Transcript
Wireless Networks EN0710 Computer Networks
W D Henderson & A P Robson
1
Introduction
2
Wireless: RF and IR z
Refers to: – – – – –
Radio frequency from long wavelengths to microwaves Infrared – long wavelength end of visible light spectrum In many respects light and radio behave similarly – both are electro-magnetic waves This similarity is useful but should not be taken too far! Radio waves will pass through walls – light will not!
3
Light and Radio Waves z z z z z z
Solid objects such as concrete attenuate (weaken) radio waves but will obscure light At short distances light and radio waves behave similarly – reflection and refraction of both Radio waves, unlike light, do not travel in straight lines but spread out For light this effect is much smaller because of the smaller wavelength of light For example, there may be line of sight between two radio stations but high attenuation because of buildings close to the line of sight – fresnel effect Radio signals can be reflected successfully from buildings – but with reduced signal strength
4
Wireless Communications z
Advantages – – – –
z
Easy to install and inexpensive Unobtrusive connection Rapid deployment Mobile applications, ubiquitous computing
Disadvantages – – – –
Low speed – but getting faster Security problems – solutions are available Interference and error – limits available speed Band allocations vary between countries
5
Differences to Wired LANs z z
Destination address/destination location Media impact the design – – – – – –
z
Boundaries Outside signals Reliability Dynamic topology Connectivity Propagation properties
Handling mobile stations
6
Connectivity is different to wired LANs Building
STA1 STA2 Barrier Malicious station STA3 STA4
7
Performance z z z z z
Example, 802.11b – Popular WiFi LAN Datarate: 11 Mb/s raw bandwidth 55 Mb/s is available with higher frequency standard Range: <= 300m Actual data rate and range are often much lower than theoretical maximum: – –
Protocol overheads Interference, lost packets
8
Unlicensed Spectrum z z z z
Interference is quite likely since many devices share the same unlicensed band ISM Bands Industrial, Scientific and Medical – 900MHz, 2.4GHz, 5GHz A cordless phone could cause data loss for your WLAN It’s the user’s responsibility to adapt to avoid interference
9
Electromagnetic Waves z z z z z z
EM waves are transmitted at the velocity of light through space – about 3 x 108 m/s Takes about 9 minutes for solar radiation to reach Earth Radio waves occupy the region from 30kHz to 300GHz Radio waves differ in one respect from other EM radiation – they can be generated and received using antenna (more about antenna later) Radio waves are generated by rapid changes in voltage/current in an antenna. Similarly, radio waves can be received by sensing the rapid changes in voltage/current in an antenna in the presence of an EM field.
10
Wavelength & Frequency The wavelength and frequency of an EM (radio) wave are related by: C=λν where C is the velocity of light – 3x108 [m s-1] λ is the wavelength [m] ν is the frequency (cycles per second) [Hz] z
11
Wavelength S(t)= A sin(π ν t)
length of one wave, λ
A
Time 12
Electromagnetic Spectrum
ν
Frequency - Hz 10
103
105
107
10-7
10-9
1021
10-11
1023
Gamma rays
10-5
1019
X rays
10-3
1017
Ultraviolet
101 10-1
Visible Light
103
1015
Infrared
105
1013
UHF Radio/TV
107
VHF Radio
SW Radio
Long radio waves
λ Wavelength - m
109 1011
10-13
13
The Radio Spectrum
λ
30km
VLF
3km
LF 30kHz
MF 300kHz
HF 3MHz
AM Broadcast
ν
10kHz
3m
10MHz
VHF 30MHz
3cm
UHF
300MHz
SHF 3GHz
FM Cellular Broadcast telephones And TV
100MHz
EHF 30GHz
RADAR
10GHz 14
Radio Bands Band name
Abbr
ITU band
Frequency
Wavelength
< 3 Hz
> 100,000 km
Example uses
Extremely low frequency
ELF
1
3–30 Hz
100,000 km – 10,000 km
