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Mobile Computing CSE 40814/60814

Fall 2014

Intro to Ad-Hoc Networks •  What is an ad-‐hoc network?

– Direct device-‐device communicaCons – No infrastructure support (what is “infrastructure”?)

– Can be homogeneous networks (e.g., wireless sensor networks) or heterogeneous networks (MANETs)

Mobile devices (laptops, smartphones) Vehicular Networks on Highways

Hybrid urban ad-‐hoc network (vehicular, pedestrian, hot spots,…)

Examples

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More Examples

Disaster & Emergency alerts

Transportation/Vehicular Networks Sensor Networks

•  No infrastructure •  Mobility and dynamics •  Higher BER and losses •  Power constraints •  Scale •  ConCnuous change of locaCon (addressing?) [wired is physically based]

•  ConnecCvity is funcCon of relaCve posiCons, radio power, etc. May be asymmetric.

•  Ease/difficulty of setup •  Self-‐organizing (self-‐healing)

Characteristics of Ad-Hoc Nets

Differences Between Cellular and Ad-Hoc Networks

Cellular Networks Ad-Hoc Networks

Fixed, pre-located cell sites and base stations

No fixed base stations, very rapid deployment

Static backbone network topology Highly dynamic network topologies, with multi-hop communications

Relatively favorable environment and stable connectivity

Hostile environment (losses, noise) and irregular connectivity

Detailed planning before base stations can be installed

Ad-hoc network automatically forms and conforms to change

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Types of Ad-Hoc Networks

•  MANET: Mobile Ad-Hoc Network •  WSN: Wireless Sensor Network •  WMN: Wireless Mesh Network •  VANET: Vehicular Ad-Hoc Network •  FANET: Flying Ad-Hoc Network

MANET

•  A Mobile Ad-hoc Network (MANET) is a collection of autonomous nodes or terminals which communicate with each other by forming a multi-hop radio network and maintaining connectivity in a decentralized manner over relatively bandwidth constrained wireless links.

•  Each device in a MANET is free to move independently in any direction, and will therefore change its links to other devices frequently.

•  The topology is highly dynamic and frequent changes in the topology may be hard to predict.

MANET Constraints and Issues •  Lack of a centralized entity •  Network topology changes frequently and unpredictably •  Routing and mobility management •  Channel access/bandwidth availability •  Hidden/exposed station problem •  Lack of symmetrical links •  Physical security is limited due to the wireless transmission •  Affected by higher loss rates, and can experience higher delays

and jitter than fixed networks due to the wireless transmission •  As nodes are battery operated (power constraint), energy savings

are an important system design criterion.

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Network Architecture •  MANETs are formed by set of mobile nodes such as

laptops, mobile phones etc. •  Mobile ad-hoc networks are based on wireless links •  Can be single-hop or multi-hop communication •  In single-hop communication, all hosts are in one

coverage area and hence communication is directly from host to host

•  In multi-hop communication, hosts communicate using intermediate hosts as many coverage areas intersect with each other

•  Packets may need to traverse mulCple links to reach desCnaCon

•  Mobility causes route changes

Multi hop communication

Classifications of MAC protocols •  Ad-hoc network MAC protocols can be classified into three types:

–  Contention-based protocols –  Contention-based protocols with reservation mechanisms –  Contention-based protocols with scheduling mechanisms – Other MAC protocols

MAC Protocols for Ad-Hoc Wireless Networks

Contention-Based Protocols

ContenCon-‐based protocols with reservaCon mechanisms

Other MAC Protocols

ContenCon-‐based protocols with scheduling mechanisms

Sender-Initiated Protocols

Receiver-Initiated Protocols

Synchronous Protocols

Asynchronous Protocols

Single-Channel Protocols

Multichannel Protocols

MACAW FAMA

BTMA DBTMA

ICSMA

RI-BTMA MACA-BI MARCH

D-PRMA CATA HRMA

RI-BTMA MACA-BI MARCH

SRMA/PA

FPRP

MACA/PR RTMAC

DirectionalAntennas

MMAC

MCSMA

PCM RBAR

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Multiple Access with Collision Avoidance (MACA) •  MACA uses signaling packets for collision avoidance

–  RTS (request to send): sender requests the right to transmit using a short RTS packet before it sends a data packet

–  CTS (clear to send): receiver grants the right to send as soon as it is ready to receive

