Wireless Local Area Networks (WLAN) Part-1: IEEE IT351: Mobile & Wireless Computing 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 –Infrastructure networks –Ad Hoc networks WLAN IEEE –Architecture –Protocols –Physical layer –MAC layer –MAC management –IEEE a/ b/... n

Overview of the main chapters Chapter 2: Wireless Transmission Chapter 3: Medium Access Control Chapter 4: Telecommunication Systems Chapter 5: Satellite Systems Chapter 6: Broadcast Systems Chapter 7: Wireless LAN Chapter 8: Mobile Network Layer Chapter 9: Mobile Transport Layer Chapter 10: Support for Mobility

Mobile Communication Technology according to IEEE Local wireless networks WLAN a b i/e/…/n/…/z/aa g WiFi h Personal wireless nw WPAN Bluetooth a/b/c/d/e/f/g ZigBee Wireless distribution networks WMAN (Broadband Wireless Access) [ (Mobile Broadband Wireless Access)] e (addition to.16 for mobile devices) + Mobility WiMAX b/c ,.6 (WBAN)

Wireless LAN (WLAN) Main uses: –Extension to existing LAN –Cross building interconnect –Nomadic access / ‘wireless hotspots’ –Ad Hoc networks Main Standard is IEEE 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 bit- rates, standards take their time (e.g. IEEE 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.) Advantages –simple, cheap, available in many mobile devices –no licenses needed –simple shielding possible Disadvantages –interference by sunlight, heat sources etc. –many things shield or absorb IR light, can not penetrate objects –low bandwidth (115kbps – 4 Mbps) Example –IrDA (Infrared Data Association) interface available everywhere Radio –typically using the license free ISM band at 2.4 GHz Advantages –experience from wireless WAN and mobile phones can be used –coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages –very limited license free frequency bands –shielding more difficult, interference with other electrical devices Example –Many different products

IEEE802.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 WLAN – (Legacy, 1997) operates at 1-2 Mbps, with 3 methods 1 infrared 2 radio access (FHSS, DSSS) Wireless Communications and Networks, W. Stallings, Prentice Hall, N.J., Two Modes: –Infrastructure Mode (LAN extension) –Ad Hoc (wireless only)

Classifications of IEEE802.11: infrastructure vs. ad-hoc networks infrastructure network ad-hoc network AP wired network AP: Access Point

System architecture infrastructure network Station (STA) –terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) –group of stations using the same radio frequency Access Point –station integrated into the wireless LAN and the distribution system Portal –bridge to other (wired) networks Distribution System –interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS –Each ESS has its own identifier ESSID Distribution System Portal 802.x LAN Access Point LAN BSS LAN BSS 1 Access Point STA 1 STA 2 STA 3 ESS

IEEE802.11: System architecture infrastructure network The distribution system is not specified in IEEE –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

– System architecture Ad-hoc network Direct communication within a limited range –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 LAN IBSS LAN IBSS 1 STA 1 STA 4 STA 5 STA 2 STA 3

IEEE802.11: Protocol architecture mobile terminal access point fixed terminal application TCP PHY MAC IP MAC PHY application TCP PHY MAC IP MAC PHY LLC infrastructure network LLC

– Protocol Architecture PLCP Physical Layer Convergence Protocol –clear channel assessment signal (carrier sense) –Service access point (SAP) PMD Physical Medium Dependent –modulation, coding PHY Management –channel selection, MIB maintenance Station Management –coordination of all management functions, higher layer functions (interaction with distribution system) PMD PLCP MAC LLC MAC Management PHY Management MAC –access mechanisms, fragmentation, encryption MAC Management –Association/de-association, synchronization, roaming, MIB (management Information Base), power management to save battery power, authentication mechanism PHY DLC Station Management

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

Physical layer (legacy) DSSS (Direct Sequence Spread Spectrum) –Spreading, despreading using 11-chip Barker code chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 –Phase Shift Keying (PSK) digital modulation –max. radiated power 1 W (USA), 100 mW (EU), min. 1mW –Robust against interference and multipath propagation –More complex compared to FHSS Infrared – nm, diffuse light, typ. 10 m range –Typically in buildings (classrooms, meeting rooms,..) –Frequency reuse is simple, a wall is enough for shielding

