Computer Networks Performance Metrics

1. Computer Networks PerformanceMetrics

2. Performance Metrics Outline • Generic Performance Metrics • Network performance Measures • Components of Hop and End-to-End Packet Delay • Other Performance Measures Computer Networks Performance Metrics

3. Computer Networks T X Z W1 W2 Host L W3 W Y AP Host M Host A 2 3 W4 4 1 Host B 5 16 nodes 14 Host J 11 17 12 Host C 15 6 13 10 7 Host H 9 Host D 8 Host G Host E Host F Computer Networks Performance Metrics

4. packet being transmitted (delay) packets queueing(delay) free (available) buffers: arriving packets dropped (loss) if no free buffers How do Loss and Delay occur? packets queue in router buffers • when the sum of the arriving packets at the router exceeds the output link capacity. 17 15 14 12 Computer Networks Performance Metrics

5. Simple Queueing Model Customer Arrivals Server Queue Computer Networks Performance Metrics

6. Router Node node15 packet packet 17 IncomingLink Outgoing Link Server Router Buffer Computer Networks Performance Metrics

7. Generic Performance Metrics Utilization :: • The percentage of time a device is busy servicing a “customer”. Throughput :: • The number of jobs processed by the “system” per unit time. Response time :: • The time required to receive the response of a request (round-trip time (RTT) ). Delay :: • The time required to perform an operation. Computer Networks Performance Metrics

8. Network Performance Measures • Channel utilization:: The average fraction of time a channel is busy [e.g. Util. = 80%] • Note: Channel utilization is often referred to as channel efficiency, if overhead is not taken into account (i.e., it is excluded from the useful bits sent), • Throughput:: The amount of data that can be sent by the channel per unit time (bits/sec) [e.g. Tput. = 10 Mbps] Computer Networks Performance Metrics

9. pipe that can carry fluid at rate Rsbits/sec) pipe that can carry fluid at rate Rcbits/sec) Throughput Throughput:: rate at which bits transferred between sender and receiver • instantaneous: rate at a given point in time • average: rate over longer period of time server, with file of F bits to send to client server sends bits (fluid) into pipe link capacity Rsbits/sec link capacity Rcbits/sec Computer Networks Performance Metrics

10. Rs > RcWhat is average end-end throughput? Rsbits/sec Rcbits/sec Rcbits/sec bottleneck link link on end-end path that constrains end-end throughput Throughput (more) Rs < RcWhat is average end-end throughput? Rsbits/sec Computer Networks Performance Metrics

11. Throughput: Internet Scenario • per-connection end-end throughput: • x connections (fairly) share backbone link Rbits/sec. • Bandwidth is divided among the connections. • What’s the main bottleneck • Min{Rc,Rs,R/x} Rs Rs Rs R Rc Rc Rc Computer Networks Performance Metrics

12. End-to-End Packet Delay End-to-end delay includes multiple hop linkdelays. The sum of delays of the connecting links. Tanenbaum Computer Networks Performance Metrics

13. Hop Delay Components • Nodal Processing • Queuing • Data Transmission • Propagation Computer Networks Performance Metrics

14. transmission A propagation B nodal processing queueing Four Sources of Packet Delay • 1.Processing delay [PROC] • Between the time the packet is correctly received at the head node of the incoming link and the time the packet is assigned to an outgoing link queue for transmission. Processing Delay • Checking for bit errors • Determination of output link • Moving packet from input queue to output queue • Table lookup time (see routing algorithms) Computer Networks Performance Metrics

15. transmission A propagation B nodal processing queueing Four Sources of Packet Delay • 2.Queuing delay[QUEUE] • Between the time the packet is assigned to a queue for transmission and the time it starts being transmitted. • During this time, the packet waits while other packets in the transmission queue are transmitted. • Queueing delay • Time waiting at output link for transmission • Depends on congestion level of router Computer Networks Performance Metrics

16. Four Sources of Packet Delay transmission A propagation B nodal processing queueing • 3.Transmission delay [TRANS] • Between the times that the first and last bits of the packet are transmitted. • Depends on available bandwidth. Transmission delay: • R=link bandwidth (bps) • L=packet length (bits) • Time required to send bits into link = L/R (sec.) Computer Networks Performance Metrics

