IT 351: Mobile & Wireless Computing Wireless Local Area Networks (WLAN) Part-1: IEEE 802. 11 Objectives: – To provide a detailed study of the WLAN architecture and system operation
Outline • Wireless LAN main uses, advantages, disadvantages • Classification of transmission technologies for WLAN • Classification of WLAN IEEE 802. 11 – Infrastructure networks – Ad Hoc networks • WLAN IEEE 802. 11 – – – Architecture Protocols Physical layer MAC management IEEE 802. 11 -a/ b/. . . n
Overview of the main chapters Chapter 10: Support for Mobility Chapter 9: Mobile Transport Layer Chapter 8: Mobile Network Layer Chapter 4: Telecommunication Systems Chapter 5: Satellite Systems Chapter 6: Broadcast Systems Chapter 3: Medium Access Control Chapter 2: Wireless Transmission Chapter 7: Wireless LAN
Mobile Communication Technology according to IEEE Wi. Fi 802. 11 a 802. 11 h 802. 11 i/e/…/n/…/z/aa 802. 11 b Local wireless networks WLAN 802. 11 g Zig. Bee Personal wireless nw WPAN 802. 15. 4 802. 15. 1 802. 15. 2 802. 15. 4 a/b/c/d/e/f/g 802. 15. 5, . 6 (WBAN) 802. 15. 3 Bluetooth Wireless distribution networks WMAN 802. 16 (Broadband Wireless Access) Wi. MAX + Mobility [802. 20 (Mobile Broadband Wireless Access)] 802. 16 e (addition to. 16 for mobile devices) 802. 15. 3 b/c
Wireless LAN (WLAN) • Main uses: – Extension to existing LAN – Cross building interconnect – Nomadic access / ‘wireless hotspots’ – Ad Hoc networks • Main Standard is IEEE 802. 11 • Wireless extension for Ethernet • Wi-Fi, Wireless-Fidelity, Alliance to certify products to the IEEE standard
Characteristics of wireless LANs • Advantages – very flexible within the reception area, – allow for design of small independent devices (e. g. to be put in pockets) – Ad-hoc networks without previous planning possible – (almost) no wiring difficulties (e. g. historic buildings, firewalls) – more robust against disasters like, e. g. , earthquakes, fire - or users pulling a plug. . . – Cost is independent of the number of users
Characteristics of wireless LANs • Disadvantages – typically very low bandwidth compared to wired networks (1 -10 Mbit/s) due to shared medium [now higher rates available] – high error rates, low quality – many proprietary solutions, especially for higher bitrates, standards take their time (e. g. IEEE 802. 11 n) – products have to follow many national restrictions if working wireless, it takes a very long time to establish global solutions – Safety & security
Design goals for wireless LANs • • global, seamless operation low power for battery use no special permissions or licenses needed to use the LAN robust transmission technology simplified spontaneous cooperation at meetings easy to use for everyone, simple management (plug & play) protection of investment in wired networks security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation) • transparency concerning applications and higher layer protocols, but also location awareness if necessary • …
Classifications of transmission technologies: infrared vs. radio transmission • Infrared (IR) – At 900 nm wavelength, uses IR diodes, diffuse light, multiple reflections (walls, furniture etc. ) • • – typically using the license free ISM band at 2. 4 GHz • Disadvantages – interference by sunlight, heat sources etc. – many things shield or absorb IR light , can not penetrate objects – low bandwidth (115 kbps – 4 Mbps) • Example – Ir. DA (Infrared Data Association) interface available everywhere Advantages – experience from wireless WAN and mobile phones can be used – coverage of larger areas possible (radio can penetrate walls, furniture etc. ) Advantages – simple, cheap, available in many mobile devices – no licenses needed – simple shielding possible • Radio • Disadvantages – very limited license frequency bands – shielding more difficult, interference with other electrical devices • Example – Many different products
IEEE 802. x standards • 802 standards specify OSI layers 1 & 2 – Physical layer • Encoding/decoding signals • Preamble (for synchronization) • Bit transmission/reception – Link layer (Medium Access Control (MAC)) • • Manage access to media Assemble/disassemble frames Addressing and error detection Interface with higher layers
ISM Unlicensed Frequency Bands
IEEE 802. 11 WLAN – 802. 11 (Legacy, 1997) operates at 1 -2 Mbps, with 3 methods • • • 1 infrared 2 radio access (FHSS, DSSS) Two Modes: – Infrastructure Mode (LAN extension) – Ad Hoc (wireless only) Wireless Communications and Networks, W. Stallings, Prentice Hall, N. J. , 2001.
