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© 2006, Monash University, Australia CSE4884 Network Design and Management Lecturer: Dr Carlo Kopp, MIEEE, MAIAA, PEng Lecture 2 Performance Criteria vs User Requirements and Expectations
© 2006, Monash University, Australia Performance criteria vs user requirements and expectations What is a communications link? Types of links? What is a network? Types of networks? What the network designer/network manager sees What the end user sees What the end user’s manager sees Measures of network performance Common problem issues for networks Reconciling the physical reality with user expectations
© 2006, Monash University, Australia What is a communications link? All communications links comprise a transmitter, a channel and a receiver. The type of channel determines the design of the transmitter and the receiver. A channel might be a radio link through the atmosphere, an optical fibre, a coaxial cable, a twisted pair cable, or a telephone wire on a pole. The transmitter takes a message, encodes it using a modulation, the modulation is sent over the channel, and the receiver demodulates the message. All networks are made up of many individual links. Links may use shared channels or individual channels. Links have performance attributes – throughput, error rates, bandwidth etc.
© 2006, Monash University, Australia Communications Link
© 2006, Monash University, Australia Types of links We can divide up links in various ways One way is by the amount of data or throughput / capacity they can carry – colloquially termed ‘speed’. We measure throughput or capacity typically in bits/sec or multiples of bits/sec, ie kilobits/s, Megabits/sec, Gigabits/s, Terabits/s. Throughput or capacity might be average, peak or otherwise measured. We can also divide links by the message format or structure – synchronous, asynchronous, character, packet, frame. Links may also be divided by channel type, ie radio- frequency, laser, electrical, waveguide, etc.
© 2006, Monash University, Australia What is a network? A network is a system in which multiple nodes are connected by some number of communications links. A network permits a node to communicate with another node, across more than one other node if required. The manner in which network nodes are connected is termed topology, which is usually characteristic for a particular networking technology. The topology of a network can strongly impact its performance and robustness. Networks may use shared channels or individual channels. Networks have performance attributes – throughput, error rates, delay etc
© 2006, Monash University, Australia Network
© 2006, Monash University, Australia Star Topology
© 2006, Monash University, Australia Linear Topology
© 2006, Monash University, Australia Types of Networks We can divide up networks in various ways One way is by the amount of data or throughput / capacity they can carry – also colloquially termed ‘speed’. We measure throughput or capacity typically in bits/sec or multiples of bits/sec, ie kilobits/s, Megabits/sec, Gigabits/s, Terabits/s, or packets/s. Throughput or capacity might be average, peak or otherwise measured. We can also divide networks by the message format or structure – synchronous, asynchronous, packet orieneted. The switching technique – circuit switched or packet switched is also used to distinguish networks.
© 2006, Monash University, Australia What the Network Designer/Manager Sees From the perspective of the designer or the manager, the network is a collection of interconnected devices which carry data between user equipment. The network has performance characteristics, reliability (‘uptime/downtime’), and quality metrics. The network has to be continuously monitored and maintained to assure service quality and availability. The network has to be continuously upgraded to meet growing user demands for load. The security of the network has to be maintained. Hardware, software and channels have to be integrated together to delivered the desired effect. The designer/manager sees the network from a technology perspective and is concerned with its function
© 2006, Monash University, Australia What the End User Sees To the end user the network is a visible as a socket on the wall or an antenna on a portable device. The end user perceives the throughput of the network as increasing or decreasing delays or response times in applications. The end user perceives the availability of the network as the presence or absence of connections, or the ability or inability to get his/her work done. The end user often has no idea of the technological complexity of the network, and often does not want to know about how it works and why. The basic question the end user most often asks is ‘does this network work, and is it running fast enough?’
© 2006, Monash University, Australia What the End User’s Manager Sees A manager is concerned with the productivity of the personnel he/she supervises, and the cost of supporting that productivity. A network is thus a ‘necessary evil’ required to do business, which costs money to provide. If a network is down with a fault, personnel are idle, running up salary costs for no output. If a network is underperforming, personnel are doing less work, and becoming frustrated, running up salary costs for diminished output. Upgrades to a network are additional expenses added to the cost of doing business. The basic questions managers ask are ‘what does it cost me to use this network?’; ‘what is the risk of its failure?’; ‘what does a failure cost me in productivity?’
© 2006, Monash University, Australia Measures of Network Performance There are various ways we can measure the performance of a network: 1. Data throughput or capacity. 2. Propagation delay between nodes; variability of delay 3. Error rates – per packet, per bit. 4. Reliability – the probability that it will not fail over a finite time interval. 5. Maintainability – time to repair faults, time to effect upgrades. 6. Security – the ability to resist unwanted eavesdropping, intrusion or theft of capacity.
© 2006, Monash University, Australia Common Problem Issues for Networks Poor design for performance resulting in poor user response times on connections. Poor design for reliability resulting in frequent outages. Poor design for growth resulting in frequent downtime to effect upgrades. Poor security resulting in theft of capacity or user data. Poor choices in technology resulting in early obsolescence and replacement. Poor compatibility with end user interfaces resulting in downtime, errors or poor performance. Poor management capabilities leaving network managers blind to existing or developing problems. Under-qualified personnel making mistakes and causing problems.
© 2006, Monash University, Australia Physical Reality versus User Expectations User: ‘we just spent two million dollars on this newfangled network and it runs slower than the one we replaced’. Designer: ‘This is because you insisted that we use voiceband modems for router interconnects and the traffic load is too big for this.’ User: ‘We had no choice, we could not afford to lease the 2 Megabit lines you wanted.’ Observation: The central task of network designers and managers is to determine what performance is required, and where feasible, deliver it within the budget. Users may often want to participate in design with unwanted collateral damage resulting.
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