Network Design
• WAN/LAN Design
– Three Layer
– Two Layer
– One Layer
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Network Design
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Scalability
• Networks that follow the hierarchical model
– can grow much larger without sacrificing
control or manageability because
• functionality is localised and
• potential problems can be recognised more easily.
• An example of a very large-scale
hierarchical network design is the Public
Switched Telephone Network.
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Ease of implementation
• A hierarchical design assigns clear
functionality to each layer, thereby making
network implementation easier.
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Ease of troubleshooting
• Because the functions of the individual
layers are well defined, the isolation of
problems in the network is less complicated.
• Temporarily segmenting the network to
reduce the scope of a problem also is easier.
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Predictability
• The behaviour of a network using
functional layers is much more predictable,
– makes capacity planning for growth
considerably easier;
• this design approach also facilitates
modelling of network performance for
analytical purposes.
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Protocol support
• The mixing of current and future
applications and protocols will be much
easier on networks that follow the principles
of hierarchical design because the
underlying infrastructure is already
logically organised.
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Manageability
• All of the listed benefits contribute to
greater manageability of the network.
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Hierarchical Design
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Core layer
• Provides fast wide-area connections
between geographically remote sites, tying
a number of “campus” networks together in
a corporate or enterprise WAN.
• Core links are usually point-to-point, and
there are rarely any hosts in the core layer.
• Core services are typically leased from a
telecom service provider (for example,
T1/E1, Frame Relay, and so on). 10
Distribution layer
• Refers to the distribution of network
services to multiple LANs within a campus
network environment.
• This layer is where the “campus backbone”
network is found, typically based on Fast
Ethernet.
• This layer is implemented on sites that are
large and is used to interconnect buildings.
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Access layer
• Usually a LAN or a group of LANs,
typically Ethernet or Token Ring, that
provide users with frontline access to
network services.
• The access layer is where almost all hosts
are attached to the network, including
servers of all kinds and user workstations.
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Core Layer
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Distribution Layer
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Access Layer
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One-Layer Design
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Two-Layer Design
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WAN Connectivity
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Enterprise Servers
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Workgroup Servers
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LAN Design
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A System Approach to Network
Design
• Requirements analysis
• Flow Analysis
• Logical Design
– Technology choices
– Interconnection mechanisms
– Network Management and security
• Physical Design
• Addressing and Routing
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Logical design: Technology
Choices
• Background
• Establishing Design Goals
• Developing Criteria for Technology
Evaluation
• Making Technology Choices for the Design
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Logical design: Technology
Choices – Background
• Input from previous stages:
– Requirements specifications
• Application,
• user and
• host requirements
– flow information
• Capacity plan
• Service plan and
• Performance characteristics
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Logical design: Technology
Choices - Background
• Determine your design goals for the
network
• Translate these goals into evaluation criteria
for making technology choices
• Determine interconnection strategy
• Integrate security and network management
into the design
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Design
• Outcome:A set of diagrams used to
– Prepare the environment for the network
• Ordering equipment
• Deploying cable
• Writing the transition plan for any existing network
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Establishing Design Goals
• Common design goals:
– Minimizing network deployment and/or
operation cost
– Maximizing one or more network performance
characteristics
– Ease of use and manageability
– Optimising security
– Adaptability to new and changing user needs
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Cost/Performance Graph
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Design Goals
• Prioritise goals
– One primary
– One or more secondary
– Wish list
• You will face Constraints
– E.g. cost
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Criteria for technology evaluation
• Starting point
– Design goals
– Flow specification
• Capacity planning
• Service planning
• Standards based and commonly available
• Commercial off the shelf (COTS) network
products
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Criteria for technology evaluation
• Maximizing performance
– Meet or exceed
• Expected capacity,
• Delay and/or
• Reliability
• Maximizing reliability
– Redundancy in the network
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Criteria for technology evaluation
• Adaptability
– Support dynamic behaviour
• Rapid reconfiguration of
– User groups
– Address assignment
– Routing,
– And location adds/drops to/from network
– Example: use a service provider for WAN
connection – VPNs!
