An Introduction to Wireless LANs Universität Karlsruhe Institut für Telematik An Introduction to Wireless LANs Lecturer: Lei Guo, Yahoo! Outline Wireless LAN architecture Medium access principle 802.11 MAC control 802.11 MAC management Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Wireless LAN Architecture Universität Karlsruhe Institut für Telematik Wireless LAN Architecture Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Comparison: infrastructure vs. ad-hoc networks Universität Karlsruhe Institut für Telematik Comparison: infrastructure vs. ad-hoc networks infrastructure network AP: Access Point AP AP wired network AP ad-hoc network Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

IEEE 802.11 - Architecture of an infrastructure network Universität Karlsruhe Institut für Telematik IEEE 802.11 - Architecture of an infrastructure network Basic Service Set (BSS) group of stations using the same radio channel 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 Portal bridge to other (wired) networks Distribution System interconnection network to form one logical network (EES: Extended Service Set) based on several BSS 802.11 LAN 802.x LAN STA1 BSS1 Portal Access Point Distribution System Access Point ESS BSS2 STA2 STA3 802.11 LAN Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 9

Wireless router in your home Universität Karlsruhe Institut für Telematik Wireless router in your home www.yahoo.com Cable modem Wireless router Internet portal AP NAT Router ESS2/BSS2 Your neighbor channel 1 ESS1/BSS1 channel 6 Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

802.11 - Architecture of an ad-hoc network Universität Karlsruhe Institut für Telematik 802.11 - Architecture of an 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 ad-hoc routing: possible but not specified in IEEE 802.11 802.11 LAN STA1 IBSS1 STA3 STA2 IBSS2 STA5 STA4 802.11 LAN Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

IEEE standard 802.11: protocol stack Universität Karlsruhe Institut für Telematik IEEE standard 802.11: protocol stack fixed terminal mobile terminal infrastructure network access point application application TCP TCP IP LLC is same for all 802.x IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik 802.11 - Layers and functions MAC control access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management PLCP Physical Layer Convergence Protocol clear channel assessment signal (carrier sense) PMD Physical Medium Dependent modulation, coding PHY Management channel selection, MIB Station Management coordination of all management functions Station Management LLC DLC MAC MAC Management PLCP PHY Management See video (in reference 4) for physical layer technologies PHY PMD Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Medium Access Principle Universität Karlsruhe Institut für Telematik Medium Access Principle Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik Motivation Can we apply media access methods from fixed networks? Ethernet: CSMA/CD Carrier Sense Multiple Access with Collision Detection send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) stop when collision is detected Problems in wireless networks signal strength decreases proportional to the square of the distance the sender would apply CS and CD, but the collisions happen at the receiver it might be the case that a sender cannot “hear” the collision, i.e., CD does not work furthermore, CS might not work if, e.g., a terminal is “hidden” Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Hidden and exposed terminals Universität Karlsruhe Institut für Telematik Hidden and exposed terminals Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails), neither can C A is “hidden” for C, and vice versa Exposed terminals B sends to A, C wants to send to another terminal D C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B, and vice versa A B C D Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

MACA - collision avoidance Universität Karlsruhe Institut für Telematik MACA - collision avoidance MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive after RTS/CTS exchange, medium is reserved Signaling packets contain sender address receiver address packet size RTS/CTS collision probability is small, since RTS/CTS packets are very short Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik MACA examples MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal D B sends RTS, and A replies CTS now C does not have to wait, for it cannot receive CTS from A A C RTS CTS CTS B A C RTS RTS CTS B D Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik Polling mechanisms If one terminal can be heard by all others, this “central” terminal (e.g., AP) can poll all other terminals according to a certain scheme, e.g., round-robin polling Polling is contention free, no collision 802.11 Infrastructure WLAN PCF (Point Coordination Function) polling to avoid collision DCF (Distributed Coordination Function) CSMA/CA with RTS/CTS to resolve hidden terminal problem no exposed terminal problem in infrastructure WLAN (why?) Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

