Wired Communication

Pros
Very reliable
For Ethernet, medium HAS TO PROVIDE a Bit Error Rate (BER) of 10 -12 (one error every one trillion bits!)  Insulated wires; wires placed underground and in walls EE 122: Wireless  Error Correction Techniques
Very high transfer rates Ion Stoica (and Brighten Godfrey)
Up to 100-Gbit/s or more
Long distance TAs: Lucian Popa, David Zats and Ganesh Ananthanarayanan
Up to 40km (~25 miles) in 10-Gbit/s Ethernet (cutting edge) http://inst.eecs.berkeley.edu/~ee122/
Cons (Materials with thanks to Vern Paxson, Jennifer Rexford,
Expensive to set up infrastructure and colleagues at UC Berkeley)
Infrastructure is fixed once set up 2
No mobility

Wireless Communication Goals for Today’s Lecture
Pros
Allows mobility
Characteristics of Wireless Media
Much cheaper and easier to deploy, change, and
802.11 Architecture and Media Access Control upgrade! Protocol
Cons
Collision Detection vs. Collision Avoidance
Exposed (unshielded) medium
Hidden Terminal and Exposed Terminal Problem
Susceptible to physical phenomena (interference)
Request To Send (RTS) / Clear To Send(CTS)
Variable BER -- Error correction may not suffice in all cases mechanism
Slower data rates for wider distances
Multihop Wireless Networks
OSI layered stack designed for wired medium
Sensor Networks
Difficult to “hide” underlying behavior
TCP over Multihop Networks
Security: anyone in range hears transmission
3 Wireless Security 4

Wireless Communication Wireless Link Characteristics Standards (Alphabet Soup) (Figure Courtesy of Kurose and Ross) Ethernet

Cellular
2G: GSM (Global System for Mobile communication), CDMA (Code division multiple access )
3G: CDMA2000
IEEE 802.11
A: 5.0Ghz band, 54Mbps ( 25 Mbps operating rate )
B: 2.4Ghz band, 11Mbps ( 4.5 Mbps operating rate )
G: 2.4Ghz, 54Mbps ( 19 Mbps operating rate )
Other versions to come.
IEEE 802.15 – lower power wireless
802.15.1: 2.4Ghz, 2.1 Mbps (Bluetooth)
802.15.4: 2.4Ghz, 250 Kbps (Sensor Networks) 5 6

1 Other Wireless Link Characteristics Path Loss

Path loss
Signal attenuation as a function of distance
Signal power attenuates by about ~r 2 factor for
Signal-to-noise ratio (SNR—Signal Power/Noise Power) omni-directional antennas in free space decreases, make signal unrecoverable
Where r is the distance between the sender and the
Multipath Propagation receiver
Signal reflects off surfaces, effectively causing self-
The exponent in the factor is different depending on interference placement of antennas
Interference from other sources
Less than 2 for directional antennas
Internal Interference
Faster Attenuation
Hosts within range of each other collide with one another’s
Exponent greater than 2 when antennas are placed on the ground transmission (remember Aloha)
Signal bounces off the ground and reduces the power of the signal
External Interference
Microwave is turned on and blocks your signal 7 8

Multipath Effects A Wireless Link? (courtesy of Gilman Tolle and Jonathan Hui, ArchRock) Ceiling

S R Floor

Signals bounce off surface and interfere with one another
What signals are out of phase?
Orthogonal signals cancel each other and nothing is received!

9

A Wireless Link! The Amoeboed “cell” (courtesy of Gilman Tolle and Jonathan Hui, ArchRock) (courtesy of David Culler, UCB)

Signal

Noise

Distance

12

2 Wireless Bit Errors 802.11 Architecture

The lower the SNR (Signal/Noise) the higher the Bit Error Rate (BER)
How can we deal with this? 802.11 frames exchanges
Make the signal stronger
Why is this not always a good idea? 802.3 (Ethernet) frames exchanged
Increased signal strength requires more power
Increases the interference range of the sender, so you
Designed for limited interfere with more nodes around you geographical area
AP’s (Access Points) are set to specific channel and broadcast
Error Correction schemes can correct some beacon messages with SSID and MAC Address periodically problems
Hosts scan all the channels to discover the AP’s
Host associates with AP (actively or passively) 13 14

Ethernet vs 802.11

Wireless MAC design
Why not just use Ethernet algorithms? 5 Minute Break
Ethernet: one shared “collision” domain
It’s technically difficult to detect collisions
Collisions are at receiver, not sender
… even if we could, it wouldn’t work Questions Before We Proceed?
Different transmitters have different coverage areas
In addition, wireless links are much more prone to loss than wired links
Carrier Sense (CSMA) is OK; detection (CD) is not

