Bandwidth versus Distance Latency
Bill Alderson, Technology Consulting Officer, NetQoS, Inc.

The first step in evaluating an application to be used across long-distance circuits is to determine if the application is bandwidth or distance sensitive. The difference between these two types of latency is the size of the packets and the ability for the transport protocol to send bursts of packets. Determine the amount of time to transmit a packet on a link with this equation:

(Packet Size in Bytes * 8)/(Link Speed in Bits per Second)

The result is the number of seconds required to transmit this packet onto the link. This number will vary depending on the speed of the link between the two sites. If the application uses a few large packets that can be sent one after another without waiting for acknowledgements from the other end, an increase in bandwidth will reduce the application response time.

Distance latency is the time it takes to propagate the signal from one point to another. This time is fixed, depending on the medium used to transport the data. An increase in bandwidth will NOT reduce this time. If an application sends a packet and must wait for an acknowledgement before sending the next packet, the response time increases as the link between the server and client increases. In our application analysis, we see client/server applications that execute one SQL call and then execute another based on the data returned by the first. When you place processes serially, increasing bandwidth has little impact on improving response times.

By developing applications that allow transport protocols such as TCP/IP and IPX Burst Mode to do their jobs, you can deploy applications across the nation and the world with somewhat consistent response times. Increasing bandwidth is a cost you must pay each month, whereas a network application written with WAN circuits in mind is a one-time cost that results in monthly savings. Keeping Distance latency and Bandwidth latency in mind when developing, implementing, and analyzing WAN applications leads to WAN Friendly Applications and reduced monthly expenditures.