Communication Networks Chapter 10 Wireless Local Area Networks

Communication Networks Chapter 10 – Wireless Local Area Networks According to IEEE 802. 11 Communication Networks: 10. IEEE 802. 11 657

10. WLANs According to IEEE 802. 11 Overview • • • Organization of a WLAN according to IEEE 802. 11 Current IEEE 802. 11 standards IEEE 802. 11 layers IEEE 802. 11 MAC synchronization, power saving and roaming WLAN and ad hoc networks Communication Networks: 10. IEEE 802. 11 658

10. 1 Working Modes 802. 11 – WLAN in Infrastructure Mode • Station (STA) § Device with access to the wireless medium and 802. 11 LAN STA 1 802. x LAN BSS 1 Portal Access Point Distribution System • Portal § Gateway to some other network • Distribution System § Connection of different WLAN cells to build an BSS 2 STA 2 • Basic Service Set (BSS) § Group of devices working on the same radio frequency • Access Point § Device that allows communication between stations and integrates them into the distribution system Access Point ESS connectivity to the access point 802. 11 LAN STA 3 Extended Service Set EES Communication Networks: 10. IEEE 802. 11 659

10. 1 Working Modes 802. 11 – WLAN in Ad-hoc Mode • Direct communication with limited range 802. 11 LAN STA 1 STA 3 BSS 1 STA 2 § Station (STA): Device with access to the wireless medium § Basic Service Set (BSS): Group of devices working on the same radio frequency (Independent Basic Service Set IBSS) BSS 2 STA 5 STA 4 802. 11 LAN Communication Networks: 10. IEEE 802. 11 660

10. 1 Working Modes Hidden Node Problem A • • B C A would like to communicate with B C is already transmitting information to B using the same channel There are collisions in B Both A and C cannot detect these collisions Communication Networks: 10. IEEE 802. 11 661

10. 1 Working Modes Exposed Node Problem A • • B C D B is already transmitting information to A C would like to communicate with D C finds the channel occupied and refrains from sending Free capacities are unused Communication Networks: 10. IEEE 802. 11 662

10. 2 The Standard IEEE 802. 11 IEEE-Standard 802. 11 Fixed Terminal Mobile Terminal Server Infrastructure Network Application TCP IP 802. 11 MAC 802. 11 PHY Access Point 802. 11 MAC 802. 3 MAC 802. 11 PHY 802. 3 PHY Communication Networks: 10. IEEE 802. 11 Application TCP IP 802. 3 MAC 802. 3 PHY 663

10. 2 The Standard IEEE 802. 11 Important Substandards • 802. 11 n • 802. 11 § Original standard from 1997 § Data rate 1 or 2 Mbit/s § Frequency range 2. 400 to 2. 485 GHz (ISM band) • 802. 11 a § Extended physical layer, published 1999 § Data rate 54 Mbit/s § Frequency range around 5 GHz • 802. 11 b § Extended physical layer, published 1999 § Data Rate 11 Mbit/s § Same frequency range as original 802. 11 • 802. 11 g § Extended physical layer, published 2003 § Data rate 54 Mbit/s § Frequency range 2. 400 to 2. 4835 GHz § § Multiple-input multiple-output antennas (MIMO) Publication by the IEEE in October 2009 Data rate up to 600 MBit/s Frequency range 2. 4 GHz or 5 GHz • 802. 11 p § Wireless access in vehicular environments (WAVE) § Data exchange between high-speed vehicles and between the vehicles and the roadside infrastructure in the licensed ITS band of 5. 9 GHz (5. 85 -5. 925 GHz) § Published in November 2010 • 802. 11 ac § Published 2013 § Theoretical max. data rate 3. 466 Gbit/s with 8 x 8 MIMO § Frequency: 5 GHz • 802. 11 ax (Wi. Fi 6) § Operates on all ISM bands upto 6 GHz § 9. 6 Gb/s maximum data rate § In store since 2019 Communication Networks: 10. IEEE 802. 11 664

10. 2 The Standard IEEE 802. 11 LLC Logical Link Control MAC Medium Access Control PLCP Physical Layer Convergence Protocol PMD Physical Medium Dependent MAC Management PHY Management Communication Networks: 10. IEEE 802. 11 Station Management PHY MAC 802. 11 Layers 665

