Advanced Topics in Next-Generation Wireless Networks

1. Advanced Topics in Next-Generation Wireless Networks Qian Zhang Department of Computer Science HKUST Basic, PHY and MAC of Ad Hoc Network

2. What is an Ad Hoc Network? • Definitions: • An ad-hoc network is one that comes together as needed, not necessarily with any assistance from the existing Internet infrastructure • Instant infrastructure • A MANET is a collection of mobile platforms or nodes where each node is free to move about arbitrarily • A MANET: distributed, possibly mobile, wireless, multihop network that operates without the benefit of any existing infrastructure (infrastructure-less), except the nodes themselves

3. Mobile Ad Hoc Networks • May need to traverse multiple links to reach a destination

4. Mobile Ad Hoc Networks (MANET) • Mobility causes route changes

5. Why Ad Hoc Networks ? • Ease of deployment • Speed of deployment • Decreased dependence on infrastructure

6. History • In 1972, DoD-sponsored Packet Radio Network (PRNet) • Initial network used centralized control station • Evolved into a distributed architecture • A network of broadcast radios • Minimal central control • Use multi-hop store-and-forward routing • Use ALOHA/CSMA, spread-spectrum • Demonstrated the technologies needed to create a MANET

7. History (Cont.) • In 1983, Survivable Radio Network (SURAN) program Motivations: • Move towards smaller size, low-cost, low power radio • Develop and demonstrate scalable algorithms, up to 10ks of nodes. • Develop and demonstrate robustness and survivability against sophisticated attacks • Technology: spread spectrum improvements, hierarchical network topology, dynamic clustering, and even cross-layer design

8. History (Cont.) • Army (Army Research Office-ARO) • The army did not embrace the new MANET technology until it was demonstrated experimentally in mid-1980s • Used it for primarily land-based applications • Utilized mainly as overlays to existing networks • Navy (Office of Naval Research-ONR) • Primarily for use by ships at sea • Network is not as dense as a ground network • Required integration with satellite links • Air Force • Explored utilizing aircrafts to provide communications between ground stations

9. Commercialization • (Arguably) two papers, both addressing the routing problem, helped spur the ad-hoc networks research • [Perkins-Bhagwat94, Johnson94] • The community started to adopt the term “Ad-Hoc Networks” • Commercial applications started to appear • A number of standards activities evolved in the mid90s— • Notably, the MANET working group (within the IETF) to standardize routing protocols for ad hoc networks • Reactive and proactive routing • The 802.11 subcommittee standardized an ad-hoc mode MAC layer • Made it possible to build ad-hoc networks using laptops

10. Fundamental Challenges It is better to know some of the questions than all of the answers. — James Thurber (1835-1910)

11. 1. Energy Efficiency • No infrastructure means must rely on batteries (or, in general, limited energy resources) • Possible solutions • Selectively sending nodes into a sleep mode • Using transmitters with variable power (the Power Control problem) • Using energy-efficient paths • Using co-operative techniques (still relatively new)

12. 2. Mobility • Mobility-induced route changes • Mobility-induced packet losses • Mobility patterns may be different • Controlled e.g. robots • Offers opportunities for improving the network functions e.g. connectivity • Uncontrolled e.g. nomadic users • Offers challenges to network design • But also offers opportunities for improvement, e.g. • Users “carry” delay-tolerant data closer to destination • Delay Tolerant Network (Challenge Networks)

13. 3. QoS • Providing QoS even in wired networks (e.g. the Internet) is a challenging problem • Wireless RF channels further complicate the problem • Unpredictability • Medium access: broadcast medium with hidden terminal problem • Possible solutions: • New MAC design • Cross-layer integration: allow different layers to adapt depending on available information at other layers

14. 4. Scalability • Limited wireless transmission range • Whether the network is able to maintain an acceptable level of service even as the number of nodes is increased • How fast the network protocol control overhead increases as N increases • Possible solutions: • Introducing hierarchy • Utilizing location information • Limiting reactions to changes • Fixing things (e.g. paths) locally

15. 5. Utilizing New Technologies • What are the gains that could be achieved by using newly available technologies such as • Smart directional (beamforming) antennas • Increases the spatial reuse in cellular, but how about ad-hoc? • Can several nodes together act as an antenna array? Practical issues? • Software Radio • The ability to quickly switch the operating frequency may provide opportunities, but also challenging • GPS • Location information may help

16. 6. Security • Ease of snooping on wireless transmissions • From crypto point of view, lack of a trusted authority is one of the main challenges • How to generate/share keys reliably • Harder to track or even detect attackers in a wireless environment, given that: • Network relies on in-situ connections to other nodes which may be malicious • Malicious nodes may be especially harmful by injecting bogus control packets • DoS attacks that deplete a node’s battery

17. 7. Lack of Reference • Lack of sufficient experimental data to confirm models • What does a multi-hop path really mean? • What is a link? • Simplistic models that do not capture the complexities, or complex models that do not lead to insights? • Are the protocols good enough, have they reached closed to the best possible? • Good balance between mathematical and experimental work

18. Multiple-Layer Problem • PHY • Adapt to rapid changes in link characteristics • MAC • Minimize collision, allow fair access, and semi-reliably transport under rapid change and hidden/exposed terminals • Network • Determine efficient transmission paths when links change often and bandwidth is at a premium • Transport • Handle delay and packet loss statistics that are very different than wired networks • Application • handle frequent disconnection and reconnection as well as varying delay and packet loss characteristics

