Computer Communication Networks, network layer performance.pptx
- 1. MAHARAJA INSTITUTE OF TECHNOLOGY MYSORE DEPARTMENT OF ELECTRONICS AND COMMUNICATION TOPIC : NETWORK-LAYER PERFORMANCE Presenting By : Shashank M 4MH21EC085
- 2. NETWORK-LAYER PERFORMANCE The collective network information to describe the quality of services delivered by the underlying computer network is known as "network performance". The network layer expect to receivean ideal service, but the network layer is not perfect. The performance of a networkcan be measured in terms of 1. Delay 2. Throughput 3. Packet Loss 4. Congestion Control (It is an issue that can improve the performance)
- 3. 1.Delay All of us expect instantaneous response from a network, but a packet, from its source toits destination, encounters delays. The delays in a network can be divided into four types: 1.Transmission Delay :A source host or a router cannot send a packet instantaneously. A sender needs to put the bits in a packet on the line one by one. If the first bit of the packet is put on the lineat time t1 and the last bit is put on the line at time t2, transmission delay of the packet is(t2 - t1). Definitely, the transmission delay is longer for a longer packet and shorter if the sender can transmit faster. Delaytr = (Packet length) / (Transmission rate). 2.Propagation Delay :Propagation delay is the time it takes for a bit to travel from point A to point B in the transmission media. The propagation delay for a packet-switched network depends on the propagation delay of each network . The propagation delay depends onthe propagation speed of the media, which is 3 X108 meters/second in a vacuum and normally much less in a wired medium; it also depends on the distance of the link. Delaypg = (Distance) / (Propagation speed). 3.Processing Delay :The processing delay is the time required for a router or a destination host to receive a packet from its input port, remove the header, perform an error detection procedure, anddeliver the packet to the output port or deliver the packet to theupper-layer protocol . The processing delay may be different from each packet, but normally it is calculated as an average. Delaypr = Time required to process a packet in a router or a destination host
- 4. 4.Queuing Delay :Queuing delay can normally happen in a router. A router has an input queue connected to each of its input ports to store packets waiting tobe processed; the router also has an output queue connected to each of its output ports to store packets waiting to be transmitted. The queuing delay for a packet in a router ismeasured as the time a packet waits in the input queue and output queue of a router. Delayqu = The time a packet waits in input and output queues in a router Total Delay :Assuming equal delays for the sender, routers, and receiver, the total delay a packet encounters can be calculated if we know the number of routers, n, in the whole path. Total delay = (n + 1) (Delaytr + Delaypg + Delaypr) + (n) (Delayqu) Note that if we have n routers, we have (n + 1) links. Therefore, we have (n + 1) transmission delays related to n routers and the source, (n + 1) propagation delays related to (n + 1) links, (n + 1) processing delays related to n routers and the destination, and only n queuing delays related to n routers.
- 5. 2.Throughput Throughput at any point in a network is defined as the number of bits passing through the point in a second, which is actually the transmission rate of data at that point. In a path from source to destination, a packet may pass through several links , each with a different transmission rate. How, then, can we determine the throughput ofthe whole path to see the situation, assume that we have three links, each with a different transmission rate, as shown in Figure. In this figure, the data can flow at the rate of 200 kbps in Link1. However, when the data arrives at router R1, it cannot pass at this rate. Data needs to be queued at the router and sent at 100 kbps. When data arrives at router R2, it could be sent at the rate of 150 kbps, but there is not enough data to be sent. In other words, the average rate ofthe data flow in Link3 is also 100 kbps. We can conclude that the average data rate for this path is 100 kbps, the minimum of the three different data rates. The figure also shows that we can simulate the behavior of each link with pipes of different sizes; the average throughput is determined by the bottleneck, the pipe with the smallest diameter. In general, in a path with n links in series, we have Throughput = minimum {TR1, TR2, . . . TRn}
- 6. 3.Packet Loss Another issue that severely affects the performance of communication is the number ofpackets lost during transmission. When a router receives a packet while processing another packet, the received packet needs to be stored in the input buffer waiting for itsturn. A router, however, has an input buffer with a limited size. A time may come whenthe buffer is full and the next packet needs to be dropped. The effect of packet loss on the Internet network layer is that the packet needs to be resent, which in turn may createoverflow and cause more packet loss. A lot of theoretical studies have been done in queuing theory to prevent the overflow of queues and prevent packet loss.
- 7. 4.Congestion control Congestion control is a mechanism for improving performance. Congestion at the network layer is related to two issues, throughput and delay, which we discussed in the previous section. A . When the load is much less than the capacity of the network, the delay is at a minimum. This minimum delay is composed of propagation delay and processing delay, both of which are negligible. However, when the load reaches the network capacity, thedelay increases sharply because we now need to add the queuing delay to the total delay. B . When the load is below the capacity of the network, the throughput increases proportionally with the load. We expect the throughput to remain constant after the load reaches the capacity, but instead the throughput declines sharply. The reason is the discarding of packets by the routers.