THE DIGITAL NETWORK AND 56K

By Dave Wallace

RIVER BENDER - July 1999

The public switched telephone network, originally designed for voice traffic, has undergone great changes since the Bell System breakup in 1984. Today, practically all calls use digital facilities once the call leaves the serving central office. This fact has enabled data speeds to increase dramatically. Not many years ago, when the world was mostly analog, 1200 bps was considered about the fastest speed one could expect over the switched network. Gradually speeds increased as facilities and modems got better but the big jump to speeds approaching 56 kbps came mainly because of the proliferation of digital facilities. Analog communications are now only a small part of the overall link, most notably in the subscribers' lines to the serving central office.

What are digital facilities? They are circuits that use pulse code modulation (PCM) to "quantize" your voice or data. Basically you start with a 4 kHz analog voice channel out of your telephone. Your signal, be it voice or data, is sampled 8000 times per second and each sample is converted to a number (the "quantization" process) that is represented by 8 bits. Eight bits times 8000 samples per second equals 64 kbps (kilo-bits per second) which is the basic channel of a PCM system. Multiplexing 24 such channels into a single digital stream on a pair of wires and you have a T-1 carrier system, which is what most Internet Service Providers (ISPs) use to connect your PC through to the Internet. The bit-rate of a T-1 carrier system is 1.544 mbps (24 times 64 kbps plus 8-bits for framing).

It makes no difference if the analog signal is data or voice. The sampling remains the same. You may have thought that your 56 kbps modem was digital but its output is really analog using phase modulation. Between the PC and the modem is digital ones and zeros represented by voltage changes but the output of the modem to the telco central office is analog frequencies at a constant voltage.

Why is digital better than analog? It is because noise has little effect on a digital signal. On analog facilities the noise builds up as the signal progresses through circuits and amplifiers. But when a digital signal is regenerated along the way it is restored to the original signal.

Ideally a data circuit would be 100% digital from a PC to the destination. If it were, it might be called an ISDN (Integrated Services Digital Network) circuit that would operate at 64 kbps or with two circuits at 128 kbps. But ISDN is very expensive so what most people have instead is a 56 kbps V.90 analog modem that doesn't run at 56 kbps but tries real hard given the circumstances and the transmission parameters of your existing telephone line.

Let's talk about your 56 kbps modem so you'll understand why it doesn't run at 56 kbps. First of all, it was never intended to run at 56 kbps in transmitting data to the ISP. Under the best of circumstances it was designed to transmit only 33.6 kbps in that direction. You don't need lots of speed to the ISP anyway because you're only typing comands or clicking your mouse. Where you really want speed is when you're receiving web pages and downloading files. But even then you'll never get 56 kbps because the FCC levies a power restriction on the manufacturer that says it cannot exceed 53 kbps. And even if you lived next door to the central office it is doubtful that you'd even get 53 kbps because the received data would probably go through at least one digital to analog (D/A) conversion that reduces bit speed.

Converting analog to digital data and vice versa is what mainly reduces the bit speed of digital data. Ordinarily over common facilities the maximum speed is 33.6 kbps because there's at least one A/D and D/A conversion in each direction of transmission. The reason you're now able to approach 56 kbps in receiving data is because the ISP's 56 kbps modem is different than yours, including the line it's connected to which is a special digital line to the telco central office. In the receive direction there is no A/D conversion required for the ISP to send you data. Only a D/A conversion is required on the last link into your modem. Eliminating the A/D conversion is what allows the speed to increase beyond 33.6 kbps. Unfortunately, however, the best speed most people can expect is somewhere in the 40-50 kbps range due to other transmission impairments. This arrangement requires the ISP to order a special line to the telco that is digital in one direction and analog in the other which is the reason you must dial into a different telephone number to connect to the ISP's 56 kbps service.