Introduction to Wireless Networks 1 & 2. Guillaume Urvoy-Keller. Laboratoire I3S, Université Nice Sophia Antipolis. September 24, 2013

Transcription

1 Introduction to Laboratoire I3S, Université Nice Sophia Antipolis September 24, 2013

2 Outline A bit of history The MAC layer Aloha Slotted Aloha CSMA 1 2

3 A bit of history

4 What is this?

5 Let us start from the beginning 1957: the technological Pearl Harbor spoutnik Eisenhower to create Advanced Research Project Agency (ARPA)

6 The origin of the beast - Scott Ruthfield - ACM Crossroads 1995 It was 1964, the height of the Cold War, and Americans spent their free time building bomb shelters and stockpiling canned food in preparation for the impending nuclear attack. The government, however, had a more pervasive problem. If war did come, how would the military be able to communicate? A centralized system might easily be destroyed in wartime, and so traditional technologies wouldn t work. This fear impressed a need on the government to do something different - to develop a whole new scheme for post-nuclear communication. Today, a descendant of that Cold War mechanism is used to track seismological phenomena, transmit pressing news bulletins, and send to mom. Does this signal a complete shift in priorities? In part, yes; more appropriately though, it is an example of a technology with more uses than anybody ever imagined.

7 A few things to keep in mind It was observed early that using telephone lines were highly inefficient for computer applications - user to computer communications Thinking that computers would connect to each other in the 60s was... outrageous J.C.R. Licklider ( ) from Massachusetts Institute of Technology (MIT) in 1962 at IPTO (nformation Processing Techniques Office) from ARPA

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9 One of the first traffic analysis study P E JACKSON C D STUBBS A study of multiaccess computer communications Proceedings of the Spring Joint Computer Conference pp AFIPS Press 1969 Type of system:

10 One of the first traffic analysis study Traffic consists of burst from user to computer, computer to user and thinking/data preparation time.

11 One of the first traffic analysis study From user to computer: 95% idle time need to share access links between users From computer to user: also very inefficient - 10 to 30%

12 If it is not the Telephone network? Kleinrock lays mathematical foundations of datagrams communications in the late 50 s A datagram is autonomous - destination No path establishment Dynamic routing - in case of failure Larry Roberts takes the lead of IPTO Former colleague of Kleinrock at MIT

13 Birth of the Internet First node at UCLA One mini-computer in charge of communication (ancestor of DSL box...) Inter-connection with other networks envisaged very early Minimum constraints to allow inter-connection Especially no constraint on access network characteristics

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17 WAN issues vs. LAN issues The ARPANET project addressed the problem of routing at large scale For the local network, Ethernet was developed in 1973 by Bob Metcalfe (Xerox Research Center - Palo Alto) Builds on Alohanet, developped at the University of Hawaii

18 Alohanet Schwartz, M.; Abramson, N.;, "The Alohanet - surfing for wireless data [History of Communications]," Communications Magazine, IEEE, vol.47, no.12, pp.21-25, Dec The Alohanet system: University of Hawaii beginning in the late 1960s 1971: UHF terrestrial data network (AlohaNet) 1973 VHF transponder in an experimental NASA satellite to demonstrate an international satellite data network (PacNet) connecting NASA in California and five universities in the US, Japan, and Australia. Key idea: random access channel Every time you power up your mobile phone or use that phone to establish a voice, SMS, or Internet connection, the very first packet transmitted is sent via an ALOHA random access channel. N. Abramson

19 AlohaNet Original goal: radio communications as an alternative to telephone University of Hawai: main campus in Manoa Valley near Honolulu and 6 colleges on the islands of Oahu, Kauai, Maui, and Hawaii Radius of about 300 km from Honolulu. Final objective: share computer resource at main campus IBM 360/65 with a 750 K byte core memory ;-)

20 AlohaNet TDMA or FDMA ruled out upfront because of: Nature of communication Varying number of clients System design: Two 100 khz bandwidth channels at MHz and MHz Dedicated communication computer: HP 2115A - 16-bit word size, a cycle time of 2 microseconds and an 8K- word core storage capacity. Design of communication node roots in the IMP. One channel for data from central location to remote consoles The other shared by the consoles Operated at 24,000 baud. Packets of fixed length: 80 8-bit characters plus 32 identification and control bits and 32 parity bits Each packet will consist of 704 bits 29 ms at 24,000 baud.

21 Aloha Protocol Sharing of the uplink No synchronization at all!!!! Ack sent only if packet received without error.

22 Aloha Either used directly or the root of: Ethernet (CSMA/CD) Wifi (CSMA/CA) Uplink sharing in DOCSIS (shared uplink of cable networks) GPRS channel establishment Two-way satellite communications Why did people improve Aloha? Performance

23 Pure Aloha Principle: If you have data to send, send the data If the message collides with another transmission, try resending "later"

24 Assumptions: Pure Aloha: performance analysis All frames have the same length - Duration T The population of stations attempts to transmit (both new frames and old frames that collided) according to a Poisson process, with mean G over a time interval T Analysis: Proba no arrival during T is e G Assume a frame sent at time t 0, no collision if no transmission during ]t 0 T,t 0 + T [ Vulnerable Time of 2T Throughput = rate of transmission proba success Throughput = Ge 2G Result: Maximum reached for G= frame per frame time 18 % of slots correspond to transmission w/o collision. The others: no transmission or collision Price of simplicity

25 Aloha vs. Slotted Aloha A bit of synchronization transmission can only occur at beginning of slot Vulnerable period is divided by 2 Throughput = Ge G Throughput multiplied by 2

26 CSMA: Carrier Sensing Multiple Access Rules: Listen before speaking: if someone talks, differ transmission. If someone else begins talking, then stop talking Still some problems: Collisions still exist, because of propagation delays When to test for next transmission if channel is busy? CSMA 1-persistent: asap! CSMA non-persistent: waits a random number of slots CSMA p-persistent: try with proba p

27 Performance Summary

28 Is it enough? Well, no ;-) How to decide the p value in CSMA p-persistent If there is a lot of active stations, p should be small If not, it should be large, otherwise a lot of empty slots! There is not only the MAC layer!!! What about the other layers?

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30 1 Outline Focus on networks Wireless LAN Ad-hoc Part 1: Foundations: Performance: Maximum Expected performance: when layering matters Performance anomaly of : why being fair is not always a good idea? Info or node moving Routing in ad-hoc network Handover, Mobile IP in WLANs Adaptation of TCP: why do we still need this old guy!

31 1 Outline Part 2: Why is unfair? The case of IdleSense, an alternative layer TCP unfairness problem. Why do uploaders achieve a better throughput at the expense of downloaders. Layer 2 and/or layer 3 approaches. Part 3: Network coding, a generalization of routing, for better resource utilization. Foundation for multicast. Application to mesh Opportunistic networking: when mobility or lack of connectivity prevents the existence of an end-to-end paths. Study of the main opportunistic routing protocols. Interest of NC. Focus on mobile social networks, where nodes cluster into communities

32 2 Outline Part 1: Measurements Measurements studies in networks Do it yourself. Labs (perf anomaly, TCP unfairness) Part 2: Performance of ad-hoc networks How does the capacity to transfer information scale with the number of nodes? What if nodes move? What if they have multiple antenna? Part 3: Energy consumption Green wireless networks The future of Internet: video streaming.