Challenges: A Radically New Architecture for Next Generation Mobile Ad Hoc Networks Ram Ramanathan Internetwork Research Department BBN Technologies.

Challenges: A Radically New Architecture for Next Generation Mobile Ad Hoc Networks

Ram RamanathanInternetwork Research Department

BBN Technologies

MANET

Any multi-hop wireless network in which nodes relay packets for each other

Examples: Military Packet Radio NetworksSensor NetworksRooftop/Mesh Networks

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

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

Future prospects

Future military networks of sensors, robots, soldiers, ground, airborne vehicles

Hybrid wired/mobile-wireless civilian networks with large number of hops

…… … .

Where do we lack then..?

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

A closer look..

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

Bottleneck: Per packet overhead at each relay node

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

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

In MANETs

3 primitive operations required1. Relaying( Most common)2. Transmitting3. Receiving

CurrentlyRelay = Receive -> Store -> Process -> Queue -> Forward -> Contend -> Transmit

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

Radical contributions..

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

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

New Physical Layer

Has 3 primitive functionsRelayTransmitReceive

Switching at physical layer itself !

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

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

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

ArchitectureNotional stack has 3 layers

1. Relay oriented Physical Layer(Relay PL)

2. Path Access Control(PAC)

3. Transport Layer

No Network Layer !!

Architecture

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 !

Lets Look in Detail…

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

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)

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

Relay-Oriented Transceiver

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

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

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!

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

Before you ask me..

There is no mention of any naming mechanism at the physical layer!!

Minor Implementation detail??

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

Sentinel

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

Are you still awake ?

Just checking ;-)

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

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.

Future work

Developing h/w (Transceiver chipset) Determining optimal segment lengths Others…

Thanks…

Ashish Sharma