Easing growing pains with TR-303, DLC systems based on TR-303 can simplify network expansion if the right test procedures are in place

Fueled by the demand for faster Internet access, ISDN has experienced a surge of growth. Unfortunately, the boom in ISDN delivery has created numerous difficulties with pricing, equipment and installation. However, one solution expands the ability of carriers to deliver ISDN service to the customer in a cost-effective, reliable manner-TR-303.

Based on Bellcore's TR-NWT-000303 specification, TR-303 technology can streamline carriers' digital loop carrier (DLC) systems. It defines an interface between a central office switch and a remote terminal to handle all call processing and operational functions for the DLC system. In comparison with today's most common DLC interface-SLC 96-TR-303 is a more powerful and versatile interface supporting a larger number of subscribers and more transport bandwidth (Table 1).

TR-303 is a protocol-driven interface. The two main protocol channels in TR-303 are the time slot management channel (TMC) and the embedded operations channel (EOC).

The TMC handles call processing operations. The DS-0 circuit for a call is dynamically assigned on demand within milliseconds after the customer goes off-hook, and the call processing functions then operate normally. The EOC is the workhorse that handles provisioning, performance monitoring, maintenance operations and alarm reporting.

The TMC and EOC protocol channels each occupy DS-0s within a DS-1 trunk, and each is backed up on another DS-1. Essentially, four DS-0s are "robbed" for the TMC and EOC datalinks and their backups.

Both the TMC and EOC use the D-channel link access protocol (LAP-D) for layer 2 operations.

For the TMC, the layer 3 protocol is a derivative of the ISDN layer 3 protocol based on the ITU-T's Q.931 specification. The majority of the message types and message elements are the same as those in ISDN.

The EOC protocol uses common management information services/remote operations service element (CMISE/ROSE) for the application layer. Between CMISE/ROSE and the LAP-D layers, there is a TR-303 specific protocol called the convergence function (Table 2).

TR-303 and basic rate ISDN Most local exchange carriers and competitive access providers have either begun deploying TR-303 or are considering it. Some major switch and remote terminal manufacturers are already shipping

TR-303 versions of their products, and some test equipment vendors now offer TR-303 capable test sets and protocol analyzers. The motivation for adopting TR-303 stems from its ability to improve basic rate ISDN operations and transport.

Figure 1 illustrates the network architecture for basic rate ISDN transport over a DLC system using TR-303 and SLC 96.

A carrier must use three ISDN U interface cards to deliver basic rate service to a customer served by a universal DLC system such as SLC 96. One U interface card is in the switch, and two are on basic rate interface transmission extension (BRITE) cards residing in a CO terminal and a remote terminal.

If a SLC 96-type system is integrated-i.e., if the DS-1s are fed directly into the switch-the carrier must groom out three DS-0s for the customer. This requires a "hairpin" configuration that consumes valuable switch ports. In addition, a U interface card and two U interface/

BRITE cards are required. Using TR-303, however, the carrier can process the ISDN interface directly from the DS-1, eliminating the need for multiple U interface cards and the need to hairpin DS-0s, which frees up valuable switch ports.

SLC 96 supports 96 subscribers over four DS-1s without concentration or over two DS-1s using a 2:1 concentration ratio. TR-303, however, has no restriction on the number of subscribers per number of transport DS-0s. TR-303 enables a carrier to choose a concentration ratio that best meets its traffic engineering guidelines.

This flexibility is essential, given the tremendous growth in residential second telephone lines requested as a result of the Internet and in-home offices. With TR-303 systems, a carrier can more easily add customers without adding transport bandwidth.

When compared with SLC 96, the TR-303 interface also offers improved operations, including provisioning, performance monitoring, and alarm reporting and retrieval. To carry out operational functions, the SLC 96 interface uses one of the DS-1 datalinks, which has very limited information transfer capability. It offers no provisioning or performance monitoring capabilities, and the alarm classifications include only major, minor, and miscellaneous alarm types.

Test equipment A service provider planning to deploy TR-303 will need two types of test instruments to effectively evaluate, install and maintain its TR-303 equipment: a TR-303 protocol analyzer and a TR-303 field test set.

Initially, product evaluation engineers and upper-level technical support engineers will need a TR-303 capable protocol analyzer. It should support the capture, decoding and analysis of both the TMC and EOC protocols, as well as report and timestamp every message transmitted by the switch and remote terminal. Ideally, the analyzer should decode multiple protocols simultaneously. And because the TMC carries the call processing information for the entire system, the analyzer must be able to filter out specific calls or message types.

To assist engineers in troubleshooting TMC problems, the protocol analyzer should be able to automatically report the number of call attempts and connections, as well as any abnormal call releases. This data should be stored in a captured trace file and log from which engineers can monitor event trends in the network, tracking incompatibilities and focusing on problem areas.

It is also desirable for the TR-303 protocol analyzer to have dual port capabilities with simultaneous monitoring of both active and standby data channels. These capabilities are essential for testing the path protection switching operations. Because path protection switching is non-revertive, the TMC and EOC often can reside on two separate T-1s. With dual port capability, the analyzer is capable of troubleshooting on one line while monitoring the other.

Once the TR-303 equipment has been evaluated and is ready for deployment, it is handed off to CO and DLC technicians responsible for installation and maintenance. These technicians typically need to know call status, the DS-1 or DS-0 to which a call has been assigned, why a call was dropped and which entity-CO or remote terminal-dropped it. A TR-303 field test set can answer these questions by performing simple call-tracing tasks.

Because most CO and DLC technicians need to support multiple services and technologies, a TR-303 field set also should contain a variety of other DS-1 and DS-0 test capabilities. Some test equipment vendors are adding TR-303 test capability into multifunction DS-1/DS-0 test sets. Together with accurate protocol analysis in the CO, these basic transmission test capabilities constitute a thorough and effective TR-303 testing strategy.

TR-303 offers carriers significant enhancements over SLC 96. As carriers deploy TR-303 systems, however, they must also equip their engineers and technicians with the proper equipment to support installation and maintenance activities.

Stephen Zielke and Jonathan Casey are Product Marketing Managers for Telecommunications Techniques Corp., Germantown, Md.