WAN Design
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WAN Design Requirements
Gathering & Analyzing Requirements
The Three-Layer WAN model
WAN Layer Functions
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WAN Design Requirements
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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
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 serversThe number on intra- and extranets continues to rise
LANs connected through WANs is expected to increase WAN traffic 300% in the next 5 years.
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 conferencingOn-line training
Increased business critical data access
Overriding Goal in WAN Design
Minimize Cost While Increasing Network
Availability
Gathering & Analyzing Requirements
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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 What’s the topology? What is the traffic’s characteristics?
Traffic Characterization is critical to successful WAN design and implementation, but it is seldom done.
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
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.
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
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.
Analyzing Requirements
Sensitivity Testing Evaluate how a network will behave under certain
conditions. Involves breaking stable links and observing the results
how is traffic reroutedspeed of convergenceis connectivity lost?is some traffic sensitive to the break?
Increase traffic loads to media saturation point and observe results.
The Three-LayerWAN Model
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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.
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 constructedManageability All the above aids net. admin. in overall management of the network
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
Core Layer
Fast WAN connections between remote sitesCore links are normally point-to-point with no host
devicesCore services include:
T1/T3Frame RelayATMSMDS
Distribution Layer
Provides WAN services to multiple LANsUsually the campus backboneUses Fast Ethernet (or Gigabit Ethernet)Used on large sites to interconnect buildings
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
WAN Layer Functions
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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 andBlocking local traffic access to the core
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
Access Layer Functions
Workgroup & User Access to the Network Isolation of Broadcast Traffic Shared and Switched Bandwidth MAC-layer filtering Microsegmentation
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.
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 serversServer 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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