computer networks performance metrics. performance metrics outline generic performance metrics...
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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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nodes
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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
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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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