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Oct 23, 2013 (3 years and 7 months ago)

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Guide to Net working Essentials, Fifth Edition



5
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Chapter 5


Making Networks Work




At a Glance


Instructor’s Manual Table of Contents




Overview




Objectives




Teaching Tips




Quick Quizzes




Class Discussion Topics




Additional Projects




Additional Resources




Key Terms




Technical Notes for Hands
-
On Project
s


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Lecture Notes


Overview


Chapter 5 describes the role of the OSI reference model and the IEEE 802 networking
model. Students learn about the importance of layered network designs and understand
how each of the layers of the OSI model works. They lear
n about the encapsulation
process as data moves across layers and how frames function and are created. Finally,
students learn about the IEEE 802 networking model and related standards, including its
relationship with the OSI reference model.



Objectives




Explain the OSI reference model layers and their relationship to hardware and software



Describe the function and creation of a data frame



Explain the IEEE 802 networking model and related standards



Teaching Tips


Understanding the OSI and 802 Networking

Models


1.

Explain the role of the
OSI

model (proposed by the
ISO
). Explain why the attempt to
develop a working set of protocols and technologies based on the OSI model, and to put
those efforts into common use, never materialized.


Teaching

Tip


Explain
that ISO is not an acronym; it comes from the Greek prefix iso, which
means “equal” or “the same.” The ISO, based in Geneva, Switzerland, is a
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Teaching

Tip


Note that the set of protocols that was developed to conform to the OSI model is
call
ed ISO. You can view the fruits of those labors at:
www.protocols.com/pbook/iso.htm
.




2.

Explain that the IEEE 802 networking model provides detailed implementation
specifications for a number of netw
orking technologies.


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Role of a Reference Model


1.

Explain the importance of having a networking reference model.


OSI Reference Model


1.

Provide a brief history of how the OSI reference model came to be.


2.

Explain that the model’s foundation is that networkin
g can be separated into a series of
related tasks. Explain that layering follows a “divide and conquer” approach.


3.

Understanding Layers
. Explain the importance of having a layered networking
reference model. Use an analogy of creating and delivering a lett
er through the U.S.
mail to help explain the layered communication process.


Structure of the OSI Reference Model


1.

Use Figure 5
-
1 to describe the structure of the OSI reference model. Introduce the term
protocol suite
. Explain that protocols plus drivers e
qual network access.


Teaching

Tip


Here are two good mnemonics to remember the seven layers of the OSI reference
model. From the bottom up, starting with the Physical layer, the acronym is
“Programmers Do Not Throw Sausage Pizza Away.” From the top down,

starting
with the Application layer, it’s “All People Seem To Need Data Processing.”





Use Figure 5
-
2 to describe the role of OSI layers in an operating system context.


3.

Explain that rigidly defined boundaries called “interfaces” separate layers in the O
SI
reference model.


4.

Explain that in general, the purpose of any layer in the model is to provide services to
the next higher layer, but also to shield that higher layer from the details of how its
services are carried out.


5.

Use Figure 5
-
3 to introduce the

concept of “peer layers”. Explain that communication
between peer layers is “virtual”. Introduce the terms
PDU
,
encapsulation
, and
decapsulation
.


6.

Simulation 5
-
1 shows how data generated from an application travels down through the
OSI model layers, with
encapsulations added where necessary, and how the process is
reversed on the receiving computer.


Teaching

Tip


For another animation of the encapsulation process, visit:
www.humbo
ldt.edu/%7Eaeb3/telecom/Encapsulation.html
.




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7.

Describe the role of each of the seven OSI layers (in reverse order). For each layer,
describe the tasks performed by the layer, provide examples of implementations of the
layer, and describe possible proble
ms that can occur at that layer:

a.

Application layer
. Note that its PDU is called “data”.

b.

Presentation layer
. Note that its PDU is called “data”. Don’t forget to introduce
the term
redirector
.

c.

Session layer
. Note that its PDU is called “data”.

d.

Transport laye
r.
Note that its PDU is called “segment” (see Figure 5
-
4). Don’t
forget to introduce the term
flow control
.

e.

Network layer.
Note that its PDU is called “packet” (see Figure 5
-
5). Don’t
forget to introduce the terms
routing

and
access control
.

f.

Data link laye
r.
Note that its PDU is called “frame” (see Figure 5
-
6). Don’t
forget to introduce the term
Cyclic Redundancy Check (CRC)
.

g.

Physical layer.
Don’t forget to introduce the term
encoding
.


Teaching

Tip


For an animation of the encoding process visit:
http://netbook.cs.purdue.edu/anmtions/anim03_1.htm
.




Teaching

Tip


For a humorous analogy to the OSI model, read “James Bond Meets The 7 Layer
OSI Model” at:
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.




