Driven by benefits such as telecommuting, video conferencing and Internet access, integrated services digital network (ISDN) telephone lines are projected to jump from 650,000 lines in 1995 to 5 million by 1998, according to Bellcore in Morristown, NJ. This increased demand for ISDN service pressures communications providers to accelerate customer service as quickly and efficiently as possible.
Unfortunately, configuring and provisioning ISDN service can sometimes be difficult. These problems often give the wrong impression that ISDN as a technology is to blame. In fact, if installed and configured properly, ISDN is a robust technology offering high-quality, advanced capabilities.
EZ-ISDN, a specification approved in early 1996 by the North American ISDN Users Forum, may simplify the process. EZ-ISDN standardizes configuration parameters for ISDN equipment and service to simplify service ordering and decrease installation problems.
While EZ-ISDN appears to have a bright future, it does not eliminate the need for an effective ISDN test strategy using equipment that:
Tests all deployed service configurations.
Is easy to use.
Includes a family of test sets allowing a tight coupling of work-group needs and equipment features.
This testing strategy produces cost reductions, especially by reducing callbacks for installation problems, which is critical in today’s competitive telecommunications environment.
Network Prequalification
As more of the existing telephone copper wire to the home is converted from voice service to Basic Rate Interface (BRI) ISDN service, prequalification of long cable loops becomes important (Figure 1). Standards bodies have set parameters for loop performance to 18,000 ft of cable with a loop loss of no more than 42 dB.
As estimated loss calculations approach 36 dB, prequalification is recommended. These calculations are based on plant records. A 6-dB margin is recommended to account for the effects of temperature changes or record accuracy.
A 40-kHz analog test tone is a common way to test the loop.
While a 40-kHz analog test tone emulates the digital signal and measures loss, placing an actual 2 binary 1 quaternary (2B1Q) ISDN signal on the loop and establishing layer-one communications is the best method to use in characterizing loop quality (Figure 2). The 2B1Q ISDN encoding technique uses two bits to represent four variations in amplitude and polarity. It is the most common signaling method on U-interfaces.
Portable test equipment with a line termination or a network emulation mode at one end of the line-under-test generates an actual BRI (2B + D) layer-one signal. 2B+D represents the two 64-kb/s bearer channels (B-channels) for data communications and a third 16-kb/s data channel (D-channel) for signaling.
Portable ISDN equipment at the opposite end establishes layer-one communications. Once the test equipment is in place, full bit error rate (BER) testing is conducted to establish line quality. Also, selectively adding loss to the U-interface in the tester can establish the signal margin (the difference between the actual signal measured and the level at which service fails in decibels).
As these loops exceed the recommended 18,000 ft of cable, mid-span U-repeaters are available to extend the reach of BRI service. As the name suggests, these repeaters accept the BRI signal, regenerate it and send it on. This, however, is a costly approach.
T1 carrier systems (a digital transport system at 1.544 Mb/s) using Basic Rate Interface Transmission Extension (BRITE) cards are more cost-effective when extending service to remote areas with more than one subscriber loop. BRITE cards convert the two-wire U-interface ISDN signal into a format compatible for transmission over a T-carrier system. Whichever method is used, testing must be conducted from the central office (CO) all the way to the customer premise through these extension systems.
Verifying Proper Configuration and Initialization
While ISDN service is robust, it is unusable without proper installation. Because ISDN service is still relatively new, service orders often do not match the customer requirements. EZ-ISDN might alleviate some of these issues; however, a service order that matches customer requirements is critical. Assuming the service order is correct, service translations must be verified at turn-up.
During turn-up, the programming of all critical service parameters in the ISDN CO must be verified to the customer line-under-test. This includes verifying service profile identifier (SPID) assignments; bearer services, such as voice, switched data or packet data; and call routing for both inbound and outbound calls.
SPIDs help identify a specific ISDN set when more than one ISDN set is attached to the same CO line. Without the proper SPID, the ISDN switch cannot establish communications with the telephone or other terminal equipment (TE) at the premise. The TE is unable to place or receive a call.
Placing a Call
Test calls should be placed to an automated test-line unit for each bearer service. It also is recommended that calls be placed to both test-line units within the serving CO and another CO to check interoffice trunking.
The test line should have a callback feature in which the automated test line calls back to the line-under-test using the calling party ID data from the original test call. This allows a field technician to verify outbound as well as inbound service translations without requiring the help of other personnel.
When ISDN service supports video applications, both B-channels are used at the same time. Special testing is recommended since the network treats the two channels as independent calls in BRI service.
The customer premise equipment (CPE) must perform the bonding or making the two separate B-channel calls act as one 128-k call. As calls leave, the CPE also must be capable of inverse multiplexing or splitting the 128-k call into two separate B channels. If the differential delay between channels is too large (300 to 500 ms or more), the CPE cannot successfully establish proper communications.
ISDN test equipment should include the capability to measure interchannel differential delays. This is done by placing two calls, one on each B-channel, to a far-end loop-back point and comparing the round-trip delay on each channel.