•  Signaling packets contain –  sender address –  receiver address –  packet size

•  Neighboring nodes that overhear an RTS/CTS reservation have to defer their own transmissions

MACA Examples •  MACA avoids the problem of hidden terminals

– A and C want to send to B

– A sends RTS first

–  C waits after receiving CTS from B

•  MACA avoids the problem of exposed terminals

–  B wants to send to A and C wants to send to another terminal

–  now C does not have to wait because it cannot hear the CTS from A

A B C

RTS

CTS CTS

A B C

RTS

CTS

RTS

–  MACA does not provide ACK –  RTS-CTS approach does not always solve the hidden node problem –  Example

•  A sends RTS to B •  B sends CTS to A; At the same time, D sends RTS to C •  The CTS & RTS packets collide at C •  A transmits data to B; D resends RTS to C; C sends CTS to D •  The data & CTS packets collide at B

Limitations

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MACAW •  MACAW (MACA for Wireless) is a revision of MACA (without ACK).

–  The sender senses the carrier and transmits an RTS (Request To Send) frame if no other nearby station transmits an RTS

–  The receiver replies with a CTS (Clear To Send) frame – Neighbors:

•  see CTS, then keep quiet •  see RTS but not CTS, then keep quiet until the CTS is sent back to the

sender –  The receiver sends an ACK when receiving a frame

•  Neighbors keep silent until they see an ACK –  Collisions

•  There is no collision detection •  The senders detect collision when they don’t receive CTS frames •  They each wait for the exponential backoff time

MACAW (MACA for Wireless)

•  RTS-CTS-DS-DATA-ACK –  RTS from A to B –  CTS from B to A –  Data Sending (DS) from A to B –  Data from A to B –  ACK from B to A –  Random wait after any successful/unsuccessful

transmission •  Significantly higher throughput than MACA •  Does not completely solve hidden & exposed node problems

Power Aware MAC Protocols

•  Minimize expensive retransmissions due to collisions •  Transceivers should be kept in standby mode as much as

possible •  Switch to low power mode sufficient for the destination to

receive the packet •  Two categories

–  Alternate between sleep and awake cycles –  Vary transmission power

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PAMAS (Power aware medium access control with signaling)

•  RTS-CTS exchanges over a signaling channeling •  Data transmission over a separate data channel •  Receiver sends out a busy tone, while receiving a data packet over

the signaling channel •  Nodes listen to the signaling channel to determine when it is

optimal to power down transceivers •  A node powers itself off if it has nothing to transmit and its

neighbor is transmitting •  A node powers off if at least one neighbor is transmitting and

another is receiving

PCM: Power Control Medium access control

•  Send RTS & CTS packets using max available power •  Send DATA & ACK with the min power required to communicate

between the sender and receiver •  Based on the received signal strength of the RTS/CTS packet,

adjust the power level for DATA transmission •  Drawbacks

–  Requires rather accurate estimation of the received signal strength, which is hard in wireless communication

–  Difficult to implement frequent changes in the transmission power level

MANET Routing Protocols

•  Proactive Protocols

–  Table driven

–  Continuously evaluate routes

–  No latency in route discovery

–  Large capacity to keep

network information current

–  A lot of routing information

may never be used!

•  Reactive Protocols

–  On Demand

–  Route discovery by some

global search

–  Bottleneck due to latency of

route discovery

–  May not be appropriate for

real-time communication

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–  Send periodic updates of the routes –  Each node uses routing information to store the location

information of other nodes in the network and this information is then used to move data among different nodes in the network

–  May have low latency since routes are maintained at all times

Table-Driven Routing Protocols

Destination Sequenced Distance Vector (DSDV)

•  Each route is tagged with a sequence number originated by destination

•  Hosts perform periodic & triggered updates, issuing a new sequence number

•  Sequence number indicates the “freshness” of a route –  Routes with more recent sequence numbers are preferred

for packet forwarding –  If same sequence number, one having smallest metric used

Topology Changes •  Broken links assigned a metric of ∞ •  Any route through a hop with a broken link is also assigned a

metric of ∞ •  “∞ routes” are assigned new sequence numbers by any host

and immediately broadcast via a triggered update •  If a node has an equal/later sequence number with a finite

metric for an “∞ route”, a route update is triggered

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DSDV Operation

Dynamic Source Routing (DSR)