Summary of the OSI Layers


1.

Use Table 5
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1 to provide a summary of the role of each of the OSI layers.


Teaching

Tip


Stress that although not all network
ing protocols adhere to the OSI model, a
network administrator’s clear understanding of the functions at each layer is
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“Layer 3 switch”.





Stress that no protocol suite developed after the OSI reference model was introduced
has been free of its influence.



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Quick Quiz 1


1.

Any computer that can access
a network must have a protocol stack, also known as a(n)
____________________ because it consists of a collection of related software elements
and services that correspond to the layers of the OSI model, instead of a single massive
program.

Answer: protoco
l suite


2.

The Presentation layer (Layer ____________________) handles data
-
formatting
information for network communications.

Answer: 6


3.

The Transport layer handles ____________________, which ensures that the recipient
of transmitted data isn’t overwhelmed

with more data than it can handle.

Answer:
flow control


4.

____________________ is handled at the Network layer during the routing process; the
router consults a list of rules before forwarding an incoming packet to determine
whether a packet meeting certai
n criteria (such as source and destination address)
should be permitted to reach the intended destination.

Answer:
Access control



Function of Data Frames in Network Communications


1.

A
frame

is the basic unit for network traffic as it travels across the me
dium.


Teaching

Tip


Note that you often hear the term “packet” used instead of frame, but because
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term is “frame.”





Explain why networks split data into small pieces.


E
xamining the Structure of a Data Frame


1.

Use Figure 5
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7 to describe the structure of a data frame.


Creating a Data Frame


1.

Use Figure 5
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8 to describe how header/trailer information is added or removed as data
passes from layer to layer.


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Teaching

Tip


Expl
ain that most protocols, such as TCP/IP, add header information only at the
Transport, Network, and Data Link layers.



Understanding Types of Data Frames


1.

Describe the difference between unicast, broadcast, and multicast frames.


Teaching

Tip


Explain t
hat the types of frames discussed in this section, unicast, broadcast, and
multicast, can also refer to the Network layer PDU (packets). Packets have their
own header information containing source and destination addresses that can also
be unicast, broadca
st, or multicast. The difference is that the packet header
contains logical addresses, such as TCP/IP addresses, assigned to the computer
manually, whereas the frame header contains the computer’s physical address
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Qui
ck Quiz 2


1.

What is a frame?

Answer: A frame is the basic unit for network traffic as it travels across the medium.


2.

What does a frame header contain?

Answer: The frame header usually contains the address of the sender (source) and the
address of the receiv
er (destination), information indicating the frame’s size or content,
an alert signal to indicate data transmission, and clocking information to synchronize
the transmission.


3.

In a(n) ____________________ frame, the frame’s destination address is a value o
f all
binary 1s.

Answer:
broadcast


4.

____________________ frames are created for any computers on a network that
“listen” to a shared network address.

Answer:
Multicast



Understanding the IEEE 802 Networking Specifications


1.

Explain how the IEEE
802

project

came to be. Explain that it concentrates on standards
that describe a network’s physical elements.


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Teaching

Tip


For more information on the IEEE and its standards, visit:
www.ieee.org

and
www.ieee802.org/
.




2.

Explain that although IEEE 802 standards predate the OSI reference model, both were
developed in collaboration and are compatible with one another.


IEEE 802 Specifications


1.

Use Table 5
-
2 to describe the role of the IEEE

802 standards categories. Explain that
these categories, which encompass a large body of standards, are the focus of ongoing
development and extension efforts at the IEEE through its working groups.


Teaching

Tip


For more information on the IEEE 802 fam
ily of standards, visit:
http://en.wikipedia.org/wiki/IEEE_802
.




IEEE 802 Extensions to the OSI Reference Model


1.

Remind students that that the two lowest layers of the OSI model define how computers

attach to specific network media, and specify how more than one computer can access
the network without causing interference with other computers on the network.


2.

Use Figure 5
-
9 to explain that Project 802 took this work further to create the
specificatio
ns (primarily 802.1 through 802.5) that define the most successful LAN
technologies, including Ethernet and token ring, which together dominate the LAN
world.


3.

Describe the role of the LLC (defined by 802.2) and MAC sublayers. Don’t forget to
introduce the

term
Service Access Point (SAPs)
.


4.

Use Figure 5
-
10 to show how the IEEE 802 specifications map to the LLC and MAC
sublayers.



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Quick Quiz 3


1.

The IEEE ____________________ standard covers all forms of Ethernet media and
interfaces, from 10 Mbps to 10 Gbps

(10 Gigabit Ethernet).

Answer: 802.3


2.