19. Topics in PHY and MAC • Fairness • Energy efficiency • Power save • Power control • Adaptive modulation (multi-rate)

20. Fairness Issue • Assume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collide • Nodes A and B then choose from range [0,63] • Node A chooses 4 slots and B choose 60 slots • After A transmits a packet, it next chooses from range [0,31] • It is possible that A may transmit several packets before B transmits its first packet A B Unfairness occurs when one node has backed off much more than some other nodes Two flows C D

21. Fairness in Multi-Hop Networks • Several definitions of fairness [Ozugur98,Vaidya99MSR,Luo00Mobicom, Nandagopal00Mobicom] • Hidden terminals make it difficult to achieve a desired notion of fairness

22. Topics in PHY and MAC • Fairness • Energy conservation • Power save • Power control • Adaptive modulation (multi-rate)

23. Energy Conservation • Since many mobile hosts are operated by batteries, MAC protocols which conserve energy are of interest • Two approaches to reduce energy consumption • Power save: turn off wireless interface when desirable • Power control: reduce transmit power Power Characteristics for a Mica2 Mote Sensor

24. Power Save in IEEE 802.11 Ad Hoc Mode • Time is divided into beacon intervals • Each beacon interval begins with an ATIM window • ATIM = ATIM window Beacon interval

25. Power Save in IEEE 802.11 Ad Hoc Mode (Cont.) • If host B has a packet to transmit to A, B must send an ATIM Request to A during an ATIM Window • On receipt of ATIM Request from A, B will reply by sending an ATIM Ack, and stay up during the rest of the beacon interval • If a host does not receive an ATIM Request during an ATIM window, and has no pending packets to transmit, it may sleep during rest of the beacon interval Node A ATIM Req ATIM Ack Data Ack Node B Sleep Node C

26. Power Save in IEEE 802.11 Ad Hoc Mode (Cont.) • Size of ATIM window and beacon interval affects performance • If ATIM window is too large, reduction in energy consumption reduced • Energy consumed during ATIM window • If ATIM window is too small, not enough time to send ATIM request • How to choose ATIM window dynamically? • Based on observed load [Jung02infocom]

27. Power Save in IEEE 802.11 Ad Hoc Mode (Cont.) • How to synchronize hosts? • If two hosts’ ATIM windows do not overlap in time, they cannot exchange ATIM requests • Coordination requires that each host stay awake long enough (at least periodically) to discover out-of-sync neighbors [Tseng02infocom] ATIM ATIM

28. Impact on Upper Layers • If each node uses the 802.11 power-save mechanism, each hop will require one beacon interval • This delay could be intolerable for some applications • Allow upper layers to dictate whether a node should enter the power save mode or not [Chen01mobicom]

29. Power Control Recall: Power control has two potential benefit • Reduced interference & increased spatial reuse • Energy saving

30. Power Control with 802.11 • Transmit RTS/CTS/DATA/ACK at least power level needed to communicate with the receiver • A/B do not receive RTS/CTS from C/D. Also do not sense D’s data transmission • B’s transmission to A at high power interferes with reception of ACK at C A B C D

31. A Plausible Solution • RTS/CTS at highest power, and DATA/ACK at smallest necessary power level • A cannot sense C’s data transmission, and may transmit DATA to some other host • This DATA will interfere at C • This situation unlikely if DATA transmitted at highest power level • Interference range ~ sensing range Data sensed A B C D RTS Data Interference range Ack

32. Other Cons • Transmitting RTS at the highest power level also reduces spatial reuse • Nodes receiving RTS/CTS have to defer transmissions

33. Modification to Avoid Interference • Transmit RTS/CTS at highest power level, DATA/ACK at least required power level • Increase DATA power periodically so distant hosts can sense transmission [Jung02tech] • Need to be able to change power level rapidly Power level

34. Topics in PHY and MAC • Fairness • Energy efficiency • Power save • Power control • Adaptive modulation (multi-rate)

35. Adaptive Modulation • Channel conditions are time-varying • Received signal-to-noise ratio changes with time • Multi-rate radios are capable of transmitting at several rates, using different modulation schemes • Choose modulation scheme as a function of channel conditions Modulation schemes provide a trade-off between throughput and range Throughput Distance

36. Adaptive Modulation (Cont.) • If physical layer chooses the modulation scheme transparent to MAC • MAC cannot know the time duration required for the transfer • Must involve MAC protocol in deciding the modulation scheme • Some implementations use a sender-based scheme for this purpose • Receiver-based schemes can perform better

37. Sender-Based “Autorate Fallback” [Kamerman97] • Probing mechanisms • Sender decreases bit rate after X consecutive transmission attempts fail • Sender increases bit rate after Y consecutive transmission attempt succeed

38. Autorate Fallback • Advantage • Can be implemented at the sender, without making any changes to the 802.11 standard specification • Disadvantage • Probing mechanism does not accurately detect channel state • Channel state detected more accurately at the receiver • Performance can suffer • The sender will periodically try to send at a rate higher than optimal • When channel conditions improve, the rate is not increased immediately

39. Receiver-Based Autorate MAC [Holland01mobicom] C • Sender sends RTS containing its best rate estimate • Receiver chooses best rate for the conditions and sends it in the CTS • Sender transmits DATA packet at new rate • Information in data packet header implicitly updates nodes that heard old rate RTS (2 Mbps) A B CTS (1 Mbps) D Data (1 Mbps) NAV updated using rate specified in the data packet