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Networks: Wireless MAC and Routing Nick Feamster CS 7260 April 10, 2006 Today s Lecture Overview What s different about wireless networks? Challenges Media Access Control (MAC) Hidden terminal and Exposed terminal CSMA/CD Reservation-based MAC: RTS/CTS Routing Traditional routing protocols Biswas et al., Opportunistic Routing in Multi-hop Wireless Networks 2 What is a Wireless Network? Wireless: without wires Many ways to communicate without wires Optical Acoustic Radio Frequency (RF) Many possible configurations Point-to-point (e.g., microwave communications links) Point-to-multipoint (e.g., cellular communications) Ad-hoc, (e.g., sensor networks) 3 Wireless Communications Networks Wireless LANs: 802.11 Cellular Networks 2G, 3G, 4G Networks Voice and data (e.g., EVDO) Point-to-Point Microwave Networks Satellite Communications Short-Range: Bluetooth, etc. Ultra-wideband Networks 4 Applications of Wireless Networking Cellular phones Wireless LANs Metro-area networks Medical equipment 5 802.11 ( Wi-Fi ) Overview Throughput 802.11b 11 Mb/s 802.11a 54 Mb/s Frequency Bands of Operation 802.11b, g: 2.400 2.4835 GHz 802.11a: 5-6 GHz range RF Formats 802.11b utilizes frequency hopping, CDMA, and CCK modulations 802.11a utilizes Orthogonal Frequency Division Multiplexing (OFDM) 6 Differences from the Wired Network Sharing and resource management Wired network: no interference below network layer Wireless networks: interference can occur at the physical layer Closest analog in the wired network: Ethernet on a hub-based network Difference: Collision detection easier in wireless network 7 Challenges in Wireless Networking Resource sharing Routing Challenge: coping with probabilistic packet reception Achieving high throughput Challenge: determining capacity of a wireless network Mobility TCP performance Energy-efficiency 8 Carrier Sense Multiple Access (CSMA) Listen to medium and wait until it is free (no one else is talking) Wait a random backoff time Advantage: Simple to implement Disadvantage: Cannot recover from a collision 9 Wireless Interference Two transmitting stations interfere with each other at the receiver Receiver gets garbage A B C 10 Carrier Sense Multiple Access with Collision Detection (CSMA-CD) Procedure Listen to medium and wait until it is free Start talking, but listen to see if someone else starts talking too If collision, stop; start talking after a random backoff time Used for hub-based Ethernet Advantage: More efficient than basic CSMA Disadvantage: Requires ability to detect collisions More difficult in wireless scenario 11 Collision Detection in Wireless No fate sharing of the link High loss rates Variable channel conditions Radios are not full duplex Cannot simultaneously transmit and receive Transmit signal is stronger than received signal 12 Solution: Link-Layer Acknowledgments Absence of ACK from receiver signals packet loss to sender Sender interprets packet loss as being caused by collision Problem: Does not handle hidden terminal cases. 13 Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) Similar to CSMA but control frames are exchanged instead of data packets RTS: request to send CTS: clear to send DATA: actual packet ACK: acknowledgement 14 Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) Small control frames lessen the cost of collisions (when data is large) RTS + CTS provide virtual carrier sense protects against hidden terminal A B 15 Random Contention Access Slotted contention period Used by all carrier sense variants Provides random access to the channel Operation Each node selects a random backoff 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 16 802.11 DCF B1 = 25 wait data B2 = 20 B2 = 15 B1 = 5 data wait B2 = 10 cw = 31 B1 and B2 are backoff intervals at nodes 1 and 2 17 802.11 Contention Window Random number selected from [0,cw] Tradeoffs in setting cw Less wasted idle time Large number of collisions with multiple senders (two or more stations reach zero at once) Optimal cw for known number of contenders, packet size Computed by minimizing expected time wastage (by both collisions and empty slots) Problem: can t estimate number of contenders very easily