Evaluation Performance Networking In Latency &Jitter note test equipment application the in ( and how to test latency. distribution. It also presents how Xena interprets and i inter T definition of latency and jitter can vary according to the RFC standard. the devices, network different benchmarking for However, engineers. and operators, are often considered the basic benchmarking indicators network by administrators, delay and jitter. As key metrics of Quality of Service (QoS)validation, latency and jitter to tolerant more is transfer data while jitter, and latency to sensitive are services (VoIP) IP over voice example, For jitter. and latency traffic on requirements different have applications Different device. or system network, a of performance the benchmarking and evaluating when parameters measurement key two are jitter and Latency OVERVIEW U his White P White his Xena Networks Networks Xena sers can can sers - frame gap, latency resolution, speed mismatch, latency over time, and
also the terms used in time mea time in used terms the clarifies aper find detailed guidelines and examples of time measur time of examples and guidelines detailed find –
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Xena Networks Networks Xena Conclusion OVERVIEW Contents
Latency and Jitter Distribution and Histograms and Distribution Jitter and Latency Time Over Jitter and Latency Accurate Latency Measurement Speed Mismatch Speed Resolution Measurement Latency Jitter Latency Introduction IFG IFG 10Compliance MEF Xena Jitter Measurement Jitter Xena First Last First Last
...... Histogram Jitter and Analysis - - - - to to
to to – ......
......
- - Global Price/PerformanceLeaders in Gig - -
Last Latency Measurement for Layer 3 Packet 3 Layer for Measurement Latency Last - Store for Measurement Latency First Last Latency Measurement of Frame Insertion Time Insertion Frame of Measurement Latency Last First Latency Measurement for Cut for Measurement Latency First
...... Networking Performance Evaluation
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17 10 13 13 12 13 9 8 8 7 6 5 4 3 3 3 1
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Xena Networks Networks Xena method commonly used for tester verification: tester for used commonly and one method and one method from RFC 4689: RFC from method one and nt types ofdiffere devices, which are: for 1242 RFC on based latencies measuring of methods two will describe sections The following frame reaches the input port and ending when the first bit of the output frame is seen the on forward devices, latency is defined as the time interval starting when the last bit of the input - store For switches. common and router, a e.g. network, the in see to common is device port and the Store Last store (SUT), definitions of latency depending on the type of the device under test (DUT) or system under test nt differe two provides Devices) Interconnection Network for Terminology (Benchmarking 1242 RFC configuration. device the changing without sizes packet of range a for taken be should a the reflect only should measurement The rate. data the of effect the eliminate should one latency, measuring When videoconferencing. and VoIP e.g. latency, network to sensitive are applications Some Latency performances. network affect jitter and important to clarify the terms first and then carry on the in- devices, the definition of latency and jitter can vary according to the RFC standard. Thus, it is network different benchmarking for However, engineers. and network administrators, operators, (QoS) validation, latency and jitter are often consideredbasic the benchmarking indicators by Service of Quality of metrics key As jitter. and delay to tolerant more is transfer data while jitter, servi (VoIP) IP over voice example, For jitter. and latency traffic on requirements different have applications Different device. or system network, a of performance Latency and jitter are two key measurement parameters when evaluating and benchmarking the What Latency are Jitter? and Introduction
• • • • - -
to and -
First Last First Last - Store for Measurement Latency First - forward devices, as the name indicates, first cache the entire packet from the input input the from packet entire the cache first indicates, name the as devices, forward –
-
and Global Price/PerformanceLeaders in Gig n process and forward it to the correct output port based decisions. This kind of - - - - to to
to to - - - - - Last for Layer 3 devices (e.g. for routers) 3 Layer devices Last First forFirst store Last for tester verification tester for Last First for cut for First ctual latencywithin the device. In order to generate accuracy, measurements forward device or cut or device forward
- through devices (e.g. cut (e.g. devices through
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Xena Networks Networks Xena egress sides of the DUT the of sides egress starting and ending measurement points are at the beginning of the packet on the ingress and ( output frame is seen the on output port first bit of the input frame reaches the input port andending when the start of the first bit of the For such cut “cut as known are devices These received. fully is it Cut First last output port. has received the last bit of the packet and stops whenDUT the begins to transmit the packet.
