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Network Analysis and Design. Introduction to Network Design. 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. Elements of Good Network Design.
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Network Analysis and Design Introduction to Network Design
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
Elements of Good Network Design • Deliver the services requested by users • Deliver acceptable throughput and response times • Cost efficiency • Reliable • Expandable • Manageable • Well-documented
Network Design Issues • User requirements • Locations of devices • Characteristics of applications • Types of traffic • Topologies • Routing protocols • Budget • Performance • Etc.
Classifications of Network Design • Build a new network • Expand or upgrade the existing network • Create the overlay network • Virtual Private Network (VPN)
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
Objectives • How to design a network using the correct techniques? • Some common guidelines applicable for all types of network design
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
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
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
Network Design Phases • Requirement analysis • Logical network design • Physical network design
Phase I - Requirement Analysis Phase • Analyze goals and constraints • Characterize the existing network • Characterize network traffic
Phase II - Logical Network Design Phase • Map the requirements into the conceptual design • Design a network topology • Node locations • Capacity assignment
Phase III - Physical Network Design Phase • Select technologies and devices for your design • Implementation
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
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
Business Constraints • Budget • Staffing • Schedule • Politics and policies
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?
Information (Contd.) • Applications, protocols, and services • Current logical and physical architecture • Current performance
Technical Goals • Scalability • Availability • Performance • Security • Manageability • Usability • Adaptability • Affordability
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
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%
Availability (Contd.) • Different applications may require different levels • Some enterprises may want 99.999% or “Five Nines” availability
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
Availability (Contd.) • System availability (R) is calculated from the component availability (Ri) • Series: • R = Ri • Parallel: • R = 1 – (1 – Ri)
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
Availability (Contd.) • 99.999% may require high redundancy (and cost) ISP 1 ISP 2 ISP 3 Enterprise
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
Network Performance • Common performance factors include • Bandwidth • Throughput • Bandwidth utilization • Offered load • Accuracy • Efficiency • Delay (latency) and delay variation • Response time
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
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.
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
Bandwidth, Throughput, Load 100 % of Capacity Throughput Actual Ideal 100 % of Capacity Offered Load
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
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
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
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?
Efficiency (Contd.) Small Frames (Less Efficient) Large Frames (More Efficient)
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
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
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
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
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
Queuing Delay and Bandwidth Utilization Number of packets in a queue increases exponentially as utilization increases
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
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
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
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
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)