Lecture 6 Objectives • Understand WLANs in terms of: o Components & characteristics o Standards & architecture o Different operating modes & protocol operation o Challenges & solutions affecting performance ▪ Channel allocation ▪ Rate adaptation ▪ Densification & enhanced spatial reuse • Lecture maps onto CLO 3: – “Explain protocols underlying the design of modern communication systems” Definition • WLAN is a wireless network that allows two or more devices to be wirelessly connected to form a local area network on a limited scale • Through a gateway, WLAN often enables Internet connection Wired Vs. Wireless LANs Parameter Wired LAN Wireless LAN Medium Wires Air Access type Multiple access Or Point-toPoint access Multiple access Hosts Host is always physically connected to its network Not physically connected Connection to Other Networks Can be connected to another network or an internetwork such as Internet using router Can be connected to wired or wireless infrastructure network, or to another Wireless LAN Topology Fixed Dynamic Outside Signal Interference Protected from outside signals Unprotected Host can move from location to location, but is only used while at fixed location Host can access WLAN while in motion Mobility 4 Wireless LANs: Characteristics • Attenuation: ❖The strength of electromagnetic signals decreases rapidly because signal disperses in all directions; only a small portion of it reaches receiver • Interference: ❖ receiver may receive signals not only from intended sender, but also from other senders if they are using same frequency band • Multipath Propagation: ❖ Receiver may receive more than one signal from same sender because electromagnetic waves can be reflected back from obstacles such as walls, ground, or objects • Error: ❖ Errors & error detection are more serious issues in a wireless net than in wired net Source: Slides accompanying textbook by Forouzan 5 WLANs – Success Story • Captured huge market share, widespread adoption • Enterprises, Educational, Medical establishments • • • • Public Access (hotels, airports, transport hubs etc.) Manufacturing facilities, Retail environments Connectivity in Home Mostly bandwidth hungry services with short range o802.11 a, b/g, n, ac, ax …. • New standard 802.11ah designed for IoT applications oDesigned to operate in sub-GHz bands ▪ Longer range (up to a km) o large scale, low power & low data rate applications Wireless LANs - IEEE 802.11 Project • IEEE defined specifications for WLAN, called IEEE 802.11 o Covers both Physical & Data Link Layers o A.k.a. “Wireless Ethernet” • Often, the term WiFi (short for wireless fidelity) is used as synonym for WLAN 7 IEEE 802.11 Family Within the IEEE 802.11 Working Group,[6] the following IEEE Standards Association Standard & Amendments exist: • IEEE 802.11-1997: The WLAN standard was originally 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and infrared (IR) standard (1997), all the others listed below are Amendments to this standard, except for Recommended Practices 802.11F and 802.11T. • IEEE 802.11w: Protected Management Frames (September 2009) • IEEE 802.11y: 3650–3700 MHz Operation in the U.S. (2008) • IEEE 802.11z: Extensions to Direct Link Setup (DLS) (September 2010) • IEEE 802.11-2012: A new release of the standard that includes amendments k, n, p, r, s, u, v, w, y and z (March 2012) • IEEE 802.11aa: Robust streaming of Audio Video Transport Streams (June 2012) • IEEE 802.11ac: Very High Throughput <6 GHz;[44] potential improvements over 802.11n: better modulation scheme (expected ~10% throughput increase), wider channels (estimate in future time 80 to 160 MHz), multi user MIMO;[45] (December 2013) • IEEE 802.11a: 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001) • IEEE 802.11b: Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999) • • IEEE 802.11c: Bridge operation procedures; included in the IEEE 802.1D standard (2001) IEEE 802.11ad: Very High Throughput 60 GHz (December 2012) see WiGig • IEEE 802.11ae: Prioritization of Management Frames (March 2012) • IEEE 802.11d: International (country-to-country) roaming extensions (2001) • IEEE 802.11af: TV Whitespace (February 2014) • IEEE 802.11mc: Roll-up of 802.11-2012 + aa, ac, ad, ae & af to be published as 802.11-2015 (~ December 2015) • IEEE 802.11e: Enhancements: QoS, including packet bursting (2005) • IEEE 802.11F: Inter-Access Point Protocol (2003) Withdrawn February 2006 • • IEEE 802.11g: 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003) IEEE 802.11ah: Sub 1 GHz sensor network, smart metering. (~ March 2016) • IEEE 802.11ai: Fast Initial Link Setup (~ November 2015) • IEEE 802.11h: Spectrum Managed 802.11a (5 GHz) for European compatibility (2004) • IEEE 802.11aj: China Millimeter Wave (~ June 2016) • IEEE 802.11i: Enhanced security (2004) • IEEE 802.11aq: Pre-association Discovery (~ July 2016) • IEEE 802.11j: Extensions for Japan (2004) • IEEE 802.11ak: General Links (~ May 2016) • IEEE 802.11-2007: A new release of the standard that includes amendments a, b, d, e, g, h, i and j. (July 2007) • IEEE 802.11ax: High Efficiency WLAN (~ 2020) • IEEE 802.11k: Radio resource measurement enhancements (2008) • IEEE 802.11n: Higher throughput improvements using MIMO (multiple input, multiple output antennas) (September 2009) • IEEE 802.11p: WAVE—Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) (July 2010) • IEEE 802.11r: Fast BSS transition (FT) (2008) • IEEE 802.11s: Mesh Networking, Extended Service Set (ESS) (July 2011) • IEEE 802.11T: Wireless Performance Prediction (WPP)—test methods and metrics Recommendation cancelled • IEEE 802.11u: Improvements related to HotSpots and 3rd party authorization of clients, e.g. cellular network offload (February 2011) • IEEE 802.11v: Wireless network management (February 2011) In process • IEEE 802.11be: Extremely High Throughput (~ 2024) o 802.11F and 802.11T are recommended practices rather than standards, and are capitalized as such. o 802.11m is used for standard maintenance. 802.11ma was completed for 802.11-2007, 802.11mb was completed for 802.11-2012 and 802.11mc is working towards publishing 802.11-2015. http://en.wikipedia.org/wiki/IEEE_802.11 IEEE 802.11: Architecture • Standard defines two types of services: BSS & ESS • Basic Service Set (BSS): Comprises of stationary or mobile wireless stations & optional central base station, known as access point (AP) A BSS without AP is a stand-alone net & cannot send data to other BSSs. It is called an ad hoc architecture A BSS with AP is called infrastructure BSS 9 IEEE 802.11: Architecture o Extended Service Set (ESS): Comprises of two or more BSSs with APs ▪ The BSSs are connected through distribution system, which is wired or wireless net ▪ The distribution system connects APs in BSSs Stationary Stations mobile stations 10 Ad Hoc Mode BSS WLAN • Is a P2P mode: nodes talk to each other directly oAP not required • Network setup on the fly oNetwork (re)configures itself • Typically limited backhaul • Example applications: oVehicular communication oMilitary oDisaster response communication oUnmanned Aerial Vehicle (UAV) communication Video illustration: https://www.youtube.com/watch?v=SLSOZokbFfM&ab_channel=SunnyClassroom Infrastructure Mode BSS WLAN • Nodes need infrastructure element to enable connectivity oNodes talk to AP • AP provides backhaul connectivity & net management • Examples Applications: oHome/Office nets oPublic nets, e.g., in coffee shops / airports / malls etc. IEEE 802.11 PHY Layer • Legacy 802.11 supports: o Diffused infrared transmission o FHSS o DSSS o Operates in the 2.4 GHz band • 802.11a introduced OFDM (5GHz operation): o Different modulation techniques, big jump in data rate ▪ At 6 Mbps ➔ PSK ▪ At 12 Mbps ➔ QPSK ▪ At 24 Mbps ➔ 16-level QAM ▪ At 54 Mbps ➔ 64-level QAM IEEE 802.11 PHY Layer • 802.11g (WiFi3): best of both (popularity of 11b + speed of 11a): oOperates in 2.4GHz band, oBackwards compatible with 802.11b • 802.11n (WiFi4): exploited