1. 1 CSCD 439/539 Wireless Networks and Security CSDA/CA, Frames Lecture 4 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material copyright 1996-2007

2. 2 Overview MAC Layer –CSMA/CA as opposed to Ethernet’s CSMA/CD –Mandatory and optional implementations of MAC schemes –Practical notes on MAC and alternative schemes

3. 3 Background Last time –Looked at architecture, how stations and AP’s connect within a network –Typically, 802.11 networks are attached in some way to an Ethernet or wired network Said they differ because many things can interfere with the signal which is broadcast –Compared with wired networks where signal is contained … in the wires

4. 4 802.11 MAC Introduction Key to 802.11 specification is the Medium Access Control Layer (MAC) –Recall Ethernet nodes sense the medium, transmit if the medium is idle, and listen for collisions (CSMA/CD) –If a collision is detected, after a back-off period, node retransmits the frame –Collision detection is not feasible in WLANs –Node cannot know whether signal was corrupted due to channel interference or actual collision –Thus, IEEE 802.11 uses Carrier Sense Multiple Access Collision Avoidance (CSMA/CA) Adopts collision avoidance, rather than collision detection

5. 5 Why CSMA/CD Does Not Work Collision detection problems –Hidden terminal Terminals are hidden from each other kind –Exposed terminal Because of interference, terminals don ’ t think they can send to a terminal when they can A B C A B C D HiddenExposed C can’t see A A can’t see C C doesn’t transmit to D because it senses B and A

6. 6 Hidden Terminal Effect Hidden terminals: A, C cannot hear each other –Obstacles in the way or distance –Both transmit to B, collisions at B –Collision if 2 or more nodes transmit at same time CSMA makes sense if: –You’re the only one transmitting –Able to avoid collision if you sense another transmission Yet, collision detection doesn’t work in this case, CSMA/CD – Ethernet style But, simple collision avoidance in CSMA/CA isn’t so good either bits Crash

7. 7 Exposed Terminal Effect Do this on the board …

8. 8 802.11 MAC Specifications Because there are still problems with collisions, 802.11 has several MAC choices In 802.11 there are several flavors of Access methods –MAC-DCF CSMA/CA (mandatory) Physical channel sensing Collision avoidance via randomized “back-off” mechanism Minimum distance between consecutive packets ACK packet (not for broadcasts) –MAC-DCF w/ RTS/CTS (optional) Physical AND Virtual channel sensing Distributed Foundation Wireless MAC Avoids hidden terminal problem –MAC- PCF (optional) Access point polls terminals according to a list

9. 9 802.11 MAC Specifications The DCF is the basis of the standard CSMA/CA access mechanisms –Like Ethernet, first checks that radio link is clear before transmitting –To avoid collisions, stations use a random backoff after each frame First transmitter seizes the channel –DCF may use CTS/RTS clearing technique to further reduce the possibility of collisions

10. 10 802.11 MAC Specifications DCF (Distributed Coordination Function) details 1. Carrier Sense Multiple Access Collision Avoidance CSMA/CA –Each unit senses medium before it starts to transmit Called physical channel sensing –If medium is free for several microseconds (DIFS), unit can transmit for a limited time –If medium is busy, unit will back off for a random time before it senses again

11. 11 IEEE 802.11 Medium Specifications CSMA Version of the Protocol Sense channel idle for DIFS sec (Distributed Inter Frame Space) transmit frame (no Collision Detection) Receiver returns ACK after SIFS (Short Inter Frame Space) if channel sensed busy => binary backoff (like Ethernet) NAV: Network Allocation Vector – essentially a timer is set for other stations See next slide …

12. 12 Carrier Sensing Details 802.11 requires carrier sensing to determine if medium available. Two types of carriersensing functions in 802.11 manage this process: –physical carrier-sensing –virtual carrier-sensing functions If either carrier-sensing function indicates medium is busy, MAC reports this to higher layers

13. 13 Carrier Sensing Details Physical carrier-sensing functions provided by physical layer –Depend on medium and modulation used –It is difficult or expensive to build good physical carrier-sensing hardware for RF-based media Transceivers can transmit and receive simultaneously only if they incorporate expensive electronics Most 802.11 hardware is half-duplex, can’t send/receive at the same time Plus, hidden nodes still possible so physical carrier-sensing cannot provide all the necessary information

14. 14 Carrier Sensing Details Virtual carrier-sensing is provided by the Network Allocation Vector (NAV) –Most 802.11 frames carry a duration field –Can reserve medium for a fixed time –The NAV is a timer that indicates the amount of time the medium will be reserved –Stations set NAV to time for which they expect to use the medium, –Other stations count down from the NAV to 0 –When NAV is nonzero, virtual carrier-sensing function says medium is busy –When NAV reaches 0, virtual carrier-sensing function says medium is idle

15. 15 Virtual Carrier Sensing with RTS/CTS and NAV NAV is carried in frame headers on the RTS and CTS frames; it is depicted on its own line show NAV relating to actual transmissions in the air. When NAV bar is present on NAV line, stations defer access to medium because virtual carrier-sensing mechanism indicates busy medium

16. 16 To ensure sequence is not interrupted, Node1 sets the NAV in its RTS to block access to the medium while the RTS transmitted All stations hear RTS defer access to medium until NAV elapses RTS frames not necessarily heard by every station in network So, Node2, the receiver of intended transmission responds with CTS that includes shorter NAV This NAV prevents other stations from accessing medium until transmission completes After sequence completes, the medium can be used by any station after distributed interframe space (DIFS), shown by contention window, Backoff Node1 Node2

17. 17 Interframe Spacing Like Ethernet, interframe spacing plays a large role in coordinating access to the transmission medium –802.11 uses four different interframe spaces –Three are used to determine medium access SIFS, DIFS, PIFS

