Abstract:
A method and apparatus for monitoring traffic in telecommunications networks, in particular applied to the problem of recognizing real-time traffic such as voice traffic in packet- or cell-based networks through network traffic characterization. The method classifies data traffic in a packet-based communications network conveying different classes of data through the monitoring of data traffic so as to identify sequences of data packets of unknown class transmitted between a source address and a destination address, subsequently measuring parameters of at least a significant part of the packet sequence and deriving from the measured parameters a probable classification of the data conveyed in the packet sequence.

Description:
[0001]    The invention relates to a method and apparatus for monitoring traffic in telecommunications networks. The invention may in particular be applied to the problem of recognising real-time traffic such as voice traffic in packet- or cell-based networks through network traffic characterisation.  
           [0002]    In modern telecommunications networks there are a variety of different types of information transmitted and received, commonly classified as either real-time (RT) traffic or non-real-time (non-RT), data traffic. Voice and video are examples of ‘real-time’ traffic. Internet web pages, file transfer, remote log-in and e-mail are examples of non-real-time. Different types of network can be used to carry the different traffic types from source to destination.  
           [0003]    For example, a data network packages data in segments, labels it, and delivers it as a series of ‘packets’ to the network for transmission. The packaged data may be examined and routed at inter-mediate network stages, or nodes. In this description, the term ‘packet’ is used to include the more specific terms such as ‘packet’, ‘cell’, ‘frame’ or ‘datagram’, which are conventionally used in relation to certain of these protocols. Examples of such packet transmission systems include Asynchronous Transfer Mode (ATM), Frame Relay and Internet Protocol (IP). In contrast, when using a traditional voice network (PSTN), the caller signals to the network the requirement for connection (dialling). The network then establishes an end-to-end connection path, which can be talked over, and the connection released back to the network on hanging up.  
           [0004]    Packet based networks, although designed for the needs of data, are capable of carrying real-time traffic. Where data networks carry RT traffic, for example internet telephony and video conferencing, the RT data is encoded in packets according to the packet network protocols. However, the different transmission requirements of RT and non-RT traffic make it desirable to identify the traffic types for various reasons. For example, network characterisation and planning, so that the network can be correctly extended to carry the different traffic types. Identification of traffic types may also be desirable for other requirements, for example where voice and data are subject to different tariffs.  
           [0005]    This information is also useful when developing new networks, in setting and validating network policies and in measuring application conformance.  
           [0006]    Accordingly, there is a desire to distinguish between ‘normal’ and real-time traffic, including voice, over data networks using packet or cell based protocols. However, only the source and destination stations typically ‘know’ what type of traffic is actually being carried.  
           [0007]    The inventors have recognised that it may be possible recognise real-time traffic in a cell or package based network carrying mixed payloads of real-time and non-real-time data through traffic characterisation based on statistical measures of the unknown data traffic, even in the absence of explicit type data.  
           [0008]    The invention provides a method of classifying data traffic in a packet-based communications network conveying different classes of data, including the steps of:  
           [0009]    (a) monitoring a communications network for data traffic to identify a sequence of data packets of unknown class transmitted between a source address and a destination address;  
           [0010]    (b) measuring at least one parameter of at least a significant part of the packet sequence, said parameter being any one of:  
           [0011]    coding attributes of packets in the sequence;  
           [0012]    type of transport protocol used;  
           [0013]    type of error protection protocol used;  
           [0014]    duration of said sequence; and  
           [0015]    correlation between traffic in said sequence and traffic in a further sequence being transported from said destination address back to said source address; and  
           [0016]    (c) deriving from the measured parameter a probable classification of the data conveyed in the packet sequence.  
           [0017]    The classification may distinguish between real-time data traffic and other data in the network. The classification may in particular distinguish between voice traffic and other traffic. How much of the sequence needs to be measured for the results to be significant will clearly depend upon the purpose of the classification, the nature of the parameters being measured, and whether they are fixed or subject to change in the course of a connection. Video traffic similarly can be detected.  
           [0018]    Preferably in step (b) a plurality of different parameters are measured, while in step (c) said classification is determined by a combination of processes based on different parameters, the results of each process being combined with the others in accordance with a specific weighting.  
