15-446 Networked Systems Practicum

Lecture 4 – Wireless Basics

1

2

Overview

• Internet mobility

• TCP over noisy links

• Link layer challenges

3

Cellular Reuse

• Transmissions decay over distance• Spectrum can be reused in different areas• Different “LANs”• Decay is 1/R2 in free space, 1/R4 in some situations

4

IEEE 802.11 Wireless LAN

• 802.11b• 2.4-2.5 GHz unlicensed radio

spectrum

• up to 11 Mbps

• direct sequence spread spectrum (DSSS) in physical layer

• all hosts use same chipping code

• widely deployed, using base stations

• 802.11a • 5-6 GHz range

• up to 54 Mbps

• 802.11g • 2.4-2.5 GHz range• up to 54 Mbps

• 802.11n• 2.4 or 5Ghz, • multiple antennas

(MIMO), up to 450Mbps (for 3x3 antenna configuration)

• All use CSMA/CA for multiple access

• All have base-station and ad-hoc network versions

5

IEEE 802.11 Wireless LAN

• Wireless host communicates with a base station• Base station = access point (AP)

• Basic Service Set (BSS) (a.k.a. “cell”) contains:• Wireless hosts• Access point (AP): base station

• BSS’s combined to form distribution system (DS)

6

• Ad hoc network: IEEE 802.11 stations can dynamically form network without AP

• Applications:• Laptops meeting in conference room, car• Interconnection of “personal” devices

Ad Hoc Networks

7

CSMA/CD Does Not Work

• Collision detection problems• Relevant contention

at the receiver, not sender

• Hidden terminal• Exposed terminal

• Hard to build a radio that can transmit and receive at same time

A

B

C

A

B

C

D

Hidden Exposed

8

Hidden Terminal Effect

• Hidden terminals: A, C cannot hear each other• Obstacles, signal attenuation• Collisions at B • Collision if 2 or more nodes transmit at

same time• CSMA makes sense:

• Get all the bandwidth if you’re the only one transmitting

• Shouldn’t cause a collision if you sense another transmission

• Collision detection doesn’t work• CSMA/CA: CSMA with Collision

Avoidance

9

Medium Access Control

• Think back to Ethernet MAC:• Wireless is a shared medium• Transmitters interfere• Need a way to ensure that (usually) only one person

talks at a time.• Goals: Efficiency, possibly fairness

• But wireless is harder!• Can’t really do collision detection:

• Can’t listen while you’re transmitting. You overwhelm your antenna…

• Carrier sense is a bit weaker:• Takes a while to switch between Tx/Rx.

• Wireless is not perfectly broadcast

10

Example MAC Protocols

• Pure ALOHA• Transmit whenever a message is ready• Retransmit when ACK is not received

• Slotted ALOHA• Time is divided into equal time slots• Transmit only at the beginning of a time slot• Avoid partial collisions• Increase delay, and require synchronization

• Carrier Sense Multiple Access (CSMA)• Listen before transmit• Transmit only when no carrier is detected

11

“Wireless Ethernet”

• Collision detection is not practical.• Signal power is too high at the transmitter• So how do you detect collisions?

• Signals attenuate significantly with distance.• Strong signal from nearby node will overwhelm the weaker signal

from a remote transmitter• Capture effect: nearby node will always win in case of collision -

receiver may not even detect remote node• Hidden transmitter

• Two transmitters may not hear each other, which can cause collisions at a common receiver.• Hidden terminal problem• RTS/CTS is designed to avoid this

12

A B C

Hidden Terminal Problem

• B can communicate with both A and C• A and C cannot hear each other• Problem

• When A transmits to B, C cannot detect the transmission using the carrier sense mechanism

• If C transmits, collision will occur at node B

• Solution• Hidden sender C needs to defer

13

A B C

Possible Solution: RTS/CTS

• When A wants to send a packet to B, A first sends a Request-to-Send (RTS) to B

• On receiving RTS, B responds by sending Clear-to-Send (CTS), provided that A is able to receive the packet

• When C overhears a CTS, it keeps quiet for the duration of the transfer• Transfer duration is included in both RTS and CTS

14

Collision Detection & Reliability

• Impossible to detect collision using half-duplex radios

• Wireless links are prone to errors. High packet loss rate detrimental to transport-layer performance.

• Mechanisms needed to reduce packet loss rate experienced by upper layers

15

Simple Solution

• When B receives a data packet from A, B sends an Acknowledgement (ACK) to A.

