Digital Imaging Tutorial

6D. Technical Infrastructure:
DELIVERY

Key Concepts

introduction
networks
  concerns
  speed
  trends
monitors
  evaluation
  image quality
printers
  technologies
  evaluation

NETWORKS: SPEED
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-192	1250
OC-48 (Abilene backbone)	300
1000BaseT Ethernet	125
vBNS (NSF/MCI backbone)	77.8
FDDI	12.5
100BaseT Ethernet	12.5
DS-3 (T-3)	5.6
10BaseT Ethernet	1.25
Cable modem (downstream)	.2-.5
ADSL (downstream)	.19 -1
DS-1 (T-1)	.19
ISDN (home use)	.018
v.90 modem	.007

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)

Example:
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.