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ITEC 275 Computer Networks – Switching, Routing, and WANs. Week 2 Robert D’Andrea 2013. Some slides provide by Priscilla Oppenheimer and used with permission. Agenda. Review Chapter #1 Business Goals Business Constraints Analyzing Technical Goals Chapter #2 Technical Goals

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ITEC 275 Computer Networks – Switching, Routing, and WANs

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Itec 275 computer networks switching routing and wans

ITEC 275 Computer Networks – Switching, Routing, and WANs

Week 2

Robert D’Andrea 2013

Some slides provide by Priscilla Oppenheimer and used with permission



  • Review Chapter #1

    • Business Goals

    • Business Constraints

  • Analyzing Technical Goals Chapter #2

    • Technical Goals

    • Technical Constraints

  • Introduce homework problems

Business goals

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

Top down network design steps

Top-Down Network Design Steps

Analyze requirements

Monitor and optimize network performance

Develop logical design

Develop physical design

Implement and test network

Test, optimize, and document design

Network design steps

Network Design Steps

  • Phase 1 – Analyze Requirements

    • Analyze business goals and constraints

    • Analyze technical goals and tradeoffs

    • Characterize the existing network

    • Characterize network traffic

Network design steps1

Network Design Steps

  • Phase 2 – Logical Network Design

    • Design a network topology

    • Design models for addressing and naming

    • Select switching and routing protocols

    • Develop network security strategies

    • Develop network management strategies

Network design steps2

Network Design Steps

  • Phase 3 – Physical Network Design

    • Select technologies and devices for campus networks

    • Select technologies and devices for enterprise networks

Network design steps3

Network Design Steps

  • Phase 4 – Testing, Optimizing, and Documenting the Network Design

    • Test the network design

    • Optimize the network design

    • Document the network design

Top down software design steps

Top-Down Software Design Steps

The pdioo network life cycle

The PDIOO Network Life Cycle







Recent business priorities

Recent Business Priorities

  • Mobility

  • Security

  • Resiliency (fault tolerance/robustness)

  • 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

Business constraints

Business Constraints

  • Budget

  • Staffing

  • Schedule

  • Politics and policies

Network technical goals

Network Technical Goals

  • Scalability

  • Availability

  • Performance

  • Security

  • Manageability

  • Usability

  • Adaptability

  • Affordability



  • Scalability refers to the ability to grow a network with existing hardware and software.

  • How much growth is anticipated within the next 5 years?

    Large companies expand more rapidly (users, applications, external networks, and new sites) than smaller ones.

  • Expanding Access to Data

    1970 -1980 data stored on mainframes

    1980 – 1990 data stored on servers

    1990 – present data stored on centralized mainframes and servers



  • 80/20 Rule

    80 percent local use and 20 percent external use. At the present time, the 80/20 Rule is moving to the other side of the scale. There is more external Internet access by employees on a daily basis (20/80).

    Some companies allow access with other companies, resellers, suppliers, and strategic customers. Introduction of extranet.

    Extranet is used to describe an internal internetwork that is accessible by outside users.



The business goal of making data available to more departments, employees, and off site offices often results in a technical goal of using the mainframe as a powerful database server.

  • Some technologies are more scalable than others.

    For example: Flat network designs at Layer 2 switches, and do not don’t scale well.

    Top-down network design is an iterative process. Scalability goals and solutions are re-evaluated on a regular basis throughout the phases of the network design process.

Scalability constraints

Scalability Constraints

Constraints often affect scalability inherent in network technologies.

Selecting technologies that meet the customers scalability goals is a difficult process, especially if it is done without planning, could result in a costly re-design process later down the road.



  • Extract from the customer information about their site. Both current and future network information.

    - Number of sites to be added in the next 5 years

    - What functionality will be needed at each of these sites?

    - How many users will be added in the next 5 years?

    - How many more servers will be added to a server farm or individual departments?



