PG 1 Advances in Wireless Networks Case Study: Mesh Networks Max Lakshtanov

Agenda Wireless Mobile Networking AP-based and ad hoc networking Wireless Mesh Networks 3G / 4G Cellular Networks

Mobile Ad Hoc Networks (MANET) Autonomous systems of mobile routers (and associated hosts) connected by wireless links Without (necessarily) using a pre-existing infrastructure Routes between nodes may potentially contain multiple hops

Mobile Ad Hoc Networks (MANET) May need to traverse multiple links to reach a destination

Mobile Ad Hoc Networks (MANET) Mobility causes route changes

Mobile Ad Hoc Networks (MANET) Three types: Autonomous – no external connections or gateways – all traffic remains in the ad hoc network Stub – limited number of gateways – all traffic originates or terminates in the ad hoc network Transit – external traffic can flow through the ad hoc network without terminating

Why Ad Hoc Networks? Rapid deployment Lower cost of infrastructure Lower cost of operations Improved reliability and robustness Mobility Supports peer-to-peer applications

Many Applications Personal area networking –cell phone, laptop, ear phone, wrist watch Military environments –soldiers, tanks, planes Civilian environments –taxi cab network –meeting rooms –sports stadiums –boats, small aircraft Emergency operations –search-and-rescue –policing and fire fighting

Many Variations Fully Symmetric Environment – all nodes have identical capabilities and responsibilities Asymmetric Capabilities – transmission ranges and radios may differ – battery life at different nodes may differ – processing capacity may be different at different nodes – speed of movement Asymmetric Responsibilities – only some nodes may route packets – some nodes may act as leaders of nearby nodes (e.g., cluster head)

Many Variations Traffic characteristics may differ in different ad hoc networks –bit rate –timeliness constraints –reliability requirements –unicast / multicast –host-based addressing / content-based addressing May co-exist (and co-operate) with an infrastructure-based network

Challenges Limited wireless transmission range Broadcast nature of the wireless medium –Hidden terminal problem Packet losses due to transmission errors Mobility-induced route changes Mobility-induced packet losses Battery constraints Potentially frequent network partitions Ease of snooping on wireless transmissions (security hazard)

Wireless Mesh Network (WMN) A 3G Alternative?

What is a Wireless Mesh Network (WMN) ? Consist of mesh routers and mesh clients Mesh routers have minimal mobility and form the backbone of WMNs Provide network access for both mesh and conventional clients

Mesh Routers PowerPCAdvanced Risc Machines

Mesh Clients LaptopPDA Wi-Fi Phone

Infrastructure / Backbone WMNs

Client WMNs

Hybrid WMNs

Why Wireless Mesh Networks ? Key characteristics –Auto-discovery of nodes and routes –Auto-configuration of network components –Mesh topology –Wireless interconnection Advantages –Rapid network deployment –Reduced infrastructure costs –Reduced engineering and operational costs –Increased network reliability

The Value of Wireless Mesh Networks Provides a value-added entry into the high-speed wireless packet data business – Utilizes technology – the interface of choice for high- speed wireless packet data Offers high-speed wireless packet data access across wider coverage areas – Today’s cellular systems don’t provide the bandwidth available in Wireless LANs – Today’s isolated hot-spot deployments don’t satisfy user desire for ubiquitous access or for mobility

The Value of Wireless Mesh Networks Minimizes cost of capital, installation and commissioning – Utilizes low cost technology – Uses wireless links for backhaul to eliminate costs associated with installation of wired interconnect – Auto-configuration algorithms in Wireless AP eliminate costs associated with engineering and organization of the wireless backhaul network

The Value of Wireless Mesh Networks Minimizes cost of operations –Uses wireless links for backhaul to eliminate costs associated with ongoing leasing of facilities –Auto-configuration, self-organizing and self healing are essential to the Wireless Mesh Network Highly flexible in terms of capacity, coverage, and availability –Increasing capacity, coverage, and/or availability simply means deploying more Wireless Access Points –Wireless Access Points may be deployed indoors or outdoors

Deploying a WMN Required components are already available –IEEE MAC protocol –ad hoc network routing protocols –wired equivalent privacy (WEP) security

Medium Access Control Protocol IEEE MAC protocol

Medium Access Control Wireless channel is a shared medium Need access control mechanism to avoid interference MAC protocol design has been an active area of research for many years

MAC: A Simple Classification Wireless MAC CentralizedDistributed Guaranteed or controlled access Random access

ABC Hidden Terminal Problem Node B can communicate with A and C both A and C cannot hear each other When A transmits to B, C cannot detect the transmission using the carrier sense mechanism If C transmits, collision will occur at node B

Busy Tone A receiver transmits busy tone when receiving data All nodes hearing busy tone keep silent Avoids interference from hidden terminals Requires a separate channel for busy tone

MAC Solution for Hidden Terminal Problem When node A wants to send a packet to node B, node A first sends a Request-to-Send (RTS) to A On receiving RTS, node A responds by sending Clear-to- Send (CTS), provided node A is able to receive the packet When a node (such as C) overhears a CTS, it keeps quiet for the duration of the transfer Transfer duration is included in RTS and CTS both ABC