The ____________________ (LLC) sublayer (defined by 802.2) controls data
-
link
communication and defines the use of logical interface points, called Service Access
Points (SAPs), that other computers can use to transfe
r information from the LLC
sublayer to the upper OSI layers.

Answer: Logical Link Control


3.

What is the role of the MAC sublayer of the Data Link layer?

Answer: The Media Access Control (MAC) sublayer manages access to the physical
medium and, therefore, co
mmunicates with the Physical layer. It communicates directly
with a computer’s NIC and is responsible for physical addressing. The physical address
burned into every NIC is called a MAC address because it operates at this sublayer of
the 802.2 specificatio
n.


4.

The ____________________ (IEEE) defined a set of LAN standards to ensure that
network interfaces and cabling from multiple manufacturers would be compatible as
long as they adhered to the same IEEE specification.

Answer:
Institute of Electrical and Ele
ctronics Engineers



Class Discussion Topics


1.

Ask students to form groups and review all the tasks and problems associated with the
data link layer of the OSI reference model (see pages 170
-
172). Then, ask them to
identify which of those tasks and problems

belong to the LLC sublayer and which to the
MAC sublayer. They should discuss and compare their lists in class.


2.

Have students heard about any other networking protocols besides the ones mentioned
in this chapter? If so, at which layers do they think thos
e protocols operate? Tip:
Students should be able to list a few other protocols/technologies, such as PPP, SLIP,
ATM, ARP, SMTP, SNMP, POP, etc.



Additional Projects


1.

The TCP/IP protocol stack differs from the OSI, but the layers of the former can be
mapp
ed to layers from the OSI reference model. Ask students to do some research to
find out about this mapping. They should hand in a report indicating the mapping of
these layers, including one or more graphics and a list of protocols that work at each
layer.


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2.

Ask students to use Ethereal (
www.ethereal.com
) to identify the fields of an Ethernet
(Ethernet II or IEEE 802.3) frame and compare them with the ones studied in this
chapter. Do they have a one
-
to
-
one relationshi
p?



Additional Resources


1.

How OSI Works:

http://computer.howstuffworks.com/osi.htm



2.

Open System Interconnection Reference Model:

www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm#xtocid5



3.

The 7 Layers of the OSI Model:

www.webopedia.com/quick_ref/OSI_Layers.asp



4.

OSI Referen
ce Model Illustrated:

http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci523729,00.html



5.

Data Link Layer:

http://en.wikipedia.org/wiki/Data_link_layer



6.

Ethernet frame types and the EtherType field:

http://en.wikipedia.org/wiki/Ethernet#Ethernet
_frame_types_and_the_EtherType_field



7.

IEEE 802 LAN/MAN Standards Committee:

www.ieee802.org/



8.

IEEE 802:

http://en.wikipedia.org/wiki/IEEE_802




Key Terms




8
02.2


The IEEE specification in Project 802 for the Logical Link Control (LLC)
sublayer of the OSI model’s Data Link layer.



802.3


The IEEE specification in Project 802 for Carrier Sense Multiple
Access/Collision Detection (CSMA/CD) networks (more common
ly called “Ethernet”).
Ethernet users can attempt to access the medium any time it’s perceived as “quiet,” but
they must back off and try to transmit again if they detect any collisions after
transmission begins.



802.5


The IEEE specification in Project
802 for token ring LANs, which map a
circulating ring structure onto a physical star and circulate a token to control access to
the medium.



802.11


The IEEE specification in Project 802 for wireless networks.



802.15


The IEEE specification that covers em
erging standards for wireless personal
area networks (PANs).

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802.16


The IEEE specification that covers wireless metropolitan area networks
(MANs).



access control


In the context of the Network layer and routing, the process whereby
a router consults a l
ist of rules before forwarding an incoming packet. The rules
determine whether a packet meeting certain criteria (such as source and destination
address) should be permitted to reach the intended destination.



Application layer


Layer 7 in the OSI referenc
e model provides interfaces that
enable applications to request and receive network services.
See also
OSI reference
model.



broadcast frames


Data frames with destination addresses that specify that all
computers on a network must read and process these f
rames.



Cyclical Redundancy Check (CRC)


A mathematical recipe that generates a
specific value, called a checksum, based on a frame’s contents. The CRC is calculated
before frame transmission and then included with the frame; on receipt, the CRC is
recalcu
lated and compared to the sent value. If the two agree, it’s assumed that the data
frame was delivered intact; if they disagree, the frame must be retransmitted.