Number of contenders could also change 18 802.11 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 cw larger 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) 19 802.11 DCF (CSMA-CA) Full exchange with virtual carrier sense Other nodes defer access for NAV A Sender Sender Receiver A B RTS CTS NAV (RTS) NAV (CTS) 20 B Receiver DATA ACK Virtual Carrier Sense Provided by RTS & CTS Prevents hidden terminal collisions Typically unnecessary RTS CTS A B C 21 Physical Carrier Sense Energy detection threshold Monitors channel during idle times between packets to measure the noise floor Energy levels above the this 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 22 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 Receive Range Carrier Sense Range Long carrier sense range helps protect from interference 23 Hidden Terminal Revisited Virtual carrier sense no longer needed in this situation RTS CTS A B C Physical Carrier Sense 24 Initial approach: Traditional routing packet packet A B packet src C dst Identify a route, forward over links Abstract radio to look like a wired link ExOR Slides adapted from http://pdos.csail.mit.edu/papers/roofnet:exor-sigcomm05/ 25 Radios aren t wires A B 3 4 56 56 1 2 3 45 6 s C d Every packet is broadcast Reception is probabilistic 26 ExOR: Probabilistic Broadcast packet A B src packet dst C Decide who forwards after reception Goal: only closest receiver should forward Challenge: agree efficiently and avoid duplicate transmissions 27 Why ExOR might increase throughput src 75% N1 50% 25% N2 N3 N4 N5 dst Best traditional route over 50% hops: 3(1/0.5) = 6 tx Throughput 1/# transmissions ExOR exploits lucky long receptions: 4 transmissions Assumes probability falls off gradually with distance 28 Why ExOR might increase throughput N1 5% 2 25% N2 N3 N4 src 25% 25 % 0% 100% 100% 10 dst % 00 1 Traditional routing: 1/0.25 + 1 = 5 tx ExOR: 1/(1 (1 0.25)4) + 1 = 2.5 transmissions Assumes independent losses 29 Batch Maps 100 tx: 9 src rx: 88 99 N1 tx: 8 rx: 57 85 N2 tx: 57 -23 24 N3 N4 rx: 23 53 tx: 23 rx: 0 40 tx: 0 dst rx: 0 22 Challenge: finding the closest node to have rx d Send batches of packets for efficiency Node closest to the dst sends first Other nodes listen, send remaining packets in turn Repeat schedule until dst has whole batch 30 Reliable summaries s t N s N N N d t s Repeat summaries in every data packet Cumulative: what all previous nodes rx d This is a gossip mechanism for summaries 31 Priority ordering N s N N N d Goal: nodes closest to the destination send first Sort by ETX metric to dst Nodes periodically flood ETX link state measurements Path ETX is weighted shortest path (Dijkstra s algorithm) Source sorts, includes list in ExOR header Details in the paper 32 Using ExOR with TCP C T N P T G W E E W P Batching requires more packets than typical TCP window 33 ExOR Evaluation Does ExOR increase throughput? When/why does it work well? 34 65 Roofnet node pairs 1 kilometer 35 Evaluation Details 65 Node pairs 1.0MByte file transfer 1 Mbit/s 802.11 bit rate 1 KByte packets Traditional Routing 802.11 unicast with linklevel retransmissions Hop-by-hop batching UDP, sending as MAC allows ExOR 802.11 broadcasts 100 packet batch size 36 ExOR: 2x overall improvement Cumulative Fraction of Node Pairs 1.0 0.8 0.6 0.4 0.2 ExOR Traditional 0 0 200 400 600 Throughput (Kbits/sec) 800 Median throughputs: 240 Kbits/sec for ExOR, 121 Kbits/sec for Traditional 37 25 Highest throughput pairs 3 Traditional Hops 2.3x Throughput (Kbits/sec) 1000 800 600 400 200 0 Node Pair ExOR Traditional Routing 2 Traditional Hops 1 Traditional Hop 1.7x 1.14x 38 25 Lowest throughput pairs Throughput (Kbits/sec) 1000 800 600 400 200 0 ExOR Traditional Routing 4 Traditional Hops 3.3x Node Pair Longer Routes 39 ExOR uses links in parallel Traditional Routing 3 forwarders 4 links ExOR 7 forwarders 18 links 40 ExOR moves packets farther 58% of Traditional Routing transmissions Fraction of Transmissions 0.6 ExOR Traditional Routing 0.2 0.1 0 25% of ExOR transmissions 0 100 200 300 400 500 600 700 800 900 1000 Distance (meters) ExOR average: 422 meters/transmission Traditional Routing average: 205 meters/tx 41

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