Communication with submarines
Super low frequency
SLF
2
30–300 Hz
10,000 km – 1000 km
Communication with submarines
Ultra low frequency
ULF
3
300–3000 Hz
1000 km – 100 km
Communication within mines
Very low frequency
VLF
4
3–30 kHz
100 km – 10 km
Submarine communication, avalanche beacons, wireless heart rate monitors, geophysics
Low frequency
LF
5
30–300 kHz
10 km – 1 km
Navigation, time signals, AM longwave broadcasting
Medium frequency
MF
6
300–3000 kHz
1 km – 100 m
AM (Medium-wave) broadcasts
High frequency
HF
7
3–30 MHz
100 m – 10 m
Shortwave broadcasts, amateur radio and over-thehorizon aviation communications
Very high frequency
VHF
8
30–300 MHz
10 m – 1 m
FM, television broadcasts and line-of-sight groundto-aircraft and aircraft-to-aircraft communications
Ultra high frequency
UHF
9
300–3000 MHz
1 m – 100 mm
television broadcasts, microwave ovens, mobile phones, wireless LAN, Bluetooth, GPS and TwoWay Radios such as FRS and GMRS Radios
Super high frequency
SHF
10
3–30 GHz
100 mm – 10 mm
microwave devices, wireless LAN, most modern Radars
Extremely high frequency
EHF
11
30–300 GHz
10 mm – 1 mm
Radio astronomy, high-speed microwave radio relay
Above 300 GHz
< 1 mm
15
Example z z z
Short wave broadcast radio transmissions have frequencies between 3 MHz and 30 MHz Worldwide communications This is a range in wavelength of: 3 . 108 / 3 . 106 to 3 . 108 / 3 . 107 or 100 m to 10 m
16
Examples z z z
VHF FM broadcast radio transmissions have a frequencies between 87.5 and 108.0 MHz Communication to just beyond the horizon A 100 MHz wave has a length of: λ =C/ν = 3 . 108 / 100 . 106 =3m
17
Example Some Wireless LANs employ radio transmissions with a frequency of between 2.4 and 2.483 GHz z This corresponds to a range of wavelength of: 3 . 108 / 2.4 . 109 to 3 . 108 / 2.483 . 109 Or from 0.125 m to 0.121 m (about 12 cm) z
18
Example z z z z z z z
Line-of-site microwave communication may use radio waves of between 4 and 10 GHz These 10 GHz signals have a wavelength of 3 . 108 / 1 . 1010m = 0.03 m or 3 cm – several cm These signals are attenuated rapidly Particularly by rainfall Higher frequency microwave (20 GHz) is used for short point-to-point links between buildings
19
Generation of Radio Waves z
A rapidly changing current in an antenna generates EM waves that spread out from the antenna in every direction
Radio waves
Rapidly changing currents
Data Tx Radio Transmitter
Antenna
Feeder 20
Reception of Radio Waves z
EM waves arriving at an antenna generate small signals that can be detected and amplified by a receiver.
Radio waves
Antenna
Rapidly changing currents
Feeder
Rx Radio Receiver
Data
21
Propagation z z
EM waves propagate in every direction through space from an antenna The waves rapidly get weaker as they get further from the transmitting antenna
22
Signal level generated by a typical 802.11b card - mW
Signal level [mW]
Signal level, mW, as a function of propagation distance 0.002
Inverse square law
0.0015 0.001 0.0005 0 0
20
40
60
80
100
120
140
160
Propagation distance [feet] 23
IEEE 802.11 - WLAN
24
802.11 standards z
Some of the 802.11 standards: – – – – – – – – – –
802.11a 802.11b 802.11e 802.11f 802.11g 802.11h 802.11i 802.11k 802.11m 802.11n
5GHz band <= 54Mbps Widely used 2.4GHz band <= 11MBps – Wi-Fi Adds QOS Interaction of Access Points defined Increased data rate of 54Mbps on 2.4GHz Dynamic frequency selection for 802.11a Security upgrade Management improvement – location-based services Maintenance Higher throughput – 100Mbps - under development 25
Physical Layer Implementations z
Available Techniques –
Infrared – 802.11 z
–
FHSS (Frequency Hopping SS) – 802.11 z z
–
2.4 GHz ISM Band 1 or 2 Mbps
DSSS (Direct Sequence SS) – 802.11b z z
–
1 or 2 Mbps – abandoned – IrDA standard in use
2.4 GHz ISM Band 1,2,5.5 or 11 Mbps