           [0019]    As noted above, the measured parameter may be coding attributes of packets in the sequence, type of transport protocol used or error protection protocol For example, certain types of transport protocol used and specified within the packets, and/or certain error protection protocols, may be particularly associated with real-time traffic, while others are generally used for general data traffic, without the need for actual knowledge of the data type.  
           [0020]    Where the measured parameter is the duration of said sequence, longer duration sequences (connections active for a duration of minutes, rather than a few seconds or less) in particular will indicate a high probability of real-time traffic.  
           [0021]    In the case where the measured parameter is correlation between traffic in said sequence and traffic in a further sequence being transported from said destination address back to said source address, periods of activity in one direction may be correlated with periods of inactivity in the reverse direction as an indicator of duplex voice or videoconference traffic.  
           [0022]    The measured parameters may in addition comprise timing parameters of individual packets within the sequence. Statistical analysis of such parameters can be used to determine said classification.  
           [0023]    In one embodiment, the step (b) includes measuring certain timing parameters of events in the sequence, while step (c) includes (i) deriving from the measured timing parameters one or more statistical properties of packet timing during at least a part of the sequence; and (ii) using said statistical properties to determine said classification.  
           [0024]    Said events may in particular comprise the arrival of each new packet for the sequence under investigation.  
           [0025]    The measured parameters may include the intervals between events within said sequence. In such a case, for example, uniformity of said intervals over a significant part of the sequence may be used to indicate a high probability of real-time traffic. Voice data standards typically define a maximum lag of 20 ms in the audio codec, implying that new packets will arrive at approximately 20 ms intervals.  
           [0026]    The step (c)(i) may include filtering events to restrict the events whose measured parameters are included in the derivation of said statistical properties.  
           [0027]    In the case where sequences to be classified as carrying real-time traffic are expected to include distinct periods of relative activity interspersed with periods of relative inactivity, said filtering may be performed so as to eliminate periods of inactivity from consideration in deriving said statistical properties. Such filtering can be used, for example, to deal with problems associated with periods of silence in voice traffic, when there may be an absence of detected data packets.  
           [0028]    In one embodiment, the sequence is divided for analysis into a sequence of shorter measurement periods, and said filtering is performed by reference to measurements from one measurement period at a time. For voice connections, a suitable period may be for example 100 ms.  
           [0029]    A number of strategies may be employed to limit erroneous measurements. The filtering may operate simply to omit or delete certain measurements or samples from the derivation of the statistical properties, or may modify those measurements and/or the statistical properties in some predetermined way.  
           [0030]    For example, in a first embodiment, only measurement periods preceded by periods containing significant activity are included in the statistical derivation. In an alternative embodiment, he first measurement within each measurement period is discarded to eliminate bias in the measurements made. As an alternative measure having the same effect, a separate record of the first measurement can be kept which is later removed from the population collected in the normal manner.  
           [0031]    In obtaining measurements, steps can be taken to improve measurement reliability and to reduce the processing burden involved in traffic type evaluation. The amount of raw data obtained by monitoring network traffic can be reduced by aggregating data for each measurement period, and optionally for groups of measurement periods. For example, summing the measurements and the measurements squared, while maintaining a count of the measurements summed in this way, enables a variety of statistical processes to be performed.  
           [0032]    The method may include steps to compensate for variations in the packet arrival (or other event) times caused by outside influences and to accommodate specific traffic patterns, such as when the absence of a signal can lead to a misinterpretation of the data. To alleviate errors due to such factors the inter-packet arrival time can be normalised against an expected arrival time. All internal time reference can be compared with the arriving data packets so as to measure differences between actual arrival times and predicted arrival times.  
           [0033]    The measurement may include measurements of correlation between parameters of traffic in opposite directions of a duplex connection. In particular correlation between periods of high traffic in one direction and low traffic in the other may be used as a strong indicator of voice traffic. In a telephone conversation, for example, talk in one direction is generally correlated with silence in the other.  
           [0034]    Additional measurements car be used to distinguish multiple from single voice circuits and, where a connection is carrying multiple voice circuits, to take account of the particular attributes of such a connection.  
