Mobile/Wireless Networking: Overview and Principles

1. Mobile/Wireless Networking: Overview and Principles DimitriosKoutsonikolas 02/03/2016 These slides contain material developed by Chunyi Peng for CSE 5469 at OSU and by Kurose-Ross

2. Family of Networks Core Network (tier-1 ISP) Data center ATM CDN … Ethernet (LAN) Cable, DSL … Access (Edge) networks Wireless networks Mobile networks 4G/3G: LTE, HSPA, EVDO, UMTS,… Internet WiFi (802.11a/b/g/n/ac…) Fiber optic WiMax, Satellite …. whitespace 60GHz Bluetooth, NFC (RFID), WSN, … (not strictly)

3. Family of Networks Interconnection end-to-end layering: TCP/IP packet switched … Edge last hop Internet Wireless networks Mobile networks Access networks Wireless broadcast Interference coverage … Mobility

4. Design Guidelines for Internet & Wireless Mobile Networks

5. Design Guidelines • The foundation for wireless networking is the Internet design guidelines • End-to-end argument • Always applicable?

6. Key Design Decision • How do you divide functionalities among layers and across different components in the network? • Given the freedom to implement a few functionalities in multiple “places” of the system (physical devices, or protocol layers), where to implement them? • Goals: • Correctness, completeness, performance tradeoffs

7. Options • Telcom approach: “Smart CORE, Dumb Terminal” • The core ensures reliability • TCP/IP approach: “Smart Terminal, Dumb CORE” • The terminal ensures reliability, while the core retains simplicity • Implicit assumption made: terminals have more capabilities: computing power, storage, memory, etc.

8. End-to-End Argument • Think twice before implementing a functionality that is useful to an application at a lower layer • If the application can implement a functionality correctly, implement it a lower layer only as a performance enhancement

9. OK Example: Reliable File Transfer • Solution 1: make each step reliable, and then concatenate them • Solution 2: end-to-end check and retry Host A Host B Appl. Appl. OS OS

10. Discussion on Solution 1 • Ensuring reliability at every step is incomplete • Why? • The receiver has to do the check anyway! • Thus, full functionality can be entirely implemented at application layer; no need for reliability from lower layers

11. More Discussions • Is there any need to implement reliability at lower layers? • Yes, but only to improve performance • Example: • Assume a high error rate on a wireless channel • Then, a reliable communication service at link layer might help • Assume high error rate writing to disk • Then, a reliable service at the OS level would help

12. Tradeoffs • Application has more information about the data and the semantics of the service it requires (e.g., can check only at the end of each data unit) • A lower layer has more information about constraints in data transmission (e.g., packet size, error rate) • Note: these trade-offs are a direct result of layering!

13. Summary: End-to-End Argument • Add functionality in lower layers iff it is (1) used by and improves performance of a large number of applications, and (2) does not hurt other applications • Success story: Internet

14. Two Forms of E2E Guideline • Horizontal: Push complexity outside the network core, into the end systems • Simple IP routers, complex TCP end hosts • Vertical: Push design to higher layers of the protocol stack • End-to-end reliability at the transport layer in TCP/IP • Hop-by-hop reliability at the link layer in telcom

15. Remarks • Challenge of building a network system: find the right balance between: Reuse, implementation effort (apply layering concepts) End-to-end argument Performance No universal answer: the answer depends on the goals and assumptions!

16. The Problem: What is New Compared to the Wired Internet? • Fundamental challenges for wireless and mobile networking design: • WIRELESS • MOBILITY • Is it so obvious and too trivial??? • Map onto each layer of the protocol stack

17. Wireless Impact on Protocol Stack Application • Partial network connectivity • Changing network quality: delay, throughput Transport Layer • Diverse data losses • Opportunistic connectivity • Time-varying link bandwidth Network Layer • Location-dependent error • Hidden terminals Link/MAC Layer

18. Mobility Impact on Protocol Stack • Connection, disconnection • Mobility-induced data losses • Topology change • Time-varying capacity • Link-layer handoff • Varying link quality Application Transport Layer Network Layer Link/MAC Layer

19. Example: TCP in wireless/mobile networks

20. Review: TCP Congestion Control • Send as fast as possible, but not causing network congestion • Probe and adapt • AIMD: additive increase, multiplicative decrease multiply

21. TCP congestion control over the Internet • Premise • Packet loss is caused by congestion • So, loss -> reducing sending rate • Cwnd (congestion window size) reduction • Timeout update

22. Issues for Wireless TCP • Different packet loss behavior violates the assumption of TCP that all packet losses are due to congestion control: • congestion-induced loss: new flow joins, etc. • channel-error-induced loss: bursty or random channel error • handoff-induced packet loss: happens during handoff transition • routing-induced packet loss: stale routing tables (in a dynamic ad hoc network) • “Uniform”reaction to different losses in TCP: • in TCP, reduce congestion window by half upon packet loss • Does “one-fit-all” work in the wireless scenario ?

