Abstract:
A method of testing a digital mobile phone network such as a GPRS or 3G network comprises creating test traffic using an unmodified test mobile phone coupled to a computer, and using the computer to measure a parameter associated with the network&#39;s response to the test traffic. The measurements made by the computer are encoded into the test traffic to create a data stream within the mobile phone network comprising test traffic, measurements relating to the test traffic, and signalling relating to the test traffic, whereby this data stream can be captured at points within the network and analysed to investigate the functioning of the network dynamically as the network is exercised with the test traffic. Software and test equipment for performing the method are also described.

Description:
CLAIM OF PRIORITY 
   This application claims priority under 35 USC 119 from United Kingdom Application No. 0128168.2, filed Nov. 23, 2001. 
   FIELD OF THE INVENTION 
   This invention is concerned with methods, apparatus, and software for testing mobile phone networks and is particularly suitable for testing data transmission over so-called 2.5G and 3G mobile phone networks. 
   BACKGROUND OF THE INVENTION 
     FIG. 1   a  shows a generic structure of a conventional mobile phone network such as a GSM-type mobile phone network. The network comprises a plurality of radio masts  102  serving a corresponding plurality of network cells  100 . A base station (not shown in  FIG. 1   a ) comprising a plurality of rf transmitters and receivers is colocated with each mast  102  and each base station is connected to one of a plurality of base station controllers  104 . In a GSM-type network the base station is referred to as a Base Transceiver Station (BTS). The base stations and masts  102  provide two-way radio communication with mobile stations such as mobile station  116  within the cells  100 . This allows two-way transmission of voice and data traffic to and from a mobile station. 
   The radio link between a base station and a mobile station is primarily managed by a base station and its associated base station controller. Together these handle radio channel set-up, cell-to-cell hand-overs (in the USA referred to hand-offs) and other radio resource control functions. The radio link carries both traffic, such as speech and data traffic, and control information used to dynamically control transmit power, to allocate radio channels to mobile stations and for signalling functions such as paging a mobile station to alert it to an incoming call. 
   The network has a hierarchical structure in which a plurality of base station controllers  104  is connected to a Mobile services Switching Centre (MSC)  106  for routing calls between cells served by different base station controllers. The MSCs  106  are in turn connected to a gateway MSC (GMSC)  108 , which is connected to the standard Public Switched Telephone Network PSTN  114 . A home location register (HLR)  110  and Visitor Location Register (VLR)  112  manage call routing and mobile station roaming; other systems not shown in  FIG. 1   a  provide functions such as security and authentication and billing. 
   The basic structure of  FIG. 1   a  is common to all mobile phone networks whether or not they are based on GSM, but the nomenclature may differ. For example in a 3G network a Base Transceiver Station is referred to as a Node B, and a Base Station Controller is referred to as a Radio Network Controller (RNC). 
   In  FIG. 1   a  the cells  100  are shown schematically as a set of interlocking, non-overlapping coverage areas but in practice the coverage of neighbouring cells will overlap, particularly at the edges. The coverage may also have gaps where none of the local base stations provide sufficient signal for a mobile to operate adequately. Although in  FIG. 1   a  the cells have been depicted as all being roughly the same size in practice cell size varies from several kilometres diameter down to pico cells, which are mainly indoor cells, with a diameter of less than 100 m. Interference between neighbouring cells is controlled by, among other things, controlling the transmission frequency and power of the base station and by using modelling programmes to carefully site the base station antennas. 
   It will be appreciated, even from this brief discussion, that network planning and management is complex. Although modelling can be of great assistance inevitably there is a heavy reliance upon practical network testing, particularly at the early stages of network design and implementation. Once a network has been established there is a continuing need for practical mobile phone network testing, both for trouble-shooting complex problems, such as problems which might only appear in 1 in 1000 calls, and for competitive analysis, that is analysing the performance of a competitor&#39;s mobile phone network. 
   At present many mobile phone network operators test their networks by means of so-called drive testing. A mobile phone is loaded with dedicated drive testing software and connected via a serial cable to a portable computer running additional drive testing software. This is used to control the mobile to cause calls to be established in regular patterns to test network. Special instructions may be issued to the phone, for example to prevent hand-over, to find the edge of a cell, or the mobile may be instructed to make repeated calls in an attempt to reproduce a fault. During these test calls the portable computer gathers diagnostic information from the phone using the serial cable and stores this for later analysis. This diagnostic information generally includes air interface messaging sent and received by the phone in normal operation, that is during call set-up, call clear down, hand-over and the like. Typically a GPS receiver is also connected to the portable computer so that this diagnostic information can be indexed by position and subsequently mapped. 
     FIG. 1   b  shows an example of the type of map  120  which can be generated using such a drive testing procedure. Geographical information such as road  134  is overlaid with results of individual measurements, such as measurements  136 , and a desired and/or measured pattern of network cells, such as cells  122 ,  124 ,  126  and  128 . Measurements  136  may be colour coded, for example to indicate signal strength. In the map of  FIG. 1   b  region  132  indicates a hole in the network coverage where calls could be dropped. Region  130  indicates an area where overlapping coverage from two different cells operating at the same frequency could cause interference. Examples of drive testing systems are the TEMS (Test Mobile System) investigation system of Ericsson and the E-7478A GPRS drive test system of Agilent Technologies. 
   U.S. Pat. No. 6,266,514 (and related patent applications WO 00/28755 and WO 00/28756) describes a system for monitoring a cellular network without need for drive testing, by making use of data which can be collected from mobile phone users. Events such as a quality measurement dropping below a predetermined threshold are detected and the location of the mobile station at the time is then used to construct a map, thus automatically mapping areas of poor coverage. The mobile station position is determined by triangulation from at least three base stations. In a variant of the technique a GPS receiver is located in the mobile station and a mobile position report is transmitted to the base station as part of the network signalling (as a conventional IS136 RQL radio quality message), and thus does not interfere with the traffic. 
   In another system, described in WO 99/12228, a master automatically initiates calls to multiple automatic mobile responders dispersed within the coverage area of a wireless mobile phone network. This provides a real time indication of the network quality. In a preferred embodiment the responders are each equipped a GPS receiver which provides position, and optionally time and velocity information for the mobile responders. The responders are self-sufficient and may be placed in vehicles which are not dedicated to testing, such as postal or public transit vehicles. The matter is connected to a conventional fixed, land telephone line. The responders check network parameters, in particular audio quality (using 23-tone testing), and transmit the results back to the master mobile station  116  to via the mobile phone network and PSTN. The arrangement of this system simplifies testing in that the responders are essentially self-sufficient and automatic, thus facilitating the monitoring of a network performance over an extended area from a single master location. 
