Network Analysis and Design

Introduction to Network Design

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Network Design

A network design is a blueprint for building a network

The designer has to create the structure of the network [and] decide how to allocate resources and spend money

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Elements of Good Network Design

Deliver the services requested by users

Deliver acceptable throughput and response times

Cost efficiency Reliable Expandable Manageable Well-documented

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Network Design Issues

User requirements Locations of devices Characteristics of applications Types of traffic Topologies Routing protocols Budget Performance Etc.

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Classifications of Network Design

Build a new network Expand or upgrade the existing

network Create the overlay network

Virtual Private Network (VPN)

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Types of Networks

Access network: The ends or tails of networks that

connect the small sites into the networkLAN, campus network

Backbone network:The network that connects major sitesCorporate WAN

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Objectives

How to design a network using the correct techniques?

Some common guidelines applicable for all types of network design

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Top-Down Network Design Methodology

A complete process that matches business needs to available technology to deliver a system that will maximize an organization’s success

Don’t just start connecting the dots In the LAN, it is more than just buying a

few devices In the WAN, it is more than just calling the

phone company

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Top-Down Network Design Methodology (Contd.)

Analyze business and technical goals first

Explore divisional and group structures to find out who the network serves and where they reside

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Top-Down Network Design Methodology (Contd.)

Determine what applications will run on the network and how those applications behave on a network

Focus on applications, sessions, and data transport before the selection of routers, switches, and media that operate at the lower layers

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Network Design Phases

Requirement analysis Logical network design Physical network design

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Phase I - Requirement Analysis Phase

Analyze goals and constraints Characterize the existing network Characterize network traffic

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Phase II - Logical Network Design Phase

Map the requirements into the conceptual design

Design a network topology Node locations Capacity assignment

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Phase III - Physical Network Design Phase

Select technologies and devices for your design

Implementation

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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 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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Information

Goals of the project What problem are they trying to solve? How will new technology help them be more

successful in their business? Scope of the project

Small in scope: Allow sales people to access network via a VPN

Large in scope: An entire redesign of an enterprise network

Does the scope fit the budget, capabilities of staff and consultants, schedule?

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Information (Contd.)

Applications, protocols, and services Current logical and physical architecture Current performance

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Technical Goals

Scalability Availability Performance Security Manageability Usability Adaptability Affordability

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Scalability

Scalability refers to the ability to grow Network must adapt to increases in

network usage and scope in the future Flat network designs don’t scale well Broadcast traffic affects the scalability of

a network

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Availability

Availability is the amount of time a network is available to users

Availability can be expressed as a percent up time per year, month, week, day, or hour, compared to the total time in that period 24/7 operation Network is up for 165 hours in the 168-

hour week Availability is 98.21%

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Availability (Contd.)

Different applications may require different levels

Some enterprises may want 99.999% or “Five Nines” availability

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Availability (Contd.)

An uptime of 99.70 % Downtime = 0.003 x 60 x 24 x 7 30.24 mins per week

An uptime of 99.95 % Downtime = 0.0005 x 60 x 24 x 7 5.04 mins per week

An uptime of 99.999 % Downtime = 0.00001 x 60 x 24 x 365 5.256 mins per year

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Availability (Contd.)

System availability (R) is calculated from the component availability (Ri)

Series: R = Ri

Parallel: R = 1 – (1 – Ri)

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Availability (Contd.)

R1 = 99.95%, R2 = 99.5%

Series: R = 0.9995 x 0.995 = 99.45% Decreases system availability

Parallel: R = 1 – [(1 – 0.9995) x (1 – 0.995)] =

99.99975% Increases system availability

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Availability (Contd.)

99.999% may require high redundancy (and cost)

Enterprise

ISP 1 ISP 2 ISP 3

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Availability (Contd.)

Availability can also be expressed as a mean time between failure (MTBF), and mean time to repair (MTTR)

Availability = MTBF / (MTBF + MTTR) A typical MTBF goal for a network that is

highly relied upon is 4000 hours. A typical MTTR goal is 1 hour.

4000 / 4001 = 99.98% availability

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Network Performance

Common performance factors include Bandwidth Throughput Bandwidth utilization Offered load Accuracy Efficiency Delay (latency) and delay variation Response time

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Bandwidth Vs. Throughput

They are not the same thing Bandwidth is the data carrying capacity

of a circuit Usually specified in bits per second Fixed

Throughput is the quantity of error free data transmitted per unit of time Measured in bps, Bps, or packets per

second (pps) Varied

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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 Protocols Distance Errors Time of day etc.

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Throughput of Devices

The maximum PPS rate at which the device can forward packets without dropping any packets

Theoretical maximum is calculated by dividing bandwidth by frame size, including any headers, preambles, and interframe gaps

SizeHeaderSizeFrame

BandwidthPPS

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Throughput of Devices (Contd.)