•  Each packet header contains a route, which is represented as a complete sequence of nodes between a source-destination pair

•  Protocol consists of two phases –  route discovery –  route maintenance

•  Optimizations for efficiency –  Route cache –  Piggybacking –  Error handling

DSR Route Discovery

•  Source broadcasts route request (id, target) •  Intermediate node action

–  Discard if node is source or node is in route record –  If node is the target, route record contains the full route to

the target; return a route reply –  Else append address in route record; rebroadcast

•  Use existing routes to source to send route reply; else piggyback

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Route Discovery in DSR

B

A

S E F

H

J

D

C

G

I K

Z

Y

Represents a node that has received RREQ for D from S

M

N

L


B

A

S E F

H

J

D

C

G

I K

Represents transmission of RREQ

Z

Y Broadcast transmission

M

N

L

[S]

[X,Y] Represents list of identifiers appended to RREQ


B

A

S E F

H

J

D

C

G

I K

•  Node H receives packet RREQ from two neighbors: potential for collision

Z

Y

M

N

L

[S,E]

[S,C]

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B

A

S E F

H

J

D

C

G

I K

•  Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

Z

Y

M

N

L

[S,C,G]

[S,E,F]


B

A

S E F

H

J

D

C

G

I K

Z

Y

M

•  Nodes J and K both broadcast RREQ to node D •  Since nodes J and K are hidden from each other, their transmissions may collide

N

L

[S,C,G,K]

[S,E,F,J]


B

A

S E F

H

J

D

C

G

I K

Z

Y

•  Node D does not forward RREQ, because node D is the intended target of the route discovery

M

N

L

[S,E,F,J,M]

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•  Destination D on receiving the first RREQ, sends a Route Reply (RREP)

•  RREP is sent on a route obtained by reversing the route appended to received RREQ

•  RREP includes the route from S to D on which RREQ was received by node D

Route Reply in DSR

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

RREP [S,E,F,J,D]

Represents RREP control message

Dynamic Source Routing (DSR)

•  Node S on receiving RREP, caches the route included in the RREP

•  When node S sends a data packet to D, the enCre route is included in the packet header –  hence the name source rouCng

•  Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded

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Data Delivery in DSR

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

DATA [S,E,F,J,D]

Packet header size grows with route length

DSR Optimization: Route Caching •  Each node caches a new route it learns by any means •  When node S finds route [S,E,F,J,D] to node D, node S also learns

route [S,E,F] to node F •  When node K receives Route Request [S,C,G], node K learns

route [K,G,C,S] to node S •  When node F forwards Route Reply RREP [S,E,F,J,D], node F

learns route [F,J,D] to node D •  When node E forwards Data [S,E,F,J,D] it learns route [E,F,J,D] to

node D •  A node may also learn a route when it overhears data •  Problem: Stale caches may increase overheads

Dynamic Source Routing: Advantages

•  Routes maintained only between nodes who need to communicate –  reduces overhead of route maintenance

•  Route caching can further reduce route discovery overhead

•  A single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches

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DSR: Disadvantages •  Packet header size grows with route length due to source

routing •  Flood of route requests may potentially reach all nodes in

the network •  Potential collisions between route requests propagated by

neighboring nodes –  insertion of random delays before forwarding RREQ

•  Increased contention if too many route replies come back due to nodes replying using their local cache –  Route Reply Storm problem

•  Stale caches will lead to increased overhead

AODV •  Route Requests (RREQ) are forwarded in a manner similar to

DSR

•  When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source –  AODV assumes symmetric (bi-directional) links

•  When the intended destination receives a Route Request, it replies by sending a Route Reply (RREP)

•  Route Reply travels along the reverse path set-up when Route Request is forwarded

AODV Forward Path Setup •  RREQ arrives at a node that has current route to the

destination ( larger/same sequence number) •  Unicast request reply (RREP) <source_addr, dest_addr,

dest_sequence_#, hop_cnt, lifetime> to neighbor •  RREP travels back to the source along reverse path •  Each upstream node updates dest_sequence_#, sets up a

forward pointer to the neighbor who transmit the RREP

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AODV Reverse Path Setup •  Counters : Sequence number, Broadcast id •  Reverse Path

