Emulating ATM network impairments in the laboratory

Hewlett-Packard Journal, April, 1997 by Robert W. Dmitroca, Susan G. Gibson, Trevor R. Hill, Luisa Mola Morales, Chong Tean Ong

This article discusses a new product for the HP Broadband Series Test System. The HP E4219 ATM network impairment emulator allows telecommunication network and equipment manufacturers to emulate an Asynchronous Transfer Mode network in the laboratory.

ATM (Asynchronous Transfer Mode) is a new networking technology that is currently being deployed for time-sensitive traffic in high-speed local and wide area networks. Information is encoded into short, fixed-length cells of 53 bytes, consisting of 48 bytes of payload and five bytes of header, as illustrated in Fig. 1.

[Figure 1 ILLUSTRATION OMITTED]

An ATM connection can have multiple virtual connections because of a two-part address field in the cell's header--the VPI (virtual path indicator) and the VCI (virtual channel indicator). These fields identify the channel to which the cell belongs and consequently the quality of service it will receive.

ATM has many advantages over other existing networking technologies, such as Frame Relay and SMDS. Its short, fixed-length cell allows switches and multiplexers to route data directly in hardware, as opposed to the slower, software-driven approach required with variable-length packets. ATM's bandwidth-on-demand feature means that a single network can carry all types of traffic--voice, video, image, and data. In summary, ATM is an efficient, flexible technology with the ability to support multimedia traffic at extremely high rates.

Since ATM network implementation is still in its preliminary stages, equipment manufacturers and service providers have a tremendous need for test solutions--in particular, a way to emulate ATM networks in the laboratory so that services and equipment can be tested before deployment. The HP Communications Measurements Division (CMD) has identified three major benefits that ATM network emulation can provide: risk reduction, design verification, and robustness.

Risk Reduction. Full deployment of any network is a very expensive proposition, especially since some planned networks are national in size. Even if a network is available to test, creating maximum delays or errors is difficult since these impairments occur only under congested or abnormal conditions. Network providers can use emulation to test their networks in a well-controlled environment.

Design Verification. Running time-sensitive voice and video data over a packet-switching network such as ATM is an active area of research. Because ATM networks have inherent delays, encoders must be able to handle both variable and fixed delays gracefully. Network emulation allows equipment vendors such as encoder manufacturers to verify their designs in the laboratory in a relatively inexpensive way.

Robustness. Network equipment manufacturers also need to confirm that their algorithms and designs work under stress. For example, some early ATM switches had buffers smaller than a single TCP/IP datagram. When they used AAL-5 (the ATM adaptation layer protocol selected to carry TCP/IP datagrams), the buffers overflowed and dropped cells. Because AAL-5 employs a high-layer, end-to-end error correction scheme, the AAL-5 packets could not be reconstructed and consequently the TCP/IP datagrams were also lost. Almost no data throughput occurred even when error rates were low. Network emulation detects these kinds of unexpected behaviors before they become costly mistakes.

Development Partner

Because CMD develops state-of-the-art test equipment for emerging telecommunication technologies, short development times are crucial. When CMD management made the decision to develop a network emulation module, it became clear that we would need a development partner with more experience in this area if we were going to ship on time.

We were very fortunate to find that Telefonica Investigacion y Desarrollo (TI D) in Madrid, Spain had previously conducted research in network emulation and was willing to cooperate with us to turn their research into a manufacturable product. TI D is the research and development arm of Spain's national telephone system.

TI D had developed a network emulator that showed some promise as a commercial product. However, the module's impairment specifications were not quite right for two reasons: the module was designed to interface to a proprietary TI D line interface, and it was not designed to be manufactured in volume. However, we were confident that we could use the basic technology of this board and redesign it to work in the HP Broadband Series Test System. Development of the HP E4219A network impairment emulator began in the fall of 1994.

Network Impairment Emulator

The HP Broadband Series Test System (BSTS) is a comprehensive platform for testing high-speed local and wide area networks. Designed primarily for laboratory research and early network deployment, the BSTS consists of a VXIbus mainframe, an HP-UX* controller, between one and 12 specialized VXIbus modules for different physical interfaces and convergence layers, and a variety of software applications for testing ATM, SMDS, Frame Relay, and other protocols.