   The above prior art techniques seek to monitor a mobile phone network performance solely by making measurements at one or more mobile stations. Parameters relating to a user&#39;s perception of network performance, such as audio quality, the number of dropped calls and the like are measured but the detailed technical information which engineers setting up and optimising a phone network would ideally like to have access to are not available through such tests. 
   In a typical third generation CDMA mobile phone network there are some 700 parameters which may be adjusted to affect the performance of any given cell, and a further approximately 300 parameters associated with GPRS data transmission. As well as the problems of poor network coverage and interference from adjoining cells mentioned above, network operators also have complex heuristics for frequency planning and radio resource usage, to attempt to maximise traffic and/or revenue. These considerations are further complicated by variations in traffic load with time of day and other factors. 
   By only measuring at a mobile station the above described prior art techniques are not able to access details of the network functionality and in particular they are not able to determine the response of the network to an individual call. 
   The mobile station and network function in some respects as a single complex entity, affected by other mobile stations connected to the network and other traffic carried by the network. It is therefore desirable to be able to monitor the interaction of a mobile station with a network and to investigate how the network responds to attempts by the mobile station to drive traffic through the network in the context of other traffic being carried by the network. It is further desirable to be able to monitor such interactions dynamically since traffic on a digital mobile phone network is managed dynamically on a timescale of a few milliseconds. 
   GSM-type digital mobile phone networks include an Operation and Maintenance Centre (OMC) which collects statistics from network infrastructure elements such as base stations and switches and compiles these into a database. This provides network operators with a high level view of the network performance which can complement the data obtained by drive testing. Thus, for example, the OMC typically includes counters for every dropped call split out by cell, and time. Several companies, for example ADC Telecommunications of Minneapolis, USA provide systems for analysis of this OMC data. However because the OMC data is aggregated into statistics it cannot provide information relating to an individual mobile station. Data of this type such as the number of protocol errors of an individual mobile station, is only available at a lower level within the network. 
   In addition to OMC data, call trace and cell trace data is also sometimes available. This data essentially comprises a diagnostics log containing messaging, including air-interface messaging, relating to a single call or cell. These logs are produced by the base station controllers of some of vendor&#39;s equipment, and can be helpful in tracking down specific problems with a user or a type of handset. 
   A third source of data relating to the operation of a mobile phone network infrastructure is provided by protocol analysers. A protocol analyser comprises equipment to tap a link or interface between infrastructure elements (either logical or physical. Broadly speaking a protocol analyser simply records all the data flowing on such a link or across such an interface as “trace file”. Such trace files can contain all the messaging between the two elements connected by the link being tapped, for example all the messaging between a base station controller and a switch. Protocol analysers are available from companies such as Tektronics, Agilent and Edixia of Telecom Technologies, France, Europe. One model “Océan” available from Edixia captures data on 300 E1 (2 Mbps) connections and provides this data over an SDH (Synchronous Digital Hierarchy) link, to allow it to be transferred over a high band width optical network to a data store. 
   Referring now to  FIG. 2 , this shows a generic structure  200  for a digital mobile phone network, showing the type of prior art tests which can be carried out. 
   A mobile station  202  is connected to a base station  204 , serving the cell in which the mobile station is located, across an air interface Uu  216 . The base station  204  is coupled to a base station controller  206  across interface Iub  218 . The base station controller  206  is connected to a voice switch  208  via interface Iuc  220 , and thence to a voice phone network  210 . These elements correspond to the network elements shown in  FIG. 1   a . Successively higher nodes concentrate the traffic and omit unnecessary operational messaging, and functionality is generally delegated so that, for example hand-overs between base stations coupled to the same BSC are not seen by higher levels. 
   Mobile cellular communications systems such as GPRS (General Packet Radio Service) and 3G systems add packet data services to the circuit switched voice services. Thus base station controller  206  is also coupled to a packet switch  212  via Iup interface  222 , and thence to a packet data network such as the Internet  214 . 
   In  FIG. 2  mobile station is shown connected to a laptop computer  224 . This in turn is coupled to a GPS receiver  226  to allow user level drive testing, as shown schematically by box  228 . Subscriber level protocol tracing  230  can be performed by capturing data from Iub interface  218  and area level protocol tracing can be performed by capturing data from interface Iu  220 , interface Iup  222 , and/or interfaces (not shown) within voice switch  208  or packet switch  212 . At a higher level Call Detail Records (CDRs) and SS 7  (Signalling System No. 7, an international standard used for the ISDN backbone) data  234  may also be collected from voice switch  208  or packet switch  212  for analysis. Vendor specific OMC data  236  provides network statistics as described above. Broadly speaking data collected from elements to the right of dashed line  238  is useful for diagnostic purposes whilst data collected from elements to the left of line  238  provides statistical information on how the network is performing but does not generally allow the reasons for a particular level of performance to be discerned. 
     FIG. 2  is representative of a range of digital mobile phone networks including so-called 2.5G networks such as GSM/GPRS and third generation mobile communications networks as encompassed by the International Mobile Telecommunications IMT-2000 standard (available from the International Telecommunications Union, ITU at www.itu.int and hereby incorporated by reference). 
   Unlike GSM, third generation technology users CDMA (Code Division Multiple Access) rather than TDMA (Time Division Multiple Access) and the IMT-2000 standard encompasses three modes of operation, WCDMA (wideband CDMA) Direct Spread FDD (Frequency Division Duplex) in Europe and Japan, CDMA2000 multicarrier FDD for the USA and TD-CDMA (Time Division Duplex CDMA) for China. 
   Collectively the radio access portion of a 3G network (RNCs and node Bs) is referred to as UTRAN (Universal Terrestrial Radio Access Network) and a network comprising UTRAN access networks is known as a UMTS (Universal Mobile Telecommunications System) network. 
   The UMTS telecommunications systems are the subject of standards produced by the 3 rd  Generation Partnership Project (3GPP), including Technical Specifications 23.101, 25.410, 25.420, 25.430 and 25.931, which are hereby incorporated by reference. The GSM standard and aspects of GPRS are defined in ETSI (European Technical Standard Institute) standards GSM 01 to 12, which are hereby incorporated by reference; details of the GPRS radio interface are described in particular in GSM 03.64 and GSM 04.60. Further aspects of the GPRS service are described in 3GPP Technical Specification 23.060 (version 4.1.0), which is also hereby incorporated by reference, and associated quality of service concepts are defined in 3G TS 23.107 (version 3.0.0) again hereby incorporated by reference. 
   In  FIG. 2  the interfaces shown have different names depending upon the precise form of mobile phone network. The names of the interfaces in the different networks are shown in the following table. 
   
