WiFi / IEEE 802.11 - WLAN Lecturer: Carlos Rey-Moreno carlos.reymoreno@gmail.com Networking Course Honors on Computer Science University of the Western Cape 04 Feb - 2013
Why Wireless? A lot of pros... Less deployment cost No need to bury infrastructure Links can be easily and cheaply reused Deployment costs irrespective the distance Faster deployments Users can be nomadic and mobile Coverage in low density populated areas Less maintenance costs
Why Wireless? Also some cons... Limited capacity Capacity degradation with increasing number of users Shared medium Security problems QoS constraints but still... more than
Many technologies, many purposes Focus on 802.11 (WiFi). It allows WLAN, WMAN & even WWAN at a very low-cost & low power consumption
Introduction to WLANs IEEE 802.11 = WLAN =WiFi! Great success of the standard Guarantees compatibility among vendors Present worldwide, integrated in user devices Cheap, good and efficient Evolved during the years: 802.11n, 802.11ac In short / medium distance allows: Wireless Local area networks indoors Wireless bridges between buildings Hot-Spots Wireless Internet Service Providers
Benefits of WLANs Wires disappear: flexibility, mobility and corporate image Networks deployment when it is not possible to use wires like parks Conventional LAN extension indoors & outdoors Low speed mobility without losing connection Transparent for users, system and software
A bit of history 1979: 1st WLAN experiment (Switzerland, IBM) 1985: ISM bands regulation by FCC 1991: 1st works on WLAN over 1Mbps 1997: Release of the standard IEEE 802.11 1999: Release of the standards IEEE 802.11b (11 Mbps in 2.4 GHz) y 802.11a (54 Mbps in 5 GHZ) 2003: Release of the standard 802.11g (54 Mbps in 2.4 GHz) 2005: Release of the standard 802.11e (QoS) 2009: Release of the standard 802.11n (300Mbps)
Standards 802.11
WiFi Introduction Basic element of a WiFi network: Station (STA): any laptop, PDA, smartphone etc. able to use 802.11 technology Access Point (AP): Wi-Fi node that provides connectivity between STA connected to him and the rest of the network Wireless Medium: The way devices access to the, different PHY layers are defined (rates, frequency, modulations) Distribution System (DS): Logical component for the commutation of frames among APs connected to the same backbone.
WiFi Introduction
WiFi Introduction Basic Service Set (BSS): Ways STAs can communicate to each other: Independent BSS: STAs communicate among them directly. Known as Ad-hoc or Mesh Mode. Infrastructure BSS: STAs communicate through an AP. Known as Infrastructure Mode. Each BSS is defined by a Service Set Identifier (SSID) Many BSS can associate in a Extended Service Set (ESSID)
WiFi Introduction
WiFi Introduction WiFi devices can be operated in one of these modes: Master (access point) Managed (also known as client or station) Ad-hoc (used for mesh networks) Monitor (not normally used for communications) Other proprietary non-802.11 modes (e.g. Mikrotik Nstreme or Ubiquiti AirMAX) Each mode has specific operating constraints, and radios may only operate in one mode at a time. Source:WTKIT
WiFi Introduction Master mode Master mode (also called AP or infrastructure mode) is used to provide an infrastructure with an access point connecting different clients. The access point creates a network with a specified name (called the SSID) and channel, and offers network services on it. WiFi devices in master mode can only communicate with devices that are associated with it in managed mode. Source:WTKIT
WiFi Introduction Managed Mode Managed mode is sometimes also referred to as client mode. Wireless devices in managed mode will join a network (specifying its SSID) created by a master, and will automatically change their channel to match it. Clients using a given access point are said to be associated with it. Managed mode radios do not communicate with each other directly, and will only communicate with an associated master (and only with one at a time). Source:WTKIT
WiFi Introduction Ad-hoc Mode Ad-hoc mode is used to create mesh networks with: No master devices (APs) Direct communication between neighbors Devices must be in range of each other to communicate, and they must agree on a network name (SSID) and channel. Source:WTKIT
WiFi Introduction Monitor Mode Monitor mode is used to passively listen to all radio traffic on a given channel. It is useful for: Analyzing problems on a wireless link Observing spectrum usage in the local area Performing security maintenance tasks Source:WTKIT
WiFi Introduction Source:WTKIT
WiFi Introduction Modes of connecting APs Wired Wireless: Wireless Distribution System (WDS) protocol. Useful but with limitations
Standards 802.11
Standards 802.11 WLAN standards evolution 1997: IEEE 802.11 is published MAC based in CSMA/CA 3 possible PHYs, rates 1 and 2 Mbps: Infrared (IR) Spread Spectrum by frequency hoping (FHSS) Spread Spectrum by direct sequence (DSSS) From 1997 many extensions & recommendations for improving rate, security, performance, etc.