–  Broadcast route request (RREQ) < source_addr, source_sequence-# , broadcast_id , des t_addr, dest_sequence_#, hop_cnt >

–  RREQ uniquely identif ied by <source_addr , broadcast_id>

–  Route reply (RREP) if neighbor is the target, or knows a higher dest_sequence_#

–  Otherwise setup a pointer to the neighbor from whom RREQ was received

–  Maintain reverse path entries based on timeouts

Route Requests in AODV

B

A

S E F

H

J

D

C

G

I K

Z

Y

Represents a node that has received RREQ for D from S

M

N

L


B

A

S E F

H

J

D

C

G

I K

Represents transmission of RREQ

Z

Y Broadcast transmission

M

N

L

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B

A

S E F

H

J

D

C

G

I K

Represents links on Reverse Path

Z

Y

M

N

L

Reverse Path Setup in AODV

B

A

S E F

H

J

D

C

G

I K

•  Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

Z

Y

M

N

L


B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

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B

A

S E F

H

J

D

C

G

I K

Z

Y

•  Node D does not forward RREQ, because node D is the intended target of the RREQ

M

N

L

Forward Path Setup in AODV

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

Forward links are setup when RREP travels along the reverse path Represents a link on the forward path

Route Request and Route Reply •  Route Request (RREQ) includes the last known sequence

number for the destination •  An intermediate node may also send a Route Reply (RREP)

provided that it knows a more recent path than the one previously known to sender

•  Intermediate nodes that forward the RREP, also record the next hop to destination

•  A routing table entry maintaining a reverse path is purged after a timeout interval

•  A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval

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Link Failure •  A neighbor of node X is considered active for a routing table

entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry

•  Neighboring nodes periodically exchange hello message

•  When the next hop link in a routing table entry breaks, all active neighbors are informed

•  Link failures are propagated using Route Error (RERR) messages, which also update destination sequence numbers

AODV: Summary

•  Routes need not be included in packet headers •  Nodes maintain routing tables containing entries only for routes

that are in active use •  At most one next-hop per destination maintained at each node

–  DSR may maintain several routes for a single destination •  Sequence numbers are used to avoid old/broken routes •  Sequence numbers prevent formation of routing loops •  Unused routes expire even if topology does not change

DSR vs. AODV DSR AODV

routing table format full path next hop

route checking passive acks ‘hello’ pings

rate of propogation of topology changes

fast slower

ability to handle frequent topology change

good fair

CPU / memory usage high low

scalability poor fair

53

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Location-Based Routing •  Also referred to as geographic routing•  Used when nodes are able to determine their (approximate)

positions•  Nodes use location information to make routing decisions

–  sender must know the locations of itself, the destination, and its neighbors

–  location information can be queried or obtained from a location broker

•  Types of geographic routing:–  unicast: single destination–  multicast: multiple destinations–  geocast: data is propagated to nodes within certain geographic

area

Unicast Location-Based Routing •  One single destination•  Each forwarding node makes localized decision based on the

location of the destination and the node’s neighbors (greedy forwarding)

•  Challenge: packet may arrive at a node without neighbors that could bring packet closer to the destination (voids or holes)

Source Destination

Greedy Perimeter Stateless Routing •  In GPRS, a node forwards packet to neighbor that is geographically

closest to the destination•  Challenge of voids/holes (example: x is closer to the destination

than its neighbors w and y)•  GPRS uses right-hand rule to traverse graph

–  when a packet arrives at node x from node y, the next edge traversed is the next one sequentially counterclockwise about x from edge (x,y)

–  right-hand rule traverses interior of a polygon in clockwise edge order and exterior region in counterclockwise edge order

(b)

VOID

x

y

z

1.

2.

3.

(a)

w

v z

y

x

Destination

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Forwarding Strategies •  Greedy: minimize distance to destination in each hop•  Nearest with Forwarding Progress (NFP): nearest of all neighbors that make

positive progress (in terms of geographic distance) toward destination•  Most Forwarding Progress within Radius (MFR): neighbor that makes

greatest positive progress (progress is distance between source and its neighbor node projected onto a line drawn from source to destination)

•  Compass Routing: neighbor with smallest angle between a line drawn from source to the neighbor and the line connecting source and destination