• If node A fails to receive an ACK, it will retransmit the packet

A B C

16

802.11 Frame Priorities

• Short interframe space (SIFS)• For highest priority frames (e.g., RTS/CTS,

ACK)

• DCF interframe space (DIFS)• Minimum medium idle time for contention-

based services

Time

Busy SIFS

DIFS

contentwindow

Frame transmission

17

SIFS/DIFS

SIFS makes RTS/CTS/Data/ACK atomic

RTS

CTS

Data

ACKSIFS SIFS SIFS DIFS

Time

Time

Sender1

Receiver1

RTS

TimeSender2

DIFS

18

802.11 RTS/CTS

• RTS sets “duration” field in header to• CTS time + SIFS + CTS time + SIFS + data pkt

time

• Receiver responds with a CTS• Field also known as the “NAV” - network

allocation vector• Duration set to RTS dur - CTS/SIFS time• This reserves the medium for people who hear

the CTS

19

C FA B EDRTS

RTS = Request-to-Send

IEEE 802.11

assuming a circular range

20

C FA B EDRTS

RTS = Request-to-Send

IEEE 802.11

NAV = 10

NAV = remaining duration to keep quiet

21

C FA B EDCTS

CTS = Clear-to-Send

IEEE 802.11

22

C FA B EDCTS

CTS = Clear-to-Send

IEEE 802.11

NAV = 8

23

C FA B EDDATA

•DATA packet follows CTS. Successful data reception acknowledged using ACK.

IEEE 802.11

24

IEEE 802.11

C FA B EDACK

25

C FA B EDACK

IEEE 802.11

Reserved area

26

IEEE 802.11

C FA B EDDATA

Transmit “range”

Interference“range”

Carrier senserange

FA

27

IEEE 802.11 MAC Protocol: CSMA/CA

802.11 CSMA: sender- If sense channel idle for DISF

(Distributed Inter Frame Space)

then transmit entire frame(no collision detection)

- If sense channel busythen binary backoff

802.11 CSMA receiver:- If received OK

return ACK after SIFS(Short IFS)(ACK is needed due tolack of collision detection)

28

IEEE 802.11 MAC Protocol

802.11 CSMA Protocol: others

• NAV: Network Allocation Vector

• 802.11 frame has transmission time field

• Others (hearing data) defer access for NAV time units

29

Collision Avoidance Mechanisms

• Problem: • Two nodes, hidden from each other, transmit

complete frames to base station• Wasted bandwidth for long duration !

• Solution: • Small reservation packets• Nodes track reservation interval with internal

“network allocation vector” (NAV)

30

Collision Avoidance: RTS-CTS Exchange

• Explicit channel reservation• Sender: send short RTS:

request to send• Receiver: reply with short CTS:

clear to send• CTS reserves channel for

sender, notifying (possibly hidden) stations

• RTS and CTS short:• collisions less likely, of shorter

duration• end result similar to collision

detection• Avoid hidden station collisions• Not widely used/implemented

• Consider typical traffic patterns

31

Backoff Interval

• When transmitting a packet, choose a backoff interval in the range [0, CW]• CW is contention window

• Count down the backoff interval when medium is idle• Count-down is suspended if medium becomes

busy

• Transmit when backoff interval reaches 0

32

DCF Example

data

waitB1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervalsat nodes 1 and 2cw = 31

B2 = 10

33

Backoff Interval

• The time spent counting down backoff intervals is a part of MAC overhead

• Important to choose CW appropriately• large CW large overhead• small CW may lead to many collisions (when

two nodes count down to 0 simultaneously)

34

Backoff Interval (Cont.)

• Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed

• IEEE 802.11 DCF: contention window CW is chosen dynamically depending on collision occurrence

35

Binary Exponential Backoff in DCF

• When a node fails to receive CTS in response to its RTS, it increases the contention window• CW is doubled (up to an upper bound)• More collisions longer waiting time to reduce

collision

• When a node successfully completes a data transfer, it restores CW to CWmin

36

Random backoff

Data Transmission/ACKRTS/CTS

802.11 Overhead

• Channel contention resolved using backoff• Nodes choose random backoff interval from [0,

CW]• Count down for this interval before transmission

• Backoff and (optional) RTS/CTS handshake before transmission of data packet

• 802.11 has large room for improvement

37

DCF Operation

38

Discussion

• RTS/CTS/Data/ACK vs. Data/ACK• Why/when is it useful?• What is the right choice

• Simulation vs. reality?

• Understand Fig 4.

