A Software Architecture for QoS Surveillance for Short Message ...

A Software Architecture for QoS Surveillance for Short Message Services in GSM Networks Waadt, A., Bruck, G., Jung, P. University Duisburg-Essen [email protected]

Kowalzik, M., Trapp, T. TynTec GmbH [email protected]

Abstract In mobile environments, packet transmission services suffer from packet losses due to e.g. inadequate received signal quality, or forced by protocols in the signaling domain of the infrastructure of a mobile communications network. To reduce the occurrence of packet losses and, hence, to improve the quality of packet transmission services, such as e.g. the Short Message Service (SMS) deployed in GSM (Global System for Mobile Communications) based mobile communications networks, a quantitative analysis of the Quality of Service (QoS) in the signaling domain is mandatory. In this communication the authors propose software architecture for QoS monitoring in distributed SMS Centers. Operating in near real-time, this helps to maintain a high QoS level. Selected monitoring results gathered during real world network operation are presented.

1. Introduction Packet transmission services have continuously gained importance in mobile communications. Such services include e.g. the SMS (Short Message Service), first introduced in GSM (Global System for Mobile Communications) 0[2] and TCP/IP services. In mobile communications, the SMS used in GSM and in GPRS (General Packet Radio Service) [3] mobile communications networks has become the most important economical success during the past decade. To enable the deployment of packet services like SMS in professional environments, complying with sustained high Quality of Service (QoS) levels is mandatory [4]. QoS levels are measured by evaluating QoS parameters, like a pre-determined level of reliability and well defined attributes for precedence and delay of the service. Unfortunately, sustained high QoS levels can usually not be guaranteed for the SMS, and SMS

Begall, C. evision GmbH [email protected]

Centers (SMSC) therefore cannot provide a high service quality in all cases, today. In particular, SMS suffers from packet losses due to e.g. inadequate received signal quality, which is caused by the time and frequency selectivity of the mobile radio channel and by imperfections of mobile terminals. However, packet losses are also produced by time-outs, entailing forced packet deletions. These forced packet deletions are the consequences of protocols in the signaling domain of the infrastructure of a mobile communications network, including the Base Station System Application Part + (BSSAP+) and the Mobile Application Part (MAP) protocols [4]. The mentioned time-outs occur for various reasons. For instance, when a mobile terminal, which shall be contacted by means of SMS, stays outside the coverage area or has been switched off for a certain period of time, the allocated Mobile Switching Center (MSC) will detect an absent subscriber and may force the deletion of packets. Another possible cause for defying the delivery of packets, which comes into the play when the origin and the destination terminals are subscribed to different network operators, lies in the absence of roaming agreements between these network operators. When SMS is used for non-professional, i.e. private, communication from one subscriber to another, the lack of sustained high QoS levels may be acceptable. However, this is no longer the case for the deployment of SMS in professional scenarios. To reduce the effect of packet losses and, hence, to improve the quality of the packet transmission service, a quantitative analysis of the QoS in the aforementioned signaling domain is a must. However, only few QoS parameters were defined by e.g. ETSI [5][6] to monitor the transmission quality, only, covering mobile originated and mobile terminated SMS. These definitions are tailored for the end-to-end transmissions, including Delivery Time and Completion Rate as QoS parameters. After

