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Chapter 4
Panko and
andPanko
Panko
Panko
th Edition
th
Business
Data
Networks
and
Telecommunications,
8
Business
Data Networks and Telecommunications, 8 Edition
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© 2011 Pearson Education, Inc. Publishing as Prentice Hall
Core concerns
Quality of service (QoS)
Network design
Selection among alternatives
Ongoing management (OAM&P)
Network visibility (SNMP)
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Networks today must work well.
Companies measure quality-of-service (QoS)
metrics to measure network performance.
Examples:
◦ Speed
◦ Availability
◦ Cost
◦ And so on
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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)
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Prefix
Meaning
Example
kbps*
1,000 bps
17,000 bps is 17 kbps
3 kbps is 3,000 bps
34.7 kbps is 3,700 bps
Mbps
1,000 kbps
8,720,000 bps is 8.7 Mbps
14.75 Mbps is 14,750,000 bps
Gbps
1,000 Mbps
87 Gbps = 87,000,000,000
bps
Tbps
1,000 Gbps
*Note that the metric prefix kilo is
abbreviated with a lowercase k
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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.
As Written
23.72 Mbps
Places
before
decimal
point
2
2,300 kbps
4
Yes
2.3 Mbps
0.5Mbps
0
No
500 kbps
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Space
Properly
between
written
number and
prefix?
Yes
OK as is
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Doing Conversions
◦ Improperly written: 3,625 Mbps
◦ Four places before the (implicit) decimal point
◦ Must divide the number by 1,000: 3.625
 (Shift the decimal point three places to the right)
◦ Therefore, must multiply the metric prefix by
1,000: So Mbps  Gbps
◦ Properly written: 3.625 Gbps
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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
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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
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Rated Speed
◦ The speed a system should achieve,
◦ According to vendor claims or the standard that
defines the technology.
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Throughput
◦ The speed a system actually provides to users
◦ (Almost always lower)
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Aggregate Throughput
◦ The aggregate throughput is the total throughput
available to all users.
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Individual Throughput
◦ An individual’s share of the aggregate throughput
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Individual
throughput
Aggregate
throughput
Rated
speed
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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
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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.
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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.
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© 2011 Pearson Education, Inc. Publishing as Prentice
Hall
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Panko and Panko Business Data Networks and
Telecommunications, 8th Edition © 2011 Pearson
Education, Inc. Publishing as Prentice Hall
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Jitter
◦ Jitter is variation in latency between successive
packets.
◦ Makes voice and music speed up and slow down
over milliseconds—sounds jittery.
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Application Response Time
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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.
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Service Level Agreements (SLA)
◦ Guarantees for performance
◦ Increasingly demanded by users
◦ Penalties if the network does not meet its QoS
metric guarantees
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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
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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?
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Examples
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Jitter
◦ No higher than 2% variation in packet arrival time
99% of the time
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Latency
◦ No higher than 125ms 99% of the time
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Availability
◦ No lower than 99.99%
◦ Availability is a percentage of time, so its SLA
does not include a percentage of time
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Topologies describe the physical
arrangement of nodes and links.
◦ “Topology” is a physical layer concept.
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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.
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How many possible paths are
there between A and B?
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How many possible paths are
there between A and B?
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In a hierarchy,
each node has
one parent.
How many possible
paths are there
between A and B?
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3
1
2
How many possible paths
are there between A and B?
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What do you think will happen if A and B
would transmit at the same time?
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Many real networks have complex topologies
incorporating the pure topologies we have just
seen.
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n sites:
n(n-1)/2 lines
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n sites:
n-1 lines
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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.
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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.
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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.
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Overprovisioning is providing far more capacity
than the network normally needs.
This avoids nearly all momentary traffic peaks but
is wasteful.
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With priority, latency-intolerant traffic, such as
voice, is given high priority and will go first if there
is congestion.
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Latency-tolerant traffic, such as e-mail, must wait.
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More efficient than overprovisioning; also more
labor-intensive.
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QoS guarantees reserved capacity for some traffic,
so this traffic always gets through.
Other traffic, however, must fight for the remaining
capacity.
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Overprovisioning, priority, and QoS reservations
deal with congestion; traffic shaping prevents
congestion by limiting incoming traffic.
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Filtering out or limiting undesirable incoming
traffic can also substantially reduce overall network
costs.
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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.
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Core concerns
Quality of service (QoS)
Network design
Selection among alternatives
Ongoing management (OAM&P)
Network visibility (SNMP)
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Described as OAM&P
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Operations
◦ Moment-by-moment traffic management
◦ Network operations center
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Administration
◦ Paying bills, administering contracts, and so on
◦ Dull but necessary
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Described as OAM&P
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Maintenance
◦ Fixing things that go wrong
◦ Also, preventative maintenance
◦ Maintenance staff should be separate from the
operations staff
 Different skill set
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Described as OAM&P
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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
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Core concerns
Quality of service (QoS)
Network design
Selection among alternatives
Ongoing management (OAM&P)
Network visibility (SNMP)
© 2011 Pearson Education, Inc. Publishing as Prentice Hall
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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.
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Central manager program communicates with each
managed device.
Actually, the manager communicates with a
network management agent on each device.
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The manager sends commands and gets responses.
Agents can send traps (alarms) if there are
problems.
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Information from agents is stored in the SNMP
management information base.
Management Information Base (MIB)
(a conceptual database)
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MIB/ SMI
Management
Station
Network
Element
SNMP
Agent
Manager
Set/Get/GetNext Request
SNMP
SNMP
UDP
Get Response / Trap
UDP
IP
IP
網路介面
網路介面
Managed Resources
IP Network (Internet)
MIB: Management Information Base
SMI: Structure of Management Information
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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.
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