Standards 802.11 IEEE802.11a 5GHz band, up to 54 Mbps IEEE802.11b 2,4GHz band, up to 11 Mbps. IEEE802.11c Wireless bridges with 802.11. IEEE802.11d 802.11 harmonization in different countries. IEEE802.11e QoS support. IEEE802.11f Interoperability among APs. IEEE802.11g 2,4GHz band, up to 54 Mbps. IEEE802.11h 802.11a adaptation to European regulation. IEEE802.11i Security in 802.11 networks. IEEE802.11j Extensions for Japan.
Standards 802.11 IEEE802.11k Exchange of capacity information. IEEE802.11m Maintenance, updates. IEEE802.11n MIMO, up to 300 Mbps, @2.4 & 5 GHz. IEEE802.11p Vehicle to vehicle communications IEEE802.11r Fast Roaming. IEEE802.11s Mesh networks. IEEE802.11T WPP test methods and metrics IEEE802.11u Interconnection with non-802 networks IEEE802.11v Wireless networks management. IEEE802.11w Protected management frames. IEEE802.11y Extension for USA.
PHY Layer Tasks Provide services to the MAC layer for exchange of frames Exchanges frames with remote PHYs using different methods of modulation and coding Provide the MAC layer with information regarding the availability of the channel
PHY Layer Architecture PMD (Physical Medium Dependent): Real tools for Tx/Rx in the physical medium. Clear Channel Assessment (CCA). PLCP (Physical Layer Convergence Procedure): Provide MAC layer with an unique interface independent of the specific PMD used.
PHY Layer 802.11. (1997) 3 possible PHYs, rates 1 and 2 Mbps: Infrared, IR. Discarded Spread Spectrum by freq hoping, FHSS. Discarded Spread Spectrum by direct sequence, DSSS. Active 802.11a (1999) OFDM PHY; @5GHz. 6 54 Mbps. 802.11b (1999) HR/DSSS PHY; @2.4 GHz; 5.5/11 Mbps. 802.11g (2003) ERP PHY(Extended Rate PHY), includes the previous PHY with their rates; @ 2.4GHz: ERP-DSSS, ERP-HR/DSSS, ERP-OFDM 802.11n (2009) Introduce MIMO, rates up to hundreds of Mbps
PHY Layer Compatibility of the standars AP 802.11a C L I E N T 802.11a 802.11b 802.11g 802.11n Yes @5GHz Yes 802.11b Yes Yes (slower) Yes @2.4GHz 802.11g Yes (slower) Yes Yes @2.4GHz Yes @2.4GHz Yes @2.4GHz Yes 802.11n Yes @5GHz Source:WTKIT
WiFi Channels WiFi Channels 2.4 GHz (2.412-2.472 GHz) 22 MHz width 3 non-interferent channels 5 GHz (5.15-5.35 / 5.470-5.825 USA) 20 MHz width 8 non-interferent channels WiFi devices must use the same channel in order to communicate with each other. Tx/Rx on the same channel half dupplex Source:WTKIT
WiFi Channels Central Freq only 5 MHz apart Channels 1, 6 and 11 non-interferent Source:WTKIT
WiFi Channel: Channel reuse Source:WTKIT
Protocol Overhead Data rates Note that the data rates quoted in the WiFi specifications refer to the raw radio symbol rate, not the actual TCP/IP throughput rate. The difference is called protocol overhead, and is needed by the WiFi protocol to manage collisions, retransmissions, and general management of the link. In the exercises you are going to be able to calculate the real data rate
PHY Overhead DSSS PHY (802.11) Total PLCP Overhead = 192 μs
PHY Overhead HR/DSSS PHY (802.11b) Two options: Long Preamble 192 μs PLCP Overhead Compatibility with 802.11 Short Preamble 96 μs PLCP Overhead
PHY Overhead OFDM PHY (802.11a) Total PLCP Overhead = 24 μs
PHY Overhead ERP-OFDM PHY (802.11g) Total PLCP Overhead = 26 μs (20 μs Preamble + 6 μs Signal Extension)
PHY Overhead ERP-DSSS / ERP-CCK PHY (802.11g) Total PLCP Overhead (Long Preamble) = 192 μs Total PLCP Overhead (Short Preamble) = 96 μs
ERP PHY Network with only 802.11g devices ERP-OFDM Network with both 802.11b & 802.11g devices. Both devices need to understand each other. That means: 11g devices must transmit at the maximum rate common to ALL the station those frames need to be understood by all the stations (ACK, RTS, CTS). ERP-DSSS PHY similar to DSSS PHY (same overhead) ERP-CCK PHY similar to HR/DSSS PHY (same overhead) Protection for activating detection in 11b devices before transmitting with ERP-OFDM. Two solutions: RTS/CTS (for more info go to slide 46) CTS-to-self (for more info go to slide 47)