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modification, they can be used to observe the quality from the SMS Center’s point of view [9]. The corresponding QoS aspects have also been treated in previous research publications, which usually set out from the statistical modeling of the packet traffic and present results on the related efficiency parameters [7][8]. The forced packet deletions mentioned above have not been taken into consideration, yet. Also, the defined QoS parameters [5][6] do not completely fulfill the needs arising from a professional SMS. Therefore, additional QoS parameters, which complement the presently existing set of QoS parameters defined by e.g. ETSI [5][6], have been defined in [9]. Since SMS Centers communicate with service providers’ infrastructures by using the SS7 (Signaling System 7) protocol [5], the additional QoS parameters consider the signaling domain explicitly. They have been applied to the SMS in real GSM networks. To quantify the corresponding QoS levels, the signals and error acknowledgements, made available through the SS7 protocol, have been evaluated. These signals and error acknowledgements also cover SMS related services, which use signals from the MAP layer of the SS7 protocol. Selected monitoring results gathered during real world network operation are presented and discussed in this manuscript. Section 2 gives a technical overview about the SMS Center and its used services. In Section 3 the authors introduce a software architecture to monitor the QoS within SMS Centers, in order to control and care a homogeneous high QoS level. Measurement results of a number of QoS parameters are shown in Section 4 before the manuscript is concluded in Section 5.

2. Packet Transmission Services in SCS Centers 2.1.

Short Message Services

To be reachable, a MS (Mobile Station) notifies the HLR (Home Location Register) of its home NSS (Network and Switching Subsystem) about its location, when it is turned on or changes the NSS. This is done by a location update procedure at which the global titles (GT) of the MSC and VLR (Visitor Location Register) of the NSS, where the MS is located, are transmitted to the HLR. These global titles (MSC/VLR-GT) are routing information to the subscriber’s location. They are stored into the HLR for the MS and can be used to route signals to the MS.

Short Messages, originated by another client, are send to the SMSC (SMS-Center) which is responsible for the home NSS of the message originating MS. After acknowledging the receipt of the short message, the SMSC queries for the routing information (the MSCGT) to the destination of the Short Messages and forwards them to the destination MS. From the point of view of the SMS Center’s Gateway MSC several commands out of the MAP of the SS7 protocol are used. The two most important are: 1) MAP-SEND-ROUTING-INFO-FOR-SM (srism): This command is used between the gateway MSC and the HLR to retrieve the routing information, needed for routing the short message to the servicing MSC, where the subscriber roams. This request for routing information from the SMS Gateway MSC contains the MSISDN (Mobile Subscriber ISDN Number) of the subscriber, whereas the result contains the MSC-GT. This address is used to forward the short message in a forward SM process. In the following this command is called srism. 2) MAP-FORWARD-SHORT-MESSAGE (fwsm): This command is used to forward mobile originated or mobile terminated short messages between the SMS Gateway MSC, which has a connection to the SMS Center, and the Servicing MSC, where the subscriber roams. This command is abbreviated with fwsm. Every command delivers a status report. The interpretation of these status reports at the SMS Center facilitates the generation of various error codes, including the acknowledgement of the successful delivery of the SMS. The mapping of error codes to their verbose description has not been standardized. TABLE I shows an abstract of a conceivable mapping of some error code numbers to verbose descriptions as they are used in the SMS Center of TynTec. There are more than 100 error codes with different meanings. Since the error code descriptions are not standardized, yet, other error codes descriptions are conceivable, as well. TABLE I: Examples of error codes for MAP commands in the SMS Center of TynTec Error code -0xb 0x0000 0x0009 0x000d 0x001b 0x6000 0x8010 0xe040

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Verbose description Internal transmission error Successful SMS Without Errors illegalSubscriber callBarred absentSubscriber memoryCapacityExceeded Node not reachable (routing failure) Dialogue timed out

If a signaling MAP command is not answered by an acknowledgment, it times out after 35 seconds and gets the error code 0xe040. The reasons of such timeouts may be network errors, corrupted network equipment or missing roaming agreements between involved network operators.

2.2.