Classifications of IEEE 802. 11: infrastructure vs. ad-hoc networks infrastructure network AP AP ad-hoc network wired network AP: Access Point AP
802. 11 - System architecture infrastructure network • 802. 11 LAN STA 1 – terminal with access mechanisms to the wireless medium and radio contact to the access point 802. x LAN • BSS 1 Portal Access Point ESS • 802. 11 LAN Access Point – station integrated into the wireless LAN and the distribution system • Portal – bridge to other (wired) networks • BSS 2 STA 2 Basic Service Set (BSS) – group of stations using the same radio frequency Distribution System Access Point Station (STA) STA 3 Distribution System – interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS – Each ESS has its own identifier ESSID
IEEE 802. 11: System architecture infrastructure network • The distribution system is not specified in IEEE 802. 11. – It could consist of IEEE LANs, wireless links or any other networks – It handles data transfer between different APs – To participate in a WLAN, you need to know the ESSID • Stations can select an AP and associate with it – The AP supports roaming (changing access points) – APs provide synchronization within a BSS, support power management, and can control medium access
802. 11 – System architecture Ad-hoc network • Direct communication within a limited range 802. 11 LAN STA 1 – Station (STA): terminal with access mechanisms to the wireless medium – Independent Basic Service Set (IBSS): group of stations using the same radio frequency – No specific node for data routing, or forwarding or exchange of topology information STA 3 IBSS 1 STA 2 IBSS 2 STA 5 STA 4 802. 11 LAN
IEEE 802. 11: Protocol architecture fixed terminal mobile terminal infrastructure network access point application TCP IP IP LLC LLC 802. 11 MAC 802. 3 MAC 802. 11 PHY 802. 3 PHY
802. 11 – Protocol Architecture • MAC • PLCP Physical Layer Convergence Protocol – access mechanisms, fragmentation, encryption – clear channel assessment signal (carrier sense) – Service access point (SAP) • MAC Management LLC MAC Management PLCP PHY Management PMD • PMD Physical Medium Dependent – modulation, coding • PHY Management – channel selection, MIB maintenance • Station Management PHY DLC – Association/de-association, synchronization, roaming, MIB (management Information Base), power management to save battery power, authentication mechanism – coordination of all management functions, higher layer functions (interaction with distribution system)
802. 11 - Physical layer (legacy) • 3 versions: 2 radio (typ. 2. 4 GHz ISM), 1 IR – data rates 1 or 2 Mbit/s • All physical variants include the provision of the clear channel assessment (CCA). This is needed for MAC mechanisms. • The Physical layer a service access point (SAP) with 1 or 2 Mbits/s transfer rate to the MAC layer. • FHSS (Frequency Hopping Spread Spectrum) – spreading, despreading using different hopping sequences (79 hopping channels for North America and Europe) – Frequency Shift Keying (FSK) digital modulation
802. 11 - Physical layer (legacy) • DSSS (Direct Sequence Spread Spectrum) – Spreading, despreading using 11 -chip Barker code chipping sequence: +1, -1, +1, +1, -1, -1 – Phase Shift Keying (PSK) digital modulation – max. radiated power 1 W (USA), 100 m. W (EU), min. 1 m. W – Robust against interference and multipath propagation – More complex compared to FHSS • Infrared – 850 -950 nm, diffuse light, typ. 10 m range – Typically in buildings (classrooms, meeting rooms, . . ) – Frequency reuse is simple, a wall is enough for shielding
802. 11 - MAC layer - DFWMAC • The MAC mechanisms are called Distributed Foundation Wireless Medium Access Control (DFWMAC) • Functions: medium access, support for roaming, authentication and power conservation • Traffic services – Asynchronous Data Service (mandatory) • exchange of data packets based on “best-effort” – no delay bounds • support of broadcast and multicast • Implemented using distributed coordination function (DCF) OR Point Coordination Function (PCF) • For both infrastructure and ad Hoc – Time-Bounded Service (optional) • implemented using PCF (Point Coordination Function) • Provides delay guarantees • For infrastructure 802. 11 only
