Paul Zawada writes:

[snip ...] You can only fit 1.536 Mbps of real data down a T1 unless you've figured out a funky way to send it across using the framing bits.

8 b per channel * 24 channels per frame * 8000 frames per s = 1536000 bps 28.8 kbps * 53 = 1.526 Mbps 28.8 kbps * 54 = 1.555 Mbps

So technically they should have used the numbers 52 and 53. :-)

There is more to this than meets the eye -- 28.8K is asynchronous and has start and stop bits for every byte, so there are a maximum of 2880 bytes/sec available over 28.8K. Then there is the issue of latency and IP overhead which tends to approximate 89% of the available bandwidth, or 2880 x .89 = 2563 bytes/sec. The T1 circuit normalized to bytes/sec is (1536000 / 8) = 192000 bytes/sec, so accounting for apples and oranges shows 192000 / 2563 = 74.91 sessions. Ofcourse, this assumes that all 75 28.8K modems are pulling data in the same direction at the same time. In reality, traffic for most ISPs runs about 4:1 to 5:1 inflows vs outflows. Because T1 circuits are full duplex (1536000 bps in two directions at the same time), and assuming a 4:1 ratio of inflows to outflows, this would allow 75 + (75 / 4) = 93 28.8K connections. Fortunately, customers don't actually saturate the lines the entire time they are connected -- some actually pause to read the screen once in a while -- so the real usage on the line has a tendency to approach 8:1, 10:1, or even more sessions. With a conservative number of 8:1 inactive to active sessions, 8 x 93 = 744 sessions. The Starr Report and other nonsence have a tendency to blow these types of calculations out of the water every now and then, but then no provider has built their network on the worst case scenareo. YMMV. Dave Stoddard dgs@us.net

Miquel van Smoorenburg writes:

In article <199809182139.RAA10174@us.net>, David Stoddard <dgs@us.net> wrote:

...

There is more to this than meets the eye -- 28.8K is asynchronous and has start and stop bits for every byte, so there are a maximum of 2880 bytes/sec available over 28.8K. Then there is the issue

Almost every modem supports V42 error correction, which makes the modems speak a sort of synchronous with each other (actually data is transmitted in blocks with a start-of-block and end-of-block marker, and a checksum). That gets you 8 bits in a bith minus some negligeble V42 overhead. So an 28k8 modem can actually transfer almost 3.6 Kbytes/sec. Because of the block-oriented approach you do get a bit higher latency on interactive connections, which is why gamers often turn of V42.

As long as we are being specific about this, I omitted data compression too, which has a real effect on the data transfer rate as well. I also omitted 56K modems, ISDN dial-up, and other aspects of dial-up communications (like MPP). The point I was trying to make was not to disassemble 28.8K modem protocols, but to point out the fallacy in believing that a T1 line can only support 53 modems. As far as the start and stop bit issues go, they are controlled in the UART on the PC serial port, not the modem. Regardless of how the modem encodes and transmits data over the wire, the start and stop bits remain in the asynchronous serial protocol on PCs -- years ago, PC terminal programs used to let you control the number of start and stop bits you used to communicate with a device. The fact that DCE/DTE rates are usually higher that the actual speed of the modem connection negates some of the effect of start and stop bits, but they do exist. For modem to modem communications, start and stop bits were used up through 2400 baud when one bit = one tone. The method of one bit = one tone was dropped for other forms of encoding when modems started supporting connection speeds in excess of 2400 baud. While the 8000 bit/sec sampling rate of POTS lines can theoretically support up to 4000 Hz bandwidth, it is technically impossible to get this (reliably) out of the analog phone system. 28.8K modems need at least 3200 Hz of bandwidth on a POTS line to support 28.8K. All said, I still stand by my comment that 53 modems per T1 is ludicrous. Dave Stoddard dgs@us.net

At 11:22 AM 9/22/98 , you wrote:

For modem to modem communications, start and stop bits were used up through 2400 baud when one bit = one tone. The method of one bit = one tone was dropped for other forms of encoding when modems started supporting connection speeds in excess of 2400 baud.

I assume you mean 2400 bps modems, i.e., v.22bis? Admittedly, it's been a while since I used one, but my recollection is that 2400 bps modems operate at 600 baud, with 4 bits/baud using QAM.

All said, I still stand by my comment that 53 modems per T1 is ludicrous.

This, without a doubt, is true. :) -- Robert Szarka Managing Partner, Operations DownCity, LLC +1 860 823 3000