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TRANSCRIPT
NETWORK DESIGN METHODOLOGY
TOP-DOWN NETWORK DESIGN
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Top-Down Network Design, 3rd Edition
Priscilla Oppenheimer
Designing and Supporting Computer Networks (CCNA)
Four phases in the top-down network design methodology
I: Identifying Your Customer's Needs and Goals
II: Logical Network Design
III: Physical Network Design
IV: Testing, Optimizing, and Documenting Your Network Design
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PART 1IDENTIFYING YOUR CUSTOMER’S NEEDS AND GOALS
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Analyzing Business Goals and Constraints Analyzing Technical Goals and Tradeoffs Characterizing the Existing Internetwork Characterizing Network Traffic
CHAPTER ONEANALYZING BUSINESS GOALS AND CONSTRAINTS
Systematic, Top-down network design methodology
Analysing your customer’s business objectives
Analysing the business constrains; budgets, timeframes, workplace politics.
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NETWORK DESIGN
Good network design must recognizes customer’s requirements.
Network design choices and tradeoffs must be made when designing the logic network before any physical devices are selected.
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STRUCTURED NETWORK DESIGN
Four fundamental network design goals: Scalability Availability Security Manageability
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STRUCTURED NETWORK DESIGN
Core Layer: connects Distribution Layer devices Distribution Layer: interconnects smaller LANs Access Layer: provides connections for hosts
and end devices
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START FROM THE TOP
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Application
Presentation
Session
Transport
Network
Data Link
PhysicalLayer 1
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2
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SYSTEMS DEVELOPMENT LIFE CYCLES(SDLC)
Typical systems are developed and continue to exist over a period of time, often called a systems development life cycle (SDLC).
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Analyze requirements
Develop logical design
Develop physical design
Test, optimize, and document
design
Monitor and optimize network
performance
Implement and test network
TOP-DOWN NETWORK DESIGN STEPS
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systems development life cycle (SDLC).
THE PDIOO NETWORK LIFE CYCLE
Plan
Design
Implement
Operate
OptimizeRetire
Plan Design Implement Operate Optimize
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NETWORK DESIGN STEPS
Phase 1 – Analyze RequirementsAnalyze business goals and constraintsAnalyze technical goals and tradeoffsCharacterize the existing networkCharacterize network traffic
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NETWORK DESIGN STEPS
Phase 2 – Logical Network DesignDesign a network topologyDesign models for addressing and namingSelect switching and routing protocolsDevelop network security strategiesDevelop network management strategies
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NETWORK DESIGN STEPS
Phase 3 – Physical Network DesignSelect technologies and devices for
campus networksSelect technologies and devices for
enterprise networks
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NETWORK DESIGN STEPS
Phase 4 – Testing, Optimizing, and Documenting the Network DesignTest the network designOptimize the network designDocument the network design
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BUSINESS GOALS
Increase revenue Reduce operating costs Improve communications Shorten product development cycle Expand into worldwide markets Build partnerships with other companies Offer better customer support or new
customer services
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RECENT BUSINESS PRIORITIES Mobility Security Resiliency (fault tolerance) Business continuity after a disaster Network projects must be prioritized
based on fiscal goals Networks must offer the low delay
required for real-time applications such as VoIP
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BUSINESS CONSTRAINTS
Budget Staffing Schedule Politics and policies
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COLLECT INFORMATION BEFORE THE FIRST MEETING
Before meeting with the client, whether internal or external, collect some basic business-related information
Such asProducts produced/Services suppliedFinancial viabilityCustomers, suppliers, competitorsCompetitive advantage
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MEET WITH THE CUSTOMER
Try to getA concise statement of the goals of
the project What problem are they trying to solve? How will new technology help them be
more successful in their business? What must happen for the project to
succeed?
