Metrodata Limited

The drive towards converged network infrastructure means that there is often a requirement to transport legacy non-ATM applications such as PBX or broadcast video over a converged ATM network. Two options exist: either to replace or upgrade the legacy equipment to support native ATM interfaces which can be very expensive and disruptive, or to use an ATM Circuit Emulation (CE) DSU to provide the conversion function at an economic cost. ATM circuit emulation products are available for converting a wide range of legacy interfaces including E1, E2, E3 and DS3 to the common fibre and copper interfaces supported by ATM equipment including E3, DS3 and STM1/OC-3.

The ATM circuit emulation DSU is a 2-port device that segments the legacy traffic into a stream of ATM cells, which are then transported via a permanent virtual connection (PVC) across the ATM network. At the far end the ATM circuit emulation DSU recover the original clock and reassembles the original bit-stream. An integral ATM cell buffer allows for transient traffic bursts without loss of data. This segmentation and reassembly processes is defined in the ITU I 363.1 AAL.1 specification.

The ATM circuit emulation DSU has built in diagnostic loops which segment the link and assist in locating device, cabling or network faults. Extensive performance monitoring statistics are also gathered and stored for 24 hours. This provides a substantial database for assessing the performance of the network, which can be used to identify alarms, errors or data loss. Front panel LEDs also give indication of major and minor alarms.

The default configuration is sufficient to get most installations applications operational, making this a truly plug and play device. Configuration can be performed via a menu driven console port, or via an integrated LM1100 SNMP enabler port which offers substantial Telnet and SNMP capability.

The main fibre interfaces supported are as follows:

• 1300nm multimode offering transmission distances up to 5km over multimode fibre
• 1300nm singlemode short haul offering transmission distances up to 30km over singlemode fibre
• 1300nm singlemode long haul offering transmission distances up to 70km over singlemode fibre

Using fibre optics these products extend the operating distance for an STM-1/OC-3 interface to 5km using low cost multimode fibre, or to either 30km or 70km using singlemode fibre. Despite these quoted maximum operating distances, specific installations may only operate over lower distances due to the variations in losses of different types of fibre cable, the use of connecting fibre cables, or patch panels. When operating over 50% of the maximum distances it is advisable to calculate all fibre optic connector and cable losses and compare these with the fibre optic loss budget specified for this product.

As standard fibre ports are supplied with SC connectors and operate at a nominal wavelength of 1300nm. Other wavelengths are available on request, e.g. 850nm, 1550nm, 1550/1300 WDM. Other connector options are also available on request, e.g. ST, FC.

This application shows how a pair of ATM circuit emulation DSU products can be used to transport an E1 link between two PBX systems over an ATM network, even though the connections into the ATM network at either end are different.