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 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

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 t medium busy SIFS PIFS DIFS next framecontention direct access if medium is free  DIFS

t medium busy DIFS next frame contention window (randomized back-off mechanism) 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) slot time (20µs) direct access if medium is free  DIFS

competing stations - simple version t busy bo e station 1 station 2 station 3 station 4 station 5 packet arrival at MAC DIFS bo e busy elapsed backoff time bo r residual backoff time busy medium not idle (frame, ack etc.) bo r DIFS bo e bo r DIFS busy DIFS bo e busy bo e bo r

– 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)

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) t SIFS DIFS data ACK waiting time other stations receiver sender data DIFS contention

– 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) t SIFS DIFS data ACK defer access other stations receiver sender data DIFS contention RTS CTS SIFS NAV (RTS) NAV (CTS)

– 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 best- effort 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.

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 Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 DataCRC bytes Protocol version TypeSubtype To DS More Frag Retry Power Mgmt More Data WEP 2241 From DS 1 Order bits111111

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 Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 DataCRC bytes Protocol version TypeSubtype To DS More Frag Retry Power Mgmt More Data WEP 2241 From DS 1 Order bits111111

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

MAC management MAC management plays a central role in an IEEE 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

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 (20ms – 1s) t medium access point busy B BBB 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 beaco n t medium station 1 busy B1B1 beacon interval busy B1B1 value of the timestamp B beacon frame station 2 B2B2 B2B2 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 power- save 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 t medium access point busy D TTD T TIM D DTIM DTIM interval BB B broadcast/multicast station awake p PS poll p d d 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 e replacing above schemes

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 – r –e.g. for vehicle-to-roadside networks

WLAN: IEEE b Data rate –1, 2, 5.5, 11 Mbit/s, depending on SNR –User data rate max. approx. 6 Mbit/s Transmission range –300m outdoor, 30m indoor –Max. data rate ~10m 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 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 –100m outdoor, 10m indoor E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40 m, 12 up to 60 m Frequency –Free , , GHz ISM-band Security –Limited, WEP insecure, SSID Availability –Some products, some vendors Connection set-up time –Connectionless/always on Quality of Service –Typ. best effort, no guarantees (same as all products) Manageability –Limited (no automated key distribution, sym. Encryption) 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 QoS

WLAN: IEEE – current developments j: Extensions for operations in Japan –Changes of a for operation at 5GHz in Japan using only half the channel width at larger range : Current “complete” standard –Comprises amendments a, b, d, e, g, h, i, j 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 m: Updates of the standard n: Higher data rates above 100Mbit/s –Changes of PHY and MAC with the goal of 100Mbit/s at MAC SAP –MIMO antennas (Multiple Input Multiple Output), up to 600Mbit/s are currently feasible –However, still a large overhead due to protocol headers and inefficient mechanisms p: Inter car communications –Communication between cars/road side and cars/cars –Planned for relative speeds of min. 200km/h and ranges over 1000m –Usage of GHz band in North America 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 i) plus incompatible devices from different vendors are massive problems for the use of, e.g., VoIP in WLANs –Handover should be feasible within 50ms in order to support multimedia applications efficiently

WLAN: IEEE – current developments s: Mesh Networking –Design of a self-configuring Wireless Distribution System (WDS) based on –Support of point-to-point and broadcast communication across several hops T: Performance evaluation of networks –Standardization of performance measurement schemes u: Interworking with additional external networks v: Network management –Extensions of current management functions, channel measurements –Definition of a unified interface w: Securing of network control –Classical standards like , but also i protect only data frames, not the control frames. Thus, this standard should extend i in a way that, e.g., no control frames can be forged y: Extensions for the MHz band in the USA z: Extension to direct link setup aa: Robust audio/video stream transport ac: Very High Throughput <6Ghz ad: Very High Throughput in 60 GHz Note: Not all “standards” will end in products, many ideas get stuck at working group level Info: 802wirelessworld.com, standards.ieee.org/getieee802/