![11/20/2019
IEEE 802.11 Family
Protocol Release
Data
Freq. Rate
(typical)
Rate
(max)
Range
(indoor)
Legacy 1997 2.4 GHz 1 Mbps 2Mbps ?
802.11a 1999 5 GHz 25 Mbps 54
Mbps
~30 m
802.11b 1999 2.4 GHz 6.5 Mbps 11
Mbps
~30 m
802.11g 2003 2.4 GHz 25 Mbps 54
Mbps
~30 m
802.11n 2008 2.4/5 GHz
20/40 MHz
200 Mbps 600
Mbps
~50 m
802.11ac 2013 5 GHz
20→160 MHz
100s Mbps
per user
1.3
Gbps
~50 m
802.11ad 2016 60 GHz Gbps 7 Gbps Short - room
A Factor of 1000+ Speedup?
• 802.11b: first WiFi to be standardized and widely deployed
• Used 20MHz channels, 2.4 GHz only, inefficient modulation
• 802.11a and g: increases rates from 11 to 54Mbit/sec
• Key factor is better modulation (“OFDM”)
• They are the same standard, but 802.11a runs in 5GHz band
• 5GHz band is wider and has lower utilization – more capacity!
• 802.11n: runs in both 5 and 2.4GHz bands – significant speed up
• How? Better modulation, channel bonding, and MIMO
802.11b Channels
• In the UK and most of EU: 13 channels, 5MHz apart, 2.412 – 2.472 GHz
• In the US: only 11 channels
• Each channel is 22MHz
• Significant overlap
• Non-overlapping channels are 1, 6 and 11
• 1, 2, 5.5 and 11 Mbps rates using DSSS technology
5150 [MHz]
5180 5350
5200
36 44
center frequency =
5000 + 5*channel number [MHz]
channel#
40 48 52 56 60 64
149 153 157 161
5220 5240 5260 5280 5300 5320
5725 [MHz]
5745 5825
5765
channel#
5785 5805
Indoor
Point-Point
802.11a Physical Channels*
* example; not all channels are shown](https://image.slidesharecdn.com/wifinetworks-220505200823-9288b569/85/WiFi-Networks-pdf-8-320.jpg)
WiFi Networks.pdf
- 1. 11/20/2019 15-441/641: WiFi Networks 15-441 Fall 2019 Profs Peter Steenkiste & Justine Sherry Fall 2019 https://computer-networks.github.io/fa19/ 2 Overview • Basic WiFi concepts • Some deployment issues • WiFi versions Standardization Local Area Networks • Wireless networks are standardized by IEEE • Under 802 LAN MAN standards committee Application Presentation Session Transport Network Data Link Physical ISO OSI 7-layer model Logical Link Control Medium Access (MAC) Physical (PHY) IEEE 802 standards The 802 Class of Standards • List on next two slides • Some standards apply to all 802 technologies • E.g. 802.2 is LLC • Important for inter operability • Some standards are for technologies that are outdated • Not actively deployed anymore • Many of the early standards are obsolete
- 2. 11/20/2019 802 Standards – Part 1 802 Standards – Part 2 Some IEEE 802.11 Standards • IEEE 802.11a • PHY Standard : 8 channels : up to 54 Mbps : widely deployment • IEEE 802.11b • PHY Standard : 3 channels : up to 11 Mbps : widely deployed. • IEEE 802.11d • MAC Standard : support for multiple regulatory domains (countries) • IEEE 802.11e • MAC Standard : QoS support : supported by many vendors • IEEE 802.11f • Inter-Access Point Protocol : deployed • IEEE 802.11g • 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 • IEEE 802.11i • Suppl. MAC Standard: Alternative WEP : standard • IEEE 802.11n • MAC Standard: MIMO : significant improvements in throughput • IEEE 802.11ac • Support for multi-user MIMO • IEEE 802.11ad • WiFi in the 60 GHz band • IEEE 802.11ax • Improved version of 802.11ac • IEEE 802.11ay • Improved version of 802.11ad Frequency Bands Low Medium High Very High Ultra High Super High Infrared Visible Light Ultra- violet X-Rays AM Broadcast Short Wave Radio FM Broadcast Television Infrared wireless LAN Cellular (840MHz) NPCS (1.9GHz) 2.4 - 2.4835 GHz 83.5 MHz (IEEE 802.11b and later) 902 - 928 MHz 26 MHz • Industrial, Scientific, and Medical (ISM) bands • Generally called “unlicensed” bands 5 GHz IEEE 802.11a and later Millimeter wave 60 GHz IEEE 802.11ad
- 3. 11/20/2019 IEEE 802.11 Overview • Adopted in 1997 with goal of providing • Giving wireless users access to services in wired networks • High throughput and reliability • Continuous network connection, e.g. while mobile • The protocol defines • MAC sublayer • MAC management protocols and services • Several physical layers: IR, FHSS, DSSS, OFDM • Wi-Fi Alliance is industry group that certifies interoperability of 802.11 products Features of 802.11 MAC protocol • Supports MAC functionality • Addressing – based on 48-bit IEEE addresses • CSMA/CA • Error detection (checksum) • Error correction (ACK frame) • Flow control: stop-and-wait • Fragmentation (More Frag) • Collision Avoidance (RTS-CTS) Infrastructure and Ad Hoc Mode • Infrastructure mode: stations communicate with one or more access points which are connected to the wired infrastructure • What is deployed in practice • Two modes of operation: • Distributed Control Functions - DCF • Point Control Functions – PCF • PCF is rarely used - inefficient • Alternative is “ad hoc” mode: multi-hop, assumes no infrastructure • Rarely used, e.g. military • Hot research topic! Our Focus 802.11: Infrastructure Mode • Station (STA) • terminal with access mechanisms to the wireless medium and radio contact to the access point • Access Point • station integrated into the wireless LAN and the distribution system • Basic Service Set (BSS) • group of stations using the same AP • Portal • bridge to other (wired) networks • Distribution System • interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS Distribution System Portal 802.x LAN Access Point 802.11 LAN BSS2 802.11 LAN BSS1 Access Point STA1 STA2 STA3 ESS