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MEET WITH THE CUSTOMERGet a copy of the organization chart
This will show the general structure of the organization It will suggest users to account for It will suggest geographical locations to account for
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MEET WITH THE CUSTOMERGet a copy of the security policy
How does the policy affect the new design? How does the new design affect the policy? Is the policy so strict that you (the network
designer) won’t be able to do your job?Start cataloging network assets that security
should protect Hardware, software, applications, and data Less obvious, but still important, intellectual
property, trade secrets, and a company's reputation
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THE SCOPE OF THE DESIGN PROJECT
Small in scope?Allow sales people to access network via a VPN
Large in scope?An entire redesign of an enterprise network
Use the OSI model to clarify the scopeNew financial reporting application versus new
routing protocol versus new data link (wireless, for example)
Does the scope fit the budget, capabilities of staff and consultants, schedule?
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GATHER MORE DETAILED INFORMATION
ApplicationsNow and after the project is completed Include both productivity applications and
system management applications User communities Data stores Protocols Current logical and physical architecture Current performance
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SUMMARY
Systematic approach Focus first on business requirements and
constraints, and applications Gain an understanding of the customer’s
corporate structure Gain an understanding of the customer’s
business style
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REVIEW QUESTIONS
What are the main phases of network design per the top-down network design approach?
What are the main phases of network design per the PDIOO approach?
Why is it important to understand your customer’s business style?
What are some typical business goals for organizations today?
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CHAPTER TWO
ANALYZING TECHNICAL GOALS AND TRADEOFFS
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Copyright 2010 Cisco Press & Priscilla Oppenheimer
TECHNICAL GOALS
Scalability Availability Performance Security Manageability Usability Adaptability Affordability
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Your lab report should reflect some of these goals of your own designed network.
SCALABILITY Scalability refers to the ability to grow Some technologies are more scalable
Flat network designs, for example, don’t scale well
Try to learnNumber of sites to be addedWhat will be needed at each of these sitesHow many users will be addedHow many more servers will be added
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AVAILABILITY
Availability can be expressed as a percent uptime per year, month, week, day, or hour, compared to the total time in that periodFor example:
24/7 operation Network is up for 165 hours in the 168-hour week Availability is 98.21%
Different applications may require different levels
Some enterprises may want 99.999% or “Five Nines” availability
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AVAILABILITY
Availability can also be expressed as a mean time between failure (MTBF) and mean time to repair (MTTR)
Availability = MTBF/(MTBF + MTTR)For example:
The network should not fail more than once every 4,000 hours (166 days) and it should be fixed within one hour
4,000/4,001 = 99.98% availability
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AVAILABILITY
DOWNTIME IN MINUTES
4.32
1.44
.72
.01
30
10
5
.10
1577(26
H)
99.70%
52699.90%
26399.95%
599.999%
Per Hour Per Day Per Week Per Year
.18
.06
.03
.0006
.29 2 10599.98% .012
99.999% AVAILABILITY MAY REQUIRE TRIPLE REDUNDANCY
Can the customer afford this?37
Enterprise
ISP 1 ISP 2 ISP 3
SERVER FARMS
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Many enterprise networks provide users with Internet-accessible services, such as email and e-commerce. The availability and security of these services are crucial to the success of a business.Managing and securing numerous distributed servers at various locations within a business network is difficult. Recommended practice centralised servers in server farms. Server farms are typically located in computer rooms and data centres.
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BENEFITS OF CREATING A SERVER FARM
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1. Network traffic enters and leaves the server farm at a defined point. This arrangement makes it easier to secure, filter and prioritise traffic.
2. Redundant, high-capacity links can be installed to the servers and between the server farm network and the main LAN. This configuration is more cost-effective than attempting to provide a similar level of connectivity to servers distributed throughout the network.
3. Load balancing and failover can be provided between servers and between networking devices.
4. The number of high-capacity switches and security devices is reduced, helping to lower the cost of providing services.
