Some problems in ad hoc wireless networking. Balaji Prabhakar

Transcription

1 Some problems in ad hoc wireless networking Balaji Prabhakar

2 Background Example scenarios for ad hoc packet networks - sensor networks (many nodes, low data rates) - wireless LANs (fewer nodes, high data rates) - mobile nodes, e.g. automobiles (many nodes, high(?) data rates) Sensor networks - development of low-power digital sensors makes large wireless sensor networks feasible - thriving research area in academia (Berkeley, MIT, UCLA, USC) - tremendous support from government agencies (DARPA, NSF, etc...) Wireless LANs - IEEE b and HiperLAN are getting widely deployed - need to understand their performance, scalability, and interoperability with wireline networks Ad hoc mobile networks - great interest in providing networking capability to automobiles

3 Issues Metrics - power/energy consumption - node throughput (user cares about this) - network throughput (network operator cares about this) - packet delay Problems - limited energy supply (for sensor networks) - interference: limits throughput - lack of protocols for coordination Mobility: friend or foe? - friend: it mixes fading states - friend: it can increase throughput - foe: it introduces many network-layer problems

4 Our interests Networks of Wireless Image Sensors (El Gamal and P) - El Gamal et al have developed a single chip digital camera - we have been interested in building wireless imaging networks Ad hoc networks (Bambos, El Gamal, Goldsmith and P) - it is known that interference severely limits their throughput - we re interested in how the throughput and delay can be improved using clever physical and networking layer tricks, e.g. 1. variable rate transmission, multihop routing, spatial reuse, successive interference cancellation (Goldsmith) 2. power control for limiting interference (Bambos and Goldsmith) 3. low-energy transmission, relaying, distributed transmission and routing protocols (Bambos, El Gamal, Goldsmith and P) Ad hoc mobile networks - mobility is known to improve throughput - at the expense of high delays - mobility is also difficult to manage (Tobagi s talk)

5 Rest of the talk Is a technical presentation - that takes sensor networks as a primary motivation - and considers minimizing the communication energy (joint work with E. Uysal and A. El Gamal) I will conclude by considering ad hoc networks - what is known - what can be done about improving performance - what are some larger unresolved, interesting issues

6 Communication Energy Energy is a key concern in wireless networks - the life-time of sensor networks depend on energy consumption - the mobility of hand-held devices is limited by battery drain-time Initial questions - what are fundamental limits on energy/bit needed for reliable communication? - what role does networking play in energy efficiency? Key Observation: Increasing transmission time reduces transmission energy speak slowly if you want to conserve energy

7 Example: Optimal coding Energy, w Transmissions, s

8 Example: A sub-optimal scheme Antipodal signaling and block error correction coding Energy, w Transmissions, s

9 Results Information Theoretic - formulate the channel coding problem directly in terms of energy-per-bit - establish general relationships to capacity - in practice: interested in energy needed to transmit a finite amount of info with given reliability paper submitted to IEEE IT Trans related work of Goldsmith et al for fading channels Networking So, - while taking time to transmit reduces energy, - delay goes to hell - we stipulate a deadline or a delay constraint - and come up with transmission schemes to minimize energy appears in Infocom extensions to variable-length packets, multiple users, relaying, etc

10 Ad hoc networks Destination Source Interference Source Destination The Setup - many nodes - relaying, hence multihop possibilities - but, as opposed to planned networks, - no networking and higher layer functionalities are specified - that is, issues like routing, per-hop and end-to-end behaviors are completely unspecified - for usual reasons this is both good and bad definitely interesting!

11 Initial questions Since there is a wireless interface... - there is interference - what is best interference-limited throughput? An answer (Gupta and Kumar) - with n nodes, randomly placed in a region (say, disk) - and the best centralized scheduling - the best network throughput goes like O( n) - hence, per user it is like O( 1 n ) pessimistic But, by taking advantage of - variable rate transmission, spatial reuse and successive interference cancellation at the link layer - the throughput can be improved result of Goldsmith, et al

12 Another answer - Grossglauser and Tse Introduce mobility and take advantage of it - n source-destination pairs, randomly moving in a region - transmit packets to nodes only when they are close The best throughput - for the network goes like O(n) - hence, per source-destination pair it is like O(1) optimistic But... - delay is bad (grows without bound)

13 Ideas for further work A recurring theme in networks - throughput limited due to some intrinsic feature of problem e.g. intereference - some new aspect allows throughput improvement e.g. mobility - but delay is poor To control latency - need clever algorithms that adapt well to changing conditions - they need to be implementable low polynomial complexity algorithms may not be good enough Complexity can mean - protocol complexity (for exchanging information) - centralized vs. distributed good centralized but undistributable algorithms not useful

14 Future work (contd.) Routing - for conserving energy - distributed protocols - adapts to variations in load and connectivity Consider the really big question - ad hoc (sensor and mobile) networks enjoy serious academic attention - at the moment the work is either at the device end or at the theory end - the motivation seems clear for sensor networks - what is a compelling reason for mobile ad hoc networks?