1. CMPE 80N - Introduction to Networks and the Internet 1 CMPE 80N Winter 2004 Lecture 9 Introduction to Networks and the Internet

2. CMPE 80N - Introduction to Networks and the Internet 2 Medium Access Control: Summary

3. CMPE 80N - Introduction to Networks and the Internet 3 Centralized versus Distributed MAC Centralized approaches: –Controller grants access to medium. –Simple, greater control: priorities, qos. –But, single point of failure and performance bottleneck. Decentralized schemes: –All stations collectively run MAC to decide when to transmit.

4. CMPE 80N - Introduction to Networks and the Internet 4 Round-Robin MAC Each station is allowed to transmit; station may decline or transmit (bounded by some maximum transmit time). Centralized (e.g., polling) or distributed (e.g., token ring) control of who is next to transmit. When done, station relinquishes and right to transmit goes to next station. Efficient when many stations have data to transmit over extended period (stream).

5. CMPE 80N - Introduction to Networks and the Internet 5 Scheduled Access MAC Time divided into slots. Station reserves slots in the future. Multiple slots for extended transmissions. Suited to stream traffic.

6. CMPE 80N - Introduction to Networks and the Internet 6 Contention-Based MAC No control. Stations try to acquire the medium. Distributed in nature. Perform well for bursty traffic. Can get very inefficient under heavy load. NOTE: round-robin and contention are the most common.

7. CMPE 80N - Introduction to Networks and the Internet 7 802.11 IEEE standard for wireless medium access control. Similar to Ethernet, I.e., contention-based. But, instead of collision detection, performs collision avoidance. Why? –Full-duplex radios are expensive. How is collision avoidance performed? –Transmitter and receiver exchange small control frames to reserve the medium for data exchange.

8. CMPE 80N - Introduction to Networks and the Internet 8 Wireless Channel Contention BA

9. CMPE 80N - Introduction to Networks and the Internet 9 The Hidden Terminal Problem BAC A sends to B, C cannot receive A C wants to send to B If use CSMA/CA: C senses a “free” medium, thus C sends to A Collision at B, but A cannot detect collision Therefore, A is “hidden” for C

10. CMPE 80N - Introduction to Networks and the Internet 10 The Exposed Terminal Problem BAC B sends to A, C wants to send to D If use CSMA/CA C senses an “in-use” medium, thus C waits But A is outside the radio range of C, therefore waiting is not necessary Therefore, C is “exposed” to B D

11. CMPE 80N - Introduction to Networks and the Internet 11 Wireless LANs Wireless LANs use radio bridges to transmit data –Example: 802.11 (up to 11 Mb/s) An Access Point (a.k.a. base station) is connected to the wired LAN and communicates with the wireless card of nearby computers For 802.11: each Access Point only covers a radius of no more than 200 feet (and less if there are walls or obstacles) –Therefore, to allow for LAN access within a building, several Access Points need to be installed in the different rooms/corridors

12. CMPE 80N - Introduction to Networks and the Internet 12 Wireless Local Area Networks Example: WaveLan, Aironet Wireless LAN may be used for –Last hop to a wireless host. –Wireless connectivity between hosts on the LAN.

13. CMPE 80N - Introduction to Networks and the Internet 13 Other Wireless Networks

14. CMPE 80N - Introduction to Networks and the Internet 14 Cellular Networks Cellular phones:voice. Cellular networks: shift from voice to data. New wireless devices: pagers, PDAs. New services: Web access, e-mail, instant messaging, etc.

15. CMPE 80N - Introduction to Networks and the Internet 15 Cellular Concept: Motivation Early mobile radio systems: –Large coverage with single, high-powered transmitter. –But, no frequency re-use due to interference. Since finite spectrum allocation, need: high capacity (number of users) with limited spectrum and wide coverage.

16. CMPE 80N - Introduction to Networks and the Internet 16 Some Cellular Terminology Mobile. Base station. Mobile Switching Center (MSC). Handoff. Cell.

17. CMPE 80N - Introduction to Networks and the Internet 17 Cellular Architecture cell BS mobile

18. CMPE 80N - Introduction to Networks and the Internet 18 Cellular Fundamentals System-level idea, no major technological changes. –Many low-power transmitters instead of single, high power on (large cell). –Service area divided into small cells covered by each low power transmitter. –Each transmitter (or base station) allocated a portion of the spectrum. –Nearby BSs assigned different channel group to minimize interference. –Scalability: as more users subscribe, more BSs can be added using lower transmission power).

19. CMPE 80N - Introduction to Networks and the Internet 19 Frequency Reuse A B C D E F G G E F

20. CMPE 80N - Introduction to Networks and the Internet 20 Handoff/Handover Mobile hosts can change cells while communicating. Hand-off occurs when a mobile host starts communicating via a new base station. Handoff decision made based on signal strength.

21. CMPE 80N - Introduction to Networks and the Internet 21 MANETs Mobile, (wireless), multi-hop ad-hoc networks. Formed by wireless hosts which may be mobile. Without (necessarily) using a pre-existing infrastructure. Routes between nodes may potentially contain multiple hops. Mobilitty cause routes to change.

22. CMPE 80N - Introduction to Networks and the Internet 22 Multi-hop May need to traverse multiple hops to reach destination.

23. CMPE 80N - Introduction to Networks and the Internet 23 Why MANETs ? Ease of deployment. Speed of deployment. Decreased dependence on infrastructure.

24. CMPE 80N - Introduction to Networks and the Internet 24 Many Applications Personal area networking. –Cell phone, laptop, ear phone, wrist watch. Military environments. –Soldiers, tanks, planes. Civilian environments. –“Smart” environments. Emergency operations –Search-and-rescue –Policing and fire fighting –Monitoring and surveillance.