NETWORK PERFORMANCE Common performance factors include
BandwidthThroughputBandwidth utilizationOffered loadAccuracyEfficiencyDelay (latency) and delay variationResponse time
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BANDWIDTH VS. THROUGHPUT
Bandwidth and throughput are not the same
Bandwidth is the data carrying capacity of a circuit, fixed.
Usually specified in bits per second-bps
Throughput is the quantity of error free data transmitted per unit of time
Measured in bps, Bps, or packets per second (pps) Depend on offered load, access method and error
rate
Throughput < Bandwidth 43
BANDWIDTH, THROUGHPUT, LOAD
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Offered Load
Throughput
Actual
Idea
l
100 % of Capacity
100 % of Capacity
OTHER FACTORS THAT AFFECT THROUGHPUT
The size of packets Inter-frame gaps between packets Packets-per-second ratings of devices that forward
packets Client speed (CPU, memory, and HD access speeds) Server speed (CPU, memory, and HD access speeds) Network design MAC Protocols (ALOHA 18.4%) Distance Errors Time of day, etc., etc., etc.
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THROUGHPUT VS. GOODPUT Are you referring to bytes per second,
regardless of whether the bytes are user data bytes or packet header bytesOr are you concerned with application-layer
throughput of user bytes, sometimes called “goodput” In that case, you have to consider that bandwidth is
being “wasted” by the headers in every packet
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PERFORMANCE (CONTINUED)
EfficiencyHow much overhead is required to deliver an
amount of data?How large can packets be?
Larger better for efficiency (and goodput) But too large means too much data is lost if a
packet is damaged How many packets can be sent in one bunch
without an acknowledgment?
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EFFICIENCY
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Small Frames (Less Efficient)
Large Frames (More Efficient)
DELAY FROM THE USER’S POINT OF VIEW
Response TimeA function of the
application and the equipment the application is running on, not just the network
Most users expect to see something on the screen in 100 to 200 milliseconds
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DELAY FROM THE ENGINEER’S POINT OF VIEW
Propagation delayA signal travels in a cable at about 2/3 the
speed of light in a vacuum (3×108 m/s) Transmission delay (also known as
serialization delay)Time to put digital data onto a transmission
line For example, it takes about 5 ms to output a 1,024
byte packet on a 1.544 Mbps T1 line Packet-switching delay Queuing delay
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QUEUING DELAY AND BANDWIDTH UTILIZATION
Number of packets in a queue increases exponentially as utilization increases
Queue depth = utilization/(1- utilization)
0
3
6
9
12
15
0.5 0.6 0.7 0.8 0.9 1
Average Utilization
Ave
rag
e Q
ue
ue
De
pth
EXAMPLE A packet switch has 5 users, each offering
packets at a rate of 10 packets per second The average length of the packets is 1,024
bits The packet switch needs to transmit this
data over a 56-Kbps WAN circuitLoad = 5 x 10 x 1,024 = 51,200 bpsUtilization = 51,200/56,000 = 91.4%Average number of packets in queue =
(0.914)/(1-0.914) = 10.63 packets
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SECURITY
Focus on requirements first Detailed security planning later
(Chapter 8) Identify network assets
Including their value and the expected cost associated with losing them due to a security problem
Analyze security risks
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MANAGEABILITY
Fault management Configuration management Accounting management Performance management Security management
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USABILITY
Usability: the ease of use with which network users can access the network and services
Networks should make users’ jobs easier
Some design decisions will have a negative affect on usability:Strict security, for example
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ADAPTABILITY
Avoid incorporating any design elements that would make it hard to implement new technologies in the future
Change can come in the form of new protocols, new business practices, new fiscal goals, new legislation
A flexible design can adapt to changing traffic patterns and Quality of Service (QoS) requirements
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AFFORDABILITY
A network should carry the maximum amount of traffic possible for a given financial cost
Affordability is especially important in campus network designs
WANs are expected to cost more, but costs can be reduced with the proper use of technologyQuiet routing protocols, for example
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MAKING TRADEOFFS (EXAMPLE)
Scalability 20 Availability 30 Network performance 15 Security 5 Manageability 5 Usability 5 Adaptability 5 Affordability 15Total (must add up to 100) 100
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SUMMARY
Continue to use a systematic, top-down approach
Don’t select products until you understand goals for scalability, availability, performance, security, manageability, usability, adaptability, and affordability
Tradeoffs are almost always necessary
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REVIEW QUESTIONS
What are some typical technical goals for organizations today?