) measurement, also known as first as known also measurement, FTF) Figure 1. Last Figure 2. First 2. Figure
- - through devices, in contrast to store in, first in, - to - ingress ingress First Latency Measurement for Cut for Measurement Latency First – -
out (LIFO) measurement as shown in F packet
- Global Price/PerformanceLeaders in Gig In otherIn words, store the through devices, latency is defined as the time interval starting when the end of the
- - to to - - First (LTF) Latency for Store for Latency (LTF) First First (FTF) Latency for Cut for Latency (FTF) First
.
t 0 t 0 packet DUT/SUT DUT/SUT - and - . In other. In words in, first in, - and - forward latency is alast
- - packet through devices forward devices, begins to transmit a packet before before packet a transmit to begins devices, forward - packet and - out (FIFO),as shown in Figure 2, where both the
igure 1, because the timer starts when the DUT DUT the when starts timer the because 1, igure - - Forward devices through” devices, e.g. a cut - Through Devices Through abit abit , cut , t 1 t 1 E - thernet through latency is a first
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Xena Networks Networks Xena are only passed upand down the protocol stackc afterare they completely received. network a way the simulates closely method LTL moment the packet is fully received and the the time, of instants two between interval time the is delay forwarding the 3, Figure in shown As packet is received from the output port of the DUT/SUT packet is offered to the input port of latency. Forwarding delay is defined as the time interval starting when the last bit of the input IP DUTs. of chain sum the 4, Figure in shown example For delay. forwarding whole the equal delay forwarding each of sum The Furthermore, the LTL method has an additive property, which the LTF method in RFC1242 lacks. measurement, alsolast known as Network Benchmarking In addition to the two latency measurement definition above, RFC 4689 (Terminology for Last throughput rate. (BenchmarkingMethodology for Network Interconnect Devices) that latency be measured at the RFC 2544 in is recommended it practice, in is used definition latency which of Regardless
Figure 3. Last
- to ingress - Last Latency Measurement for Layer 3 Packet 3 Layer for Measurement Latency Last –
Global Price/PerformanceLeaders in Gig
- to
of forwarding delay of DUT A, B, and C equal the forwarding delay of the whole whole the of delay forwarding the equal C and B, A, DUT of delay forwarding of - Last Forwarding Delay (RFC 4689) Delay Forwarding Last
t - 0 layer Traffic Control Mechanisms) defines a last a defines Mechanisms) Control Traffic layer packet DUT - in, last in,
the DUT/SUT andending when the last bit of the output IP / SUT - out (LILO), which is called forwarding called delay is which (LILO), out
moment the packet is completely transmitted. The The transmitted. completely is packet the moment - layer device processes IP an packet. IP packets
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Xena Networks Networks Xena Ethernet link is calculated: link Ethernet 64 is that frame Ethernet minimum a of time insertion frame the Thus, (bps). rate the output packet is receivedfrom the output port of the DUT/SUT, as shown in Figure 5. bit of the input packet is offered to the input port of the DUT/SUT and ending when the last bit of first or measurement, where below: equation the with match should measurements time insertion frame Ethernet the theory in and medium the on frame a place to required time the is FIT (FIT). time insertion frame corr is instrument test a verifying when useful is method This The last in the permutation, not defined in any RFC, is the first First
Figure 4. Additive PropertyLast of Figure 5. First 5. Figure
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packet 0 0 packet Global Price/PerformanceLeaders in Gig
DUT/SUT A - to - Last Latency -
in, last in, t A t 1 0 packet
egress - out (FILO). This latency is as the time interval starting when the first first the when starting interval time the as is latency This (FILO). out t
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t C
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t t C 1 1 packet egress
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Xena Networks Networks Xena where as: packets belonging to the same expressed stream also , absolute value of the difference between the Forwarding Delay of two consecutive received the of out received be to packets causes it because service a of impact negative have can jitter Large constant. remaining of instead vary can packet each between delay the or lumpy, become can stream steady this errors, configuration congestion network to Due apart. evenly spaced packets the with stream measure. astreaming In network (VoIP or video packets streaming), are sent in acontinuous Packet jitter, as known as delay packet variation, is another important metric and to monitor Jitter Summary 1. Table methods: measurement latency different four the of summary a shows table following The year but is off by10 hours is extremely precise but not accurate. i Accuracy measure. can it value minimum the or point, reference some around variation measurement’s sometimes Precision, measurement. to comes it when things different purposes in order to achieve different to according methods correct the configure to important is it latency, measuring When medium. the traverse to frames the by required time the is it because 64 for latency FTL The