           [0035]    In one embodiment the timestamping of the captured and re-assembled data packets can be carried out on closure of the bit stream representing a data packet and written contiguous with the packet. This can be carried out by a specialist interface card and has the advantage of producing accurate timestamps for the packets which in turns reduces potential errors.  
           [0036]    Alternatively, the process of timestamping packets can be carried out separately from the detection and assembling of data packets. Packet capture can be carried out by standard interface means and timestamped by separate means.  
           [0037]    The method according to the invention may further comprise using the determined classification automatically to control configuration of the network.  
           [0038]    The method may alternatively or yet further comprise using the determined classification to report usage of the network for accounting purposes.  
           [0039]    The invention further provides an apparatus for classifying data traffic in a packet-based communications network conveying different classes of data, comprising:  
           [0040]    (a) monitoring means for monitoring a communications network for data traffic to identify a sequence of data packets of unknown class transmitted between a source address and a destination address;  
           [0041]    (c) measuring means for measuring at least one parameter of at least a significant part of the packet sequence, said parameter being any one of:  
           [0042]    coding attributes of packets in the sequence;  
           [0043]    type of transport protocol used;  
           [0044]    type of error protection protocol used;  
           [0045]    duration of said sequence; and  
           [0046]    correlation between traffic in said sequence and traffic in a further sequence being transported from said destination address back to said source address; and  
           [0047]    (c) classification means for deriving from the measured parameter a probable classification of the data conveyed in the packet sequence.  
           [0048]    The steps of the method as set forth above can conveniently be implemented in a single digital processor chip, although of course hard-wired or “pure software” arrangements could be used instead.  
           [0049]    The above and other features, together with their respective advantages, will be apparent to the skilled reader from the description of specific embodiments which follow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0050]    Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which:  
         [0051]    [0051]FIG. 1 shows a simplified model of a mixed telecommunications and data network;  
         [0052]    [0052]FIG. 2 illustrates differences in call lengths between realtime and non real-time traffic within a mixed network;  
         [0053]    [0053]FIG. 3 illustrates differences in packet delays between real-time and non real-time traffic packets within a mixed network;  
         [0054]    [0054]FIG. 4 shows schematically a process for the recognition of real-time traffic, according to one embodiment of the invention;  
         [0055]    [0055]FIG. 5 illustrates a first measurement scheme implemented within the process of FIG. 4; and  
         [0056]    [0056]FIG. 6 illustrates the occurrence and compensation of cell delay variation within the process of FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0057]    [0057]FIG. 1 shows a simplified model of a typical cell- or packet-based telecommunications and data network  5 . Such a transport network typically comprises cell or packet switches  10  connected by bearers  15 . Each bearer is in this example an optical fibre, carrying packetised data with both routing information and data “payload” travelling in the same data stream.  
         [0058]    Various devices are attached to the network. For example, local area networks (LANs)  20  and private branch exchanges (PBXs)  25 . Over this network the payload may comprise a mixture of voice traffic and/or other data types. Different cell and packet transmission systems protocols are accommodated Examples shown are connections  35  to Frame Relay Gateway and asynchronous transfer mode (ATM) equipment, while other possibilities are so-called datagram networks and Internet Protocol (IP). Hereinafter the term ‘packet’ shall be taken to include such equivalents as ‘cell’, ‘frame’ and ‘datagram’, which are often used in relation to these different protocols.  
         [0059]    Possible monitoring sites on various bearers in the network are shown at  40  and elsewhere. A network probe  45  can be attached at such sites in order to monitor and analyse network traffic characteristics over a circuit connecting a source address and a destination address.  
         [0060]    In developing and maintaining a mixed services network it is desirable to monitor the type of traffic. This may be to allow the service provider to apply different tariffs to different traffic types, or for technical reasons to ensure adequate network performance.  
         [0061]    By processing and analysing traffic packet data gathered the inventors have recognised that it is possible to establish the probability of whether or not the traffic is real-time or data traffic. To recognise RT traffic within a network, there is proposed a process for recognising automatically the patterns that are common within RT traffic that are absent within non-RT traffic and vice-versa. RT traffic has the following generalised characteristics, any or all of which can be exploited in such a process:  
         [0062]    1. Long Duration of the ‘Call’.  
