Network Layer

uptightexampleNetworking and Communications

Oct 24, 2013 (3 years and 11 months ago)

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Computer

Networks

Chapter 6

云南农业大学

主讲:李艳

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6.1 NETWORKS, LARGE AND SMALL


A computer network is a combination of systems (e.g., a
computer) connected through transmission media (e.g., a
wire, a cable, or air). A computer network can span a small,
medium, or large geographical area. Respectively called:
LAN, MAN and WAN. These three types of networks can
also be connected using connecting devices to form an
internetwork (or an internet).


MODEL AND PROTOCOL



A model is the specification set by a standards
organization as a guideline for designing networks. A
protocol, on the other hand, is a set of rules that controls
the interaction of different devices in a network or an
internetwork.

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6.2 OSI MODEL


SEVEN LAYERS

The Open Systems Interconnection model is a
theoretical model that shows how any two different
systems can communicate with each other.

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Flow of data in the OSI model

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FUNCTIONS OF THE LAYERS


Physical Layer



The physical layer is responsible for transmitting a bit
stream over a physical medium.


The mechanical and physical specifications of the physical
devices are determined by the physical layer.


Data
-
Link Layer



The data
-
link layer organizes bit into logical units called
frames.


Note that the data
-
link layer is responsible only for node
-
to
-
node delivery of the frame (from one station to another).

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Network Layer



The network layer is responsible for delivery of a packet
(the data unit handled by network layer is called a packet)
between the original source and final destination.


Note that when a packet moves from a source to a desti
-
nation, the physical address (added at the data
-
link layer)
changes from station to station, but the logical address
remains unchanged from the source to the destination.


Transport Layer



The transport layer is responsible for source
-
to
-
destination
(end
-
to
-
end) delivery of the entire message.


The network layer is responsible for end
-
to
-
end delivery of
individual packets. The transport layer, in contrast, is
responsible for end
-
to
-
end delivery of the whole message.

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Session Layer



The session layer is designed to control the dialog between
users. It establishes, maintains, and synchronization the
dialog between communicating systems.


Presentation Layer



The presentation layer is concerned with the syntax (format)
and semantics (meaning) of the information exchanged
between two systems. It compresses and decompresses
data for efficiency. It encrypts and decrypts data for
security.


Application Layer



The application layer enables the user, whether human or
software, to access the network. It defines common
applications that can be implemented to make the job of
the user simpler.

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6.3 CATEGORIES OF NETWORKS


We can divide the networks into three broad categories:
local area networks (LANs), metropolitan area networks
(MANs), and wide area networks (WANs).


LOCAL AREA NETWORK (LAN)



A local area network (LAN) is designed to allow resource
sharing (hardware, software, and data) between computers.
A LAN can be simply defined as a combination of
computers and peripheral devices (e, g., printers)
connected through a transmission medium (e.g., cable).

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METROPOLITAN AREA NETWORK (MAN)



A metropolitan area network (MAN) uses services provided
by a common carrier (network service provider) such as a
telephone company. It spans a city or a town and provides
services to individual users or organizations.

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WIDE AREA NETWORK (WAN)



A wide area network (WAN) is the connection of individual
computers or LANs over a large area (state, country, and
world).
Note that a person using a telephone line to connect
to an Internet service provider (ISP) is using a WAN.

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6.4 CONNFECTING DEVICES


Connecting devices can be divided into four types based on
their functionality as related to the layers in the OSI model:
repeaters, bridges, routers, and gateways. Repeaters and
bridges typically connect devices in a network. Routers and
gateways typically connect networks into internetworks.

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REPEATERS



A repeater is an electronic device that regenerates data. It
extends the physical length of a network. As a signal is
transmitted, it may lose strength, and a weak signal may
be interpreted erroneously by a receiver. A repeater can
regenerate the signal and send it to the rest of the network.

Repeaters operate at the first layer of the OSI model.

