Introductory Example
Mesh clients (mobile)
Gateway
Internet
Mesh router (stationary)
Wireless Mesh Networks: Introduction
Basic Concepts
Eduard Glatz (eglatz@hsr.ch)
Mesh network
Example Scenario: Infrastructure/backbone Wireless Mesh Network
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Objectives
Multi-hop network: one or more routers on path between source and destination node
Mesh network: multipoint to multipoint connectivity
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Agenda
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You can define and characterize a wireless mesh network
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Definition and characteristics of a wireless mesh network
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You can illustrate five different application areas of wireless mesh networks
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Typical application scenarios
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You know how to calculate a wireless communication range
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RF communication basics
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You can explain the relationship between coding/modulation schemes and capacity
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Channel access schemes in wireless communications
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You can give an overview about wireless channel access schemes and their properties
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Multi-hop wireless networking and formation guidelines
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You know the history and formation guidelines of wireless mesh networks
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The path length and power control problem
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You know the path length and power control problem
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Routing in mesh networks: requirements, taxonomy, example
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You can name five wireless-specific routing problems
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You know the taxonomy of wireless routing protocols
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You can explain the principle of the ad-hoc wireless distance vector (AODV) routing
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NB: Not covered topic is security
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Mesh Networking
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Characteristics of a WMN
Network Topologies:
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WMN‘s are considered to be a subclass of ad hoc networking
--> Routing nodes are stationary (unlike in Mobile Ad Hoc Networks, MANET‘s)
Full mesh: each node
is directly connected
to all other nodes
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- Self-Configuration
- Self-Healing (redundant, decentralized, no central point of failure)
- Self-Managment
- Self-Optimization
--> Challenging tasks for a good system design
Partial mesh: not all
nodes are directly
interconnected
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High overall capacity:
- Spatial diversity
- Power management
Definition of a Wireless Mesh Network (WMN):
In short: Multi-hop network built from wireless routers
More detailed: Multi-hop peer-to-peer wireless network in which nodes connect with
redundant interconnections and cooperate with one another to route packets
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WMN‘s have properties of an autonomic system:
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Important constraints:
- Shared bandwidth & interference
- Number and location of nodes
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Application Scenario
Application Scenario
Broadband Internet Access (last mile)
Community Mesh Network
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ADSL
Backbone
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Middle Mile
Last Mile
Cable
Wired infrastructure often too expensive for last and middle mile:
- Rural areas
- Weakly populated countries
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Cost factors in wired infrastructure:
- Number of endpoints
- Cable costs (length, unfriendly terrain)
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WiMAX
Issues in wireless solutions:
- Range and bandwidth (mesh networking is the key)
- Costs and maintenance requirements of hardware
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Mesh infrastructure owned by participants
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WiMAX
ADSL
Cablemodem
Mesh network
Wholesale local loop
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Application Scenario
Application Scenario
City-wide Wireless Coverage (Blanket)
Spontaneous Mesh Network
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Example: Roofnet Project
Temporary mesh network for collaboration (usually with portable wireless devices)
Scenarios:
Real-time advisory
(e.g. traffic information)
- 802.11b mesh network
- volunteer user host nodes
- omnidirectional antennas
- only up/downlink to wired
ethernet
- goal: high TCP throughput
(average realized 627 kbps,
routing queries failed for
10% of source-destination
pairs)
Public safety
(emergency teams, fire, rescue)
Peer-to-peer calling within
local groups (events, university campus, conference
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Application Scenario
RF Communications Basics
Industry Breakdown
Transmission Formula (ideal conditions)
Neighborhood Mesh Network
Internet
Wireless mesh based
broadband access
architecture:
101
Bus Stop
206
Master node: connects
to wired internet
Gas Station
(Internet TAP)
Mesh Router 7
Mini base station: mesh
router rented to subscriber
EXIT
Mesh Router 5
Mesh Router 2
Mesh Router 3
Mesh Zone
Mesh Router 1
Mesh End Device
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End Device
(Guest to Router 1)
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Franchising: system and
business model rented to
local service provider
(usually as a „side business“)
[image source: V. Bahl, MSR]
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Transmitter
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Receiver
Friis transmission formula (free space, ideal isotropic antennas):
2
c
P r = -------------------- ˜ P t
2
4 S df [1]
