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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