Voice Over Ip Delay Measurement Information Technology Essay

Transmit existent clip informations is needed some technique to convey between transmitter and receiving system. If there is any clip hold on transmittal between transmitter and receiving system so signal can non make finish in proper clip. To manage existent clip informations, The Real-time Transport Protocol is developed. RTP is enhanced by another control protocol name RTCP usage to monitoring and bringing informations in a scalable manner.

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The receiving system studies contain 16 spot sequence figure Fieldss which is usage to cipher the package loss. In a peculiar session sequence figure generates. Sequence figure incremented by one each package send. The fraction of doomed

packages is defined to be the figure of packages lost divided by the figure of packages expected, which are calculated based on really received packages and the highest sequence figure received in RTP packages. [ 2 ]

Loss fraction= figure of packages lost/ figure of packages expected.

Where:

figure of packages lost = figure packages expected – figure packages received,

figure of packages expected = EHSNR – initial sequence figure

EHSNR = extended highest sequence figure received

= figure of sequence figure cycles*216 + last sequence figure received

2.2 Inter-arrival Jitter

There is continuance of clip to finish the session between packages over web. In existent clip there is no warrant the package can present seasonably over web. Because of some ground hold can be occur because of this hold a spread create between package sequences. The hold fluctuation between packages is known as Jitter. To manage the jitter each package contains a timestamp that tells the receiving system at which clip the informations in the package should be played back.

2.3 Inter-arrival jitter computation

The receiving systems observe continuously the discrepancy of the inter-arrival clip of incoming RTP packages. An estimation for inter-arrival jitter is calculated as follows.

If Si is the RTP timestamp from package I, and Ri is the clip of reaching in RTP timestamp units for package I, so for two packages one and J, D expressed as

D ( I, J ) = ( Rj – Rhode island ) – ( Sj – Silicon ) = ( Rj – Sj ) – ( Ri -Si ) ……….. ( 1 )

The inter-arrival jitter should be calculated continuously as each information package is received from beginning SSRC_n, utilizing this difference D for that package and the old package i-1 in order of reaching ( non needfully in sequence ) , harmonizing to the expression

J ( I ) = J ( i-1 ) + ( |D ( i-1, I ) | – J ( i-1 ) ) /16 ………………….. ( 2 )

2.4 Calculating directing Timestamp

A timestamp is a thirty-two spot field in RTP that gives the clip at which the first eight of informations was sample ; timestamp is incremented continuously even when no information is sent. For illustration clip distance at the transmitter between each package is 20ms and each package contains 160 eights of voice informations. Table 1 shows the RTP timestamp for the first 10 packages at the transmitter side.

Table ( 1 ) Timestamp when informations send

Sequence No.

Timestamp

1

0

2

160

3

320

4

480

5

640

6

800

7

960

8

1120

9

1280

10

1440

2.5 Calculating Receiver Timestamp

Receiver stamp the clip at RTP package when it received, receiver timestamp calculated harmonizing to the expression.

If Ri is the clip of arrival RTP package I, so for two packages one and J, D may be expressed as

Ri = ( Rj – Rhode island ) + Last Ri Value………………………… . ( 3 )

Rec. timestamp = ( Ri X Sampling frequence ) /1000…… ( 4 )

Calculation timestamp at receiver side shows in the tabular array.

Table ( 2 ) Time cast when informations received

Seq.

Arrival Time

Ri MS

Rec. timestamp

1

01:02:01:20

0

0

2

01:02:01:43

23

184

3

01:02:01:63

43

344

4

01:02:01:84

64

512

5

01:02:01:109

89

712

6

01:02:01:130

110

880

7

01:02:01:150

130

1040

8

01:02:01:170

150

1200

9

10

01:02:01:230

210

1680

Rec. timestamp measured merely when the package is arrive, packet nine is non arrive no computation done for package nine.

2.6 Jitter Calculation

Assume that the first package is sent at clip t=01:02:02:00. The clip distance at the transmitter between each package is 20 MS and each package contains 160 eights of voice informations. The sampling frequence of the voice signal is 8 kilohertz ( each sampled unit contains one eight ) . [ 1 ]

Formula

Sender timestamp computation defined in Table -1.

Receiver timestamp computation defined in equation ( 3 ) and ( 4 ) , Delay computation defines in equation ( 1 ) ; Jitter computation defines in equation ( 2 ) .