Pr: Signal power available at receiver antenna output [W]
Pt: Signal power fed to transmitter antenna input [W]
d: Distance between antennas [m]
f: Frequency [Hz]
c: Speed of light (3 x 108 m/s)
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Multipath Fading and Shadowing
Transmissions under Non-ideal Conditions
Effects to consider
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LOS (Line-Of-Sight)
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Distance between transmitter and receiver (Friis transmission law): Path loss
Receiver sensitivity: Thermal noise
Multipath fading: Reflection (eg. ground, buildings, water surface), diffraction, refraction
Shadowing: Absorption (e.g. walls, buildings, rain, windows)
Interference: same source or different source
- May annihilate signal (same frequency & amplitude, 180o fixed phase relation)
- May amplify signal (same frequency, 0o fixed phase relation)
NLOS (Non Line-Of-Sight)
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Multipath fading: diffraction, reflection, scattering
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Shadowing: absorption
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- May be caused by other radio sources (e.g. microwave oven, WLAN, WPAN)
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Technical: Antenna Gains, Cable Losses
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Communication Range Calculations
Communication Range Calculations
Use of the Decibel (dB)
Strength of received signal
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Ratio between two quantities expressed by a dimensionless logarithmic unit [dB]
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May be used in different application areas (acoustics, physics, electronics)
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P r > dBm @ = P t > dBm @ – L fs > dB @ + G t > dBi @ + G r > dBi @
Usage in calculations for RF communications:
Ratio between two power values = 10 log10(P2/P1)
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Special use: [dBi] = Antenna gain relative to an isotropic antenna (one-point source)
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Path loss (free space)
§ Pt ·
4 S df
L fs > dB @ = 10 ˜ log ¨ ------¸ > dB @ = 20 ˜ log § ------------· > dB @
© c ¹
© P r¹
[3]
d: Distance between antennas
f: Frequency
c: Speed of light
Examples: 2 dBi (simple antenna), 5 dBi (omnidirectional), 18-27 dBi (parabolic)
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[2] „Link Budget“
Pr: Signal power available at receiver antenna output
Pt: Signal power fed to transmitter antenna input
Lfs: Free space loss
Gt, Gr: Gain of transmit antenna, of receive antenna
Special use: [dBm] = Power level relative to a reference value of 1 mW
Examples:
- Typical 802.11 receiver sensitivity -60...-80 dBm
- Typical 802.11 maximum transmitter power ~14 dBm
- Typical minimal Signal-to-Noise (S/N, SNR) values for BPSK modulation ~6 dB
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Receive signal strength expressed in dBm (non isotropic antennas):
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Communication Range Calculations
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Antenna Types
Maximum usable communication range is given by:
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- Signal strength at receiver
- Noise level at receiver
- Minimum required S/N (given by modulation and coding used)
- omnidirectional radiation pattern
- sectorial radiation pattern
- directive radiation patterns
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Signal-to-Noise Ratio (SNR):
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SNR > dB @ = P r > dBm @ – N > dBm @
Thermal noise level:
[4]
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[5a]
N = k B ˜ T ˜ fbw > W @
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kB: Boltzmann‘s constant [J/K]
T: Temperature [K]
fbw: Bandwidth [Hz]
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Formula for room temperature
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(20o
Antenna: reciprocity property
(same behavior for transmit/receive)
Basic reference is ideal isotropic antenna
(radiates equally in all directions)
Antenna gain: expressed in dBi relative to
an ideal isotropic antenna
Example „Cantenna“:
- Uses a tin can as a wave guide
- Cheap solution for developing countries
C equiv. 293 K) for a non-ideal receiver:
N > dBm @ = – 174 + 10 ˜ log f bw + NF
Many different forms:
[5b]
NF (Noise Figure): Ratio of actual receiver noise to ideal receiver noise
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MIMO (Multiple-Input Multiple-Output) Antennas
MIMO
SIMO
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Communication Range and Modulation/Coding
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Maximum communication range is given by minimal required S/N
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Minimal required S/N is determined by an acceptable BER (Bit Error Rate)
MISO
Modulation scheme and code
rate determine SNR requirements
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Modulation schemes (e.g. 802.16):
BPSK: Binary Phase-shift keying
QPSK: Quadrature Phase-shift
Keying
QAM: Quadrature amplitude
modulation
MIMO is a promising multi-antenna systems approach to:
increase link capacity (eg. 802.11n)
improve robustness
benefit from „constructive“ interference
avoid „destructive“ interference
MIMO allows for:
FEC (Forward Error Correction):
Code rate = useful bits/total bits
e.g., code rates in 802.16:
1/2, 2/3, 3/4
- multiple radio links (spatial multiplexing)
- space-time coding
- beamforming
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Adaptive Modulation and Coding
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Overview: Spectrum Usage Regulations
Constellation diagram: shows a digital modulation scheme in the complex plane
--> distances between constellation points are a measure for „robustness“
Examples:
BPSK
,
QPSK
16QAM
,
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Government regulations restrict frequency spectrum usage
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Regulations are not equal in all countries (e.g. Europe, USA, Asia)
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Frequency spectrum is „overloaded“
- For example the 2.4 GHz band („junk band“):
WLAN 802.11b&g, WPAN 802.15 Bluetooth, 15.245 sensors, Part18-ISM, amateur, ....