larger channels (20 or 40 MHz) & Multiple Input Multiple Output (MIMO) technique o Works in both 2.4 & 5 GHz, o Backwards compatible with 802.11 IEEE 802.11 PHY Layer • Prior to 802.11n, two antennas were used for antenna diversity – one radio with two antennas o Antenna that receives strongest signal is used to send response frames (e.g. ACK) • In 802.11n (MIMO): o MIMO: using multiple radios & multiple antennas o MIMO devices use multiple radios each with own antenna o Employ beamforming to direct transmissions to device from which frame was received o uses spatial multiplexing - frames are broken up & sent in multiple parts from different radios IEEE 802.11 PHY Layer • 802.11ac (WiFi3): oIntroduced channel bonding + MU-MIMO oUses beamforming, MU-MIMO in Downlink oSupports up to 4 users simultaneously oSupports higher order QAM modulation 256 QAM • 802.11ax (WiFi6/6E): oUses beamforming, supports MU-MIMO in uplink oSupports OFDMA in uplink oSupports up to 8 users simultaneously oSupports enhanced Spatial Reuse (SR) & higher order QAM modulation 1024 QAM Analogy Source: https://www.networkworld.com/article/3258807/what-is-80211ax-wi-fi-6-and-what-will-it-mean-for-80211ac.html IEEE 802.11 PHY Layer • Analogy: o 802.11: Long line of customers in bank with 1 teller o MU-MIMO (802.11ac&ax): 4 tellers serving 4 lines of customers o OFDMA: Each teller can serve 4 customers simultaneously Analogy Source: https://www.networkworld.com/article/3258807/what-is-80211ax-wi-fi-6-and-what-will-it-mean-for-80211ac.html IEEE 802.11 MAC Sublayer • IEEE 802.11 defines two MAC sublayers: o Distributed Coordination Function (DCF) o Point Coordination Function (PCF) Source: Data Communications and Networking, Forouzan 18 How do you coordinate access to the medium? Distributed Coordination Function (DCF) • One of two protocols defined by IEEE at MAC sublayer • Devices listen before talking (CSMA/CA) oAll devices wait random amount of time (number of slots) after the medium is clear • Receiver acknowledges explicitly upon receipt • Provides option for reservation too o Request to Send (RTS) / Clear to Send (CTS) Video illustrating how DCF works: https://youtu.be/MgbLTgao3yw WLAN: Hidden Terminal Problem • Terminals B & C thinks the medium is free, and both send data to terminal A simultaneously causing collision at terminal A Source: Data Communications and Networking, Forouzan 21 DCF with RTS-CTS Enabled DIFS: distributed interframe space RTS: request to send SIFS: short interframe space CTS: clear to send Source: Data Communications and Networking, Forouzan Network Allocation Vector (NAV) ▪ When station sends RTS frame, it includes duration of time it needs to occupy channel ▪ Stations affected by this transmission create timer called network allocation vector (NAV) that shows how much time must pass before these stations are allowed to check channel for idleness ▪ Each time station accesses channel & sends RTS frame, other stations start their NAV ▪ Each station: ▪ First checks its NAV to see if it has expired ▪ Then sense physical medium to see if it is idle ▪ Backoff if collision & try again Source: Data Communications and Networking, Forouzan 23 Point Coordination Function (PCF) • The point coordination function (PCF) is optional access method implemented in an infrastructure net (not for Ad Hoc mode) • Used mostly for time-sensitive transmission • PCF has centralized, contention-free polling access • Stations that support PCF are polled oMixed mode deployment with DCF & PCF stations can exist • The stations are polled one after another, sending any data they have to AP Source: Data Communications and Networking, Forouzan 25 Point Coordination Function (PCF) • Due to priority of PCF over DCF, stations that only use DCF may not gain access to medium • To prevent this, a repetition interval (“Super Frame”) is designed to cover both contention-free PCF & contentionbased DCF traffic 26 Point Coordination Function (PCF) • The repetition interval