Chapter 4 Panko and Panko Business Data Networks and Telecommunications, 8 th Edition © 2011 Pearson Education, Inc. Publishing as Prentice Hall Panko and Panko Business Data Networks and Telecommunications, 8 th Edition © 2011 Pearson Education, Inc. Publishing as Prentice Hall Panko and Panko Business Data Networks and Telecommunications, 8 th Edition © 2011 Pearson Education, Inc. Publishing as Prentice Hall

Core concerns Quality of service (QoS) Network designSelection among alternativesOngoing management (OAM&P)Network visibility (SNMP) © 2011 Pearson Education, Inc. Publishing as Prentice Hall 2

 Networks today must work well.  Companies measure quality-of-service (QoS) metrics to measure network performance.  Examples: ◦ Speed ◦ Availability ◦ Cost ◦ And so on © 2011 Pearson Education, Inc. Publishing as Prentice Hall 3

 Normally measured in bits per second (bps) ◦ Not bytes per second ◦ Occasionally measured in bytes per second  If so, labeled as Bps ◦ Metric prefixes increase by factors of 1,000 (not 1,024 as in computer memory) © 2011 Pearson Education, Inc. Publishing as Prentice Hall 4

PrefixMeaningExample kbps*1,000 bps17,000 bps is 17 kbps 3 kbps is 3,000 bps 34.7 kbps is 3,700 bps Mbps1,000 kbps8,720,000 bps is 8.7 Mbps Mbps is 14,750,000 bps Gbps1,000 Mbps87 Gbps = 87,000,000,000 bps Tbps1,000 Gbps © 2011 Pearson Education, Inc. Publishing as Prentice Hall 5 *Note that the metric prefix kilo is abbreviated with a lowercase k

 Expressing speed in proper notation ◦ There must be one to three places before the decimal point, and leading zeros do not count. ◦ There must be a space before the metric suffix. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 6 As WrittenPlaces before decimal point Space between number and prefix? Properly written Mbps2YesOK as is 2,300 kbps4Yes2.3 Mbps 0.5Mbps0No500 kbps

 Doing Conversions ◦ Improperly written: 3,625 Mbps ◦ Four places before the (implicit) decimal point ◦ Must divide the number by 1,000:  (Shift the decimal point three places to the right) ◦ Therefore, must multiply the metric prefix by 1,000: So Mbps  Gbps ◦ Properly written: Gbps © 2011 Pearson Education, Inc. Publishing as Prentice Hall 7

 Doing Conversions ◦ Improperly written: 0.3 Mbps ◦ Zero places before the decimal point ◦ Must multiply the number by 1,000: 300  (Shift the decimal point three places to the left) ◦ Therefore must divide the metric prefix by 1,000: So Mbps  kbps ◦ Properly written: 300 kbps © 2011 Pearson Education, Inc. Publishing as Prentice Hall 8

 Perspective ◦ If the number has one to three places before the decimal point, it is fine. ◦ Otherwise, you must multiply or divide the number by 1,000. ◦ You do the opposite to the metric prefix. ◦ This leaves the number the same  0.4 Mbps = 400,000 bps  400 kbps = 400,000 bps © 2011 Pearson Education, Inc. Publishing as Prentice Hall 9

 Rated Speed ◦ The speed a system should achieve, ◦ According to vendor claims or the standard that defines the technology.  Throughput ◦ The speed a system actually provides to users ◦ (Almost always lower) © 2011 Pearson Education, Inc. Publishing as Prentice Hall 10

 Aggregate Throughput ◦ The aggregate throughput is the total throughput available to all users.  Individual Throughput ◦ An individual’s share of the aggregate throughput © 2011 Pearson Education, Inc. Publishing as Prentice Hall 11

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 12 Individual throughput Aggregate throughput Rated speed

 Availability ◦ The time (percentage) a network is available for use  Example: 99.9% ◦ Downtime is the amount of time (minutes, hours, days, etc.) a network is unavailable for use.  Example: An average of 12 minutes per month © 2011 Pearson Education, Inc. Publishing as Prentice Hall 13

 Error Rates ◦ Errors are bad because they require retransmissions. ◦ More subtly, when an error occurs, TCP assumes that there is congestion and slows its rate of transmission. ◦ Packet error rate: the percentage of packets that have errors. ◦ Bit error rate (BER): the percentage of bits that have errors. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 14

 Latency ◦ Latency is delay, measured in milliseconds. ◦ When you ping a host’s IP address, you get the latency to the host. ◦ When you use tracert, you get average latency to each router along the route. ◦ Beyond about 250 ms, turn-taking in conversations becomes almost impossible. ◦ Latency hurts interactive gaming. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 15

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 16

Panko and Panko Business Data Networks and Telecommunications, 8th Edition © 2011 Pearson Education, Inc. Publishing as Prentice Hall 17

 Jitter ◦ Jitter is variation in latency between successive packets. ◦ Makes voice and music speed up and slow down over milliseconds—sounds jittery. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 18

 Application Response Time © 2011 Pearson Education, Inc. Publishing as Prentice Hall 19

 Application Response Time ◦ Not purely a network matter. ◦ To control application response time, networking, server, and application people must work together to improve user experiences. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 20

 Service Level Agreements (SLA) ◦ Guarantees for performance ◦ Increasingly demanded by users ◦ Penalties if the network does not meet its QoS metric guarantees © 2011 Pearson Education, Inc. Publishing as Prentice Hall 21