Let ‘s cipher the inter-arrival jitter for the package 3, 4

Sender timestamp: 0+160+160+160 = 480

Receiver timestamp: ( ( ( 84-63 ) +43 ) *8000 ) /1000 = 512

Delay: D ( 3,4 ) = ( 512-344 ) – ( 480-320 ) = 8

Jitter: J ( 4 ) = 1.406+ ( 8-1.406 ) /16 = 1.818 timestamps =.227 MSs

Table ( 3 ) Jitter computation

Seq. No

Sender

Timestamp

Arrival

Time

Rhode island

Multiple sclerosis

Receiver

Timestamps

D ( i-1, I )

timestamp

D ( I-1, I )

MS

J ( I )

Timestamps

Jemaah islamiyah

MS

1

0

01:02:01:20

0

0

0

0

0

0

2

160

01:02:01:43

23

184

24

3

15

.188

3

320

1:02:01:63

43

344

0

0

1.406

.176

4

480

01:02:01:84

64

512

8

1

1.818

.227

5

640

01:02:01:109

89

712

140

17.5

4.204

.526

6

800

01:02:01:130

110

880

8

1

4.442

.555

7

960

01:02:01:150

130

1040

0

0

4.164

.521

8

1120

01:02:01:170

150

1200

0

0

3.904

.488

9

1280

10

1440

01:02:01:230

210

1680

160

20

13.66

1.708

3. Describe and explicate how receiving system and transmitter studies harmonizing to RTCP can be used to describe losingss and jitter

Harmonizing to RTCP, transmitter and receiving systems generates studies of assorted session statistics and multicast to the group

The intent of RTCP is to look into is any mistakes take topographic point in the multicast distribution tree. It controls the congestion over informations transmittal. It takes attention about 3rd party public presentation monitoring and entree hallmark. [ 3 ]

The chief function of RTCP is to response on the quality of service ( QoS ) in media distribution by clip to clip directing statistics information to participants in a streaming multimedia session. RTCP collects statistics for a media connexion and information such as familial eight and package counts, lost package counts, jitter, and round-trip hold clip.

3.1 RTCP heading

RTCP defines five types of package. All the types portion a common heading construction. Packages types are Sender study ( SR ) ,

Receiver study ( RR ) , Source description ( SDES ) , Membership expiration ( BYE ) and Application-specific maps ( APP ) .

Figure.RTCP Header construction

The RTCP heading construction can be define as

V = version figure which is same as RTP,

P = embroidering index,

RR = response study count ( 5 spots ) ,

RTCP message type ( 8 spots ) ,

RTCP message length ( 16 spots ) ,

SSRC for the transmitter of this study ( 32 spots )

3.2 RTCP send study package format

The transmitter study package format is describe as follows

The heading contains the heading type for a transmitter study is 200.

NTP timestamp: It denotes about the wall-clock clip when package was send with 64 spot words less important words and most important words, which can combination with timestamps returned in response studies from other receiving systems to mensurate round-trip extension to those receiving systems [ 2 ] .

RTP timestamp: It shows the same clip case as the NTP timestamp but in RTP clock units.Which is used for synchronism factor.

Sender ‘s cumulative package count: Count the figure of RTP packages sends by this node in the peculiar session.

Sender ‘s cumulative byte count: Count the figure of bytes sends by this node in this peculiar session. Average payload information rate and the mean package rate can be calculated with this information over a specific clip interval without holding received any informations. Using these two values and the mean package size it is possible to cipher the norm available spot rates at the receiving system side

3.3 RTCP receive study package format

The RTCP receiving system study package format describes holla.

The package format of RR is about same as SR. in the RR package format the package type is 201 and the five words of transmitter information are omitted.

Synchronization beginning ( SSRC ) : To specify enchantress transmitter is described the Reportee SSRC field is used. [ 3 ]

Loss fraction: The figure of packages lost since the last SR or RR package was sent is expressed as a fraction.It is defined as the figure of packages lost in this interval divided by the figure of expected packages.

Loss fraction= nbr of packages lost/ figure of packages expected.

where:

nbr of packages lost = nbr packages expected – nbr packages received

figure of packages expected = EHSNR – initial sequence figure

EHSNR = extended highest sequence figure received

= figure of sequence figure rhythms 216 + last sequence figure received

Accumulative figure of packages lost: Accumulative figure of packages lost can be summarized as the entire figure of lost packages from this SSRC since the transmittal started.

Extended highest sequence figure received: The highest sequence figure found in this RTP watercourse. [ 3 ]

Interarrival jitter: Interarrival clip jitter is an estimation of the statistical discrepancy of RTP informations package interarrival times.

The smoothened mean absolute value of the difference between the directing interval at a beginning and the interarrival clip at a receiving system jitternew = jitterold + ( instantaneous jitter – jitterold/16 )

instantaneous jitter = | ( reci – reci-1 ) – ( senti – senti-1 ) |

3.4 Delay since last transmitter study received ( DLSR )

A clip value can be expressed in 1/65536 seconds.