is repeated continuously & starts with special control frame called beacon frame • When stations hear beacon frame, they start their NAV for the duration of contention-free period of repetition interval 27 How device finds a network to connect to? Association in WLANs • Device wants to join WLAN must first listen/scan wireless medium: oPassive or oActive scan • Passive Scan: oClient listens on each channel for finite period of time oListens for Beacon frame sent periodically (100ms) by each AP or ad hoc devices oBeacon frame includes AP’s service set identifier (SSID) & basic SSID (BSSID) oClient follows authentication procedure & joins net Association in WLANs • Active Scan: oClient sends probe request frame on each channel ▪ Contains client ID, capabilities, supported rates oWaits (10-20ms) for probe response frame from AP Reassociation in WLANs • Reassociation happens when roaming in ESS: oMake-before-break, client reassociates with new AP, new AP sends disassociation frame to old AP Associating & disassociating in an ESS Issues affecting WLAN Performance Channel Selection/Allocation • Trend: density of WLANs increasing o Increase in no. of users & APs o Limited amount of operating channels implies potential for conflict • How should AP choose its operating channel? o Easy in a planned setup o Difficult in unplanned setup ▪ APs belong to different owners/operators • Different administrative authority • Operation under imperfect knowledge of neighbourhood ▪ Typically configured to a fixed random channel Channel Allocation Considerations • Assignment (Planned setup: tell AP which channel to use) o Graph colouring technique, measurements/site surveys • Traffic awareness: o Collect traffic demand information at AP • Coordination: o Make measurements at UE/AP & report to AP o Terminal side changes undesirable, high overheads o Energy consumption issues • Selection (Unplanned setup: AP chooses its own channel) o Standard approach in low cost off-the-shelf devices ▪ Typically static or random selection approach Channel Allocation – Wish List • Ability to communicate across administrative domains oUse beacons for advertising information • Regular monitoring of network dynamics oCapture spatial & temporal traffic patterns • Reduce channel switching overheads oAvoid unnecessary switching • Reduce service disruption oUse channel switch announcement (CSA) feature Rate/Link Adaptation • IEEE 802.11 provides multi-rate capability: o Lower rates are robust but provide less throughput ▪ Preferable when channel conditions are BAD o Higher rates provide high throughput but robustness depends on channel quality ▪ Preferable when channel conditions are GOOD • Adaptation mechanisms left out of the standard • Desirable: Way to infer & differentiate between channel conditions to guide Rate Adaptation (RA) • Design Goals & Constraints of RA mechanisms: o Should be responsive o Interoperable with legacy devices ▪ Shouldn’t require changes to 802.11 standard o Simple to implement Rate/Link Adaptation – Issues to consider Proposed rate adaptation approaches Use channel metrics such as signal strength, SNR, etc Use statistical data such as loss ratio, no. of success/failures Proposed rate adaptation approaches Entirely Senderbased implementation Sender-recipient co-ordination based implementation HYBRID techniques • Important considerations for RA mechanisms: • How quickly can it adapt to changing channel conditions? • React to any type of loss? • Differentiate between channel related & collision related losses? • Requires changes to the standard? • Changes at AP side, terminal side, or both? Summary We discussed: oWLAN characteristics & architecture oDifferent standards & their capabilities oOperating modes & protocol operation oIssues affecting performance & solutions Other Resources • IEEE 802.11 Working Group ohttp://www.ieee802.org/11/ • A tutorial on High Efficiency WLANs (802.11ax) ohttps://ieeexplore.ieee.org/stamp/stamp.jsp?arnu mber=8468986