Data Link layer


Layer 2 in the OSI reference model is responsible for managing
access to the

networking medium and ensuring error
-
free delivery of data frames from
sender to receiver.
See also
OSI reference model.



data section


The frame component that’s the actual data being sent across a
network. The size of this section can vary from less tha
n 50 bytes to 16 KB, depending
on the network type.



decapsulation


The process of stripping the header from a PDU as it makes its way
up the communication layers before being passed to the next higher layer.
See also
protocol data unit (PDU).



encapsulatio
n


The process of adding header information to a PDU as it makes its
way down the communication layers before being passed to the next lower layer.
See
also
protocol data unit (PDU).



encoding


The representation of 0s and 1s as a physical signal, such as

electrical
voltage or a light pulse.



flow control


A process designed to regulate information transfer between a sender
and a receiver. Flow control is often necessary when there’s a speed differential
between sender and receiver.



frame


The basic unit
for network traffic as it travels across the medium. Data is
broken into these smaller, more manageable pieces for faster, more efficient delivery.



frame header


Information added to the beginning of data being sent, which
contains, among other things, ad
dressing and sequencing information.



frame trailer


Information added to the end of the data being sent in a frame; it
generally contains error
-
checking information, such as the CRC.



International Organization for Standardization (ISO)


The international

standards
-
setting body based in Geneva, Switzerland, which sets worldwide technology
standards.



layers


The functional subdivisions of the OSI reference model.
See also
OSI
reference model.



Logical Link Control (LLC)


The upper sublayer of the IEEE Proj
ect 802 model for
the Data Link layer of the OSI model. It handles error
-
free delivery and controls the
flow of frames between sender and receiver across a network.

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Media Access Control (MAC)


The lower sublayer of the IEEE Project 802 model
for the Data
Link layer of the OSI model. It handles access to network media and
mapping between logical and physical network addresses for NICs.



multicast frames


Frames that use a special destination address so that any computer
listening for this address can read a
nd process the frame’s data.



Network layer


Layer 3 of the OSI reference model handles addressing and routing
PDUs across internetworks in which sender and receiver must traverse multiple
networks.
See also
protocol data unit (PDU)
and
OSI reference model
.



Open Systems Interconnection (OSI)


The family of ISO standards developed in
the 1970s to facilitate functionality of networking services among dissimilar computers
on a global scale. The OSI initiative was unsuccessful, owing to a fatal combination of
an all
-
inclusive standards
-
setting effort and a failure to develop standard protocol
interfaces to help developers implement its manifold requirements.



OSI reference model


ISO Standard 7498 defines a frame of reference for
understanding networks by divid
ing the process of network communication into seven
layers. Each layer is defined in terms of the services and data it handles on behalf of the
layer directly above it and the services and data it needs from the layer directly below it.
The OSI reference m
odel remains the OSI initiative’s most enduring legacy.



Physical layer


Layer 1, the bottom layer of the OSI reference model, transmits and
receives signals and specifies the physical details of cables, adapter cards, connectors,
and hardware behavior.
Se
e also
OSI reference model.



Presentation layer


At Layer 6 of the OSI reference model, data can be encrypted
and/or compressed to facilitate delivery. Platform
-
specific application formats are
translated into generic data formats for transmission or from
generic data formats into
platform
-
specific application formats for delivery to the Application layer.
See also
OSI
reference model.



Project 802


The IEEE effort that produced the collection of 802 networking
specifications and standards.



protocol data un
it (PDU
)


A unit of information passed as a self
-
contained data
structure from one layer to another on its way up or down the network protocol stack.



protocol suite


A family of related protocols in which higher
-
layer protocols provide
application servic
es and request handling facilities, and lower
-
layer protocols manage
the intricacies of Layers 1 to 4 in the OSI reference model.



redirector


A software component that intercepts requests for service from a
computer and redirects requests that can’t be ha
ndled locally across the network to a
networked resource that can handle the request.



routing


A Network
-
layer service that determines how to deliver an outgoing packet
of data from sender to receiver. Routing entails several methods for managing delivery
,
and requires error and status reporting so that senders can determine whether packets
are reaching the receivers.



Service Access Points (SAPs)


Logical interface points used to transfer information
from the LLC sublayer to the upper OSI layers.
See also

Logical Link Control (LLC).



Session layer


Layer 5 of the OSI reference model is responsible for setting up,
maintaining, and ending ongoing sequences of communications (called sessions) across
a network.
See also
OSI reference model.



Transport layer


L
ayer 4 of the OSI reference model is responsible for fragmenting
large PDUs from the Session layer for delivery across the network, inserting integrity
controls, and managing delivery mechanisms to allow for error
-
free reassembly on the
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receiving end of a
network transmission.
See also
OSI reference model
and
protocol
data unit (PDU).



unicast frame


A data frame addressed to a single recipient.



Technical Notes for Hands
-
On Projects


Hands
-
On Project 5
-
1: In this project, students use the ipconfig command
-
line utility.


Hands
-
On Project 5
-
2: This project requires a Web browser and Internet access.