10. 2 The Standard IEEE 802. 11 Functions • MAC • PMD § Medium Access § Segmentation/Reassembly § Ciphering § Modulation § Coding • PHY Management • MAC Management § § § Channel Selection § MIB Synchronization Roaming MIB Power Control • Station Management § Coordination of Management Functions • PLCP § Clear Channel Assessment Signal (Carrier Sense) Communication Networks: 10. IEEE 802. 11 666

10. 3 802. 11 MAC www. rhyshaden. com/ images/80211 fh. gif 802. 11 MAC Frame Format Communication Networks: 10. IEEE 802. 11 667

10. 3 802. 11 MAC Fields in the IEEE 802. 11 MAC Frame • Synch – Preamble: § for FH PHY 80 bits § for DSSS PHY 128 bits § alternating '0's and '1's • SFD – Start Frame Delimiter: § 16 bits “ 0000 1100 1011 1101” • PLW – PLCP-PDU Length Word: § 12 bits indicating the number of bytes in the packet § first portion of the PLCP header § PLCP header is transmitted only at 1 Mbps! • PSF – PLCP Signaling Field: § 4 bits to show the rate of the MAC payload transmission § Bit 0 is reserved and is always '0' § Bits 1 to 3 indicate the data rates Communication Networks: 10. IEEE 802. 11 668

10. 3 802. 11 MAC 802. 11 – MAC • Distributed Foundation Wireless MAC (DFWMAC) • Different traffic types § Asynchronous data transmission (standard) v Exchange of MAC frames without Quality of Service („best-effort”) v Broadcast and multicast § Time-limited transmission (optional) v Point Coordination Function (PCF) only in infrastructure mode Communication Networks: 10. IEEE 802. 11 669

10. 3 802. 11 MAC Procedures • Distributed Coordination Function: Carrier Sense Multiple Access with Collision Avoidance (DFWMAC-DCF CSMA/CA) (standard) § Collision avoidance based on arbitrary backoff algorithm § Minimum time span between two MAC frames (so called inter frame spacing) § Correct transmission signaled with ACK-frame (except Broadcast or Multicast) • Distributed Coordination Function with “Request to Send” / “Clear To Send” Frames (DFWMAC-DCF with RTS/CTS) (optional) § Avoidance of hidden node problem • Point Coordination Function (DFWMAC-PCF) (optional) § List-based polling done in the Access Point Communication Networks: 10. IEEE 802. 11 670

10. 3 802. 11 MAC 802. 11 – MAC: Inter Frame Spacing • Implementation of Priorities § No guarantees § Shorter inter frame spacing allows earlier sending time for the frame: v SIFS (Short Inter Frame Spacing) - Highest priority, for ACK, CTS, response to polling v PIFS (PCF IFS) - Medium priority, for time limited services in PCF v DIFS (DCF, Distributed Coordination Function IFS) - Lowest priority, for asynchronous data transmission DIFS medium busy DIFS PIFS SIFS competition direct access, if medium is free DIFS Communication Networks: 10. IEEE 802. 11 next frame t 671

10. 3 802. 11 MAC 802. 11 – CSMA/CA I DIFS medium busy waiting time • • • Competition Window (arbitrary backoff time) DIFS PIFS SIFS next frame time slot t Carrier sense based on clear channel assessment signal Station may send, if medium is free for the appropriate IFS If medium is busy, station sets backoff time to an arbitrary number of time slots After the medium is free again, station waits the appropriate IFS and the backoff time If medium gets busy during backoff time, backoff time is frozen Communication Networks: 10. IEEE 802. 11 672

10. 3 802. 11 MAC 802. 11 – CSMA/CA II DIFS Station 1 Station 2 DIFS boe bor boe busy boe bor boe busy Station 3 Station 4 boe bor Station 5 busy bor t busy medium busy (frame, ack etc. ) boe elapsed backoff time data arrival at MAC-SAP bor residual backoff time Communication Networks: 10. IEEE 802. 11 673

10. 3 802. 11 MAC 802. 11 – CSMA/CA III • Sending unicast frames § Frames can be sent after DIFS (plus backoff time as described before) § Receiver responds immediately (after SIFS), if frame has been correctly received (CRC) § If an error occurs, the frame is automatically repeated DIFS Sender Data SIFS Receiver Further Stations Ack DIFS waiting time Data t competition Communication Networks: 10. IEEE 802. 11 674

