Microsoft Windows 2000 Server Documentation: Choosing the Right Connection

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Windows 2000 Server Documentation


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

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

  1. Divide the connection speed per second by the estimated file size to get
    the following results:

    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
  2. 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

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