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BRIDGES



A bridge is a traffic controller. It can divide a long bus into
smaller segments so that each segment is independent
trafficwise. The bridge uses a table to decide if the frame
needs to be forwarded to another segment. With a bridge,
two or more pairs of stations can communicate at the
same time.

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In addition to its traffic controlling duties, a bridge also
functions as a repeater by regenerating the frame. This
means that a bridge operates at the physical layer. But
because a bridge needs to interpret the address embedded
in the frame to make filtering decisions, it also operates at
the data
-
link layer of the OSI model.

Bridges operate at the first two layers

of the OSI model.


In recent years, the need for better performance has led to
the design of a new device referred to as a second
-
layer
switch, which is simply a sophisticated bridge with multiple
interfaces. The media are not shared; each station is directly
connected to the switch.

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ROUTERS



Routers are devices that connect LANs, MANs, and WANs.
A router operates at the third layer of the OSI model.
Whereas a bridge filters a frame based on the physical
(data
-
link layer) address of the frame, a router routes a
packet based on the logical (network layer) address of the
packet.

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GATEWAYS



Traditionally, a gateway is a connecting device that acts
as a protocol converter. It allows two networks, each with
a different set of protocols for all seven OSI layers, to be
connected to each other and communicate.


A gateway is usually a computer installed with the
necessary software. The gateway understands the
protocols used by each connected network and is
therefore able to translate from one to another. For
example, a gateway can connect a network using the
Apple Talk protocol to a network using the Novell
Netware protocol.

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6.5 THE INTERNET AND TCP/IP


The Internet was originally a research internetwork designed
to connect several different heterogeneous networks. It was
sponsored by the Defense Advanced Research Projects
Agency (DARPA). Today, however, the Internet is an
internetwork that connects millions of computers throughout
the world.


The Transmission Control Protocol/Internet Protocol (TCP/IP)
is a suite or a stack of protocols that officially controls the
Internet.


TCP/IP was developed before the OSI model. Therefore, the
layers in the TCP/IP protocol do not exactly match those in
the OSI model.

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TCP/IP and OSI model

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PHYSICAL AND DATA
-
LINK LAYERS


At the physical and data
-
link layers, TCP/IP does not
define any specific protocol. It supports all of the standard
and proprietary protocols.


NETWORK LAYER


At the network layer (or more accurately, the internetwork
layer or the internet layer), TCP/IP supports the Internet
Protocol (IP). IP is an unreliable protocol and a best
-
effort
delivery service. The term best
-
effort means that IP
provides no error checking or tracking.


The data unit at the IP layer is called an IP datagram, an
independent packet that travels from the source to the
destination.

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Addressing



TCP/IP requires that every computer connected to the
Internet be identified by a unique international address.
This address is sometimes referred to as the Internet
address, or IP address.


Each Internet address consists of 4 bytes (32 bits). To
make the 32
-
bit form shorter and easier to read, Internet
addresses are usually written in decimal form with decimal
points separating the bytes: dotted
-
decimal notation.

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TRANSPORT LAYER



At the transport layer, TCP/IP defines two protocols:
Transmission Control Protocol (TCP) and User Datagram
Protocol (UDP).


The TCP provides full transport layer services to
applications. TCP is a reliable transport protocol.


APPLICATION LAYER


Communication on the Internet uses the client
-
server
model. A client, an application program running on a local
machine, requests a service from a server, an application
program running on a remote machine. Usually, the
service program is always running, and the client
program runs only when needed.

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Client
-
server model

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File Transfer Protocol (FTP)


The standard protocol on the Internet for transferring a file
from one machine to another is the File Transfer Protocol
(FTP). FTP was designed to respond to traditional
problems related to file transfer, one problem is the
different coding systems in use; one machine may use
ASCII, and the other may use Unicode. Another problem
is the different file formats in use. FTP was designed to
resolve these problems.