MAC Layer • Asynchronous Data Service access method – DFWMAC-DCF CSMA/CA (mandatory) • collision avoidance via randomized „back-off“ mechanism • minimum distance between consecutive packets • ACK packet for acknowledgements (not for broadcasts) – DFWMAC-DCF w/ RTS/CTS (optional) • avoids hidden terminal problem – DFWMAC-PCF (optional) • Time-bounded Service access method – DFWMAC- PCF (optional) • access point polls terminals according to a list
802. 11 - MAC layer • Priorities – defined through different inter frame spaces – no guaranteed, hard priorities – SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response – PIFS (PCF IFS) • medium priority, for time-bounded service using PCF – DIFS (DCF, Distributed Coordination Function IFS) • lowest priority, for asynchronous data service DIFS medium busy DIFS PIFS SIFS direct access if medium is free DIFS contention next frame t
802. 11 - CSMA/CA access method I • station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) • if the medium is free for the duration of a DCF Inter-Frame Space (DIFS), the station can start sending • if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) • if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) DIFS medium busy direct access if medium is free DIFS contention window (randomized back-off mechanism) next frame t slot time (20µs)
802. 11 - competing stations - simple version DIFS boe bor DIFS boebusy boe busy station 1 boe bor boe busy boebor boe busy station 2 busy station 3 station 4 boe bor station 5 t busy medium not idle (frame, ack etc. ) boe elapsed backoff time packet arrival at MAC bor residual backoff time
802. 11 – CSMA/CA • The contention window (CW) size affect the performance of the MAC scheme • A small CW ensures shorter access delay but the probability of collision increases (more than one station can have the same backoff time) • The contention window starts with a minimum value then doubles each time a collision occurs up to a maximum value (e. g. 7, 15, 31, 63, 127, 255). • This is called the exponential backoff algorithm (already used in CSMA/CD)
802. 11 - CSMA/CA • Sending unicast packets – station has to wait for DIFS before sending data – receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) – automatic retransmission of data packets in case of transmission errors (The sender has to compete again) DIFS sender data SIFS receiver ACK DIFS other stations waiting time data t contention
802. 11 – DFWMAC with RTS/CTS (method II) • Sending unicast packets – To solve the problem of hidden terminal – station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) – acknowledgement via CTS after SIFS by receiver (if ready to receive) – sender can now send data at once, acknowledgement via ACK – other stations store medium reservations distributed via RTS and CTS in the NAV (net allocation vector) DIFS sender RTS data SIFS receiver other stations CTS SIFS ACK DIFS NAV (RTS) NAV (CTS) defer access data t contention
802. 11 – DFWMAC with RTS/CTS (cont. ) • The scheme reserves the medium for one user (virtual reservation scheme) • RTS/CTS can result in a non-negligible overhead causing a waste of bandwidth and higher delay • A threshold based on frame size can be used to determine when to use the additional mechanism and when to disable it • To reduce the bit error-rates in transmission, fragmentation can be used. However, for RTS/CTS scheme all fragments are sent by one RTS. Each fragment reserve the medium for the next fragment.
DFWMAC-PCF with polling – Method III (almost never used) • The two previous methods cannot guarantee a maximum delay or minimum bandwidth • PCF provides time-bounded service • It requires an access point that control medium access and polls the single nodes • Ad Hoc network can’t use this function so it provides only besteffort service • The point coordinator in the access point splits the access time into super frame periods. • A super frame comprises an contention-free period and a contention period • If only PCF is used and polling is distributed evenly, the bandwidth is also distributed evenly – static centrally controlled TDMA with TDD transmission • Much overhead if nodes have nothing to send.
