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WANs need to be developed to meet the
Optimize WAN bandwidth
Maximize the effective service to end users
LANs & shared media networks are being
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.
LANs and WANs, previously logically
separated, must now be fully integrated for
The LAN/WAN network (or corporate internet)
now must be able to handle...
Voice traffic (VoIP)
Bandwidth intensive multimedia applications
Increased business critical data access
Overriding Goal in WAN Design
Minimize Cost While
Gathering & Analyzing
Table of Contents
Factors Affecting Design
Where are all the nodes?
What level of reliability? Host/client speeds? Traffic
What’s the topology? What is the traffic’s characteristics?
Traffic Characterization is critical to successful
WAN design and implementation, but it is
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
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.
Evaluate how a network will behave under certain
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
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
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
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
behavior of functional layers can be estimated and planned for
allows easier implementation of future technologies because the
network has been logically constructed
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
Fast WAN connections between remote sites
Core links are normally point-to-point with no host
Core services include:
Provides WAN services to multiple LANs
Usually the campus backbone
Uses Fast Ethernet (or Gigabit Ethernet)
Used on large sites to interconnect buildings
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
We study the design of this layer in Semester 3, LAN Design
Core Layer Functions
Optimize Transport Between
Redundant paths to guard against circuit
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
Boundary definition & packet manipulation
Control access to services of the core layer and other
distribution layer routers
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
Other Hierarchical Options
Only a few remote sites
need to be connected
Servers are placed in farms
or in each workgroup to
reduce traffic on the
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
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.