T&L Publications

Hi.
Downloading files from the net, if the prompt shows "Transfer rate 4.75KB/sec"
Does it mean 47500 bits per second if no parity and 52250 bits per second if yes parity ?

The dialup connection speed showed "44000 bps", but I do not believe that being accurate at all, seen many akward figures sometimes. Does someone agree with the connection figures prompted being usually imprecise ?

Miguel

Miguel,

Transfer rate really is completely different than download speed. For example, I'm on 1.5MB DSL, but that doesn't mean it copies 1.5MB of data per second. In reality, my connection connects at 1.3MB, and I download files at about 160-170k/second.

4.75k is prettty darn good for dialup. I BELIEVE (check with other first) dialup works is the connection speed windows shows is the speed at time the connection is made. The speed may improve after the connection has been established. As long as that is correct, you could conenct at, say, 28k, but download files as if you were connected at 50k. Windows would still show 28k, though.

To estimate speeds for broadband downloads (I am not sure if the same applies for dialup due to diff packet sizes):

Divide your line speed (lets say its 5000 kilobits / sec) by 8, you should get the theoretical your theoretical max download speed in kilobytes.

So:
5000 kilobits / 8 = 625 kilobytes (which means theoretical max download speed will be 625kilobytes / sec)

Notice that the normal line speed units are in megabits and the normal download unit speed is in kilobytes.

Harrison

no one gets the theoretical DL rate. There are multiple things that affect DL speed - noise, server utilization, packet overhead, there are multiple links you have to share with others and so on.

Frankly, I just wouldn't worry about.

Thanks fellows.

I meant if that "4.75 KB/sec" download speed reported in my dialup connection corresponds to:

4.75 times 10 (for a byte with 8 bits + 1 start bit + 1 stop bit) and times 1000 for the Kilo prefix would actually mean 47500 bits per second.

In the parity case, 4.75 times 11 (for a byte with 8 bits + 1 start bit + 1 stop bit + 1 parity bit) and times 1000 for the Kilo prefix would actually mean 52250 bits per second.

Are these numbers correct? I do not think a simple 8 bit byte is to be taken in account here.

How is it known/chosen if the download bytes use the parity bit or not?

And, is the K prefix in this case 1024 instead of 1000?

Just trying to clarify basic concepts. Nothing to worry about.
Miguel

"k" in the case of bit rate is 1000, not 1024.

If memory serves me correctly, when parity is used it "steals" one of the 2 stop bits so you end up with 10 bits total for the transmitted byte.

Modems usually have hardware compression. If you're downloading a zip file or a jpg image you'll usually not benefit from compression, but if you're downloading an exe (slightly compressible) or text (heavily compressible) you can get rates higher than the theoretical throughput. Usually your serial port is the limiter at 115kbps.

Here's how I calculate:
If you connect at 44,000 bps you will get exactly 44,000 bps, or (44,000bps/8b/B) = 5.5KB/s (note B not b).

However, the IP protocol uses a little bit of this bandwidth and then depending on the protocol used to download (TCP or UDP) some more is used. Not counting compression, I generally divide by 10 to get a close ball park.

I've noticed that modems tend to do better than broadband and getting close to the theoretical throughput of the link. I have two theories on why this is so, and I think they both apply:

#1 - With modem connections, more effort has been taken with the service providers equipment vendors to squeeze every ounce of the link's efficiency since with such slow connections, even 200b/s improvement is major.

#2 - A DSL/Cable modem can easily saturate a moderately loaded server's bandwidth. For example, a server with a T3 connection may limit throughput to .5 - 2MB/s per connection to keep from getting hit with extremely expensive bandwidth charges (T3 connections are usually paid for by their 95% usage rate and exceeding your agreed upon estimate is very very very expensive). Therefore your 3MB/s cable modem will only download from them at 2MB/s.

Oh, and number 3 is that mismatched MTU settings at one time were very common. Having mismatched MTU settings on the ethernet interface and the broadband modem causes a lot of packet fragmentation which will have a serious impact on download speed. Since Windows 2000 came out it has not been an issue for most people. If you're using Windows 98, there are some registry patches that will change your settings that will give you a dramatic impact on broadband speed. I think if you google for speed booster broad band registry setting you might find what I'm referring to.