,
- For example the 5.8 GHz band:
WLAN 802.11a, WiMAX 802.16a, 15.209 generic unlicensed, satellite, aviation,...
Adaptive modulation and coding: Switch between modulation/coding scheme based on actual SNR
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Coverage Area
Regulations differentiate between „licensed“ and „unlicensed“ spectrum usage
Capacity
available SNR
r
min. SNR
r (radius)
2.4 GHz Band (USA)
5.8 GHz Band (USA)
Optimization: automate adaptation (SNR-based, packet loss-based, ...)
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CSMA and Wireless Communication
CSMA and Wireless Communication
The Collision Detection Problem
The Hidden Terminal Problem
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Wired ethernet
B
A
B
C
A
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A
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C
C
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A wants to send to B: if the channel is clear then the transmission starts
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C wants also to send (A is still sending) and checks if channel is free
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Since C can not hear A it assumes the channel is free
Example CSMA/CD situation:
- A and C both want to transmit
- If channel is idle, then both A and C start to transmit --> Collision
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B
Wireless LAN
Wired Ethernet: A and C detect collision during sending (stop and backoff)
Wireless LAN: Sender cannot detect collision during sending (technically not feasible)
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--> Collision at B!
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A is a hidden station (hidden terminal) from the viewpoint of C
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CSMA and Wireless Communication
MACA (Multiple Access Collision Avoidance)
The Exposed Terminal Problem
D
RTS
A
RTS
B
CTS
B
D
C
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A starts a transmission to B
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C detects an occupied channel and waits with its transmission to D (until A-->B ends)
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Since D cannot receive A this waiting is not really necessary
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Backoff Procedure
A
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A waits until the channel is clear
A announces its transmission intent with a Request-To-Send (RTS)
The addressed node B responds with a Clear-To-Send (CTS)
A transmits the data
Behavior of C and D in this example:
- The RTS and CTS PDU‘s contain the length of the transmission
- Hidden node C overhears the CTS and does not send during A‘s data transmission
- Exposed node D overhears RTS (but not CTS) and may send if he wants
(NB: depending on WLAN standard this may be treated alike a CTS reception)
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RTS
Before a data transmission starts a RTS/CTS handshaking is done
Example: A wants to send data to B
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D
CTS
DATA
A
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C
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Carrier Sense in Wireless Systems
RTS
B
(CTS)
RTS
Physical Carrier Sensing
C
(CTS)
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Done at the air interface
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Carrier sensing by CCA (Clear Channel Assessment):
--> Process of detecting transmitting stations inside of the CCA detection range
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Channel access:
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before transmission choose a backoff time tb in the range (0, cw); cw = contention window
count down tb when channel is idle
suspend countdown when channel is busy
when tb=0 then start transmission
Collision example: A and C want to send data to B
- A and C send both RTS to B at the same time --> collision at B!
