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Wireless Networks the 802 Class of Standards 4/28/2019 Standardization Local Area Networks • Wireless networks are standardized by IEEE • Under 802 LAN MAN standards committee Application 15-441/641: Wireless Networks Presentation ISO 15-441 Spring 2019 OSI Session IEEE 802 Profs Peter Steenkiste & Justine Sherry 7-layer Transport standards model Network Logical Link Control Data Link Medium Access (MAC) Spring 2019 https://computer-networks.github.io/sp19/ Physical Physical (PHY) • 802.1 Overview Document Containing the Reference Model, Tutorial, and Glossary • 802.1 b Specification for LAN Traffic Prioritization • 802.1 q Virtual Bridged LANs “Ethernet” • 802.2 Logical Link Control • 802.3 Contention Bus Standard 1 Obase 5 (Thick Net) The 802 Class of Standards • 802.3a Contention Bus Standard 10base 2 (Thin Net) • 802.3b Broadband Contention Bus Standard 10broad 36 • 802.3d Fiber-Optic InterRepeater Link (FOIRL) • 802.3e Contention Bus Standard 1 base 5 (Starlan) • List on next slide • 802.3i Twisted-Pair Standard 10base T • 802.3j Contention Bus Standard for Fiber Optics 10base F • Some standards apply to all 802 technologies • 802.3u 100-Mb/s Contention Bus Standard 100base T • 802.3x Full-Duplex Ethernet • 802.3z Gigabit Ethernet • E.g. 802.2 is LLC • 802.3ab Gigabit Ethernet over Category 5 UTP • 802.4 Token Bus Standard • Important for inter operability • 802.5 Token Ring Standard • 802.5b Token Ring Standard 4 Mb/s over Unshielded Twisted-Pair • Some standards are for technologies that are outdated • 802.5f Token Ring Standard 16-Mb/s Operation • 802.6 Metropolitan Area Network DQDB • 802.7 Broadband LAN Recommended Practices WiFi Family • Not actively deployed anymore • 802.8 Fiber-Optic Contention Network Practices • 802.9a Integrated Voice and Data LAN • E.g. 802.6 • 802.10 Interoperable LAN Security • 802.11 Wireless LAN Standard Bluetooth, Zigbee, .. • 802.12 Contention Bus Standard 1 OOVG AnyLAN • 802.15 Wireless Personal Area Network • 802.16 Wireless MAN Standard 1 4/28/2019 Overview Wireless Communication • Link layer challenges and WiFi • Wireless communication is based on E • WiFi broadcast C • Basic WiFi design • A, B, and C can all hear each A other’s signal • Some deployment issues B • Looks like Ethernet! • WiFi version D • Why not use CSMA/CD? • Cellular • Carrier-sense Multiple Access / Collision Detection A B C D E • Well, it is not that easy 5 What is the Problem? Implications for Wireless Ethernet There are no Wires! • Collision detection is not practical • Attenuation is very high! A B C D E • Ratio of transmitted signal power to received power is way too • Signal is not contained in a wire high at the transmitter • Attenuation is 1/D2 for distance D • Transmitter cannot detect colliding transmissions (deaf while transmitting) • In addition, there is significant noise and interference • So how do you detect collisions? A B C D E • Not all nodes can hear each other • No wire to protect the signal • “Listen before you talk” often fails • Much higher error rates • Hidden and exposed terminals • Not all nodes in the wireless network can hear each other • Made worse by fading • Wireless communication range is shorter • Changes over time! • Standard cannot limit the length of the wires 7 2 4/28/2019 Hidden Terminal Problem Exposed Terminal Problem R1 S1 RTS CTS CTS S1R1 S2 S2 R2 • Lack signal between S1 and S2 and cause collision at R1 • Carrier sense prevents two senders from sending simultaneously although they • Severity of the problem depends on the sensitivity of the do not reach each other’s receiver carrier sense mechanism • Severity again depends on CCA threshold R2 • Clear Channel Assessment (CCA) threshold • Higher CCA reduces occurrence of exposed terminals, but can create hidden terminal scenarios Spectrum Allocation in US History • Aloha wireless data network • Car phones • Big and heavy “portable” phones • Limited battery life time • But introduced people to “mobile networking” • Later turned into truly portable cell phones • Wireless LANs • Originally in the 900 MHz band • Later evolved into the 802.11 standard • Later joined by the 802.15 and 802.16 standards • Cellular data networking • Data networking over the cell phone • Many standards – throughput is the challenge 12 3 4/28/2019 Some IEEE 802.11 Standards Spectrum Use Comments • IEEE 802.11a • PHY Standard : 8 channels : up to 54 Mbps : some deployment • IEEE 802.11b • Each country is in charge of spectrum allocation and use • PHY Standard : 3 channels : up to 11 Mbps : widely deployed. • IEEE 802.11d internally • MAC Standard : support for multiple regulatory domains (countries) • IEEE 802.11e • MAC Standard : QoS support : supported by many vendors • Federal Communication Commission (FCC) and National • IEEE 802.11f Telecommunication and