For example, it might be easy to determine that the network must be able to support up to 100 Mbps of bandwidth, but you need to know how, when, and where that bandwidth is used.. Determ
Trang 1When assessing needs, you are trying to come up with detailed answers to the following questions:
N How much storage space is required?
N How much bandwidth is required?
N What network services are required?
N What is the budget for the project?
These basic questions are fairly easy to answer as a whole, but you need to break them down further to make sure no holes in the network design could lead to problems For example, it might be easy to determine that the network must be able to support
up to 100 Mbps of bandwidth, but you need to know how, when, and where that bandwidth is used If the accounting department is using 90 percent of the bandwidth when communicating to the accounting server, for example, then naturally you want
to put the accounting system’s server and its users on their own network segment You won’t recognize such issues and how to address them unless your assessment leads you
to determine with some degree of detail how the network resources will be used
The following sections discuss what you should examine as you learn what a given network must be able to do No particular order exists in which you should examine these issues, and you might find that you need to cycle through the list several times
to get a complete picture You also might find a particular company’s needs require more or less analysis in each category Common sense is required when you design a network The following suggestions are guidelines to start you on the right path
Applications
A good place to start with a network design is to list and understand the applications that will run on the network Ultimately, a network is only as good as the work it helps people accomplish, and people do their work most directly through the application software they use If the applications don’t work right, then the users won’t work right Most networks have both common applications and department- and user-specific applications Most companies usually meet the common application needs through a suite of desktop applications, such as Microsoft Office or Lotus SmartSuite The following
is a list of applications that most companies install for all users, whether or not each user needs each one:
N Word processor
N Spreadsheet
N End-user database
N Presentation graphics
N E-mail
N Personal information manager (calendar, contact list, and so forth)
N Virus-scanning software
Trang 2Your first order of business is to determine just how the common applications will
be used Determine whether all users need to have the entire suite installed, how often
different users plan to use the different applications, how many files they will create and
store, how large those files might be, and how those files will be shared among users
For example, in a 1,000-user population, you might determine that 90 percent will
use word processing to generate an average of ten documents a month, with each
document averaging 100KB, and the users probably will want to keep two years’
worth of documents on hand at any given time Yes, these will be educated guesses,
but it’s important to come up with reasonable estimates Experience with similar user
populations and companies can pay off handsomely in determining these estimates
With this information alone, you know immediately that you need about 24MB of
storage per user, or 21.6GB for the word processing population of 900 users, just for
word processing documents For applications where users frequently will share files,
you might need to factor in that most users keep personal copies of some files that they
also share with others
TIP You can help reduce overall network storage requirements by establishing shared directories in
which different groups of people can store and access shared files
Then you come up with the same estimates for the other applications, taking into
account their expected size, frequency of creation, and long-term storage requirements
After determining the common applications, move on to department-specific
applications This step gets trickier for new networks in new companies, because you
might not know which applications will be used For existing companies, you have the
advantage of already knowing which departmental applications you must support
Different departmental applications can have wildly different impacts on the network
For example, an accounting system designed around shared database files needs a
different network design than one using a client/server database design The former
relies more on file server performance and is more likely to be bandwidth-sensitive than
an efficient client/server application that runs on a dedicated server If a departmental
application is not yet selected, talk with the managers of that department to get their best
estimates and then proceed
Following are common departmental applications you should consider:
N Accounting
N Distribution and inventory control
N Manufacturing/material requirements planning (MRP)
N Information technology
N Electronic commerce
N Human resources
N Payroll and stock administration
Trang 3N Publishing
N Marketing support
N Legal
N Other line-of-business applications specific to the company’s industry
For each of the departmental applications you identify, you need to ask several questions: How much storage will they consume? From where will the applications
be run (from local computers with data on a server or completely centralized, where both the data and the application run on a central computer)? Will they have their own dedicated servers? How much network bandwidth will the application need? How will all these factors change as the company grows?