Using BERT to Check Line Quality
In practice, most turn-up problems relate to service translation issues, but line quality also must be tested. Conducting a bit error rate test (BERT) to an automated test-line unit on each B-channel is standard using a 2,047 pseudorandom bit pattern. A pseudorandom bit pattern is a test message consisting of 511 or 2,047 bits, ensuring that all possible bit combinations can pass through a network without error.
End-to-end testing also is part of turn-up procedures in specific critical service situations. When service supports video applications, special testing is recommended. In this case, using a unit that simultaneously generates BER testing on both B-channels is required. This method most closely looks for problems that may appear when actual service is established by the customer.
Trouble Resolution
Service quality also relates to quickly resolving customer problems. In this case, the capability to monitor D-channel messages and to detect CPE configuration and operation errors is critical.
A special U-monitor capability observes an active line and identifies error conditions. A test instrument with this feature and the capability to display the English-language translation of the D-channel messages by a knowledgeable technician are important elements in an ISDN test strategy.
Service Changes
ISDN is still an evolving service since new capabilities are routinely being deployed. For example, B-channel packet data service is becoming available. Service profile management (SPM) features are being deployed on more CO platforms. SPM, which allows the technician to query the CO and identify button assignments on the CPE, is especially valuable in a business environment. The complexities of CPE button mapping are critical when button assignments include commercial transactions or time- sensitive information.
Because of evolving services, test equipment that handles field upgrades with minimal expense or technical knowledge is best suited to meet the needs of this rapidly changing technology.
Testing Benefits
With deregulation taking hold in the telecommunications industry, it is imperative for service providers to quickly and successfully turn-up new services. This proven test strategy combined with specialized test equipment allows for swift service turn-up.
Thorough service turn-up procedures also reduce costs and improve customer satisfaction. When service is properly tested at installation to verify all critical service items, callbacks to correct problems are reduced. This is a significant cost savings to a service provider. When callbacks are reduced, customer satisfaction improves since the customer does not become involved with trouble reporting and service interruption.
Finally, ISDN has enormous capabilities for today’s changing workplaces. By eliminating installation and maintenance problems in the beginning, the ISDN service user gets the full benefits of this exceptional service.
About the Author
John Williams is director of product marketing at Tele-Path Instruments. He has been with TPI for more than three years and in the telecommunications industry for more than 13 years. Williams holds a B.A. degree in economics from the University of Washington. Tele-Path Instruments, Division of TTC, 221 S. Yorkshire St., Salem, VA 24153, (540) 375-0530.
SIDEBAR
ISDN Delivers the Goods
From small business to residential customers, ISDN delivers the bandwidth for applications such as telecommuting, high-speed Internet access and LAN interconnect. ISDN allows you to do things that an analog modem just cannot handle—transferring graphical applications, large text retrievals, multimedia and imaging applications.
Introduced in the 1980s, ISDN was hyped then as much as it is now. Except for one thing: The product was good, but the applications were missing.
As a result, service providers did not make ISDN widely available to customers.
By the early 1990s, the standards and applications were developed enough to provide true value. But implementation was widespread only in Europe. It was not until the Internet explosion of 1994 and 1995 that service providers bought into ISDN domestically as an answer to high-speed Internet access.
So here we are again. The hype is back, but this time there are applications to back it up. Take a closer look at the technology that drives ISDN and a clear picture begins to emerge on how and why it can help personal or business communications. At five times the speed of a standard analog phone line, ISDN is not only faster than a 28.8-k modem, but it is also robust and reliable because it is digital.
ISDN comes in two interfaces: BRI and Primary Rate Interface (PRI). A BRI provides two 64-kb/s B-channels for data and voice communications and a third 16-kb/s D-channel used for signaling and secondary low-speed data transmission. A PRI bundles 23 B-channels for speeds up to 1.54 Mb/s with a 64-kb/s D-channel. All this occurs over standard telephone facilities.
So where does the extra speed and performance come from if we are still communicating over traditional copper phone lines? First, ISDN allows multiple digital channels to operate through regular phone lines by using a phone company’s digital switches. The same physical facilities are used, but a digital signal is transmitted instead of an analog signal. This provides a much higher data transfer rate than analog. The digital signal also is far less susceptible to noise and interference than the analog.
These two factors make downloading Internet pages or large text files a snap. And while ISDN’s bandwidth falls short of what is needed to transmit raw video, you can still take advantage of compressed video and multimedia currently available with streaming technologies. These technologies can supply acceptable frame rates over a 128-kb/s ISDN connection.
Whether or not ISDN lives up to its hype will not be determined for some years. There are competing technologies such as asymmetric digital subscriber line, cable modems and asynchronous transfer mode, which provide digital quality and even higher speeds than ISDN. But ISDN is far less expensive at this point. It all comes down to customer demand, pricing and implementation. When service providers strike the right mix, ISDN will be unleashed.
Copyright 1997 Nelson Publishing Inc.
May 1997
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