Frame Size

(Bytes)

Theoretical Max PPS

(100-Mbps Ethernet)

64 148,800

128 84,450

256 45,280

512 23,490

768 15,860

1024 11,970

1280 9,610

1518 8,120

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Bandwidth, Throughput, Load

Offered Load

Throughput

Actual

Idea

l

100 % of Capacity

100 % of Capacity

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Throughput Vs. Goodput

Most end users are concerned about the throughput for applications

Goodput is a measurement of good and relevant application layer data transmitted per unit of time

In that case, you have to consider that bandwidth is being “wasted” by the headers in every packet

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Utilization

The percent of total available capacity in use

For WANs, optimum average network utilization is about 70%

For hub-based Ethernet LANs, utilization should not exceed 37%, beyond this limit, collision becomes excessive

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Utilization (Contd.)

For full-duplex Ethernet LANs, a point-to-point Ethernet link supports simultaneous transmitting and receiving

Theoretically, Fast Ethernet means 200 Mbps available Gigabit Ethernet means 2 Gbps available 100% of this bandwidth can be utilized

Full-duplex Ethernet is becoming the standard method for connecting servers, switches, and even end users' machines

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Efficiency

Large headers are one cause for inefficiency

How 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 (Contd.)

Small Frames (Less Efficient)

Large Frames (More Efficient)

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Delay from the User’s Point of View Response Time

The time between a request for some service and a response to the request

The network performance goal that users care about most

A 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 ms

The 100-ms threshold is often used as a timer value for protocols that offer reliable transport of data

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Delay from the Engineer’s Point of View Propagation delay

Signal travels in a cable at about 2/3 the speed of light in a vacuum

Relevant for all data transmission technologies, but especially for satellite links and long terrestrial cables

Geostationary satellites: propagation delay is about 270 ms for an intercontinental satellite hop

Terrestrial cables: propagation delay is about 1 ms for every 200 km

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Delay from the Engineer’s Point of View (Contd.)

Transmission delay Also known as serialization delay Time to put digital data onto a transmission

line Depends on the data volume and the data

rate of the line It takes about 5 ms to output a 1,024 byte

packet on a 1.544 Mbps T1 line

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Delay from the Engineer’s Point of View (Contd.) Packet-switching delay

The latency accrued when switches and routers forward data

The latency depends on the speed of the internal circuitry and CPU the switching architecture of the internetworking

device the type of RAM that the device uses

Routers tend to introduce more latency than switches

QoS, NAT, filtering, and policies introduce delay

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Delay from the Engineer’s Point of View (Contd.)

Queueing delay The average number of packets in a queue

on a packet-switching device increases exponentially as utilization increases

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Queuing Delay and Bandwidth Utilization

Number of packets in a queue increases exponentially as utilization increases

0

3

6

9

12

15

0.5 0.6 0.7 0.8 0.9 1

Average Utilization

Ave

rage

Que

ue D

epth

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Delay Variation (Jitter)

The amount of time average delay varies Users of interactive applications expect minimal

delay in receiving feedback from the network Users of multimedia applications require a

minimal variation in the amount of delay Delay must be constant for voice and video

applications Variations in delay cause disruptions in voice

quality and jumpiness in video streams

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Delay Variation (Jitter) (Contd.)

Short fixed-length cells, for example ATM 53-byte cells, are inherently better for meeting delay and delay-variance goals

Packet size tradeoffs Efficiency for high-volume applications

versus low and non-varying delay for multimedia

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Delay Variation (Jitter) (Contd.)

Audio/video applications minimize jitter by providing a buffer that the network puts data into

Display software or hardware pulls data from the buffer

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Accuracy

Data received at the destination must be the same as the data sent by the source

Error fames must be retransmitted, which has a negative effect on throughput

In IP networks, TCP provides retransmission of data

For WAN links, accuracy goals can be specified as a bit error rate (BER) threshold Fiber-optic links: about 1 in 1011

Copper links: about 1 in 106

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Accuracy (Contd.)

On shared Ethernet, errors often result from collisions Collisions happen in the 8-byte preamble

of the frames (not counted) Collisions happen past the preamble and

somewhere in the first 64 bytes of the data frame (legal collision)

Collisions happen beyond the first 64 bytes of a frame (late collision)

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Accuracy (Contd.)

Late collisions are illegal and should never happen (too large network)

A goal for Ethernet collisions: less than 0.1% affected by a legal collision

Collisions should never occur on full-duplex Ethernet links

In wireless LAN 802.11 CSMA/CA, collisions can still occur

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Security

Security design is one of the most important aspects of enterprise network design

Security problems should not disrupt the company's ability to conduct business

The cost to implement security should not exceed the cost to recover from security incidents

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Security (Contd.)