     
       
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               Type of 
               Interface 
                 
             
           
        
         
             
                 
               Network 
               Iup 222 
               Iuc 220 
               Iub 218 
               Uu 216 
             
             
                 
                 
             
             
                 
               GSM/GPRS 
               Gb 
               A 
               Abis 
               Um 
             
             
                 
               cdma2000 
               Iup 
               Iuc 
               Aquater 
               Uu 
             
             
                 
               WCDMA 
               Iup 
               Iuc 
               Iub 
               Uu 
             
             
                 
                 
             
           
        
       
     
   
   Referring now to  FIG. 3   a , this shows details of the base station controller  206  and packet switch  212  in a GSM (and/or UMTS) network supporting GPRS functionality. 
   The elements labelled in  FIG. 2  as base station controller  206  and base station  204  make up a Base Station System (BSS) comprising a BSC (or RNC)  300  coupled via an Abis interface  308  to base station (or Node B)  204 . The BSC (RNC)  300  is also coupled via an A interface  312  to voice switch  208 , and via a PCU.A disk interface  310  to a packet control unit (PCU)  302 . The PCU  302  is in turn connected, via a Gb interface  314  to a Serving GPRS Support Node (SGSN)  304 . 
   A plurality of SGSNs  304  are connected via a Gn interface  316  to a Gateway GPRS Support Node (GGSN)  306 , which in turn is connected via a Gi interface  318  to Internet  214 . The SGSNs  304  and GGSN  306  are connected together by means of an IP-based packet switched network, and together make up part of what is referred to as packet switch  212  in FIG.  2 . The SGNS and GGSN functionalities (and the functionalities of other elements within the network) may reside on a single physical node or on separate physical nodes of the system. In a GPRS network security and access control functions and tracking the location of an individual mobile station are also performed by the SGSNs. 
     FIG. 3   b  shows user end equipment  320  for use with the mobile phone networks of  FIGS. 2 and 3   a . This equipment comprises a mobile station or handset  322 , in the context of data services sometimes referred to as a mobile terminal (MT), incorporating a SIM (Subscriber Identity Module) card  324 . Handset  322  is coupled to a personal computer  326 , sometimes referred to as Terminal Equipment (TE), by means of a serial connection  328 . 
   Once a handset has attached to a GPRS network it is effectively “always on” and user data can be transferred transparently or non transparently between the handset and an external data network. Personal computer  326  communicates with handset  322  using standard AT commands as defined, for example, in 3GPP Technical Specification 27.007, hereby incorporated by reference. Handset  322  may require a terminal adaptor, such as a GSM datacard (not shown) in  FIG. 3   b.    
   User data is transferred transparently between handset  322  and an external IP network such as Internet  214  by means encapsulation and tunnelling. Where a reliable data link is not required, UDP (User Datagram Protocol, as defined in RFC 768) may be used instead of tunnelling. A packet Temporary Mobile Subscriber Identity (Packet TMSI) is allocated to each GPRS-attached handset and a packet domain subscriber identified by an International Mobile Subscriber Identity (IMSI) is allocated a Packet Data Protocol (PDP) address, which is an IP address specifying a GGSN node to access, so that data from a mobile subscriber can be “tunnelled” to the handset&#39;s point of attachment to the network. Radio interface resources are shared dynamically between speech and data as a function of service load and operator preference, as described in more detail below. The GPRS specification separates the radio sub-system from the rest of the network to allow the radio access technology to be changed or upgraded. 
   It will be appreciated that in a network of the general type shown in  FIG. 2 , as well voice and/or data traffic each interface carries signalling comprising control messages for managing the network. These messages fall into one of three broad categories, call control signalling, mobility management signalling, and radio resource signalling, although the information available from this signalling depends upon the interface concerned. For example at Abis interface  308  and PCU Abis interface  310  information is available relating to the relative allocation of radio resources to voice and data traffic, but such detailed radio resource signalling is not available at higher levels within the network. 
   Referring now to  FIG. 4   a , this shows a system  400  for capturing and analysing data from Abis interface  308  using a protocol analyser. The same principles apply to capturing data at other interfaces within the network. 
   A plurality of E1 (2.048 Mbps) or T1 (1.544 Mbps) connections  402 - 410  connect base station controller  300  to base station  204 , one E1/T1 data feed being allocated to each base station transceiver. Each of these data feeds is coupled to protocol analyser  414 , which writes the captured data into a plurality of data files  416   a-c  at, for example 15 minute, intervals. These data files may be physically located within the protocol analyser  414  or at some separate, remote location. In a subsequent step a data analyser  418  reads the data in data files  416  and analyses the data for diagnostic purposes. In variants of the technique the captured data is made available on a computer network rather than written to files. 
   Each 2 Mbps E 1  data feed comprises 32 time domain multiplexed 64 Kbps PCM channels, each PCM channel comprising two logical traffic channels and two logical control channels, one each way per call. Data from an E 1  data feed is captured and streamed into the data file by protocol analyser  414 . Data analyser  418  then implements the appropriate protocol stack for the interface from which the data has been captured in order to convert the data to a useful form. Data analyser  418  may be configured to associate traffic data with signalling data for a single voice call so that the progress of the call, cell-to-cell hand-overs and the like can be monitored. At a low level such as Abis interface  308  information such as RF signal level measurement reports are available whereas data collected at a higher level interface such as the Iup or Gb interface  314  omits such detailed operational signalling. Likewise at the Iub/Abis interface data is available for all subscribers attached to base station  204  but at higher level interfaces such as Iup/Gb data for a larger number of subscribers is available. 
   Actix Limited of London, UK has a commercial product, CallTracker (trade mark) which can be used to analyse data collected by a protocol analyser in this way. The Actix CallTracker works by monitoring Abis messages from multiple transceivers belonging to a number of cells. Each transceiver can handle multiple simultaneous calls. In order to track every call, the CallTracker analyses all the Abis messages for call initiation sequences, that is call set-ups, and remembers the timeslot information assigned to each new call. If a timeslot assigned to a call is to be changed, for example, at a handover, then the CallTracker interprets the contents of the resulting Abis command sequence in the existing timeslot and uses them to link to the new timeslot that matches the signalling information. The CallTracker then tracks the sequence of messages from the old timeslot and the new timeslot to determine whether the handover is successful or not and updates its internal records accordingly. This process repeats until the call terminates, and in this way the CallTracker is able to determine the timeslot information for each call and hence is able to associate each Abis message with a particular call. 
   By using a GPS receiver coupled to a personal computer a set of timed position measurements for a mobile station can be logged in a data file. The CallTracker software is able to combine this position information with data from the protocol analyser in order to generate a map showing the position dependence of call-related parameters. However there remains a need for still more detailed information, in particular relating to the dynamic behaviour of the network, especially where the transmission of packet switched data is concerned. This can be illustrated by considering the allocation of radio resources in a GSM-based GPRS packet data transmission network. 
   In a GSM-type network  124  carrier frequencies are used each carrying TDMA (Time Divsion Multiple Access) data. This TDMA data is arranged in burst periods of 156.25 bits each lasting 0.577 ms, eight burst periods constituting a TDMA frame (which lasts approximately 4.6 ms). One physical channel comprises one burst period per TDMA frame, so that each frame carries 8 channels. Some channels are used to carry traffic, such as voice and data traffic, and other channels are used for signalling or control messages, such as the broadcast control channel, the paging channel used to alert the mobile station to an incoming call, and other channels. A group of 26 frames defines a multiframe in which 24 frames are allocated to traffic channels, one frame is allocated to signalling, and one frame is unused. 
     FIG. 4   b  shows four TDMA frames  422 ,  424 ,  426  and  428  at successive time intervals, each frame comprising 8 timeslots a to h, during each of which data can either be transmitted or received. In  FIG. 4   b  a time slot occupied by data traffic is indicated by “D”, a time slot occupied by speech traffic is indicated by “X”, and an unused time slot is left blank. In GSM-GPRS rules govern which slots may be occupied by data, these rules specifying that for a given upstream/downstream transmit/receive data stream the slots must be contiguous and must not cross the middle point of a frame. Speech data is given priority and when a call initially set up speech may occupy any time slot, although the network may then contrive to shuffle the time slot occupied by a speech connection to create a desired number of data timeslots. Typically a greater data bandwidth is needed for downstream data than for upstream data. In the example shown in  FIG. 4   b  initially in frame  422  three timeslots,  422   a-c  are occupied by downstream data and one timeslot,  422   f  is occupied by upstream data. Then, in frame  424 , two voice calls occupy timeslots  424   a  and  424   c  thus leaving only one downstream timeslot for data, timeslot  424   b . In frame  426  the call previously occupying timeslot  424   a  has been moved to  426   h , freeing up two contiguous timeslots for downstream data. Finally in frame  428  the speech channel occupying timeslot  426   c  has been reallocated to timeslot  428   d  to once again achieve three contiguous timeslots available for downstream data, timeslots  428   a-c.    
   It will be appreciated that to properly understand the behaviour of a network it is desirable to be able to monitor how individual timeslots are being allocated to data channels, in response to both the demands made by an individual mobile station and the load imposed on the network by other traffic in the same or nearby cells. A packet data transmission has an associated quality of service (QoS) profile which is negotiated with the network in accordance with the available GPRS resources. The network always attempts to provide adequate resources to support the negotiated quality of service and the data transmission radio priority is determined based upon this. The quality of service is also classified depending upon whether the traffic is delay sensitive (for example video) or relatively delay insensitive (for example, web browsing). Generally quality requirements such as delay and reliability only apply to incoming traffic up to a guaranteed bit rate. 