802.11 Wireless LANs: MAC Control Universität Karlsruhe Institut für Telematik 802.11 Wireless LANs: MAC Control Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik 802.11 - MAC layer I - DFWMAC Traffic services Asynchronous Data Service (infrastructure/ad-hoc, mandatory) station sends packets with “best-effort”, no upper bound for delay support of broadcast and multicast implemented using DCF (Distributed Coordination Function) Time-Bounded Service (infrastructure only, optional) implemented using PCF (Point Coordination Function) with polling DFWMAC (Distributed Foundation Wireless MAC) DFWMAC-DCF CSMA/CA (mandatory) collision avoidance via randomized “back-off” mechanism reliable delivery with packet ACK (for unicast only) DFWMAC-DCF w/ RTS/CTS (optional) based on CSMA/CA, avoids hidden terminal problem optional, but supported by most commodity products DFWMAC-PCF (optional) AP polls terminals according to a list, contention free optional, not supported by most commodity products Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik 802.11 - MAC layer II Frame types data frame: normal data packets control frame: ACK, RTS, CTS management frame: beacon, probe, association, authentication, … Priorities for different frames defined through different IFS (Inter Frame Spacing) duration SIFS (Short IFS): highest priority, for ACK, CTS, polling response, … PIFS (PCF IFS): medium priority, for time-bounded service (PCF) DIFS (DCF IFS): lowest priority, for asynchronous data service (DCF) no guaranteed, hard priorities DIFS DIFS PIFS SIFS medium busy contention next frame t direct access if medium is free  DIFS random Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

802.11 - CSMA/CA: sending unicast data frame Universität Karlsruhe Institut für Telematik 802.11 - CSMA/CA: sending unicast data frame contention window (randomized back-off mechanism) DIFS DIFS medium busy next frame direct access if medium is free  DIFS t total waiting time when has data to send: starts sensing the medium (Carrier Sense) if the medium is free for the duration of DIFS, starts sending if the medium is busy, wait for a free DIFS, then do back-off: wait a random back-off time (contention window) to avoid collision if the medium is busy again during the back-off time: stop back-off, wait for a free DIFS, then continue back-off (the contention window is set to the residual back-off time) if collision: stop back-off, wait for a free DIFS, then redo back-off by doubling contention window size (until maximum), re-send data Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller 12

802.11 - competing stations - simple version Universität Karlsruhe Institut für Telematik 802.11 - competing stations - simple version DIFS DIFS DIFS DIFS boe bor boe bor boe busy station1 boe busy station2 busy station3 boe busy boe bor station4 boe bor boe busy boe bor station5 t busy medium not idle (frame, ack etc.) boe elapsed backoff time packet arrival at MAC bor residual backoff time Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

802.11 - CSMA/CA: data frame acknowledgment Universität Karlsruhe Institut für Telematik 802.11 - CSMA/CA: data frame acknowledgment Sending a unicast packet: complete procedure station has to wait for DIFS before sending data receiver send ACK at once if the packet was received correctly ACK cannot be delayed since it just wait a SIFS automatic retransmission of data packets in case of transmission errors Compared with CSMA/CD (Ethernet) no collision detection (Ethernet has CD) use ACK to confirm delivery except multicast/broadcast (Ethernet no ACK) atomic operation DIFS data sender SIFS ACK receiver DIFS data other stations t waiting time contention Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik 802.11 – DFWMAC with RTS/CTS Sending unicast packets A sends RTS with net allocation vector (NAV, amount of time to use the medium) after waiting for DIFS, to reserve medium B replies CTS after SIFS (if ready to receive) A sends data to B, B replies ACK other stations know medium reservation through NAV in RTS and CTS atomic operation DIFS RTS data A sender SIFS SIFS SIFS CTS ACK B receiver DIFS NAV (RTS) data other stations NAV (CTS) C t defer access contention Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

