15-441/641: Wireless Networks Application Presentation ISO 15-441 - PowerPoint PPT Presentation

4/28/2019 Standardization Local Area Networks • Wireless networks are standardized by IEEE • Under 802 LAN MAN standards committee 15-441/641: Wireless Networks Application Presentation ISO 15-441 Spring 2019 Session IEEE 802 OSI Profs Peter Steenkiste & Justine Sherry Transport standards 7-layer model Network Logical Link Control Data Link Medium Access (MAC) Spring 2019 Physical (PHY) https://computer-networks.github.io/sp19/ Physical • 802.1 Overview Document Containing the Reference Model, Tutorial, and Glossary 802.1 b Specification for LAN Traffic Prioritization • “Ethernet” 802.1 q Virtual Bridged LANs • The 802 Class of Standards • 802.2 Logical Link Control 802.3 Contention Bus Standard 1 Obase 5 (Thick Net) • • 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 • • E.g. 802.2 is LLC 802.3z Gigabit Ethernet • 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• • 802.5b Token Ring Standard 4 Mb/s over Unshielded Twisted-Pair • Some standards are for technologies that are outdated • 802.5f Token Ring Standard 16-Mb/s Operation • 802.6 Metropolitan Area Network DQDB WiFi Family • Not actively deployed anymore 802.7 Broadband LAN Recommended Practices • 802.8 Fiber-Optic Contention Network Practices • 802.9a Integrated Voice and Data LAN • E.g. 802.6 • 802.10 Interoperable LAN Security • Bluetooth, Zigbee, .. • 802.11 Wireless LAN Standard 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! D • WiFi version • 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! • Ratio of transmitted signal power to received power is way too A B C D E high at the transmitter • Signal is not contained in a wire • Attenuation is 1/D 2 for distance D • Transmitter cannot detect colliding transmissions (deaf while transmitting) • In addition, there is significant noise and • So how do you detect collisions? interference 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 Exposed Terminal Problem Hidden Terminal Problem S1 R1 RTS CTS CTS S1 R1 S2 S2 R2 Lack signal between S1 and S2 and cause collision at R1 • Carrier sense prevents two senders from sending simultaneously although they • do not reach each other’s receiver Severity of the problem depends on the sensitivity of the • carrier sense mechanism Severity again depends on CCA threshold • R2 Higher CCA reduces occurrence of exposed terminals, but can create hidden terminal • Clear Channel Assessment (CCA) threshold • 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 • Federal Communication Commission (FCC) and National • MAC Standard : QoS support : supported by many vendors • IEEE 802.11f Telecommunication and Information Administration in the US • Inter-Access Point Protocol : deployed IEEE 802.11g • • Spectrum allocation differs quite a bit – implications for mobile users? PHY Standard: 3 channels : OFDM and PBCC : widely deployed (as b/g) • 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 IEEE 802.11 Overview Frequency Bands Industrial, Scientific, and Medical (ISM) bands • • Adopted in 1997 with goal of providing Generally called “unlicensed” bands • • Giving wireless users access to services in wired networks Short Wave Radio FM Broadcast Infrared wireless LAN AM Broadcast Television • High throughput and reliability Cellular (840MHz) NPCS (1.9GHz) • Continuous network connection, e.g. while mobile Medium High Very Super Millimeter Infrared Visible Ultra- Ultra • The protocol defines Low violet X-Rays High High wave Light 60 GHz High IEEE 802.11ad • MAC sublayer • MAC management protocols and services • Several physical layers: IR, FHSS, DSSS, OFDM 902 - 928 MHz 2.4 - 2.4835 GHz 5 GHz 26 MHz 83.5 MHz IEEE 802.11a • Wi-Fi Alliance is industry group that certifies (IEEE 802.11b and later interoperability of 802.11 products and later) 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: Our Focus CSMA/CA • Distributed Control Functions - DCF • • Error detection (checksum) Point Control Functions – PCF • • Error correction (ACK frame) PCF is rarely used - inefficient • • Alternative is “ad hoc” mode: multi-hop, assumes no • Flow control: stop-and-wait infrastructure • Fragmentation (More Frag) Rarely used, e.g. military • • Collision Avoidance (RTS-CTS) Hot research topic! • 802.11: Infrastructure Mode Wireless Collision Avoidance • Station (STA) 802.11 LAN 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 Collision detection not reliable: “listen before talking” canfail STA 1 • • Access Point BSS 1 Solution: rely on ACKs instead to detect packet loss Portal • station integrated into the wireless LAN and the Access • Point Collisions waste bandwidth for long duration ! distribution system • 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 • network (ESS: Extended Service Set) STA 2 STA 3 802.11 LAN based on several BSS 20 5