10. 3 802. 11 MAC 802. 11 – RTS / CTS • Sending unicast frames using RTS/CTS § Before transmitting a data frame, an RTS frame has to be sent including the duration of the data frame (after DIFS) § Receiver acknowledges RTS frame with a CTS frame (after SIFS) § Sender may then send the data frame after SIFS, which is acknowledged as usual § Other stations store the time the medium is busy (as contained in the RTS and CTS frames) DIFS Sender Receiver Further Stations NAV = Network Allocation Vector RTS data SIFS CTS SIFS DIFS NAV (RTS) NAV (CTS) waiting time ACK data t competition Communication Networks: 10. IEEE 802. 11 675

10. 3 802. 11 MAC 802. 11 – RTS / CTS: Fragmentation DIFS Sender Receiver RTS frag 1 SIFS CTS SIFS frag 2 SIFS NAV (RTS) NAV (CTS) Further Stations ACK 1 SIFS NAV (frag 1) NAV (ACK 1) ACK 2 DIFS data t competition Communication Networks: 10. IEEE 802. 11 676

10. 3 802. 11 MAC DFWMAC-PCF I t 0 t 1 Medium busy Point Coordinator Stations NAV of the stations PIFS Super Frame SIFS D 1 SIFS D 2 SIFS U 1 U 2 NAV Point Coordination Function Communication Networks: 10. IEEE 802. 11 677

10. 3 802. 11 MAC DFWMAC-PCF II t 2 t 3 Point Coordinator D 3 PIFS SIFS D 4 SIFS Stations NAV of the stations t 4 CFend U 4 NAV period without competition Communication Networks: 10. IEEE 802. 11 competition t 678

10. 3 802. 11 MAC Address Format Frame Type Ad-hoc Network Infrastructure Network, from AP Infrastructure Network, to AP Infrastructure Network, in DS to DS from DS Address 1 Address 2 Address 3 Address 4 0 0 DA SA BSSID - 0 1 DA BSSID SA - 1 0 BSSID SA DA - 1 1 RA TA DA SA DS AP DA SA BSSID RA TA Communication Networks: 10. IEEE 802. 11 : Distribution System : Access Point : Destination Address : Source Address : Basic Service Set Identifier : Receiver Address : Transmitter Address 679

10. 4 802. 11 MAC Management 802. 11 – MAC Management • Synchronization § Finding and staying in a WLAN § Timer etc. • Power Management § Sleep modus without loosing a frame § Periodically sleeping, buffering of frames, traffic map • Association / Reassociation § Associating with a distribution system § Roaming, i. e. changing networks when changing access points § Scanning, i. e. actively looking for a WLAN • MIB - Management Information Base § Administering, reading, writing Communication Networks: 10. IEEE 802. 11 680

10. 4 802. 11 MAC Management MAC Synchronization in Infrastructure Mode beacon interval (20 ms – 1 s) access point medium B B busy value of the timestamp B beacon frame Communication Networks: 10. IEEE 802. 11 B busy t 681

10. 4 802. 11 MAC Management MAC Synchronization in Ad hoc Mode beacon interval station 1 B 1 B 2 station 2 medium busy value of the timestamp B 2 busy B beacon frame Communication Networks: 10. IEEE 802. 11 busy t random delay 682

10. 4 802. 11 MAC Management Power Saving • Idea • Infrastructure § switch the transceiver off if not needed • States of a station § sleep § awake • Timing Synchronization Function (TSF) § stations wake up at the same time § Traffic Indication Map (TIM) v list of unicast receivers transmitted by AP § Delivery Traffic Indication Map (DTIM) v list of broadcast/multicast receivers transmitted by AP • Ad-hoc § Ad-hoc Traffic Indication Map (ATIM) v announcement of receivers by stations buffering frames v more complicated - no central AP v collision of ATIMs possible (scalability? ) • APSD (Automatic Power Save Delivery) § new method in 802. 11 e replacing above scheme Communication Networks: 10. IEEE 802. 11 683

