Outline

1. Outline Wireless introduction Wireless cellular (GSM, CDMA, UMTS) Wireless LANs, MAC layer IEEE 802.11 Bluetooth ZigBee Wireless Ad hoc networks routing: proactive routing, on-demand routing, scalable routing, geo-routing wireless Ad hoc multicast TCP in ad hoc networks QoS, adaptive voice/video apps Sensor networks CS219

2. 802.11 Architecture CS219

3. IEEE 802.11 Standard Why we study this standard: • overall architecture • MAC layer spec. • channel access • mobility support • physical layer spec. • direct sequence • frequency hopping CS219

4. 802.11 Features • CSMA/CA based MAC protocol - DCF (Distributed Coordination Function) • support for both time-critical - PCF( Point Coordination Function) and non-critical traffic (DCF) • support multiple priority levels • spread spectrum technology (no licensing) • power management allows a node to doze off CS219

5. 802.11 Protocol Entities • MAC entity • basic access mechanism • fragmentation & encryption • MAC layer management entity • synchronization • power management • roaming • Physical layer convergence protocol (PLCP) • PHY-specific, common PHY SAP support • provides carrier sense • Physical medium dependent sublayer (PMD) • modulation & coding • PHY layer management • channel tuning & PHY MIB MAC Sublayer MAC layer Management PLCP sublayer PHY layer Management PMD sublayer CS219

6. PHY spec • Infrared PHY (No products !) • diffuse infrared • 1 and 2Mbps • Radio PHY • Frequency hopping PHY • Direct Sequence PHY • CCA (clear channel assessment) - how to sense a channel is clear: • energy level is above a threshold • can detect a signal • use both CS219

7. Frequency Hopping CS219

8. Frequency Hopping Spread Spectrum • Pseudo-random frequency hopping • 2.4Ghz ISM band, 1-2Mbps; 2GFSK (2 level Gaussian frequency shift keying), 4GFSK; hop over 79 channels • spreads the power over a wide spectrum -> spread spectrum • narrowband interference cannot jam • developed initially for military CS219

9. Direct Sequence Spread Spectrum CS219

10. Direct Sequence Spread Spectrum • Spreading factor = code bits/data bit, 10-100 commercial (min 10 by FCC) • Signal bandwidth>10*data bandwidth • code sequence synchronization • correlation between codes -> interference: orthogonal • 2.4Ghz band, 1,2Mbps; DBPSK(differential binary phase shift keying), DQPSK(differential quadrature phase shift keying); 11 chip barker sequence CS219

11. Multiple Access Control (MAC) Protocols • MAC protocol: coordinates transmissions from different stations to minimize/avoid collisions • (a) Channel Partitioning MAC protocols: TDMA, FDMA, CDMA • (b) Random Access MAC protocols: CSMA, MACA • (c) “Taking turns” MAC protocols: polling • Goal: efficient, fair, simple, decentralized CS219

12. Basic MAC Features • DCF: Carrier sense multiple access with collision avoidance (CSMA/CA) based • based on carrier sense function in PHY called Clear Channel Assessment (CCA) • CSMA/CA+ACK for unicast frames, with MAC level recovery • parameterized use of RTS/CTS to protect against hidden nodes • frame formats to support both infrastructure and ad-hoc networks • PCF (option, not been widely implemented) • centralized, polling based • restricted to infrastructure network CS219

13. CSMA/CA+ACK: 4-way handshake • MAC headers format differs per type • control frames: RTS, CTS, ACK • management frames, e.g. beacon, probe/probe response, (re)-association request/response, • data frames CS219

14. Frame Format Frame Control Field • Addressing: Address 1 Address 2 Address 3 Address 4 • Ad hoc: DA SA BSSID - • From AP: DA BSSID SA - • To AP: BSSID SA DA - • AP to AP: RA TA DA SA CS219

15. 802.11 frame priorities CS219

16. CSMA/CA+ACK explained • Reduce collision probability where mostly needed • defer access based on carrier sense • CCA from PHY and virtual carrier sense state • direct access when medium is sensed free • longer than DIFS, otherwise defer and backoff • receiver of directed frames to return ACK when • CRC correct CS219