17. Four Sources of Packet Delay transmission A propagation B nodal processing queueing 4. Propagation delay[PROP] Between the time the last bit is transmitted at the head node of the link queue and the time the last bit is received at the next router. Propagation delay: • d = Length of physical link (met.) • s = Propagation speed in medium (met./sec.) • Propagation delay = d/s (sec.) • Depends on the physical distance and medium Computer Networks Performance Metrics

18. End-to-end Delay End-to-end packet delay :: • The time required for delivering a packet from the source node to the destination Host (or node). • Most often, we are interested in the packet delay within the communication subnets. • This delay is the sum of the delays on each subnet link traversed by the packet. Computer Networks Performance Metrics

19. Link (Nodal) Delay • dproc = processing delay • typically a few microsecs or less • dqueue = queuing delay • depends on congestion • dtrans = transmission delay • = L/R, significant for narrow bandwidth channels • dprop = propagation delay • a few microsecs to tens of msecs Computer Networks Performance Metrics

20. End-to-End Packet Delay E-T-E delay= (Link Delay) Link Delay= PROC+QUEUE+TRANS+PROP End-to-End delay = Sum of ALL link packet delays. End-to-End can be defined either: • From Host-to-Host • From end-to-end nodes within the subnet. Computer Networks Performance Metrics

21. End-to-End Packet Delay End-to-end delay includes multiple hop linkdelays. Computer Networks Performance Metrics

22. Queueing Delay • R=Link (Channel) bandwidth (bits/s) • L=Packet length (bits) • a=Average packet arrival rate (p/s) Traffic Intensity = La/R • La/R~0: Average queueing delay small • La/R1: Delays become large • La/R> or =1: More work arriving than can be handled. The rate at which bits arrive exceeds the rate bits can be transmitted. Computer Networks Performance Metrics

23. Network Performance Measures • Latency:: • usually implies the minimum possible delay. Latency assumes no queueing and no contention encountered along the path. • Goodput:: • measured at the receiver • the number of useful traffic delivered by the network to a certain destination, per unit of time. • File transmission (file size/transfer time) • No Retransmitted packets • No protocol overhead • No dropped packets • GoodputThroughput Computer Networks Performance Metrics

24. Network Performance Measures • Fairness:: • Fairness determines whether users or applications are receiving a fair share of system resources. • There are several mathematical and conceptual definitions of fairness. Jain’s fairness (Raj Jain) • n : number of users • xi : the resource share allotted to ith user • ranges from 1/n (worst case) to 1 (best case) • it is maximum when all users receive the same allocation. • is k/n when k users equally share the resource, and the other n − k users receive zero allocation. Computer Networks Performance Metrics

25. Network Performance Measures • Other fairness Definitions (e.g., Max-min) ? Computer Networks Performance Metrics

26. Network Performance Measures • Quality of Service (QoS):: • QoS refers to a broad collection of networking technologies and techniques. • The goal of QoS is to provide guarantees on the ability of a network to deliver predictable results. • Several QoS-based network performance factors are • availability (uptime) • error rate (reliability) • bandwidth (throughput) • latency (delay) Computer Networks Performance Metrics

27. Network Performance Metrics Packet Delivery Ratio (PDR):: • The ratio of the number of received packets to the total number of sent packets • PDR= Packets Received / Packets Sent • Packets sent/received values should be from the same layer. i.e. all from application layer or network layerNo Duplicate, No Retransmission Packet loss rate:: • The percentage of the lost or dropped packets. Link layer retransmission rates:: • The percentage of DL layer frames that are retransmitted. Computer Networks Performance Metrics

28. Packet Loss • Packet loss is the failure of one or more transmitted packets to arrive at their destinations. • Packet loss reasons: • Low signal strength • Interference • Congestion • Overburdened network • Queue (buffer) capacity is finite, and the packets arriving to the full queue are dropped. buffer (waiting area) packet being transmitted A B packet arriving to full buffer is dropped. Computer Networks Performance Metrics

29. Performance Metrics Summary • The three most general performance measures are : utilization, throughput and response time. • In computer networks, end-to-end delay is an important performance metric. • Queuing models are used to analyze and estimate computer network performance. Computer Networks Performance Metrics

30. Performance Metrics Summary • Other useful metrics include: latency, goodput, fairnessand QoS metrics such as Availability, reliability. • In wireless networks, delivery ratio, packet loss rate and link layer retransmission ratesare valuable network measures. Computer Networks Performance Metrics