Network Assets Hardware Software Applications Data Intellectual property Trade secrets Company’s reputation

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Affordability

Affordability is sometimes called cost-effectiveness

A network should carry the maximum amount of traffic for a given financial cost

Financial costs include nonrecurring equipment costs and recurring network operation costs

Campus networks: low cost is often more important than availability and performance.

Enterprise networks: availability is usually more important than low cost

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Affordability (Contd.)

Monthly charges for WAN circuits are the most expensive aspect of running a large network

How to save Use a routing protocol that minimizes WAN traffic Improve efficiency on WAN circuits by using such f

eatures as compression Eliminate underutilized trunks Use technologies that support oversubscription

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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 traffic patterns

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Usability

The ease of use with which network users can access the network and services

Usability might also include a need for mobility

Some design decisions will have a negative affect on usability: Strict security, for example

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Characterizing a Network (Why?)

Verify that a customer's technical design goals are realistic

Understand the current topology Locate existing network segments and

equipment Locate where new equipment will go Develop a baseline of current

performance

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Characterizing a Network (What?)

Infrastructure Addressing and naming Wiring and media Architectural and environmental

constraints Health

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Infrastructure

Develop a set of network maps Learn the location of major

internetworking devices and network segments

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Infrastructure (Contd.)

Information to collect Geographical locations LAN, WAN connections Buildings and floors, and possibly rooms Location of major servers or server farms Location of routers and switches Location of mainframes Location of major network-management stations Location and reach of virtual LANs (VLANs) Etc.

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Infrastructure (Contd.)

Gigabit Ethernet

Eugene Ethernet20 users

Web/FTP server

Grants PassHQ

16 MbpsToken Ring

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

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Addressing and Naming

IP addressing for major devices, client networks, server networks

What to consider? Private/public address Classless/classful addressing Variable-length subnet mask (VLSM) Route aggregation or supernetting Discontiguous subnets

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Discontiguous Subnets

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

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Wiring and Media

Document the types of cabling in use as well as cable distances

Distance information is useful when selecting data link layer technologies based on distance restrictions

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Wiring and Media (Contd.)

Single-mode (SM) fiber Multi-mode (MM) 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 sufficient Air conditioning Heating Ventilation Power Protection from electromagnetic

interference

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Architectural Constraints (Contd.)

Make sure there’s space for: Cabling conduits Patch panels Equipment racks Work areas for installing and troubleshooti

ng equipment

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Wireless Installations

Reflection Signal bounces back and interferes with its

elf Metal surfaces such as steel girders, scaff

olding, shelving units, steel pillars, and metal doors

Implementing a WLAN across a parking lot can be tricky because of metal cars that come and go

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Wireless Installations (Contd.)

Absorption Energy of the signal can be absorbed by the

material in objects through which it passes Reduces signal level Water has significant absorption properties, and

objects such as trees or thick wooden structures can have a high water content

Implementing a WLAN in a coffee shop can be tricky if there are large canisters of liquid coffee

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Wireless Installations (Contd.)

Refraction RF signal is bent when it passes from a

medium with one density into a medium with another density

The signal changes direction and may interfere with the nonrefracted signal

It can take a different path and encounter other, unexpected obstructions, and arrive at recipients damaged or later than expected

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Wireless Installations (Contd.)

Diffraction Similar to refraction Like refraction, the signal is bent around

the edge of the diffractive region and can then interfere with that part of the signal that is not bent

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Wireless Installations (Contd.)

Boost the power level to compensate for variable environmental factors

The additional power added to a transmission is called the fade margin

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Health

Performance Availability Bandwidth utilization Accuracy Efficiency Response time Status of major routers, switches, and

firewalls

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Develop a Performance Baseline How much better the new internetwork p

erforms once your design is implemented

Baseline of normal performance should not include nontypical problems caused by exceptionally large traffic loads

The decision whether to measure normal performance, performance during peak load, or both, depends on the goals of the network design

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Characterize Availability

Enterprise

Segment 1

Segment 2

Segment n

MTBF MTTRDate and Duration of Last Major Downtime

Cause of Last Major Downtime

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Utilization

Measurement of how much bandwidth is in use during a specific time interval

Different tools use different averaging windows for computing network utilization

Trade-off between amount of statistical data that must be analyzed and granularity

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Utilization in Minute Intervals

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 (%)

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Utilization in Hour Intervals

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 (%)

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Utilization (Contd.)

