1. Shared vs. Switched Ethernet (Modeling and Simulation using OPNET)
Dr. Khaled Salah

2. Objectives Show basic features and capabilities of OPNET by example
Ethernet background
How to:
Build a small Ethernet LAN types
Generate traffic: Poisson & Bursty
Examine performance
Choose statistics
Data collection
Animation
K. Salah

3. Issues to Investigate How many users/node can be supported?
Identify congestion and bottlenecks
Node placement and load balancing
Assess the health of existing network
Resource utilization
Delays
Capacity planning
Test different vendor Ethernet products
“what-if” scenarios
Resource utilizations: Which nodes or links are being over- or under-utilized
Capacity planning: not to overbuild or over-engineer a network by adding more capacity or pre-matuarly upgrading nodes, without justifications.
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4. Ethernet Network Topology
Point-to-point
Bus
Ring
Star
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5. Shared Ethernet Protocol – CSMA/CD
Station is ready to send
Station is
ready to send
New attempt
Station is ready to send
Wait according to backoff strategy
(6)
Sense channel (1)
Channel busy (3)
Channel free (2)
Station is ready to send
Transmit
jam signal
(5)
Station is ready to send
Transmit data and sense channel
(4)
Collision detected
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6. CSMA/CD Step 1. “Listen before talking”.
Step 2. If channel is quiet for a certain time, called interframe gap (IFG), then transmit. “Talk if quiet”.
Step 3. If channel is busy, monitor the channel until it is quiet for IFG period before transmitting. “Wait for quiet before talking.”
Step 4. Monitor the channel continuously during transmission to detect collisions. “Listen while talking”.
Step 5. If collision occurs, the first node recognizes it will send a jam signal to ensure that all other stations detect the collision. Transmitters should stop immediately, receivers should reject data, and others wishing to transmit should recalculate their backoff period. “A buzzer sounds off indicating we have more than one talker at a time.”
Step 6. All nodes wish to transmit must now wait a random period called “backoff” and attempt again to spread out collisions. “Backoff” is based on BEB algorithm (Binary Exponential Backoff). After 16 collisions, drop the frame to be transmitted.
K. Salah

7. Shared Ethernet Component
Coaxial Cable
Shared medium with BNC or vampire taps
Repeater
Layer 1 device that provides physical and electrical connections.
It receives signals from one cable segment, regenerates, retimes, and amplifies them, and then transmits these “revitalized” signals to another cable segment.
Transmits in both directions
Joins two segments of cable
No logical isolation of segments
Greater distance is achieved
Hub
Used to describe a repeater
Can be “repeater hub”, “switching hub”, bridging hub”.
NIC
Performs layer-2 functions: framing, error detection, and flow control.
Performs layer-1 functions by converting the bits into electrical signals using appropriate coding scheme.
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8. Switched Ethernet Point-to-point No Collision Higher Throughput
Larger Network diameter
10GbE that can go more than 50 km over SMF.
Higher Throughput
Smaller Latencies
No Congestion Collapse
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9. Bursty Traffic
For long time, traffic was thought to be Poisson. It is non-poisson, but rather bursty. Characterized by on periods, and off periods. What causes this, C/S modeling. ON no web activities, followed by off where user watching it. FTP transfer also is an example.
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10. CSMA/CD Model in OPNET Process Model Node Model
Node model: a block structured data flow, process model: stare transition diagram (STD) which is part of proto-C language.
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11. Bursty traffic model in OPNET
Node Model
Process Model
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12. Packet-Size Distribution
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13. Packet Distribution in OPNET
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14. Creating Network Topologies
Importing the topology
Placing each individual node from the Object Palette into the workspace
Crating a new network topology by using Rapid Configuration
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15. Statistics Two ways to collect statistics Object statistics
from individual nodes
Global statistics
From the entire network as whole
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16. Data Collection Vector Scalar Animation
Time-dependent series of values
Scalar
Parametric relationships
Animation
visualization
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17. Vector vs. Scalar
Vector data graphs
Scalar data graph
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18. OPNET Animation Used for debugging
Extremely useful for new or modified protocols
Allows to visualize a simulation while running it.
Gives insight how a certain protocol works (RIP, BGP, OSPF, IPv4, IPv6, etc.)
The animation can also be run after the simulation is over, from a history file.
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19. Setup Animation
To view animation, click on Simulation and select Record Animation For Subnet. You have to do this before you do a simulation run.
Animation Viewer/Player
Slow
Fast
Step
Pause
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20. An Example
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21. Debugging
A good debugging tool is the Simulation Log. You can check for warnings and errors
Click on Results
Then Click on Open Simulation Log
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22. Coming up examples Shared Ethernet with bus topology
Coaxial cable
Shared Ethernet with star topology
Hub
Switched Ethernet with star topology
Switch
Mix
Hub and Switch
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23. Shared with Coaxial
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24. Shared with Hub
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25. Shared and Switched
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26. demos (step-by-step instructions are available)