Transcript public network

Computer Networks and Internets with
Internet Applications, 4e
By Douglas E. Comer
Lecture PowerPoints
By Lami Kaya, [email protected]
© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
1
Chapter 15
Network Characteristics
Ownership, Service Paradigm,
And Performance
© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Topics Covered
• 15.1 Introduction
• 15.2 Network Ownership
– 15.2.1 Private Networks
– 15.2.2 Public Networks
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15.3 Privacy And Public Networks
15.4 Advantages And Disadvantages
15.5 Virtual Private Networks
15.6 Guaranteeing Absolute Privacy
15.7 Service Paradigm
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Topics Covered (cont)
• 15.8 Connection-Oriented Service Paradigm
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15.8.1 Continuous And Bursty Traffic
15.8.2 Simplex And Full Duplex Connections
15.8.3 Connection Duration And Persistence
15.8.4 Service Guarantees
15.8.5 Stream Or Message Interface
15.9 Connectionless Service Paradigm
15.10 Interior And Exterior Service Paradigms
15.11 Comparison Of Service Paradigms
15.12 Examples Of Service Paradigms
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Topics Covered (cont)
• 15.13 Addresses And Connection Identifiers
• 15.14 NW Performance Characteristics
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15.14.1
15.14.2
15.14.3
15.14.4
Delay
Throughput
The Relationship Between Delay And Throughput
Delay-Throughput Product
• 15.15 Jitter
– 15.15.1 Isochronous Networks
– 15.15.2 Asynchronous Networks
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15.1 Introduction
• Each technology is classified into one of three
categories: LAN, MAN, or WAN,
• Three additional characteristics of networks:
– network ownership,
– the type of service (both the service that the network provides to
attached computers and the service it uses internally),
– and the performance of the resulting system.
• These characteristics permit more accurate comparisons
among technologies
– because they provide detail about similarities and differences
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15.2 Network Ownership
• NW HW and SW can be owned by a company or
individual
• A NW owned and used by a single company or an
individual is
– said to be “private network”
• A NW owned by common carriers are
– called “public network”
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15.2.1 Private Networks (1)
• LAN technologies comprise the most common form of
private network.
• To run a private network, a corporation
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hires employees who create and operate the network
The necessary HW and software are purchased outright
And employees install the wiring, connect computers
and manage the resulting system
• A private corporation can install cables only on property
that the corporation owns.
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15.2.1 Private Networks (2)
• A large corporation may also use private WAN
technologies to connect computers at multiple sites:
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The corporation purchases WAN HW such as packet switches
Hires employees to operate the NW
The employees design network interconnections
Attach computers to NW, assign addresses, and control routing
• To form a private WAN, a corporation must lease
connections between its sites from public carriers
– Such WAN is still considered private because the leased
connections carry data directly between the corporation's sites
– No other corporations have access to the wires or the data
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15.2.2 Public Networks
• A public network is analogous to a telephone system
– NW is run as a service available to subscribers
• Any one can subscribe to the service
• A feature of a public NW is universal communication
– A given subscriber's computer can communicate with any other
• A public NW that is available to many subscribers in
many locations is more attractive than one that only
serves a small geographic area
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15.3 Privacy And Public Networks
• The term “public” refers to availability of the service
– not to the data transferred
• Public networks provide private communication
• Some public networks permit a group of computers to
communicate
– analogous to a telephone conference call
• Public network does not use broadcast technology
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15.4 Advantages And Disadvantages
• Advantage of a private NW is that the owner has
complete control:
– Chooses HW to use, the capacity, redundancy/backup
– Sets policies
– Can guarantee that NW is isolated from computers outside
• Isolation helps enforce security
• Disadvantages:
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A large private NW can be expensive to install and maintain.
Purchasing the NW HW
Hire/train a staff to plan/install/manage/operate
Special tools for installation/maintenance
Keeping up with rapid change
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15.4 Advantages And Disadvantages (cont)
• The chief advantages of a public NW
– are flexibility and the ability to use state-of-the-art networking
without maintaining technical expertise
– A subscriber at an arbitrary location can connect to the NW at
any time
• Connections between a host owned by one organization
and a computer owned by another can be made
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15.5 Virtual Private Networks (VPN)
• VPN technology allows a company with multiple sites to
have a private NW
– but use a public NW as a carrier
• VPN technology restricts traffic
– so that packets can travel only between the company's sites
• If an outsider accidentally receives a copy of a packet
– VPN technology ensures that they cannot understand the
contents
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Building a VPN
• Special HW and SW system for each site
• The system is placed between private/public NW.
• Each VPN system configured with the addresses of the
VPN systems at other sites
• Additional details can be found in Chapter 40
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Operation of a VPN
• Once VPN configured, the company's sites can only
communicate with one another
– they are cut off from the rest of the NW.