23. The Goals • Hide impact of wireless • SAME QUALITY AS WIRED LINK!! • Offer seamless services while mobile • Overall, “Anytime, anywhere” services

24. Two Popular Design Approaches • Adaptation high-dimension dynamics • Coordination coherent system

25. Adaptation As the Guideline • Many concrete forms/instantiations of adaptations • Adaptation to channel variations • Adaptation to mobility • … • Adaptation at different layers of protocol stacks • From PHY, LINK, to TRANSPORT and APP layers • Numerous solutions/papers published • 333000 entries for google search “wireless adaptation” • 956000 entries for google search “mobility adaptation”

26. Research Issues in Adaptation • What to adapt? • Transmission power, transmission rate, # of retries, …? • When to adapt? • when to invoke specific adaptation? • stability versus responsiveness • How to adapt? • specific mechanisms/algorithms in adaptation

27. Forms of Adaptation • Opportunistic design approach • Opportunistically adapt • Model-referenced design • Adapt to trace a reference model

28. Opportunistic Design • Exploit the system population • Leverage system diversity • Multiple receivers, multiple devices, multiple applications/flows, …

29. Example: Opportunistic Scheduling • How to maximize system throughput by exploiting time-varying channels for each user in a fair way? • Each active user gets a share of the channel

30. Dynamics for Each User • Each user’s channel varies independently over time due to fading etc. • In a large network, it is very likely to find a user with a very good channel at any time. • Long-term total throughput can be maximized by opportunistically serving user with the strongest channel

31. Resulting Algorithm: Proportional Fair Scheduler • (Used by Qualcomm EVDO system) • Schedule the user with the highest ratio • Rk = current requested rate of user k • Tk = average throughput of user k in the past tc time slots

32. Opportunistic Performance Gain Increases with # of Users

33. Model-Referenced Adaptation • Ideal model to capture expected behaviors under idealized situation • e.g., error-free, static settings • Track the reference model under realistic conditions/scenarios • Mobility, wireless channel dynamics, …

34. backbone 3 4 1 1 2 2 Example: Scheduling over Channel Errors Goal: Each user gets 50% of channel Reference Model for Error-Free Channels Time  Channel status 1 MH #1 Sender 2 Base Station MH #2

35. backbone 3 4 1 1 2 2 Example: Scheduling over Channel Errors Idea: Lead/Lag to track difference with ref. model & Swap scheduling order for 1 and 2 Reference Model for Error-Free Channels Time  Time  3 1 Channel status MH #1 Sender 4 2 Base Station MH #2

36. Forms of Coordination • Cross-Layer design • Enable close interactions across non-adjacent layers in the layered protocol stack • Coordination via “indirection” • Adaptation-aware proxy provides indirection

37. Cross-layer Design Information sharing, informed decision at other layers Merging layers

38. Example of Cross-layer Feedback • PHY info to higher layers • Link/MAC layer • Control transmit power, modulations to reduce error rate or retransmit • Network layer • Bit-error rate information in order to switch another network interface with lower bit-error-rate • Application layer • Channel condition information • Various standard coding techniques for multi-media applications

39. Any Bad Effects? • Undesirable consequences on overall system performance • The importance of architecture • Stability • Robustness • Spaghetti design – hard to upkeep • …

40. Indirection via “Proxy” • Proxy bridges the server and the client • Move complexity away from both server and client • Generalized end-to-end argument: “edge” rather than “end” systems • Little changes at server & client server proxy client

41. Driving Factors for Wireless (Mobile) Networking Research New Applications, Services, Requirements Top Up Transport Layer Network Layer Down Link Layer Bottom New Wireless Communications Technology

42. Bottom Up Driver: Wireless Communications • Many of them: • Antenna arrays, Smart antennas, … • Adaptive modulation, OFDM, MIMO • Spectrum sharing, cognitive radios, channel management • Multi-interface radios, device heterogeneity • … Challenge: How to exploit these new PHY communication capabilities in the protocols?

43. Root Cause of Problems • two largely disconnected communities • speak different terminologies • wireless communications: • Symbols, signals • probabilistic terms: • information theoretic bounds • confidence factor on symbol reception, … • wireless networking • Packets, bits • deterministic terms • Correct/wrong binary reception

44. Root Cause of Problems (2) • Two largely disconnected communities • different methodologies • wireless communications • solid theoretic foundation on information theory • a set of well known assumptions: noises, interferences, etc. • Theory Design-->Analysis-->prototype in chips-->experiments • wireless networking • mostly on heuristics • network setting “ad hoc”: no agreed benchmarks/base settings • Heuristic Design-->Simulations--Network Prototype-->Experiments

45. Perspective From Wireless Networking • We are not on the driver’s seat so far • communication has driven the technology so far • we are followers • Still plenty of space • the direct communication almost NEVER works in reality at the 1st place!

46. Top Down Driver: User Demands • New applications • MMS, P2P image/video sharing, IP TV streaming, … • New requirements • Security, privacy, robustness/dependability, distributed management • New services • Location-based service, Personalized service, … • New trends • Interoperability of different wireless technologies Challenge: How to support such new demands?

47. Cellular Networks: Overview

48. Cellular Networks • To date, the only operational large-scale wireless network with wide-area coverage and mobility support

49. Key Services: Connectivity and More • Pervasive connectivity: anyone, anytime, anywhere • Device -> Base station -> Cellular core Network -> External network or internal devices) • Carrier-network services: data, voice, messaging, … • Two salient features • Wide-area (e.g. nationwide) coverage: cells • Mobility support: seamless service as we go

50. Mobile Network Evolution 1G AMPS, NMT, TACS 1980s 3G WCDMA/HSPA+ CDMA2000/EVDO TD-SCDMA 2000s 2G GSM/GPRS/EDGE cdmaOne 1990s 4G LTE LTE-advanced 2010s Digital voice + Simple data Mobile broadband More and Faster analog voice APP