Transcript CHAP10

Business Data
Communications and
Networking, 6th ed.
FitzGerald and Dennis
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Chapter 10
Network Design
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Objectives of Chapter 10
Become familiar with…
 the overall process of design and
implementing a network
 commonly used MAN and WAN designs
Understand
 commonly used backbone designs
 commonly used LAN designs
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INTRODUCTION
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Introduction
The traditional network design approach
follows a structured systems analysis and
design process similar to that used to build
application systems.
• The network analyst met with users to determine the
needs and applications.
• The analyst estimated data traffic on each part of the
network.
• The analyst designed circuits needed to support this
traffic and obtains cost estimates.
• Finally, a year or two later, the network is implemented.
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Introduction
Three forces are making the traditional design
approach less appropriate for many of
today’s networks:
1. The underlying technology of computers,
networking devices and the circuits themselves
if rapidly changing.
2. The growth in network traffic is immense.
3. The balance of costs has changed
dramatically over the last 10 years.
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Introduction
While some organizations still use the
traditional approach, many others use a
simpler approach to network design, the
building block approach.
Begin by classifying users as “typical” or “high
volume.”
The network is then planned by adding
circuits using a few standard network
designs, also classified as “typical” or “high
volume.”
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THE NETWORK DESIGN
PROCESS
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The Network Design Process
The basic process involves three steps that
are performed repeatedly:
 Needs analysis
 Technology design
 Cost assessment
By cycling through these three processes, the
network design settles on the final network
design.
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The Network Design Process
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The Network Design Process
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Needs Analysis
The goal of needs analysis is to understand
why the network and what users and
applications it will support.
Much of the work may have already been
done monitoring existing systems, which
can provide a baseline against future
design requirements.
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Geographic Scope
The first step is to define the geographic
scope of the network.
A data communication network can have four
basic levels of geographic scope:
• Wide area networks (within several states,
provinces or countries)
• Metropolitan area networks (within a city)
• Campus area networks (within a series of
buildings located in the same general area)
• Local area network (within one building)
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Application Systems
The designers must review the list of
applications that currently use the network
and identify the location of each one so that
all of them will be interconnected by the
planned network (baselining). In many
cases, the applications will be relatively well
defined.
It is also helpful to identify the hardware and
software requirements of each application
that will use the network, and , if possible,
the message type each application uses. 10-15
Network Users
In the past, applications system accounted for
the majority of network traffic. Today, much
network traffic is produced by the
discretionary use of the Internet (I.e. e-mail
and WWW).
Therefore, you must also assess the number
and type of users that will generate and
receive network traffic.
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Categorizing Network Needs
The next step is to assess the relative amount
of traffic generated in each segment, based
on some rough assessment of the relative
magnitude of network needs (I.e. typical vs.
high volume).This assessment can be
problematic, but the goal is some relative
understanding of the network needs.
Once the network requirements have been
identified, they also should be organized
into mandatory requirements, desirable
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Deliverables
The key deliverables for the needs
assessment stage are a set of network
maps, showing the applications and the
circuits, clients, and severs in the proposed
network, categorized as “typical” or “high
volume”.
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Technology Design
Once the needs have been defined, the next
step is to develop a technology design (or
set of possible designs) for the network.
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Designing Clients and Servers
“Typical” users are allocated the “base level”
client computers, as are servers supporting
“typical” applications. “High volume” users
and servers are assigned some “advanced”
computers.
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Designing Circuits and
Devices
There are two interrelated decisions in
designing network circuits and devices:
the fundamental technology and protocols
the capacity of each circuit.
Designing the circuit capacity means capacity
planning, estimating the size and type of the
“standard” and “advanced” network circuits
for each type of network.
This requires some assessment of the current
and future circuit loading (average vs peak).10-22
Designing Circuits and
Devices
The designer usually starts with the total
characters transmitted per day on each
circuit, or if possible, the maximum number
of characters transmitted per two second
interval if peaks must be met.
Although no organization wants to overbuild
its network and pay for more capacity than
it needs, in most cases, going back and
upgrading a network significantly increases
costs.
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Network Design Tools
Network modeling and design tools can
perform a number of functions to help in the
technology design process.
Some modeling tools require the user to
create the network map from scratch.
Other tools can “discover” the existing
network.
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Network Design Tools
Once the map is complete, the next step is to
add information about the expected network
traffic and see if the network can support
the level of traffic that is expected. This
may be accomplished through simulation
models.
Once simulation is complete, the user can
examine the results to see the estimated
response times and throughput.
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Deliverables
The key deliverables at this point are a
revised set of network maps that include
general specifications for the hardware and
software required.
In most cases the crucial part is the design of
the network circuits.
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Cost Assessment
The purpose of cost assessment is to assess
the costs of various network alternatives
produced from the previous step.
Some of the costs to consider are:
Circuit costs
Internetworking devices
Hardware costs
Software costs
Network management costs
Test and maintenance costs
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Request for Proposal (RFP)
Although some network components can be
purchased “off-the-shelf”, most
organizations develop an RFP before
making large network purchases.
Once the vendors have submitted their
proposals, the organization evaluates them
against specific criteria, and selects the
winner(s).
One of the key decisions in the RFP process
is the scope (one vendor or multi-?).
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Request for proposal

Information in a Typical RFP
Background Information
•
•
•
•

Organizational Profile
Overview of current network
Overview of new network
Goals of new network
Network Requirements
• Chose sets of possible network designs (hardware, software,
circuits)
• Mandatory, desirable, and wish list items
• Security and control requirements
• Response time requirements
• Guidelines for proposing new network designs
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Request for proposal

Service Requirements
•
•
•
•

Bidding Process
•
•
•
•

Implementation time plan
Training courses and materials
Support services (e.g. spare parts on site)
Reliability and performance guarantees
Time schedule for the bidding process
Ground rules
Bid evaluation criteria
Availability of additional information
Information required from vendor
•
•
•
•
Vendor corporate profile
Experience with similar networks
Hardware and software benchmarks
Reference lists
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Selling the Proposal to
Management
One of the main problems in network design
is obtaining the support of senior
management.
The key to gaining senior management
acceptance lies in speaking their language.
A focus on network usage, budgets, and
reliability are easily understandable issues.
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Deliverables
There are three key deliverables for this step:
• An RFP that goes to potential vendors.
• After the vendor has been selected, the revised
set of network set of maps with the technology
design component complete.
• The business case that provides support for the
network design, expressed in business
objectives.
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COMMON WIDE AREA
NETWORK DESIGNS
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Common Wide Area Network
Designs
Most organizations do not build their own
WANs by laying cable, building microwave
towers, or sending up satellites. Instead
most organizations lease circuits from
interexchange carriers, and use those to
transmit their data.
Once the major connection points one the
WAN have been identified, the next step is
to design the circuits that will connect those
locations.
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Ring-Based WAN Design
A ring-based WAN design connects all
computers in a closed loop, with each
computer linked to the next, usually with a
series of point-to-point dedicated circuits.
One disadvantage is of the ring topology is
that messages can take a long time to
travel from the sender to the receiver.
In general, the failure of one circuit or
computer in the network means that the
network can continue to function.
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Ring-Based WAN Design
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Star-Based WAN Design
A star-based WAN design connects all
computers to one central computer that
routes messages to the appropriate
computer, usually via a series of point-topoint dedicated circuits. It is easy to manage
because the central computer receives and
routes all messages in the networks.
In general, the failure of any one circuit or
computer affects only the one computer on
that circuit.
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Mesh-Based WAN Design
Mesh-based WAN designs: full or partial mesh.
The effects of the loss of computers or circuits
in a mesh network depend entirely on the
circuits available in the network.
In general, mesh networks combine the
performance benefits of both ring networks,
and star networks. The drawback is that
mesh networks use decentralized routing so
that each computer in the network performs
its own routing.
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Mesh-Based WAN Design
Cloud-based mesh designs are becoming
very popular. With this design all computers
are simply connected into a packet
switched network provided by a common
carrier.
Cloud-based designs are simpler for the
organization because they move the burden
of network design and management from
the organization to the common carrier.
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Fish & Richardson’s WAN
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COMMON BACKBONE
NETWORK DESIGNS
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Common Backbone Network
Designs
With WANs, the most important issues are
centered around the geographic layout of
the network and the basic topology. With
backbone networks, the most important
characteristic is the way in which packets
are moved across the backbone.
There are three basic approaches used in
backbone networks to move packets from
one segment to another:
Routing, Bridging and Switching
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Routed Backbone Design
Routed backbones move packets based on
their network layer address.
The primary advantage is that routed
backbones clearly segment each part of the
network connected to the backbone.
There are two disadvantages to routed
backbones.
1. The routers in the network impose time delays
2. Routed networks require a lot of
