1 Wireless Networking Primer (few topics that may help in understanding other lectures) Nitin Vaidya University of Illinois at Urbana-Champaign

2 What Makes Wireless Interesting?  Absence of wires facilitate mobility  Signal attenuation  Spatial reuse  Diversity Multi-user diversity Antenna diversity Time diversity Frequency diversity  Wireless devices often battery-powered  Broadcast medium makes it easier to snoop on, or tamper with, wireless transmissions

3 Transmission “Range” Whether a transmission is received reliably or not depends on  Transmit power level  Channel conditions (time-varying)  Interference (time-varying)  Noise (not deterministic)  Packet size  Modulation scheme (bit rate)  Error control coding  Transmission rate  Transmission not received by all “neighbors” reliably  Not all nodes can “hear” each other  Time-varying outcome of transmissions

4 Medium Access Protocol (MAC) Wireless channel is a shared medium, requiring suitable MAC protocol. Performance of the MAC protocol depends on  Channel properties  Physical capabilities  Single interface?  One packet at a time?  One channel at a time?  Antenna diversity? Assume single interface, single channel, single antenna, one packet at a time, small propagation delay

5 “Basic” Protocol  Simple rule (a distributed protocol): Transmit packet immediately (if not transmitting already) Shortcomings  No provision for reliability  No detection of “collisions”

6 Reliability: A Retransmission Protocol  Stop-and-wait

7 A Mechanism to Reduce Collision Cost Packet loss may occur due to collisions. To reduce cost:  “Reserve” the wireless channel before transmitting data  Send short control packets for reservation  Collision may occur for control packets, but they are short  lower collision cost  Once channel reserved, data transmission (hopefully) reliable

8 RTS-CTS Exchange  Node A sends RTS to B Duration of proposed transmission specified in RTS  B responds with CTS  Host A sends data  Other hosts overhearing RTS keep quiet for duration of proposed transmission  Works alright if all nodes within “range” of each other

9 RTS-CTS  RTS-CTS reduce collision cost  If data packets too small, sending RTS-CTS not beneficial A possible implementation: Send RTS-CTS only for data packets with size > RTS-threshold

10 Carrier Sense Multiple Access (CSMA) (to reduce collisions)  Listen-before-you-talk  A host may transmit only if the channel is sensed as idle

11 Carrier Sensing (Approximation) Implementation using Carrier Sense (CS) threshold Pcs  If received signal power < CS threshold  Channel idle  Else channel busy In reality, efficacy of carrier sensing affected by noise & interference.

12 Carrier Sense Multiple Access (CSMA)  D perceives idle channel although A is transmitting A B C D distance power D’s CS Threshold

13 Carrier Sense Multiple Access (CSMA)  D perceives busy channel when A transmits A B C D distance power D’s CS Threshold

14 Trade-Off  Large carrier sense threshold  More transmitters  Greater spatial reuse & more interference  Trade-off between spatial reuse and interference

15 Impact of CS Threshold on Interference  Suppose C transmits even though A is already transmitting A B C D Path gain g = received power / transmit power

16 Hidden Terminals

17 Hidden & Exposed Terminals  Collisions may occur despite carrier sensing  Smaller carrier sensing threshold can help  But increases the incidence of exposed terminals ?

18 Hidden & Exposed Terminals  Cannot eliminate all collisions using carrier sensing  Trade-off between hidden and exposed terminals  Optimal carrier sense threshold function of network “topology” and traffic characteristics

19 Collision Detection  Ethernet uses carrier sensing & collision detection (CSMA/CD)  Transmitter also listens to the channel  Mismatch between transmitted & received signal indicates mismatch  Stop transmitting immediately once collision is detected  Reduces time lost on a collision

20 Collision Detection in Wireless Networks  Receiving while transmitting: Received signal dominated by transmitted signal  Collision occurs at receiver, not the transmitter  Collision detection difficult at the transmitter without feedback from the receiver