  • Availability can be expressed as a percent of uptime per year, month, week, day, or hour, compared to the total time in that period.

    For example:

    • 24/7 operation

    • Network is up for 165 hours in the 168-hour week

    • Availability is 98.21%



  • Different applications and areas of campus may require different levels of availability. Availability could be considered a critical goal for a network design customer.

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



From a customers perspective, they want to know how much time the network will be operational.

Availability is linked to reliability.

  • Reliability addresses a list of issues, which include accuracy, error rates, stability, and the time between failures.



  • Redundancy is a solution to a goal of high availability. In this manner, redundancy means adding duplicate links or devices to a network to avoid network outages.

  • Disaster Recovery

    Natural disaster – floods, fires, hurricanes, and earth quakes.

    Satellite outages – meteorite storms, collisions in space, solar flares, and system failures



Unnatural disaster – bombs, terrorist attacks, riots, or hostage situation.

Resiliency is the amount of stress a network can handle over time and how quickly the network can rebound or string back from security breaches, natural and unnatural disasters, human error, and catastrophic software or hardware failures.



Note: Bank check clearing process after 9/11.

A main goal in the planning process would be to recognize which parts of the network are critical and must be maintained.

The disaster recovery plan should include keeping data backed up in one or more places that are unlikely to be affected by the disaster. Secondly, the technologies affected by the disaster should be switched to another site with similar technologies.

Note: Canada’s underground facility.



Personnel must be considered an important resource when planning for a disaster recovery.

Consider using VPV to access the corporate office when a disaster recovery occurs.

Provide VPN service to mission critical staff to work from home or a remote location. VPN service in the case of a disaster would allow this staff to begin building the damaged system without being involved at the site where there may be contamination or disease present.



  • Testing

    It is important to require employees to be part of drills in the event of a disaster. This includes visiting remote sites, and utilizing the available equipment. Keeping the remote equipment hardware and software at release levels similar to the main operations center.

  • Availability Requirements

    Uptime 99.95 % - network is down 5 minutes per week

    Uptime Five Nines - hard to achieve. Involves staff, equipment redundancy, and software.



  • 24/7 equals 8760 hours

    - Hot swappable boards

    - No maintenance window

    - In-service updates

    - Triple Redundancy

    One active

    One active standby

    One standby or maintenance



  • Cost of Downtime

    • Each critical application should be documented. How much money the company loses per minute/hour of downtime.

    • Third party network management



  • MTBF is mean time before failure

    • 4000 hours goal

  • MTTR is mean time to repair

    • One hour goal

  • MTBF and MTTR are used to calculate available goals when the customers wants to specify explicit periods of uptime and downtime, rather than a simple percent uptime value.

    Availability = MTBF / (MTBF + MTTR)



  • A typical MTBF equals 4000hours.

  • A typical MTTR is 1 hour

    Availability = MTBF / (MTBF + MTTR)

    Availability = 4000 / (40000 + 1)

    Goal 99.98 percent

  • Mean times might be different in different parts of the network. The goal of a Cisco core layer in an enterprise network are more strigent than those goals for a switch.



  • Vendors provide MTBF and MTTR estimates for their products.

  • It is advisable to research for independent lab results from MTBF and MTTR estimates before making a final conclusion about the product.

Network performance

Network Performance

  • Performance of a network includes accuracy, efficiency, delay, and response time.

  • Common performance factors include

    • Bandwidth (capacity)

    • Throughput

    • Bandwidth utilization

    • Offered load

    • Accuracy

    • Efficiency

    • Delay (latency) and delay variation

    • Response time

Network performance1

Network Performance

  • Utilization is normally specified as a percent of capacity.

  • Optimum average network utilization is approximately 70 percent. This means that peaks in the network traffic can probably be handled without noticeable performance degradation.

  • Normally, WANs have less capacity than LANs. When setting up the utilization estimate for a WAN links, more consideration is required regarding the bandwidths. WAN links are designed with bandwidths that offer little, if any extra capacity for incidental traffic because WAN links are expensive.