The size of the averaging window depends on your goals When troubleshooting network problems,

keep the interval very small, either minutes or seconds

For performance analysis and baselining purposes, use an interval of 1 to 5 minutes

For long-term load analysis, to determine peak hours, days, or months, set the interval to 10 minutes

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Bandwidth Utilization by Protocol

Protocol 1

Protocol 2

Protocol 3

Protocol n

Relative Network Utilization

Absolute Network Utilization

Broadcast Rate

Multicast Rate

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Accuracy

Bit error rate (BER) Frame error rate (FER) Packet loss Collision Runt (partial) frame Healthy network should not have more

than one bad frame per megabyte of data

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Characterize Packet Sizes

Increasing the maximum transmission unit (MTU) on router interfaces can also improve efficiency

Increasing MTU can increase serialization delay

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Characterize Packet Sizes (Contd.)

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Characterize Packet Sizes (Contd.)

Small frames consist of control information and acknowledgments

Data frames fall into the large frame-size categories

Frame sizes typically fall into what is called a bimodal distribution

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Characterize Response Time

A more common way to measure response time is to send ping packets and measure the round-trip time (RTT)

Variance measurements are important for applications that cannot tolerate much jitter

You can also document any loss of packets

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Characterize Response Time (Contd.)

Node A

Node B

Node C

Node D

Node A Node B Node C Node D

X

X

X

X

node = router, server, client, or mainframe

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Checking Status of Major Devices

CPU utilization How many packets it has processed How many packets it has dropped Status of buffers and queues You can use SNMP or commands in the

devices

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Characterizing Network Traffic (Why?)

Analyze network traffic patterns to help you select appropriate logical and physical network design solutions to meet a customer's goals

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Network Traffic Factors

Location of traffic sources and sinks Traffic load Traffic behavior

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Traffic Flow

Information transmitted between communicating entities during a single session

Flow attributes: addresses for each end of the flow direction symmetry path number of packets or bytes

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Traffic Flow Types

Terminal/host Client/server Peer-to-peer Server/server Voice over IP

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Terminal / Host

Examples: Telnet, ssh Usually asymmetric: terminal sends a few

characters and the host sends many characters In some full-screen terminal applications, the ter

minal sends characters typed by the user and the host returns data to repaint the screen

The screen is usually 80 characters wide by 24 lines long, which equals 1920 characters

The full transfer is a few thousand bytes

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Client / Server

Examples: FTP, HTTP Usually bidirectional and asymmetric Requests are typically small frames exce

pt when writing data to the server Responses range from 64 bytes to 1500

bytes or more, depending on the MTU of the data link layer

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Peer-to-Peer

Examples: Workgroup, videoconferencing, P2Ps

No hierarchy and no dedicated server Usually bidirectional and symmetrical Another example is a meeting between b

usiness people at remote sites using videoconferencing equipment

Information dissemination in a class is a client/server model

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Server / Server

To implement directory services, to cache heavily used data, to mirror data for load balancing and redundancy, to back up data, and to broadcast service availability

Generally bidirectional With most server/server applications, the flow is

symmetrical, but in some cases there is a hierarchy of servers, with some servers sending and storing more data than others

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VoIP

The flow associated with transmitting the audio voice is separate from the flows associated with call control The voice flow for transmitting the digital

voice is essentially peer-to-peer The call control flow for call setup and

teardown is a client/server flow

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Traffic Load

Network capacity is sufficient to avoid bottleneck

Key parameters: Number of stations Average time that a station is idle between

sending frames Time required to transmit a message once

medium access is gained Application usage patterns

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Traffic Load (Contd.)

Traffic load caused by applications Terminal screen: 4 Kbytes Simple e-mail: 10 Kbytes Simple web page: 50 Kbytes High-quality image: 50,000 Kbytes Database backup: 1,000,000 Kbytes or

more

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Traffic Load (Contd.)

Protocol overhead IPX: 30 bytes TCP: 20 bytes IP: 20 bytes Ethernet: 18 + 8-byte preamble + 12-byte

interframe gap (IFG) HDLC: 10 bytes

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Traffic Behavior

Broadcast Goes to all network stations on a LAN 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

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Traffic Behavior (Contd.)

Multicast Goes to a subset of stations 01:00:0C:CC:CC:CC (Cisco Discovery

Protocol) Should just disturb NICs that registered to

receive it Requires multicast routing protocol on

internetworks

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Traffic Behavior (Contd.)

Broadcast/multicast traffic is necessary and unavoidable share topology information advertise services locate services addresses and names

No more than 20% of the network traffic, otherwise segment the network using routers or VLANs

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Traffic Behavior (Contd.)

Layer 2 devices, such as switches and bridges, forward broadcast and multicast frames out all ports

Router does not forward broadcasts or multicasts

All devices on one side of a router are considered part of a broadcast domain

VLANs can also limit the size of a broadcast domain based on membership