How do bandwidth and throughput differ? How can one improve network efficiency? What tradeoffs may be necessary in order
to improve network efficiency?
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CHAPTER THREE
CHARACTERIZING THE EXISTING INTERNETWORK
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Copyright 2010 Cisco Press & Priscilla Oppenheimer
WHAT’S THE STARTING POINT? According to Abraham Lincoln:
“If we could first know where we are and whither we are tending, we could better judge what to do and how to do it.”
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WHERE ARE WE?
Characterize the exiting internetwork in terms of: Its infrastructure
Logical structure (modularity, hierarchy, topology) Physical structure
Addressing and namingWiring and mediaArchitectural and environmental constraintsHealth
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DIAGRAM A PHYSICAL NETWORK AND DOCUMENT THE EXISTING NETWORK
Network documentation: Logical and physical diagrams Floor plans Complete lists for equipments and
applications Current network configuration files
GET A NETWORK MAP (PHYSICAL)
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Gigabit Ethernet
Eugene Ethernet20 users
Web/FTP server
Grants PassHQ
Gigabit Ethernet
FEP (Front End Processor)
IBMMainframe
T1
MedfordFast Ethernet
50 users
RoseburgFast Ethernet
30 usersFrame Relay
CIR = 56 KbpsDLCI = 5
Frame RelayCIR = 56 Kbps
DLCI = 4
Grants PassHQ
Fast Ethernet75 users
InternetT1
DIAGRAM A PHYSICAL NETWORK AND DOCUMENT THE EXISTING NETWORK
Identify and document the strengths and weaknesses of the existing network
Focus on finding ways to overcome weaknesses
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CHARACTERIZE ADDRESSING AND NAMING
IP addressing for major devices, client networks, server networks, and so on
Any addressing oddities, such as discontiguous subnets?
Any strategies for addressing and naming?For example, sites may be named using airport
codes San Francisco = SFO, Oakland = OAK In LSBU, T-tower block; K-keyworth building; B-
Borough road building; L- london road building
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DISCONTINUOUS SUBNETS – make problems for some routing protocols
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Area 1Subnets 10.108.16.0 -
10.108.31.0
Area 0Network
192.168.49.0
Area 2Subnets 10.108.32.0 -
10.108.47.0
Router A Router B
CHARACTERIZE THE WIRING AND MEDIA
Single-mode fiber Multi-mode fiber Shielded twisted pair (STP) copper Unshielded-twisted-pair (UTP) copper Coaxial cable Microwave Laser Radio Infra-red
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ARCHITECTURAL CONSTRAINTS
Make sure the following are sufficientAir conditioningHeatingVentilationPowerProtection from electromagnetic interferenceDoors that can lock
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ARCHITECTURAL CONSTRAINTS
Make sure there’s space for:Cabling conduits Patch panelsEquipment racksWork areas for technicians installing and
troubleshooting equipment
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CHECK THE HEALTH OF THE EXISTING INTERNETWORK
Performance Availability Bandwidth utilization Accuracy Efficiency Response time Status of major routers, switches, and
firewalls74
CHARACTERIZE AVAILABILITY
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Enterprise
Segment 1
Segment 2
Segment n
MTBF MTTRDate and Duration of Last Major Downtime
Cause of Last Major Downtime
Fix for Last Major Downtime
Mean time between failures (MTBF)Mean time to recovery (MTTR)
Network Utilization
0 1 2 3 4 5 6 7
17:10:00
17:07:00
17:04:00
17:01:00
16:58:00
16:55:00
16:52:00
16:49:00
16:46:00
16:43:00
16:40:00
Tim
e
Utilization
Series1
NETWORK UTILIZATION IN MINUTE INTERVALS
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Network Utilization
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
17:00:00
16:00:00
15:00:00
14:00:00
13:00:00
Tim
e
Utilization
Series1
NETWORK UTILIZATION IN HOUR INTERVALS
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BANDWIDTH UTILIZATION BY PROTOCOL