  • LANs are overbuilt with full-duplex Giga-bit Ethernet links to servers and 100-Mbps Giga-bit Ethernet links to clients.

Network performance2

Network Performance

  • Point-To-Point transmission is a full duplex link that connects a switch to a server or some other switch. It is possible to consume all the bandwidth, depending on the traffic load or behavior. Network traffic is normally bursty.

Network performance3

Network Performance

  • Throughput is the quantity of error-free data that is transmitted per unit of time. The assessment of the amount of data that can be transmitted per unit of time. Throughput is typically the same as capacity. Customers specify throughput goals in terms of number packets per second (pps).

  • Vendor use either pps od cps from their independent tests conducted on their product(s). Many internetwork devices can forwardpackets a theoretical maximum, which is called wire speed.

Network performance4

Network Performance

  • Bandwidth is a means capacity and is normally fixed. A measure of the width of a range of frequencies.

    Example: PVC pipe with water running through it.

  • Capacity depends on the physical ISO layer. The capacity of a network should be adequate to handle bursts of data.

Network performance5

Network Performance

  • Goodput is the number of useful bits of information at the application layer considered throughput. This information is delivered by the network to a certain destination, per unit of time. This is relate to the amount of time from the first bit of the first packet is sent until the last bit of the last packet is delivered. Goodput is a measure of good and relevant application layer data transmitted per unit of time.

Network performance6

Network Performance

  • Application Layer Throughput

    Vendors refer to the application layer throughput as goodput. Being called goodput, heightens the fact that it is a measure of good and relevant application layer data transmitted per unit of time. Throughput means bytes per second. Applications using throughput as goodput would file transfers and data base applications.

Network performance7

Network Performance

  • See page 37 for factors that constrain application layer throughput.

  • Accuracy is paramount when sending and receiving data. The data sent over the wire is expected to be identical to the data received at the destinamtion.

  • Typical causes of data errors.

    - Power surges or spikes

    - Impedance mismatches

    - Poor physical connections

    - Failing devices

    - Noise from electrical devices

    - Some specific software bugs

Network performance8

Network Performance

  • WANs links accuracy is based on bit error rate (BER). WAN links are on a serial interface, and collision errors should never occur.

    Analog links BER threshold 1 in 105 (100,000)

    Copper links BER threshold 1 in 106 (1,000,000)

    Digital circuits BER threshold 1 in 101

    Fiber-optic BER threshold 1 in 10 to 11th

Network performance9

Network Performance

  • LANs links accuracy is based on frames and not bits. A good threshold is 1 in 106

Network performance10

Network Performance

  • Ethernet errors usually result from collisions. The error is termed, cyclic redundancy check (CRC).

  • Errors can occur at the preamble, past the preamble, and beyond the 64 bytes after the preamble.

Network performance11

Network Performance

Not registered-First eight byte preamble of a frame

Registered – First sixty four bytes of a data frame (considered a runt frame)

Illegal – after the first 64 bytes

Collisions should never occur when using full-duplex Ethernet

WAM collisions should never occur.

Network performance12

Network Performance

  • Accuracy refers to the number of error-free frames transmitted relative to the total number of frames transmitted.

  • Efficiency is a measurement of how effective an operation is in comparison to the cost in effort, energy, time, and money.

    Note: Large and small frame sizes.

    Large frame make better use of bandwidth and improve application throughput. Bigger frames do introduce more chance for bit errors and a need for an elaborate recovery procedure.

  • Response delays are expected to be minimal.

    • Variations in delay, called jitter

Network performance13

Network Performance

- Jitter causes disruptions in voice and video streams.

- Telnet protocol

- Customer perspective on running any delay-sensitive applications

Delays in voice and video streams will be a major consideration to be discussed with the customer.

Network performance14

Network Performance

Serialization delay is the time to put digital data on a transmission line. Using too large of data frame (FTP), can cause delays if the shared transmission line includes time sensitive data (like voice or video).

