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

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

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. K. Salah

Ethernet Network Topology Point-to-point Bus Ring Star K. Salah

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 K. Salah

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

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. K. Salah

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 K. Salah

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. K. Salah

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. K. Salah

Bursty traffic model in OPNET Node Model Process Model K. Salah

Packet-Size Distribution K. Salah

Packet Distribution in OPNET K. Salah

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 K. Salah

Statistics Two ways to collect statistics Object statistics from individual nodes Global statistics From the entire network as whole K. Salah

Data Collection Vector Scalar Animation Time-dependent series of values Scalar Parametric relationships Animation visualization K. Salah

Vector vs. Scalar Vector data graphs Scalar data graph K. Salah

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. K. Salah

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 K. Salah

An Example K. Salah

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 K. Salah

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 K. Salah

Shared with Coaxial K. Salah

Shared with Hub K. Salah

Shared and Switched K. Salah

demos (step-by-step instructions are available)