As the ATM Forum sat down to debate the proposals for a 25Mbps Asynchronous Transfer Mode standard and whether one is even needed, PMC-Sierra Inc announced a capability for its SONET-based UNI-PHY specification, which it hopes will tip the balance in its favour. The new capability is said to enable users to incorporate voice or Basic Rate Integrated Services Digital Network services into Asynchronous Transfer Mode wiring, without any loss of bandwidth. Sierra plans to make the technology available freely, and was also planning to issue technical details at the Forum meeting. The technology works by mapping the ISDN/voice service into unallocated bytes in the SONET overhead, the Line Data Communications Channel: according to Vernon Little, Sierra’s manager of local network products, through the magic of numerology at 12.96Mbps, the lowest speed defined by the Forum’s Mid-Range PHY Specification, this provides 144Kbps of bandwidth, which is precisely the bandwidth needed for a Basic Rate ISDN service or two voice channels. Since the rival specification from the Desktop ATM25 Alliance does not have a synchronous frame structure, the same approach cannot be used there, said Little, and would require isochronous mappers to achieve the same effect. Sierra envisages two main applications for the approach: in an office environment, it would mean that just one wiring link would be needed to the desktop for Asynchronous Transfer and voice (by connecting the Asynchronous Mode wiring to the PABX); while in a residential environment, a pedestal-based broadband service unit could pull the ISDN or voice streams off a digital Loop Carrier or Optical Loop Carrier link, incorporate them into Data Communications Channel bytes, and then separate them off again into an appropriate line interface at the network termination unit – again resulting in just one set of wires into the home.
