In OPNET IT Guru Laboratory assignment 3 we will create different scenarios in concerned with Switched LANs. Scenarios will be different on bases of the switching devices are used and configuration of the network. We will observe the performance of network for different scenarios which are designed with devices such as HUB and SWITCH.
Every network has its own boundaries on bases of number of hosts in a network, geographical area, traffic in a network and devices used in a network, etc.
Switching devices like Hub, Switch, Router, Bridge or Gateways are used to provide communication links among different nodes. In this Laboratory we will use Hub and Switch with nodes to provide communication link and we will observe the performance of it.
Switch is a device which provides communication link between nodes connected to its input and output line. In Switch each line has full access of bandwidth. It will collect the packets from its input link and forward them to its destination from its output link. It will start buffering the input packets if output lines are not free. In such case it will wait till the output line become free and then it will start forwarding files.
Hub is device which serves the same purpose as the switch. Only difference is, all the nodes connected to Hub shares the bandwidth among themselves. Hub will send all the data received from input line to all the output lines regardless the specific address of nodes.
First Scenario: OnlyHub
In this scenario, LAN is created using 16 nodes which are ethernet_station and Ethernet_16Hub. All 16 nodes are connected to Hub with STAR topology. 10BaseT links are used to connect each node to Hub. Implemented network will look likes a Figure 1.
Figure 1: 16 ethernet_stations connected to Ethernet_16Hub using 10BaseT link
Second Scenario: HubandSwitch
In this scenario we will add Switch to the network of Figure 1. First we will divide the 16 nodes between 2 ethernet_16Hub. Each Hub will have connection to 8 nodes. Now we will connect two Hubs with Ethernet_16Switch. For all connections 10BaseT link will be used. The implemented network will look like Figure 2.
Figure 2: Two Ethernet_16Hub connected via Ethernet_16Switch
After implementing the above two scenarios, we will take results for the Traffic Sent, Traffic Received, Delay and Collision Count.
In both scenarios Traffic Sent from workstations is almost identical which can be seen in Figure 3.
Figure 3: Traffic Sent for scenarios OnlyHub and HubandSwitch.
When we observe the Traffic Received in both scenarios, we can see in Figure 4 that we can get better throughput when we add a Switch between two Hubs
Figure 4: Traffic Received for scenarios OnlyHub and HubandSwitch.
As Switch transmits packets to the destination node on bases of MAC, it will increase the throughput of the network. Adding the Switch we are reducing the sharing of bandwidth which provides better availability of transfer media for each node.
Figure 5 shows the delay in both scenarios. For OnlyHub scenario, delay is increasing in the beginning as Hubs are not self configured so the starting delay is expected. When we add a Switch, delay is very less – almost negligible.
Figure 5: Delay for scenarios OnlyHub and HubandSwitch.
Figure 6 shows the collision count at Hub1 for both scenarios. In OnlyHub scenario, Collision count is high of almost 2400 collision. But in the scenario of HubandSwitch the collision count is very low compare to OnlyHub scenario. Protocols used in switching network helps to reduce the collision of packets in the network.
Figure 6: collision Count for scenarios OnlyHub and HubandSwitch.
Above results shows that Switch increase the performance of the network in compare to Hub. This proves that adding a switch in a network will make a remarkable change in terms of performance.
Hub will just sent data packets to all other output lines, which means that when one node receives a packet on an input line the hub will forward that packet to all other nodes, while Switch will send the received data packet to the dedicated line from all other nodes. This functionality of Switch helps in increasing the throughput.
Delay is also very important for any network. As above result for delay shows that after adding a switch delay will be reduced. So thus Switch not just improves the throughput it also reduces the delay.
Another important constraint for any network is Collision Count. All the nodes connected to the hub share the bandwidth among them, while in switch nodes have the full bandwidth. So by adding a Switch in a network we are increasing the bandwidth for each of the node and this will reduce the collision count as each node as more bandwidth.
Collision is directly propositional to the volume of the traffic. In OnlyHub scenario Hub is serving 16 nodes while in second scenario there are two Hubs- each serves 8 nodes. So by reducing the number of nodes for each hub and adding a switch between them chance for collision is reduced and performance of network is increased.
Question / Answer:
Explain why adding a switch makes the network perform better in terms of throughput and delay.
Hub forwards the packet to all its connection without taking consideration of addressing for packet transfer. Such it will send a packet to its all link. Apart from this, there is no traffic inspection and it will just forward the packet. This is very inefficient way of communication. Hub is also not self configured device. So there is always a starting delay.
Switch is a smart network device compare to hub. It serves the several dedicated links independent to each other. This feature improves the throughput of the network. Switch is self configured device so network downtime is less and received packet will be sent to its destination only on bases of its address. This characteristic of switch reduces the delay.
So when we add a switch in our second scenario, traffic generated at any node will be directed to its destination node. So there is no traffic propagation. Such switch makes the network performance better in terms of throughput and delay.
We analyzed the collision counts of the hubs. Can you analyze the collision count of the Switch? Explain your answer.
Hubs are “shared media” and switches are not. We cannot analyze the collision count of the Switch.
Switch will divides the network into multiple segments and maintain simultaneous connection of multiple pairs of the computer which do not compete with other pairs of computers for network bandwidth . When switch receives the packet it will forward the packet to its destination using the information of destination address in a header. So in Switch each port has its designated bandwidth. Multiple connections can be handle separately using switch. As Switch does not share the transmission media, network do not experience collision .
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Create two new scenarios. The first one is the same as the OnlyHub scenario but replace the hub with a switch. The second new scenario is the same as the HubAndSwitch scenario but replace both hubs with two switches, remove the old switch and connect the two switches you just addes together with 10baseT link. Compare the performance of the four scenarios in terms of delay, throughput and collision count. Analyze the results.
In this section, we are creating two more scenarios. Both scenarios have same configuration as previous ones but the hub will be replaced by switch. So, two new scenarios will be OnlySwitch and SwitchAndSwitch. Figure 7 and Figure 8 shows the OnlySwitch and SwitchAndSwitch network respectively.
Figure 7: 16 ethernet_stations connected to Ethernet_16Switch using 10BaseT link
Figure 8: 8 Ethernet_stations connected to each Ethernet_16Switch
When we compared the performance of four scenarios in terms of delay, throughput and collision count, we got following results.
1. Traffic Sent:
Traffic Sent for all four scenarios is identical. There is no big difference among them. This can be seen in Figure 9.
2. Traffic Received:
It can be seen in Figure 10 that traffic received is high in scenarios where Switch is present. This proves that Switch improves the throughput of the network.
Figure 9: Traffic Sent for all Four Scenarios.
Figure 10: Traffic Received in all Four Scenarios.
When we compare the delay for all four scenarios, following result came as shown in Figure 11. It can be seen that Switches reduce the delay in network as it as self configured device and use dedicated bandwidth for each nodes. When there is only hub in a network, delay is high compare to the network with switches.
Figure 11: Delay for all Four Scenarios.
4. Collision Count:
Figure 12 shows the result for comparison of Collision count of all four scenarios. It can be seen from the graph that collision count for OnlyHub scenario is higher than any other scenario. As we add a Switch with hub, Collision count is decreased. There is no Collision Count for scenarios where only switches are used because switch does not share the transferring media among nodes.
Figure 12: Collision Count for all Four Scenarios
By performing this laboratory we learned that how different devices used in a network affects the performance of the network in terms of throughput, delay and collision. We also learned that Switch is better network device than hub as it improves the performance of the network. We learned that use of switch in implementation of a network will reduce the collision and delay, and increase the throughput.
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