21 Solutions for Hidden Terminals  Busy-tone  Virtual carrier sensing  Carrier sensing mechanism discussed earlier will be referred to as physical carrier sensing, to differentiate with virtual carrier sensing

22 Virtual Carrier Sensing  RTS specifies duration of transmission  CTS also includes the duration  Any host hearing RTS or CTS stay silent as shown CTS RTS

23 Virtual Carrier Sensing  Host C may not receive RTS and still cause collision at host B  SINR = Signal-to-interference-and-noise ratio = S / (I + N)  Assume “SINR-threshold model”  assume that SINR  necessary/sufficient for reliable delivery (approximation of reality)

24  SINR for RTS reception at C is upper bounded as  If C transmits while A is receiving an Ack from B, SINR for Ack reception at A is upper bounded as CTS RTS

25  It is possible to find path gains for which we have and 

26 Virtual Carrier Sensing  C’s silent interval below is not adequate to ensure reliable Ack reception at A  Similarly, D’s silent interval not adequate to ensure reliable data reception at B CTS RTS

27 Virtual Carrier Sensing - Modification  Greater protection from interference  Reduce book-keeping with multiple nearby transmitters

28 “Space Reserved” by Virtual CS RTS Reminder: “Range” not necessarily circular in practice

29 Physical & Virtual CS  Physical carrier sensing (PCS) & virtual carrier sensing (VCS) may be used simultaneously  Channel assumed idle only if both PCS and VCS indicate that the channel is idle

30 Backoff Intervals  Channel sensing not enough to prevent multiple nodes to start transmitting “at nearly the same time”  Reduce such collisions by controlling access probability  Implementation using backoff intervals:  Choose backoff interval uniformly in range [0, cw-1]  Initialize a counter by this value  Decrement counter after each slot if channel detected idle  Transmit when counter reaches 0

31 Responding to Packet Loss  To reduce collisions due to excessive load on the channel, access probability should be reduced  May be achieved by increasing the window over which backoff interval is chosen  Exponential backoff : [0,c-1]  [0,2c-1]

32 IEEE Distributed Coordination Function (DCF)  Physical & virtual carrier sensing (RTS-CTS)  Contention window (cw) : Backoff chosen uniformly in [0,cw]  Exponential backoff after a packet loss  Contention window reset to CWmin on a success

33 Infrastructure-Based Networks

34 Hybrid Networks Ad Hoc Networks

35 Routing Protocols for Mobile Ad Hoc Networks (MANET)  Proactive protocols  Determine routes independent of traffic pattern  Traditional link-state and distance-vector routing protocols are proactive (and could be extended for MANET)  Reactive protocols  Maintain routes only if needed  Hybrid protocols Similar solutions may be used in “mesh” networks

36 Example of Reactive Routing: Dynamic Source Routing (DSR) [Johnson96]  When node S wants to send a packet to node D, but does not know a route to D, node S initiates a route discovery  Source node S floods Route Request (RREQ)  Each node appends own identifier when forwarding RREQ

37 Route Discovery in DSR B A S E F H J D C G I K Z Y Represents a node that has received RREQ for D from S M N L

38 Route Discovery in DSR B A S E F H J D C G I K Represents transmission of RREQ Z Y Broadcast transmission M N L [S] [X,Y] Represents list of identifiers appended to RREQ

39 Route Discovery in DSR B A S E F H J D C G I K Node H receives packet RREQ from two neighbors: potential for collision Z Y M N L [S,E] [S,C]

40 Route Discovery in DSR B A S E F H J D C G I K Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once Z Y M N L [S,C,G] [S,E,F]

41 Route Discovery in DSR B A S E F H J D C G I K Z Y M Nodes J and K both broadcast RREQ to node D Since nodes J and K are hidden from each other, their transmissions may collide N L [S,C,G,K] [S,E,F,J]

42 Route Discovery in DSR B A S E F H J D C G I K Z Y Node D does not forward RREQ, because node D is the intended target of the route discovery M N L [S,E,F,J,M]

