Interfacing Serial Encrypters to ATM Switches


The diagram below shows a requirement to convert serial EIA530/TTL to OC-3 ATM, and thereby transport this across an ATM network.

The key technical problem was supporting the exact clocking rates specified by the satellite equipment which relates to the operating speed of the encrypter and end user equipment.

The AP3000 is used to convert the serial data stream from the satellite demodulator to ATM, for transport via an ATM switch across an ATM network. At the far end of the ATM Network, a second AP3000 was used to reconstruct the exact original clock rate to connect into the EIA530/TTL serial encrypter.

This is a typical Government system, and Metrodata were the only company that could provide a solution, since the EIA530/TTL module for the AP3000 can be slaved off the satellite demodulator. Satellite modems can support serial data rates with 1bps granularity and serial encrypters will slave off these. For a fully functionality solution the AP3000 not only had to provide a clock to 1bps granularity (which it can do for each direction independently), but it also had to exactly match the satellite modem data rate and clock phasing and pass this to the serial encrypter, so that no bit slips could occur in the data stream.

Other manufacturers of similar products support data rate granularities of 8kbps or more, and cannot exactly lock onto the satellite clock, and will therefore suffer regular bit slips in the data stream.

The Metrodata AP3000 that was used in this application could also have supported HSSI or ECL serial encrypter equipment (up to 100Mbps), and a wide range of ATM switch interfaces, e.g. E1, E2, E3, DS3 and OC-3/STM-1.

A simple diagram of this application is shown below.

ATM Network

Using larger members of the Access Processor product range, several serial connections can be multiplexed onto a single ATM Trunk. For example, we could multiplex up to 8 serial connections with EIA530 or HSSI interfaces into an E3, DS-3 or OC-3/STM-1 ATM switch. This is a very effective method of multiplexing, as our ability to exactly match the clock rates of the serial connections means that bandwidth on the ATM network is used more efficiently, and no loss of data occurs despite the use of multiple clock domains.

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