   It will be understood from the foregoing discussion that an important problem arising in the context of 2.5G and 3G mobile phone networks is presented by the need to be able to analyse the dynamic behaviour of the phone network as the network is being exercised. In a circuit switched network, once a circuit has been established it is relatively straightforward to test the characteristics of the channel by making measurements at one end, such as the measurements described above with reference to drive testing. Likewise in a circuit switched network data parameters such as round trip delay are meaningful and useful information regarding data throughput and delay can be obtained simply by making measurements at the mobile station end. 
   By contrast in a packet switched network different packets may take different routes to their destination, and may be delayed or even lost entirely, depending upon other traffic within the network. Moreover, because the circuit switched voice connections take priority at the radio interface, and because radio channels may be occupied or become free according to whether or when other users in the cell place voice calls, meaningful data about the network performance must take into account not only what is taking place at the mobile station end, but must also take account of what is taking place at other points within the network. In a further complication the priority given to data traffic depends upon the negotiated quality of service and upon the class of traffic being sent or received. There are additional complicating factors imposed by the network operators, such as data rate limitations placed on users trying to send or receive large volumes of data (to stop low rate traffic being denied access) which means that packets are not necessarily allocated slots on a random basis. 
   There is therefore a need for improved systems and methods for testing and monitoring digital mobile phone networks, and in particular 2.5G and 3G networks configured for the transmission of packet data. 
   SUMMARY OF THE INVENTION 
   According to the present invention there is therefore provided a method of testing a digital mobile phone network, the network comprising, a communications network infrastructure, the infrastructure having a plurality of elements, including a plurality of radio communications base stations, and having interfaces between said elements; and a plurality of mobile communications devices for radio communications with said base stations, communications between a said mobile communications devices and said base stations, and signals on interfaces within the network infrastructure, comprising traffic and signalling data; the method comprising, creating test traffic between a test one of said mobile communications devices and said communications network infrastructure, measuring at least one parameter associated with said traffic to provide measurement data, coding said measurement data representing said measured parameter into said test traffic, demultiplexing traffic and associated signalling data for said test mobile communications device from traffic and signalling data for others of said mobile communications devices on a test interface selected from said infrastructure element interfaces, decoding said measurement data from said demultiplexed traffic for said test mobile communications device; and combining said decoded measurement data and said demultiplexed signalling data for said test mobile communications device to determine a response of said digital mobile phone network to said test traffic. 
   By driving test traffic over the network and then measuring the performance of the network in response to the created traffics and inserting this measurement data into the traffic stream itself, a traffic and signalling thread is created within the network which can be tapped to retrieve not only the signalling information for a given test voice or data call but also information about the performance of the network in response to the test traffic as seen from the mobile subscriber end. The type of test and/or parameters characterising the test traffic driven onto the network can also be determined because the type of test being run will be known or because in embodiments of the method, parameter characterising the test are encoded within the test traffic. 
   The method may involve pulling or pushing test voice or data traffic to or from the test mobile communications device. Since the test device is creating the traffic, when the traffic and signalling data is monitored within the network subscriber-end measured parameters can be linked with network behaviour. Thus the effect of the allocation of radio resources as illustrated in  FIG. 4   b  or, at a higher level, the loss of packets due to a buffer overflow, can be seen. 
   Important parameters which can be measured for data traffic include data throughput and data delay. Although round trip delay can be measured from a mobile station with a “ping” test this is not a reliable indicator of one-way throughput or delay, and is also unrepresentative of typical traffic because of the very small data payload such a function requires. By contrast the present method allows a realistic test of, for example, web page download, video streaming or an FTP session or, at a lower level, TCP or UDP data transmission protocols. 
   The method allows information about the data being used to exercise the network and about the bit rate and time delay as seen from the subscriber end to be collected from traffic within the network, concurrently with network control signals. The method allows this information to be collected from any point within the network at which traffic and signalling information relating to the test mobile communications device is available. At a low level such as the Abis allocation of radio resources can be tied to data throughput and delay whilst at a higher level such as the Gb interface other parameters such as bandwidth allocation can be monitored. 
   The method can be used with either voice or data test traffic but is particularly useful for testing the response of the network to a demand for packet data transmission. This is because with a circuit switched voice call a radio time slot is always allocated to the circuit whereas with a data call the allocation of time slots depends upon demand. 
   With a 3G CDMA network users other than the test device effectively constitute interference to communications of the mobile test device with the network. Rather than monitoring the allocation of timeslots the radio performance and characteristics of a cell may be determined using the method, but the same general principles apply. In a CDMA system users other than the test mobile communications device effectively add background noise and CDMA systems therefore attempt to limit the number of users of a given cell to maintain signal quality. 
   The signal quality thresholds for voice and data will in general be different and the tolerable signal to noise ratio generally depends upon the class of data traffic and the use to which the data is being put. For example when downloading a large file, although a relatively slow throughput and long latency may be tolerable as compared with, for example, video streaming, a packet failure rate of 1 in 100 may be adequate but a packet failure rate of 1 in 2 with retry-on-failure may prevent a large file from ever being downloaded. The present method allows a 3G network to be tested by, for example, downloading a large file and then allows the systems performance to be monitored at different levels within the network. In particular it allows the dynamic performance of the network, as measured at the test mobile communications device, to be associated with the signalling to control the test traffic taking place at the same time. All these can be done within different parts of the network. 
   It will be appreciated that the test interface may comprise any interface within the digital mobile phone network at which traffic and associated signalling data is available, and is not restricted to an interface defined by a formal specification for the network. It will likewise be appreciated that the test interface may be either a logical or a physical interface. The signalling data may comprise call control, mobility management, or radio resource signalling data or other network signalling or control data. 
   In a preferred embodiment the at least one measured parameter is a parameter of traffic received at the test mobile communications device in response to test traffic transmitted from that device, although in other embodiments one or more parameters of test traffic sent from another mobile communications device may be measured. Preferably the measurement is performed on traffic received from the test mobile communications device by, for example, a terminal connected to the device, rather than on raw data extracted at a low level from within the mobile communications device. 
   The measurement data may comprise data characterising traffic rate, traffic signal to noise ratio or bit error ratio, and traffic delay or latency; other data such as position data (derived, for example, from a GPS receiver) may also be inserted into the traffic. Preferably the demultiplexing extracts a traffic and signalling thread for the test mobile communications device from other traffic and signalling data within the network, although in other embodiments the traffic and signalling data for the test device may simply be labelled to allow it to be identified for subsequent decoding. Similarly the decoding, in the case of data test traffic, may also simply comprise labelling the relevant data items. Once demultiplexed and decoded the measurement data and signalling data are combined or associated in such a way that the data can be read together, to assist in interpreting the behaviour of the network in response to the test traffic. Thus, again, the combining may simply comprise labelling corresponding data items to allow decoded measurement data and corresponding demultiplexed signalling data for the test device to be identified. 
   The method may be used with any digital mobile phone network including, but not limited to, a GSM/GPRS mobile phone network (and related networks such as PCS-1800 and PCS-1900 networks), cdmaOne and cdma2000 networks, IS136 (digital amps), iDEN (integrated Digital Enhanced Network) and TETRA, and WCDMA 3G networks. 
   The test traffic may be created by either pulling or pushing data to or from the test device, for example by requesting data from an external packet data network address or from another mobile communications device. A transparent data transmission protocol such as TCP (Transmission Control Protocol) or a non-transparent protocol such as UDP (User Datagram Protocol) may be employed. Where data communications are “always on” establishing a data communications session (in GPRS, a Packet Data Protocol Context Request) may comprise requesting or negotiating a desired quality of service. Measurements may then include quality of service-related parameters such as latency—the time it takes for a packet to cross a network connection from a sender to a receiver. The measurements may comprise, for example, time measurements, data throughput (for example, maximum, minimum or average bit rate) and bit error ratio (residual or other BER). Measurements may also be made on packets or service data units (SDUs) to determine, for example, SDU loss probability, SDU error probability, SDU transfer delay, and SDU transfer rate. In the case of a voice mode connection audio parameters such as signal to noise ratio may be measured and the measurements encoded as, for example, audio tones. 
