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

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

2. MANET • Any multi-hop wireless network in which nodes relay packets for each other • Examples: • Military Packet Radio Networks • Sensor Networks • Rooftop/Mesh Networks

3. 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

4. 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

5. Future prospects • Future military networks of sensors, robots, soldiers, ground, airborne vehicles • Hybrid wired/mobile-wireless civilian networks with large number of hops • …… • … • .

6. Where do we lack then..?

7. 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

8. A closer look..

9. 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

10. Bottleneck: Per packet overhead at each relay node

11. 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

12. 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

13. In MANETs • 3 primitive operations required • Relaying( Most common) • Transmitting • Receiving • Currently Relay = Receive -> Store -> Process -> Queue -> Forward -> Contend -> Transmit

14. 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

15. Radical contributions..

16. Next generation MANET architecture Three key features • Physical Layer optimized for multi-hop wireless networking • Access to medium for entire path (as opposed to single hop) • Cooperative transport of packets

17. 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

18. New Physical Layer • Has 3 primitive functions • Relay • Transmit • Receive • Switching at physical layer itself !

19. 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

20. 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

21. 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

22. Architecture Notional stack has 3 layers • Relay oriented Physical Layer (Relay PL) • Path Access Control (PAC) • Transport Layer No Network Layer !!

23. Architecture

24. 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 !

25. Lets Look in Detail…

26. 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

27. Relaying problems • Routing & Forwarding • Essentially to decide at node X, for a packet destined S -> D, whether to • Keep packet (X=D) • Discard it (X is not on path S -> D) • Re-broadcast (Relay)

28. 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

29. Relay-Oriented Transceiver

30. 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

31. 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

32. Routing features • “Capturing” of nodes ensures reliable broadcast of LSUs • As data rates increase what matters is • Propagation time of updates • Reliability of updates • Not how many control messages were sent!

33. 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

34. Before you ask me..

35. There is no mention of any naming mechanism at the physical layer!!Minor Implementation detail??

36. 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

37. Sentinel

38. Important Issue • Setting up of frequencies of each node’s RX and TX to enable full-duplex operation • Select TX frequency and let RX “auto-tune” • (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

39. Are you still awake ? Just checking ;-)

40. 3. Cooperative Transport • Cooperative Diversity: Operates entirely at the Physical Layer • Near 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

41. 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.

43. Future work • Developing h/w (Transceiver chipset) • Determining optimal segment lengths • Others…

44. Thanks… Ashish Sharma