Voice over DSL is a derivative of Digital Subscriber Line (DSL) technology, which is currently being deployed in several countries worldwide. DSL enables the existing copper wires to transmit data at substantially faster speeds than possible by modem, typically up to forty times faster than an industry standard 56k modem. Voice over DSL extends DSL’s capabilities by encoding several voice or fax calls as data, and using the enormous bandwidth inherent in DSL to transmit up to 20 calls simultaneously with very high speed data. (VoDSL differs from ADSL – which also permits voice and data to be shared over the same phone line- in that ADSL as currently offered by Telstra transmits data only, and uses standard analogue telephony techniques to transmit voice signals. It is therefore limited to a single voice or fax call. In VoDSL, both voice and data are transmitted digitally, and multiple voice or fax calls may be made simultaneously.
VoDSL is deployed in the same way as other DSL technologies. A DSLAM (DSL Access Multiplexer) is placed in the telephone company’s exchange, and an IAD (Integrated Access Device) is installed in the customer’s premises.
An IAD typically has an Ethernet port, and between four and twenty telephone ports, to which the user connects standard analogue telephones (or a small PABX). The user experience when making a call is exactly the same as when using standard telephony: the only difference is that 20 or more calls may be made over the same line simultaneously.
VoDSL is only now being deployed in the US, and is led by competitive local exchange carriers (CLECs) such as Picus. Furthermore, although Covad, Northpoint and Rhythms focussed initially on data applications such as wholesale internet access, each has recently announced plans to adopt VoDSL.