802.11 Wireless LANs: MAC Management Universität Karlsruhe Institut für Telematik 802.11 Wireless LANs: MAC Management Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik 802.11 - MAC management Synchronization synchronize internal clocks functions to support finding a WLAN Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements Roaming scanning changing access points joining a network MIB - Management Information Base managing, read, write Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Synchronization using a Beacon (infrastructure) Universität Karlsruhe Institut für Telematik Synchronization using a Beacon (infrastructure) each station maintains an internal clock synchronized clocks are needed for power management PCF coordination hopping sequence of radio frequency in physical layer (see video) how to synchronize: AP broadcasts quasi periodic beacon frames (100 ms) beacon interval B B B B access point busy busy busy busy medium t B value of the timestamp beacon frame synchronization in ad-hoc WLAN: more complex Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Power management: motivation Universität Karlsruhe Institut für Telematik Power management: motivation Mobile devices are usually driven by battery powers Wireless card consumes a great amount of energy in mobile devices over 50% total energy for PDA up to 10% total energy for laptop Power modes of wireless card Transmit mode data transmission (sending) power consumption: high Receive mode data receiving and listening power consumption: medium Sleep mode power consumption: low power consumption of a wireless card Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

802.11 power management mechanism Universität Karlsruhe Institut für Telematik 802.11 power management mechanism Idea: switch the transceiver off if not needed power saving mode: a station sleep and awake sender knows when to wake up for transmission receiver needs to know when to wake up for receiving unicast in infrastructure WLAN AP buffers data for a station working in power saving mode AP periodically broadcasts with beacon a Traffic Indication Map (TIM), indicating which station has data to receive station periodically wakes up to receive TIM station polls AP to receive data broadcast/multicast in infrastructure WLAN similar but no buffer and no poll (broadcast/multicast has no ACK) needs another Traffic Indication Map (called Delivery TIM, DTIM) period of DTIM is multiple of TIM ad-hoc WLAN more complex Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Power saving with wake-up patterns (infrastructure) Universität Karlsruhe Institut für Telematik Power saving with wake-up patterns (infrastructure) before sleep: station notifies AP to have data buffered TIM interval DTIM interval D B T T d D B access point busy busy busy busy medium p d station t t0 t1 t2 t3 T TIM D DTIM awake d data transmission to/from the station B broadcast/multicast p PS poll Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik 802.11 – Roaming Scan passive scan: listen into the medium for beacon signals active scan: send probes into the medium and wait for an answer Reassociation station sends a Reassociation Request to new AP(s) new AP replies Reassociation Response success: join the new AP failure: continue scanning Handover new AP: signals the new station to the distribution system (DS) DS: updates data base (i.e., location information) old AP: sends buffered data to new AP through DS, release resources Advantage seamless to end users, IP layer not affected TCP/IP socket not broken (roaming in an ESS) Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik IEEE 802.11 family 802.11: legacy (1997) 2.4 GHz 1, 2 Mbps 802.11a: not widely used (2001) 5-6 GHz range 6, 9, 12, 18, 24, 36, 48, and 54 Mbps 802.11b: widely used before (1999) 1, 2, 5.5, 11 Mbps 802.11g: most popular now (2003) backwards compatible with b 802.11n: MIMO antenna (2010) up to 300 Mbps, 600 Mbps in the future backwards compatible with a, b, g product for 802.11n draft already on market And others: 802.11 d, e, f, h, i, … Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik Summary: Key points Architecture 802.11 protocol stack Access principle of radio medium hidden terminal problem RTS/CTS, polling 802.11 MAC control DCF CSMA/CA DCF with RTS/CTS 802.11 MAC management synchronization (infrastructure) power saving mechanism (infrastructure) References Basic: J. F. Kurose, K. W. Ross, Computer Networking: A Top-Down Approach (4 ed.), Chapter 6 Advanced: J. Schiller, Mobile Communications (2 ed.), Chapter 7 Lab book: M. Gast, 802.11® Wireless Networks: The Definitive Guide, O'Reilly Physical layer video: http://public.yahoo.com/~lguo/download/cisco-aironet.zip, click radio frequency technology to watch (optional) Modern Networking Concepts: An Introduction to Wireless LANs Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller