March 2002

GL Communications
207A Perry Pkwy., Ste. 1
Gaithersburg, MD 20877
Phone: 301-670-4784
Fax: 301-670-9187
Web site: www.gl.com

Price: $20, 000�$120,000 depending on configuration. Average is $50,000�$60,000.

GL Communications� Digital Central Office Switch Simulator (DCOSS) platform converts a Pentium PC into a digital central office switch simulator, including T1, E1, SS7, and POTS interfaces. It can also test for fax signaling, DTMF/MF tones and line noise/energy. It can also measure voice quality via the PSQM, PAMS, and PESQ+ voice quality standards, which is significant for VoIP testing. 

The DCOSS system we had ran on Windows NT 4.0 and utilizes NMS Communications (www.nmscommunications.com) hardware. Using the GUI, the DCOSS may be initialized for a variety of protocols such as Robbed-Bit R1 (wink) (along with additional CAS protocols), MFC-R2, PRI-ISDN (includes T1 ISDN-NFAS), SS5, or SS7 (ISUP or TUP). Also, analog telephones and/or BRI ISDN (National and International) telephones may be used for placing or receiving calls. (The DCOSS in a typical test scenario can be seen in Figure 4.)

FEATURES
Some of the more important functions include the ability to send/record PCM voice files, send/detect DTMF/MF digits, send/detect single or dual frequency tones, send/receive faxes, and send/receive modem traffic. The nice thing about DCOSS is that these functions can be performed simultaneously. In addition, these functions can be executed either on an individual timeslot or all the timeslots on all the trunks.

DCOSS features both manual and bulk call generation. Manual call generation allows the user to place calls on any timeslot/trunk manually simply by clicking the mouse on the timeslot or trunk. Calls may be made to/from a standard phone, fax machine, or modem without physical connections. Once the manual calls are established, the user may talk over the phones, send/capture PCM voice files, send/ detect DTMF/MF digits, send/detect frequency tones, send or receive faxes (requires optional DCOSS Fax licenses), and send/receive modem traffic (V.90).

Bulk call generation allows the user to automatically generate a multitude of calls (for single, many, or all specified timeslots). The user may load multiple trunks with a variety of calls such as send/record PCM voice file, send DTMF/MF Digits, send single/dual frequency tones, send/receive fax in both �Transmit� and �Receive� directions, send/receive modem traffic. In addition, the DCOSS features a bulk call scheduler to allow the user to set up a series of actions or events, which can be executed immediately or at user-defined start time. If the turnkey tests are not adequate, users can write scripts using the DCOSS Bulk Call Script Editor for more complex and flexible testing.

Other features include:

• Call Status Screen � provides Real Time Status of all calls on a particular trunk. This screen also displays the current function being performed on any given timeslot (send file, record file, send tones, send fax, receive fax, send modem, receive modem).
• Phone Handset status � Phone Handset status displays the Real Time status of the associated analog or BRI-ISDN phones. Information such as On Hook/Off Hook/Monitor mode as well as the current timeslot and trunk are displayed. The phone initiating the call will display the phone number dialed.
• Bulk Call Status � The Bulk Call Status screen displays to the user Start / Stop Bulk Call activity from the Bulk Calling functionality.
• Captured SS7 Events � The Captured SS7 Events lists chronologically all the SS7 messages generated by the trunk and timeslot. Each SS7 event displays timeslot/trunk, timestamp, SS7 event, OPC, DPC, and CIC.
• Trunk Status � All digital trunks are displayed on a single screen showing alarm status, current protocol and protocol configuration, as well as additional information concerning the configuration of the trunk.
• Timeslot Status � A single screen showing all timeslots of all trunks and the current status of each timeslot.
• DID Routing, switch routing, and dedicated switching.
• Fax Generation/Reception (optional).

OPERATIONAL TESTING
The DCOSS comes as a turnkey solution, so no software installation was required. We connected the various physical connections pretty easily. We soon discovered that the DCOSS has more features than you can shake a stick at! Although we did test the traditional T1/E1 simulation and bulk call generation, we focused most of our testing on checking out the PSQM, PAMS, and PESQ voice quality measurement benchmarks, which are essential tools to test VoIP applications.

Voice files may be sent or recorded using the DCOSS PCM Voice File Generation/Reception Function, which can continuously transmit multiple PCM voice files or WAV voice files. Both A-Law and m-Law PCM Voice Files or WAV Voice Files may be transmitted or received. Fortunately, the DCOSS comes with several reference PCM files to use for testing voice quality across the system under test, so we didn�t need to record our own reference files.