   The method may further comprise inserting test characterising data representative of the type of test traffic into the test traffic, and then decoding this test characterising data and combining the decoded data with the decoded measurement data and the demultiplexed signalling data. The test characterising data may comprise one or more of traffic type data (voice or data), traffic class such as conversational (where time relation preservation and a low delivery delay time are important), streaming (for example video streaming, where the time relationship or sequence of packets is important but the delivery time is less important), interactive (for example web browsing, where the request-response pattern and data content are important but not the time relation between individual packets) and background (for example e-mail download, where timing is of little importance but the data content must be preserved). Other test characterising data may comprise, for example file size data, the-address of the IP network with which communication is taking place, and time of day, week or month data. More precise time data, for example derived from the network itself or from a GPS receiver, may be included with the measurement data and may include timing data with a precision of better than 100 ms or in some circumstances better than 1 ms. 
   In a preferred embodiment the test mobile communications device comprises an unmodified consumer mobile communications device. This allows the network to be tested and its performance evaluated by simulating a subscriber to the network and by measuring the response of the network to the test traffic under realistic conditions simulating actual use of network. 
   Preferably the traffic and signalling demultiplexing comprises recording substantially all the traffic and signalling data at a point within the network and then demultiplexing or dethreading this recorded data to extract a single thread comprising traffic data and associated signalling data for the test mobile communications device. Preferably this demultiplexed traffic and signalling data is decoded according to a protocol stack associated with the point at which the information was collected, that is, the relevant protocol stack is implemented in reverse to decode the captured and demultiplexed traffic and signalling data. For this reason the method is preferably applied (but need not exclusively be applied) at an interface or reference point at which signals within the network are defined according to an agreed standard for the network. 
   Preferably the decoded measurement data and demultiplexed signalling data are stored in a time series database so that the measurement data and corresponding signalling data are retrievable as a time series for ease of interpretation and graphical presentation. Where the test traffic comprises packet data traffic, in a preferred embodiment the method further comprises outputting a graphical representation of the combined data to provide representation of radio interface resources and the at least one measured parameter over time. Preferably the radio interface resources are graphically displayed, for example as a bar chart, to show what fraction of the resources are devoted to data traffic and what fraction are devoted to voice traffic. In other embodiments, however, the graphical representation may simply indicate a variation in the radio resources allocated to data over time. A particularly preferred embodiment displays data throughput and/or delay parameters over time in association with the radio resources to allow a side by side comparison of network function and a measure of perceived subscriber quality of service. 
   In another aspect the invention provides mobile communications equipment for connecting to a mobile communications device for testing a digital mobile phone network, said mobile communications equipment comprising a mobile communications device driver having a traffic input for driving traffic to said mobile communications device and a traffic output for outputting a traffic received from said mobile communications device; a test traffic supply to supply test traffic; a traffic parameter measurer configured to receive an input from said device driver traffic output and to provide traffic parameter measurement data representing a measured traffic parameter; and a combiner configured to combine said test traffic from said test traffic supply and measurement data from said traffic parameter measurer and to provide a combined traffic output to said traffic input of said device driver; whereby, in operation, the equipment outputs traffic data comprising a combination of test traffic for testing said digital mobile phone network and traffic parameter measurement data to said mobile communications device, said traffic parameter measurement data representing a measured parameter of traffic received from said digital mobile phone network via said mobile communications device as a response to said test traffic. 
   The mobile communications equipment can be used with the above described method for testing a digital mobile phone network and provides corresponding advantages and benefits. In a preferred embodiment the mobile communications equipment comprises a suitably programmed general purpose computer having a serial or other port for connecting the computer to a test mobile communications device. Thus the mobile communications device driver, test traffic supply, traffic parameter measurer, and combiner preferably each comprise a portion of a programme code for controlling the computer to provide the required function. Similarly the traffic input and output of elements such as the device driver may comprise, for example, registers or memory areas. 
   In a preferred embodiment the combiner interleaves the measurement data with the test traffic to form a single output traffic stream. The test traffic may be generated randomly or loaded from a data store, and is preferably packetised for presentation to the device driver. The equipment is preferably controlled by a test sequence controller, in a preferred embodiment comprising a state machine. Where voice rather than data test traffic is employed test samples may be stored as, for example, “wave” files and the measurement encoded as (digitised) audio tones. 
   Preferably the device driver is suitable for driving an unmodified consumer mobile communications device. In this way the equipment can be arranged to simulate a subscriber to the digital mobile phone network, preferably using realistic test traffic and the performance of a network under these conditions can be evaluated. 
   The invention also provides computer readable programme code to, when running, implement the functions of the above described mobile communications equipment. 
   The invention also provides a carrier medium carrying computer readable code for controlling a computer coupled to a mobile communications device to test a digital mobile phone network, the code comprising computer code for, a mobile communications device driver having a traffic input for driving traffic to said mobile communications device and a traffic output for outputting a traffic received from said mobile communications device, a test traffic supply to supply test traffic, a traffic parameter measurer configured to receive an input from said device driver traffic output and to provide traffic parameter measurement data representing a measured traffic parameter and a combiner configured to combine said test traffic from said test traffic supply and measurement data from said traffic parameter measurer and to provide a combined traffic output to said traffic input of said device driver, whereby, in operation, the computer outputs traffic data comprising a combination of test traffic for testing said digital mobile phone network and traffic parameter measurement data to said mobile communications device, said traffic parameter measurement data representing a measured parameter of traffic received from said digital mobile phone network via said mobile communication device; as a response to said test traffic. 
   The carrier medium may comprise a data carrier or storage medium such as a hard or floppy disk, ROM or CD-ROM, or an optical or electrical signal carrier. 
   In another aspect the invention provides a method of using a mobile communications device to facilitate testing of a digital mobile phone network, the method comprising, controlling said mobile communications device to send test traffic over said digital mobile phone network, receiving traffic from said digital mobile phone network using said mobile communications device, measuring at least one parameter associated with said received traffic to provide traffic parameter measurement data; and inserting said traffic parameter measurement data into said test traffic, to thereby facilitate testing of said digital mobile phone network. 
   Preferably the method further comprises providing stored or random test traffic data from a test traffic data supply, coding this test traffic for transmission, coding the measurement data and interleaving the coded test traffic and measurement data, and providing this interleaved data to the mobile communications device driver. 
   The invention also provides computer readable programme code to, when running, implement this method. In a related aspect the invention provides test equipment for testing a digital mobile phone network, the network comprising, a communications network infrastructure, the infrastructure having a plurality of elements, including a plurality of radio communications base stations, and having interfaces between said elements; and a plurality of mobile communications devices for radio communications with said base stations, communications between a said mobile communications devices and said base stations, and signals on interfaces within the network infrastructure, comprising traffic and signalling data; the test equipment comprising, an input to receive data collected at a test one of said interfaces, said received data comprising traffic and signalling data for mobile communications devices using said network, a demultiplexer to identify test traffic and associated signalling data for a test one of said mobile communications devices from said received data, a decoder to identify measurement data representing at least one measured parameter associated with said test traffic embedded in said test traffic, and a data store to store at least said test traffic signalling data in association with said measurement data in such a way that time series measurement data and corresponding signalling data are retrievable from the data store. 
   This test equipment can be used for implementing the above described method of testing a digital mobile phone network, and provides corresponding advantage and benefits. 
   Preferably data is collected from the test interface using a protocol analyser or similar equipment and is stored in a data store. The test equipment input may then comprise a computer network connection to the data store, although in other embodiments data may be transferred from the protocol analyser data store to the test equipment on disk. 
   In a preferred embodiment the test equipment comprises a suitably programmed general purpose computer and the demultiplexer, decoder and data store comprise portions of programme code. Preferably the demultiplexer and decoder separate or extract the desired data from the remainder of the data, but in other embodiments the data of interest may simply be suitably labelled. The data store preferably comprises a time series data base, and preferably the equipment includes an output device to output a graphical representation of time series measurement data and corresponding signalling data. 
   The invention also provides computer readable programme code to, when running, implement the functions of the above described test equipment. 