- A and C can not detect collision during sending
- B will not send CTS --> A and C back off
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CCA detection range depends on detector implementation
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Interference may be interpreted as non-clear channel
Virtual Carrier Sensing
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Done at the MAC layer
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Transmission duration info is extracted from MAC PDU header of RTS/CTS
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Stations use so-called NAV (Network Allocation Vector) to store reservation periods
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NAV reservation periods represent a virtually detected carrier
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For carrier sensing both methods are combined in an OR‘ed fashion
Binary exponential backoff:
- each time expected CTS is not received: cw is doubled (up to a maximum size cwmax)
- upon each successful transmission cw is restored to cwmin
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Communication and Interference Range
Lost Packet Detection
Interference
range
D
RTS
RTS
A
B
C
ri
A
B
CTS
CTS
DATA
rc
ACK
Communication range
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Typical reasons for packet loss:
- poor signal quality (path loss, multipath fading, scattering, absoprtion, ...)
- interference
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TDMA (Time Division Multiple Access)
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Dynamic TDMA:
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DoD: Office environment multimedia communications with handheld devices
- GloMo (Global Mobile Information Systems) 1995 - 2000
Characteristics: CSMA/CA and TDMA, several routing and topology control schemes,
frequency 225-450 MHz, data rate 300 kbps
- 802.16 (WiMAX), combined with TDD
(Time Division Duplexing) or FDD (Frequency
Division Duplexing)
- 802.15 (Bluetooth), combined with FHSS
(Frequency Hopping Spread Spectrum)
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DoD: Battlefield communications in infrastructureless hostile environments
- PRNET (Packet Radio Network) 1972 - 1983
- SURAN (Survivable Adaptive Radio Network) 1983 - 1992
Characteristics (both): Combined Aloha & CSMA, distance vector routing,
frequency 1.78 - 1.84 GHz, data rates 100..400 kbps
Dynamic TDMA application examples:
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Historical perspective
- alternative to CSMA/CA in wireless networks
- dynamically assigns a variable number of time
slots per frame to each data flow
- combines characteristics of circuit switching
and packet switching
- data flows may differ in guaranteed capacity
(and other QoS properties)
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Multi-hop wireless networking
- used in circuit switched networks
- examples: GSM, DECT
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MACAW (Multiple Access Collision Avoidance for Wireless LAN):
- extends basic MACA sequence by ACK
Dynamic TDMA
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Example: A transmits to B
- Transmission is successfully completed when A receives ACK
- a missing ACK would cause a timeout and a retransmission
- Announced transmission time includes ACK (NAV reservation covers RTS/CTS,DATA,ACK)
Communication range rc: Area for possible communication links
Interference range ri: Area of interfered stations
ri/rc ratio: depends on radio technology, typical values are 1.5 - 3
rc, ri may vary between stations (eg. A hears B, but B does not hear A)
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IETF: „ad hoc networks“
- Mobile ad hoc networking (MANET) working group, since 1997
IEEE:
- 802.11s working group, since 2004
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Mesh formation guidelines & results
Performance: Packets in Flight Problem
RTS
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1
Problem:
How many nodes have to sign up
before a viable mesh network forms?
1
1
5
6
7
8
4
5
6
7
8
3
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0
20 40 60 80 100 120 140 160
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[image source: V. Bahl, MSR]
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11
9
10
11
9
10
CTS
RTS
RTS
4
5
6
7
CTS
8
RTS
11
CTS
Using MACAW (Multiple Access Collision Avoidance for Wireless):
- Step 1: RTS 3-> 4 (inhibits 2), RTS 9-> 8 (inhibits 10)
- Step 2: CTS 4-> 3 (inhibits 5), CTS 8->9 (inhibits 7)
- Step 3: RTS 1->2 (can‘t proceed), RTS 11->10 (can‘t proceed), RTS 6->5 (can‘t proceed)
2 packets in flight
Only 4 out of 11 nodes are active
Backoff algorithm hurts (binary exponential backoff)
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Traffic flow through chain (starting from node 1)
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Sending node 1 has least interference resulting in highest throughput
A
B
C
D
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Reducing transmit power decreases interference (eg. C does not interfere with B)
--> Increases throughput
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Example situation:
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Performance: Power Control Problem
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5
10
RTS
RTS
2
9
Backoff window doubles
Performance: Path Length Problem
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3
Zeit
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2
RTS
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3
4
CTS
Experimental results for mesh formation (MS Research):
- at least 5-10% subscription rate required with wireless ranges > 100 m
- if a mesh forms, then it is typically
well connected (node degree > 2)
- increasing range is a key for
good mesh connectivity
2
3
RTS
Answer: Depends on
- Interpretation of „viable“
- Topology
- Wireless range
- Probability of participation
1
2
RTS
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However: Collision at C when B and D transmit simultaneously
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Performance: Power Control Problem
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Routing in Wireless Mesh Networks
Solution for example situation
Differences to Wired Networks
- Reduced transmit power for B
- Collision problem solved
- However: network disconnected
--> What power setting is optimal?
