實驗五：媒介存取協定模擬

1. 實驗五：媒介存取協定模擬 教師： 助教：

2. Outline • Background • Transmission Protocols • ALOHA • CSMA/CD • CSMA/CA • Network Devices • Hub • Switch • Access Point (AP) • Simulation Software - NCTUns • Background • Supported simulation function • User interface • Experiment • Goals & Experiment methods • Wired Networking • Wireless Networking • Simulation result

3. Background (1/2) • Why simulations are needed • Plan a new networking topology • Estimate the efficiency of networks • Evaluate new protocols or devices • Advantages • Lower cost • Easy to build • Avoid environmental interference

4. Background (2/2) • Goal of this experiment • Learn to use the tool of NCTUNS • Realize the mechanisms of • Transmission protocols • Network devices • Methodlogy of designing an experiment

5. Transmission Protocols (1/2) • ALOHA • Transmit data only • Two types • Pure-Aloha • Slotted-Aloha • No sensing for collision and congestion • Channel throughput: 18% (Stallings, 2000) • CSMA (Carrier Sense Multiple Access) • Sense the media and transmit data if it’s idle • Two implementations: • Persistent CSMA • Non-persistent CSMA • Throughput is much better: over 40%

6. Transmission Protocols (2/2) • CSMA/CD • CSMA with Collision Detection • Wait a random time if collision and then transmit • Used in wired networks (ex: IEEE 802.3) • CSMA/CA • CSMA with Collision Avoidance • RTS/CTS mechanism for hidden terminals problems • Used in wireless networks (ex: IEEE 802.11)

7. Network Devices • Hub • Broadcast all packets to every port • Switch • Build a table for mapping between MAC addresses and ports • Broadcast packets at first time to verify the target port • Send packets to the target port instead of broadcasting • Access Point (AP) • A bridge for wired and wireless networks • Use RTS/CTS mechanism on wireless network • Always broadcast because of the media

8. Simulation Software - NCTUns • Background • Developed by the Network and System Laboratory (NSL) of NCTU CS • Used in both wired and wireless IP networks • Directly uses the Linux ’s TCP/IP protocol stack • Open-Source Simulation engine • Provides complete and high-quality documentations • Continuously supported, maintained and improved

9. Supported Simulation Functions • Flexibility of creating user’s own • Traffic generator • Configuration Tool • Monitor Tool • Different and new types of networks • Various networking devices and protocols • Generate repeatable simulation results • Remote and concurrent simulations

10. User Interface (1/2) Assignment tools Network device Mode selector Zoom tools Topology plane Location info. Time bar Playback tools

11. User Interface (2/2) • Four modes: • Draw Topology • Draw topology by using the network device toolbox • Edit Property • Enter each device and set their property and reporting information • Run Simulation • Start simulation and recording the report • Play Back • Use GUI interface to replay the simulation

12. Experiment • Goals : • Find the relationship between the offered load and throughput • Experiment methods • Use different offered loads to run simulations and compare the results • Offered load: • Capacity of the line divided by the amount of input data • Throughput : • The maximum of data rate the network can transmit

13. Experiment: Wired Networking • Use hubs and switches with different offered loads to run simulations • Example topologies:

14. Experiment: Wireless Networking • Use mobile hosts, access points (AP) with different offered loads to run simulations • Example topologies:

15. Results • Use [plot graph] tool to draw the throughput graph • Example graphs: The unit of values depend on the log file you selected