From SR and RR we can acquire the undermentioned information:

Packet loss rate over the interval between two RR, Number of packages expected during the interval, Packet loss fraction over the interval -the ratio of the two above, Loss rate per second, Number of packages received = figure of packages expected minus the figure of package lost, Statistical cogency of any loss estimations, Judged utilizing the figure of packages expected. E.g. 1 out of 5 packages lost has a lower significance than 200 out of 1000, Inter-arrival Jitter.

4. One manner hold measuring:

The technique usage to protect hold between asynchronous connexions in existent clip applications Voip and picture is one manner hold measuring.

The redstem storksbills in the web are non synchronized which create job to mensurating one manner holds. How can we acquire the one manner hold measuring that is, A nod direct a package to following node set clip cast on it. When the following node received the package it set its ain clip cast on it. The difference between the clip cast is one manner hold measuring. This measuring will be equal to the matching one manner hold, if the redstem storksbills of both the nodes are synchronized. Otherwise the one manner hold set clock beginning and matching hold between the nodes. [ 4 ]

Requirement to mensurate the OWD

OWD = Ts – Tr

Tr = Packet having timestamp

Ts = Packet directing timestamp

In one manner hold measuring there is two factors involve the edict and truth.

( 1 ) Synchronism mistake. ( 2 ) The intrinsic mistake of the measuring device itself. With the aid of this tow factors must be minimized to supply a meaningful one-way hold measuring, if the mistake approaches even a ten percent of a msec, the truth will be deficient to reliably detect SLA public presentation issues. [ 4 ]

Some criterion have been developed to administer a clock synchronism signal by conveying mention packages through the web. The popular criterion known as Network Timing Protocol ( NTP ) , package timing Protocol ( PTP ) , Outline in a figure of ITU and IEEE criterion. [ 4 ]

4.1 Active measuring:

Active measuring can be described by the undermentioned manner. [ 9 ]

1. To Synchronize between two end systems utilizing GPS

2. Host1 sends one-way traffic to host2 through assorted theodolite webs

3. Host2 receives traffics, and steps delay

4. To compare mensural consequences for each Transit webs.

It will supply us a high declaration, spatially drawn-out dynamic image of fast alterations in the web traffic. This can open up the possibility to new sort of web imaging, where cross correlativity between measuring flows can be measured on a all right timescale and the internal province of the web, far off from the terminals of the web, where the measuring devices are located, can be reconstructed and its clip behaviour can be studied, and the informations can be analyzed with methods developed in the complexness. [ 8 ]

5. Describe chief part to a entire hold budget terminal to stop

A web public presentation can be improved by right numbering all optional holds. Overall voice quality is a map of many factors delay is one of them.

There is a bound for a good quality voice connexion. This can be described as, 0ms-150ms suitable for user applications. 150ms-400ms acceptable provided that decision makers are cognizant of the transmittal clip and the impact it has on the transmittal quality of user applications. Above 400ms is unacceptable for general web planning intents. However, it is recognized that in some exceeding instances this bound is exceeded.

For private webs for good quality voice 200 MS one manner hold is acceptable and 250 MS a bound. If the hold is rises over the talker and hearers become un-recognized and speaker overlap status is occur which can be observe on international telephone calls which travel over satellite connexions. [ 6 ]

Two different type of hold can be take castle on web.

1. Fixed hold constituents add straight to the overall hold on the connexion.

2. Variable holds arise from line uping holds in the egress bole buffers on the consecutive port connected to the WAN. These buffers create variable holds, called jitter, across the web. Variable holds are handled through the de-jitter buffer at the having router/gateway. The de-jitter buffer is described in the De-jitter Delay ( ?n ) subdivision of this papers.

Coder Delay: Coder hold can be express as, the clip taken by the digital signal processor to compact a block of PCM samples. This hold is varies with the voice programmer used and processor velocity.

Packetization Delay: Packetization hold is the clip taken to make full a package warhead with encoded/compressed address.

Serialization Delay: Serialization hold is the fixed hold required to time a voice or informations frame onto the web interface. It is straight related to the clock rate on the bole.

Queuing/Buffering Delay: Queuing hold is variable hold and is dependent on the bole velocity and the province of the waiting line.

Network Switching Delay: The public frame relay or ATM web that interconnects the end point locations is the beginning of the largest holds for voice connexions. Network Switching Delays are besides the most hard to quantify.

De-Jitter Delay: The de-jitter buffer transforms the variable hold into a fixed hold. It holds the first sample received for a period of clip before it plays it out. This keeping period is known as the initial drama out hold.

