Department of Computer and IT EngineeringDepartment of Computer and IT EngineeringUniversity of KurdistanUniversity of Kurdistan
Computer Networks IIWireless Networks
By: Dr. Alireza AbdollahpouriBy: Dr. Alireza Abdollahpouri
Outline
Basic Concepts of Wireless Networks
Applications of Wireless Networks
Overview of Research Topics
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Outline
Basic Concepts of Wireless Networks
Applications of Wireless Networks
Overview of Research Topics
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Wireless Communications
There is no physical link in wireless networks. Signals are transmitted on a certain frequency, propagate in the space and are captured by the receiver tuned to the same frequency.
Wireless communication is normally broadcast communication, i.e., all nodes within the transmission range of a particular node can receive the transmitted packets.
Transmissions in a common neighborhood will interfere with each other. If the Signal-to-Interference-Noise-Ratio (SINR) in the receiver is large enough, a packet can be correctly decoded.
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Wireless Communications
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Electromagnetic wave
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ELECTROMAGNETIC SPECTRUM
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I’ve Upgraded
To Wireless
Wireless Attracts Many Users
Elements of a Wireless Network
network infrastructure
wireless hosts laptop, PDA, IP phone run applications may be stationary (non-
mobile) or mobile wireless does not
always mean mobility
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network infrastructure
base station typically connected
to wired network relay - responsible
for sending packets between wired network and wireless host(s) in its “area” e.g., cell towers,
802.11 access points
Elements of a Wireless Network
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network infrastructure
wireless link typically used to
connect mobile(s) to base station
also used as backbone link
multiple access protocol coordinates link access
various data rates, transmission distance
Elements of a Wireless Network
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Mobility vs. Throughput
Wireless Propagation Channel(s)
Multipath propagation: signals reach the receiver via multiple paths.
Outdoor indoor
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Shadowing
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A
A B C D
B
CD
Radio Propagation
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(Reflection effect)اثر بازتابي : اين اثر نيز به خاطر وجود موانع بزرگ )در مقايسه با طول موج سيگنال تابش شده(
است. در اين حالت سيگنال تابش شده پس از برخورد به مانع بزرگ منعكس مي گردد. اما سيگنال منعكس شده داراي توان كمتري نسبت به سيگنال اصلي است.
(Scattering effect)اثر پخشي : اگر اندازه مانع در حدود طول موج يا كمتر از طول موج سيگنال تابيده شده باشد، مانع مي تواند باعث پخش شدن موج تابيده شده شود. بنابراين موج تابيده شده به چند موج
ضعيف تر شكسته مي شود. (Refraction effect)اثر شکست :
اين اثر به دلیل ورود سیگنال از یک محیط به محیط دیگر به وجود می آید.
16QAM
64QAM
SNR
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15
10
5
0
-5
Distance
Distance Sensitivity in Wireless Networks
QPSK
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Adaptive Modulation and Coding
To provide a tradeoff between throughput and
robustness
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Antennas
Limitations of the Wireless Environment Limitations of the Wireless Network
limited communication bandwidth frequent disconnections heterogeneity of fragmented networks
Limitations Imposed by Mobility route breakages lack of mobility awareness by system/applications
Limitations of the Mobile Device short battery lifetime limited capacities
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Wireless Networks
Single-hop wireless networks: cellular network, wireless LAN.
Multi-hop wireless networks: mobile ad hoc network, wireless mesh network, wireless sensor network.
Wireless Mesh Networks (WMN)
Mesh nodes
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Wireless Mesh Networks (WMN)
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Wireless Sensor Networks (WSN)
Sensor nodes
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Wireless Sensor Networks (WSN)
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Wireless Multi-hop (Mesh vs. Sensor)
Bandwidth is limited (tens of kbps)
In most applications, fixed nodes
Energy efficiency is an issue
Resource constrained Most traffic is user-to-
gateway
Wireless Sensor Networks Wireless Mesh Networks
Bandwidth is generous (>1Mbps)
Some nodes mobile, some fixed
Normally not energy limited
Resources are not an issue
Most traffic is user-to-gateway
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Relaying for (a) Throughput enhancement, and (b) Coverage extension
Relaying
Outline
Basic Concepts of Wireless Networks
Applications of Wireless Networks
Overview of Research Topics
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Applications
Broadband home networking.
