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![Page 1: Challenges: A Radically New Architecture for Next Generation Mobile Ad Hoc Networks Ram Ramanathan Internetwork Research Department BBN Technologies.](https://reader036.fdocuments.us/reader036/viewer/2022062713/56649ccb5503460f94993ae2/html5/thumbnails/1.jpg)
Challenges: A Radically New Architecture for Next Generation Mobile Ad Hoc Networks
Ram RamanathanInternetwork Research Department
BBN Technologies
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MANET
Any multi-hop wireless network in which nodes relay packets for each other
Examples: Military Packet Radio NetworksSensor NetworksRooftop/Mesh Networks
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Motivation
Despite decades of research, MANETs continue to lag behind wireline networks in terms of Latency Capacity Robustness
Need for Low-latency, High bandwidth wireless networks
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Goals
Network with 1000+ Mobile Ad Hoc nodes Diameters (path-lengths) = 50-100 hops!! Transport capacity of 1 Gbps !! End-to-end latency less than 10ms Wireline robustness
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Future prospects
Future military networks of sensors, robots, soldiers, ground, airborne vehicles
Hybrid wired/mobile-wireless civilian networks with large number of hops
…… … .
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Where do we lack then..?
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Reasons for severe under-utilization of performance potential
Hop Centric approach Unsuitable Physical Layer for
multi-hop/relay-based communications Failure to utilize broadcast nature of
MANETs
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A closer look..
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Hop-centric approach
Processes are terminated and re-initiated at every hop
Large amount of processing, queuing and contention at each hop, for every packet
Each packet processed at 3 layers for header stripping
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Bottleneck: Per packet overhead at each relay node
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Subway train analogy
Its like getting off at each intermediate station en-route to one’s station
Going outside the station Waiting in line for fresh ticket Waiting for the next train Boarding it
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Unsuitable Physical Layer
We still use Physical Layer suited for single-wireless-hop networks (WLAN/Cellular)
Current Physical Layer optimized for 2 primitives
Receiving Transmitting
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In MANETs
3 primitive operations required1. Relaying( Most common)2. Transmitting3. Receiving
CurrentlyRelay = Receive -> Store -> Process -> Queue -> Forward -> Contend -> Transmit
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Failure to utilize Broadcast
We actually try to curb it by imposing wireline-like thinking
Most (traditional) routing protocols transmit to a single neighboring node
Broadcast can be used To increase signal quality End-to-end path capacity
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Radical contributions..
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Next generation MANET architecture
Three key features
1. Physical Layer optimized for multi-hop wireless networking
2. Access to medium for entire path (as opposed to single hop)
3. Cooperative transport of packets
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1. Physical Layer restructuring
Move “Routing” and “Forwarding” – functions to the physical layer!Routing:
To determine which set of nodes relay the packet from source to destination
Forwarding: To transport along this chosen path
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New Physical Layer
Has 3 primitive functionsRelayTransmitReceive
Switching at physical layer itself !
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2. Path-Centric hops
Atomic unit of operation = multiple hops
Medium Access Control is path-oriented
Packet does not have to re-contend at every hop
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3. Cooperative Transport
Harness unused resources to increase capacity of path
Concept of “Cooperative Diversity”Nodes simultaneously retransmit the same
packet on different frequencies/channels to be diversity combined at receivers
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How does this improve performance?
Reduced processing and elimination of re-contending at every hop will reduce latency
Cooperative transport increases capacity Path diversity increases path robustness
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ArchitectureNotional stack has 3 layers
1. Relay oriented Physical Layer(Relay PL)
2. Path Access Control(PAC)
3. Transport Layer
No Network Layer !!
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Architecture
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Important features
Paths are composed of “segments” A packet never leaves physical layer
throughout a segment PAC only invoked between segments Segment length: Interesting research
problem !
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Lets Look in Detail…
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1. Relay-oriented Physical Layer
Based on a multi-frequency/multi-band system Full-duplex operation: Simultaneously
transmitting and receiving using multiple frequencies
Start transmitting while you are still receiving the rest of the packet
Transit Routing Table at Physical Layer for routing decisions
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Relaying problems
Routing & Forwarding Essentially to decide at node X, for a
packet destined S -> D, whether toKeep packet (X=D)Discard it (X is not on path S -> D)Re-broadcast (Relay)
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Mechanism
Extract certain information (destination/signal strength/..) from Front of the packet
Use it to decide whether to keep/drop/relay, while still receiving remaining packet
Shunt the incoming stream to transmit chain
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Relay-Oriented Transceiver
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Routing Decisions ?
Transit Control Table at a node X contains mappings from every source (S), destination (D) pair to one of keep/drop/relay
Proactive Link-State Routing run at Physical Layer
Routing updates and Neighbor discovery probes do not use the MAC layer
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Link State Routing
Link State Updates (LSU) flooding when a link goes up or down
Flooding consists of a multihop network preamble followed by the actual LSU
Network Preamble “Captures” all nodes i.e. it gets them to ignore data transmission or reception and tune in to LSU
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Routing features
“Capturing” of nodes ensures reliable broadcast of LSUs
As data rates increase what matters isPropagation time of updatesReliability of updates
Not how many control messages were sent!
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Infrastructure for Relay PL
Hardware components well within scope of current technology
Routing logic & algorithms can be placed in Flash ROM (which are increasing in size & decreasing in cost)
Flexibility to use Software Radios – switching functionality can be in software
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Before you ask me..
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There is no mention of any naming mechanism at the physical layer!!
Minor Implementation detail??
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2. Path Access Control (PAC)
Acquires the floor for multiple hops, namely a segment, within which packets are relayed at physical layer
Segment Access Request (SAR) = multi-hop RTS
Segment Access Clear (SAC)= multi-hop CTS
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Sentinel
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Important Issue
Setting up of frequencies of each node’s RX and TX to enable full-duplex operation
1. Select TX frequency and let RX “auto-tune”
2. (Less efficient) Always use SAR/SAC and decide a priory in half duplex mode
Any path can be full-duplexed using no more than 3 frequencies
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Are you still awake ?
Just checking ;-)
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3. Cooperative Transport
Cooperative Diversity: Operates entirely at the Physical LayerNear simultaneous transmission of the same
information by multiple nodes that is coherently combined at the receiver
Gives much better SNR at receiver as essentially power of many nodes is added up
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Cooperative Diversity Level of synchronization required for
decoding depends upon the receiver technology e.g. MIMO
MIMO or equivalent technology required to diversity-combine the simultaneous transmissions
frequency diversity: receiving multiple versions of the same signal, being transmitted at different carrier frequencies.
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Future work
Developing h/w (Transceiver chipset) Determining optimal segment lengths Others…
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Thanks…
Ashish Sharma