217
Chapter 15: Designing a Network
because the data might be static and easy to restore from tape if the disk array is lost.
Other applications might require the highest level of data-loss safety possible, with
failover servers each having mirrored RAID 1 or RAID 10 arrays and online tape
backup systems updating a backup tape every hour or for every transaction. Similarly,
some companies might work with data that is so sensitive that they must install the
best firewalls, perhaps even two levels of firewalls, and hire full-time professionals
dedicated to keeping the data secure. Other companies might be satisfied if they are
only reasonably secure.
The point is that you must determine how important these issues are to the company
for which you are designing the network. Then you can propose different solutions to
address these needs and factor them into the rest of your design.
Growth and Capacity Planning
The final area to consider is the expected growth of the network, particularly if the
company expects this growth to be substantial. As mentioned earlier in this chapter, a
network designed for a rapidly growing company looks different from one for a slowly
growing company, even if both companies start out at the same size. In the former case,
you want a design that you can quickly and easily expand without needing to replace
much of the existing hardware and software. In the latter case, you can get by with a
simpler network design.
Consider the impact of growth on the different parts of the network that you’ve
already examined (applications, users, and services), because linear growth does not
always mean a matching linear impact to the network. Assuming linear growth, the
impact to the network might be much lower or much higher than the curve.
For example, you saw in Chapter 4 how Ethernet uses a collision detection mechanism
to manage network traffic. In that chapter, you also learned that Ethernet scales linearly,
but only up to a point. Once the network starts to become saturated, performance begins
to drop rapidly because of the chaotic nature of Ethernet’s collision detection scheme.
Consider a 10 Mbps Ethernet network transmitting 3 Mbps of traffic. This traffic probably
flows smoothly, with few collisions and few retransmissions required. Push the network

demand up to 4 or 5 Mbps, however, and its performance grinds to a halt as the network
becomes saturated, and you end up with as many collisions and retransmissions as real
data. In fact, the total amount of good data flowing over a saturated Ethernet network will
be less than the amount flowing over a less-saturated network.
You can also find examples where an increase in demand doesn’t cause a
corresponding increase in network or server load. For example, the server load for
a complex e-mail system might increase only by a small amount if you doubled the
number of users, because the system’s overhead generates most of the load. The storage
requirements for an accounting system might not double just because you keep twice as
much data in it to accommodate the overhead that might consume most of the existing
space. Alternatively, that same accounting system might consume four times as much
storage space if you double the data storage, because it might have a relatively inefficient
indexing scheme. The point is that you need to know how different applications scale
218
Networking: A Beginner’s Guide
with increased use. The vendors of the main applications you will use should be able to
provide useful data in this regard.
TIP Be careful not only to consider how applications behave as they scale, but how they behave
as they are scaled in your planned network environment. Different NOSs, network topologies, and
client and server computers will all affect how well a particular application can support growth.
Meeting Network Needs
Once you complete your assessment (by this point, you’re probably sick of the assessment
process!), you can then start working on finding ways to meet all the needs you’ve
identified. This process is largely holistic and is not worked through by following a series
of steps and ending up with a single answer, like an equation. Instead, you should start by
mapping out the various parts of the network, considering the three main topics discussed
in this section, and “build a picture” of the network design. The design that you create
will incorporate all you learned during the assessment process, taking into account your
experience and the advice you have received to devise a concrete design that results in an
equipment list, specifications, and a configuration.

Seeking criticism of your design from other network professionals, who might have
valuable experience that you can then factor into your design, is important. No single
networking professional has seen and had to cope with all possible design needs, so
you want to combine the advice of as many seasoned people as you can.
Choosing a Network Type
You probably want to start the design by choosing a network type. This should be a
relatively straightforward decision, based on the overall bandwidth requirements for
the network. For most new networks, you almost certainly will decide to use one of the
flavors of Ethernet. Ethernet is by far the most common type of network installed today,
and it’s an easy default choice.
You also need to decide what level of Ethernet you need. For wiring to the desktop,
you should choose 100Base-T. It’s reliable and provides plenty of capacity for most
needs. For your network backbone, you can usually use a higher-bandwidth connection,
such as 1000Base-T, without incurring too much additional cost.
Structuring the Network
Next, decide how you plan to structure the network. In other words, how will you
arrange and wire the various hubs, switches, and routers that the network needs? This
is probably the trickiest part to determine, because it’s hard to predict how much data
must flow from any given set of nodes to any other set of nodes. The estimates you have
based on your assessment work will help. If you can identify expected heavy traffic
219
Chapter 15: Designing a Network
patterns, you should also draw a network schematic with these patterns indicated to
help you sort it out. Remember the following tips:
N Ethernet’s CDMA/CD collision handling means that an Ethernet network will
handle only about one-third of its rated speed. In other words, a 100Base-T
segment, which is rated at 100 Mbps, will handle about 33 Mbps of actual data
before starting to degrade.
N Whenever possible, use “home-run” wiring (in which each network cable runs
from each workstation to a single location) for all nodes to a single wiring