- 4. 11/20/2019 13 Wireless Collision Avoidance • Problem: two nodes, hidden from each other, transmit complete frames to base station • Collision detection not reliable: “listen before talking” canfail • Solution: rely on ACKs instead to detect packet loss • Collisions waste bandwidth for long duration ! • Plus also exponential back off before retransmissions – collisions are expensive! • Solution: “CA” using small reservation packets • Nodes track reservation interval with internal “network allocation vector” (NAV) • This is called “virtual carrier sense” • Note that nodes still do “physical” carrier sense • “Listen before you talk” often works and is cheap 14 Collision Avoidance: RTS-CTS Exchange • Explicit channel reservation • Sender: send short RTS: request to send • Receiver: reply with short CTS: clear to send • CTS reserves channel for sender, notifying (possibly hidden) stations • RTS and CTS are short: • collisions are less likely, of shorter duration • end result is similar to collision detection • Avoid hidden station collisions • Not widely used (not used really) • Overhead is too high! • Not a serious problem in typical deployments 15 IEEE 802.11 MAC Protocol • RTS/CTS implemented using NAV: Network Allocation Vector • NAV is also used with data packets • 802.11 data frame has transmission time field • Others (hearing data header) defer access for NAV time units • But why do you need NAV if you can hear the header? • Fading? • Header is sent at lower bit rate – more likely to be correctly received DCF mode transmission without RTS/CTS source destination other DIFS Data Ack SIFS NAV Must defer access DIFS Congestion Window Random backoff Not used in Ethernet WiFi is more concerned about collisions
- 5. 11/20/2019 How About Exposed Terminal? • Exposed terminals result in a lost transmission opportunity • Reduces capacity – no collisions • Exposed terminals are difficult to deal with • Even hard to detect them! • Good news – they are very rare! • So we live with them 17 A B C D Exposed Exponential Backoff • Force stations to wait for random amount of time to reduce the chance of collision • Backoff period increases exponential after each collision • Similar to Ethernet • Also used when the medium is sensed as busy: • Wait for medium to be idle for a DIFS (DCF IFS) period • Pick random number in contention window (CW) = backoff counter • Decrement backoff timer until it reaches 0 • But freeze counter whenever medium becomes busy • When counter reaches 0, transmit frame • If two stations have their timers reach 0 at same time; collision will occur; • After every failed retransmission attempt: • increase the contention window exponentially • 2i –1 starting with CWmin up to CWmax e.g., 7, 15, 31, … What about PCF? • IEEE 802.11 combines random access with a “taking turns” protocol • DCF (Distributed Coordination Mode) – Random access • CP (Contention Period): CSMA/CA is used • PCF (Point Coordination Mode) – Polling • CFP (Contention-Free Period): AP polls hosts • Basestation can control who access to medium • Can offer bandwidth guarantees • Rarely used in practice CP CFP CFP Super-frame Frame PCF Operation Overview • PC – Point Coordinator • Uses polling – eliminates contention • Polling list ensures access to all registered stations • Over DCF but uses a PIFS instead of a DIFS – gets priority • CFP – Contention Free Period • Alternate with DCF • Periodic Beacon – contains length of CFP • NAV prevents transmission during CFP • CF-End – resets NAV • CF-Poll – Contention Free Poll by PC • Stations can return data and indicate whether they have more data • CF-ACK and CF-POLL can be piggybacked on data
- 6. 11/20/2019 21 Overview • Basic WiFi concepts • Some deployment issues • WiFi versions Association Management Stations must associate with an AP before they can use the wireless network • AP must know about them so it can forward packets • Often also must authenticate Association is initiated by the wireless host – involves multiple steps: 1. Scanning: discover available access points based on periodic beacons 2. Selection: deciding what AP (or ESS) to use 3. Association: protocol to “sign up” with AP – share configuration info 4. Authentication: needed to gain access to secure APs – many options Disassociation: station or AP can terminate association “Static” Channel – Bitrate Adaptation 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Lower signal rates enable coverage of large additional area Mobile Channel – Pedestrian 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps 18 Mbps 24 Mbps 36 Mbps 48 Mbps 54 Mbps