802. 11 - Frame format • Types: control frames, management frames, data frames • Sequence numbers – important against duplicated frames due to lost ACKs • Addresses – receiver, transmitter (physical), BSS identifier, sender/receiver (logical) • Miscellaneous – Duration (to set the NAV), checksum, frame control, data bytes 2 Frame Control bits 2 2 6 6 6 2 6 0 -2312 Duration/ Address Sequence Address Data ID 1 2 3 Control 4 2 4 1 Protocol To Type Subtype version DS 1 1 From More DS Frag 1 Retry 1 1 Power More WEP Order Mgmt Data 4 CRC
802. 11 - Frame format • Frame Control – Protocol version: 2 bits – Type (management 00, control 01, data 10) – Subtype (e. g. Management- association 0000, beacon 100 Control – RTS 1011, CTS 1100) – More fragments: 1 if another fragment to follow – Retry: 1 if retransmission of an earlier frame – Power Management: 1 if the station will go to power save mode – More Data: A sender has more data to send – Wired Equivalent Privacy (WEP): Standard security mechanism applied – Order: frame must be processed in strict order bytes 2 Frame Control bits 2 2 6 6 6 2 6 0 -2312 Duration/ Address Sequence Address Data ID 1 2 3 Control 4 2 4 1 Protocol To Type Subtype version DS 1 1 From More DS Frag 1 Retry 1 1 Power More WEP Order Mgmt Data 4 CRC
MAC address format DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address
802. 11 - MAC management plays a central role in an IEEE 802. 11 as it controls all the functions related to system integration, i. e. , integration of a wireless station into a BSS, formation of an ESS, synchronization of stations, . . etc The major functions are: • Synchronization – try to find a WLAN and stay within it – synchronization of internal clock (timing synchronization function (TSF) • For power management • For coordination of PCF (super frame) • For synchronization of hopping sequence in FHSS systems – Generation of beacon signals
802. 11 - MAC management (cont. ) • Power management – To control transmitter activity for power conservation – sleep-mode without missing a frame – periodic sleep, frame buffering, traffic measurements • Association/Re-association – integration into a WLAN – roaming, i. e. change networks by changing access points – scanning, i. e. active search for a network • MIB - Management Information Base – managing, read, write, update
Synchronization using a Beacon (infrastructure) • Within a BSS, timing is conveyed by the periodic transmission of a beacon frame • A beacon contains a timestamp and other management information (identification of BSS, power management, roaming) • In infrastructure-based networks, the beacon is sent by the access point periodically. However, it may be delayed if medium is busy, but beacon interval is not shifted if one beacon is delayed. • The time stamp is used by a node to adjust its local clock beacon interval (20 ms – 1 s) access point medium B B busy t value of the timestamp B beacon frame
Synchronization using a Beacon (ad-hoc) • Each node maintains its own timer and starts transmission of a beacon frame after the beacon interval • Using random backoff algorithm, one beacon only wins • All other stations adjust their internal clock according to the received beacon interval station 1 B 1 B 2 station 2 medium busy value of the timestamp B 2 busy B beacon frame busy t random delay
Power management • Power-saving mechanisms are crucial for wireless devices • Standard WLAN protocols assume that stations are always ready to receive data. This permanent readiness consumes much power • Idea: switch the transceiver off if not needed • States of a station: sleep and awake • Timing Synchronization Function (TSF) – stations wake up periodically at the same time • Buffering of data at senders • Senders announce destination during wake periods • Longer off periods save battery life but reduce average throughput and increase delay
Power Management • Infrastructure – Access point buffers all frames destined for stations operating in powersave mode – With every beacon sent, a Traffic Indication Map (TIM) is transmitted • TIM contains a list of unicast receivers transmitted by AP • Beacon interval = TIM interval – Additionally, the AP maintains a Delivery Traffic Indication Map (DTIM) • list of broadcast/multicast receivers transmitted by AP • DTIM interval = multiple of TIM interval – The TSF assures that sleeping stations will wake-up periodically and listen to the beacon and TIM – If TIM indicates a unicast frame buffered for a station, the station stay awake to receive it – Stations always stay awake for muti-cast/ broadcast transmission – Stations also wake-up when they have frames to be transmitted
Power saving with wake-up patterns (infrastructure) TIM interval access point medium DTIM interval D B T busy T d D B busy p station d t T TIM D DTIM B broadcast/multicast awake p PS poll d data transmission to/from the station
Power saving with wake-up pattern (Ad hoc) • Ad-hoc – Ad-hoc Traffic Indication Map (ATIM) • announcement of receivers by stations buffering frames • more complicated - no central AP • collision of ATIMs possible (scalability? ) • APSD (Automatic Power Save Delivery) – new method in 802. 11 e replacing above schemes