Frame Types

• Protocol Version• Frame Type and

Sub Type• To DS and From

DS• More Fragments • Retry • Power

Management • More Data • WEP• Order

FCDuration

/IDAddress

1Address

2Address

3SequenceControl

Address4

DATA FCS

2 2 6 6 6 2 6 0-2312 4 bytes

NAV informationOr Short Id for PS-

Poll

BSSID –BSS Identifier

TA - Transmitter RA - Receiver SA - Source DA - Destination

IEEE 48 bit address

Individual/Group Universal/Local 46 bit address

MSDU Sequence Number Fragment Number

CCIT CRC-32 Polynomial

Upper layer data 2048 byte max 256 upper layer

header

39

Frame Subtypes

• RTS• CTS• ACK• PS-Poll• CF-End & CF-End

ACK

Data Data+CF-ACK Data+CF-Poll Data+CF-ACK+CF-Poll Null Function CF-ACK (nodata) CF-Poll (nodata) CF-ACK+CF+Poll

Beacon Probe Request & Response Authentication Deauthentication Association Request & Response Reassociation Request &

Response Disassociation Announcement Traffic Indication

Message (ATIM)

CONTROL DATA MANAGEMENT

40

41

802.11 Management Operations

• Scanning

• Association/Reassociation

• Time synchronization

• Power management

42

Scanning & Joining

• Goal: find networks in the area

• Passive scanning• No require transmission saves power• Move to each channel, and listen for Beacon frames

• Active scanning• Requires transmission saves time• Move to each channel, and send Probe Request frames to solicit

Probe Responses from a network

43

Association in 802.11

AP

1: Association request

2: Association response

3: Data trafficClient

44

Time Synchronization in 802.11

• Timing synchronization function (TSF)• AP controls timing in infrastructure networks• All stations maintain a local timer• TSF keeps timer from all stations in sync

• Periodic Beacons convey timing• Beacons are sent at well known intervals• Timestamp from Beacons used to calibrate

local clocks• Local TSF timer mitigates loss of Beacons

45

Power Management in 802.11

• A station is in one of the three states• Transmitter on• Receiver on• Both transmitter and receiver off (dozing)

• AP buffers packets for dozing stations• AP announces which stations have frames

buffered in its Beacon frames• Dozing stations wake up to listen to the beacons• If there is data buffered for it, it sends a poll frame

to get the buffered data

46

Important Lessons

• Many assumptions built into Internet design• Wireless forces reconsideration of issues

• Link-layer• Spatial reuse (cellular) vs wires• Hidden/exposed terminal• CSMA/CA (why CA?) and RTS/CTS

• Network• Mobile endpoints – how to route with fixed identifier?• Link layer, naming, addressing and routing solutions

• What are the +/- of each?

• Transport• Losses can occur due to corruption as well as congestion

• Impact on TCP?• How to fix this hide it from TCP or change TCP

47

802.11 modes

• Infrastructure mode• All packets go through a base station• Cards associate with a BSS (basic service set)• Multiple BSSs can be linked into an Extended

Service Set (ESS)• Handoff to new BSS in ESS is pretty quick

• Wandering around CMU• Moving to new ESS is slower, may require re-

addressing• Wandering from CMU to Pitt

• Ad Hoc mode• Cards communicate directly.• Perform some, but not all, of the AP functions

48

Discussion

• RTS/CTS/Data/ACK vs. Data/ACK• Why/when is it useful?• What is the right choice• Why is RTS/CTS not used?

49

802.11 Rate Adaptation

• 802.11 spec specifies rates not algorithm for choices• 802.11b 4 rates, 802.11a 8 rates, 802.11g 12

rates• Each rate has different modulation and coding

Transmission Rate then Loss Ratio

throughput decreases either way – need to get it just right

Transmission Rate then Capacity Utilization

An IntuitionSISO Single Input Single Output

Disclaimer: This Intuition is incomplete with respect to how communication signals are actually analyzed

Forget about noise for now and the frequency domain transformation. Assume we have an

antenna, which transmits a signal x at a frequency f. As the signal propagates through an environment, the

signal is faded, which is modeled as a multiplicative coefficient h. The received signal y will be hx.

fading h1

y1 = h1x1

transmit receive

x1

50

An Intuition SIMO Single Input Multiple Output

Now assume we have two receiving antennas. There will be two received signals y1 and y2 with different fading coefficients h1 and h2. The effect upon the signal x for a given path (from a

transmit antenna to a receive antenna) is called a channel.

The channel capacity has not increased

The multiple receive antennas can help us

get a stronger signal through diversity

fading h2

y1 = h1x1

transmit receive

x1

fading h1

y2 = h2x1

51

An Intuition MISO Multiple Input Single Output

Assume 2 transmitting antennas and 1 receive antenna. There will be one received signal y1 (sum of x1h1 and x2h2). In order to separate x1 and x2 we will need to also transmit, at a different time, -x1* and x2*.The channel capacity has not really increased because we still have to transmit -x1* and x2* at time 2. (Alamouti scheme)

fading h2

y1 = h1x1+ h2x2

transmit receive

x1

fading h1

x2

y2 = h1x2*+ h2-x1*

Time 1 Time 2

-x1*

x2*Time 1 Time 2

52

An Intuition MIMO Multiple Input Multiple Output

With 2 transmitting antennas and 2 receiving antennas, we actually add a degree of freedom! Its quite simple and intuitive. However, in this simple model, we are assuming that the h coefficients of fading are independent, and uncorrelated. If they are correlated, we will have a hard time finding an approximation for the inverse of H. In practical terms, this means that we cannot recover x1 and x2.

fading h2

y1 = h1x1+ h2x2

transmit receive

x2

x1

y2 = h3x1+ h4x2fad

ing h3

fading h1

fading h4

y1

y2 Finally Assume there is some white Gaussian Noise, and we have a set of linear equations

y = Hx + w

All 2 degrees of freedom are being utilized in the MIMO case, giving us Spatial Multiplexing.