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Criteria for technology evaluation
• Flowspec
– Capacity Plan
• Minimizing cost (design goal)
– Service Plan
• Maximizing performance (design goal)
• Adaptability (design goal)
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Criteria for technology evaluation
• Characteristics of technologies
– Non-broadcast multiple access (NBMA)
capability
– Broadcast capability
– Performance upgrade paths
– Flow considerations
– Meeting capacity and service plans
requirements
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Criteria for technology evaluation
• NBMA: ATM, Frame Relay, HiPPI
• Broadcast LAN technologies: Ethernet,
Token ring, FDDI (using ARP, RARP)
• NBMA vs Broadcast technology
– Native broadcast support
– Connection support: connection
oriented/connectionless
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Criteria for technology evaluation
• Functions and features of technology
– Adapting to a lack of communications infrastructure
• Use of wireless techniques + protocols
– Adapting to the mobility of users or their resources
• Wireless networks, multiple access points + protocols
– Adapting to users/applications that have strict
performance requirements
– For asymmetric flows, technology should optimize
directionality of flow
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Criteria
• NBMA technologies do not inherently have
a broadcast mechanism
• NBMA: flexible handling of broadcast
• Use hierarchies
– Background broadcast traffic will be less than
2% of the capacity of the technology; e.g. 200
Kb/s for 10Mb/s Ethernet
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Criteria
• Upgrade paths
– NICs supporting multiple bit rates
– Cable types
– Distances
– SONET: 51.84 Mb/s to 9.953 Gb/s
– Ethernet 10 Mb/s to 1 Gb/s
– FDDI 100 Mb/s to 1 Gb/s
– Frame Relay 56 Kb/s to 45 Mb/s
– ATM 1.5 Mb/s to 622 Mb/s
– HiPPI 800 Mb/s – SuperHiPPI 6.4 Gb/s
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Criteria
• Scalability
• Backbone flows
– Use scaling depending on the hierarchy
• Hierarchy < 4:1 scaling factor = 1
• Hierarchy 4:1 to 6:1 scaling factor = 1.5
• Hierarchy 7:1 to 9:1 scaling factor = 2
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Criteria
• Flow considerations
– Capacity planning
• required capacities of candidate technologies
and
– Service planning will be used in the selection of
technology
• ability to provide specified service support
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Evaluation Criteria for Selected Technologies
• Ethernet
– Broadcast
– Connectionless
– 10 Mb/s nominal, 100Mb/s to 1 Gb/s available
– Shared & switched versions available
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Evaluation Criteria for Selected Technologies
• Token Ring
– Broadcast
– Connectionless
– Deterministic for high traffic loads
– 4 Mb/s and 16 Mb/s available
– Shared & switched versions available
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Evaluation Criteria for Selected Technologies
• FDDI
– Broadcast
– Connectionless
– Deterministic for high traffic loads
– 100 Mb/s, 1 Gb/s expected
– Shared & switched versions available
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Evaluation Criteria for Selected Technologies
• ATM
– Non-Broadcast Multiple Access (NBMA)
– Connection oriented
– Specified support through QoS
– T1 (1.5 Mb/s), T3 (45 Mb/s), SONET OC-3c
(155 Mb/s), OC-12c, OC-48c, and OC192c
(9.953 Gb/s - planned)
– Shared & switched versions available
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Evaluation Criteria for Selected Technologies
• High Performance Parallel Interface
– NBMA
– Connection oriented
– High performance LAN technology
– 800 Mb/s, 1.6 Gb/s, SuperHiPPI 6.4 Gb/s –
planned
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Evaluation Criteria for Selected Technologies
• Frame Relay
– NBMA
– Connection oriented
– Support through Committed Informtion Rates
– 56 Kb/s to T3 (45 Mb/s)
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Evaluation Criteria for Selected Technologies
• SMDS
– NBMA
– Connection oriented
– 56 Kb/s, 4, 10, 16, 25, 34 Mb/s
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Guidelines and Constraints on Technology
Evaluation
Rule 1: If specified requirements are specified
in the flow specification then either the
technology or a combination of technology
and supporting protocols or mechanisms
must support specified services
Rule 2: If best effort or specified capacities
are specified in the flow specification then
the selection of technology may also be
based upon capacity planning for each flow.
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Constraints on Candidate
Technologies
1. The cost
2. pre-existing network
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Next Week
• Logical Design: Interconnection
Mechanisms
• Logical Design: Network Management and
Security
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