17. •Duration field in RTS and CTS frames distribute Medium Reservation information which is stored in a Net Allocation Vector(NAV) •Defer on either NAV or “CCA” indicating Medium Busy •Use of RTS/CTS is optimal but must be implemented •Use is controlled by a RTS -Threshold parameter per station -To limit overhead for short frames CS219

18. Time-critical service via PCF CS219

19. PCF Access Procedure • Point Coordinator (PC) senses the medium at the beginning of each CFP • PC in Access Point transmits a beacon containing “CF parameter set element” when idle > PIFS • each station presets its NAV to the CFPMaxDuration from the CF Parameter Set Element in beacons from the PC CS219

20. PCF Access Procedure(cont) • after a SIFS period, PC sends one of the following: a data frame, CF-Poll frame, Data+CF-Poll frame, CF-end frame (when no traffic buffered & no polls to send at the PC) • PC maintains a polling list to select stations that are eligible to receive CF-Polls during contention-free periods. • A CF-Pollable station always responds to a CF-Poll: if no data from the station, responds with a Null Frame or a CF-ACK (no data) frame (when ACK is required); • “piggyback” ACK or Poll in the data frame whenever possible CS219

21. Further details • Alternating Contention free and contention operations under PCF control • NAV prevents contention traffic until reset by the last PCF transfer -> variable length contention free period per interval • both PCF and DCF defer to each other causing PCF burst start variations • CF-burst by polling bit in CF-down frame • immediate response by station on a CF_Poll CS219

22. Synchronization in 802.11 • All stations maintain a local timer • Timing synchronization function (TSF) • keeps timers from all stations in synch • AP controls timing in infrastructure networks • timing conveyed by periodic beacons • beacons contain timestamp for the entire BSS • timestamp from beacons to calibrate local clocks • not required to hear every beacon to stay in synch • used for power management • beacons sent at well known intervals • all station timers in BSS are synchronized CS219

23. Roaming in 802.11 CS219

24. Roaming Approach • Station decides that link to its current AP is poor • station uses scanning function to find another AP • station sends Reassociation Request to new AP • if Reassociation Response is successful • then station has roamed to the new AP • else station scans for another AP • if AP accepts Reassociation Request • AP indicates Reassociation to the Distribution System • Distribution System information is updated • normally old AP is notified thru distributation system CS219

25. Scanning • Scanning required for many functions • finding and joining a network • finding a new AP while roaming • initializing an ad hoc network • 802.11 MAC uses a common mechanism • Passive scanning • by listening for Beacons • Active Scanning • probe + response CS219

26. Steps to Association: Station sends Probe APs send Probe Response Station selects best AP: Station sends Association Request to select AP AP sends Association Response Active scanning CS219

27. Power Management • A station can be in one of three states: - Transmitter on - Receiver only on - Dozing: Both transmitter and receivers off • Access point (AP) buffers traffic for dozing stations • AP announces which stations have frames buffered. Traffic indication map included in each beacon. All multicasts/broadcasts are buffered. • Dozing stations wake up to listen to the beacon. If there is data waiting for it, the station sends a poll frame to get the data. CS219

28. Congestion Avoidance:IEEE 802.11 DCF • Before transmitting a packet, randomly choose a backoff interval in the range [0,cw] • cw is the contention window • Direct access when medium is sensed free longer than DIFS, otherwise defer and backoff • “Count down” the backoff interval when medium is idle • Count-down is suspended if medium becomes busy • When backoff interval reaches 0, transmit packet (or RTS) CS219

29. B1 = 25 B1 = 5 wait data data wait B2 = 10 B2 = 20 B2 = 15 DCF Example (count down) Let cw = 31 B1 and B2 are backoff intervals at nodes 1 and 2 CS219

30. Congestion Avoidance • The time spent counting down backoff intervals contributes to MAC overhead • Choosing a large cwleads to large backoff intervals and can result in larger overhead • Choosing a small cw leads to a larger number of collisions (more likely that two nodes count down to 0 simultaneously) CS219

31. Congestion Control • Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage congestion is needed • IEEE 802.11 DCF: Congestion control achieved by dynamically adjusting the contention window cw CS219

32. Binary Exponential Backoff in DCF • When a node fails to receive CTS in response to its RTS, it increases the contention window • cw is doubled (up to an upper bound – typically 5 times) • When a node successfully completes a data transfer, it restores cw to CWmin CS219