Community networking.
Applications - Biomedical
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Habitat Monitoring on Great Duck Island http://www.greatduckisland.net/
Intel Research Laboratory at Berkeley initiated a collaboration with the College of the Atlantic in Bar Harbor and the University of California at Berkeley to deploy wireless sensor networks on Great Duck Island, Maine (in 2002)
Monitor the microclimates in and around nesting burrows used by the Leach's Storm Petrel
Goal : habitat monitoring kit for researchers worldwide
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Applications - Habitat monitoring
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FireBug
Wildfire Instrumentation System Using Networked Sensors Allows predictive analysis of evolving fire behavior Firebugs: GPS-enabled, wireless thermal sensor motes based on TinyOS that
self-organize into networks for collecting real time data in wild fire environments
Software architecture: Several interacting layers (Sensors, Processing of sensor data, Command center)
A project by University of California, Berkeley CA.32
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Metropolitan area networks
Transportation systems
Applications
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Applications
Emergency ResponseSource: www.meshdynamics.com
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Many Other Applications
Remote monitoring and control
Public transportation Internet access
Multimedia home networking
Source: www.meshnetworks.com
(now www.motorola.com).
Outline
Basic Concepts of Wireless Networks
Applications of Wireless Networks
Overview of Research Topics
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Overview of Research Topics
Physical Layer
MAC Layer
Network Layer
Transport Layer
Application Layer
Security
Network Management
Cross-layer design
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Advanced Physical Layer Techniques
Multiple-antenna systems
Combination of different modulation and coding rates
Using OFDM and UWB for high speed transmission
Using Multi-antenna systems like: MIMO, Smart antenna
Software Antenna: Programmable RF bands, Channel access modes and channel modulation
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PHY - Modulation
Existing modulations work well (OFDM, DSSS, FSK, etc.).
UWB may be an interesting alternative for short distances (480 Mbps up to 1.6 Gbps at distances up to a few meters)
Spread spectrum solutions are preferred as they tend to have better reliability in the face of Fading (very important for mobile applications) Interference (more of a factor than in any other
wireless system)
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PHY – Smart Antennas
Background Implemented as an array of
omnidirectional antennas By changing the phase,
beamforming can be achieved
The result is a software steered directional antenna
Omnidirectionalantenna
Variabledelay
Signal totransmit
Radiation Pattern
Directionchanged bythe delays
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PHY-Smart Antennas Advantages
Low power transmissions Battery not a big
concern in many applications
Enables better spatial reuse and, hence, increased network capacity
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PHY-Smart Antennas Advantages (cont.)
Punch-through links Better delays Less packet loss Better data rates Less power
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PHY-Smart Antennas Advantages (cont.)
Better SNR Better data rates Better delays Better error rates
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PHY-Smart Antennas Disadvantages
Specialized hardware Specialized MAC (difficult
to design) Difficult to track mobile
users
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PHY – Transmission Power Control
Too low Too high Just right
GW GWGW
Transmission power can control network topology
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PHY – Transmission Power Control (cont.)
Optimization CriteriaNetwork capacityDelayError ratesPower consumption
The ideal solution will depend onNetwork topologyTraffic load
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Overview of Research Topics Physical Layer
MAC Layer
Network Layer
Transport Layer
Application Layer
Security
Network Management
Cross-layer design
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MAC: A Simple Classification
WirelessMAC
Centralized Distributed
Guaranteedor
controlledaccess
Randomaccess
On Demand MACs, Polling
SDMA, FDMA, TDMA
Aloha, CSMA/CA
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MAC – MultichannelWhat?
Channels can be implemented by: TDMA (difficult due to lack
of synchronization) FDMA CDMA (code assignment is
an issue) SDMA (with directional
antennas) Combinations of the above
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Increases network capacity
MAC – MultichannelWhy?
