WAN Design

1. WAN Design Semester 4, Chapter 3 Allan Johnson

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

3. WAN Design Requirements Table of Contents

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

5. 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.

6. 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

7. Overriding Goal in WAN Design Minimize Cost While Increasing Network Availability

8. Gathering & Analyzing Requirements Table of Contents

9. 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.

10. 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 Traffic Characterization

11. 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.

12. 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

13. 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.

14. 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.

15. The Three-LayerWAN Model Table of Contents

16. 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.

17. 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

18. 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

19. 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

20. 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

21. 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

22. WAN Layer Functions Table of Contents

23. 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

24. 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

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

26. 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.

27. 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.

28. Table of Contents End Slide Show