Transcript Offered load

實驗五：媒介存取協定模擬
教師：
助教：
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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
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Goals & Experiment methods
Wired Networking
Wireless Networking
Simulation result
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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
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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
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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%
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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)
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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
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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
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Supported Simulation Functions
 Flexibility of creating user’s own
 Traffic generator
 Configuration Tool
 Monitor Tool
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Different and new types of networks
Various networking devices and protocols
Generate repeatable simulation results
Remote and concurrent simulations
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User Interface (1/2)
Assignment tools
Network device
Mode selector
Zoom tools
Topology plane
Location info.
Time bar
Playback tools
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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
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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
offered _ load 
input (kbps)
capacity(kbps)
 Throughput :
 The maximum of data rate the network can transmit
throughput 
output (kbps)
capacity (kbps)
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Experiment: Wired Networking
 Use hubs and switches with different offered loads to run simulations
 Example topologies:
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Experiment: Wireless Networking
 Use mobile hosts, access points (AP) with different offered loads to run
simulations
 Example topologies:
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Results
 Use [plot graph] tool to draw the throughput graph
 Example graphs:
The unit of values depend on the log file you selected
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