The resurgence of the integrated services digital network (ISDN) has been the most interesting phenomena in what was once a fading technology and service. The idea of having a bandwidth of more than 28.8 kbps is especially appealing to users of the Internet.
With the possibility of high-speed communications, comes the logistics of integrating equipment and services into a functional telecommuting and home office solution. One challenge is the ability of the Internet service providers to satisfy and verify that the user has the service. The other is the telco’s ability to assure that the ISDN service is functional.
The Internet service provider must test the service offering through the ISDN and perform that test at a moment’s notice. ISDN is a service provided by the telco in which the user has two lines to carry voice and data traffic. In the case of a corporation, 23 lines are available in the public switched-telephone network. The current service offering is for Basic Rate Access (BRA) and Primary Rate Access.
In brief, the user requesting BRA needs an ISDN phone with an access for a computer. The ISDN service provider can supply the ISDN telephone and an NT1, network termination equipment.
The connection between the telephone and the switch must be tested before the service is made available. This connection can be tested in one of two ways.
First, the field engineer can set up equipment identical to that which the user will be connecting and test every function the user will use. From a telco’s point of view, this is not the most feasible or efficient way of activating a consumer service.
Second, the field engineer can test the services requested with a protocol analyzer. This is the most likely mode of activating ISDN service to the user. In this test, the field engineer must verify that the two lines provided in the BRA service can be accessed.
The interface where the telephone connects to the NT1 is known as the Basic Rate Interface (BRI). The BRI interface has the capability of 2B+D. The 2B+D, translated to its capability, provides 2 Bearer Channels (B1 and B2), each capable of supporting 64 kbps of data or voice, and 1 D channel for setting up and releasing the call.
The D channel sends and receives messages governed by a specific protocol, part of the National ISDN 1 specification. This specification makes the integration of switch and customer-provided equipment a more manageable task. Without this specification, there would be chaos in trying to interconnect a multivendor environment.
With the telco’s ability to activate the ISDN service, the next task is for the Internet service provider to test its service function. This is a difficult task which focuses on three generalized issues: a 64-kbps connection, a 128-kbps connection, and an underlying and overlaying protocol.
The first service function, the 64-kbps connection, is the most common connection into the Internet and gives you more than twice the speed of a 28.8-kbps modem connection. The underlying protocol standard is the point-to-point protocol (PPP) which ensures data-link capability. On top of the PPP is the internetworking protocol (IP). IP allows you to access applications such as electronic mail, file transfer and remote login.
The task of verifying the functionality is simple-execute a remote login or send electronic mail. The problem arises when the electronic mail is not sent or when a remote login is refused.
This is where a protocol analyzer is most handy, either at the user site or at the service provider’s site. The ability to monitor the flow of traffic on the B channel is crucial to identifying the problem and resolving it in a timely manner to satisfy the user’s need. Here is an example of a problem:
Isolating a problem such as a File Transfer Application that did not complete was solved by analyzing the flow of data from the file server and the user. Analysis of the data flow indicated that the user did not acknowledge the last frame being received. It also showed that the file server tried transmitting the frame three times, got no response and then aborted the file transfer.
Further analysis of the data was still needed, but the problem was isolated to the user not responding to the last frame. Later, it was discovered that the user’s file transfer application was two version levels behind the current release and did not support a short frame. This was easily fixed by updating the customer to the current release.
The solution would not be that simple if you were required to support this older version. Instead, it would require a change on the service-provider side.
The second service function is the 128-kbps connection. This involves the setup of both bearer channels (B1 and B2). From the ISDN service point of view, two calls are being initiated–one for B1 and the other for B2.
The Internet service provider combines these two channels into one with the multilink point-to-point protocol (MLPPP or MP). This type of function allows the user to transfer files at 128 kbps. Without physical access into the BRI with a protocol analyzer, the service cannot be tested.
MLPPP combines both B channels for a higher throughput by sending the same amount of frames on an equal basis on the B channels. To decode the upper layer protocol, the protocol analyzer must support MLPPP decoding.
The underlying and overlaying protocols are extensive and the same protocol analyzer must handle the decoding of the upper-layer protocol to help the Internet service provider supply the necessary service functions. There is no doubt that the task at hand is complicated, but manageable; and in time, it will require a few new twists and turns in the protocol to handle future service function needs.
In conclusion, the future of ISDN is constantly evolving in the types of services and applications being implemented. With these changes, the challenges of testing the new capabilities of the ISDN services are more complex, requiring a good understanding of the protocol and a tool to complement the skill of the engineer.
About the Author
Norm Meritt has been at Tekelec for more than five years. As Product Marketing Specialist, he oversees product features and standards of wide area network and ATM signaling products. Tekelec, 26580 W. Agoura Rd., Calabasas, CA 91302, (818) 880-5656.
Copyright 1996 Nelson Publishing Inc.
February 1996
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