1. Nitin Vaidya University of Illinois at Urbana-Champaign
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 B responds with CTS
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 D B C A
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
RTS
CTS

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
RTS
CTS

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
RTS
CTS

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

28. “Space Reserved” by Virtual CS
RTS
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 802.11 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 Y Z S E F B C M L J A G H D K I N
Represents a node that has received RREQ for D from S

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

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

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

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

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

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

50. Use of Route Caching: Can Speed up Route Discovery
[S,E,F,J,D]
[E,F,J,D] S E
[F,J,D],[F,E,S] F B
[J,F,E,S] C M L
[G,C,S] J A G
[C,S] H D K
[K,G,C,S] I N
RREP
RREQ Z
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

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” l 1
Feasible
Rate vector 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 CancellationB 2 1
At node 1, treat other
Signal as interference

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

62. Downlink Scenario: Successive Interference CancellationB 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