B = bandwidth of a channel
User bandwidth = B/2
Ch-1
Ch-1
1 2
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User bandwidth = B
Ch-1
Ch-2
1 2
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Chain bandwidth = B
Ch-1
Ch-2
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2
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Multi-Hop Networks with Single Radio
Source Mesh Router Destination
With a single radio, a node can not transmit and receive simultaneously.
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Multi-Hop Networks with Multiple Radios
SourceMesh Router Destination
With two radios tuned to non-interfering channels, a node can transmit and receive
simultaneously.
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MAC – Research challenges
The scalability issue in multi-hop ad hoc networks has not been fully solved yet.
A MAC protocol for WNs must consider both scalability and heterogeneity between different network nodes.
Advanced bridging functions must be developed in the MAC layer
Development of MAC protocols with multiple QoS metrics such as delay, packet loss ratios and jitter
MAC/Physical Cross-layer design
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Overview of Research Topics Physical Layer
MAC Layer
Network Layer
Transport Layer
Application Layer
Security
Network Management
Cross-layer design
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Routing
Finds and maintains routes for data flows
The entire performance of the WN depends on the routing protocol
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Routing
An optimal routing protocol must capture the following features:
Multiple performance metrics Scalability Robustness (to link failure and congestion)
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Routing – different routing protocols
Routing protocols with various performance metrics
example : LQSR Multi-radio routing (using WCETT)
example: MR-LQSR Multi-path routing for load balancing and fault
tolerance Hierarchical routing Geographic routing
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Existing Routing Protocols Internet routing
protocols (e.g., OSPF, BGP, RIPv2) Well known and trusted Designed on the
assumption of seldom link changes
Without significant modifications are unsuitable for WNs in particular or for ad hoc networks in general.
Ad-hoc routing protocols (e.g., DSR, AODV, OLSR, TBRPF) Newcomers by
comparison with the Internet protocols
Designed for high rates of link changes; hence perform well on WNs
May be further optimized to account for WNs’ particularities
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Routing Protocols
Proactive protocols (OLSR , TBRPF) Determine routes independent of traffic pattern Traditional link-state and distance-vector routing
protocols are proactive
Reactive protocols (DSR , AODV) Maintain routes only if needed
Hybrid protocols
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Power-Aware Routing
Define optimization criteria as a function of energy consumption.
Minimize energy consumed per packet Maximize duration before a node fails due
to energy depletion
Example: LEACH Algorithm
Routing – Hierarchical Routing
Organizes the mobile nodes into clusters
Each cluster is governed by a cluster-head
Only heads send messages to a BS
Suitable for data fusion Self-organizing
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It is assumed that every node knows it own and its network neighbors positions.
Compared to topology-based routing schemes, geographic routing schemes forward packets by only using the position information of nodes in the vicinity and the destination node.
Topology change has less impact on the geographic routing than other routing protocols.