Modulations Different rates, because each symbol can transport, different amount of information The more information a symbol can transport, the better the signal received must be in order to understand its content
MAC Layer The main goal of the MAC layer is controlling the access the physical medium (air) to be as efficient as possible. Tasks: Creation of the MAC frame Bridging Fragmentation / Re-ensambling Error check (via CRC) Medium access (CSMA / CA) Stations mobility
MAC Layer Challenges The fluctuation in the quality of the RF link (noise, interference, multipath...) requires unicast data frames to be confirmed. Not forever Acktimeout. If expires, frame is ReTx Hidden nodes problem, solved with the RTS/CTS mechanism.
MAC Layer. Coordination Types PCF (Point Coordination Function). Centralized. The CP (coordination point) reserves preiodically the channel for contention free phases during which it polls the stations. It has not been implemented DCF (Distributed Coordination Function). Distributed. Each station uses CSMA/CA and, optionally, RTS/CTS for accessing the channel. All the station, including the AP have the same rates. It is worldwide used
MAC Layer (CSMA vs TDMA) 802.11 WiFi uses Carrier Sense Multiple Access (CSMA) to avoid transmission collisions. Before a node may transmit, it must first listen for transmissions from other radios. The node may only transmit when the channel becomes idle. Other technologies (such as WiMAX, Nstreme, and AirMAX) use Time Division Multiple Access (TDMA) instead. TDMA divides access to a given channel into multiple time slots, and assigns these slots to each node on the network. Each mode transmits only in its assigned slot, thereby avoiding collisions. Source:WTKIT
MAC Layer. Temporization IFS (Inter Frame Space): Separation times among frames SIFS (Short IFS): Separation between the end of the reception and the Tx of the Ack. PIFS (PCF IFS): Used in PCF Mode PIFS=SIFS+SlotTime DIFS (DCF IFS): Used in DCF Mode DIFS=PIFS+SlotTime=SIFS+2*SlotTime The earlier you get access to the channel, the more data you will transmit DSSS HR/DSSS OFDM ERP-DSSS SIFS 10 μs 10 μs 16 μs 10 μs ERP-CCK ERP-OFDM 10 μs 10 μs
MAC Layer. CSMA/CA WiFi stations sense the channel until it is idle Once it is free, it waits DIFS without doing nothing After that, it waits a random time before transmitting. This time is defined an integer number of slots. The integer number is contained between 0 and the minimum Contention Window (CW_min) parameter. The length of each slot is called SlotTime. The smallest the number & the shortest the SlotTime the least the MAC overhead DSSS HR/DSSS OFDM ERP-DSSS ERP-CCK ERP-OFDM SlotTime 20 μs 20 μs 9 μs 20 μs 20 μs 9 μs CW_min 31 31 15 31 31 15 CW_max 1023 1023 1023 1023 1023 1023
MAC Layer. CSMA / CA If before the CW finishes, a frame is detected, the CW countdown freezes. Once, the channel is idle again, it waits again DIFS and restart the countdown where the CW was frozen. When the CW arrives at zero, the frame is transmitted.
MAC Layer. CSMA/CA If the frame was unicast, the Transmitter starts its AckTimeOut (DIFS+SIFS=2*SIFS+2*SlotTime) and waits for the Ack. Boradcast & multicast frame are not confirmed The Receiver receive the frame, checks its CRC while waits SIFS, once it is finished sends the Ack. Being SIFS < DIFS no collision is possible. If the Transmitter receive the ACK correctly and before the AckTimeout expires, the process is completed
MAC Layer. CSMA / CA If the ACK does not arrive or arrives late, the CW_min is doubled, a new CW is obtained and the process restart. It the maximum number of ReTx is reached (7 by default, 4 in RTS/CTS mode), the frame is discarded and the process start again with Cwmin. All this is considered MAC Overhead and is added to the PHY Overhead. The more the number of STAs the bigger the MAC overhead
MAC Layer. Fragmentation If a frame is too long, it can be fragmented and transmitted sequentially (each fragment is confirmed individually) separated by SIFS, so no other station can win the channel (but a bit more overhead is added).