First Last First Last Method
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Global Price/PerformanceLeaders in Gig
of four different latency measurement methods measurement latency four of different FILO LILO FIFO LIFO As Known Also -
byte
frames on 1G Ethernet link thus has a minimumvalue ofns 512
optimal precision and accuracy. Precision and accuracy are two two are accuracy and Precision accuracy. and precision optimal
| is the order in which packets were received. were packets which in order the is 𝐷𝐷 𝑇𝑇
𝑖𝑖 + = 1 − RFC 4689 RFC 1242 RFC 1242 Standard 512 10 range of a buffer. RFC 4689 defines jitter as the as jitter defines 4689 RFC buffer. a of range
𝐷𝐷 9 𝑖𝑖
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= (RFC 4689) (RFC 512 abit abit
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Xena Networks Networks Xena is the time when the first bit the of packet leaves the output port, and the definit the first bit of the packet enters input the port. Since jitter is always calculated based on two packets, first, Xena calculate the timestamps accordingly. For exa For accordingly. timestamps the calculate Xena first, timestamps are are: are timestamps important two These implemented. is measurement jitter how to comes it when mentioning first) and For the four different latency measurements, i.e. first i.e. measurements, latency different four the For Gbps. 100 to up rates line at running testers, Xena in accordingly is implemented 10speciation MEF how illustrates 6 Figure in below chart flow The jitter. or (FDV), variation delay frame measure 10 MEF the released (MEF) Forum Ethernet Metro The accuracy. 8ns with jitter measures and Gbps, 100 40, 25, 10, 2.5, 1, e.g. rates, line at time real in processes Real MEF Compliance 10 where where (last The term jitter (packet delay variation ) used byXena not only includes the definition in RFC 4689 Measurement Jitter Xena previously. Thus, jitter calculated byXena can be expressedmathematically as below:
𝑱𝑱𝑱𝑱𝑱𝑱𝑱𝑱𝑱𝑱𝑱𝑱 - - • • to time jitter measurement at high resolution requires specialized hardware that accurately accurately that hardware specialized requires resolution high at measurement jitter time
𝑖𝑖 𝐿𝐿 -
last latency variation), but also other types of latency measurements asment
is the order in which packets were received. were packets which in order the is is latency value based on user selection (first selection user on based value is latency 𝑇𝑇𝑇𝑇 port) 𝑅𝑅
𝑖𝑖 𝒊𝒊 ion can be expressed with the timestamps as below
–
is the orde , Timestamp of the transmitted packet (when the packet is going out of the egres the of out going is packet the (when packet transmitted the of Timestamp , , Timestamp of the received packet (when the packet is going into the ingress port) ingress the into going is packet the (when packet received the of Timestamp ,
= Global Price/PerformanceLeaders in Gig
| ( 𝑹𝑹𝑹𝑹
𝒊𝒊 + r in which packets were received. There are two timestamps worth worth timestamps two are There received. were packets which in r
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𝒊𝒊
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( www.xenanetworks.com - - to to 𝑹𝑹𝑹𝑹 - - last, last last, last, last last, 𝒊𝒊
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− 𝑹𝑹𝑹𝑹 - - to to 𝒊𝒊 - s - ) 𝑇𝑇𝑇𝑇 |
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Xena Networks Networks Xena time analysis (charting and logging) and in a histogram graph to analyze with distribution graphs. Another advantage of real parameters. device or traffic in changes dynamic therefore there will be difference in the IFG between the received packets. received the between IFG the in difference be will there therefore value on one interval.But in many cases, the DUT/SUTwill introduce jitter to the trafficand t and same the be should values IFG received the all jitter, introduce packets should be measuredand displayed in a histogram. Ideally, if the DUT/SUT does not packets are transmitted with a fixed and known IFG value,and only the IFG of the received that requires This jitter. of view indirect an generate to used be can it therefore and value IFG prepare for reception of the next packet. An IFG histogramshows the distribution of the every known as the interframe gap packets Ethernet of transmissions between period idle minimum a allow must devices Ethernet IFG Histogram and Jitter Analysis lost out and sent with any uniform or bursty traffic pattern. Furthermore, this method c be can packets the that is measurement jitter frame time real 10 MEF of advantages main The
Figure 6. MEF 10 frame delay variation (jitter) measurement implementation
Yes No Calculate Jitter –
- Update min,maxand Global Price/PerformanceLeaders in Gig of
Tx avg jitterstatistics - sequence packets and it provides real First instream? Frame arrives? i In sequence? ) –(Rx Start
i+1 Yes No i - -Rx =|(Tx time jittermeasurement is jitter that data can be included in an ov (IFG). A brief recovery time between packets allows devices to to devices allows packets between time recovery brief A (IFG). i )| i+1 –