5.1 De- Jitter buffer

Variable Arrival Rate= Codec Frame Rate +/-

Over Flow Queue Fills if voice frame arrive excessively fast

F

Line uping Point Specified in MS

Video Frame from Network

Normal Operating Mode

Normal Operating Mode

Fixed Play out Rate = Codec Frame Rate

Under flow Queue empties if voice frame arrive excessively slow

Because address is a changeless bit-rate service, the jitter from all the variable holds must be removed before the signal leaves the web. In Cisco router/gateways this is accomplished with a de-jitter ( ?n ) buffer at the far-end ( having ) router/gateway. The de-jitter buffer transforms the variable hold into a fixed hold. It holds the first sample received for a period of clip before it plays it out. This keeping period is known as the initial drama out hold.

Figure 5-1-a: De-Jitter Buffer Operation

It is indispensable to manage decently the de-jitter buffer. If samples are held for excessively short a clip, fluctuations in hold can potentially do the buffer to under-run and cause spreads in the address. If the sample is held for excessively long a clip, the buffer can infest, and the dropped packages once more cause spreads in the address. Last, if packages are held for excessively long a clip, the overall hold on the connexion can lift to unacceptable degrees. [ 6 ]

5.2 Adaptive Jitter Buffers

Adaptive jitter buffers by and large react to either discard events or measured addition in jitter degree. When a discard event is detected so the jitter buffer size is increased and when there is no discard event so the jitter buffer size is reduced. [ 7 ]

If jitter events are widely spaced, as could happen with LAN congestion or frequent path alterations, so the addition in size of the jitter buffer may be counter-productive, hold may increase but it may be sufficiently long until the following event that the buffer size has reduced. An adaptative buffer may be more aid if jitter events are clustered, as would be typical for entree nexus congestion.

As adaptative jitter buffers are going widely used, and their operation is rather sensitive to the distribution of jitter events, it would look indispensable to mensurate jitter in a manner which reflects this clip distribution. A histogram based attack would non be sufficient to supply the information needed. Jitter buffer emulation can nevertheless supply a matter-of-fact solution and allows discard events to be straight estimated.

6. Measure package loss and unit of ammunition trip hold between routers utilizing IP-SLA

The Cisco IOS Software is use determine VoIP application public presentation on the information web. The ability to garner informations in existent clip and on demand makes it executable for IT groups and service suppliers to make or verify SLAs for IP applications ; baseline values can so be used to confirm an IP SLA for VoIP. Cisco IOS Service Assurance Agent ( SAA ) engineering is a constituent of an IP SLA solution and the Round Trip Time Monitor ( RTTMON ) MIB, which Enable the testing and aggregation of hold, jitter, and package loss measuring statistics. Active monitoring with traffic coevals is used for edge-to-edge measurings in the web to supervise the web public presentation. [ 6 ]

Routers may take 10s of msecs to treat incoming packages, due to other high precedence processes. This hold affects the response times because the answer to prove packages might be sitting on waiting line while waiting to be processed. In this state of affairs, the response times would non accurately represent true web holds. Cisco IOS IP SLAs minimise these processing holds on the beginning router every bit good as on the mark router ( if Cisco IOS IP SLAs Responder is being used ) , in order to find ( RTT ) true round-trip times. IP SLAs trial packages use clip stomping to minimise the processing holds. When enabled, the IP SLAs Responder allows the mark device to take two clip casts both when the package arrives on the interface at interrupt degree and once more merely as it is go forthing, extinguishing the processing clip. This clip stamping is made with a coarseness of sub msec. At times of high web activity, an ICMP Ping trial frequently shows a long and inaccurate response clip, while an IP SLAs trial shows an accurate response clip due to the clip stomping on the respondent. Figure 6 demonstrates how respondent works.

Beginning Router

T4

T1

Target Router

T2

Respondent

T3

= T3-T2

Figure 6. Cisco IOS IP SLAs Responder

Four timestamps are taken to do the computation for ( RTT ) round-trip clip. At the mark router, with the respondent functionality enabled, Timestamp 2 is subtracted from Timestamp 3 to bring forth the clip spent treating the trial package. This is represented by delta ( ? ) . This delta value is so subtracted from the overall round-trip clip. Notice that besides the same rule is applied by Cisco IOS IP SLAs on the Source Router where the entrance Timestamp 4 is besides taken at the interrupt degree to let for greater truth.

Now the RTT can be calculated as follows.

RTT = T4 ( Time stamp 4 ) – T1 ( Time stamp 1 ) – ? , Where ?=T3-T2.

An extra benefit of the two clip casts at the mark router is the ability to track one-way hold, jitter, and directional package loss.