Wireless Communication and Networks

1. Wireless Communication and Networks Applications of Wireless Communication Wireless Communication Technologies Wireless Networking and Mobile IP Wireless Local Area Networks Student Presentations and Research Papers Wireless Medium Access http://web.uettaxila.edu.pk/CMS/AUT2012/teWCNms/

2. Outline • Medium Access Control • CSMA • CSMA-CD • CSMA-CA • Random Contention Access • 802.11 Multiple Access schemes • PCF • DCF • HCF • BA and WMM • Virtual Carrier Sense • Physical Carrier Sense • Multiple Access Problems

3. Multi-transmitter Interference Problem • Similar to multi-path or noise • Two transmitting stations will constructively/destructively interfere with each other at the receiver • Receiver will “hear” the sum of the two signals (which usually means garbage)

4. Medium Access Control • Protocol required to coordinate access • i.e. transmitters must take turns • Similar to talking in a crowded room • Also similar to hub based Ethernet

5. Carrier Sense Multiple Access (CSMA) • Procedure • Listen to medium and wait until it is free (no one else is talking) • Wait a random back off time then start talking • Advantages • Fairly simple to implement • Functional scheme that works • Disadvantages • Can not recover from a collision (inefficient waste of medium time)

6. Medium Access Control • Media Access Control (MAC) describes how the media (wired or wireless) is used • Polling, token passing, contention based • CSMA/CD is for ethernet wired networks • CSMA/CA for wireless 802.11 • Stations using either access method must first listen to see whether any other device is transmitting. If another device is transmitting, the station must wait until the medium is available.

7. Carrier Sense Multiple Accesswith Collision Detection • CSMA-CD Procedure • Listen to medium and wait until it is free • Then start talking, but listen to see if someone else starts talking too • If a collision occurs, stop and then start talking after a random back off time • This scheme is used for hub based Ethernet • Advantages • More efficient than basic CSMA • Disadvantages • Requires ability to detect collisions

8. Collision Detection Problem • Transmit signal is MUCH stronger than received signal • Due to high path loss in the wireless environment (up to 100dB) • Impossible to “listen” while transmitting because you will drown out anything you hear • Also transmitter may not even have much of a signal to detect due to geometry

9. Carrier Sense Multiple Accesswith Collision Avoidance • CSMA-CA Procedure • Similar to CSMA but instead of sending packets control frames are exchanged • RTS = request to send • CTS = clear to send • DATA = actual packet • ACK = acknowledgement

10. Carrier Sense Multiple Accesswith Collision Avoidance • Advantages • Small control frames lessen the cost of collisions (when data is large) • RTS + CTS provide “virtual” carrier sense which protects against hidden terminal collisions (where A can’t hear B) A B

11. Carrier Sense Multiple Accesswith Collision Avoidance • Disadvantages • Not as efficient as CSMA-CD • Doesn’t solve all the problems of MAC in wireless networks

12. CSMA-CD vs CSMA-CA • The difference is when the coast is clear • CSMA/CD node can immediately begin transmitting. • If a collision occurs while a CSMA/CD node is transmitting, the collision will be detected and the node will temporarily stop transmitting. • 802.11 wireless stations are not capable of transmitting and receiving at the same time, so they are not capable of detecting a collision during their transmission. • 802.11 wireless networking uses CSMA/CA instead of CSMA/CD to try to avoid collisions.

13. CSMA-CA • IF CSMA/CA station sees no transmissions it will wait a random interval • Keep watching the medium • If still clear after interval, transmit • If not, start over • Because only 1 station can use the frequency at a time • Half duplex

14. CSMA-CA • Standard Distributed Coordination Function (DCF) is defined to allow access of multiple stations • Checks and balances to keep the line clear • Minimize chances of collision

15. CSMA-CA • Since 802.11 stations cannot detect collisions, how do they know when they happen? • Every unicast frame requires an acknowledgement • Broadcast and multicast don’t • ACK Frame • When ACK Frame is received, original station knows the frame is received

16. Collision Detection • Receiving device will also check the CRC • If frame is corrupt, no ACK frame • IF no ACK frame, sender assumes delivery failure

17. Random Contention Access • Slotted contention period • Used by all carrier sense variants • Provides random access to the channel • Operation • Each node selects a random back off number • Waits that number of slots monitoring the channel • If channel stays idle and reaches zero then transmit • If channel becomes active wait until transmission is over then start counting again

18. Distributed Coordination Function (DCF) • Fundamental method of 802.11 communication • Mandatory access method • Point Coordination Functions (PCF) is optional • 802.11 also has Hybrid Coordination Function (HCF) • Four Main parts • Interframe Space • Virtual Carrier Sense • Physical Carrier Sense • Random back-off timer Pg 254

19. Interframe Space (IFS) • The amount of time between transmissions • Actual length of time depends on network speed • Short interframe space (SIFS), highest priority • PCF interframe space (PIFS), middle priority • DCF interframe space (DIFS), lowest priority • Arbitration interframe space (AIFS), used by QoS stations • Extended interframe space (EIFS), used with retransmissions Pg 254

20. Interframe Space (IFS) • Only certain types of frames are sent after certain interframe spaces • Only ACK and CTS after SIFS Pg 254