S

A

B

D

E

C

Geographic Adaptive Fidelity •  GAF was primarily designed for networks with mobile nodes•  Network region is divided into virtual grid•  In each cell, only one node is a forwarder at any given time (all other

nodes can sleep)•  Assumption: all nodes within a cell can communicate with all nodes

within all adjacent cells

Base Station

Multicast Location-Based Routing •  Multicast: deliver the same packet to multiple

receivers•  Simple solution 1: deliver copies of same packet to

each individual receiver via unicast routing–  resource-inefficient

•  Simple solution 2: flood the entire network–  resource-inefficient

•  Concerned with efficiently delivering the same packet to receivers, i.e., minimize the number of links the packet has to travel

•  Common approach: multicast tree rooted at source and destinations are leaf nodes

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Scalable Position-Based Multicast •  SPBM relies on group management scheme to maintain list of all

destinations for a packet•  Packet header carries group membership information instead of all

destination’s addresses to ensure efficiency•  Network is represented as a quad-tree with pre-defined number of

levels L•  Example: L=3 (levels 0..L-1)•  All nodes within level-0 square are in radio range of each other

442

Level 3

1

2 3

41

42 43

443

441444

1

2 3

41

42 43

Level 0

Level 1

Level 2

443

444

Source

441

442

Geocasting •  Packet is sent to all or some nodes within

specific geographic region•  Example: query sent to all sensors within

geographic area of interest

•  Routing challenge:–  propagate a packet near the target region (similar

to unicast routing)–  distribute packet within the target region (similar

to multicast routing)

Geographic-Forwarding-Perimeter-Geocast

Source Source

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Wireless routers

Gateways

Printers, servers

Mobile clients

Stationary clients

Intra-mesh wireless links

Stationary client access

Mobile client access

Internet access links

Node Types Link Types

Mesh Networks

Gateways

•  Multiple interfaces (wired & wireless)

•  Mobility –  Stationary (e.g., rooftop) –

most common case –  Mobile (e.g., airplane,

busses/subway) •  Serve as (multi-hop)

“access points” to user nodes

•  Relatively few are needed, (can be expensive)

GW

Wireless Routers Ø  At least one wireless interface Ø  Mobility

Ø  StaConary (e.g., roocop) Ø  Mobile (e.g., airplane, busses/

subway). Ø  Provide coverage (acts as a mini-‐

cell-‐tower) Ø  Do not originate/terminate data

flows Ø  Many needed for wide areas,

hence, cost can be an issue

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Users •  Typically one interface. •  Mobility

–  Stationary –  Mobile

•  Connected to the mesh network through wireless routers (or directly to gateways)

•  The only sources/destinations for data traffic flows in the network.

User – Wireless Router Links •  Wired

–  Bus (PCI, PCMCIA, USB) –  Ethernet, Firewire, etc.

•  Wireless –  802.11x –  Bluetooth –  Proprietary

•  Point-to-Point or Point-to-Multipoint

•  If properly designed is not a bottleneck.

•  If different from router-to-router links we’ll call them access links

Router to Router Links •  Wireless

–  802.11x –  Proprietary

•  Usually multipoint to multipoint –  Sometimes a collection of

point to point

•  Often the bottleneck •  If different from router-

to-user links we’ll call them backbone links

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Gateway to Internet Links •  Wired

–  Ethernet, TV Cable, Power Lines

•  Wireless –  802.16 –  Proprietary

•  Point to Point or Point-to-Multipoint

•  We’ll call them backhaul links

•  If properly designed, not the bottleneck

How it Works •  User-Internet Data Flows

–  In most applications the main data flows

•  User-User Data Flows

–  In most applications a small percentage of data flows

Taxonomy Wireless

Networking

MulC-‐hop

Infrastructure-‐less (ad-‐hoc)

Infrastructure-‐based (Hybrid)

Infrastructure-‐less (MANET)

Single Hop

Cellular Networks Wireless Sensor

Networks Wireless Mesh

Networks

Car-‐to-‐car Networks (VANETs)

Infrastructure-‐based (hub&spoke)

802.11 802.16 Bluetooth 802.11

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Mesh vs. Ad-Hoc Networks

•  Multihop •  Nodes are wireless,

possibly mobile

•  May rely on infrastructure

•  Most traffic is user-to-user

Ad-Hoc Networks Wireless Mesh Networks

•  Multihop •  Nodes are wireless,

some mobile, some fixed

•  It relies on “infrastructure”