Routing

The imperfections of the infrastructure of mobile communications networks, e.g. disturbances in gateway MSCs or due to bottlenecks in connections between networks of different operators and countries, cause time outs and routing failures while an SMSC, which is located in a first mobile network, attempts to deliver messages into a second, foreign mobile network. In many cases, this disturbance can be evaded, by routing the signals through another gateway MSC, located in a third mobile network, into the second mobile network. For this purpose, signaling units of the different operators, which are connected to the SS7 network by different gateway MSCs, can be bonded to one distributed SMSC, which is controlled by a central routing logic, containing a QoS surveillance system. Another motivation for a dynamic signaling routing is the lack of adequate roaming agreements between network operators. When the network operator owning the Servicing MSC and the network operator owning the Gateway MSC do not have any roaming agreement for the SMS, a subscriber will not be reachable for the delivery of a short message. In this case, the provision of the SMS may be possible through a different network operator who also provides service at the temporary location of the aforementioned subscriber. The In order to compare the available routes, the central routing logic needs information about the QoS of the different network connections. Therefore a monitoring and analysis of the QoS of several connections from different gateway MSCs to a number of mobile networks is mandatory.

2.3.

QoS Parameters

Several QoS parameters for the SMS have been standardized, e.g. by the ETSI [5][6]. Not all of them are applicable to the QoS evaluation in an SMS Center because they are tailored to the mobile originated SMS. However some of the QoS parameters are applicable after modifications. These modified parameters have been introduced in [9]. Furthermore additional QoS parameters have been defined,

evaluating the acknowledging network responses, given to the SMS Center. Some important QoS parameters, monitored by the SMSC, are [9]: 1) Error-#-Ratio SMS 2) Success-Ratio SMS 3) Short Time Effort SMS. The Short Time Effort STESMS is defined as the number N MAP-ALL of MAP commands, sent in a short time interval, divided by the number N SMS,ALL of SMS for which the MAP commands were sent: STESMS =N MAP,ALL / NSMS,ALL

(1)

This parameter reacts faster but less violent as the common Effort from [9], which regards larger time intervals and is more interesting for strategic analyses of the characteristics of the SMS traffic of different user groups or the performance of delivery retry schemes of the SMSC.

3. QoS Control in an SMS Center 3.1. QoS Surveillance System For surveillance and visualization of the mentioned QoS parameters, which vary depending on e.g. the called party HPLMN, the traffic characteristics of different calling parties or the Servicing MSC, a QoS monitoring system has been developed, containing Java programs and a graphical analysis tool Value ranges for the QoS parameter values are set in a parameter supervision block depending on the traffic, which is expected from e.g. the various SMS customers, or on the error behavior of the Servicing MSC. The values of the QoS parameters are then measured and compared to the value ranges in the parameter supervision block. In the case of a QoS parameter value being out of range, an alerting message is sent to the operators of the SMS Center. If an adaptation of the system framework at the SMS center leads to an improvement of the QoS level, an automatic reconfiguration can be initiated from the reconfiguration block. This reconfiguration may be e.g. a modification of the scheduling of the retry cycles in the short message transmission system or a change of the Servicing MSC, if the mobile station is reachable via a different Servicing MSC as the already attempted.

3.2.

Thresholds and Alerting

The QoS parameters, introduced in Section 2.3 and in [9], are monitored in near real-time for a number of

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Obs 3.1 Properties Timer

Engine Controller Synchronization, Evaluation & Report, Reconfiguration

Queue allocate sql connection;

FIFO (A) cyclic buffer

Obs 3.203 Properties Timer

QoS reports alerting database entries

Obs 2.1 Properties Timer

graphical visualization

Obs 1.1 Properties Timer

FIFO (B)

while (FIFO is not empty){ get element; refer a sql connection; start sql query; return QoS parameter } synchronize, wait & restart;

cyclic buffer

Query 2 (duration) Query m

sql connection (B)

