PDF of this tutorialATM technology is being designed to perform a complex set of functions. The rapid pace of ATM development has resulted in publication of specifications that are indefinite, subject to interpretation, not always backwards compatible, and, in some cases, prove to be impractical to implement. Regardless of these difficulties, manufacturers are meeting these challenges by proceeding with product development—often by making assumptions about future ATM services.
This challenge extends to testing ATM products and ATM networks. Initially, conformance testing must be performed to verify that new ATM equipment works according to a given specification. Following this demonstration, the equipment must be proven to interoperate with other equipment in an ATM network. Finally, performance testing is required to establish product benchmarks such as throughput and latency.
Conformance Testing
Conformance testing is used to validate a specific product according to a standard or specification. The equipment is always tested under out-of-service conditions. The ATM test technician must simulate an operating environment by generating traffic flows or emulating specific protocols. This should include simulating valid traffic flows and network errors to ensure that the device functions correctly under a range of operating conditions.
Figure 9 describes a conformance test case for a signaling protocol to establish a call.
Interoperability Testing
Interoperability testing must be performed to ensure that products from multiple vendors will function properly together in a network. With ATM specifications being constantly updated to meet the increasing demands of users, interoperability testing is the most critical phase of ATM system verification.
Performance Testing
Performance testing provides essential measurement criteria for evaluating ATM switch operation. Conformance verification alone does not guarantee that the same equipment will operate satisfactorily under all anticipated network conditions.
Testing Challenges
There are three main reasons why ATM technology is currently very difficult to test:
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ATM is an emerging technology—Products are being developed based on specifications that may have been drafted based only on theoretical projections. As actual working models are produced, the specifications must be modified to match the realities of contemporary production methods. It is expected to take a number of years before ATM reaches a stable state comparable to today's local-area network (LAN) technology.
- complexity of the standards and specifications—The concept of ATM appears relatively simple: a protocol designed to provide high data throughput at negotiated rates (services). The primary function of an ATM switch is to receive a 53-byte ATM cell on an input port and then retransmit the same cell on a second port based on the VPI and the VCI. The high degree of complexity is introduced from the numerous control protocols required to set up and maintain these VPI/VCI pairs. Many interoperability problems are caused by timing relationships between these signaling protocols. Another factor is support for multiple priority levels for different traffic types based on service classes (i.e., UBR or ABR).
- optional parameters—Since ATM has widespread appeal for many diverse traffic types and applications, the specifications contain many optional parameters. If switches are connected together which support different optional features, they may have problems working together despite the fact they both pass complete conformance tests.
Migration of the Standards
Interoperability testing is critical for ATM because of this extensive migration of the standards (see Figure 10). The perceived slow pace of activities at the international standards bodies (i.e., International Telecommunications Union–Telecommunications Standardization Sector[ITU–T]) has prompted companies anxious to develop cutting-edge ATM products to create the ATM Forum. The purpose of the ATM Forum is to expedite development and deployment of ATM products and services. The ATM Forum has developed an aggressive schedule for the rapid development of ATM specifications (not standards) based on a two-thirds (members) majority rule for approval. These specifications are often treated as de facto standards even though all issues are not yet resolved.
However, the ATM Forum is not a standards body—it only issues specifications. These specifications are supplied both for implementing products and to influence the ITU–T. The ITU–T is under no obligation to implement standards that are compatible with any specification issued by the ATM Forum. The migration of user network interface (UNI) 3.0 signaling to UNI 3.1 is one example where the specifications were not made backwards compatible (i.e., UNI 3.0 equipment will not interoperate with UNI 3.1 equipment).
Summary
