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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 Goals & Experiment methods Wired Networking Wireless Networking Simulation result

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Background (1/2)

Why simulations are neededPlan a new networking topologyEstimate the efficiency of networksEvaluate new protocols or devices

AdvantagesLower costEasy to buildAvoid environmental interference

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Background (2/2)

Goal of this experimentLearn to use the tool of NCTUNSRealize the mechanisms of

Transmission protocolsNetwork 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/CDCSMA with Collision DetectionWait a random time if collision and then transmitUsed in wired networks (ex: IEEE 802.3)

CSMA/CACSMA with Collision AvoidanceRTS/CTS mechanism for hidden terminals problemsUsed 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

BackgroundDeveloped by the Network and System Laboratory (NSL)

of NCTU CSUsed in both wired and wireless IP networks Directly uses the Linux ’s TCP/IP protocol stack Open-Source Simulation engineProvides complete and high-quality documentationsContinuously supported, maintained and improved

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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

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User Interface (1/2)

Network device

Assignment tools

Zoom tools

Mode selector

Time bar Playback tools

Location info.

Topology plane

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User Interface (2/2)

Four modes:Draw Topology

Draw topology by using the network device toolboxEdit Property

Enter each device and set their property and reporting informationRun Simulation

Start simulation and recording the reportPlay 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

Throughput : The maximum of data rate the network can transmit

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kbpscapacity

kbpsoutputthroughput

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kbpscapacity

kbpsinputloadoffered

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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