   The invention further provides a carrier medium carrying computer readable code for controlling a computer to test a digital mobile phone network, the network comprising, a communications network infrastructure, the infrastructure having a plurality of elements, including a plurality of radio communications base stations, and having interfaces between said elements, and a plurality of mobile communications devices for radio communications with said base stations, communications between a said mobile communications devices and said base stations, and signals on interfaces within the network infrastructure, comprising traffic and signalling data, the code comprising computer code for providing, an input to receive data collected at a test one of said interfaces, said received data comprising traffic and signalling data for mobile communications devices using said network, a demultiplexer to identify test traffic and associated signalling data for a test one of said mobile communications devices from said received data, a decoder to identify measurement data representing at least one measured parameter associated with said test traffic embedded in said test traffic; and a data store to store at least said test traffic signalling data in association with said measurement data in such a way that time series measurement data and corresponding signalling data are retrievable from the data store. 
   The carrier medium may comprise any conventional data carrier or storage medium or an optical or electrical signal carrier. 
   In a corresponding aspect the invention provides a method of processing data from a digital mobile phone network to facilitate testing of the network, the network comprising, a communications network infrastructure, the infrastructure having a plurality of elements, including a plurality of radio communications base stations, and having interfaces between said elements; and a plurality of mobile communications devices for radio communications with said base stations, communications between a said mobile communications devices and said base stations, and signals on interfaces within the network infrastructure, comprising traffic and signalling data, the method comprising, inputting data from a test one of said interfaces, said inputted data comprising traffic and signalling data for mobile communications devices of said plurality of mobile communications devices, demultiplexing said inputted data to identify test traffic and associated signalling data for a test one of said mobile communications devices, identifying measurement data representing at least one measured parameter associated with said test traffic embedded in said test traffic; and storing said test traffic signalling data in association with said measurement data so as to be able to retrieve a time series of measurement data and the corresponding test traffic signalling data. 
   The invention also provides a system for testing a digital mobile phone network comprising a combination of the mobile communications equipment for connecting to a mobile communications device for testing a digital mobile phone network and the above described test equipment for testing a digital mobile phone network. 
   In a further aspect the invention provides a carrier medium carrying computer readable code for testing the performance of a mobile communications system as perceived by a subscriber to the system using a subscriber mobile communications device, the computer readable code comprising code for running on a computer system coupled to said subscriber mobile communications device, said code being for controlling the computer system to, send traffic over said mobile communications system using said subscriber mobile communications device, said traffic including test traffic and coded information; and to code said coded information to allow said information to be identified within said traffic and extracted from said traffic; and wherein said information comprises information relating to a test activity performed by said computer system. 
   In another aspect the invention provides a carrier medium carrying computer readable code for analysing the performance of a mobile packet data communications system, the mobile packet data communications system including a plurality of base stations for radio communication with a plurality of mobile communications system devices, the system being logically divided into portions linked at interfaces at which measurements may be made; the computer readable code comprising code to, when running, analyse data captured at a said interface, said code being configured to control a computer system to, read data captured at a said interface, extract traffic data and associated mobile communications system operation information for one of said communications devices from said read data, decode coded information from said traffic data, and output a linked combination of said decoded information and said mobile communications system operation information associated with said traffic from which said information was decoded. 
   The decoded information and system operation may be linked by time, for example entries being made in a database at regular intervals, each entry having a defined time, or in some other way, for example as linked lists. The functional requirement is that the decoded information can be linked to the system operation information associated with or corresponding to the traffic from which the information was decoded. This permits the two sets of data to be matched for example at times which correspond to within a predetermined degree of accuracy. 
   Preferably the code is further configured to control the computer system to provide a graphical representation of the decoded information and the mobile communications system operation information associated with the traffic from which the information was decoded. 
   In a further aspect the invention provides a method [as claim 59]. Preferably the method includes outputting a graphic representation of both the decoded information and the system operation information associated with the traffic from which the information was decoded. 
   The invention may be embodied in computer programme code provided to a computer by a conventional carrier medium or by a communications network such as the Internet. As is well know to those skilled in the art, such programme code may be implemented on a single computer or in a client-server system on one or more computers. The computer programme code may be implemented as a single programme or as a number of separate applications. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the present invention will now be further described, by way of example only, with reference to the accompanying figures in which: 
       FIGS. 1   a  and  1   b  show, respectively, a generic structure of a conventional mobile phone network, and an example of a map generated by drive testing; 
       FIG. 2  shows a generic structure for a digital mobile phone network illustrating prior art test data sources; 
       FIGS. 3   a  and  3   b  show, respectively, details of a mobile phone network supporting GPRS functionality, and user end equipment for a digital mobile phone network; 
       FIGS. 4   a  and  4   b  show, respectively, digital mobile phone network testing using a protocol analyser according to the prior art, and dynamic reallocation of packet data traffic time slots in a GSM-GPRS mobile phone network; 
       FIG. 5  shows, conceptually, digital mobile phone network testing according to the present invention; 
       FIG. 6  shows user end equipment for testing a digital mobile phone network with voice traffic; 
       FIG. 7  shows user end equipment for testing a digital mobile phone network with data traffic; 
       FIG. 8  shows test equipment for decoding test traffic created by the user end equipment of  FIG. 6  or  7 ; 
       FIG. 9  shows a general purpose computer suitably for implementing digital mobile phone test equipment software; and 
       FIG. 10  shows an exemplary graphical output from the test equipment of FIG.  8 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring now to  FIG. 5 , this conceptually illustrates a method of the present invention. A mobile station or handset  502  is in two-way radio communication with a base station  504 , which in turn communicates with and is controlled by a base station controller  506  across an Iub interface. A protocol analyser  512  is coupled to the Iub interface connection between base station  504  and base station controller  506 , and is thus able to capture all the signals flowing between the base station and the base station controller and to record these in a series of data files  514  spaced at, for example, 15 minute intervals. 
   A terminal  508 , such as a laptop computer, is connected to mobile station  502  for sending and receiving commands and data to and from the mobile station and thence to another device (not shown). This further device may be another mobile communications device on the same or another network or it may be a device connected to an external network such as the Internet. Optionally a GPS receiver  510  is also connected to terminal  508  to provide information on the position of mobile station  502  which can be used in later analysis. 
   The combination of terminal  508  and mobile handset  502  simulates a subscriber using a standard mobile phone to make voice or data calls over the mobile phone network. Terminal  508  generates calls and creates test voice or data traffic, such as traffic sequences  520  and  516 , in which is embedded measurement data or statistics and other information such as GPS position information. The test traffic may be sent to or received from the further device by the terminal  508 . The statistical information, measurements, GPS locations and other data are not stored locally but instead are incorporated within the traffic and sent through the network. The statistical information and/or measurements may include one or more of data throughput rates and voice quality measures. 
   The embedded information is encoded to allow it to be identified from among the remaining traffic data. The embedded information may be encoded by, for example, tagging the information with unique coding keys or with keys which are expected to occur only rarely within the test traffic. In the case of data traffic the information may be embedded into IP data packets; in the case of voice traffic the information is encoded into the voice channel using a channel coding scheme. 
   As illustrated in  FIG. 5  a test data traffic stream  516  comprises a plurality of data segments  516   a-c ,  516   f  into which are interleaved GPS position information  516   d  and statistical information  516   e  derived from measurements made by terminal  508  on the test traffic. Similarly in the case of voice test traffic  520 , voice segments  520   a-c ,  520   f  are interleaved with GPS segments  520   d  and statistics segments  520   e.    
   A protocol analyser  512  or similar equipment is used to record all of the messaging, both traffic and signalling, at a test interface. In  FIG. 5 , the Iub interface is illustratively shown as the test interface but other interfaces such as Iup and Iuc could also be used. The recorded messaging contains messaging for mobile phones served by the relevant portion of the network, and in the case of a tap on the Iub interface as shown this information comprises messaging, including air interface messaging, for all the mobile phones attached to the cell served by base station  504  (assuming that all the Iub links for that cell are tapped). 
   Data processing software is used to process the data files  514  collected by protocol analyser  512 , to demultiplex the traffic and signalling streams for mobile station  512  and terminal  508  from the remainder of the recorded data. The demultiplexed traffic streams such as data traffic stream  518  and voice traffic stream  522  correspond to the traffic streams  516  and  520  sent by terminal  508  and handset  502 . The traffic for the test mobile station  502  thus contains the statistical  518   e ,  522   e  and other data  518   a  to  d,    518   f ,  522   a-d ,  522   f , embedded into the traffic by terminal  508 . The data processing software extracts the statistics and other data from the recorded traffic data streams by recognising the keys used to tag this data, and this information is combined with network performance information which is extracted from signalling on the same link. This combined information may then be used for diagnosing faults, network optimisation and the like. 
