ADSL Modem Interfacing

Receiving and sending vast amounts of information is all well and good, but the data stream needs to be interfaced to the transmitting and receiving sections of the modem.

At the present time, modems are (in the main) equipped with either a V.35 or 10BaseT Ethernet interface.

This is all well and good if you are connecting to a Local Area Network (LAN), but what happens if the modem is going to hook into a network designed to pass voice signals as well as data.

The delays incurred in transmitting and receiving large 1024 bytes (1kbyte) frames will result in gaps appearing when speech is transmitted, so in these cases an Asynchronous Transfer Mode (ATM) interface is more appealing, owing to its much smaller frame size of 48bytes.

As a comparison, 1kbyte frames will result in delays of around 250ms, which is enough for full duplex speech transmission to become broken and stilted, hence the need to use ATM which with round-trip delays measurable in low milliseconds, which will allow a conversation to be carried without perceptible pauses.