- time jitter (FDV)time statistics reflecting abit abit E thernet
Testing Testing he histogram will only have have will only histogram he
ompensates for the the for ompensates – www.xenanetworks.com
er
the the -
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Xena Networks Networks Xena received IFG histogram. IFG received and jitter increase.Thus, for these reasons, it is useful to monitor latency and jitter over t latency introduce also can traffic of Bursts latency. increased in resulting time, fillover to buffers situations, other In milliseconds. few for test the of beginning the in overflow to buffers cause may congestion QoS, the testing When when understand to is critical it circumstances, many Under l how i.e. lost, is information some reduction, can be used for comparisonswith other DUTs or as benchmarks. However, due to the data and jitter values. By doing this, the humongous quantity of data can be reduced and the values extended duration, and results are summarized into minimum, av an over measured is received frame single every of latency equipment, test a for Typically, Latency and Jitter Over Time arrive out will indicate an erroneously high jitter value due to the dropped packet. Similar, when packet packets these because histogram, IFG the from deducted be can that value jitter maximum the lo the after and before packets the because is This results. jitter the corrupt An important flaw of using IFG histogram method to analyze jitter is apacket that loss will jitter. average and dete be can that since all produce any detailed analysis such as a jitter histogram or how the jitter occurred over time, cannot but bucket, IFG each in received were packets many how show will histogram IFG The patterns. traffic varying rate (bursty) r on jitte measure to impossible also is It only. patter traffic uniform to measurement jitter the restricts which intervals, equal at sent be must packets that is limitation method This histogram. IFG received the analyzing by jitter average and minimum maximum, approximate the determine to possible is it values, IFG received the all of distribution the shows histogram IFG received the Since
Figure 7. Interframe
packet
- of –
Global Price/PerformanceLeaders in Gig - however cannot provide an accurate view jitter of due to its limitations. Theprimary order, this will also corrupt th corrupt will also this order, IFG
IFG histogram packet
gap histogram andjitter analysis
rmined from the received IFG histogram are the maximum, minimum minimum maximum, the are histogram IFG received the from rmined IFG packet ingress
a minor speed mismatch between two interfaces may cause cause may interfaces two between mismatch speed minor a
e jitter results, which can be deducted from the
atency changes over time. over changes atency DUT / SUT abit abit
E thernet increased latency or jitter occurs. occurs. jitter or latency increased egress packet erage, and maximum latency latency maximum and erage,
Testing Testing
IFG st packet will corrupt will corrupt st packet – packet www.xenanetworks.com IFG histogram
IFG ime. ime. packet
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Xena Networks Networks Xena and displayed in real time over the test duration, as shown the in exampl summary Xena provides many ways of tracking changes in latency and jitter. One way is short frame transmitted with fields including timestamp, sequence number, t number, sequence timestamp, including fields with transmitted frame proprietary test payload of either normal or micro length. The test payload is embedded every in with frame generate can Xena inspection. level frame for payload test Xena is way Another Figure 8. Latency andjitter –
, where average, minimum and maximum latency or jitter is calculated every second second every calculated is jitter or latency maximum and minimum average, where , –
Global Price/PerformanceLeaders in Gig
1 sec average over time
abit abit E thernet
Testing Testing
est ID, etc. Xena offers a a offers Xena etc. ID, est e in Figure 8. Figure e in – www.xenanetworks.com
- term term
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Xena Networks Networks Xena Figure 9. Latency andjitter histogram each divided into a series of intervals. Intervals are adjacent and equal size.The number of cases in are values all histogram, a Toconstruct variable. of continuous a distribution probability the A histogram agraphical is representation of the distribution of numerical data. It is an estimate of analysis. in for knowledge such collecting of way common a is histograms Using circumstances. different under behaves DUT the how about information gives it because parameters other over maximum,and are it but is important, also important to know how the me minimum, average, i.e. reduction, data with or level frame per a on jitter and latency measure To Latency and Jitter Distribution and Histograms jitterlatency and over du the test display timing information for every frame. It is thus possible to detect the smallest changes in plug
- in for Wireshark, the open interval is thus drawn in a rectangle shown as the example in Figure 9.