         [0063]    [0063]FIG. 2 illustrates schematically differences between RT traffic A and non-RT traffic B within a mixed network, showing RT traffic call lengths  50  and non-RT traffic call lengths  55 . Whereas a voice or video (real-time) call can be of minutes duration a data (non-real-time) call is typically less than a second, or only a very few seconds. Well known examples of data calls such as internet web connections may appear to last a long time. However, because loading a web page involves loading of many separate elements, each one of which is a TCP connection of its own, the duration of the calls is short.  
         [0064]    [0064] 2 . Non-Error Corrected Coding.  
         [0065]    Whereas most data calls use an error correcting transmitting coding, for example TCP (transmission control protocol), real-time calls use a non-error correcting coding such as User Datagram Protocol (UDP).  
         [0066]    3. Regular traffic.  
         [0067]    [0067]FIG. 3 illustrates differences in packet delays between RT traffic A and non-RT traffic B within a mixed network. Traffic packets  60  and various intervals  65 ,  70 ,  75  between successive packets are shown . The nature of RT traffic by its very nature imposes time constraints on the transmission of packets. In he case of real time traffic A the packets  60  are separated by regular intervals  65 , typically of  20 ms for voice traffic. For non-real-time traffic the intervals  70 ,  75  can be widely variable between packets. The requirements of data within a network impose few, if any, real-time constraints upon delivery as data integrity is a more important requirement. Non-RT traffic has none of the constraints that RT traffic has as far as being delivered regularly are concerned. Data transfer integrity is ensured by re-transmitting data that was lost or corrupted by the network. Such re-transmissions take extra time, and delay the delivery of later data whilst the current data is corrected. Real time data such as voice or video on the other hand requires real time delivery of subsequent data, even at the expense of non-delivery or delivery of corrupt data. In a cell or packet network, real-time traffic is coded (by a “codec”) and emitted into the network at regular intervals. This interval must be kept small to give the real-time nature of the traffic, even if it results in data units that are smaller than would be chosen for transmission efficiency reasons. For voice an inter-packet delay of  20 ms is mandated. The non-RT traffic packets may also be transmitted in grouped bursts with very small intervals between packets in a group but with comparatively large intervals between groups.  
         [0068]    4. Silence.  
         [0069]    Voice is a special form of RT data flow, in that the roles of the two end points swap many times during the connection. Whilst one party talks the other listens, and then the roles are reversed. Therefore, voice traffic can be characterised by periods of activity (high throughput) followed by periods of silence (low throughput).  
         [0070]    Although each of the characteristics listed can be interpreted as an indicator of voice traffic is being carried over a bearer, no single one can give a 100% certainty of this. In order to achieve a reasonable degree of certainty the different indicators are weighted and evaluated together, in embodiments of the present invention.  
         [0071]    Implementation  
         [0072]    A probe  45  can be connected to the network  15  at various points  40  in order to monitor a network circuit. The term “probe” is used herein to refer to a functionally self-contained system or subsystem designed to carry out the required analysis for a bearer in a network circuit. A probe unit can be implemented in different ways with differing levels of hardware and software implementation to carry out the packet capture and analysis. A probe may include separate modules to carry out such operations as capturing the data packet and then interpreting the actual packet or other data analysis.  
         [0073]    A number of issues are raised when such analysis is considered. For example, in the recognition of real-time traffic as opposed to data traffic, a related sequence of packets must first be identified and analysed. Capture and analysis of such a sequence or “flow” of packets requires a great deal of probe system resources for measurement storage. Other measurement issues must also be addressed, such as establishing accurate measurement methods to deal with variations in packet arrival times and inter-packet delays, forms of “jitter” commonly referred to herein as cell delay variation (CDV).  
         [0074]    [0074]FIG. 4 is a top level block diagram showing one example of how the process for the recognition of real-time traffic can be implemented. A network signal  80  is monitored by Media Access Controller (MAC) hardware  82 . A clock  84  provides the necessary timing requirements for the probe. The MAC module  82  captures the packets on the network circuit being monitored. A timestamping component  86  then timestamps each recognised packet on arrival and a flow recognition component  88  recognises and labels packets belonging to the same data sequence, or flow.  
