Semester 4, Chapter 3 Allan Johnson WAN Design Semester 4, Chapter 3 Allan Johnson

Table of Contents WAN Design Requirements Go There! WAN Design Requirements Go There! Gathering & Analyzing Requirements Go There! The Three-Layer WAN model Go There! WAN Layer Functions

WAN Design Requirements Table of Contents

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 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.

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

Overriding Goal in WAN Design Minimize Cost While Increasing Network Availability

Gathering & Analyzing Requirements Table of Contents

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 rerouted speed of convergence is connectivity lost? is some traffic sensitive to the break? Increase traffic loads to media saturation point and observe results.

The Three-Layer WAN Model Table of Contents

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 constructed Manageability 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 sites Core links are normally point-to-point with no host devices Core services include: T1/T3 Frame Relay ATM SMDS

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

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 Table of Contents

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

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 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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