Transcript The Concept of Universal Service

The Concept of Universal Service
• The chief problem with multiple networks is obvious:
– A computer attached to a given network can only communicate with
other computers attached to the same network
– The problem became evident in the 1970s as large organizations
began to acquire multiple networks
– Each network in the organization formed an island
• In many early installations
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each computer attached to a single network
and employees had to choose a computer appropriate for each task
an employee was given access to multiple screens and keyboards
the employee was forced to move from one computer to another to
send a message across the appropriate network
– Users are neither satisfied nor productive when they must use a
separate computer for each network
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Upper Saddle River, NJ. All
rights reserved.
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Internetworking
• Despite the incompatibilities among network technologies
– researchers have devised a scheme that provides universal
service among heterogeneous networks, called
internetworking
• The scheme uses both hardware and software
– Additional systems are used to interconnect a set of networks
– Software on the attached computers provides universal
service
– The resulting system of connected physical networks is known
as an internetwork or internet
• An internet is not restricted in size
– internets exist that contain a few networks
– the global Internet contains tens of thousands of networks
– the number of computers attached to each network can vary
• some networks have no computers attached
• while others have hundreds
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rights reserved.
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Physical Network Connection with
Routers
The basic component used to connect heterogeneous
networks is a router
• Physically a router is
– an independent hardware system dedicated to the task of
interconnecting networks
– contains a processor and memory as well as a separate I/O interface
for each network to which it connects
• Figure 20.1 illustrates that the physical connection of networks
with a router is straightforward
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Upper Saddle River, NJ. All
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Internet Architecture
• Figure 20.2 shows each router with exactly two connections
– commercial routers can connect more than two networks
– a single router could connect all four networks in the example
• An organization seldom uses a single router to connect all of its
networks
• There are reasons for multiple connections:
• Load-balancing and speed
– the processor in a given router is insufficient to handle the traffic
passing among an arbitrary number of networks
• Redundancy improves internet reliability
– To avoid a single point of failure
• The protocol software continuously monitors internet connections
• It instructs routers to send traffic along alternative paths when a network
or router fails
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Upper Saddle River, NJ. All
rights reserved.
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Internet Architecture
• An organization must choose a design that meets
the organization's need for
– Reliability
– Capacity
– Cost
• The exact details of internet topology to be chosen
often depend on the following
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bandwidth of the physical networks
expected traffic
organization's reliability requirements
cost
performance of available router hardware
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Upper Saddle River, NJ. All
rights reserved.
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A Virtual Network
• Internet provides the appearance of a single
seamless communication system
– a combination of hardware and software provides the
illusion of a uniform network system
• Internet software hides the details of
– physical network connections
– physical addresses
– routing information
• Users/application programs are not supposed to be
aware of the underlying physical networks or the
routers that connect
• We say that an internet is a virtual network system
– because the communication system is an abstraction
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Upper Saddle River, NJ. All
rights reserved.
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Review of TCP/IP Layering
• Recall from Chapter 1 that the Internet protocols
use a five-layer reference model as Figure 20.4
illustrates
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Upper Saddle River, NJ. All
rights reserved.
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Review of TCP/IP Layering
• Physical Layer (Layer 1)
– Specify details about the underlying transmission medium
and hardware.
– All specifications related to electrical properties, radio
frequencies, and signals belong to layer 1
• Network Interface Layer (Layer 2)
– Specify details about communication between higher layers
of protocols (implemented in SW) and the underlying
network (implemented in hardware).
– Specifications are about network addresses, maximum
packet size, and hardware addressing E-services, and video
teleconferencing
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•
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Review of TCP/IP Layering
• Internet Layer (Layer 3)
– Specify communication across the Internet (spanning multiple
interconnected networks)
– Layer 3 (IP) specifies the format of packets sent across the Internet
– Also specifies mechanisms used to forward packets
• Transport Layer (Layer 4)
– Provide for communication from an application program on one
computer to an application program on another.
– Includes specifications on controlling the maximum rate a receiver
can accept data,
– Provides procedures that are used to insure reliable transfer
– mechanisms to avoid network congestion, and
– techniques to insure that all data is received in the correct order
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Review of TCP/IP Layering
• Application Layer (Layer 5)
– Specify how a pair of applications interacts when
they communicate.
– Specify details about the format and the meaning of
messages that applications can exchange.
– Examples of network applications in layer 5 are:
email exchange, file transfer, web browsing
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Upper Saddle River, NJ. All
rights reserved.
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Addresses for the Virtual Internet
• IP address is divided into two parts:
• A prefix
– identifies the physical network to which the host is attached
– Each network in the Internet is assigned a unique network number
• A suffix
– identifies a specific computer (host/node) on the network
– Each computer on a given network is assigned a unique suffix
• IP address scheme guarantees the following properties:
– Each computer is assigned a unique address
(i.e., a single address is never assigned to more than one computer)
– Network number (prefix) assignments must be coordinated globally
– Suffixes are assigned locally without global coordination
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rights reserved.
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Dotted Decimal Notation
• A notation more convenient for humans to understand is used
• Notation that has been accepted is
– express each 8-bit section of a 32-bit number as a decimal value
– use periods to separate the sections
– The scheme is known as dotted decimal notation
• Figure 21.2 illustrates examples of binary numbers and the equivalent
dotted decimal notation
• Dotted decimal treats each octet (byte) as an unsigned binary integer
– the smallest value, 0
• occurs when all bits of an octet are zero (0)
– the largest value, 255
• occurs when all bits of an octet are one (1)
– dotted decimal addresses range
0.0.0.0 through 255.255.255.255
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Upper Saddle River, NJ. All
rights reserved.
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