         [0075]    A flow characterisation component  88  characterises the timestamped sequence. This is carried out by making a series of measurements relating to the flow that are indicative of real-time traffic. Other calculations relating to measurements taken can also be performed at this stage. A selection of results of measurements of parameters such as call duration, coding type and jitter are fed to a correlator component  92  which produces an output  94  which indicates whether the flow does, or does not, contain real-time data such as voice or video data.  
         [0076]    A ‘flow’ is defined here as a sequence of packets delivered between the same source address and destination address, constituting a network circuit, or part thereof. A flow commences with the first packet and terminates with the last packet. The termination condition, for identifying occurrence of the last packet, is when the connection times out.  
         [0077]    A typical example of such an implementation might include MAC or other suitable interface hardware, an embedded processor, and a real-time clock of appropriate resolution and stability for the network circuit under scrutiny. The MAC hardware or the processor can be used to produce a timestamp for each data packet received and these timestamps converted to flows. The processor can then characterise the flows accordingly, either as real time (for example voice) or non-real time (for example web data).  
         [0078]    The embedded processor controls the MAC and analyses the packets captured by the MAC. The processor is capable of carrying out flow recognition, protocol decoding, and traffic characterisation, using the routing information and traffic type carried in the packets. The timestamps are also used to characterise the flow.  
         [0079]    A number of methods can be implemented by the flow characterisation component  90  of the probe. The characterisation of real-time traffic can generally be carried out by monitoring flow types and flow duration between two end points. Data calls can be distinguished from ‘real-time’ calls by the protocol being used to transmit them and the duration they are transmitted for. One can determine voice or video from data traffic using indicators such as, for example, transmission error coding (TCP for data, UDP for voice) and call duration (sub-one) second for data compared with minutes for voice) Measurement methods can also be split broadly into simplex and duplex types, which shall now be discussed.  
         [0080]    Simplex Connections  
         [0081]    For cases of transmission of data in one direction only, that is simplex connections, a number of measurements are carried out in order to ascertain whether or not the flow is carrying real-time data traffic or not.  
         [0082]    Characterisation Of RT Traffic By Inter-Packet Delay  
         [0083]    It is possible to recognise real-time traffic by monitoring packet inter-arrival time (inter-packet delay) measurements of the traffic. This can distinguish real-time traffic from non-RT traffic because RT traffic, when carried over a packet network, has a source codec outputting data at a regular rate. Hence the inter-arrival time of a RT data stream will be highly correlated to a constant value. All packet inter-arrival times can be considered as a set upon which standard statistical analysis can be performed. This follows from the need to transmit packets at regular intervals. For example, for voice the mean inter-arrival time will be 20 ms and will have a very low standard deviation. More particularly in ‘dimensionless units’ the ratio of the mean, μ, to standard deviation will be very high for traffic that has a regular arrival time so that a high value of (μ/standard deviation) indicates a very high probability of RT-traffic. The converse will also hold true for non-RT traffic.  
         [0084]    Reduction of Memory Requirements in Algorithm  
         [0085]    In order to carry out the necessary measurements and calculations for flow characterisation one must process the raw data obtained from the timestamps and packet inter-arrival times, Measuring and recording the individual timestamps or inter-packet arrival time of each packet would require a great deal of measurement storage. A reduction in the amount of processing power and time needed to complete the task can be achieved by using a summation of measurements.  
         [0086]    Hence only the following are required:  
         [0087]    the sum of values of the inter-packet delay, Σd,  
         [0088]    the sum of squares of the value of inter-packet arrival delay, Σd 2 ,  
         [0089]    the number of instances of the measure which are recorded, N.  
         [0090]    This method minimises the amount of data that need be held at any one time and the amount of processing power needed to analyse the data recovered.  
         [0091]    A further advantage of these measures is scalability. It is possible to accumulate the values of multiple periods and the aggregate will deliver the required measures for the whole period. In this way the average and standard deviation values are determined:  
         [0092]    The mean i.e. average:  
         μ   =       ∑   x     N       ,                         
 
         [0093]    and hence Nμ=Σμ 
         [0094]    The variance,  
         σ   2     =       ∑       (     x   -   μ     )     2         N   -   1                             
 
         [0095]    The standard deviation,  
       SD   =       √   σ     =         ∑       (     x   -   μ     )     2         N   -   1                                 
 
         [0096]    Other derived measurements can also be deduced, such as the maximum and minimum inter-arrival time, max(d) and min(d) respectively.  