–
Global Price/PerformanceLeaders in Gig
- source protocol analyzer, to decode the test fields payload and ration by examining the capture frames.
abit abit E thernet
Testing Testing
asurements change
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- depth depth
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Xena Networks Networks Xena roceed with accurate latency measurement. the fix latency delay, and it accurate is with ready to proceed ports based on the measuredpropagation delay, overall latency can be adjusted to account for ports using last using ports of propagation latency of a40 Xena testmodules have a latency measurement resolutionof ±8 ns, which corresponds to the equipment. test the of precision the to refers It measurement. measure propagation delay using Xena testers. A back transparent materials isslower than the speed of light in vacuum c n index refractive In addition to the port inherent latency, propagation latency should also be measured. Due to the statistics. collecting for traffic generate and in order to verify that the test setup is homogenous. To calibrate a test port, set ports test the to out carried be should calibrations measurement, latency the performing Before measurement. latency overall the affects the correct test methodology be used so that no congestio that important is also It reliability. high and jitter, low resolution, sufficient accuracy, high provide low Measuring Accurate Latency Measur measurement. the on buffers DUT’s the of effect the consider mixed a for test jitter or latency a designing when will occur congestion traffic whether Thus, consider it to is important measurement, but a buffer capacity. latency desire the reflect correctly will not situation a such in measured The latency will occur. loss frame and overflow will buffer egress the rate, line the of 10% above rate any at interface Ethernet Gigabit a to traffic offers interface Ethernet 10G a if instance, For measurement. Interfaces operating at different speeds can cause congestion, which can affect the latency Mismatch Speed time any for concept key a is resolution, timestamp or resolution, measurement Latency Latency Measurement Resolution
Figure 10. Calibrating test ports in local loop-
1000 ns may be anywhere in the range of 1008 to 992ns. 1008to of range the in anywhere be may 1000 ns –
Global Price/PerformanceLeaders in Gig
- - latency switch fabric accurately requires that the measurement equipment to
of the fiber medium, where where medium, fiber of the - last modecan quickly determine the latency of the fiber. By calibrating the - speed environment. If congestion occurs, the result analysis should then
-
meter optical fiber. This means that, for example, the actual latency latency actual the example, for that, means This fiber. optical meter Test port Test ement
n>1 back mode. back
, the speed v
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n or backpressuren is created, which E thernet Test port k connection between two test test two between connection k
at which light propagates through
(
v =c/n v Testing Testing
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the loop
- - based back on
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Xena Networks Networks Xena and the latency is accurately measured. output port of the switch is not oversubscribed, no congestion will occur asshown in Figure 13 occurs to affect latency measurement. Ideally, when the test ports are synchronized and the transmission at the test ports is synchronized so that no queuing or congestion inside switch the low of latency measuring when challenge major A = N(N L as increases links of number The test. forwarding capabi The switch fabricshould be tested with a full mesh topology in order to thoroughly examine its
Figure 11. Measuring propagation delay with back Figure 12. Physical andlogical
– 1
Global Price/PerformanceLeaders in Gig
2 lity. Figure 12 shows the full mesh topology of a 4- a of topology mesh full the 12 shows Figure lity. 3
4 Test port Test
a four - port full mesh topology
1 2
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. Test port latency switches is to ensure that packet packet that ensure is to switches latency
- to abit abit - back connectionback E thernet
4 3
Testing Testing port switch fabric for the the for fabric switch port
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Xena Networks Networks Xena 1 packet of size the 15, Figure in example For results. measurement latency the skew also addition,In varying packet sizes can also cause ashort of port period output congestion. operate at stable synchronized and transmit clocks as well allow as PPM adjustments. adde will be queue until packet 0 synchronized and congestion occurs at the output port 0. Packet 1 Figur in example For latency. measured congestion will occur and packets will be buffered at the output port, which will increasethe However, if the transmissionclocks are not properly synchronized or drifted over time, internal packet 0 smaller packet than 0 sure that no packet is dropped. and sequence. The pattern testshould run at the repeatable a linewith rate and makestraffic network real simulate to order in sizes packet mixed with tested be must switches However, turn to transmit. This extra amount of latency will be to added the latency measurement.