33. MILD Algorithm in MACAW[Bharghavan94Sigcomm] • When a node fails to receive CTS in response to its RTS, it multiplies cw by 1.5 • Less aggressive than 802.11, which multiplies by 2 • When a node successfully completes a transfer, it reduces cw by 1 • More conservative than 802.11, where cw is restored to Cwmin • 802.11 reduces cw much faster than it increases it • MACAW: cw reduction slower than the increase • Exponential Increase Linear Decrease • MACAW can avoid wild oscillations of cw when congestion is high CS219

34. A B C D Fairness Issue • Many definitions of fairness plausible • Simplest definition: All nodes should receive equalbandwidth • Observation: unfairness occurs when one node has backed off much more than some other node Two flows CS219

35. A B C D Fairness Issue • Assume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collide • Nodes A and B then choose from range [0,63] • Node A chooses 4 slots and B choose 60 slots • After A transmits a packet, it next chooses from range [0,31] • It is possible that A may transmit several packets before B transmits its first packet Two flows CS219

36. MACAW Solution for Fairness • When a node transmits a packet, it appends its current cw value to the packet • All nodes hearing that cw value use it for their future transmission attempts • The effect is to reset all competing nodes to the same ground rule CS219

37. Distributed Fair Scheduling (DFS) [Vaidya Mobicom00] • A fully distributed algorithm for achieving weighted fair queueing: Assign a weight to each node • Goal: bandwidth used by each node should be proportional to the weight assigned to the node • Chooses backoff intervals proportional to (packet size / weight) • DFS attempts to mimic the centralized Self-Clocked Fair Queueing algorithm • Works well on a LAN CS219

38. B1 = 10 B1 = 5 B1 = 15 wait wait Collision ! data data B2 = 5 B2 = 5 B2 = 5 Distributed Fair Scheduling (DFS) B1 = 15 (DFS actually picks a random valuewith mean 15) B2 = 5 (DFS picks a value with mean 5) Weight of node 1 = 1 Weight of node 2 = 3 Assume equal packet size CS219

39. Performance Improvement for 802.11-based Wireless Networks [L. Zhang ICC06] • Problem with WLANs • Every packet need the AP to forward • The AP has the same priority with wireless stations to access the wireless channel • Motivation • Make the AP with higher priority • The AP send a frame immediately after receiving a frame from the WS CS219

40. Action for the AP • The AP must be involved in any communication. • If the AP is the receiver, it will set its backoff time counter to be zero • the AP should obtain the channel immediately and send the data, since its backoff time counter is zero. • As all wireless stations has increased their backoff time counter by one after the communication, there is no collision. • As a result, the AP can send one frame, after any wireless station sending a frame. It will not be the bottleneck anymore. CS219

41. Action for Wireless Stations • In backoff procedure, the backoff counter is • decremented while the medium is sensed idle, • frozen when a transmission is detected on the channel. • increased by one If the sender is one of other wireless stations (except when the backoff counter is already at its maximum) • reactivated when the channel is sensed idle again • The station transmits a frame when the backoff counter reaches zero. CS219

42. Model: a discrete-time Markov chainfor two-dimensional process {s (t), b (t)} s (t) - stochastic process - backoff stage b (t) - stochastic process - backoff-time counterq - probability that at least one station transmits CS219

43. Goodput Analysis • Throughput • Goodput G – sum of the end-to-end throughput in WLAN CS219

44. Results - UDP Goodput performance compare for UCP pair scenario Fairness performance compare CS219

45. MAC Enhancements for QoS: IEEE 802.11e • The major enhancement of 802.11e • Traffic differentiation • Concept of transmission opportunity (TXOP) • Enhanced DCF (contention-based) • HCF(Hybrid Coordination Function) controlled channel access (contention free) • Burst ACK (optional) • Direct link protocol (DLP) CS219

46. IEEE 802.11e MAC Architecture • Hybrid Coordination Function (HCF): TGe (Group E) proposes HCF to provide QoS for real-time applications CS219

47. HCF - Introduction • HCF combines functions from the DCF and PCF with enhanced QoS-specific mechanisms • HCF consists of • Enhance DCF (EDCF) for contention-based access: provides differentiated access to the WM (Wireless Mobility) for 8 priorities for non-AP STAs (stations) • Controlled Access for contention-free access CS219