   The statistical information embedded into the traffic may comprise a statistical evaluation of measured test traffic characteristics, such as data throughput or latency averaged over a time period. Additionally or alternatively the embedded information may comprise individual measurements. The statistics may be interpreted differently depending upon the test application, and may take into account factors such as the potential need to sequence out-of-order packets. Inserting statistical data into the test traffic not only allows the performance to be measured but also permits an engineer to find out why the performance is as it is. 
     FIG. 6  shows user end equipment  600  for voice traffic testing of a digital mobile phone network. The equipment comprises a mobile station  602  coupled to a general purpose computer  604  running software components to provide the illustrated functions. A data store  606  stores audio test traffic files in, for example, .wav format. Data store  606  is coupled to a coder  608  which reads data from the data store and provides a digitised audio output  626  to a software switch  610 , and thence to a voice phone device driver  612 . The device driver  612  interfaces (via a physical serial port) to mobile station  602  to provide MS  602  with digitised audio signals and to control MS  602  to make calls. Device driver  612  also receives digitised audio data received over the digital mobile phone network back from MS  602  and provides this data as an output to a decoder  618 , which converts the data to a suitable form for comparison with the stored test audio data. 
   Sometimes, depending upon the level support of the mobile station for ISDN, a digital audio connection is not available. In this case the user end equipment may include a digital-to-analogue converter, for example on a sound card, for driving mobile station  602  with analogue audio signals. Typically a mobile phone will provide an interface for a hands-free kit which can be used for this purpose. 
   Comparator code  620  compares the output of decoder  618  with the stored audio data in store  606  and provides an output indicative of the degree of similarity of the two signals and/or signal-to-noise ratio or other audio quality data. The output of comparator code  620  is process by a second coder  622  which provides a coded output  624  to another “terminal” of software switch  610 . 
   The comparator  620  may employ any one of a number of published algorithms for the evaluation of audio samples. Such speech quality algorithms generally compare a measured speech sample with an original version of the speech and provide a score; one such algorithm is ITU-T P.861, also known as PSQM (perceptual speech quality metric). A preferred algorithm is the PAMS algorithm planned for ITU-T P.862, available from Psytechnics of Ipswich, UK which compares reference and degraded signals and returns quality scores from 1 to 5 on two scales, listening quality and listening effort. 
   Coder  622  encodes measurement/statistical data output from comparator  620  using a channel coding scheme, in a preferred embodiment essentially frequency shift keying based upon a knowledge of the voice coder in use in MS  602  and, optionally, in other parts of the network. For example in a GSM network voice coding is performed using a Regular Pulse Excited-Linear Predictive Coder (RPE-LPC) in which frequencies near pre-determined key tone frequencies are transmitted only slightly changed. For example, a 300 hertz tone might be received as a 347 hertz tone with 2 dB attenuation. Preferably one, two or more of such tones are identified and used to encode data “1”s and “0”s. In practice the volume of statistical/measurement data sent is low—for example, 20 to 30 bits may be sent over a period of 1 sec—and it has been discovered that when operating at speeds of less than 100 bps conventional frequency shift keying can be used successfully encode the embedded data. 
   The software switch  610  interleaves test data  626  and encoded measurement data  624  in accordance with a control signal  628  provided by a state machine  616 . The state machine  616  is itself controlled by autocaller code  614  which controls phone device driver  612  to control MS  602  to make (and/or receive) voice calls. 
   The voice calls may either be made between two mobile stations each connected to user end equipment as shown in  FIG. 6 , one acting as a master, the other as a slave, or voice calls may be made to or from a server. In this latter case similar functionality may be provided by means of a voice card, for example from Dialogic of Parsippany, MJ, USA, installed in the server with an interface using Microsoft&#39;s TAPI (telephony API). The TAPI allows a number to be dialled and provides, for example, either PCM (pulse code modulation) data bytes comprising the voice traffic or, with an analogue line and card, packets of wave format data. In this way functionality equivalent to that of  FIG. 6  can be implemented on a server. 
   State machine  616  controls coder  608  to read data from a set of test traffic files in data store  606 , reading each test file in turn in a repeating loop. State machine  616  also provides a control output for coder  622  and a control output  628  for software switch  610 . The state machine controls coder  608 , coder  622  and switch  610  in co-ordination such that when coder  622  has enough data to constitute a segment of the test traffic switch  610  is thrown so that device driver  612  receives data  624  from coder  622  rather than test traffic  626  from coder  608 . Once the statistical or measurement data from coder  622  has been passed to voice phone device driver  612  the switch  610  is returned to its original position again output traffic from coder  608 . In this way test traffic and statistical/measurement data are interleaved, typically in a ratio of 5:1 to 100:1, preferably in a ratio in the region of 20:1. Graphical user interface code  630  is also provided to allow a test equipment operator to select test parameters such as a traffic/measurement interleaver ratio, an FSK (Frequency Shift Keying) encoding method/audio codec employed, and/or a sequence of one or more test files to employ. 
     FIG. 7  shows user end equipment  700  similar to that shown in  FIG. 6 , for testing a digital mobile phone network using packet data test traffic. As described with reference to  FIG. 6 , the equipment comprises a mobile station  702  coupled to a suitably programmed general purpose computer  704  running software to provide the functions illustrated by the functional blocks within dashed line  704 . 
   A data generator  706  generates test traffic, which may be randomly generated traffic, but which preferably comprises data from one or more test data files. These test data files may comprise data to be transmitted over the network or they may comprise instructions, for example to download data from a website or to set up a video phone call or to send or receive TCP, UDP, or other data packets. 
   Data from data generator  706  is processed by packetiser and rate controller code  708  to generate TCP, UDP or other protocol data packets on output  724  and, similarly to the arrangement in  FIG. 6 , these packets are passed via a software switch  710  to a data phone device driver  712 . Data phone device driver  712  sends and receives data and commands to and from MS  702 , in a GPRS or 3G network using a standard set of AT commands. Data received by MS  702  and passed to device driver  712  is depacketised by code block  716  and processed by code block  718 . In the illustrated embodiment code block  718  comprises a bit or packet rate counter and a bit or packet error counter, although other data processing functions may be additionally or alternatively be employed. A bit or packet error counter may be employed with a non-transparent protocol such as UDP. Optionally processing code  718  may also output information about the data from data generator  706  being used to exercise the mobile phone network. The raw measurements, or statistics compiled from the raw measurements, are provided to a coder/packetiser  720  which has an output  722  to switch  710 . 
   Coder/packetiser  720  also adds a tag data sequence to the packetised measurement and other data to allow this data to be retrieved from the test traffic. The tag data sequence is selected to be one which is known not to occur in the test traffic or, where the test traffic composition is not known because, for example, it comprises data downloaded from the website, the tag data sequence is selected to be one which is very unlikely to occur by chance within the test traffic. For example a repeated “101010 . . . ” pattern may be used, with a sequence length of more than 30, and preferably more than 100 bits. 
   As with the arrangement of  FIG. 6 , a state machine  714  controls packetiser and rate controller  708 , coder/packetiser  720 , and switch  710  in co-ordination to interleave measurement data with the test data traffic. The state machine  714  controls the test sequence (and may also control data generator  706 ), as well as test traffic parameters such as block length and data rate. State machine  714  may also interface to device driver  712  in order that the data rate can be controlled to be less than, equal to, or greater than a maximum data rate allowed for a packet data session over the mobile phone network. Since MS  702  is effectively in an “always on” state for data traffic once it has attached to the mobile phone network there is no need for an autocaller. As with the voice test traffic system of  FIG. 6 , however, a graphical user interface  728  is provided to control the data generator  706  and state machine  714  to set the required type of test, to select or programme a test sequence, to enter test data and/or instructions, and/or to set other test parameters. 
   In the arrangements of both  FIGS. 6 and 7  a GPS device driver code portion (not shown) may be employed to interface with a GPS receiver to provide GPS position data. This is encoded and embedded into the test traffic, by coder  622  or coder  720  respectively, in a similar way to the measurement/statistical data. 
   Referring now to  FIG. 8 , this shows test equipment for decoding test traffic  800  created by the subscriber end equipment of  FIG. 6  or  FIG. 7. A  protocol analyser  802  captures data from an interface, reference point, or other point within a digital mobile phone network and writes the captured data into a set of data files  804   a-c , in a similar way to the prior art method described with reference to  FIG. 4   a . The data files  804  may be accessed directly via a computer network or indirectly by copying the data into a removal storage medium for later processing and analysis. 
   Data from one or more data feeds tapped by protocol analyser  802  is demultiplexed by data feed demultiplexer  806  and passed to a protocol stack decoder/demultiplexer  808  which extracts messaging, comprising voice/data test traffic and associated signalling, for a test device such as MS  602 ,  702  of  FIG. 6  or  7  from the remainder of the captured data. At the Abis interface this can be performed by the conventional CallTracker software available from Actix Limited, which analyses all the Abis messages for call initiation sequences. The phone number of MS 602 ,  702 , in a GSM network the IMSI (International Mobile Subscriber Identity), is known and this can be used to identify call initiation from or to the test mobile device. Once call initiation from the mobile has been detected the time slot information allocated to the call is logged and hand over and other time slot control messages are interpreted using a protocol stack decoder to track the time slot allocated to the call and hence thread together test traffic and signalling data associated with the call. The techniques applied at other interfaces within the system correspond although extracting the data is simplified at the higher levels because time slot and radio resource allocation is generally omitted. 
