Approach to Solving the Problems with IP-based Telephone Systems in the Field

3.1 Background

Problems that occur in the field are diversifying because of the widespread use of cloud services and multi-functional equipment. Customer networks have also become complex due to the integration of several services, such as telephony and Internet. Thus, it is sometimes difficult to determine the causes of problems that occur only occasionally. In such cases, investigation is mainly conducted by interviewing customers, and only preventive measures, such as replacing equipment and/or wiring, are carried out by service personnel.

Regarding problems with key telephone systems, factors related to such problems exist not only in the system but also in usage (such as incorrect telephone operations), and such combined factors may lead to a longer investigation. Given these circumstances, we developed the α-Command Analysis Support Tool for monitoring exchange commands between the master equipment unit of the key telephone system and telephones.

3.2 Overview of functions

This tool can decode various information, such as communication commands (α-command), from IP packets sent between the master equipment unit and telephones of the “α series” key telephone system sold by NTT EAST and NTT WEST. By inputting the IP-packet information acquired by packet-capture devices or other software to this tool, the α-command sequence, parameters, telephone operations executed by the user, content of the telephone displays shown at those times, and operation details of the master equipment unit are outputted as analyzed results. Based on this output, it is possible to understand the system status when a problem occurred. The usage of this tool is illustrated in Fig. 2.

Fig. 2. Using α-Command Analysis Support Tool.

This tool also has the following functions for supporting detailed data analysis.

• “Save as CSV”: Save the analysis result displayed in the tool as a comma-separated value (CSV) file. Depending on the user’s preference, other tools that can handle the CSV data can be used to edit the analysis results.
• “Jump to”: Specify a date and time in minutes and seconds within a large amount of data and spot the data at the time the problem occurred.
• “Search”: Search by specifying the telephone equipment (by extension number or process identifier (PS) number) or a specific α command.

4.1 Summary of problem and capture points

A customer who uses cordless telephones (NX2-<24> CCLBTEL-<1>) with an αNX2-L-type master equipment unit reported that a telephone rebooted when making outgoing calls. Although the on-site service personnel replaced the master equipment unit and a telephone, the problem persisted; therefore, TASC investigated the cause of the problem by capturing IP packets in cooperation with the service personnel. During that investigation, to detect the problem reported by the customer, IP packets were captured at the point shown in Fig. 3 (TCCU port of communication control unit (CCU)) with the packet capture device Cap-two (see Fig. 1).

Fig. 3. Capture point.

4.2 Occurrence status of problem and analysis results

During the capture period (about one month), we received five notifications of the problem from the customer, and the captured IP packets were analyzed using the α-Command Analysis Support Tool focusing on the times that the problem occurred. The operation details of a certain telephone (extension number 203) when the problem occurred are listed in Table 1. From the analysis results, the problem reported by the customer occurred when the kana (Japanese syllabary)-search function of the phonebook was used. With this in mind, we executed the same number-search operation to reproduce the same problem with the same model telephone at our laboratory. This clarified the conditions under which the problem occurred.

Table 1. Occurrence status of problem.

4.3 Causes and countermeasures

The reported problem was caused by internal processing in the telephone, as explained below. The mechanism that caused the problem is illustrated in Fig. 4.

Fig. 4. Mechanism of problem occurrence (data processing in the telephone).

• The amount of data processed by the telephone increased compared to that processed with previous models.
• During the kana-search operation in the phonebook, a large amount of data to be displayed on the telephone accumulated in the receive-data buffer. Although the buffered data are sequentially processed, the receive-data buffer was not emptied.
• While the telephone was processing such a large amount of data, the watchdog timer (568.9 ms), which detects a terminal stack and resets (restarts) the telephone, was activated.

Since the specifications of the telephones caused this problem, and it was impossible to change the telephone software, we took measures to change the manner in which the telephones are used. After explaining the cause of the problem to the customer, we asked the customer to take more than one second when operating the kana search to avoid this problem.

When problems occurred because a customer operated telephones in the manner described above, the only way to identify the problem had been by service personnel visually checking the operation of the telephones in question or by monitoring telephone operation with a camera or other means. However, by using the a-Command Analysis Support Tool, we can reliably identify the cause without visual inspection or monitoring with cameras.

4.4 How to obtain the α-Command Analysis Support Tool

Since September 2019, the α-Command Analysis Support Tool has been downloadable from the website of TASC^* (in Japanese only). We believe that using this tool will make it possible to easily investigate causes of problems and prevent unnecessary replacement of equipment, devices, and/or wiring.