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

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

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. 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

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-Layer WAN 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. , 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
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