   In the case of a data call a packet domain subscriber identified by an IMSI has one or more associated PDP (Packet Data Protocol) addresses either temporarily or permanently associated with it. These addresses are either IP version  4  addresses or IP version  6  addresses, and are activated and deactivated through mobility management procedures according to PDP context activation procedures described in, for example, the 3GPP technical specification 23.060. 
   In a GPRS packet data network user data is transferred between the MS and an external data network by means of encapsulation and tunnelling, in which data packets are equipped with packet-switched-specific protocol information and transferred between the MS and the GGSN. Packets are transferred between the MS and SGSN either using SNDCP (Sub-network Dependent Convergence Protocol), or in 3G networks, GTPU (GPRS Tunnelling Protocol for User Plane) and PDCP (Packet Data Convergence Protocol). Between the SGSN and the GGSN packets are transferred using UDP-IP protocols, through tunnels identified by a TEID (Tunnel End Point Identifier) and a GSN (GPRS Support Node) address. A Network Layer Service Access Point Identifier (NSAPI) is used in conjunction with the ISMI to assign a Tunnel End Point Identifier (TEID). 
   The NSAPI is used in association with a Temporary Logical Link Identity (TLLI) for network layer routing, and an NSAPI/TLLI pair is unambiguous within a routing area. The TLLI (Temporary Logical Link Identity) identifies a GSM user but the relationship between the TLLI and IMSI is known only in the MS and in the SGSN, to preserve user identity confidentiality (this applies in a GSM-type network; similar considerations apply in a UMTS based network). However the TLLI can be captured by programme code (not shown) running on the computer  604  of  FIG. 6  or computer  704  of  FIG. 7 , and this can be provided to protocol stack decoder/demultiplexer  808  to extract the packet data traffic for the test mobile communications device. Since this information is available once the MS has attached to the network. The information may be provided before the test sequence begins, to allow for real time decoding and analysis of the captured test traffic and associated signalling. This allows packets of a session to be tracked and pulled together. 
   In an alternative approach a characteristic data pattern is inserted into one or more data packets sent from the mobile station, to allow at least one packet sent from the MS to be picked out or at least provisionally selected as a candidate data packet from the MS. Such a fingerprint bit pattern preferably comprises a sequence of bits, for example a random or pseudo-random bit sequence. The bit sequence is preferably long enough to make it unlikely that the sequence will occur by chance within the intercepted data; preferably the sequence comprises at least four bytes, and more preferably more than ten bytes. In one embodiment a sequence of 24 bytes is employed. In practice the length of the sequence is limited by the length of a packet, and this can be up to 1500 bytes for an IP packet, although normally packets are split down into shorter lengths for transmission using a mobile phone network. The maximum packet length is generally operator dependent but is normally ample for inserting such characteristic data into a packet. 
   Once a candidate packet has been identified from amongst the captured data the TLLI for the packet can be read and all the corresponding (either later and/or earlier) packets with that TLLI can then also be selected to reconstruct a datastream. Where a relatively short fingerprint bit pattern is employed there is a possibility that an incorrect datastream will have been picked out due to the chance occurrence of the fingerprint bit pattern in data from a mobile station other than that which is exercising the network and measuring its characteristics. However the question of whether or not the correct datastream has been identified can be resolved by attempting to decode encoded measurement data from within the data packets since, if the wrong datastream has been identified, this will not be successful. 
   Use of a fingerprint test pattern or characteristic data to identify the datastream of interest is generally the preferred method of picking out the data relating to the test MS as in this case all the relevant information can be derived from data tapped from an interface within the network. It will be appreciated that the fingerprint pattern need only be inserted into one IP packet of a session since once this packet has been identified the rest of the session can be extracted by threading along the TLLI. 
   In the case of a UMTS-type network a Radio Network Temporary Identity (RNTI) performs a similar role to the TLLI and identifies a UMTS user. The relationship between the RMTI and the IMSI is known in the MS and in the UTRAN (Universal Terrestrial Radio Access Network) and may therefore be furnished to the decoder/demultiplexer  808  in a similar way to the TLLI. A P-TMSI (Packet Temporary Mobile Subscriber Identity) identifies the UMTS user between the MS and the SGSN and, again, the relationship between the T-TMSI and the IMSI is known in the MS and the SGSN. 
   From the foregoing discussion it will be appreciated that sufficient information is available to the protocol stack decoder/demultiplexer to extract the test traffic data packets and associated signalling from the data files  804  captured and recorded by the protocol analyser  802 . 
   The demultiplexed or “dethreaded” test traffic and signalling data from decoder/demultiplexer  808  relating to the test MS and its driver terminal is stored in a time series database  810 . At a later time, or substantially concurrently, a data decoder/resequencer code portion  812  operates on the data stored in time series database  810  to extract the statistics or other measurement data, and optional GPRS position data, from the test traffic and resequences this data along-side the test traffic and associated signalling data. Graphical user interface  814  provides a plurality of user display options for the data in time series database  810 . These options may include a simple list of the test traffic, associated network signalling and control data and statistics/measurement/position data in a time-ordered sequence, as well as one or more graphical presentations of the data. 
   It will be appreciated that the functional blocks  806 ,  808 ,  810 ,  812 , and  814  of  FIG. 8  preferably comprise portions of programme code running on a general purpose computer such as a laptop computer. The main components of exemplary general purpose computer  900  which may be employed for such a purpose are shown in FIG.  9 . In  FIG. 9  the computer is shown having programme code elements corresponding to the data test traffic user equipment of  FIG. 7 , but code to implement the user equipment of  FIGS. 6 and 8  can be implemented on a similar general purpose computer in a corresponding manner. 
   The computer  900  of  FIG. 9  comprises a data and address bus  902  to which are connected a serial interface  904  for interfacing to a mobile station such as mobile station  702  of  FIG. 7 , a pointing device  906  such as a mouse, a keyboard  908 , and a display  910 . Permanent programme memory  918  provides local data storage for programme code for controlling the computer to perform the desired functions. In the embodiment of  FIG. 9 , the programme code comprises data generator code, packetiser and rate controller code, software switch code, device driver code, state machine code, depacketiser code, rate/error counter code, coder/packetiser code, and graphical user interface code. Permanent data memory  916  stores test data files comprising test traffic data. The permanent programme memory  918  and permanent data memory  916  may comprise non-volatile storage such as a hard disk. Some or all of the contents of the permanent programme memory and permanent data memory may also be provided on portable storage media such as floppy disk  920 . The computer also includes working memory  914 , illustrated storing test data. The permanent programme memory  918 , permanent data memory  916 , and working memory  914  are all coupled to bus  902  and a processor  912  is also coupled to bus  902  to load and implement code from the permanent programme memory. As illustrated processor  912  implements the code to provide a data generator, a packetiser and rate controller, a switch, a device driver, a state machine, a depacketiser, a rate/error counter, a coder/packetiser, and a graphical user interface. 
   Referring now to  FIG. 10 , this shows a particularly preferred graphical presentation of the data in the time series database  810  of  FIG. 8 , provided by graphical user interface  814 . 
   A display  1000  comprises a time axis  1002  and a radio resources axis  1004  which, in a case of a GSM-type network, is graduated in (frame) timeslots. In other networks usage of radio resources may be displayed differently. For each of a series of consecutive and sequential times, such as times  1002   a  and  1002   b , the display shows a level of radio resources  1008   a ,  1008   b  allocated to data transmission and a level of radio resources  1010   a ,  1010   b  used by voice calls on the same interface. 
   The radio resources allocated to data packets are indicated by bars  1008 , which grow upwards as more radio resources are allocated and the resources allocated to voice users are indicated by bars  1010 , which grow downwards from a maximum capacity level indicated by line  1006 . It will be appreciated that when bars  1008  and  1010  meet the available radio resources are being utilised to their maximum capacity. 
   Display  1000  also depicts the one or more measured parameters or statistics extracted from the test traffic, for example a line such as line  1012  in FIG.  10 . This allows a side-by-side visual comparison of the subscriber end measurements with the allocated data and voice radio resources, simplifying interpretation of the data and facilitating network fault diagnosis and optimisation. The display  1000  shows the network&#39;s dynamic response to the test traffic, and the time intervals at which successive radio resource allocation and measured parameters are presented may be selected according to the type of diagnostic information required. Thus they may range from, for example, time intervals of the order of a burst period, frame or multiframe, that is less than 200 ms, up to time periods of the order of seconds, minutes, or even hours. The display  1000  of  FIG. 10  may be presented in pseudo-real time. 
   It will be appreciated that the precise form of the data presented will depend upon the interface being tapped, and the format of  FIG. 10  is particularly suitable where the Abis or PCU Abis (or corresponding interfaces) have been tapped. It will be recognised that for the display format of  FIG. 10  to be employed radio resource allocation data must be available at the tapped interface. 
   The display format of  FIG. 10  may be varied whilst retaining its fundamental value, which arises from being able to see network radio performance in comparison with data metrics such as data throughput and/or data delay. Thus, for example, the axis and bar-chart type format in display  1000  are optional and a plurality of lines  1012  (or other graphical formats) may be provided to display a plurality of measured parameters. Other data may also be included on display  1000  such as, for example, an indication of the negotiated quality of service for a packet data session. 
   No doubt many other effective alternatives will occur to the skilled person and it will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.