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Protocol 1
Protocol 2
Protocol 3
Protocol n
Relative Network Utilization
Absolute Network Utilization
Broadcast Rate
Multicast Rate
CHARACTERIZE PACKET SIZES
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CHARACTERIZE RESPONSE TIME
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Node A
Node B
Node C
Node D
Node A Node B Node C Node D
X
X
X
X
CHECK THE STATUS OF MAJOR ROUTERS, SWITCHES, AND FIREWALLS
show buffers show environment show interfaces show memory show processes show running-config show version
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Use Cisco IOS show command
TOOLS
Protocol analyzers Multi Router Traffic Grapher (MRTG) Remote monitoring (RMON) probes Cisco Discovery Protocol (CDP) Cisco IOS NetFlow technology CiscoWorks
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SUMMARY
Characterize the exiting internetwork before designing enhancements
Helps you verify that a customer’s design goals are realistic
Helps you locate where new equipment will go
Helps you cover yourself if the new network has problems due to unresolved problems in the old network
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REVIEW QUESTIONS
What factors will help you decide if the existing internetwork is in good enough shape to support new enhancements?
When considering protocol behavior, what is the difference between relative network utilization and absolute network utilization?
Why should you characterize the logical structure of an internetwork and not just the physical structure?
What architectural and environmental factors should you consider for a new wireless installation?
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CHAPTER FOUR
CHARACTERIZING NETWORK TRAFFIC
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Copyright 2010 Cisco Press & Priscilla Oppenheimer
NETWORK TRAFFIC FACTORS
Traffic flow Location of traffic sources and data
stores Traffic load Traffic behavior Quality of Service (QoS) requirements
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USER COMMUNITIES, a set of worker who use a particular application or set of applications.
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User Community Name
Size of Community (Number of Users)
Location(s) of Community
Application(s) Used by Community
DATA STORES (SINKS), an area in a network where application layer data resides. Server, or any device where large quantities of data are stored.
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Data Store Location Application(s) Used by User Community(or Communities)
TRAFFIC FLOW, involves identifying and characterizing individual traffic flows between traffic source and stores.
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Destination 1 Destination 2 Destination 3Destination MB/sec MB/secMB/sec MB/sec
Source 1
Source 2
Source 3
Source n
TRAFFIC FLOW EXAMPLE
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Administration
Business and Social Sciences
Math and Sciences
50 PCs 25 Macs50 PCs
50 PCs30 PCs
30 Library Patrons (PCs) 30 Macs and 60 PCs in Computing Center
Library and Computing Center
App 1 108 KbpsApp 2 60 KbpsApp 3 192 KbpsApp 4 48 KbpsApp 7 400 KbpsTotal 808 Kbps
App 1 48 KbpsApp 2 32 KbpsApp 3 96 KbpsApp 4 24 KbpsApp 5 300 KbpsApp 6 200 KbpsApp 8 1200 KbpsTotal 1900 Kbps
App 1 30 KbpsApp 2 20 KbpsApp 3 60 KbpsApp 4 16 KbpsTotal 126 Kbps
App 2 20 KbpsApp 3 96 KbpsApp 4 24 KbpsApp 9 80 KbpsTotal 220 Kbps
Arts and Humanities
Server Farm
10-Mbps Metro Ethernet to Internet
TYPES OF TRAFFIC FLOW
Terminal/host Client/server Thin client Peer-to-peer Server/server Distributed computing
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TRAFFIC FLOW FOR VOICE OVER IP
The flow associated with transmitting the audio voice is separate from the flows associated with call setup and teardown. The flow for transmitting the digital
voice is essentially peer-to-peer.Call setup and teardown is a
client/server flow A phone needs to talk to a server or
phone switch that understands phone numbers, IP addresses, capabilities negotiation, and so on.
