This is the single most important question to answer when starting a network design: why do you want to build a network? It sounds a little silly, but frequently people seem confused about this point. Often they start building a network for some completely valid and useful reason and then get bogged down in technical details that have little or nothing to do with the real objectives. It is important to always keep these real objectives in mind throughout the process of designing, implementing, and operating a network.
Too often people build networks based on technological, rather than business, considerations. Even if the resulting network fulfills business requirements, it will usually be much more expensive to implement than is necessary.
If you are building a network for somebody else, then they must have some reason why they want this done. Make sure you understand what the real reasons are. Too often user specifications are made in terms of technology. Technology has very little to do with business requirements. They may say that they need a Frame Relay WAN, or that they need switched 100Mbps Ethernet to every desk. You wanted them to tell you why they needed these things. They told you they needed a solution, but they didn't tell you what problem you were solving.
It's true that they may have the best solution, but even that is hard to know without understanding the problem. I will call these underlying reasons for building the network "business requirements." But I want to use a very loose definition for the word "business." There are many reasons for building a network, and only some of them have anything to do with business in the narrow sense of the word. Networks can be built for academic reasons, or research, or for government. There are networks in arts organizations and charities. Some networks have been built to allow a group of friends to play computer games. And there are networks that were built just because the builders wanted to try out some cool new technology, but this can probably be included in the education category.
What's important is that there is always a good reason to justify spending the money. And once the money is spent, it's important to make sure that the result actually satisfies those requirements. Networks cost money to build, and large networks cost large amounts of money.
So the first step in any network design is always to sit down and list the requirements. If one of the requirements is to save money by allowing people to do some task faster and more efficiently, then it is critical to understand how much money is saved.
Money is one of the most important design constraints on any network. Money forms the upper limit to what can be accomplished, balancing against the "as fast as possible" requirement pushing up from below. How much money do they expect the network to save them? How much money do they expect it will make for them? If you spend more money building this network than it's going to save (or make) for the organization, then it has failed to meet this critical business objective. Perhaps neither of these questions is directly relevant. But in that case, somebody is still paying the bill, so how much money are they willing to spend?
Geography is the second major requirement to understand. Where are the users? Where are the services they want to access? How are the users organized geographically? By geography I mean physical location on whatever scale is relevant. This book's primary focus is on Local Area Network (LAN) design, so I will generally assume that most of the users are in the same building or in connected building complexes. But if there are remote users, then this must be identified at the start as well. This could quite easily spawn a second project to build a Wide Area Network (WAN), a remote-access solution, or perhaps a Metropolitan Area Network (MAN). However, these sorts of designs are beyond the scope of this book.
One of the keys to understanding the local area geography is establishing how the users are grouped. Do people in the same area all work with the same resources? Do they need access to the same servers? Are the users of some resources scattered throughout the building? The answers to these questions will help to define the Virtual LAN (VLAN) architecture. If everybody in each area is part of a self-contained work group, then the network could be built with only enough bandwidth between groups to support whatever small amounts of interaction they have. But, at the opposite extreme, there are organizations in which all communication is to a centralized group of resources with little or no communication within a user area. Of course, in most real organizations, there is most likely a mixture of these extremes with some common resources, some local resources, and some group-to-group traffic.
The next major business requirement to determine is the installed base. What technology exists today? Why does it need to be changed? How much of the existing infrastructure must remain?
It would be extremely unusual to find a completely new organization that is very large, has no existing technology today, and needs it tomorrow. Even if you did find one, chances are that the problem of implementing this new technology has been broken down among various groups. So the new network design will need to fit in with whatever the other groups need for their servers and applications.
Installed base can cause several different types of constraints. There are geographical constraints, such as the location and accessibility of the computer rooms and LAN rooms. There may be existing legacy network technology that has to be supported. Or it may be too difficult, inconvenient, or expensive to replace the existing cable plant or other existing services.
Constraints from an existing installed base of equipment can be among the most difficult and frustrating parts of a network design, so it is critical to establish them as thoroughly and as early as possible.
Now that you understand what you're connecting and to where, you need to figure out how much traffic to expect. This will give the bandwidth requirements. Unfortunately, this often winds up being pure guesswork. But if you can establish that there are 50 users in the accounting department who each use an average of 10kbps in their connections to the mainframe throughout the day, plus one big file transfer at 5:00 P.M., then you have some very useful information. If you know further that this file transfer is 5 gigabytes and it has to be completed by 5:30, then you have another excellent constraint.
The idea is to get as much information as possible about all of the major traffic patterns and how much volume they involve. What are the expected average rates at the peak periods of the day (which is usually the start and end of the day for most 9-5 type operations)? Are there standard file transfers? If so, how big are they, and how quickly must they complete? Try to get this sort of information for each geographical area because it will tell you not only how to size the trunks, but also how to interconnect the areas most effectively.
In the end it is a good idea to allow for a large amount of growth. Only once have I seen a network where the customer insisted that it would get smaller over time. And even that one got larger before it got smaller. Always assume growth. If possible, try to obtain business-related growth projections. There may be plans to expand a particular department and eliminate another. Knowing this ahead of time will allow the designer to make important money-saving decisions.
Last among the top-level business requirements is security. What are the security requirements? This is even important in networks that are not connected to anything else, like the Internet or other shared networks. For example, in many organizations the servers in the Payroll Department are considered sensitive, and access is restricted. In investment banks, there may be regulations that require the trading groups to be separate from corporate financing groups. The regulatory organizations tend to get annoyed when people make money on stock markets using secret insider information.
The relationship between security and geography requirements may make it necessary to implement special encryption or firewall measures, so these have to be understood before a single piece of equipment is ordered.
Besides the business requirements, there could be philosophical requirements. There may be a corporate philosophy that dictates that all servers must be in a central computer room. Not all organizations require this, but many do. It makes server maintenance and backups much easier if this is the case. But it also dictates that the network must be able to carry all of the traffic to and from remote user areas.
There may be a corporate philosophy that, to facilitate moves, adds, and changes, any PC can be picked up and moved anywhere else and not require reconfiguration. Some organizations insist that all user files be stored on a file server so that they can be backed up. Make sure that you have a complete list of all such philosophical requirements, as well as the business requirements, before starting.