Query 1 (CR) sql connection (A)

different networks, Signaling Units and groups of SMS customers. An alert is given, when a measured QoS parameter crosses one of several defined thresholds. Analyzing the acknowledging responses of the signals, which led to the alert, the main error codes can be found and anticipated with a reason of the trouble. This leads to a classification of the alerts, which is important for the evaluation to be done later. The alerts are classified into four groups: 1) Network and routing errors 2) User related trouble 3) Internal problems 4) Uncategorized disturbances Network and routing errors are anticipated with such error codes, which concern the connection between the signal transmitting SMS Center and the called HLR or MSC. Examples for such error codes are 0x000d, 0x8010 and 0xe040 in TABLE I. Reasons could be troubles in gateway MSCs, network errors or blocked operators for instance. Blocked operator means that the network operator, where the called HLR or MSC is located, does temporally not accept any signals from the network operator, where the SMS Center is located. These errors can be handled by a changed routing, i.e. the signals are sent through another signaling unit. User related troubles are specific for groups of called parties or message originating parties. In order to discern several groups of called or calling parties, they are identified with business partner ID numbers. Those groups are denoted by business partners. Examples for user related troubles are invalid MSISDNs or subscribers which are not able to receive a message, since they are corrupted, their memory capacity is exceeded or they are turned off or out of the coverage area. Those errors occur very often. Their frequency depends on the characteristics of the SMS traffic. Internal problems may occur within the SMS Center, which tried to transmit the signals. Other disturbances are classified as uncategorized. They have to be examined separately until their characteristics are fully analyzed. All classes of alert lead to trouble reports, which are generated by the mentioned monitoring system and sent to the QoS surveillance system. By changing the routing, it is possible, to react to network or routing errors and to internal errors within SS7 signaling units. An automatic changed routing does not help in case of user related trouble. However, the trouble reports help to analyze reasons for

database SS7-BMD

Fig. 1: Software architecture of the QoS Engine disturbances and e.g. to categorize the kind of messaging traffic, generated by the different business partners. The kind of traffic affects the signaling effort, which has to be done by the SMS Center in order to deliver messages. This can be taken into account by adjusting the billing for SMS according to the expected signaling effort or by adjusting the retry scheme for delivery attempts of messages. The retry scheme determines the frequency of retrying to deliver an SMS after failed delivery attempts. Therefore, four actions can be done, if a QoS parameter crosses an alerting threshold, namely 1) Change routing, 2) Adjust delivery retry cycles, 3) Change billing and/or 4) Search for and eliminate the reason of the detected trouble.

3.3.

Software Architecture

To manage the monitoring of different QoS parameters for a large number of business partners and

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signaling units and their connections to several mobile networks, sophisticated software architecture is necessary. The problem of monitoring several parameters for several signaling connections is divided into a number of small observation jobs. Every observation job is done by a single observation station, which runs in a separate thread and observes one QoS parameter for one network connection or one business partner. The observation stations are characterized by a set of properties, which contain information about the controlled QoS parameter, prefix domains of the monitored network subsystem and identifiers for business partners or signaling units. These parameters are stored in databases and can be edited with a graphical user interface. The synchronization and management of the resulting observation stations is done by a central QoS Engine Controller, which is connected with the observations stations and generates all alerts and QoS reports. Fig. 1 illustrates the concept of the used software architecture. The observation stations are scheduled by a timer module and ask in equidistant time intervals for new data about transmitted SS7 signals. The queries are sent into a queue, which controls the data access to the SS7 log database (SS7BMD in Fig. 1). After processing the query, the queue sends the result back to the Engine Controller, which evaluates the result according to the properties of the responsible observations station. The processing of the mentioned queries are done by a combination of several program modules, representing the different QoS parameters, introduced in Section 2.3 and SQL statements, which are sent to the SQL database, where data of transmitted SS7 signals are temporally stored. The SQL queries are stored into a FIFO buffer before they are processed by e040 Ratios Italy 90 80

0xe040-Ratio Vodafone Italy 0xe040-Ratio Wind Italy 0xe040-Ratio Telecom Italy

Percentage

70 60 50 40 30

the database. Furthermore, the monitoring jobs are partitioned in two groups, for tactical and strategic QoS analyses. Tactical QoS analyses are used for automatic reconfigurations of the SMSC. They are time critical and have to be done in near real-time. Strategic QoS analyses are used to create reports about the progression of the performance of the SMSC and the general behavior of different business partners and networks. Since their processing is more time consuming but less time critical, they are scheduled in a second FIFO buffer B, while tactical QoS analyses with higher priority are scheduled in a separate FIFO buffer A. The SQL queries from FIFO buffer A and FIFO buffer B use different connections to the SS7 log database.