Reliability 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

A Simple Solution to Improve Reliability When node B receives a data packet from node A, node B sends an Acknowledgement (Ack). This approach adopted in many protocols If node A fails to receive an Ack, it will retransmit the packet ABC

IEEE Wireless MAC Distributed and centralized MAC components – Distributed Coordination Function (DCF) – Point Coordination Function (PCF) DCF suitable for multi-hop ad hoc networking DCF is a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol

IEEE DCF Uses RTS-CTS exchange to avoid hidden terminal problem – Any node overhearing a CTS cannot transmit for the duration of the transfer Uses ACK to achieve reliability Any node receiving the RTS cannot transmit for the duration of the transfer – To prevent collision with ACK when it arrives at the sender When B is sending data to C, node A will keep quite ABC

Collision Avoidance With half-duplex radios, collision detection is not possible CSMA/CA: Wireless MAC protocols often use collision avoidance techniques, in conjunction with a (physical or virtual) carrier sense mechanism Carrier sense: When a node wishes to transmit a packet, it first waits until the channel is idle. Collision avoidance: Nodes hearing RTS or CTS stay silent for the duration of the corresponding transmission. Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit.

CFABED RTS RTS = Request-to-Send IEEE Assuming a circular range

CFABED RTS RTS = Request-to-Send IEEE NAV = 10 NAV = remaining duration to keep quiet

CFABED CTS CTS = Clear-to-Send IEEE

CFABED CTS CTS = Clear-to-Send IEEE NAV = 8

CFABED DATA DATA packet follows CTS. Successful data reception acknowledged using ACK. IEEE

CFABED ACK

CFABED IEEE Reserved area

IEEE CFABED DATA Transmit range Interference range Carrier sense range FA

CSMA/CA Physical carrier sense Virtual carrier sense using Network Allocation Vector (NAV) NAV is updated based on overheard RTS/CTS/DATA/ACK packets, each of which specified duration of a pending transmission Nodes stay silent when carrier sensed (physical/virtual) Backoff intervals used to reduce collision probability

Unicast Routing in Mobile Ad Hoc Networks

Why is Routing in MANET different ? Host mobility – link failure/repair due to mobility may have different characteristics than those due to other causes Rate of link failure/repair may be high when nodes move fast New performance criteria may be used – route stability despite mobility – energy consumption

Unicast Routing Protocols Many protocols have been proposed Some have been invented specifically for MANET Others are adapted from previously proposed protocols for wired networks No single protocol works well in all environments – some attempts made to develop adaptive protocols

Routing Protocols Proactive protocols – Determine routes independent of traffic pattern – Traditional link-state and distance-vector routing protocols are proactive Reactive protocols – Maintain routes only if needed Hybrid protocols

Trade-Off Latency of route discovery – Proactive protocols may have lower latency since routes are maintained at all times – Reactive protocols may have higher latency because a route from X to Y will be found only when X attempts to send to Y Overhead of route discovery/maintenance – Reactive protocols may have lower overhead since routes are determined only if needed – Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating Which approach achieves a better trade-off depends on the traffic and mobility patterns

New Breed of WLAN Attributes Traditional WLAN has wired connection to the network In Wireless Mesh Networks, Access Points connected via wireless Mesh topology Auto-discovery of nodes & routes Auto-configuration of Network Elements Self-healing Centralized management End-to-end security Ad Hoc + WLAN = Wireless Mesh

Current and Future Applications Public “Hotzone” Applications Downtown areas Neighborhoods Airports Train stations Hotels Conference centers Recreation area's Shopping malls Sports stadiums Libraries Enterprise Applications University Campus Office buildings Industrial Parks Military bases Factories Hospitals Universities Museums Railway / Shipping yards Retail

WMNs for Broadband Home Networking

WMNs for Community Networking

WMNs for Enterprise Networking

WMNs for Metropolitan Area Networks

3G and WiFi 3G –use licensed spectrum to provide wireless telephone coverage over some relatively large contiguous geographic serving area. –vertically-integrated, top-down, service-provider approach to delivering wireless Internet access WiFi – wireless Ethernet b standard for WLANs –end-user-centric, decentralized approach to service provisioning –operate using unlicensed spectrum in the 2.4GHz band.

3G and WiFi Both 3G and WiFi are access or edge-network technologies. They offer alternatives to the last-kilometer wireline network Beyond the last-kilometer, both rely on similar network connections and transmission support infrastructure. –For 3G, the wireless link is from the end-user device to the cell base station which may be at a distance of up to a few kilometers –For WiFi, the wireless link is a hundred meters from the end-user device to the base station.

Cellular + Nomadic Public WLANs: Unique Opportunity for high speed mobile data services for cost-effective, extended coverage

Summary Ad hoc Wireless Mesh Networks under development today Huge market potential Tremendous innovation in IEEE Very broad application / service usage High degree of customer interest Key technical enablers – Radio / antenna technology – Networking technology – IP, routing, security, mobile IP

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