OFDM (Orthogonal Frequency Division Multiplexing) – 802.11a z z
No spread spectrum 5 GHz, 6, 9, 12, 18, 24, 36, 48 and 54 Mbps 26
Inventor of Spread Spectrum Hedy Lamarr
American 1940’s film star
27
802.11a and b z z z z z z z z
Two task groups: A: 802.11a and B: 802.11b Group A explored 5 GHz band – more on this later Group B explored 2.4 GHz spectrum spreading techniques 1999: published 802.11b Data rates of 1, 2, 5.5 and 11 Mbps 11 channels within 2.40 to 2.47 GHz No more than 3 channels in operation to limit interference Sender and receiver must be on the same channel 28
The OSI model and 802.11 Application Presentation Session TCP/UDP
Transport
IP
Network Data Link
IEEE 802.11b
Physical
LLC Service Access Point LSAP
802.2 Logical Link Control LL2 802.11 Medium Access Control Layer MAC 802.11 Physical Link Control Protocol PLCP
29
Network Structure
802.11 Structure - IBSS Independent Basic Service Sets aka Ad-Hoc-Networks “Peer-to-peer”
IBSS1
IBSS2
STA1 STA3
Direct communication in BSS Within a limited range
STA2
STA4
•No AP •No need for infrastructure •Nodes with an IBSS can communicate with each other- peer-to-peer •Nodes must send beacons 31
802.11 Structure - BSS BSS The BSS has a unique BSSID
STA1
Basic Service Set - BSS Single access point forms a bridge between the wireless and wired LANs
AP1
AP is fixed
BSS is operating in “infrastructure mode”
STA2
All communication is through the access Point, AP No direct communication between clients
32
802.11 Structure - ESS Extended Service Set Two or more BSSs on the same wired network At least 2 APs operating in Infrastructure Mode
BSS1
BSS2
STA1 STA3
AP1
STA4
Stations using The same frequency
AP2
Wired (Ethernet) LAN Or WAN or WLAN, … STA2
Distribution System - DS
Portal (bridge)
802.x LAN
Stations can roam between BSS 33
Wired LAN
Roaming
AP2 AP1
z z
z z
z z
Move seamlessly from one cell to another APs hand the client off ensuring unbroken connection Must have cell overlap Can provide wireless roaming for an entire building or campus Requires reassociation Roaming will involve new channel selection
Inter-cell roaming and Handoff
z z
Not part of 802.11b Left to manufacturers to define how its done
34
MAC and LLC z
Medium Access Control, MAC – – –
z
On transmission, assemble data into a frame with address and error detection fields On reception, disassemble frame and perform address recognition and error detection Govern access to the LAN transmission medium
Logical Link Layer – LLC –
Provide an interface with higher layers and perform flow and error control
35
Access Control - MAC –
–
– – –
Cannot listen and transmit at the same time on radio channel – not practical Receiver would hear its own transmitter signal and nothing else CSMA/CA – collision avoidance is employed cf CSMA/CD on Ethernet Uses ACK messages to confirm reception Data Link
IEEE 802.11b
Physical
802.2 Logical Link Control LL2 802.11 Medium Access Control Layer MAC 802.11 Physical Link Control Protocol PLCP 36
Multiple access problem z z
More than one user is using the medium Arbitrary users – – –
z
Different WLAN standards Microwave ovens Other ISM devices
Different users of one system –
Stations of a WLAN
Physical Layer PHY
MAC Layer MAC
37
½ pause 38
CSMA/CA Protocol
Modes of Operation z
Distributed Coordination Function (DCF) – –
z
No central control Similar to Ethernet
Point Coordination Function (PCF) – –
Base station controlled Beacon frames z z
Invites stations to sign up for polling Can guarantee bandwidth
40
CSMA/CA z z
Standard MA for 802.11 Every station has own Network Allocation Vector (NAV) – –
z
From duration field Reduces probability of collisions
Collisions are still possible – –
ACKs for recognizing these Costly for long frames
Simplified NAV==0 ?
N
Y Medium Free?
