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 )

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) Network device Assignment tools Zoom tools Mode selector Time barPlayback tools Location info. Topology plane

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