•  Most traffic is user-to-gateway

Mesh vs. Sensor Networks

Ø  Bandwidth is limited (tens of kbps) Ø  In most applicaCons, fixed nodes Ø  Energy efficiency is an issue Ø  Resource constrained Ø  Most traffic is user-‐to-‐gateway

Wireless Sensor Networks Wireless Mesh Networks Ø  Bandwidth is generous (>1Mbps) Ø  Some nodes mobile, some fixed Ø  Normally not energy limited Ø  Resources are not an issue Ø  Most traffic is user-‐to-‐gateway

Broadband Internet Access

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Extend WLAN Coverage

Source: www.meshdynamics.com

Traffic Mgt, Law Enforcement, …

Source: www.meshnetworks.com

(now www.motorola.com).

Emergency Response


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Layer 2 Connectivity

•  The entire wireless mesh cloud becomes one (giant) Ethernet switch

•  Simple, fast installation –  Short-term events (e.g.,

conferences, conventions, shows)

–  Where wires are not desired (e.g., hotels, airports)

–  Where wires are impossible (e.g., historic buildings) Internet

Military Communications


Community Networks

Source: research.microsoft.com/mesh/

Ø  Grass-‐roots broadband Internet Access

Ø  Several neighbors may share their broadband connecCons with many other neighbors

Ø  Not run by ISPs Ø  Possibly in the

disadvantage of the ISPs

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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).

Overview •  Wireless multi-hop infra networks, where a few

nodes provide a connection to the external world (e.g., Internet) through a cable

•  Alternative wireless access technology, which can replace the traditional sets of IEEE 802.11 wireless LANs

•  Commercialized and managed ad hoc networks, which Introduce a hierarchy in the network architecture with fixed, special routers and mobile, general clients

Overview •  Many vendors have developed their own WMN

solutions and put them on the market, because they are flexible and more cost effective than the typical wired APs. –  Motorola –  Tropos –  Belair –  PacketHop

•  However, though most of them are based on the common 802.11 MAC, these products are not interoperable. –  Need for defining a standard architecture for WMNs!

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WLAN (Layer 2) Mesh Networks •  The 802.11 Task Group “s” (TGs)

–  Formed in May 2004 to design mesh networks consisting of different WLAN devices performing routing at link layer (layer 2)

–  To be based on extensions to the current IEEE 802.11 architecture and protocols:

Internet

STA

STA

STASTA

STA

APAP

BSSBSS

IBSS

ESS

DS

IEEE 802.11s: Meshed WLAN Networks

It will provide an IEEE 802.11 Wireless DS that supports both broadcast/multicast and unicast delivery at the MAC layer using radio-aware metrics over self-configuring multi-hop topologies.

•  The objectives: –  Increased range/coverage & flexibility in use –  Possibility of increased throughput –  Reliable performance –  Seamless security –  Power efficient operation –  Multimedia transport between devices –  Backward compatibility and interoperability for

interworking

802.11s WLAN Mesh - Network Architecture

•  Mesh Portal: Acting as a gateway/bridge to external networks •  Mesh STA (station): Relay frames in a router-like hop-by-hop fashion •  Mesh AP (Access Point): Mesh relaying functions + AP service for clients

MeshSTA

STA

STASTA

STA

External Internet

Mesh Portal

Mesh AP

MeshAP

MeshAP

PortalMesh

Mesh Links

Non-mesh STAs

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Mesh Basic Service Set (MBSS) •  Internal L2 behavior of WLAN Mesh is transparent to higher layers

–  An MBSS (Mesh Basic Service Set) appears as a single access domain.

Usage Models •  Residential

–  Inside home or a residential building –  High bandwidth application, such as multimedia

content distribution

•  Office –  Small to medium sized enterprise buildings

•  Campus/Community/Public access –  Out-door deployment environment –  Seamless connectivity

•  Public Safety –  Emergency sites

Functional Requirements •  The set of services provided by the WLAN Mesh that support the control,

management, and other operaCon, including the transport of MSDUs between routers within the WLAN Mesh.