N
Backoff
Y Send Frame
Wait ACK
ACK received
N
Y
41
Interframe spaces – are used to reserve the channel z
Interframe Spaces (IFS) are used to reserved the channel –
z
DIFS
They allow DCF and PCF to coexist
PIFS
Types of IFS –
Short IFS (SIFS) – z
–
z
–
SIFS Medium Busy Time
issues polls takes precedence over normal contention traffic
Distributed Coordination Function IFS (DIFS) z
–
ACK, CTS, Poll response
Point Coordination Function IFS (PIFS) z
used by ordinary asynchronous traffic
Extended IFS (EIFS) z
z
EIFS
error handling
IFS generate priorities 42
Frame Fragmentation z z
Wireless networks are very noisy Long frames are very likely to be damaged –
z
Generating lots of retransmissions
Solution –
Break down data into small pieces each with own check sum z
–
More likely to get to target with no damage
Remaining fragments have priority over new traffic z
Fragment bursts
43
Interframe Spaces SIFS PIFS DIFS EIFS
ACK
Control frame or next fragment PCF frames can be sent here DCF frames okay here
Bad frame recovery done here
44
Backoff z
z
Stations must be prevented from collisions following DIFS Uses random but short backoff time DIFS Medium In use
Backoff Data Frame Time
45
Frame transmission DIFS
SENDER
Backoff
Medium In use
Data Frame SIFS
ACK
RECEIVER DIFS
OTHER STATION
Medium In use
SIFS
Backoff
Backoff
Network Allocation Vector
46
Hidden Station Problem B can hear A and C, A cannot hear C and C cannot hear A C can disrupt transmissions from A since C is not known to A 802.11b CTS/RTS protocol solves this problem?
B A
C
Retransmissions Degraded throughput – perhaps 40% 47
RTS/CTS z z z
Avoid Hidden Station Problem RTS – Request to Send CTS – Clear to Send
RTS CTS
CTS A
B
C
48
Collision Avoidance in 802.11b Source
Destination Request to send (RTS)
Clear to send (CTS)
•
Reduces impact of Hidden Nodes
•
Usually 50% loss of throughput as a result of CSMA/CA protocol
•
RTS/CTS may further reduce throughput by 40%
DATA
ACK
49
RTS/CTS Protocol DIFS
SENDER
Control Duration
Rx ID
Tx ID
CTS
Control Duration
Rx ID
Check sum
Data Frame
RTS
SIFS
DIFS
OTHER STATION
SIFS
CTS
RECEIVER
Medium In use
Check sum
SIFS
Backoff
Medium In use
RTS
ACK
DIFS
Backoff
Backoff
Network Allocation Vector (RTS)
NAV (CTS)
50
RTS/CST Tradeoff z
Pros: – –
z
Cons: – – –
z z
Collisions are reduced Hidden station problem is resolved More latency – overhead of RTS/CTS Bandwidth reduction RTS frame can collide
CTS/RTS is not used with multi- or broadcast Usage – – –
With asymmetrical coverage ranges With large frames When collisions are likely
51
Hidden Node - Summary z
Use CTS/RTS – – –
z z z
Greater overhead On or Off or On with Threshold – set in APs and nodes – triggered by size of packet
Increase power Remove obstacles Move some nodes
52
Near/Far AP
Near
Near
• • • • •
Far
Near
All nodes are within the range of the AP However, Far client cannot be heard above Near traffic Far client may be on a low power setting, e.g., 5 mW Near clients may be on high power setting, e.g., 100 mW Administrator must be aware of this type of problem 53
Solving Near/Far z z z z
Has the Far node associated? – check association table in AP Increase power of Far nodes Decrease power of Near nodes Move the AP closer to Far nodes – this may create further problems!
54
Channel and Performance Issues
11
0
System Throughput Throughput - Mbps
z z z z z z z
The highest raw bit rate for 802.11b is 11 Mbps Spread spectrum configured to decrease bit rate in response to high error rates – in steps: 1, 2, 5.5, 11 Mbps Throughput is about 50% of data rate on a wireless LAN as a result of half-duplex channel and CSMA/CA protocol May achieve 5 Mbps in practice CTS/RTS may cause a further 40% reduction in throughput – resulting in 3 Mbps only Application of WEP and/or VPN further reduce throughput Further reduction caused by: – – – –
Great distances causing error and retransmissions Fragmentation – large packets are more efficient Mixed mode devices – 802.11b and 802.11 Large number of users
56
Co-location z z z
Co-location is the use of more than one channel in the same wireless environment Can provide more throughput and allow mobile nodes to roam There are 11 channels – –
Channel spectrum is 22 MHz wide Channel separation is 5 MHz Channel: 1
2 3 4 5
6 7
8 9 10 11
2.4 GHz spectrum
10 * 5 + 22 + 401 = 473
Frequency, GHz 2.401
2.473 57
Co-location z
Channels 1, 6 and 11 are typically used to provide 3 non-overlapping bands Channel: 1
2 3 4 5
6 7
8 9 10 11
2.4 GHz spectrum
Frequency, GHz 2.401
z z
2.473