oversubscription. Figure 13. Latency is accurately measured when test ports are synchronized without output
Input 1 Input 0 Input 3 Input 2 → 0 is completely forwarded. As a result, pack result, a As forwarded. completely is 0 –
d to the actual forwarding latency. Thus, the transmission portsmust be ableto Global Price/PerformanceLeaders in Gig
→ →
0 isfinished forwarding.This extra amount of waiting time in the queue 1. This will result in that packet 1→ packet that in will result This 1. 3→2 2→3 1→0 0→1
t 0 3→ 2→ 1→ 0→ Latency =t e 14, packets arriving at port 0 and 1 are not properly not 1are 0and port at arriving packets e 14,
1 et 1 et abit abit –t → 2→3 3→2 0→1 1→0 E 0 arrives 0 port before0 the at output thernet 0 0 has to wait in the queue until its t → 1
Testing Testing 0 has to wait in the output 3→ 2→ 0→ 1→
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Output 3 Output 2 Output 1 Output 0
This will → 1 is
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Xena Networks Networks Xena Figure 15. Latency increase dueto congestion caused by different packet sizes. Figure 14. Latency increase dueto congestion caused by lack of synchronization.
Input 1 Input 0 Input 3 Input 2
Input 1 Input 0 Input 3 Input 2 clock drift/skew Test portTX –
Global Price/PerformanceLeaders in Gig Packet size
difference Δt Δ 3→2 2→3 0 1→0 →1 t 3→2 2→3 0
1→0 →1 t 0 Latency =t 3→3 2→2 0 t 0 1→1 → Latency =t 3→3 2→2 0 → 1→1 0 0
1 –t 1
– 0 0→1
+Δt t
0→1 1→0 2→3 3→2 1→0 0
2→3 3→2 + abit abit Δ t t 1 0→0 E 0→0 t thernet 1→1 3→3 2→2 1 1→1 3→3 2→2 Packet 1→0 Waiting timeof Δt
Packet Waiting timeof Δ Testing Testing
t 1→0 Output 2 Output 3 Output 1 Output 0 – Output 3 Output 2 Output 1 Output 0 www.xenanetworks.com
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Xena Networks Networks Xena equipment. test Xena using measurement time of examples and guidelines detailed find can users where of Latency and Jitter in Networking Performance Evaluation, How to apply latency testing is included in the application note order to accurate obtain latency measurement as well as real - Low measurements. over time, and distribution. It also presents how Xena interprets and implements time inter jitter, latency, e.g. measurement, time in used terms the of some clarifies paper white This jitter. and latency traffic on requirements different have applications Different device. or system network, a of performance Latency and jitter are two key measurement parameters when evaluating and benchmarking the CONCLUSION
–
Global Price/PerformanceLeaders in Gig
latency switches should be tested with both fixed and mixed packet sizes in
- frame gap, latency resolution, speed mismatch, l mismatch, speed resolution, latency gap, frame
abit abit E thernet - life per life
Testing Testing formance results. formance
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atency
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