Routing – Geographic Routing
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Existing Routing Metrics are Inadequate
Source Mesh Router Destination
18 Mbps 18 Mbps
2 Mbps
11 Mbps 11 Mbps
Best path: 11 Mbps
Shortest path: 2 Mbps
Path with fastest links: 9 Mbps
Routing – metrics
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Link Metric: Expected Transmission Time (ETT)
Link loss rate = p Expected number of transmissions
Packet size = S, Link bandwidth = B Each transmission lasts for S/B
Lower ETT implies better link
ETXBS
ETT *
p-1
1 ETX
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ETT: Illustration
Source Destination
18 Mbps10% loss
11 Mbps5% loss
1000 Byte Packet
ETT : 0.77 ms
ETT : 0.40ms
18 Mbps50% loss
1000 Byte Packet
ETT : 0.77 ms
ETT : 0.89 ms
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Combining Link Metric into Path Metric
Add ETTs of all links on the path Use the sum as path metric
SETT = Sum of ETTs of links on path
Pro: Favors short paths
Con: Does not favor channel diversity
(Lower SETT implies better path)
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SETT does not favor channel diversity
Source
6 MbpsNo Loss
6 MbpsNo Loss
Mesh Router Destination
6 MbpsNo Loss
6 MbpsNo Loss
1.33ms 1.33ms
1.33ms1.33ms
Path Throughput SETT
Red-Red 3 Mbps 2.66 ms
Red-Blue 6 Mbps 2.66 ms
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Impact of Interference
Interference reduces throughput
Throughput of a path is lower if many links are on the same channel Path metric should be worse for non-diverse paths
Assumption: All links that are on the same channel interfere with one another Pessimistic for long paths
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Overview of Research Topics Physical Layer
MAC Layer
Network Layer
Transport Layer
Application Layer
Security
Network Management
Cross-layer design
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Impact of Transmission Errors
TCP can’t distinguish between packet losses due to congestion and transmission errors
Unnecessarily reduces congestion window
Throughput suffers
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TCP Problems
Causes for missing ACKs or loss packets in Wireless Networks:
Wireless transmission error Broken routes due to mobility (both users and wireless
routers) Delays due to MAC contention
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Impact of Multi-Hop Wireless Paths [Holland99]
0
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Number of hops
TCPThroughtput(Kbps)
TCP Throughput using 2 Mbps 802.11 MAC
Connections over multiple hops are at
a disadvantage compared to shorter connections, because they have to contend for wireless access at each hop
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TCPEfficiency Solutions
Focus on eliminating the confusion between congestion loss and all other reasons
Many approaches developed for single-hop wireless systems Snoop I-TCP M-TCP
End to end SACK Explicit error notification Explicit congestion notification
(e.g. RED) Several solutions for multi-hop
A-TCP Freeze-TCP
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Split Connection Approach
Connection between wireless host MH and fixed host FH goes through base station BS
FH-MH = FH-BS + BS-MH
FH MHBS
Base Station Mobile HostFixed Host
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Split connection results in independent flow control for the two parts
Flow/error control protocols, packet size, time-outs, may be different for each part
Examples : I-TCP and M-TCP
Split Connection Approach
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Overview of Research Topics Physical Layer
MAC Layer
Network Layer
Transport Layer
Application Layer
Security
Network Management
Cross-layer design
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Application Layer
Improve existing Internet applications in order to work under the architecture of WNs.
Propose new application-layer protocols for distributed information sharing.
Develop Innovative applications for WNs.
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Overview of Research Topics Physical Layer
MAC Layer
Network Layer
Transport Layer
Application Layer
Security
Network Management
Cross-layer design
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Security Authentication
Prevent theft of service Prevent intrusion by
malicious users
Privacy - user data is at risk while on transit in the WN due to: Wireless medium Multi-hop
Reliability – protect: Routing data Management data Monitoring data Prevent denials of service
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Overview of Research Topics Physical Layer
MAC Layer
Network Layer
Transport Layer
Application Layer
Security
Network Management
Cross-layer design
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Network Management Monitor the “health” of
the network Determine when is time
to upgrade Either hardware New gateway
Detect problems Equipment failures (often
hidden by the self-repair feature of the network)
Intruders Manage the system
Source: www.meshdynamics.com
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Overview of Research Topics Physical Layer
MAC Layer
Network Layer
Transport Layer
Application Layer
Security
Network Management
Cross-layer design
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In order to provide satisfactory performance of a wireless network, MAC, routing, and transport protocols have to work together with the physical layer.
Cross-layer Design
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Cross-layer design can be performed in two ways:
1 - To improve the performance of a protocol layer by considering parameters in other protocol layers.
Example1: the packet loss rate in the MAC layer can be reported to the transport layer so that a TCP protocol is able to differentiate congestion from packet loss. Example2: the physical layer can report the link quality to a routing protocol as an additional performance metric for the routing algorithms.
2- To merge several protocols into one component.
Example: in ad hoc networks, MAC and routing protocols can be combined into one protocol in order to closely consider their interactions.
Cross-layer Design
Questions!Questions!Questions!Questions!