These three channels can be used to co-locate multiple (up to 3) points within the same physical area For full separation, only 3 channels can co-locate
58
Co-location - Reality AP1 z
z z
z z
Should see a throughput of between 4.5 and 5.5 Mbps on each of the 3 channels In fact, channels 1, 6 and 11 do overlap to a small extent So, instead of getting normal half-duplex throughput on three channels, a detrimental effect is seen on all three. Throughput can decrease to 4 Mbps on all three channels Or perhaps 3, 4 and 5 Mbps
AP6
AP11
Channel: 1
2.401
6
11
2.473
59
Co-location – solutions? z z z
Use equipment from the same manufacturer Use max possible channel separation, e.g., 1 and 11. This provides no interference for any physical separation. Example: – –
z z
3 channels (1, 6 and 11) may provide a total throughput (3 * 4) of 12 Mbps 2 channels (1 and 11) may provide a total throughput (2 * 5.5) of 11 Mbps
Another option is to use 802.11a equipment 802.11a – – – –
Operates at 5 GHz and provides 3 bands (each wider than 802.11b), and each band allows for 4 non-overlapping channels. Could support up to 8 access points in the same space. Is more expensive but suffers less interference 802.11a and 802.11b are incompatible! 60
Channel Interference z z
z
Important in planning large networks to allocate channel use with physical space Example – Channels 1 and 2 are adjacent – they will interfere since channels are 22 MHz wide and their centre frequencies are only 5 MHz apart. Must plan coverage to avoid adjacent channel interference – – –
Locate to adjacent channel APs such that areas do not overlap Turn down power Use channels that do not overlap
61
Co-channel Interference z z z
z
These LAN access points will interfere Use non-overlapping channels Move equipment far enough apart to avoid overlap Use channel pairs 1&11 or 2&10 or 3&9
AP
Floor 2
AP
Floor 1
AP
Floor 0
Three Access Points each using Channel 1
62
Channel Reuse z
z
Can reuse channels in a large network to avoid cochannel interference Channel Reuse patterns avoid interference
CH 1 AP
CH 1 CH 6
AP
AP
AP CH 11
AP CH 1
AP CH 11
63
Range z
Transmission Power –
z
Antenna Type –
z
Some antennas provide more focused and longer range signals
Environment – –
z
High output power will increase the range of signals
Noisy environments reduce the range of wireless LANs Packet error rate is greater at the fringes of coverage – poorer SNR
Frequency –
2.4 GHz will reach further than 5 GHz systems with the same power. 64
802.11a, 802.11g and 802.11n
802.11a z z z z z z z
Uses Orthogonal Frequency Division Multiplexing, OFDM. Subcarriers modulated using BPSK, QPSK, 16-QAM or 64-QAM Used 5GHz band – little interference 12 non-overlapping channels Performance: up to 54 Mbps raw (possibly only 20 Mbps in the field) Shorter range than 11b but better noise resistance More users in the same area
66
802.11g z z z z z z
Recent compromise to lengthen life of 802.11b networks and provide greater performance Works on 2.4 GHz band – therefore noisy (microwave ovens, phones, etc) Backward compatible with 802.11b - Adopts Complementary Code Keying, CCK, used by 802.11b, providing 5.5 and 11 Mbps raw. Adopts Orthogonal Frequency Division Multiplexing, OFDM, used by 802.11a provides 54 Mbps Limited to three channels Similarly to 802.11b, the practical performance is often much less than the theoretical performance because of noise – provides about 20 Mbps throughput
67
802.11n z z z z
802.11n is proposed extension to Wi-Fi: Will support speeds of over 100 Mbps Modified PHY and MAC layers to improve effective throughput Measure performance at MAC layer not PHY layer –
z z z z
Smaller gap between theoretical performance and real-world rates
Conforms to all existing 802.11 functions Probably compatible with 2.4GHz 802.11b and 5 GHz 802.11a Multimedia applications will benefit most Now expected to be available in 2006
68
802.11n – What’s New? z z z z z z z z
Smart antenna Enhanced modulation methods Greater bandwidth Closed loop control Boost PHY rates to 250 Mbps Provide real throughput of over 100 Mbps Unlikely to use more spectrum (than 20 MHz) because spectrum is scarce Intended to be suitable for home, enterprise and hotspot markets 69
Wireless LAN Technology is Developing Rapidly z
z z z
Vendors and manufacturers are keen to bring out new faster products based on draft standards Standards are slow to develop, e.g., 802.11n Some manufacturers are jumping the gun Theoretical best performance is often not realised in the field
70