Finally, while you might not formally include them in your plan, consider user-specific applications that might be run For example, you might estimate that the people in the company’s research and development group are likely to run two or three unknown applications as part of their job If you decide that user-specific applications will have a significant impact on the network, then you should estimate their needs, just as you have for the other types of applications If you believe they will have minimal impact, then you might decide either to include a small allowance for them or none at all
TIP Don’t get bogged down in “analysis paralysis,” worrying about whether you can scientifically
prove that your estimates are accurate Instead, make sure the estimates are reasonable to other network professionals At a certain point, you need to justify the network design and cost and, to do this, having reasonable estimates is necessary Just avoid overdoing it
Users
Once you know the applications that the network must support, you can estimate how many users need to be supported and which applications each user will use Estimating total users will likely be easier because the company should already have a business plan
or long-range budget from which you can derive these estimates Your user estimates should be reasonably granular; know the number of users in each department in the company as well as the company’s total number of users
You should estimate how many users will need to be supported immediately, in one year, in three years, and in five years Even though five years is a distant horizon
to use for an estimate, this information is important to know during the design process Different growth rates suggest different network designs, even at the inception of the network A company estimating that it will have 100 users immediately, 115 users in one year, 130 users in three years, and 150 users in five years needs a different network design than a company estimating 100 users immediately, 115 users in one year, 300 users
in three years, and 1,000 users in five years In the latter case, you must invest more in a design that is more quickly scalable, and you are likely to spend much more at inception
to build the network, even though the network will have the same number of users in the first two years
Trang 4Knowing the number of users isn’t enough, though You need to know more about
the users At a minimum, consider the following questions to determine if any of the
following will be important factors for the users generally or for subgroups of users:
N Bandwidth requirements Aside from the bandwidth required to save and
retrieve files, send and receive e-mail, and perform an average amount of
browsing on the Internet, do any users need significant amounts of bandwidth?
For example, will scientists download a fresh copy of the human genome from
the Internet once a week? Will groups of users need to exchange large quantities
of data among different sites? Will any users be running videoconferencing
software over your network connection? How much web browsing do you
expect the network’s users to do? Will people be sending large e-mail attachments
frequently?
N Storage requirements Will any group of users need significantly more storage
capacity than the overall average you already determined? For instance, will
an electronic imaging group catalog millions of documents into image files on
a server? If so, how many people need access to that data? Will the accounting
group need to keep the previous ten years of financial information online?
Will the company use or install an executive information system where all the
managers have query capability into the company’s accounting, distribution,
and manufacturing systems, and, if so, how much additional bandwidth or
server performance could that capability require?
N Service requirements Will any groups of users require additional network
services not needed by most users? For example, does part of the company do
work of such sensitivity that it should be separated from the rest of the local
area network (LAN) by a network firewall? Will a subset of users need direct
inward fax capability?
When examining user bandwidth requirements, remember to look at the timeliness
of the bandwidth needs If certain known activities require a lot of bandwidth and must
be carried out during the normal workday, that high-bandwidth use might interfere
with the performance of the rest of the network Therefore, make sure to estimate both
average and peak bandwidth needs
Network Services
Next, you should look at the services that the network must provide These can vary
widely in different companies A very basic network might need only file and print
services, plus perhaps Internet connectivity A more complex network will need many
additional services Consider which of the following types of services the network you
are designing will need to provide, as well as any others that are specific to the company:
N File and print services
N Backup and restore services
Trang 5N Internet web browsing
N FTP and Telnet
N Internet or external e-mail
N Internet security services
N Remote access to the LAN through a VPN or a modem pool
N Fax into the LAN (manually distributed or automatically distributed)
N Dynamic Host Configuration Protocol (DHCP) services
N Centralized virus-protection services
N Wide area network (WAN) services to other locations
N Streaming Internet radio and other media
N Voice over IP (VoIP)
For each service, you must answer a number of questions First, you need to know the storage and bandwidth requirements for each service, and any other impacts they might have For instance, a fax-in service might itself require a small amount of storage space, but all the fax bitmaps that users will end up storing could have a large impact
on total storage needs
Second, you need to know how the service is to be provided Usually, this means that you need to know which server will host the service Some services require such little overhead that you can easily host them on a server that does other jobs A DHCP server, which requires minimal resources, is a good example of such a service On the other hand, an e-mail system might require such high resources that you must plan to host it on its own dedicated server
Third, you need to know what users or groups of users need which services This is because, to minimize backbone traffic, you might need to break down the network into smaller segments and locate frequently used services for a particular user population
on the same segment as the users use
Security and Safety
The preceding considerations are all related to the bits and bytes required by different parts of the network Security and safety concern the company’s need to keep information secure—both inside and outside an organization—and to keep the company’s data safe from loss You need to know how important these two issues are before attempting to set down a network design on paper
For both these considerations, a trade-off exists between cost and effectiveness As mentioned in earlier chapters, no network is ever totally secure and no data is ever totally safe from loss However, different companies and departments have different sensitivities
to these issues, indicating that more or less money should be spent on these areas
Some applications might be perfectly well suited to keeping their data on a striped RAID 0 array of disks, where the risk of loss is high (relative to other RAID levels),