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6D. Technical Infrastructure:

Key Concepts

  image quality




Speed and capacity issues are determined by a multitude of factors. Some are within your control, some are not. As with so many other performance issues, avoidance of bottlenecks is an important objective. Network transmission is governed by the slowest link. Factors affecting network delivery include:

  • Carrying capacity (bandwidth) of the local area network
  • Bandwidth of the institution's Internet connection
  • Speed and capacity of the network server
  • Read speed and data transfer rate of storage devices
  • Image file size
  • User demand at any particular time
  • Amount of competing network traffic (at all network levels)
  • Speed of any "on-the-fly" processing steps
  • Time required for authentication and other security checks
  • Capabilities of the end user's computer, including:
    • CPU speed
    • RAM/disk caching
    • Video subsystem performance
    • Speed of Internet connection

There are a variety of network technologies that might be encountered between an image server and the ultimate recipient. The following table presents some of the more important ones, in declining order by speed, in MB/second.

Table: Network Data Transfer Rates

Network Type
Speed in MB/sec
OC-48 (Abilene backbone)
1000BaseT Ethernet
vBNS (NSF/MCI backbone)
100BaseT Ethernet
DS-3 (T-3)
10BaseT Ethernet
Cable modem (downstream)
ADSL (downstream)
.19 -1
DS-1 (T-1)
ISDN (home use)
v.90 modem


The fastest of these networks are used only for major Internet backbones. The next tier are local area networks, while the slowest are consumer services. The speeds given are theoretical maximums, which are rarely, if ever, encountered in real installations. Note that the fastest network is more than 175,000 times faster than the slowest.

Once one knows the transmission speed of a network it is possible to compute the approximate time it will take a file of any particular size to make its way across. Use this formula:

Formula on Transmission Speed
t (time in seconds) = number of megabytes in file ÷ (transmission speed (in MB/sec) x .8)

A 1 MB file can theoretically make it across a 10BaseT Ethernet network in 1 / (1.25 x .8) = 1 second. The .8 takes into account that 80% of rated speed is about the best one can expect to realistically encounter. Since most networks share bandwidth amongst users, the more traffic they handle, the lower the overall transmission speed. When saturated, performance can fall dramatically.

Reality Check

Using the formula for transmission speed and the table of network data transfer rates shown above, calculate the least amount of time it will take a 1 MB file to be accessed via 100BaseT Ethernet and v.90 modem (round to the nearest tenth second).
seconds (Ethernet speed)
seconds (v.90 modem speed)

How much faster is 100BaseT Ethernet?
times faster

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