21. B1 = 25 B1 = 5 wait data data wait B2 = 10 B2 = 20 B2 = 15 802.11 DCF Example B1 and B2 are backoff intervals at nodes 1 and 2 B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31 cw = 31

22. 802.11 Contention Window • Random number selected from [0,cw] • Small value for cw • Less wasted idle slots time • Large number of collisions with multiple senders (two or more stations reach zero at once) • Optimal cw for known number of contenders & know packet size • Computed by minimizing expected time wastage (by both collisions and empty slots) • Tricky to implement because number of contenders is difficult to estimate and can be VERY dynamic

23. 802.11 Adaptive Contention Window • 802.11 adaptively sets cw • Starts with cw = 31 • If no CTS or ACK then increase to 2*cw+1 (63, 127, 255) • Reset to 31 on successful transmission • 802.11 adaptive scheme is unfair • Under contention, unlucky nodes will use larger cw than lucky nodes (due to straight reset after a success) • Lucky nodes may be able to transmit several packets while unlucky nodes are counting down for access • Fair schemes should use same cw for all contending nodes (better for high congestion too)

24. 802.11 PCF (CSMA-CA) • Optional access control method • Polling • AP is point coordinator • Only work in a BSS • Not in ad-hoc/IBSS • Both AP and client station must support PCF • AP will switch between DCF and PCF • PCF time is contention free period (CFP) • DCF is contention period (CP)

25. 802.11 DCF (CSMA-CA) • Full exchange with “virtual” carrier sense (called the Network Allocation Vector) A B Sender Receiver RTS DATA Sender CTS ACK Receiver NAV (RTS) A NAV (CTS) B B

26. Hybrid Coordination Function (HCF) • Added in 802.11e • Enhanced Distributed Channel Access (EDCA) • HCF Controlled Channel Access (HCCA) • DCF and PCF require contention for each frame • HCF defines ability to send multiple frames • Transmit Opportunity (TXOP) • During TXOP period, client station can send a frame burst • Uses Short Interframe Space (SIFS) Pg 259

27. Enhanced Distributed Channel Access (EDCA) • Provides differentiated access using eight user priority levels • Extension of DCF • Uses frame tags similar to 802.1D standard • QoS standard at MAC (layer 2) level • Define priority values • With priority queuing • Data waiting in higher priority queues transmits before lower priority queues Pg 259

28. Enhanced Distributed Channel Access (EDCA) Pg 259

29. Enhanced Distributed Channel Access (EDCA) • Defines four access categories base on User Priority level • From lowest to highest priority access category: • AC_BK (Background) • AC_BE (Best Effort) • AC_VI (Video) • AC_VO (Voice • For each category, Enhanced Distributed Cannel Access Function (EDCAF) is used • Frames with higher category have lower back off values and are more likely to get a TXOP Pg 259

30. Block Acknowledgement (BA) • Part of 802.11e • Allows for a single acknowledgement for multiple frames • Reduces overhead • Two Types • Immediate • For low latency traffic • Delayed • For latency tolerant traffic Pg 261

31. Block Acknowledgement (BA)-Immediate • Originator sends a block of QoS data frames to station • Originator requests acknowledgement of all outstanding QoS data • Block AckReq frame • Recipient can send a single ack frame for all received frames • Can request a single frame from block be retransmitter Pg 261

32. Block Acknowledgement (BA)-Immediate Pg 261

33. W i-Fi Multimedia (WMM) • For latency sensitive data • Real time voice and video • Voice, video, audio, have less tolerance of latency (cumulative delay) • 802.11e had layer 2 MAC methods to meet QoS requirements • WiFi Alliance created WMM • Because WMM is based on EDCA mechanisms, 802.1D priority tags from the Ethernet side are used to direct traffic to four access-category priority queues. The WMM certification provides for traffic prioritization via four access categories Pg 262

34. W i-Fi Multimedia (WMM) Pg 262

35. Virtual Carrier Sense • Provided by RTS & CTS • Designed to protect against hidden terminal collisions (when C can’t receive from A and might start transmitting) • However this is unnecessary most of the time due to physical carrier sense RTS CTS B A C

36. Physical Carrier Sense Mechanisms • Energy detection threshold • Monitors channel during “idle” times between packets to measure the noise floor • Energy levels above the noise floor by a threshold trigger carrier sense • DSSS correlation threshold • Monitors the channel for Direct Sequence Spread Spectrum (DSSS) coded signal • Triggers carrier sense if the correlation peak is above a threshold • More sensitive than energy detection (but only works for 802.11 transmissions) • High BER disrupts transmission but not detection

37. Physical Carrier Sense Range • Carrier can be sensed at lower levels than packets can be received • Results in larger carrier sense range than transmission range • More than double the range in NS2 802.11 simulations • Long carrier sense range helps protect from interference Receive Range Carrier Sense Range

38. Hidden Terminal Revisited • Virtual carrier sense no longer needed in this situation RTS CTS B A C

39. RTS CTS Still Useful Sometimes • Obstructed hidden terminal situation • Fast collision resolution for long data packets A B

40. Exposed Terminal Problem • Hidden terminal is not the only challenge for a distributed wireless MAC protocol • A blocks B, and C doesn’t know what is happening (B is exposed) D B C A

41. Double Exposure Problem • If A and C are out of phase, there is NO time D can transmit without causing a collision B A C D

42. Q&A ?