For our tests we performed a loop back configuration connecting a crossover T1 cable from trunk 0 to trunk 1. We then configured the bulk call generator to call out on trunk 0 and receive the calls on trunk 1. After assigning a specific voice file to play from trunk 0, we specified the file to receive on trunk 1 as well as selected all 24 channels. A few minor configuration settings later, we clicked Begin Bulk Calling and the system started making calls and playing the specified voice file across the T1 channels we selected. In addition, the main GUI features a scrolling test call history and up to eight analog handset devices, which can be mapped to a specific channel for listening in or for making manual outbound test calls.

We especially liked the Measurement History, which shows several bits of important voice quality information, including the trunk number/channel and PSQM, PAMS, PESQ scores in a tabular format. Each record displayed is also time/date stamped making diagnosis much easier. Another measurement of call quality can be performed from the DCOSS Call Quality Test screen. From this screen the user may define the criteria that determines if a call will yield a Pass or Fail. If a call fails, a record of that failed call will be sent to the error screen with an explanation of where the call failed.

Although it isn�t fair to make a single feature comparison of the DCOSS versus Empirix� competing Hammer system, we should point out that there was one important feature that the DCOSS has that the Hammer product does not. The DCOSS system is able to perform all the various speech quality tests (PSQM, PAMS, PESQ, etc.) using just a single set of test calls, whereas the Hammer system requires that you run separate test calls for each type of speech quality test performed. In fact, this was one of our few �Room for Improvement� items for the Empirix product reviewed in the November 2001 issue (www.tmcnet.com/it/1101/1101labs1.htm).

The way the DCOSS is able to perform multiple speech quality tests all at once is that it actually stores both the reference voice file (original file) and the corresponded degraded voice file received on the terminating side. It then performs the various speech quality tests, comparing the reference sound file and the terminating degraded sound file. In fact, the DCOSS has an interesting �manual� voice quality test. You can take any two voice files and perform manual voice quality tests without having to perform any telephony calls. As an interesting verification test that the PAMS standard does indeed work, we manually chose the same voice file for both the reference and degraded voice file and the resultant PAMS score was a perfect 5. Additionally, DCOSS has an �auto voice quality monitor� mode, which will �watch� a specified directory for voice files put there. It will then automatically apply voice quality tests on the files against the corresponding reference file.

The DCOSS features a useful pass/fail criteria setting. Thus, a user can enter the desired threshold values for PAMS LE/LQ, PSQM/PSQM+, PAMS Clipping, PAMS Latency, PESQ MOS, and PESQ Latency measurements. Then the user could select �View Failed Measurements Only� in order to view only Failed Calls and screen out calls that were successful. From this point, the user can double-click on a failed call and open it in CoolEdit (graphical oscilloscope-like sound editor) to visually compare the reference and degraded voice files.

One of the features that we liked was that DCOSS can freeze the real-time Call Records screen. This is useful for pausing the real-time flood of statistics when making several test calls. Another useful screen was the Call Statistics screen, which shows the progress of calls in real-time, as well as any detected errors. The Call Statistics screen displays statistics per timeslot, per trunk, per entire system as well as hourly statistics and may be exported to an ASCII file. One final feature of note is the Remote Access capability, which allowed us to remotely control the DCOSS using the supplied DCOSS client. The DCOSS Remote Access capability can operate over any IP connection, including a LAN, WAN, or the Internet.

ROOM FOR IMPROVEMENT
The DCOSS has an almost endless supply of �undockable� windows, which can be both a good thing and a bad thing. We found it to a bit confusing having several windows open and then trying to remember which icon at the bottom of the screen pertained to the window we wanted to switch to. We found ourselves resorting to several trial-and-error mouse clicks to find the window we wanted. Thus, we would instead prefer some sort of hierarchal tree-like structure within a single window to traverse the various configuration screens. Of course, in some cases, having undockable windows can be convenient, so we wouldn�t want to lose this capability. One minor improvement would be to see the system run on Windows 2000; besides, the Windows 2000 drivers for the NMS hardware have been out for a while now.

CONCLUSION
The DCOSS is a great platform for simulating and testing telecom networks and products, including VoIP gateways, softswitches, and PBXs. With its bulk call generator, E1/T1 plus SS7 support, voice quality measurement tools, and fairly easy to use interface, the DCOSS is a comprehensive telecom testing solution perfect for service providers, and carriers, as well as testing and product development labs.

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