4. Measurement Results 4.1.

Network errors

As introduced in Section 3.2, four kinds of disturbances can be discerned: network and routing errors, user related trouble, internal problems and uncategorized disturbances. “Uncategorized disturbances” is just a default denotation, if the automatic classification of the trouble failed. In what follows only network and routing errors, user related troubles and internal problems within an SMSC are covered. An example of how errors in network elements affects QoS parameters observed in an SMSC, can be seen in the Error-0xe040-Ratio of fwsm signals, which were sent from an SMSC over a specific gateway MSC into three different networks in Italy in November 2005 (cf. Fig. 2). The Error-0xe040-Ratio denotes the percentage quota of signals having error code 0xe040 from all sent signals. The error code 0xe040 indicates a time out of the MAP command, which has not been acknowledged (cf. TABLE I). Depending on the regarded network, the Error-0xe040-Ratio usually fluctuates in the range of less than or not much more than one percent. In Fig. 2 this ratio can be seen rising to about 41% - 78%. That was caused by a network error, which occurred on November 18 at about 10:00 GMT and ended on November 21 at about 09:00 GMT.

20 10

4.2.

0 17 00:00

17 12:00

18 00:00

18 12:00

19 00:00

19 12:00

20 00:00

20 12:00

21 00:00

21 12:00

22 00:00

Day and Time in November 2005

Fig. 2: Error-0xe040-Ratio for FWSM signals sent to Italy in November 2005

User related trouble

Such troubles, which occur for specific groups of calling parties or called parties, are termed user related troubles. An example of such a user related trouble has been observed for a business partner in November

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Success Ratio and Error 0x6000 Ratio 100

Success Ratio

90

Error 0x6000 Ratio

80

Percentage

70 60 50 40 30 20 10 0 25 12:00

25 16:48

25 21:36

26 02:24

26 07:12

26 12:00

26 16:48

Day and Time in November 2005

Fig. 3: Error-Ratios for business partner 248 in November 2005 Short Time Effort 5

Short Time Effort

4

3

2

1

0 25 12:00

25 16:48

25 21:36

26 02:24

26 07:12

26 12:00

26 16:48

Day and Time in November 2005

Fig. 4: Short-Time-Effort for business partner 248 in November 2005 2005. Fig. 3 illustrates the Success-Ratio and the Error-0x6000-Ratio of signals, which have been sent for a specific business partner. The error code 0x6000 indicates failed fwsm signals, where the short message has not been delivered due to an exceeded memory capacity of the subscriber. The percentage of this error is usually less than 5 %. However, on November 26 the Error-0x6000-Ratio increased to about 62 %. This trouble appeared only for one business partner and was the main reason for the low Success-Ratio of sent signals and therefore responsible for the increased signaling effort, which is illustrated in Fig. 4. The signaling effort is ideally about 2 signals per SMS (one srism and one fwsm). On November 2005 it increased to about 5 signals, which was caused by the delivery retries of short messages to subscribers with exceeded memory capacities.

5. Conclusion

systems improves the QoS level and, at the same time, reduces the effort of the SMS Center. The improved QoS level is mandatory if business applications shall use SMS. The authors presented a system framework for the QoS monitoring, alerting and reconfiguring an SMS Center and presented measurement results for different classes of disturbances and situations. This system framework makes use of the appropriate implementation of standardized, however modified QoS parameters as well as of newly proposed additional QoS parameters. It was illustrated that the chosen QoS parameters are a viable means for the near-real time characteristics of SMS provision.

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The observation of properly chosen QoS parameters together with a reconfiguration of the transmission

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