• The VPN system restricts packets:
– VPN system at each site restricts incoming/outgoing packets
– No packet can leave the site unless it is traveling to another one
of the company's sites
• When two computers belong to same VPN exchange
messages, routing takes place:
– VPN sends the packet across the public NW to the destination
– When the packet arrives
• the receiving VPN system verifies that it came from a valid peer site
• and then forwards the packet to its destination
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15.6 Guaranteeing Absolute Privacy
• Some corporations are unwilling to send sensitive data
across a public NW
• Absolute privacy
– preserving individual anonymity, or
– preserving the confidentiality of data being sent
• VPN system encrypts each packet before sending the
packet across the public NW
– An outsider cannot understand the contents of packets
• The receiving VPN system decrypts each incoming
packet before sending it on to the destination computer
– packet remains encrypted during its trip across the public NW
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15.7 Service Paradigm
• NW systems offer a variety of services
• The goal is to provide a higher-level interface
– Allows the computer to specify a remote destination, and
– Transfer data without worrying about packets.
• Exact details of interface mechanisms may vary
• General type of interface is known as
– an interface paradigm or
– a service paradigm
• NWs are placed in one of two broad categories:
– Connection-oriented (CO) service
– Connectionless (CL) service
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15.8 Connection-Oriented Service Paradigm
• Analogous to telephone communication:
– A connection must be established between two computers
– After connection data can be sent across
– The connection must be terminated
• CO NW is an abstract idea
– Circuit switching is a specific mechanism that provides a CO
interface
• The term CO is generic
– it applies broadly to a class of technologies
– The class encompasses many technologies
– The designs and details differ
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15.8.1 Continuous And Bursty Traffic
• CO NW designed to handle voice or video
– to accept and deliver continuous data at a fixed rate
• Some other CO NW are designed to handle burst traffic
– A computer can send data for a while,
– stop sending data
– and then resume sending
• The connection does not disappear because no data is
being sent
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15.8.2 Simplex And Full Duplex Connections
• Some CO technologies provide full duplex connections
(two-way)
• Some CO technologies provide simplex (one-way)
• To communicate using a simplex design, a pair of
computers must establish two connections:
– one from computer A to computer B and
– another from computer B to computer A
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15.8.3 Connection Duration And Persistence
• Some CO NWs are designed to use permanent
connections that persist over months or years
• Some CO technologies permit switched connections
– that can be established or terminated quickly and automatically
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15.8.4 Service Guarantees
• Some CO NWs provide guarantees about the service
that computers will receive. For example,
– Guarantee a throughput rate or
– Maximum packet loss rate.
• Other connection-oriented technologies do not provide
guarantees. For example,
– ATM provides statistical guarantees about performance, but
does not absolutely guarantee delivery (i.e., cells can be lost).
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15.8.5 Stream Or Message Interface
• Some CO NWs provide a stream interface
– Once the connection is open, the computer can send a stream of
data octets that are delivered to the other end
– With a stream interface, no boundaries are recorded
• Receiver may receive a single block of 60 characters even though
the sender generates three blocks of 20 characters.
• Other CO technologies provide a message interface
– in which the NW guarantees to deliver data in the same size
chunks that the sender transmitted
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15.9 Connectionless (CL) Service Paradigm
• CL NWs operate analogous to the postal mail system
• Whenever it has data to send, a computer must
– place the data in the appropriate frame format,
– attach the address of receiver and
– then pass the frame to the NW for delivery
• CL system transports the frame to the destination
• Details differ among connectionless technologies
– Addressing scheme;
• Length address
• Method of assigning address
– Imposing an upper bound on the size of a frame
– Impose a minimum packet size
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15.10 Interior And Exterior Service
Paradigms
• A NW that provides one service paradigm to attached
computers can use an entirely different service paradigm
internally
– For example, although it provided connectionless service to
attached computers, the ARPANET used a connection-oriented
paradigm internally
• We will see another example of mixed paradigms when
we examine the TCP/IP protocols.
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15.11 Comparison Of Service Paradigms
• Each service type has advantages and disadvantages.
• CO paradigm has ease of accounting and the ability to
inform communicating computers immediately when a
connection breaks
• Public NW that charge customers for NW use favor CO
– because less effort is required to charge for time
• Learning about NW failure immediately can help
applications that are using the NW.
• A failure in a CL may go unnoticed and unreported
• CL service paradigm has less initial overhead
– a CL NW allows a computer to send data immediately, without
waiting for a connection
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15.12 Examples Of Service Paradigms
• LAN technologies such as Ethernet use CL;
– Although a computer needs to wait for access to a shared medium
before sending a packet
• the computer does not need to establish a connection
• WANs can use CO and CL service paradigms
– Frame Relay uses CO
– ATM
• switched virtual circuit PVC ( SVCs ) and
• permanent virtual circuit (PVC)
– Switched Multi-megabit Data Service (SMDS)
• is also used in public WANs and offers CL
• Most LAN technologies are CL, but some LANs take a CO approach
– ATM was designed for both WANs and LANs and uses CO
• Figure 15.1 summarizes the service paradigms offered
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© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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15.13 Addresses And Connection Identifiers
• In a CL, each packet must contain the address of the recipient.