2

1

2

1

43

21

2

1

w

w

x

x

hh

hh

y

y

53

Multipath Fading

• There should be a significant number of multipaths for each of the coefficients

• The energy should be equally spread out

• If there are very few or no paths in some of the directions, then H will be correlated

• The antennas should be properly spaced otherwise H will be correlated

54

Conclusions

• MIMO adds a full degree of freedom

• Think of it as a dimensionality extension to existing techniques of time and frequency

• The more entropy in the fading environment, the more “richly” scattered, and less likely for zero eigenvalues

• Rayleigh fading is a reasonable estimate

55

Frequency Response Due to Multipath

56

OFDM

57

OFDM

58

59

Outline

• Wireless systems and standards• 802.11 Architecture• 802.11 MAC

• Bluetooth

• UWB

60

Bluetooth basics

• Short-range, high-data-rate wireless link for personal devices • Originally intended to replace cables in a range

of applications • e.g., Phone headsets, PC/PDA

synchronization, remote controls

• Operates in 2.4 GHz ISM band • Same as 802.11• Frequency Hopping Spread Spectrum across ~

80 channels

61

Bluetooth Basics cont.

• Maximum data rate of up to 720 Kbps • But, requires large packets (> 300 bytes)

• Class 1: Up to 100mW (20 dBm) transmit power, ~100m range• Class 1 requires that devices adjust transmit power

dynamically to avoid interference with other devices

• Class 2: Up to 2.4 mW (4 dBm) transmit power• Class 3: Up to 1 mW (0 dBm) transmit power

62

Usage Models

• Wireless audio • e.g., Wireless headset associated with a cell phone • Requires guaranteed bandwidth between headset and base • No need for packet retransmission in case of loss

• Cable replacement • Replace physical serial cables with Bluetooth links • Requires mapping of RS232 control signals to Bluetooth messages

• LAN access • Allow wireless device to access a LAN through a Bluetooth connection • Requires use of higher-level protocols on top of serial port (e.g., PPP)

• File transfer • Transfer calendar information to/from PDA or cell phone • Requires understanding of object format, naming scheme, etc.

Lots of competing demands for one radio spec!

63

Protocol Architecture

64

Piconet Architecture

• One master and up to 7 slave devices in each Piconet:

• Master controls transmission schedule of all devices in the Piconet• Time Division Multiple Access (TDMA): Only one device transmits at a time

• Frequency hopping used to avoid collisions with other Piconets • 79 physical channels of 1 MHz each, hop between channels 1600 times a

sec

65

Scatternets

• Combine multiple Piconets into a larger Scatternet • Device may act as master in one Piconet and slave in another • Each Piconet using different FH schedule to avoid interference

• Can extend the range of Bluetooth, can route across Piconets

66

Baseband Specification

• 79 1-MHz channels defined in the 2.4 GHz ISM band • Gaussian FSK used as modulation, 115 kHz frequency deviation

• Frequency Hopping Spread Spectrum • Each Piconet has its own FH schedule, defined by the master • 1600 hops/sec, slot time 0.625 ms

• Time Division Duplexing • Master transmits to slave in one time slot, slave to master in the

next

• TDMA used to share channel across multiple slave devices• Master determines which time slots each slave can occupy • Allows slave devices to sleep during inactive slots

67

Time slots

• Each time slot on a different frequency • According to FH schedule

• Packets may contain ACK bit to indicate successful reception in the previous time slot • Depending on type of connection... • e.g., Voice connections do not use ACK and retransmit

• Packets may span multiple slots – stay on same frequency

68

Physical and Logical Links

• Bluetooth supports two types of physical links. • Synchronous Connection Oriented (SCO):

• Slave assigned to two consecutive slots at regular intervals • Just like TDMA...

• No use of retransmission ... why?? • Asynchronous Connectionless (ACL)

• Allows non-SCO slots to be used for “on demand” transmissions • Slave can only reply if it was addressed in previous slot by master

70

Discussion

• Nice points• A number of interesting low power modes• Device discovery

• Must synchronize FH schemes• Burden on the searcher

• Some odd decisions• Addressing• Somewhat bulky application interfaces

• Not just simple byte-stream data transmission • Rather, complete protocol stack to support voice, data, video,

file transfer, etc. • Bluetooth operates at a higher level than 802.11 and 802.15.4