43 Route Discovery in DSR  Destination D on receiving the first RREQ, sends a Route Reply (RREP)  RREP is sent on a route obtained by reversing the route appended to received RREQ  RREP includes the route from S to D on which RREQ was received by node D

44 Route Reply in DSR B A S E F H J D C G I K Z Y M N L RREP [S,E,F,J,D] Represents RREP control message

45 Dynamic Source Routing (DSR)  Node S on receiving RREP, caches the route included in the RREP  When node S sends a data packet to D, the entire route is included in the packet header  hence the name source routing  Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded

46 Data Delivery in DSR B A S E F H J D C G I K Z Y M N L DATA [S,E,F,J,D] Packet header size grows with route length

47 DSR Optimization: Route Caching  Each node caches a new route it learns by any means  When node S finds route [S,E,F,J,D] to node D, node S also learns route [S,E,F] to node F  When node K receives Route Request [S,C,G] destined for node, node K learns route [K,G,C,S] to node S  When node F forwards Route Reply RREP [S,E,F,J,D], node F learns route [F,J,D] to node D  When node E forwards Data [S,E,F,J,D] it learns route [E,F,J,D] to node D  A node may also learn a route when it overhears Data packets

48 Use of Route Caching  When node S learns that a route to node D is broken, it uses another route from its local cache, if such a route to D exists in its cache. Otherwise, node S initiates route discovery by sending a route request  Node X on receiving a Route Request for some node D can send a Route Reply if node X knows a route to node D  Use of route cache  can speed up route discovery  can reduce propagation of route requests

49 Use of Route Caching B A S E F H J D C G I K [P,Q,R] Represents cached route at a node (DSR maintains the cached routes in a tree format) M N L [S,E,F,J,D] [E,F,J,D] [C,S] [G,C,S] [F,J,D],[F,E,S] [J,F,E,S] Z

50 Use of Route Caching: Can Speed up Route Discovery B A S E F H J D C G I K Z M N L [S,E,F,J,D] [E,F,J,D] [C,S] [G,C,S] [F,J,D],[F,E,S] [J,F,E,S] RREQ When node Z sends a route request for node C, node K sends back a route reply [Z,K,G,C] to node Z using a locally cached route [K,G,C,S] RREP

51 Route Caching: Beware!  Stale caches can adversely affect performance  With passage of time and host mobility, cached routes may become invalid  A sender host may try several stale routes (obtained from local cache, or replied from cache by other nodes), before finding a good route  Can affect higher layer performance adversely (e.g., TCP) [Holland99]

52 Rate Region  Rate region characterizes rates that can be supported simultaneously on various links  Useful in determining a transmission “schedule” 1 Feasible Rate vector l 1

53 Rate Region Rate region = all feasible rate vectors Determined by  Channel state  Power constraints  Physical capabilities & constraints: Examples: Use multiple channels simultaneously? Number of interfaces

54 Rate Region Simple example scenarios  Downlink scenario (common transmitter)  Uplink scenario (common receiver) B 2 1 B 2 1

55 Downlink Scenario  Treating interference as noise B 2 1

56 Downlink Scenario: Treating Interference as Noise W = 10 MHz P = 1 mW

57 Downlink Scenario: Treating Interference as Noise  Power-sharing

58 Downlink Scenario  Power-sharing & Time-sharing

59 Downlink Scenario: Power-sharing & Bandwidth sharing

60 Downlink Scenario: Successive Interference Cancellation B 2 1 At node 1, treat other Signal as interference

61 Downlink Scenario: Successive Interference Cancellation B 2 1 At node 2, “cancel” the interference 

62 Downlink Scenario: Successive Interference Cancellation B 2 1  Decode signal for 1, and “cancel” it  Decode signal for 2

63 Downlink Scenario: Successive Interference Cancellation

64 For more information …  See tutorials at  UIUC course ECE/CS 439 Wireless Networks slides at