Network performance15

Network Performance

  • Propagation delay  is the amount of time it takes for the head of the signal to travel from the sender to the receiver (186,000 miles per second)

  • Serial delay is the time to put digital data onto a transmission line.

  • Packet-switching delay is the latency accrued when switches and routers forward data.

    • DRAM

    • SRAM

Dynamic random access memory

Dynamic Random Access Memory

  • Dynamic random-access memory (DRAM) is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1. Since capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically. Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and other static memory.

Dynamic random access memory1

Dynamic Random Access Memory

  • The advantage of DRAM is its structural simplicity: only one transistor and a capacitor are required per bit, compared to four or six transistors in SRAM.

Static random access memory

Static Random Access Memory

  • Static Random Access Memory (Static RAM or SRAM) is a type of RAM that holds data in a static form, that is, as long as the memory has power. Unlike dynamic RAM, it does not need to be refreshed. SRAM stores a bit of data on four transistors using two cross-coupled inverters. The two stable states characterize 0 and 1. During read and write operations another two access transistors are used to manage the availability to a memory cell.

Static random access memory1

Static Random Access Memory

  • To store one memory bit it requires six metal-oxide-semiconductorfield-effect transistors (MOFSET). MOFSET is one of the two types of SRAM chips; the other is the bipolar junction transistor. The bipolar junction transistor is very fast but consumes a lot of energy. MOFSET is a popular SRAM type. The term is prononuced "S-RAM", not "sram."

Network performance16

Network Performance

  • Queuing delay  is the time a job waits in a queue until it can be executed.

    A good rule is to inform the customer that they should experience less than delay 1 or 2 percent

  • Response time is the network performance goal that users are interested in. Users begin to get frustrated if the response is longer then 1/10th (100MS) of a second.



  • Focus on requirements first (MD5 / AES combined)

  • 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

    • Hackers compromise a network device, such as a switch, router, server, firewall, or IDS.

Network assets

Network Assets

  • Hardware

  • Software

  • Applications

  • Data

  • Intellectual property

  • Trade secrets

  • Company’s reputation

Security risks

Security Risks

  • Hacked network devices

    • Data can be intercepted, analyzed, altered, or deleted

    • User passwords can be compromised

    • Device configurations can be changed

  • Reconnaissance attacks

  • Denial-of-service attacks

  • Security should not disrupt the company’s business.

    Note: BOTNETS and high capacity servers.



Some customer goals are specific. They want to visualize problems occurring on the network. They use SNMP to capture the number of bytes each router receives and sends

  • Fault management – detecting, isolating, and correcting problems.

  • Configuration management – controlling, operating, identifying, and collecting data

  • Accounting management – accounting of network usage to allocate costs to network users and/or plan for changes in capacity requirements.

  • Performance management – analyze traffic and application behavior to optimize a network, meet service-level agreements, and plan for expansion.

  • Security management- Monitoring and testing security and protection policies, maintaining passwords, encryption keys, and auditing adherence to security policies.



  • Usability: the ease of use with which network users can access the network and services. VPN might be a consideration for flexible access.

  • Networks should make users’ jobs easier

  • Some design decisions will have a negative affect on usability:

    • Strict security, for example



  • 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.



  • 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 technology

    • Quiet routing protocols, for example

Making tradeoffs

Making Tradeoffs

  • Scalability 20

  • Availability 30

  • Network performance 15

  • Security 5

  • Manageability 5

  • Usability 5

  • Adaptability 5

  • Affordability 15

    Total (must add up to 100)100

This week s outcomes

This Week’s Outcomes

  • Business Goals

  • Business Constraints

  • Technical Goals

  • Technical Constraints

Due this week

Due this week

  • 1-3 – Concept questions 1

Next week

Next week

  • Read Chapters 3 and 4 in Top-Down Network Design

  • 2-1 – Concept questions 2

Itec 275 computer networks switching routing and wans

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