Making Networks Work

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Chapter
5


Making Networks Work

Instructor: Nhan Nguyen Phuong

Guide to Networking Essentials, Fifth Edition

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Contents


1
. Understanding the OSI and
802
Networking Models


2
. Function of Data Frames in Network Communications


3
. Understanding the IEEE
802
Networking
Specifications


1
. Understanding the OSI and
802
Networking Models


1.1
. Role of a Reference Model


1.2
. OSI Reference Model


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The
Open Systems Interconnection (OSI)
reference model was proposed by the
ISO


Common framework for developers and students of
networking to work with and learn from


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


IEEE
802
networking model provides detailed
implementation specifications for a number of
networking technologies


Influential set of networking standards

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1.1
. Role of a Reference Model


Reference models and standards enable
interoperability among layers


Computer networking, computer compatibility, and
networking features and functions can be daunting
concepts to grasp


However, they would be more difficult to
comprehend if networking weren’t built on a common
framework with the process separated into layers


The OSI model and its seven
-
layer approach to
networking provides this common framework


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1.2
. OSI Reference Model


OSI reference model:

drafted in late
1970
s by ISO;
theoretical model for networks of all kinds


By
1983
, the draft became ISO Standard
7498


Model’s foundation: networking can be separated
into a series of related tasks


Each task can be conceptualized as a single aspect,
or layer, of the communication process


Reduces complexity of networked communications into
series of interconnected tasks and activities


“Divide and conquer” approach: relationship among
tasks persists, but each can be handled separately,
and its issues solved independently

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Understanding Layers


The OSI reference model for networking clarifies
many communications activities and related tasks
and requirements to help in understanding what
networks are and how they work


Breaks down all the events that must occur for data
to be addressed and formatted correctly before it
can actually be delivered to its final recipient


With a layered approach, one part of the process
can change, sometimes drastically, while the rest of
the process remains unchanged


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1.3
. Structure of the OSI Reference Model


A computer that accesses a network must have a
protocol stack (
protocol suite
)


TCP/IP


IPX/SPX


NetBEUI


AppleTalk


Protocols plus drivers equal network access

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Communication between peer layers is “virtual”


In reality, communications pass up and down the
protocol stacks on both machines


As data gets passed from layer to layer, it’s divided
into data units appropriate for the layer


Protocol data units (PDUs)

are passed as a self
-
contained data structure from layer to layer


Encapsulation
process adds “headers” to allow
successful delivery of each layer’s payload


Decapsulation

strips header information on way up


No layer can pass information directly to its peer
counterpart except for the Physical layer

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1.3.1
. Application Layer


Layer
7
; PDU: data


Set of interfaces to access networked services


E.g., networked file transfer, message handling, and
database query processing


Handles network access, moving data from sender
to receiver, and error recovery for applications


Components usually have a client and a server part


E.g., HTTP, Client for Microsoft Networks, NFS


Possible problems: missing/misconfigured client or
server SW, incompatible or obsolete commands
used to communicate between client and server

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1.3.2
. Presentation Layer


Layer
6


Data
-
formatting info for network communications


Handles: protocol conversion, character set issues,
encryption/ decryption, and graphics commands


May compress data


A
redirector

operates at this layer


Intercepts requests for service from the computer;
those that can’t be handled locally are redirected to a
networked resource that can handle the request


Usually built into the Application layer component


E.g., FTP, HTTP

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1.3.3
. Session Layer


Layer
5


Permits two parties to hold ongoing sessions


Handles session setup, data or message
exchanges, and teardown when the session ends


Monitors session identification so that only
designated parties can participate


Monitors security services for access control


Examples: name lookup and user logon and logoff


E.g., DNS name resolution, FTP’s logon/logoff


End
-
to
-
end task synchronization services


Manages mechanics of any ongoing conversation

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1.3.4
. Transport Layer


Layer
4
; PDU: segment


Manages end
-
to
-
end transfer of data


Segments long data streams into chunks


Resequences chunks into original data on receipt


Includes error checks to ensure error
-
free delivery


Handles
flow control


E.g., TCP (TCP/IP) and SPX (from IPX/SPX)


Layer
4
problems include a corrupt protocol stack
and segments that are too large for the medium
between the source and destination networks