• CO service often uses abbreviations for connections
• For a connection,
– Computer sends a message to the NW specifying the address of
destination
– After connection, the NW responds with a message that verifies the
connection and specifies a connection identifier
• Usually, a connection identifier is a small integer, much shorter than
the full destination address
– When sending/receiving data
• the computer uses the connection identifier instead of a destination address
– Using connection identifiers reduces overhead
• because it makes the header of data packets smaller
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15.14 NW Performance Characteristics
• NWs can be classified as low speed or high speed
• Such definitions are inadequate
– because NW technologies change rapidly
• Scientists or engineers need to specify NW speeds
precisely,
– They do not use informal, qualitative terms
– Instead, they use quantitative metrics
• Quantitative measures are important
– because they make it possible to compare any two NWs
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15.14.1 Delay
• The delay of a NW specifies how long it takes for a bit of
data to travel across the NW
• Delays may differ slightly
– depending on the location of the specific pair of computers that
communicate
• Users only care about the total delay of a NW
• Engineers need to make more precise measurements
– The maximum
– The average delay
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Delay Types
• Propagation Delay
– A signal requires a some time to travel, which is proportional to
the distance
• Switching Delay
– an electronic device waits until all bits of a packet have arrived
– and then takes a small amount of time to choose the next hop
before sending a packet
• Access Delay
– Most LANs use shared media, computers must delay until the
medium is available
• Queueing Delay
– Each packet switch enqueues incoming packets as part of the
store-and-forward process
• If the queue already contains packets, the new packet may wait
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15.14.2 Throughput
• Throughput is a measure of the rate at which data can be sent
through the NW, in bits per second (bps)
• Throughput capability of the underlying HW is called BW
– sometimes BW = throughput.
– Programmers and users do not care about the capability of the
underlying HW, they are interested in the actual data rate.
• A frame contains a header, which means that the effective throughput is
less than the HW bandwidth.
• NWing professionals often use the term speed = throughput.
– This can be confusing because delay and throughput are separate
– Throughput is a measure of capacity, not speed.
• imagine a NW to be a road between two places and packets traveling across
the NW to be analogous to cars traveling down the road.
• NW throughput specifies how many bits can enter NW per unit time
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15.14.3 The Relationship Between Delay
And Throughput
• In theory, the delay and throughput of a NW are
independent.
– In practice, however, they can be related
– Think of the road analogy
• If a PS has a queue of packets waiting when a new
packet arrives
– the new packet will be placed on the tail of the queue
– and will need to wait while the switch forwards the previous ones
• Excessive traffic in a NW is called congestion
• Data entering a congested NW will experience longer
delays than data entering an idle NW
• Throughput and delay are not completely independent.
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Estimating Delay
• The expected delay can be estimated from the current percentage of
the NW capacity being used, if
– D0 delay when the NW is idle
– U, utilization ( 0 1)
– D, effective delay
D0
D
(1  U )
•
•
•
•
When a NW is completely idle, U is zero, D = D0
When a NW operates at 1 / 2 of its capacity D = 2D0
As traffic approaches the NW capacity (U 1 ), the delay  ∞
As traffic increases, delays increase
– a NW that operates at close to 100% of its throughput capacity
experiences severe delay
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15.14.4 Delay-Throughput Product
• When used as a measure of the underlying HW
– Delay x Throughput (D x T)
– Delay x Bandwidth (BW)
• The (D x T) measures the volume of data that can be
present on NW
– the total # of bits in transit at any time
• This product is important for any NW with long delay
– Means a NW can generate a large volume of data before the
destination receives the first bit.
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15.15 Jitter
• NW jitter represent the variance in delay
– used for transmission of real-time voice and video
• Consider sending voice over a NW.
– On the sending side, the analog signal is digitized
– The samples are collected into packets or cells
• then transferred across the NW
– At the receiving side
• the digital values are extracted and converted back to analog output
– If the NW has zero jitter (exactly the same time to transit the NW)
• the audio output will exactly match the original input
– Otherwise, the output will be flawed
• Telephone system and data NW handle jitter differently
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15.15.1 Isochronous Networks
• To ensure digitized telephone calls are transmitted and
played back correctly
– the telephone NW is designed so that all transmissions have
exactly the same delay.
– Ex: if digitized data from a phone call is transmitted over two
paths
• HW is configured so that both paths have exactly the same delay
• Term “isochronous”, pronounced as ``eye-sock-re-nus'‘,
– to characterize a NW that has a jitter of zero
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15.15.2 Asynchronous Networks
• Asynchronous is alternative to isochronous NWs is a
NW in which delay among packets can vary
• Most current data NWs are asynchronous
– but still they can used for audio and video transmission
• Although audio and video work best when jitter is low
– additional protocols have been designed to ensure correct
playback
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