ATM runs rings around Cambridge University

ATM runs rings around Cambridge University


Computer-savvy students embrace the idea of obtaining information from online systems, the Internet and campus communications links. With a combination of an Asynchronous Transfer Mode (ATM) network and high-speed fiber-optic technology, that concept is one step closer to reality at Cambridge University in England. This technology promises to usher in a new generation of interactive distance learning.

To demonstrate how multimedia and high-speed communications systems can be tied together to allow distributed interactive language training, a consortium of vendors, including Ascom-Tech AG in Bern, Switzerland, and IBM in La Gaude, France, has developed a high-speed network that spans several departments at Cambridge University.

Funded by the European Union as part of its Research into Advanced Communications Environments (RACE) program, the innovative Hipernet ATM project (see figure) uses a gigabit-per-second ATM broadband network that interconnects remote personal computer-based multimedia platforms.

The aim of the project was to develop a system that supports direct communication among students located in different colleges of the University. Using a high-speed network enabled two-way interactive communication between students and tutors.

According to Thomas Martinson of Ascom-Tech, providing individual multimedia material for students requires a high-capacity multimedia network capable of serving a large number of terminals simultaneously.

Broadband solution

The Hipernet project offers a complete broadband networking solution, including transmission switches, a server and multimedia applications to meet the needs of the Language Centre at the University. The Ascom Strategic Technology Unit designed the ATM broadband part of the network around its proprietary ATM products.

The network delivers multimedia material and applications throughout the campus and supports both videoconferencing and screen-sharing applications. Users are connected to the network through standard 486-based multimedia PCs.

Because reasonably fast computers are required to handle the chores of real-time encoding and decoding of video and audio data, 66-megahert¥PCs were chosen. These computers are configured into the ATM system through an IBM 25.6-megabit-per-second ATM interface. Two Ascom ATM switches link the PCs across the fiber-optic ring and into an IBM server that stores the multimedia data.

Fortunately for the system developers, a large, high-speed fiber-optic network-- the Granta Backbone Network--had already been installed at the University and connects colleges at distances to 10 kilometers. For the RACE project, the Hipernet ATM network was built directly on top of the Granta network.

The completed Hipernet network comprises an optical ATM transport network running at 1.2 gigabits per second, with access nodes that function as add/drop transfer multiplexers and optical access node point-to-point optical links. ATM cells are multiplexed into Synchronous Digital Hierarchy frames at 622 Mbits/sec, which are then output onto optical fiber.

The 1.2-Gbit/sec network routes the traffic on a cell-by-cell basis, according to the ATM cell header values. A ring topology is used for this network; cells are extracted from the traffic stream by the optical access nodes and either dropped out to an ATM switch or reinserted into the optical ring. A dual redundant ring topology provides resilience.

ATM a perfect match

The Hipernet physical network was implemented using an ATM system because ATM offers the ability to transport multimedia data streams, including voice, video and data. Furthermore, different service qualities can be assigned to these data streams. For videoconferencing and audioconferencing, near real-time performance is needed from the network, but for other services--the retrieval of stored data from the server, for example--longer delays could be tolerated. Servers may require hundreds of megabits per second of system bandwidth, while user workstations may use only a few megabits per second.

Nevertheless, because delays have to be small and predictable throughout the network, ATM provides a perfect match for applications such as data retrieval. The network transports ATM information from the workstation to the high-speed optical backbone. Two alternative implementations were used: one ran at a data rate of 622 Mbits/sec and the other at 1.2 Gbits/sec.


The network transports three main applications: French Means Business courseware, an associated multimedia dictionary and a help desk. According to Martinson, the multimedia material was encoded with the Motion Picture Experts Group protocol whenever possible to optimize bandwidth use while ensuring the picture and sound quality users require.

The language course multimedia material is provided by an IBM server capable of supporting a number of these video streams simultaneously. The multimedia communications system has two layers: a lower layer encompasses the ATM network and its management facilities, and an upper layer comprises the server and workstations with associated software. On both the server and the terminals, the TCP-UDP/IP protocol stack is used for communications over ATM. The server is implemented in two different parts: a 486-based backend server in charge of administrative functions and a backend IBM RS/6000 machine responsible for workstation video retrieval.

The Hipernet ATM project was successfully completed at the end of last year. A follow-up project known as "Leverage" promises to be even more challenging. Based on similar ATM technology, it aims to provide a link connecting Cambridge, Paris and Madrid. q

Dave Wilson writes from London.

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