Bandwidth efficiency an important consideration

Over the years, one of the most important parameters in choosing technologies and equipment based on certain technologies has been efficiency. Simply put, efficiency is the percentage of bandwidth used for actual data transport as compared with the total bandwidth consumed – for both the data and the overhead – in the transport process. The bandwidth efficiency might also account for packet data efficiency.

Let’s look at some historical examples. Twenty years ago, we were seeing the dawn of a new era of data networking when end users started purchasing T-1 and E-1 services – which were originally designed for voice traffic – and using these services to transport data. The efficiency issue here was actually based on efficiency of TDM techniques. T-1, for instance, was, and for that matter still is, extremely bandwidth-efficient for transporting 24 channels of 64K bit/sec PCM voice. In fact, the transport is almost 99.5% efficient since 1.536M bit/sec of the 1.544M bit/sec transmission rate could be used for information transport.

However, the voice-centric formats, known as Superframe Format (SF) and Extended Superframe Format (ESF), were quite inefficient for data devices of the era. When using SF and ESF, 12.5% of the remaining bandwidth was lost to guaranteeing “ones density” in most cases, resulting in the standard transmission speed of 56K bit/sec rather than 64K bit/sec. And the efficiency got even worse for the typical (for the day) data device speeds of 9.6K bit/sec and 19.2K bit/sec.

A number of companies, such as Timeplex, N.E.T., and General DataComm, emerged with products that used a proprietary “flexible framing” format optimized for squeezing traffic more efficiently into a TDM stream. They also introduced some of the first sub-64K bit/sec voice algorithms.

All of this was driven by the price of WAN bandwidth. After all, at this point a coast-to-coast T-1 circuit in the U.S. cost on the order of $40,000 per month. And this was for bandwidth that is of the same order of magnitude that you get from a consumer DSL or cable modem service today.

Next time we’ll continue this discussion, moving on to packet technology.