- 7. 11/20/2019 Infrastructure Deployments Frequency Reuse in Space • Set of cooperating cells with a base stations must cover a large area • Cells that reuse frequencies should be as distant as possible to minimize interference and maximize capacity • Minimizes hidden and exposed terminals • 3D problem! • Lots of measurements Frequencies are Precious 2.4 Ghz: 3 non-overlapping channels • Plus lots of competition: microwaves and other devices 5 GHz: 20+ channels, but with constraints • Power constraints, indoor/outdoor, .. • Exact number and rules depend on the country 802.11n and ac: bonding of 2-8 channels And the world is not flat! Centralized Control • Many WiFi deployments have centralized control • APs report measurements • Signal strengths, interference from other cells, load, … • Controller makes adjustments • Changes frequency bands • Adjusts power • Redistributes load • Can switch APs on/off • Very sophisticated! Controller 28 Overview • Basic WiFi concepts • Some deployment issues • WiFi versions • Very high level
- 8. 11/20/2019 IEEE 802.11 Family Protocol Release Data Freq. Rate (typical) Rate (max) Range (indoor) Legacy 1997 2.4 GHz 1 Mbps 2Mbps ? 802.11a 1999 5 GHz 25 Mbps 54 Mbps ~30 m 802.11b 1999 2.4 GHz 6.5 Mbps 11 Mbps ~30 m 802.11g 2003 2.4 GHz 25 Mbps 54 Mbps ~30 m 802.11n 2008 2.4/5 GHz 20/40 MHz 200 Mbps 600 Mbps ~50 m 802.11ac 2013 5 GHz 20→160 MHz 100s Mbps per user 1.3 Gbps ~50 m 802.11ad 2016 60 GHz Gbps 7 Gbps Short - room A Factor of 1000+ Speedup? • 802.11b: first WiFi to be standardized and widely deployed • Used 20MHz channels, 2.4 GHz only, inefficient modulation • 802.11a and g: increases rates from 11 to 54Mbit/sec • Key factor is better modulation (“OFDM”) • They are the same standard, but 802.11a runs in 5GHz band • 5GHz band is wider and has lower utilization – more capacity! • 802.11n: runs in both 5 and 2.4GHz bands – significant speed up • How? Better modulation, channel bonding, and MIMO 802.11b Channels • In the UK and most of EU: 13 channels, 5MHz apart, 2.412 – 2.472 GHz • In the US: only 11 channels • Each channel is 22MHz • Significant overlap • Non-overlapping channels are 1, 6 and 11 • 1, 2, 5.5 and 11 Mbps rates using DSSS technology 5150 [MHz] 5180 5350 5200 36 44 center frequency = 5000 + 5*channel number [MHz] channel# 40 48 52 56 60 64 149 153 157 161 5220 5240 5260 5280 5300 5320 5725 [MHz] 5745 5825 5765 channel# 5785 5805 Indoor Point-Point 802.11a Physical Channels* * example; not all channels are shown
- 9. 11/20/2019 Aside: Why Multiple Antennas? • Access points almost always have multiple antennas • The number has increased with successive generations • Some devices also have multiple antennas (e.g., 2-3) • Original motivation: spatial diversity • Quality of the links can be very different and what link is best • Transmitter picks the antennas with the best channel to receiver • Receiver picks the best signal it receives, or it combines them Transmit Receiver How do we Go Faster? • Wired world: Pull more wires! • Wireless world: How about if we could do the same thing and simply use more antennas? MIMO: Multiple In – Multiple Out • Key idea: use multiple antenna pairs to send parallel data streams • Should give us linear capacity increase (just like the wired world) • Problem: the different transmissions interfere! • Each receiving antenna receives (weighted) sum of all transmissions • Could be viewed as noise – low S/N ration in Shannon • Solution: interference is not random but can be subtracted T R Input Streams Postprocessing Preprocessing Output Streams 4 Channels Channel Bonding • Why only use 20Mhz channels per user? • Remember Shannon? • What changes are needed? • Radios need to use a wider channel: adds complexity, cost • Interoperability between 20 and 40 MHz devices – messy • Mostly useful in 5 GHz band – more spectrum 20 MHz 40 MHz
- 10. 11/20/2019 How Do We Go Even Faster? • 802.11ac: faster, mostly by more aggressive modulation and MIMO • Also uses multi-user MIMO: AP can send packets to multiple stations simultaneously (don’t worry about the details) • 802.11ad: first WiFi to use the 60 GHz band + Lots of bandwidth available, mostly unused ‒ Transmission only over short distances ‒ Signal does not penetrate objects, i.e., mostly LOS • In practice, need to use beam forming • While standardized, lots of open questions remain 802.11ad – Beamforming • 802.11ad does very aggressive beam forming • Overcome distance limitations that are a result of high attenuation (proportional to f2) • Some background: • Antenna arrays can be used to concentrate transmit power into beams to specific receivers • Higher frequencies -> smaller antennas and narrower beams • Extends range and increases throughput • How do we find the right beams? • Iterative search process 38