Information Administration in the US • Inter-Access Point Protocol : deployed • IEEE 802.11g • PHY Standard: 3 channels : OFDM and PBCC : widely deployed (as b/g) • Spectrum allocation differs quite a bit – implications for mobile users? • IEEE 802.11h • Suppl. MAC Standard: spectrum managed 802.11a (TPC, DFS): standard • Broadly speaking two types of spectrum • IEEE 802.11i • Suppl. MAC Standard: Alternative WEP : standard • Licensed spectrum: allocated to licensed user(s) • IEEE 802.11n • MAC Standard: MIMO : significant improvements in throughput • IEEE 802.11ac • Unlicensed spectrum: no license needed but device must respect rules • Support for multi-user MIMO • IEEE 802.11ad • WiFi in the 60 GHz band 13 Frequency Bands IEEE 802.11 Overview • Industrial, Scientific, and Medical (ISM) bands • Generally called “unlicensed” bands • Adopted in 1997 with goal of providing Short Wave Radio FM Broadcast • Giving wireless users access to services in wired networks AM Broadcast Television Infrared wireless LAN Cellular (840MHz) • High throughput and reliability NPCS (1.9GHz) • Continuous network connection, e.g. while mobile Very Super Millimeter Visible Ultra- Low Medium High Ultra Infrared X-Rays High High High wave Light violet 60 GHz • The protocol defines IEEE 802.11ad • MAC sublayer • MAC management protocols and services 902 - 928 MHz 2.4 - 2.4835 GHz 5 GHz • Several physical layers: IR, FHSS, DSSS, OFDM 26 MHz 83.5 MHz IEEE 802.11a (IEEE 802.11b and later • Wi-Fi Alliance is industry group that certifies and later) interoperability of 802.11 products 4 4/28/2019 Infrastructure and Ad Hoc Mode Features of 802.11 MAC protocol • Infrastructure mode: stations communicate with one or more access points which are connected to the wired infrastructure • Supports MAC functionality • What is deployed in practice • Addressing – based on 48-bit IEEE addresses • Two modes of operation: • CSMA/CA • Distributed Control Functions - DCF Our Focus • Error detection (checksum) • Point Control Functions – PCF • Error correction (ACK frame) • PCF is rarely used - inefficient • Flow control: stop-and-wait • Alternative is “ad hoc” mode: multi-hop, assumes no infrastructure • Fragmentation (More Frag) • Rarely used, e.g. military • Collision Avoidance (RTS-CTS) • Hot research topic! 802.11: Infrastructure Mode Wireless Collision Avoidance 802.11 LAN • Station (STA) 802.x LAN • Problem: two nodes, hidden from each other, transmit complete frames to • terminal with access mechanisms to the wireless base station medium and radio contact to the access point STA1 • Collision detection not reliable: “listen before talking” canfail BSS1 • Access Point • Solution: rely on ACKs instead to detect packet loss Access Portal • station integrated into the wireless LAN and the Point distribution system • Collisions waste bandwidth for long duration ! Distribution System • Basic Service Set (BSS) • Plus also exponential back off before retransmissions – collisions are expensive! Access • group of stations using the same AP • Solution: “CA” using small reservation packets ESS Point • Portal • Nodes track reservation interval with internal “network allocation vector” (NAV) BSS 2 • bridge to other (wired) networks • This is called “virtual carrier sense” • Distribution System • Note that nodes still do “physical” carrier sense • interconnection network to form one logical • “Listen before you talk” often works and is cheap STA2 802.11 LAN STA3 network (ESS: Extended Service Set) based on several BSS 20 5 4/28/2019 Collision Avoidance: RTS-CTS Exchange IEEE 802.11 MAC Protocol • RTS/CTS implemented using NAV: • Explicit channel reservation Network Allocation Vector • Sender: send short RTS: request to send • Receiver: reply with short CTS: clear to send • NAV is also used with data packets • CTS reserves channel for sender, notifying • 802.11 data frame has transmission time (possibly hidden) stations field • RTS and CTS are short: • Others (hearing data header) defer access • collisions are less likely, of shorter duration for NAV time units • end result is similar to collision detection • But why do you need NAV if you can hear • Avoid hidden station collisions the header? • Not widely used (not used really) • Fading? • Overhead is too high! • Header is sent at lower bit rate • Not a serious problem in typical deployments 22 21 How About Exposed Terminal? DCF mode transmission without RTS/CTS • Exposed terminals result in a lost transmission opportunity DIFS Exposed Data • Reduces capacity – no collisions source A SIFS • Exposed terminals are difficult to deal with Ack destination Congestion • Even hard to detect them! Window B NAV DIFS • Good news – they are very rare! C other Must defer access Random backoff • So we live with them D Not used in Ethernet WiFi is more
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