 Service Level Agreements (SLA) ◦ Guarantees are often written on a percentage of time basis  “No worse than 100 Mbps 99.95% of the time”  As percentage of time requirement increases, the cost to provide service increases exponentially  So SLAs cannot be met 100% of the time © 2011 Pearson Education, Inc. Publishing as Prentice Hall 22

 Service Level Agreements (SLA) ◦ SLAs specify worst cases (minimum performance to be tolerated)  Penalties if worse than the specified performance  Example: latency no higher than 50 ms 99.99% of the time ◦ If specified the best case (maximum performance), you would rarely get better  Example: No higher than 100 Mbps 99% of the time. Who would want that? © 2011 Pearson Education, Inc. Publishing as Prentice Hall 23

 Examples  Jitter ◦ No higher than 2% variation in packet arrival time 99% of the time  Latency ◦ No higher than 125ms 99% of the time  Availability ◦ No lower than 99.99% ◦ Availability is a percentage of time, so its SLA does not include a percentage of time © 2011 Pearson Education, Inc. Publishing as Prentice Hall 24

 Topologies describe the physical arrangement of nodes and links. ◦ “Topology” is a physical layer concept.  Many standards require specific topologies.  In other cases, you can select topologies that make sense in terms of transmission costs, reliability through redundancy, and so on. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 25

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 26 How many possible paths are there between A and B? How many possible paths are there between A and B?

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 27 How many possible paths are there between A and B? How many possible paths are there between A and B?

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 28 In a hierarchy, each node has one parent. How many possible paths are there between A and B?

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 29 How many possible paths are there between A and B?

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 30 What do you think will happen if A and B would transmit at the same time?

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 31 Many real networks have complex topologies incorporating the pure topologies we have just seen.

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 32 n sites: n(n-1)/2 lines n sites: n(n-1)/2 lines

© 2011 Pearson Education, Inc. Publishing as Prentice Hall 33 n sites: n-1 lines n sites: n-1 lines

 Full-mesh and hub-and-spoke topologies are opposite ends of a spectrum.  Real network designers must balance cost and reliability when designing complex networks. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 34

 Normally, network capacity is higher than the traffic.  Sometimes, however, there will be momentary traffic peaks above the network’s capacity—usually for a fraction of a second to a few seconds. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 35

 This congestion causes latency because switches and routers must store frames and packets waiting to send them out.  Buffers are small, so packets are often lost. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 36

 Overprovisioning is providing far more capacity than the network normally needs.  This avoids nearly all momentary traffic peaks but is wasteful. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 37

 With priority, latency-intolerant traffic, such as voice, is given high priority and will go first if there is congestion.  Latency-tolerant traffic, such as , must wait.  More efficient than overprovisioning; also more labor-intensive. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 38

 QoS guarantees reserved capacity for some traffic, so this traffic always gets through.  Other traffic, however, must fight for the remaining capacity. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 39

 Overprovisioning, priority, and QoS reservations deal with congestion; traffic shaping prevents congestion by limiting incoming traffic. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 40

 Filtering out or limiting undesirable incoming traffic can also substantially reduce overall network costs. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 41

 Some traffic can be banned and simply filtered out.  Other traffic has both legitimate and illegitimate uses; it can be limited to a certain percentage of traffic. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 42

Core concernsQuality of service (QoS)Network designSelection among alternatives Ongoing management (OAM&P) Network visibility (SNMP) © 2011 Pearson Education, Inc. Publishing as Prentice Hall 43

 Described as OAM&P  Operations ◦ Moment-by-moment traffic management ◦ Network operations center  Administration ◦ Paying bills, administering contracts, and so on ◦ Dull but necessary © 2011 Pearson Education, Inc. Publishing as Prentice Hall 44

 Described as OAM&P  Maintenance ◦ Fixing things that go wrong ◦ Also, preventative maintenance ◦ Maintenance staff should be separate from the operations staff  Different skill set © 2011 Pearson Education, Inc. Publishing as Prentice Hall 45

 Described as OAM&P  Provisioning (providing service) ◦ Includes physical installation ◦ Includes setting up user accounts and services ◦ Reprovisioning when things change ◦ Deprovisioning when accounts and services are no longer appropriate ◦ Collectively, extremely expensive © 2011 Pearson Education, Inc. Publishing as Prentice Hall 46

Core concernsQuality of service (QoS)Network designSelection among alternativesOngoing management (OAM&P) Network visibility (SNMP) © 2011 Pearson Education, Inc. Publishing as Prentice Hall 47

 It is desirable to have network visibility—to know the status of all devices at all times.  The simple network management protocol (SNMP) is designed to collect information needed for network visibility. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 48

 Central manager program communicates with each managed device.  Actually, the manager communicates with a network management agent on each device. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 49

 The manager sends commands and gets responses.  Agents can send traps (alarms) if there are problems. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 50

 Information from agents is stored in the SNMP management information base. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 51 Management Information Base (MIB) (a conceptual database)

Set/Get/GetNext Request Get Response / Trap SNMP MIB/ SMI 網路介面 SNMP UDP IP Manager 網路介面 SNMP UDP IP Agent IP Network (Internet) Managed Resources Management Station Network Element MIB: Management Information Base SMI: Structure of Management Information

 Network visualization programs analyze information from the MIB to portray the network, do troubleshooting, and answer specific questions.  SNMP interactions are standardized, but network visualization program functionality is not, in order not to constrain developers of visualization tools. © 2011 Pearson Education, Inc. Publishing as Prentice Hall 53