management.
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Bridged Backbone Design
Bridged backbones move packets based on
their data link layer address.
Advantages: Bridges tend to be less
expensive than routers. Bridged backbones
tend to be simpler to install.
Disadvantages: Individual segment
management is difficult. Network speed is
slower than routed backbones since
broadcast messages must be permitted to
travel everywhere.
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Switched Backbone Design
Most switched backbones use the data link
layer address to move packets . A collapsed
backbone is the most common form.
Advantages: Improved performance and far
fewer networking devices.
Potential disadvantages: More broadcast traffic
and difficult to isolate and separately manage
individual LANs. They also use more cable
and if the switch fails, so does the network.
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Department of Education’s financial aid network
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COMMON LOCAL AREA
NETWORK DESIGNS
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Traditional LAN Design
Client computers are connected to a hub that
provides the physical connection.
This design has dominated for years because
it is simple, easy to install and manage.
The introduction of Switches greatly improved
response time in LANs with large traffic
flows.
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Traditional LAN Design
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Virtual LAN Design
Switches also have enables the creation of
Virtual LANs (VLANs). VLANs are usually
faster and provide greater opportunities to
manage the flow of traffic on the LAN.
VLANs are groups of computers in an
intelligent switched network.
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Basic Switches
The front of the switch contains a series of
ports exactly like the ports on the front of a
hub.
These ports are connected inside the switch
by a switching fabric which provides
connections between any two ports.
It is possible to have ports running at different
speed, but this could overwhelm the slower
port.
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Basic Switches
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Intelligent Switches
Intelligent switches support larger networks
than the basic switch’s 8- or 16- port LANs.
As well as being able to support far more
computers or network connections, the key
advantage is in the modularity of intelligent
switches.
These switches often can support several
hundred ports spread over a dozen or more
different modules.
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Intelligent Switches
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Intelligent Switches
Since there is not enough capacity in the
backplane to support all ports if they
become the switch forms groups of
connections and assigns capacity using
time division multiplexing.
This means that the switch no longer
guarantees simultaneous transmission on
all ports, but will accept simultaneous input
and will switch incoming data to outgoing
ports as fast as possible.
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Port-Based VLANs
(Layer-1 VLANs)
Port-based VLANs use the physical layer port
address to form the groups for the VLAN.
It is logical to connect computers that are
physically close together on the LAN into
ports that are physically close together on the
switch, and to assign ports that are physically
close together into the same VLAN.
This is the approach used in traditional LAN
design: physical location determines the LAN,
but is not always the most effective approach.
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Port-Based VLANs
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Building VLANs
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VLANs used to balance capacity against network traffic
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MAC-Based VLANs
Layer-2 VLANs
MAC-based VLANs use the dame data link
layer addresses to form the VLAN groups.
The advantage is that they are simpler to
manage when computers are moved.
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IP-Based VLANs
Layer-3 VLANs
IP-based VLANs use the network layer
address (i.e. TCP/IP address) to form the
VLAN groups. Layer-3 VLANs reduce the
time spent reconfiguring the network when
a computer is moved as well.
Some layer-3 VLANs can also use the
network layer protocol to create VLAN
groups. This flexibility enables manager
even greater precision in the allocation of
network capacity.
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Application-Based VLANs
Layer-4 VLANs
Application-based VLANs use the application
layer protocol in combination with the data
link layer and network layer addresses to
form the VLAN groups.
The advantage is a very precise allocation of
network capacity.
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End of Chapter 10
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