Transcript CIS 460 - Network Analysis and Design Chapter 2 Analyzing Technical Goals and
CIS 460 - Network Analysis and Design Chapter 2 Analyzing Technical Goals and Constraints
Analyzing Technical Goals and Constraints
• Goals include: – scalability – availability – performance – security – manageability – usability – adaptability – affordability – Tradeoffs associated with these goals • Provides terminology to discuss technical goals with customer
Scalability
• How much growth a network design must support – maybe a prime goal for some – Proposed design should be able to adapt to increase in usage/scope • Planning for Expansion – How many sites to add – How extensive the networks for each new site – How many more users to access the corporate network – How many more servers or hosts to add to the internetwork
Scalability (Cont’d) • Expanding the Data Available to Users
– Empowered employees make strategic decisions that require access to sales, marketing, engineering and financial data – The 80/20 rule is no longer valid (80 % stays in local LANs, 20% destine for other places) – Increasing access to WWW servers – Increasing intranet usage – Strategic alliances with suppliers/customers
Scalability (Cont’d)
• Technical goals for scaling/upgrading enterprise networks: – Connect separate department LANs in corporate internetworks – Solve LAN/WAN bottleneck problems – Provide centralized servers that reside on server farms – Merge an independent SNA network w/enterprise IP network – Add new sites to support field offices & telecommuters – Add new sites to support communication with customers, suppliers, resellers, and other business partners • Constraints – There are impediments to scalability due to incorrect technology
Availability
• Refers to the amount of time a network is available to users and is often a critical goal • Can be expressed as a percent uptime per year, month, week, day, or hour • Also lined to reliability but has a more specific meaning (percent uptime) than reliability. Reliability refers to a variety of issues, including accuracy, error rates, stability and TBF.
• Also associated with resiliency - how much stress a network can handle and how quickly a network can rebound from problem.
• Disaster recovery. Disaster recovery plan
Availability (Cont’d)
• Specifying Availability Requirements – Specify with precision (% uptime), timeframe(day or night downtime), time unit (day, week, etc.) • Cost of Downtime – For critical applications document how much lost per hour – Also helps to determine whether in-service upgrades must be supported • MTBF/MTR – Define as mean time between failures and mean time to repair – They can be used to calculate availability goals – 4000 hours MTBF is a common goal – Good idea to identify for specific applications in addition to network as a whole – Can usually use data supplied by manufacturer for equipment
Network Performance
• Criteria for accepting network performance – throughput, accuracy, efficiency, delay, and response time • Tightly tied to analyzing the existing network to determine what changes need to be made to meet performance goals.
• Also tightly linked to scalability goals • Definitions – capacity (bandwidth), utilization, optimum utilization, throughput, offered load, accuracy, efficiency, delay (latency), delay variation, response time
Network Performance (Cont’d)
• Optimum Network Utilization – measure of how much bandwidth is used during a specific time period. Commonly specified as a percentage of capacity – Various tools 7used to measure usage and averaging the usage over elapsed time.
– Customer may have a network design goal for the maximum average network utilization allowed on shared segments.
– For shared Ethernet should not exceed 37 percent else collision rate becomes excessive – Toke Ring/FDDI typical goal is 70 percent – For WANs optimum is also about 70 percent
Network Performance (Cont’d)
• Throughput – the quantity of error -free data that is transmitted per unit of time – Ideally should be the same as capacity, however nout usually – Capacity depends on the physical-layer technologies in use – Depends on the access method • Throughput of Internetworking Devices – Some specify goals as Number of packets per second – The maximum rate the device can forward packets without dropping
Network Performance (Cont’d)
• Application layer throughput – a measure of good and relevant application-layer data transmitted per unit of time (also called goodput) – Can increase throughput not goodput because extra data transmitted is overhead data – Usually measure in kilobytes or megabytes per second – Constraints • end-to-end error rates • protocol functions such as handshaking, windows, & acknowledgments protocol parameters such as frame size • protocol parameters such as frame size and retransmission timers • PPS or CPS rate of internetworking devices • Lost packets or cells at internetworking devices • Workstation and server performance factors
Network Performance (Cont’d)
• Accuracy – goal is that data received at the destination must be the same as the data sent by the source. Typical causes include power surges or spikes, failing devices, noise, impedance mismatch, poor physical connection – Accuracy goals can be specified as a bit error rate (BER) – On shared Ethernet errors are often result of collisions – Collision that happens beyond the first 64 bits of a frame is a late collision which are illegal and should never happen – In token rings accuracy goals sometimes include goals for minimizing media-access control error reports
Network Performance (Cont’d)
• Efficiency – measurement of how effective an 0operation is in comparison to the cost in effort, energy, time, or money – Provides a useful way to talk about network performance – Large frame headers are an obvious cause for inefficiency – Goal is to minimize the amount of bandwidth used by headers • Delay and Delay Variation – Users of interactive applications expect minimal delay in receiving feedback – Multimedia applications require a minimal variation in the amount of delay that packets experience – Telnet protocol applications are also sensitive to delay because of echo feedback
Network Performance (Cont’d)
• Causes of Delay – delay is relevant to all data transmission technologies, but particularly satellite links and long terrest4rial cables. Also time required to put digital data on a transmission line. Packet switching delay, router delay, queuing delay • Delay Variation – Digital voice and video applications are effected with jitter, or disrupted communications. Can be minimized using a buffer to minimize jitter • Response Time – the network performance goal that users care about most. They recognize small changes in the expected response time. If less than 100 ms most users do not notice
Security
• Overall goal is that security problems should not disrupt the company’s ability to conduct business.
• Have protection so that business data and other resources are not lost or damaged • First task is planning which involves analyzing risks and developing requirements.
– Hackers, financial costs, sensitive of customer data
Security (Cont’d)
• Security Requirements – protect resources from being incapacitated, stolen, altered, or harmed.
– Let outsiders access data on public web but not internal data – authorize/authenticate branch-office users, mobile users & telecommuters – detect intruders – physically secure hosts and internetworking devices – protect applications and data from viruses – train network users and network managers on security risks
Manageability
• Includes the following network management functions • performance management • fault management • configuration management • security management • accounting management
Usability • Refers to the ease-of-use with which network users can access the network and services • some network design components can have a negative effect on usability
Adaptability
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A good network can adapt to new technologies and changes
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New protocols, new business practices, new fiscal goals, new legislation or other possibilities
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Effects it availability
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A flexible design as enables a network to adapt to changing traffic patterns
Affordability
• Sometimes called cost-effectiveness • Primary goal is to carry the maximum amount of traffic for a given financial cost • In a campus network low cost of often the primary goal • For enterprise networks availability is usually more important than low cost • Minimizing hiring, training, and maintaining personnel to operate and manage the network is also an important goal
Affordability (Cont’d)
• Making Network Design Tradeoffs – Need to know how important affordability is to other goals – Availability often requires Redundant components which raises costs – To implement affordability might mean availability must suffer – Your customer should identify a single driving network design goal • Sometimes making tradeoffs is more complex than what has been described because goals can differ for various parts of an internetwork