802. 11 - Roaming • No or bad connection? Then perform: • Scanning – scan the environment, • Passive scanning: listen into the medium for beacon signals • Active scanning: send probes into the medium and wait for an answer • Reassociation Request – Choose best AP (e. g. based on signal strength) – station sends a request to one or several AP(s) • Reassociation Response – success: AP has answered, station can now participate – failure: continue scanning • AP accepts Reassociation Request – signal the new station to the distribution system – the distribution system updates its data base (i. e. , location information) – typically, the distribution system now informs the old AP so it can release resources • Fast roaming – 802. 11 r – e. g. for vehicle-to-roadside networks
WLAN: IEEE 802. 11 b • Data rate – 1, 2, 5. 5, 11 Mbit/s, depending on SNR – User data rate max. approx. 6 Mbit/s • Transmission range – 300 m outdoor, 30 m indoor – Max. data rate ~10 m indoor • Frequency – DSSS, 2. 4 GHz ISM-band • Security – Limited, WEP insecure, SSID (service set identifier) • Availability – Many products, many vendors • Connection set-up time – Connectionless/always on • Quality of Service – Typ. Best effort, no guarantees (unless polling is used, limited support in products) • Manageability – Limited (no automated key distribution, sym. Encryption) • Special Advantages/Disadvantages – Advantage: many installed systems, lot of experience, available worldwide, free ISM-band, many vendors, integrated in laptops, simple system – Disadvantage: heavy interference on ISM-band, no service guarantees, slow relative speed only
WLAN: IEEE 802. 11 a • Data rate – 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR – User throughput (1500 byte packets): 5. 3 (6), 18 (24), 24 (36), 32 (54) – 6, 12, 24 Mbit/s mandatory • Transmission range • – Connectionless/always on • • Frequency – Free 5. 15 -5. 25, 5. 25 -5. 35, 5. 7255. 825 GHz ISM-band • Security – Limited, WEP insecure, SSID • Availability – Some products, some vendors Quality of Service – Typ. best effort, no guarantees (same as all 802. 11 products) • Manageability – Limited (no automated key distribution, sym. Encryption) – 100 m outdoor, 10 m indoor • E. g. , 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30 m, 18 up to 40 m, 12 up to 60 m Connection set-up time • Special Advantages/Disadvantages – Advantage: fits into 802. x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band – Disadvantage: stronger shading due to higher frequency, no Qo. S
WLAN: IEEE 802. 11– current developments • 802. 11 j: Extensions for operations in Japan – Changes of 802. 11 a for operation at 5 GHz in Japan using only half the channel width at larger range • 802. 11 -2007: Current “complete” standard – Comprises amendments a, b, d, e, g, h, i, j • 802. 11 k: Methods for channel measurements – Devices and access points should be able to estimate channel quality in order to be able to choose a better access point of channel • • 802. 11 m: Updates of the 802. 11 -2007 standard 802. 11 n: Higher data rates above 100 Mbit/s – Changes of PHY and MAC with the goal of 100 Mbit/s at MAC SAP – MIMO antennas (Multiple Input Multiple Output), up to 600 Mbit/s are currently feasible – However, still a large overhead due to protocol headers and inefficient mechanisms • 802. 11 p: Inter car communications – Communication between cars/road side and cars/cars – Planned for relative speeds of min. 200 km/h and ranges over 1000 m – Usage of 5. 850 -5. 925 GHz band in North America • 802. 11 r: Faster Handover between BSS – Secure, fast handover of a station from one AP to another within an ESS – Current mechanisms (even newer standards like 802. 11 i) plus incompatible devices from different vendors are massive problems for the use of, e. g. , Vo. IP in WLANs – Handover should be feasible within 50 ms in order to support multimedia applications efficiently
WLAN: IEEE 802. 11– current developments • 802. 11 s: Mesh Networking • 802. 11 T: Performance evaluation of 802. 11 networks • • 802. 11 u: Interworking with additional external networks 802. 11 v: Network management • 802. 11 w: Securing of network control • • – Design of a self-configuring Wireless Distribution System (WDS) based on 802. 11 – Support of point-to-point and broadcast communication across several hops – Standardization of performance measurement schemes – Extensions of current management functions, channel measurements – Definition of a unified interface – Classical standards like 802. 11, but also 802. 11 i protect only data frames, not the control frames. Thus, this standard should extend 802. 11 i in a way that, e. g. , no control frames can be forged. 802. 11 y: Extensions for the 3650 -3700 MHz band in the USA 802. 11 z: Extension to direct link setup 802. 11 aa: Robust audio/video stream transport 802. 11 ac: Very High Throughput <6 Ghz 802. 11 ad: Very High Throughput in 60 GHz Note: Not all “standards” will end in products, many ideas get stuck at working group level Info: www. ieee 802. org/11/, 802 wirelessworld. com, standards. ieee. org/getieee 802/
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