         [0097]    Processed measurements arc then passed on to the correlator component  92  for further analysis.  
         [0098]    Management of Silence  
         [0099]    Monitoring “voice” (as an example of RT traffic) requires suppression of monitoring of the arrival time of packets during periods of “silence”, that is when there is little or no information being carried in the signal. This is both a necessary modification to accommodate the one-way nature of voice, and an indication that the connection does carry voice. That is, while there is traffic from A to B, there is no traffic in the direction B to A. The algorithm must not only deal with silence, but also the presence of silences in the RT traffic stream indicates that the stream is indeed voice.  
         [0100]    Similarly video conferencing will exhibit periods of silence. The active time will be whilst one camera is broadcasting to all other parties, and silence for those parties watching where there is no significant signal activity.  
         [0101]    Unless the measurement was suppressed for those measures associated with the start of traffic (after a “silence”) considerable error would be introduced into the measurement. That is, the inter-packet delay (the “silence”) would corrupt the measurement of average and standard deviation of inter-packet delay.  
         [0102]    The suppression of measurement may be accomplished in a number of ways. FIG. 5 illustrates a measurement scheme used to manage periods of silence. A typical voice signal will include a flow  100  with packets and periods of silence  105 . Measurement buckets  110 , or measurement periods, are defined which restrict when measurements can be made. The measurement buckets may contain flows of packets, flows with lost packets  115 , or have zero entries  110 .  
         [0103]    The use of appropriate measurement buckets is used in the present example to deal with problems associated with silence. Only measurement buckets preceded by non-silence are used in calculations. The total measurement period is divided into a sequence of measurement buckets, each lasting for example 100 ms for voice. The population for the statistical measurement is restricted to those buckets which are preceded by a bucket that is also active. This eliminates the bias error of a few large inter-arrival times and eliminates delays  130 .  
         [0104]    Additionally, other constraints on the measurements are made.  
         [0105]    An upper bound of acceptable inter-arrival times is configured. Only inter-arrival times meeting that criterion are counted.  
         [0106]    Silence manifests itself not only in empty buckets (such as  120 ,  130 ) but also as a very long inter-arrival time for the first traffic packet after the silence. Hence to eliminate this bias, the first sample within a bucket (for the first packet) is not recorded. Alternatively, a separate measure of the first inter-packet arrival time can be kept which can be later removed from populations collected in the normal manner.  
         [0107]    Record And Remove Longest Sample.  
         [0108]    For measurements where there is an extended period of silence present additional manipulation of the samples may be needed.  
         [0109]    In order to accommodate extended periods of silence it is preferred to eliminate bias in the inter-packet arrival time metric. In order to mininise any bias in measurement of the average and standard deviation of the inter packet arrival time, the maximum and minimum inter-packet arrival times are eliminated from the collected statistics.  
         [0110]    The maximum and minimum inter-arrival time, max(d) and min(d), are used further to ensure that periods of silence are handled properly. Upon the termination of a bucket, because measurement activity has moved to the next bucket, values for max(d) and min(d) are removed from the measurement set. Silence will be the inter-arrival time of the first element, which will also be the largest. Hence upon closure of the bucket, the N, Σd, Σd 2  of the first and last elements are removed.  
         [0111]    Eliminating Double Counting Of Cell Delay Variation (Jitter)  
         [0112]    [0112]FIG. 6 shows examples of cell delay variation, or jitter, in a mixed network for data packets.  
         [0113]    A source codec sources data packets at regular intervals  150 . However, the network can introduce a delay  155  in these packets so that their inter-arrival times differ from what was expected, as shown at  160 . Variations in the network system can lead to lengthened cell delays  165 , and shortened cell delays  170 .  
         [0114]    For voice, the source transmits all packets with a cell delay of 20 ms, but at a monitoring point, and the receiver, the delay will be different because the network may delay each packet by a different amount.  
         [0115]    Network delay can be considered to consist of three parts:  
         [0116]    the fixed ‘speed of light’ part,  
         [0117]    the fixed serialisation delay,  
         [0118]    the variable queuing delay in the network switching elements.  