The latter forces Network layer to fragment segments,
which causes performance degradation

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1.3.5
. Network Layer


Layer
3
; PDU: packet


Handles addressing messages for delivery


Translates logical addresses into physical addresses


Determines how to route transmissions from sender
to receiver (
routing

process)


Traffic cop for network activity and handles routing
and
access control

(during routing process)


E.g., IP (from TCP/IP) and IPX (from SPX/IPX)


Possible problems: incorrect IP addresses or subnet
masks, incorrect router configuration, and router
operation errors

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1.3.6
. Data Link Layer


Layer
2
; PDU: frame (has header and trailer (FCS))


Sends PDUs from/to Network to/from Physical layer


FCS contains
Cyclical Redundancy Check (CRC)


It’s the responsibility of the upper layers (e.g., Layer
4
)
to retransmit data discarded due to errors


Header contains source/destination MAC addresses


Destination address is of final destination or
intermediate device (e.g., router)


The SW component at this layer is the NIC driver


HW components include NIC and switches


Possible problems: collisions, invalid frames, trying
to use incompatible network architectures

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1.3.7
. Physical Layer


Layer
1


Converts bits into signals and vice versa


Signals generated depend on the medium


Details for creating network connection are specified


Governs the type of connector used


Regulates the transmission technique


Handles intricacies of transmitting bits


Specifies
encoding

mechanism


Tries guarantee that received bits match pattern sent


Problems: improper media termination, EMI, faulty or
misconfigured NICs and hubs

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1.4
. Summary of the OSI Layers

2
. Function of Data Frames in Network
Communications


2.1
. Examining the Structure of a Data Frame


2.2
. Creating a Data Frame


2.3
. Understanding Types of Data Frames


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


Reasons why networks split data into small pieces


Large units of data sent across a network hamper
effective communications by saturating the network


If a sender and receiver use all the available
bandwidth, other computers can’t readily
communicate


Networks can sometimes be unreliable


Retransmission of large frames (due to errors) is
inefficient

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2.1
. Examining the Structure of a Data
Frame




Header: source/destination MAC addresses,
frame’s size, description of content, clocking
information


Data (“payload”): actual data being sent along with
the headers of other PDUs in the frame


Size can vary from less than
50
bytes to
16
KB,
depending on the network type


Trailer: CRC (if the sent/received CRCs don’t
match, the receiving computer discards the frame)

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2.2
. Creating a Data Frame

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2.3
. Understanding Types of Data Frames


Unicast frame:

addressed to only one computer


Adapters read the frames and pass them to higher
layers only if the destination address in the frame
header matches their own address


Broadcast frame:
created for all computers on a
network


Destination address is a value of all binary
1
s


Multicast frame:
created for any computers on a
network that “listen” to a shared network address


A special kind of address allows any interested
receiver to read these data streams

3
. Understanding the IEEE
802
Networking Specifications


3.1
. IEEE
802
Specifications


3.2
. IEEE
802
Extensions to the OSI Reference Model


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The IEEE defined a set of LAN standards to ensure
network interface and cabling compatibility


Project
802
(inception on February (
2
) of
19
80
)


Concentrates on standards that describe a network’s
physical elements


NICs, cables, connectors, signaling technologies,
media access control, and the like


OSI model was not standardized until
1983

1984


IEEE
802
standards predate the model


Both were developed in collaboration and are
compatible with one another

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3.1
. IEEE
802
Specifications

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3.2
. IEEE
802
Extensions to the OSI
Reference Model

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Summary


The OSI reference model and IEEE Project
802
define a frame of reference for networking and
specify the lower
-
layer behaviors for most networks


Together, these models describe the complex
processes and operations involved in sending and
receiving information across a network


The OSI reference model separates networking into
seven layers, each with its own purposes/activities


From the bottom up: Physical, Data Link, Network,
Transport, Session, Presentation, and Application

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Data frames consist of three parts: frame header,
data section, and frame trailer


Classified as unicast, multicast, or broadcast frames


The IEEE
802
project elaborates on the functions of a
network’s Physical and Data Link layers by dividing
the Data Link layer into two sublayers: Logical Link
Control (LLC) and Media Access Control (MAC)


Together, these sublayers handle media access,
addressing, and control and provide reliable, error
-
free
delivery of data frames from one computer to another