closet or server room. Doing so enables you to change the network structure
more easily (for example, to break segments into smaller segments) as needs
change.
N Except in the smallest networks, plan on installing a network backbone to
which the hubs connect. An Ethernet switch rather than a nonswitching hub
should handle the backbone, so each hub constitutes a single segment or
collision domain. You still must plan to keep each segment’s traffic below the
Ethernet saturation point, but this structure will give you plenty of flexibility to
meet this goal.
N The physical building might dictate how you structure your network. For
example, a building larger than 200 meters (about 600 feet) in any dimension
probably means you won’t be able to employ a home-run wiring scheme
for all your nodes. This is because twisted-pair Ethernet usually reaches
only 100 meters (about 300 feet), which includes routing around building
obstructions, patch cables, and other things that make the actual cable distance
longer than you might measure on a map of the building.
N For multifloor buildings that are too big for a home-run wiring scheme,
consider running the backbone vertically from floor to floor, and then have
a wiring closet on each floor that contains the switches to service that floor’s
nodes. The wiring from the closet on each floor then fans out to each of the
nodes on that floor.
N Consider running the backbone speed at ten times the hub/desktop network
speed. If you’re using 100Base-T hubs to connect to the desktop computers,
plan on a 1000Base-T backbone.
N Most of the time, most nodes do the majority of their communication to one or
two servers on the network. If you are planning department-specific servers or
if you can identify similar patterns, make sure that each server is on the same
segment as the nodes that it primarily serves.
N If your servers tend not to be assigned to support departments and instead
support the entire company, make sure that the servers are directly connected

to the backbone’s Ethernet switch.
220
Networking: A Beginner’s Guide
N If you have any high-bandwidth users, consider keeping them on a segment
separate from the rest of the network (if appropriate) and also consider
upgrading the speed of that segment to 100 Mbps or 1,000 Mbps if needed.
N As you start to implement the network, carefully watch the ratio of collision
packets to data packets. If the number of collisions on any segment climbs
5 to 7 percent of the total number of packets, performance is starting to
suffer, so you need to investigate the cause and find a way to decrease this
ratio. You can usually do so by breaking the segment into smaller pieces or by
configuring capable switches into what is called a virtual LAN (VLAN), unless
you know of another way to reduce the amount of traffic.
Selecting Servers
When choosing servers for a network, start by determining which NOS you will use.
For PC-centric networks, the decision is usually between Novell NetWare and Windows
family of servers. As discussed in Chapter 13, whenever possible, avoid using both,
because supporting two NOS systems makes managing the servers much more difficult.
You’re better off compromising on a single NOS platform.
Next, list the various network services that your servers must provide. You need
to look for efficient ways to host these various services on your servers, balancing a
number of factors:
N All else being equal, using more small servers to host fewer services each is
more reliable than using fewer large servers to each host many services.
N Conversely, having more small servers increases your chance of having a server
fail at any given time.
N Using more small servers is more expensive and requires more maintenance than
using fewer large servers.
N If you plan to use more than one server, consider which services should be
redundant on another server or how you plan to deal with the failure of any

server.
Using your assessment information, you can easily determine how much storage
capacity your servers will need. However, it’s much harder to know how capable each
server should be in terms of processor power, installed RAM, and other features, such
as bus configuration. For these specifications, you need to rely on the advice of the NOS
vendor and the manufacturer of the servers that you are considering. Fortunately, both
Microsoft and Novell have published tests and recommendations for sizing servers
given different service and user loads. Many first-tier server manufacturers also have
such data to help you choose an actual server model and its specifications.
221
Chapter 15: Designing a Network
Chapter Summary
Designing an entire network can be extremely complex. If you are in the enviable
position of designing a network, your best bet is to start with the framework described in
this chapter and to use other resources to answer specific questions. Many resources are
available to help you do this, ranging from books devoted to aspects of network design,
server management, network performance tuning, and specific NOS management, to
consultants experienced with similar networks and the various vendors you are working
with on any planned purchases. In fact, so many resources exist to help you accomplish
this job, you may have trouble deciding which advice to follow!
Always remember to leave some escape hatches in any network design, so you
can respond quickly to new or changed requirements, many of which will occur while
you’re finalizing the design. The good news is that if you follow the advice in this
chapter and the rest of the book, along with the other resources mentioned, it’s a safe
bet you’ll end up with a solid, expandable, maintainable network design that meets the
needs of the company and of which you can be proud.