[This is preliminary documentation and subject to change]

# Choosing the Right Connection

You can choose the right connection for your situation by mathematically figuring the average size of your content files, the amount of time files of that size take to send, and the number of simultaneous users you want to support. This topic shows you how to perform the calculations necessary to select the right connection. For convenience, a utility is available to do these calculations for you. For more information, see Calculating Connection Performance.

## Determining Connection Type Based on File Transmission Speed

You can select a connection type based on the size of files you will be sending to users and the amount of time users are willing to wait to receive a file. Generally, HTML text pages should load within five seconds. External files, such as graphics or video, should load within 30 seconds. If you will be using modems, be aware that you must also factor in the time it takes for a data packet to make the round trip between the user and your server. Modem connections take nearly one second per connection, a significant amount of time. Faster leased-line connections take 0.1 or 0.2 seconds, which is not significant and therefore not included in calculations.

To estimate file size

1. Multiply 8 bits per character X 80 characters per line X 66 lines per page = 42,240 bits per page.
2. For every 8 bits of data transferred, there are 4 bits of overhead. Multiply 42,240 bits per page X 1.5 bits used to transfer 1 bit of data = 63,360 bits per page transferred.

Note This calculation assumes a solid text page with no graphics, most typically seen when converting text documents to HTML format. The typical home page is generally much less text intensive and is probably closer to 24,000 bits per page including overhead. However, the typical home page may also contain one or more graphics files, each requiring connections between the client and the server.

To estimate transmission time

Divide the connection speed per second by the estimated file size to get the following number of pages transmitted per second:

Connection Type | Pages Transmitted
--- | ---
Dedicated PPP/SLIP | 0.3 to 0.6
56K (Frame Relay) | 0.9
ISDN (using PPP) | 1.7
T1 | 24
T3 | 710

For modems, add additional transmission time of one second to open the connection. This means that a modem would take from 2.5 to 4 seconds to transfer one page, depending on the modem speed.

To determine the number of possible connections per day on a T1 line

1. Divide the 1,540,000 bps connection speed by the 12 bits per byte transmitted to equal 125 KB per second transmitted.
2. Multiply 125 KB per second X 86,000 seconds per day = 10,777,994 KB per day transmitted.
3. Divide 10,777,994 KB per day by 1,048,576 (1024 KB per MB X 1024 MB per GB) = 10.3 GB per day transmitted.
4. Assuming an average file size (text and graphics) of 25 Kb, divide 10,777,994 KB by 25 KB = 431,000 hits per day.

Note This calculation assumes a T1 connection dedicated to your Web server only.

To determine the number of simultaneous users a connection type can support

1. Assuming that you want to stay within the five-second transmission time for a page of text, and assuming a text file size of 63,360 bits transmitted for the user to receive the page, divide the 63,360 bits by 5 seconds = 12,672 bits per second per user.
2. Divide the connection speed by the bits per second per user. For instance, for a T1 line, divide 1,540,000 bps by 12,672 bps per user = 121 simultaneous users on a T1 connection.

Note This assumes text-only pages with no graphics and a complete page transmitted within five seconds. If more users attempt to connect, they are not refused, but the transmission speed of files may drop well below the five-second recommended time. The following table provides guidelines for the number of users expected to be supported by a connection type.

Number of Simultaneous Users Supported

Connection Type | Users Supported
--- | ---
Dedicated PPP/SLIP | 2-3
56K (Frame Relay) | 10-20
ISDN (using PPP) | 10-50
T1 | 100-500
T3 | 5000+

© 1997-2001 Microsoft Corporation. All rights reserved.