10. 4 802. 11 MAC Management Power Saving in Infrastructure Mode TIM interval access point DTIM interval D B T busy medium busy T d D B busy p station d t T TIM D B broadcast/multicast DTIM awake p PS poll d data transmission to/from the station Communication Networks: 10. IEEE 802. 11 684

10. 4 802. 11 MAC Management Power Saving in Ad hoc Mode ATIM window station 1 B 1 station 2 B beacon frame awake beacon interval A B 2 random delay B 2 D a B 1 d A transmit ATIM t D transmit data a acknowledge ATIM d acknowledge data Communication Networks: 10. IEEE 802. 11 685

10. 4 MAC Management Roaming • No or bad connection? Then perform: • Scanning § scan the environment, i. e. , listen into the medium for beacon signals or send probes into the medium and wait for an answer • Reassociation Request § 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 Communication Networks: 10. IEEE 802. 11 686

10. 5 802. 11 in Ad-hoc Mode IEEE 802. 11 s • IEEE 802. 11 amendment for mesh networking, since 2012 incorporated in 802. 11 standard • Broadcast/multicast and unicast delivery using “radio-aware metrics over selfconfiguring multi-hop topologies” • Default mandatory routing protocol Hybrid Wireless Mesh Protocol (HWMP), inspired by a combination of AODV (RFC 3561) and tree-based routing, based on MAC addresses • Peer authentication methods defined for security • “One Laptop per Child” project (laptop. org) uses the 802. 11 s draft standard for its OLPC XO laptop and OLPC XS school server networking Communication Networks: 10. IEEE 802. 11 687

10. 5 802. 11 in Ad-hoc Mode Ad-hoc Networks and Qo. S (I) • Guarantee of Qo. S even when topology keeps changing all the time! • Normal procedure: § Find route with enough resources § Reserve the required resources § Keep on controlling the achieved Qo. S • For ad-hoc networks: § § Limited range and energy Restricted availability of channels / bit rate (shared medium) Unforeseeable radio problems Vertical and horizontal handover Communication Networks: 10. IEEE 802. 11 688

10. 5 802. 11 in Ad-hoc Mode Ad hoc Networks and Qo. S (II) Node cannot supply the required Qo. S Communication Networks: 10. IEEE 802. 11 689

10. 5 802. 11 in Ad-hoc Mode Ad hoc Networks and Qo. S (III) • Trying to guarantee Qo. S through redundancy • If Qo. S cannot be supplied: best effort transmission or communication breakdown Parallel Routes Simultaneous transmission Backup routes established and reserved Communication Networks: 10. IEEE 802. 11 Backup routes selected 690

10. 5 802. 11 in Ad-hoc Mode Interoperability with Other Networks • One of the nodes in the ad-hoc network allows access to some other network • Problem: different network characteristics § Addresses § Capacity / Qo. S § Routing and Signaling • Example: WLAN-based ad hoc network Ad-hoc Network Internet LTE Cell Communication Networks: 10. IEEE 802. 11 691

10. 5 802. 11 in Ad-hoc Mode Multimedia Transmission in an Ad hoc Network • Idea: Transmitting a video stream in an ad hoc network based on AODV • Problem: Communication Networks: 10. IEEE 802. 11 692

10. 5 802. 11 in Ad-hoc Mode Summary on Ad hoc Networks • Currently many research projects in this area • Still not very well accepted § Security? § Benefits? § Standards? • Well suited to enhance an infrastructure network • Perfect for communication in underdeveloped areas Communication Networks: 10. IEEE 802. 11 693

References • Gast, Matthew S. (2017): 802. 11 Wireless Networks. The Definitive Guide. Sebastopol, CA: O'Reilly Media. • Olenewa, Jorge L. (2017): Guide to Wireless Communications. Fourth edition. Australia: Cengage Learning. • Perahia, Eldad; Stacey, Robert (2013): Next Generation Wireless LANs. 802. 11 n, 802. 11 ac, and Wi-Fi direct. 2 nd edition. Cambridge: Cambridge University Press. • Schiller, Jochen H. (2009): Mobile Communications. United Kingdom: Pearson Education Limited. • Slingerland, Janet (2018): Wi-Fi. How It Works. Lake Elmo, MN: Focus Readers. Communication Networks: 10. IEEE 802. 11 694