panko, chapter 4. core concerns quality of service (qos)network designselection among...

Panko, Chapter 4


Networking must go beyond the systems development life cycle to the full system life

cycle over the network’s life.

It also needs to understand the business system in which each network component

operates.

Networking must go beyond the systems development life cycle to the full system life

cycle over the network’s life.

It also needs to understand the business system in which each network component

operates.


User demand is growing much faster than network budgets.

Cost efficiency is always critical.

User demand is growing much faster than network budgets.

Cost efficiency is always critical.

Legacy Decisions

◦ Decisions that lock your network in for a considerable period of time

◦ Multi-year leases

◦ Decisions about alternate strategic directions to take

◦ Deserve very careful attention


Networks today must work well.

Companies measure quality-of-service (QoS) metrics to measure network performance.

Examples:◦ Speed

◦ Availability

◦ Cost

◦ And so on


Prefix Meaning Example

kbps* 1,000 bps 17,000 bps is 17 kbps3 kbps is 3,000 bps34.7 kbps is 3,700 bps

Mbps 1,000 kbps 8,720,000 bps is 8.7 Mbps14.75 Mbps is 14,750,000 bps

Gbps 1,000 Mbps 87 Gbps = 87,000,000,000 bps

Tbps 1,000 Gbps


*Note that the metric prefix kilo is abbreviated with a lowercase k

Rated Speed◦ The speed a system should achieve,

◦ According to vendor claims or the standard that defines the technology.

Throughput◦ The speed a system actually provides to users

◦ (Almost always lower)


Availability◦ The time (percentage) a network is available for

use

Example: 99.9%

◦ Downtime is the amount of time (minutes, hours, days, etc.) a network is unavailable for use.

Example: An average of 12 minutes per month


Error Rates◦ Errors are bad because they require

retransmissions.

◦ More subtly, when an error occurs, TCP assumes that there is congestion and slows its rate of transmission.

◦ Packet error rate: the percentage of packets that have errors.

◦ Bit error rate (BER): the percentage of bits that have errors.


Latency

◦ Latency is delay, measured in milliseconds.

◦ When you ping a host’s IP address, you get the latency to the host.

◦ When you use tracert, you get average latency to each router along the route.

◦ Beyond about 250 ms, turn-taking in conversations becomes almost impossible.

◦ Latency hurts interactive gaming.


Application Response Time


Service Level Agreements (SLA)◦ Guarantees are often written on a percentage of

time basis

“No worse than 100 Mbps 99.95% of the time”

As percentage of time requirement increases, the cost to provide service increases exponentially

So SLAs cannot be met 100% of the time


Service Level Agreements (SLA)◦ SLAs specify worst cases (minimum

performance to be tolerated) Penalties if worse than the specified

performance Example: latency no higher than 50 ms

99.99% of the time

◦ If specified the best case (maximum performance), you would rarely get better Example: No higher than 100 Mbps 99% of the

time. Who would want that?


To manage a network, it helps to be able to draw pictures of it.◦ Network drawing programs do this.

◦ There are many network drawing programs.

◦ One is Microsoft Office Visio.

Must buy the correct version to get network and computer templates

◦ We will show examples from OPNET IT Guru.


Nodes are hosts, switches, routers, and so on.

20© 2011 Pearson Education, Inc. Publishing as Prentice Hall

Just drag nodes onto the canvas.

Just drag nodes onto the canvas.


Then drag link icons between nodes.There are many types of link icons.

Then drag link icons between nodes.There are many types of link icons.

Congestion causes latency because switches and routers must store frames and packets waiting to send them out.

Buffers are small, so packets are often lost.


Overprovisioning is providing far more capacity than the network normally needs.

This avoids nearly all momentary traffic peaks but is wasteful.


With priority, latency-intolerant traffic, such as voice, is given high priority and will go first if there is congestion.

Latency-tolerant traffic, such as e-mail, must wait. More efficient than overprovisioning; also more

labor-intensive.


QoS guarantees reserved capacity for some traffic, so this traffic always gets through.

Other traffic, however, must fight for the remaining capacity.


Overprovisioning, priority, and QoS reservations deal with congestion; traffic shaping prevents congestion by limiting incoming traffic.


Compression can help if traffic chronically exceeds the capacity on a line.

Data often contains redundancies and can be compressed.


Often, the design of a building naturally constrains the topology of a design.

In a multistory building, for in-stance, it often makes sense to place an Ethernet workgroup switch on each floor and a core switch in the basement.


Network drawing tools show the elements of the network and how they are interconnected.

Network simulation software goes farther by creating a computer model of the network, not just a picture.◦ The model has the capacity and configuration of

each node and transmission link.

◦ Simulation can indicate congestion points, underused lines, and so on.


Comparing Alternatives

◦ Designers must select among competing approaches and even competing technologies.

◦ When learning about technologies and network designs, you need to look carefully at pros and cons.

◦ .


4.22: Scalability


Minimum Requirements

◦ Specifications that set particular requirements must be met.

◦ Noncompliant products that do not meet a minimum requirement cannot be considered further.

◦ A failure to scale to meet expected traffic would be an example.


Multicriteria decision making is a disciplined way to look at and evaluate all aspects of alternatives.


Product A Product B

Criterion CriterionWeight(Max 5)

Product Rating

(Max 10)

Criterion Score

Product Rating

(Max 10)

Criterion Score

Functionality 5 8 40 4 20

Ease of management

2 8 16 8 16

Cost* 4 2 8 8 32

Total Score 64 68*Higher cost ratings indicate lower cost.*Higher cost ratings indicate lower cost.

Cost is difficult to measure. Systems Development Life Cycle Costs

◦ Hardware: full price—base price plus necessary optional components

◦ Software: full price—base price plus necessary optional modules

◦ Labor costs: Network staff and user costs during development

◦ Outsourced development cost

◦ Total development cost


Described as OAM&P

Operations◦ Moment-by-moment traffic management

◦ Network operations center

Administration◦ Paying bills, administering contracts, and so on

◦ Dull but necessary


Described as OAM&P

Provisioning (providing service)◦ Includes physical installation

◦ Includes setting up user accounts and services

◦ Reprovisioning when things change

◦ Deprovisioning when accounts and services are no longer appropriate

◦ Collectively, extremely expensive


It is desirable to have network visibility—to know the status of all devices at all times.

The simple network management protocol (SNMP) is designed to collect information needed for network visibility.


Central manager program communicates with each managed device.

Actually, the manager communicates with a network management agent on each device.


Network visualization programs analyze information from the MIB to portray the network, do troubleshooting, and answer specific questions.

SNMP interactions are standardized, but network visualization program functionality is not, in order not to constrain developers of visualization tools.


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