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Simple Mail Transfer Protocol (SMTP)


By far the most popular application on the Internet today is
electronic mail (email). The protocol that supports email on
the Internet is Simple Mail Transfer Protocol (SMTP). To
send and receive email, the user must install both client and
server SMTP software on a computer. SMTP is often used
with another protocol such as Post Office Protocol (POP).

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Address

SMIP uses a unique addressing system that
consists of two parts: a local part and a domain name
separated by an @ sign. The local part defines the name of
a special file, called the user mailbox, where all the mail
received for a user is stored for retrieval by the user agent.
The domain name defines the computer (often symbolically)
that serves as the SMTP server.

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Hypertext Transfer Protocol (HTTP)



Hypertext Transfer Protocol (HTTP) is a client
-
server
program that is used to access and transfer documents in
the World Wide Web.


Uniform Resource locator (URL)

HTTP uses a special
kind of addressing called the Uniform Resource Locator
(URL), which is a standard for specifying any kind of
information on the Internet. The URL defines four things:
method, host computer, port, and path.


For example:
http://www.w3.org/hypertext/project.html


method
: http,
host
: www.w3.org

pathname and name of
file
: hypertext/project.html



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WWW


World Wide Web (www), or the Web, is based on the idea of
distributed information. Instead of holding all information in
one place, each entity (individual or organization) that has
information to share stores that information on its own
computer and allows Internet users to access it. The WWW
is a collection of multimedia documents.


Hypertext

The WWW uses the concept of hypertext, which
is a document containing special text, words, and phrases
that can create a link to other documents containing text,
images, audio, or video. A document of hypertext available
on the Web is called a page. The main page for an
organization or an individual is known as a home page.

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Browser

To access a page on the WWW, one needs a
browser that usually consists of three parts: a controller, a
method, and an interpreter.The controller is the heart of the
browser; it coordinates all activities. The method is a client
application program that retrieves the document. Although it
can be any of the application programs we have discussed,
it is usually HTTP. The interpreter displays the document in
the screen.

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A computer network is a combination of devices connected
by transmission media.



The Open Systems Interconnection (OSI) model is a
theoretical model that shows how any two different systems
can communicate with each other.



The seven layers of the OSI model are the physical layer,
data
-
link layer, Network layer, transport layer, session layer,
presentation layer, and application layer.



A local area network (LAN) allows resource sharing
(hardware, software, and data) between computers.



A LAN can de configured in a bus, ring, or star topology.



A metropolitan area network (MAN) uses the services
provided by a common carrier.

SUMMARY

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A wide area network (WAN) is the connection of individual
computers or LANs over a large area. WANs are installed
and run by common carriers.



A repeater is a connecting device that regenerates data
and extends the physical length of a network.



A bridge is a connecting device that filters traffic.



A router is a connecting device that routes packets.



A gateway allows two networks, each with a completely
different protocol suite, to communicate.



An internetwork is two or more LANs, MANs, or WANs.



Transmission Control Protocol/Internet Protocol (TCP/IP)
is the set of protocols used by the Internet, a worldwide
internetwork of computers.

SUMMARY (continued)

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The Internet Protocol (IP) is TCP/IP’s unreliable protocol
at the internet layer.



An IP address identifies each computer connected to the
Internet.



User Datagram Protocol (UDP) and Transmission Control
Protocol (TCP) are protocols at the transport layer.



File Transfer Protocol (FTP) is a TCP/IP client
-
server
application for coping files from one host to another.



The protocol that supports electronic mail (email) on the
Internet is Simple Mail Transfer Protocol (SMTP).

SUMMARY (continued)

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TELNET is a client
-
server application that allows a user to
log on to a remote machine, giving the user access to the
remote system.



Hypertext Transfer Protocol (HTTP) is a client
-
server
program for accessing and transferring documents on the
World Wide Web (WWW), a collection of multimedia
documents.



The Uniform Resource Locator (URL) is a standard
identifier for specifying information on the Internet.



A browser is needed to access a page on the WWW.



A document on the Internet can be classified as static,
dynamic, or active.

SUMMARY (continued)

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Thanks