15 16

Hidden Terminals Exposed Terminals

ABC D A B C

transmit range
Exposed node : B sends a packet to A; C hears this and decides not to send a packet to D (despite the fact that this will not cause interference)!
A and C can both send to B but can’t hear each other
A is a hidden terminal for C and vice versa
CSMA/ CD will be ineffective – need to sense at receiver

17 18

3 CSMA/CA: CSMA w/ Collision Multiple Access with Collision Avoidance Avoidance (MACA) other node in sender receiver sender’s range RTS
Since we can’t detect collisions, we try to avoid CTS them data
When medium busy, choose random interval ACK (contention window )
Wait for that many idle timeslots to pass before
Before every data transmission sending
Sender sends a Request to Send (RTS) frame containing the length of the transmission
When a collision is inferred, retransmit with binary
Receiver respond with a Clear to Send (CTS) frame exponential backoff (like Ethernet)
Sender sends data
Receiver sends an ACK; now another sender can send data
Use ACK from receiver to infer “no collision”
When sender doesn’t get a CTS back, it assumes collision
Use exponential backoff to adapt contention window 19 20

MACA, con’t MACA, con’t

other node in other node in receiver sender receiver sender sender’s range sender’s range RTS RTS

CTS CTS data data data

ACK

If other nodes hear RTS, but not CTS: send
If other nodes hear RTS, but not CTS: send
Presumably, destination for first sender is out of
Presumably, destination for first sender is out of node’s range … node’s range …
… Can cause problems when a CTS is lost
When you hear a CTS, you keep quiet until scheduled transmission is over (hear ACK) 21 22

RTS / CTS Protocols (MACA) 802.11 Stack View

RTS A B C D CTS

MACA = Multiple Access with Collision Avoidance Overcome exposed/hidden terminal problems with contention-free protocol
CSMA/CA runs over the 802.11 physical layer 1. B stimulates C with Request To Send (RTS) 2. A hears RTS and defers (to allow C to answer)
Link-level acknowledgements for every frame 3. C replies to B with Clear To Send (CTS) sent 4. D hears CTS and defers to allow the data 5. B sends to C 23 24

4 Link-Layer Acknowledgements Channelization of spectrum Kurose and Ross
Typically, available frequency spectrum is split into multiple channels
Receiver acks every
Some channels may overlap data packet 3 channels 8 channels 4 channels

26 MHz 100 MHz 200 MHz 150 MHz

If ACK is lost, source 915 MHz2.45 GHz 5.25 GHz 5.8 GHz tries again until a maximum 250 MHz 500 MHz 1000 MHz retransmission number is reached 24.125 GHz 61.25 GHz 122.5 GHz 25 26

Preventing Collisions Altogether Using Multiple Channels

802.11: AP’s on different channels
Frequency Spectrum partitioned into several
Usually manually configured by administrator channels
Automatic Configuration may cause problems
Nodes within interference range can use separate channels
Most cards have only 1 transceiver
B Not Full Duplex: Cannot send and receive at the A C same time D
Multichannel MAC Protocols
Automatically have nodes negotiate channels
Channel coordination amongst nodes is necessary
Now A can send B while C sends to D without any
Introduces negotiation and channel-switching latency that interference! reduce throughput
Aggregate Network throughput doubles 27 28

Wireless Multihop Networks Large Multihop Network (courtesy of Sanjit Biswas, MIT)

Vehicular Networks
Delay Tolerant (batch) sending over several hops carry data to a base station
Common in Sensor Network for periodically transmitting data
Infrastructure Monitoring
E.g., structural health monitoring of the Golden Gate Bridge
Multihop networking for Internet connection sharing
Routing traffic over several hops to base station connected to Internet

E.g., Meraki Networks 1 kilometer 29

5 Multi-Hop Wireless Ad Hoc Networks Multi-Hop Wireless Ad Hoc (Courtesy of Tianbo Kuang and Carey Williamson University of Calgary) Networks

S S

A C A C B B D D

R R

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Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc

Networks (Assume ideal world…) Networks

1 2 2 3 3 4 4 5 5 6 S 6 S 1 7 7 8 8 9 9 10 10 11 11 12 12 A C A C B B D D

R R

33 34

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

3 4 4 5 5 6 S 2 6 S 3 7 7 8 8 9 9 10 10 1 11 11 12 1 12 2 A C A C B B D D

R R

35 36

6 Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

5 6 S 4 6 S 7 7 5 8 8 9 9 10 10 11 2 11 3 12 3 1 12 4 2 A C A C B B 1 D D

R R

37 38

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

S 6 S 7 7 8 8 9 9 10 10 11 4 3 11 5 12 5 12 6 4 A C A C B B 2 D D 3 1 1 2

R R

39 40

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

S 8 S 9

9 10 10 11 6 11 7 12 7 5 12 8 6 A C A C B B D 4 D 5 1 1 2 2 3 3 4 R R

41 42

7 Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

S S 10 11

11 8 8 12 9 7 12 9 A C A 10 C B B D 6 D 7 1 1 2 2 3 3 4 4 5 5 R R 6

43 44

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

S 12 S

10 11 10 11 9 12 A C A C B B D 8 D 9 1 1 2 2 3 3 4 4 5 5 R 6 R 6 7 7 8

45 46

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

S S

12 11 12 A C A C B B D 10 D 11 1 1 2 2 3 3 4 4 5 5 R 6 R 6 7 7 8 8 9 9 10

47 48

8 Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

S S

A C A C B B D 12 D 1 1 2 2 3 3 4 4 5 5 R 6 R 6 7 7 8 8 9 9 10 10 11 11 12 49 50

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks (Reality check…) Problem 2: can’t use both 1 1 of these links at same time 2 Problem 1: node A can’t use both 2 3 3 - range overlap at A 4 of these links at the same time 4 5 5 - “hidden node” problem 6 S - shared wireless channel 6 S 7 7 - “exposed node” problem 8 - transmit or receive, but not both 8 9 9 10 10 11 11 12 12 A C A C B B D D

R R

51 52

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

1 Problem 3: LOTS of 1 2 2 RTS 3 contention for the channel 3 CTS 4 4 5 - in steady state, all want to send 5 6 S 6 S 7 - need RTS/CTS to resolve contention 7 8 8 9 9 10 10 11 11 12 12 A C A C B B D D

RTS: Request-To-Send R R CTS: Clear-To-Send

53 54

9 Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

2 2 3 3 RTS CTS 4 4 5 5 6 S 1 6 S 7 7 8 8 9 9 10 10 11 11 12 12 A C A 1 C B B D D

R R

55 56

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

2 2 3 3 4 4 5 5 6 S 6 S 7 7 8 8 9 9 10 10 11 11 12 1 12 1 A C A C B B D D

R R

57 58

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

3 3 4 4 5 5 6 S 2 6 S 7 7 8 8 9 9 10 10 11 11 12 1 12 2 A C A C B B D D 1

R R

59 60

10 Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

3 4 4 5 5 6 S 6 S 3 7 7 8 8 9 9 10 10 11 11 12 2 12 2 A C A C B B D D 1 1

R R

61 62

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

4 4 5 5 6 S 6 S 7 7 8 8 9 9 10 10 11 11 3 12 3 12 A C A C B B D 2 D 1 1 2

R R

63 64

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

5 5 6 S 4 6 S 7 7 8 8 9 9 10 10 11 11 12 3 12 4 A C A C B B D D 3 1 1 2 2

R R

65 66

11 Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

1 Problem 4: TCP uses ACKS to 2 3 indicate reliable data delivery 4 5 5 - bidirectional traffic (DATA, ACKS) 6 S 6 S 7 7 - even more contention !!! 8 8 9 9 10 10 11 11 12 4 12 A C A C B B D D 1 2 3 R R

67 68

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

2 2 3 3 4 4 5 5 6 S 1 6 S 7 7 8 8 9 9 10 10 11 11 12 12 1 A C A C B B D D

R R

69 70

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

2 3 3 4 4 5 5 6 S 6 S 2 7 7 8 8 9 9 10 10 11 11 12 1 12 1 A C A C B B D D

R R

71 72

12 Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

3 3 4 4 5 5 6 S 6 S 7 7 8 8 2 9 9 10 10 11 11 2 1 12 2 12 TCP A A C C ACK B B 2 1 D 1 D 1 1

R R

73 74

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

4 4 5 5 6 S 3 6 S 7 7 8 8 9 9 10 10 11 11 12 2 1 12 1 A C A 3 C B B D D 2 1 1

R R

75 76

Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

4 4 5 5 6 S 6 S 7 7 8 8 1 9 9 10 10 11 11 12 1 12 A 3 C A 3 C B B D 2 D 2 1 1

R R

77 78

13 Multi-Hop Wireless Ad Hoc Multi-Hop Wireless Ad Hoc Networks Networks

4 4 5 5 6 S 6 S 7 7 8 1 8 1 9 9 10 10 11 11 12 3 12 A C A C B B 3 D 2 D 2 1 1 2

R R

79 80

Summary

Wireless connectivity provides a very different set of tradeoffs from wired
Much greater ease of deployment
Mobility
But: unprotected physical signaling
Complications due to interference , attenuated range
Leading to much more frequent loss
Hidden terminal and Exposed terminal problems motivate need for a different style of Media Access Control: CSMA/CA
Multihop provides applications to sensornets, citynets
But additional complications of routing, contention
Wireless devices bring new security risks
Next lecture: Quality of Service

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