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IDENTIFYING APPLICATION IMPACTS ON NETWORK DESIGN
File transfer and email applications: Unpredictable bandwidth usage Large packet size Centralization of file and mail servers in a
secure location Redundancy to ensure reliable service
IDENTIFYING APPLICATION IMPACTS ON NETWORK DESIGN
HTTP and web traffic: Network media Redundancy Security
NETWORK APPLICATIONSTRAFFIC CHARACTERISTICS
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Name of Application
Type of Traffic Flow
Protocol(s) Used by Application
User Communities That Use the Application
Data Stores (Servers, Hosts, and so on)
Approximate Bandwidth Requirements
QoS Requirements
TRAFFIC LOAD To calculate whether capacity is
sufficient, you should know:The number of stationsThe average time that a station is idle
between sending framesThe time required to transmit a message
once medium access is gained That level of detailed information can be
hard to gather, however
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SIZE OF OBJECTS ON NETWORKS
Terminal screen: 4 Kbytes Simple e-mail: 10 Kbytes Simple web page: 50 Kbytes High-quality image: 50Mbytes Database backup: 1Gbytes or more
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TRAFFIC BEHAVIOR
Broadcasts All ones data-link layer destination address
FF: FF: FF: FF: FF: FF Doesn’t necessarily use huge amounts of
bandwidth But does disturb every CPU in the broadcast
domain Multicasts
First bit sent is a one 01:00:0C:CC:CC:CC (Cisco Discovery Protocol)
Should just disturb NICs that have registered to receive it
Requires multicast routing protocol on internetworks 98
NETWORK EFFICIENCY
Frame size Protocol interaction Windowing and flow control Error-recovery mechanisms
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QOS REQUIREMENTS
ATM service specificationsConstant bit rate (CBR)Realtime variable bit rate (rt-VBR)Non-realtime variable bit rate (nrt-VBR)Unspecified bit rate (UBR)Available bit rate (ABR)Guaranteed frame rate (GFR)
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QOS REQUIREMENTS PER IETF (Internet Engineering Task Force, develops and promotes Internet standards, It is an open standards organization, with no formal membership or membership requirements.)
IETF integrated services working group specificationsControlled load service
Provides client data flow with a QoS closely approximating the QoS that same flow would receive on an unloaded network
Guaranteed service Provides firm (mathematically provable) bounds
on end-to-end packet-queuing delays101
QOS REQUIREMENTS PER IETF
IETF differentiated services working group specificationsRFC 2475 IP packets can be marked with a
differentiated services codepoint (DSCP) to influence queuing and packet-dropping decisions for IP datagrams on an output interface of a router
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HOW QUALITY OF SERVICE IS IMPLEMENTED ON THE LAN/WAN
Where QoS can be implemented to affect traffic flow:
Layer 2 devices Layer 3 devices
DOCUMENT THE NETWORK REQUIREMENTS OF SPECIFIC CATEGORIES OF APPLICATIONS
Estimate the volume of application traffic during the initial design phase.
Document projected applications and associated hardware in a network diagram.
SUMMARY
Continue to use a systematic, top-down approach
Don’t select products until you understand network traffic in terms of:FlowLoadBehaviorQoS requirements
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REVIEW QUESTIONS
List and describe six different types of traffic flows.
What makes traffic flow in voice over IP networks challenging to characterize and plan for?
Why should you be concerned about broadcast traffic?
How do ATM and IETF specifications for QoS differ?
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