So far, IAUS has demonstrated nothing with DWM modems in a real environment, over real telephone lines. Everything they have shown has been 2 boxes, wired back-to-back, and with a peak throughput of less than 60 Kbps, based on what I've heard. How about doing a demo over real telephone lines, say from Los Angeles to New York, and witnessed by legitimate telecom experts from outside the company? (If you do this over U.S. domestic circuits, you will probably be carried by fiber or microwave, eliminating the delays found in going over a satellite link.) Here's a simple test procedure:
(1) Use compressed image files of at least 200 Kbytes, such as JPEG or GIF pictures. (The reason for this type of file is to avoid any tricks such as using text files that can be compressed by the DWM modem. Data compression is a trick that most modern modems do, with methods such as MNP.)
(2) Do binary file transfers with some file transfer method, such as FTP or ZMODEM, that have fixed numbers of bytes per packet, and a fixed number of overhead bytes per packet, so that the total amount of data sent will be known. If the file is 200 Kbytes long, and the file transfer method breaks it into 1 Kbyte packets, with 16 bytes per packet of overhead, the total number of bytes to be transmitted will be 200 packets * (1024 bytes [data] + 16 bytes [overhead] = 208000 bytes.
(3) Measure the number of seconds it takes to do the file transfer. Divide the number of bytes by the number of seconds and multiply this value by 8 to get the bits per second. (In the unlikely (??) event that the interface to the DWM modem is through a PC's asynchronous COM port, you would multiply by 10 instead of 8, because the start and stop bits are added to 8 bits of data per byte.) So, our formula is:
bits per second [bps] = (bytes / time [in seconds]) * 8
After the signal gets digitized at the phone company central offices, there is a very low probability of adding additional distortion (bit errors), so any errors would occur because of the DWM waveform.
Now, the absolute speed limit on an unmodified telephone line is 64 Kbps, that is imposed by the analog-to-digital conversion at the telco central office. (The line is sampled at twice the analog bandwidth of 4 KHz (Nyquist sampling theorem), to produce 8000 samples per second * 8 bits/sample, which gives 64000 bits per second or 64 Kbps. Higher speed telephone interfaces such as ADSL or ISDN require a change of equipment at the telco central office. Many 64 Kbps channels are multiplexed onto higher speed trunks between central offices, such as T-1, which carries 24 channels on a 1.544 Mbps link, with additional overhead.
This is a simple fact of modern telephony, and apparently nobody at IAUS even bothered to do something as simple as going to Radio Shack, and buying a copy of their book, "Understanding Telephone Electronics" to understand why they'll never get anywhere close to their claim of 600 Kbps over an unmodified telephone line.
Paul McGinnis / PaulMcG@aol.com |
