Computer Networks PerformanceMetrics

Performance Metrics Outline

Generic Performance MetricsNetwork performance MeasuresComponents of Hop and End-to-End

Packet DelayOther Performance Measures

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Computer Networks

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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.

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packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets dropped (loss) if no free buffers

15 17

Simple Queueing Model

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CustomerArrivals

Queue Server

Router Node

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packet

node 15

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Outgoing Link

Router BufferServer

packet

Incoming Link

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.

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Generic Performance Metrics

Generic Performance Metrics

Network Performance MeasuresChannel 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]

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ThroughputThroughput:: rate at which bits transferred between sender and receiver

◦instantaneous: rate at a given point in time

◦average: rate over longer period of time

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server, withfile of F bits

to send to client

link capacity

Rs bits/seclink capacity

Rc bits/sec

pipe that can carryfluid at rate

Rs bits/sec)

pipe that can carryfluid at rate

Rc bits/sec)

server sends bits

(fluid) into pipe

Throughput (more)Rs < Rc What is average end-end throughput?

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Rs bits/sec Rc bits/sec

Rs > Rc What is average end-end throughput?

Rs bits/sec Rc bits/sec

link on end-end path that constrains end-end throughput

bottleneck link

Throughput: Internet Scenario

per-connection end-end throughput:

x connections (fairly) share backbone link R bits/sec.

Bandwidth is divided among the connections.

What’s the main bottleneck Min{Rc,Rs,R/x}

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Rs

Rs

Rs

Rc

Rc

Rc

R

End-to-End Packet Delay

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End-to-end delay includes multiple hop link delays.

The sum of delays of the connecting links. Tanenbaum

Hop Delay Components

Nodal Processing Queuing Data Transmission Propagation

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Four Sources of Packet DelayProcessing Delay

Checking for bit errorsDetermination of output

linkMoving packet from input

queue to output queue Table lookup time (see routing

algorithms)

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propagation

transmission

nodalprocessingqueueing

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.

Four Sources of Packet Delay

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propagation

transmission

nodalprocessingqueueing

Queueing delay Time waiting at output

link for transmission Depends on congestion

level of router

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.

Transmission delay: R=link bandwidth (bps) L=packet length (bits) Time required to send bits

into link = L/R (sec.)

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propagationtransmission

nodalprocessingqueueing

Four Sources of Packet Delay

Four Sources of Packet Delay

3.Transmission delay [TRANS]• Between the times that

the first and last bits of the packet are transmitted.• Depends on available

bandwidth.

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

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propagationtransmission

nodalprocessingqueueing

Four Sources of Packet Delay

Four Sources of Packet Delay

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.

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.

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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

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proptransqueueprocnodal ddddd

End-to-End Packet Delay

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.

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E-T-E delay= (Link Delay) Link Delay= PROC+QUEUE+TRANS+PROP

End-to-End Packet Delay

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End-to-end delay includes multiple hop link delays.

Queueing DelayR=Link (Channel) bandwidth

(bits/s)L=Packet length (bits)a=Average packet arrival rate

(p/s)

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Traffic Intensity = La/R

La/R~0: Average queuing delay small La/R1: 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.

Network Performance MeasuresLatency ::

◦ 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◦ GoodputThroughput

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Network Performance MeasuresFairness ::

◦ 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.

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Network Performance Measures

◦ Other fairness Definitions (e.g., Max-min)

?

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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)

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Network Performance MetricsPacket 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.

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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.

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packet being transmitted

packet arriving tofull buffer is dropped.

buffer (waiting area)

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.

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Performance Metrics Summary

Other useful metrics include: latency, goodput, fairness and QoS metrics such as Availability, reliability.

In wireless networks, delivery ratio, packet loss rate and link layer retransmission rates are valuable network measures.

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