WAN Design

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WAN Design
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Table of Contents

• WAN Design Requirements

Go There!

• Gathering Analyzing Requirements

Go There!

• The Three-Layer WAN model

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• WAN Layer Functions

Go There!
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WAN Design Requirements
Table of Contents
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Network Demand

• WANs need to be developed to meet the following
  requirements
• Optimize WAN bandwidth
• Minimize cost
• Maximize the effective service to end users

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Network Demand

• LANs shared media networks are being overtaxed
  because...
• Network cost continues to escalate
• Network usage has increased
• Application requirements increasingly demand more
  network services (i.e., push technologies)
• Increased use of enterprise servers
• The number on intra- and extranets continues to
  rise
• LANs connected through WANs is expected to
  increase WAN traffic 300 in the next 5 years.

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LAN/WAN Integration

• LANs and WANs, previously logically separated,
  must now be fully integrated for seamless
  performance.
• The LAN/WAN network (or corporate internet) now
  must be able to handle...
• Voice traffic (VoIP)
• Bandwidth intensive multimedia applications
• Video conferencing
• On-line training
• Increased business critical data access

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Overriding Goal in WAN Design
Minimize Cost While Increasing Network
Availability
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Gathering Analyzing Requirements
Table of Contents
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Factors Affecting Design

• Environmental Variables
• Where are all the nodes?
• Performance Constraints
• What level of reliability? Host/client speeds?
  Traffic throughput?
• Networking Variables
• Whats the topology? What is the traffics
  characteristics?
• Traffic Characterization is critical to
  successful WAN design and implementation, but it
  is seldom done.

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Traffic Characterization

• Types of Traffic
• Voice/fax
• Client/Server data
• Messaging
• File transfers
• Batch data
• Network overhead
• Multimedia

• Traffic Characteristics
• Peak Avg. Volumes
• Connectivity volume flows
• Connection orientation
• Latency tolerance
• Network availability tolerance
• Error rate tolerance
• Priority
• Protocol type
• Avg. packet length MTUs

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Gathering User Requirements

• In general, users primarily want application
  availability in their networks. This includes...
• Response Time -- time between entry of a command
  and execution of the command
• Throughput-intensive apps. -- such as
  file-transfers and batch operations scheduled
  during low traffic periods
• Reliability -- some apps require nearly 100
  uptime such as NASDAQ and emergency services.

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Assessing User Requirements

• Three methods to assess user needs
• User community profiles--determine the needs of
  various user groups within the organization
  crucial 1st step
• Interviews, focus groups, and surveys--used to
  establish a baseline for building the network
• Human factors tests--most expensive time
  consuming of the three sampling of users
  interacting with the network from a controlled
  lab environment to determine user tolerance to
  various levels of service

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Factors That Affect Availability

• Throughput
• Response Time
• Access to Services

• You can increase availability by adding more
  resources (i.e. bandwidth, servers, etc.), but
  this drives up cost
• Network design seeks to provide the greatest
  availability for the least cost.

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Analyzing Requirements

• Sensitivity Testing
• Evaluate how a network will behave under certain
  conditions.
• Involves breaking stable links and observing the
  results
• how is traffic rerouted
• speed of convergence
• is connectivity lost?
• is some traffic sensitive to the break?
• Increase traffic loads to media saturation point
  and observe results.

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The Three-LayerWAN Model
Table of Contents
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The Importance of Layers

• Designing networks using the OSI model
• Allows the network to be designed in layers
• Uses layers to simplify the tasks required for
  internetworking
• Design elements can be replicated as the network
  grows
• Therefore, networks should be designed using a
  hierarchical model.
• Unfortunately, most networks are thrown together
  into a mesh (a mess!) with little or no vision
  of future needs.

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Benefits of Hierarchical Design

• Scalability
• allows for future growth without sacrificing
  control or functionality
• Ease of Implementation
• logically constructed layers specify the
  functions of each layer
• Ease of troubleshooting
• well-defined functions at each layer aid in the
  isolation of problems
• Predictability
• behavior of functional layers can be estimated
  and planned for
• Protocol support
• allows easier implementation of future
  technologies because the network has been
  logically constructed
• Manageability
• All the above aids net. admin. in overall
  management of the network

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The Hierarchical Design Model

• The three layers are...
• Core layer--provides transport between remote
  sites
• Distribution layer--provides policy-based
  connectivity
• Access layer--provides workgroup/user access to
  network

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Core Layer

• Fast WAN connections between remote sites
• Core links are normally point-to-point with no
  host devices
• Core services include
• T1/T3
• Frame Relay
• ATM
• SMDS

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Distribution Layer

• Provides WAN services to multiple LANs
• Usually the campus backbone
• Uses Fast Ethernet (or Gigabit Ethernet)
• Used on large sites to interconnect buildings

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Access Layer

• Usually a LAN or group of LANs
• Gives access to specific users and workgroups
• This layer is where all hosts (including servers)
  are attached to the network
• We study the design of this layer in Semester 3,
  LAN Design

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WAN Layer Functions
Table of Contents
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Core Layer Functions

• Optimize Transport Between Remote Sites
• Redundant paths to guard against circuit outages
• Provide load sharing and rapid convergence when
  link states change
• Efficient use of bandwidth by...
• Implementing scalable routing protocols and
• Blocking local traffic access to the core

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Distribution Layer Functions

• Policy-Based Connectivity
• Boundary definition packet manipulation
• Control access to services of the core layer and
  other distribution layer routers
• VLAN routing
• Address aggregation (i.e., subnets) route
  optimization
• ACLs and other security measures

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Access Layer Functions

• Workgroup User Access to the Network
• Isolation of Broadcast Traffic
• Shared and Switched Bandwidth
• MAC-layer filtering
• Microsegmentation

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Other Hierarchical Options

• One-Layer Design
• Only a few remote sites need to be connected
• Servers are placed in farms or in each workgroup
  to reduce traffic on the backbone
• Two-Layer Design
• WAN link is used to interconnect separate sites
• Link does not have to be dedicated. An
  alternative would be ISDN.

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Hierarchical Design Advantages

• Controlling data traffic patterns through
  source/destination network layer addressing
• A packet only needs to travel up the hierarchy as
  far as it needs to find the destination.
• With good design, most traffic would be contained
  in the access layer with users accessing their
  workgroup servers
• Server Placement
• Enterprise Servers needed by all workgroups
  should be placed in the Distribution Layer (e.g.
  email, DNS, etc.)
• Workgroup Servers needed by a unique set of users
  should be placed in the Access Layer, preferably
  in the same broadcast domain as the users.

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Table of Contents
End Slide Show