         [0119]    Given that the source is regular, one delay will cause first a lengthened packet delay  165  followed by a reduced inter packet delay  170  in the next. In the measurement scheme as described so far this would have a double effect in the measurements and statistics, although there is only one delay  155  as the cause.  
         [0120]    To eliminate this double error for one cause, the inter-packet arrival time is normalised against an expected arrival time. The probe  45  synchronises an internal time track against the arriving cells rather than measure the inter-cell/packet delay. Then the difference between actual arrival time and the predicted arrival is measured so that the jitter is measured directly.  
         [0121]    This measure of jitter eliminates possible double counting of cell delays  155 . The internal time track is synchronised by using a low-pass filtered version of the incoming cells.  
         [0122]    Estimating Cell Loss  
         [0123]    By noting the presence of a regular traffic flow, the absence of packet within a stream can be recognised as a measure of loss.  
         [0124]    The probe may not see all packets within a flow because the network can lose packets and the network can re-route packets around the probe since a packet switched network is not necessarily connection orientated end to end.  
         [0125]    However in certain networks that are connection orientated, such as Asynchronous Transfer Mode (ATM), the probe can be placed so that it sees all the traffic between two end points. An example of this would be probing the edge connection of a network. In these cases loss can be determined: if no packet arrives for time bucket N, and there is only 1 packet in each of buckets N−1, N+1 etc., then loss has occurred. These time buckets are synchronised with the traffic and so one measure per bucket is expected. This contrasts with the previous measures where buckets were only small enough to allow measurements during the periods of activity between periods of silence, for example, 20 ms in the first case, and is in the second. Two or more packet-based processes may thus be conducted in parallel, with different sized buckets for each.  
         [0126]    Packet Content  
         [0127]    No use is made of the contents of the packets, in the present example, only their routing information and their arrival times. In other embodiments, one could for example analyse their size to determine whether it was fast versus slow action video.  
         [0128]    Duplex Connections  
         [0129]    The following measures are possible where one has access to a bidirectional, or duplex flow.  
         [0130]    Silence as an Indication of Voice  
         [0131]    Noting that there are periods of silence, and that these periods of silence correlate with periods of activity in the reverse direction is an indication that there is voice communication between the two ends.  
         [0132]    Distinguishing Single Voice And Multiple Voice Circuit  
         [0133]    Given that a connection is deemed to be carrying voice using the aforementioned methods, it can be determined whether the circuit is being used for a single voice circuit, (a single session between two users) or many (e.g. a connection between 2 PABX&#39;s).  
         [0134]    Assume that a circuit is judged to be voice, because of its RT nature. If the connection does not exhibit silence periods, and if the data transfer content is larger than expected from a voice-coding scheme, then the connection is of multiple speech circuits. The corollary is that if it exhibits silence periods, and if the silence periods match active periods in the reverse direction, then the circuit is single channel voice.  
         [0135]    Correlation Function  
         [0136]    The organisation and comparison of measurements to evaluate the probability of traffic being real-time is carried out within the correlator component  92  of the probe.  
         [0137]    For each measurement type an estimator is produced and weighted according to relevance. Some measurements are stronger indicators of voice traffic than others, for example protocol versus packet activity (silence). For example, data packets carried using TCP error correcting transmitted over IP would strongly indicate data traffic. Similarly, a long duration call with packet inter-arrival times close to 20 ms would strongly indicate voice traffic. Such measures would then be weighted strongly for either voice or data. Other measurements may only be weak indicators of voice or data and would be weighted accordingly weakly.  
         [0138]    The skilled reader will appreciate that the invention is not limited to the specific implementations and applications detailed above. It can be implemented in various combinations of hardware and software, and with our without hardwired or programmable circuits. For example, various components of the probe processing described with reference to FIG. 2 could be replaced by a general purpose computer, for example a personal computer such as a notebook PC, interfaced to the network by the appropriate hardware. Another implementation is a hand-held device with the appropriate hardware connectivity and an embedded processor with software to support the other functions needed. The probe system, and particularly thc analysis and reporting functions, can be situated physically remote from the network circuit to be monitored or integrated into part of some other network analysis package.