18. 18 Interframe Spacing Varying interframe spacings create different priority levels for different types of traffic Logic behind this is simple: –High-priority traffic doesn’t have to wait as long after the medium is idle. –So, if any high-priority traffic waiting, it grabs network before low-priority frames have a chance –To assist with interoperability between different data rates, the interframe space is a fixed amount of time, independent of the transmission speed

19. 19 Interframe Spacing Short interframe space (SIFS) –SIFS used for highest-priority transmissions, such as RTS/CTS frames and positive acknowledgments –High-priority transmissions can begin once the SIFS has elapsed PCF interframe space (PIFS) –PIFS, sometimes erroneously called the priority interframe space, is used by the PCF during contention-free operation –Stations with data in the contention-free period can transmit after the PIFS has elapsed and preempt any contention-based traffic DCF interframe space (DIFS) –DIFS is minimum medium idle time for contention-based services –Stations may have immediate access to the medium if it has been free for a period longer than the DIFS.

20. 20 DCF Backoff After frame transmission has completed and the DIFS has elapsed, stations may attempt to transmit congestion-based data Period called, contention window or backoff window follows the DIFS Window is divided into slots –Slot length is medium dependent –Higher-speed physical layers use shorter slot times –Stations pick a random slot and wait for that slot before attempting to access the medium –All slots are equally likely selections –When several stations are attempting to transmit, the station that picks the first slot (the station with the lowest random number) wins. –As in Ethernet, backoff time is selected from a larger range each time a transmission fails

21. 21 DCF Backoff Continued … Contention window sizes are always 1 less than a power of 2 –e.g., 31, 63, 127, 255 –Each time the retry counter increases, the contention window moves to the next greatest power of two –The maximum size of the contention window is limited by the physical layer –For example, the DS physical layer limits the contention window to 1023 transmission slots

22. 22 DCF Backoff Finished When contention window reaches its maximum size, it remains there until it can be reset –Allowing long contention windows when several competing stations are trying to transmit keeps MAC algorithms stable even under maximum load –Contention window is reset to its minimum size when frames are transmitted successfully, or associated retry counter is reached, and the frame is discarded

23. 23 Practical 802.11 MAC Advice

24. 24 Practical 802.11 MAC Hidden Node Problem –Other solutions to this problem –Increase power to nodes Increasing the power of the nodes can solve the hidden node problem by allowing the cell around each node to increase in size, encompassing all of the other nodes. –Use omnidirectional antennas Since nodes using directional antennas are nearly invisible to nodes that are not positioned in the direction the antenna is aimed at, directional antennas should be used only for very small networks (e.g., dedicated point-to-point connections).

25. 25 Practical 802.11 MAC Hidden Node Problem –Other solutions to this problem Remove Obstacles –Increasing the power on your mobile nodes may not work if, for example, the reason one node is hidden is that there is a cement or steel wall preventing communication with other nodes Move the Node –Another method of solving the hidden node problem is moving the nodes so that they can all hear each other. If you have found that the hidden node problem is the result of a user moving his computer to an area that is hidden from the other wireless nodes, you may have to force that user to move again.

26. 26 Practical 802.11 MAC Frottle (Freenet throttle) is an open source GNU GPL project to control traffic on wireless networks –Such control eliminates the common hidden-node effect even on large scale wireless networks Frottle works by scheduling traffic of each client, using a master node to co-ordinate actions –Eliminates collisions, and prevents clients with stronger signals from receiving bandwidth bias Frottle developed and tested on large community wireless network of WaFreeNet We have found running frottle has given us a significant improvment in the network usability http://frottle.sourceforge.net/

27. 27 Practical 802.11 MAC More thoughts … 1. RTS/CTS has very high overhead, particularly for small packets and where stations have variable rates. May be prohibitively expensive to use 2. Most current deployments of 802.11 are based on a cellular infrastructure, and are not ad hoc Neighboring cells are usually configured to operate on different channels (frequencies) explicitly so hidden terminal problems on the downlink (to the wireless LAN clients) are actually rare On the uplink, hidden terminals could occur, but often these packets are small (e.g., TCP ACKs) and the RTS/CTS overhead is then significant

28. 28 Practical 802.11 MAC 3. In practice, many commercial WiFi cards can sense carrier as “busy” even when they can’t decode the bits, reducing the need for explicit reservations An entirely different approach to sharing wireless channels is to allocate access by time. This approach used in some cellular telephone networks, where base station determines a transmission time-schedule for clients In general, at high loads, time division makes sense; otherwise, slots are wasted Avoiding this waste in TDMA usually makes protocols more complex. At the same time, CSMA based approaches don’t perform too well when there is heavy, persistent load from a large number of nodes. Much work has been done in the community on MAC protocols, including on hybrid CSMA/TDMA protocols Ref.: http://nms.csail.mit.edu/6.829-f05/lectures/L11-wlessmac.pdf

29. 29 Measurement of RTS/CTS and CSMA/CA Other papers relating to hidden node problem and performance of MAC –http://portal.acm.org/citation.cfm?id=1143703 Looks at timing of backoff slots as solution --------------------------------------------------------------- –http://patraswireless.net/software.html Protocol booster – WiCCP is a protocol booster for 802.11b DCF based wireless networks, that provides cyclic token-passing medium access, and scheduled allocation of the available network resources, eliminating the "Hidden Node" problem.

30. 30 Summary Looked at MAC part of 802.11 –Problems encountered not found in wired networks –More complicated way of dealing with collisions on broadcast radio networks –Also, practical ways of avoiding collisions –Some research in this area …

31. 31 Finish Next time Gone on Wed. - Conference See reading on Course Notes page Assignment – Assignment page