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Implementation issues:
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A
- Set all nodes to same power level?
- Tune each node at deployment time?
- Use equipment capable for automatic
power control? Availability?
- Use directional antennas?
B
Topology changes related to environmental fluctuations
- new nodes may join
- nodes may leave network
- link qualities may vary over time (movement)
--> Dynamics may prevent convergence of routing algorithm
C
D
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Limited bandwidth and battery life
--> periodic updates are unattractive
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Partly unidirectional links
--> computed routes may not work
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Many redundant links
--> increase routing updates
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Additional factors to consider for path selection
- link quality (stability, BER, bandwidth, ...)
- interference
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Taxonomy
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Pro-active (table-driven)
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- discover routes only when needed
- overhead scales automatically with movement
Hybrid (Pro-Active/Reactive)
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Hierarchical
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Geographical
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Power aware
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Flat addressing
- node address independent of location
- each node runs routing protocol
Reactive (on-demand)
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Addressing
- routes are learned and spread out
- typically periodic updates
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Hierarchical addressing
- a subnet per cluster
- nodes acquire address of subnet
- only cluster heads run routing protocol
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Clustering
- node address independent of location
- only cluster heads run routing protocol
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Each wireless routing protocol is related to a particular addressing scheme
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Path Selection Metrics
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AODV (Ad hoc On-demand Distance Vector)
Minimal hop count metric often not optimal
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Popular but still experimental routing protocol (IETF RFC 3561)
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Routing problem divided into two parts: route discovery and route maintenance
- weights derived from low rate, available bandwidth, ...
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Route discovery: on-demand when packets have to be routed
Path metric: combine all link metrics on path
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Route maintenance: when routing failures (packet loss) occur
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Sequence numbers:
- wireless links often vary in quality
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Link metric: assign weights to links
- prefer short paths
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- route freshness
- loop prevention
Metrics:
-
hop count
ETT (Expected Transmission Time)
ETX (Expected Transmission Count)
WCETT (Weighted Cumulative ETT)
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Routing tables at nodes:
- routes are stored as long as routes are active
- timeouts: mark route(s) as inactive
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Two-dimensional routing metric: hop count, sequence number
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Basic routing messages:
- RREQ (Route Request)
- RREP (Route Reply)
- RERR (Route Error)
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AODV (Ad hoc On-demand Distance Vector)
Summary
Route discovery:
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RREQ
RREQ rebroadcast
RREP
RREP rebroadcast
Reverse route
Forward route
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A wireless mesh network is a multi-hop network built from wireless routers and has
properties of an autonomic system
Wireless mesh networks have promising application areas, e.g.
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broadband internet access (last mile)
community mesh networks
city-wide wireless voverage (blanket)
spontaneous mesh networks
industry breakdown
The wireless communication range can be calculated exactly in theory, but in practice
the topology of the terrain has often a limiting influence
Advanced wireless systems use adaptive modulation/coding schemes to leverage
range and capacity for optimum performance
Wireless networks may use several channel access schemes (CDMA/CA, MACAW,
TDM, ...), which are clearly different from wired solutions
Wireless mesh networks have evolved over more than 30 years and allow to build up
viable networks if the communication range and node density fit reasonably
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Summary (2)
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The performance of a wireless mesh network is degraded when using long paths and
in the case of interfering nodes
The routing in wireless networks is different from wired networks because of:
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Wireless mesh routing protocols may be classified into the following categories:
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topology changes related to environmental fluctuations
limited bandwidth and battery life
partly unidirectional links
many redundant links
link quality
pro-active (table-driven)
reactive (on-demand)
hybrid (pro-active/reactive)
hierarchical
geographical
power aware
The AODV routing protocol uses on-demand route discovery and is resilient by doing
route maintenance in case of routing failures
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