Hidden Node The hidden node problem is shown in the figure below. Nodes 1 and 3 do not see each other so they do not detect the transmission from the other to 2 and a collision occurs. So, the basic CSMA/CA mode does not work.
RTS / CTS
RTS/CTS RTS and CTS contains information about the total duration of the transmission Virtual Carrier Detection: every station that may interfere listens either the RTS or the CTS and avoids transmitting during that period although it senses the channel idle It is a good solution but it introduces even more MAC overhead
CTS-to-self A better solution for 11b and 11g devices in the same network, but it also introduces MAC overhead. Sometimes it is not supported
Frame Types. MAC header Frame Control: MAC control information, for all. Duration ID: only in RTS/CTS to activate the virtual carrier Sequence control: Use to identify different fragments Addresses: to bridge through the DS Address 1 & 2: Source and destination MACs Address 3: BSSID Address 4: Only used in Intra DS communication Frame body: Data to be transmitted FCS: Error check Normal header 28 bytes, DS header 34 bytes More overhead
Frames Types Ack 14 bytes @ maximum common rate RTS or CTS 20 bytes @ maximum common rate
Propagation time effects ACKTimeout: We need to allow the ACK to arrive on time ACKTimeout = SIFS+SlotTime+ACK+PLCP_Overhead As it is in the standard, many packets are retransmitted innecesarily because the ACKs do not arrive on time. Result In Basic mode the link would work very slowly In RTS/CTS mode there would be no link 55
Propagation time effects Solution ACKTimeout= ACKTimeout std +2 δ CTSTimeout=CTSTimeout δ= Propagation delay = distance [m] / speed of light [m/s]std +256δ
Collisions due to distance CSMA/CA with depreciable propagation delay: If A and B are two stations that wants to transmit a packet, both sense the channel and if its free after the contention windown countdown: They will collide if they transmit in the same slot They will not collide if they transmit in different slots. One will listen the other one and will freeze its countdown until the channel is free again CSMA/CA with non-depreciable propagation delay: If the propagation delay is too long (i.e. 200 ms), MANY slots pass since the transmission from A until B senses it. The odds of collision increase widely and performance decreases.. 57
Collisions due to the distance 58
Collisions due to distance Solution: In PtMP and Mesh In PtP SlotTime=SlotTime std +2 δ SlotTimestd +2 δ>slottime>slottime std +δ Effect in the throughput some overhead is introduced since a bigger SlotTime entails less effiency of the system As Propagation delay= distance [m] / speed of light [m/s]. In PtMP: 59 If SlotTime = 20 us Max distance ~ 3 km If SloTime = 9us Max distance ~ 1'35 km
Summary protocol issues Changes in ACKTimeout and CTSTimeout ACKTimeout std =SIFS+SlotTime+ ACK + PLCP Overhead ACKTimeout= ACKTimeout std +2 δ CTSTimeout=CTSTimeout std +2 δ Changes in SlotTime (from 20 or 9 μs depending in the PHY) PtMP & Mesh: SlotTime=SlotTime std +2 δ PtP: SlotTime=SlotTime std +δ The changes in the ACKTimeout and CTSTimeout are now embedded in the new SlotTime By knowing the distance of the link you can caltulate the propagation delay and the new values of the parameters Atheros chipsets allow changing them that and the MP too Changing the SlotTime and not the ACKTimeout has side effects ACKTimeout adjusted for 60 km 60
Other Alternatives Polling Mikrotik nstreme TDMA Ubiquiti AirMAX Keeping PHY OFDM Disabling retransmisions and contention Programing TDMA in the driver WiMAX is the obvious technology to do so, since has been create for that purpose. However, prices are much higher 61
Bonding Bonding allows increasing the capacity by using two radios: one for Tx and the other one for Rx. 62
The content of this presentations has used materials from: - F. Javier Simó-Reigadas @ URJC, Spain - Wireless Training Kit @ http://wtkit.org Thanks to them!