1. Network Analysis and Design Introduction to Network Design

2. 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

3. Elements of Good Network Design • Deliver the services requested by users • Deliver acceptable throughput and response times • Cost efficiency • Reliable • Expandable • Manageable • Well-documented

4. Network Design Issues • User requirements • Locations of devices • Characteristics of applications • Types of traffic • Topologies • Routing protocols • Budget • Performance • Etc.

5. Classifications of Network Design • Build a new network • Expand or upgrade the existing network • Create the overlay network • Virtual Private Network (VPN)

6. Types of Networks • Access network: • The ends or tails of networks that connect the small sites into the network • LAN, campus network • Backbone network: • The network that connects major sites • Corporate WAN

7. Objectives • How to design a network using the correct techniques? • Some common guidelines applicable for all types of network design

8. 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

9. 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

10. 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

11. Network Design Phases • Requirement analysis • Logical network design • Physical network design

12. Phase I - Requirement Analysis Phase • Analyze goals and constraints • Characterize the existing network • Characterize network traffic

13. Phase II - Logical Network Design Phase • Map the requirements into the conceptual design • Design a network topology • Node locations • Capacity assignment

14. Phase III - Physical Network Design Phase • Select technologies and devices for your design • Implementation

15. 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

16. 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

17. Business Constraints • Budget • Staffing • Schedule • Politics and policies

18. 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?

19. Information (Contd.) • Applications, protocols, and services • Current logical and physical architecture • Current performance

20. Technical Goals • Scalability • Availability • Performance • Security • Manageability • Usability • Adaptability • Affordability

21. 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

22. 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%

23. Availability (Contd.) • Different applications may require different levels • Some enterprises may want 99.999% or “Five Nines” availability

24. 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

25. Availability (Contd.) • System availability (R) is calculated from the component availability (Ri) • Series: • R =  Ri • Parallel: • R = 1 – (1 – Ri)

26. 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

27. Availability (Contd.) • 99.999% may require high redundancy (and cost) ISP 1 ISP 2 ISP 3 Enterprise

28. 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

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

30. 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

31. 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.

32. 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

33. Throughput of Devices (Contd.)

34. Bandwidth, Throughput, Load 100 % of Capacity Throughput Actual Ideal 100 % of Capacity Offered Load

35. 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

36. 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

37. 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

38. 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?

39. Efficiency (Contd.) Small Frames (Less Efficient) Large Frames (More Efficient)

40. 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

41. 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

42. 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

43. 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

44. 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

45. Queuing Delay and Bandwidth Utilization Number of packets in a queue increases exponentially as utilization increases

46. 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

47. 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

48. 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

49. 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

50. 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)