PHYs

Mesh Topology Learning, Routing &

Forwarding

Medium Access Coordination

Discovery & Association

802.11 service integration Mesh Configuration & Management

Mesh Measurement

Mesh Security

Mesh Interworking with other 802 networks

LAN metaphor, 802.1 bridging support

802.11i link security based

MAC enhancements

Unmanaged, autonomic management

Legacy 802.11 a/b/g/n

Single-hop/multi-hop neighbor discovery, Extensible path selection & forwarding

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Key Functionality of 802.11s Networks

•  Mesh Topology Creation –  Self-configuring neighbor discovery called “mesh peering” –  Channel selection

•  L2 Routing –  MAC address based mesh path selection and forwarding –  Radio-aware metrics for routing

•  MAC Enhancement –  For supporting QoS, and increasing the network throughput –  Power management, multi-channel operation, …

•  Security –  IEEE 802.11i as basis

Mesh Peering Mechanism •  To discover peer Mesh STA devices and their

properties: –  MSTA performs passive scanning (via periodic beacons) or

active scanning (via probe messages)

–  The received beacon or probe response frame contains mesh related information

•  Mesh ID: name of the mesh (SSID like string) •  Mesh configuration element (including version and support

functions)

–  A discovered MSTA will become a peer MSTA after peering succeeds using a 4-way handshake

•  2-way handshake in each direction

Mesh Path Selection and Forwarding

•  To select single/multi-hop path(s) and to forward data frames across these paths between routers at the link layer.

•  Extensible path selection framework –  A WLAN Mesh may include multiple path selection metrics and

protocols for flexibility. –  A mandatory protocol and a mandatory metric for all

implementations are specified. •  Hybrid Wireless Mesh Protocol (HWMP) •  Airtime link metric function

–  Only one protocol/metric will be active on a particular link at a time. –  A particular mesh will have only one active protocol at a time.

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Airtime Link Metric Function •  A default link metric to be used by a path selection protocol to select the

best paths. –  Other metrics can also be used.

•  Its cost function is based on airtime cost (Ca), which reflects the amount of channel resources consumed by transmitting the frame over a particular link.

f

ta er

BOc−⎥

⎦

⎤⎢⎣

⎡+=

11

Parameter Description

O Channel access overhead including frame headers, training sequences, access protocol frames, etc. (depending on PHY)

Bt Test frame length in bits (Constant) r Transmission data rate in Mb/s for the test

frame size Bt

ef Test frame error/loss rate for Bt

Example

48Mb/s, 10% PER

54Mb/s, 8% PER

12Mb/s, 10% PER

54Mb/s, 2% PER

54Mb/s, 2% PER

48Mb/s, 10% PER

This path having the minimum airtime cost is the best!

§  Unicast Cost FuncCon based on AirCme Link Metrics

Hybrid Wireless Mesh Protocol (HWMP)

•  A default path selection protocol for interoperability.

•  To combine the flexibility of on-demand route discovery with extensions to enable efficient proactive routing to mesh portals. –  On-demand mode

•  Used in intra-mesh routing for the route optimization •  When a root portal is not configured or it can provide a better

path even if root is configured.

–  Proactive, Tree based mode •  If a root portal is present, a distance vector routing tree is built. •  Tree based routing avoids unnecessary discovery flooding during

discovery and recovery

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HWMP: On-demand Path Selection Mode

1.  Source broadcasts PREQ (path request) with the destination and metric initialized.

2.  Upon receiving PREQ, routers update the path to source if sequence number is greater and offers a better metric

3.  If a new path is created or the existing one is modified, PREQ is forwarded

further.

4.  PREQ provides “Target only” (TO) and “Reply and Forward” (RF) flags. •  If TO=1: Only destination sends PREP (path reply) after selecting best path. •  If TO=0 and RF =0: Intermediate node with path sends a unicast PREP to

the source router and does not forward PREQ •  If TO=0 and RF =1: The first intermediate node with the path to the

destination sends a PREP and forwards PREQ setting TO =1 to avoid other intermediate nodes to send back PREP.

5. When source receives the PREP, it creates a path to the destination.

•  The existing 802.11 MAC layer is being enhanced to: – support QoS:

•  EDCA(Enhanced Distributed Channel Access) specified in 802.11e, as the 802.11s’ basic operation mechanism

•  Other features of 802.11e, like HCCA, are not considered.

–  Improve the network capacity: •  The usage of multiple channels and multiple radios •  Efficient handling of the two different kinds of traffic

(BSS traffic & forwarding mesh traffic) •  Intra-mesh congestion control

802.11s MAC Enhancements

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