4.3 How many clients can I support at the end of a pipe?

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Let us assume you have a group of users that work at the end of a 64-Kbps network link. Your design can place a server at the end of the link and either connect the server to an existing routing group or form a new routing group and connect via a routing group, SMTP, or X.400 connector. On the other hand, you can elect to connect the users into a centralized server. In most cases, designs seek to eliminate servers as far as possible in order to reduce ongoing administrative effort and cost. The question is whether the 64-Kbps link is big enough to provide reasonable response to the users at the end of the line.

The answer depends on three factors: the number of users, the type of work they do, and the software you deploy. Even if you do not have detailed analysis to back up designs, you can revert to some guidelines that are often used successfully in projects. For example, the classic rule of thumb for estimating MAPI client demand is as follows: Allow 3 Kbps for each "light" email user, 6 Kbps for every "medium" email user, and 10 Kbps for every heavy email user.

This is a simple rule that hides all sorts of complexity, since you then have to answer the question of how to define users as light, medium, and heavy. Generally, light users are people who receive far more email than they send and do not receive more than ten messages a day. Medium users are more balanced in terms of the send/receive ratio and generate up to 20 messages a day, along with some scheduling and other activities that create network traffic. Heavy users are online most of the time and probably deal with over 50 messages a day. These people tend to maintain their own distribution lists. Large distribution lists are not bad as such, but large personal distribution lists impose an extra penalty when the client expands their contents to add each recipient to the message header. This action increases the overall size of messages to a point where a simple message containing a couple of lines of text occupies 50 KB or more after all the individual recipients are in the message header. System distribution lists do not incur the same penalty, because Exchange keeps only a pointer to the list in the message header. Heavy users also tend to attach more documents than other users and are the people who explore the edge of the envelope when it comes to client options such as autosignatures. Every company varies, and you may find that your heavy users demand more than 10 Kbps.

Even the most detailed network analysis will not cover all possible situations. There are going to be times when a network connection slowly stops because of cumulative user load or due to an action taken by an individual. Early in the morning, when people log on at roughly the same time and connect to read new mail, is often a time of peak demand. Attaching a large file to a message can fully occupy a 64-Kbps link for some time. Obviously, other activities use up bandwidth too. Given the graphically rich nature of the Web, browsing often chews up much more bandwidth than you may realize, especially if users download files from the Internet.

The software factor is harder to quantify, because it depends on the intersection of user habits and corporate environment. If your user community is largely mobile, then the demands made on software are different from classic LAN-based populations, and it may be worthwhile to upgrade quickly to software designed to accommodate mobile connectivity, such as Outlook 2003. As explained previously, the success of Outlook 2003 varies depending on the version of Exchange the client connects to, so there are many different items to measure before you can get an accurate picture of the situation.

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