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IST 228\Ch1\Internetworking
Chapter 1: Internetworking
• Internetworking Basics
• Network segmentation
• How bridges, switches, and routers are
used to physically segment a network
• How routers are used to create
internetwork
• OSI model
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IST 228\Ch1\Internetworking
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Internetworking Models
• Most networks are designed as a stack of
layers, each one built upon the one below
it. Why?
Host 1
Layer 3
Host 2
Layer 3 protocol
Layer 3
Layer 2/3 interface
Layer 2 protocol
Layer 2
Layer 2
Layer 1/2 interface
Layer 1 protocol
Layer 1
Layer 1
Physical Medium
IST 228\Ch1\Internetworking
• Each layer provides services to the higher
levels.
• Each layer behaves as a black box.
• Layer n on one machine talks to layer n on
another machines.
• The corresponding layer in the layered
structure are called peers.
• The communication between peers must
follow certain rules, known as protocol.
• No data are directly transferred between
layers. Actual communication is through a
physical medium below layer 1.
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An Analogy
Professor A
Professor B
I like
rabbits
Urdu & English
Translator
L: Dutch
Ik vind
konijnen
leuk
Secretary
Fax:#
L: Ducth
Ik vind
konijnen
leuk
J’aime
bien les
lapins
Message
Information
for the
remote
translator
Chinese & French
Translator
Ik vind
konijnen
leuk
use Dutch
Information
for the
remote
secretary
use fax
L: Ducth
Secretary
Fax:#
L: Ducth
Ik vind
konijnen
leuk
IST 228\Ch1\Internetworking
The lower levels
The upper levels
Open Systems Interconnection (OSI)
Reference Model
Application
Presentation
Session
• Provides user interface
• Initiates services
• Transfer data into standard format
before transmission
• Keeps data different applications’ data
separate
• Control the data exchange
Transport
• End-to-end data error free data
transmission
Network
• Logical addressing for data packets
Routing and error handling
Data Link
• NIC software function
• How data in packaged
• Error detection
Physical
• Moves bits between devices
• Specifies voltages, cables, and cables
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IST 228\Ch1\Internetworking
Reasons for Layering
• Simplifies the network model
• Enables programmers to specialize in a
particular level or layer of the networking
model
• Provides design modularity
• Encourages interoperability
• Allows for standardized interfaces to be
produced by networking vendors
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IST 228\Ch1\Internetworking
The Application Layer (Layer 7)
• The layer where users communicate to the computer
• Contains protocols and utilities that provides services to
network applications
– (True/False) MsWord, Eudora Mail, Netscape are in the
application layer.
– Eudora (application) uses SMTP (Simple Mail Transfer Protocol)
(protocol).
• E-mail:
– Message formats such as RFC 822
– SMTP, POP3 (Post Office Protocol Version 3), IMAP (Internet
Message Access Protocol)
• WWW:
– HTML (The HyperText Markup Language), XML (eXtensible
Markup Language), XSL (eXtensible Style Language)
– HTTP (The HyperText Transfer Protocol)
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IST 228\Ch1\Internetworking
The Presentation Layer (Layer 6)
• The presentation layer prepares the data from
the application layer for transmission over the
network or from the network to the application
layer.
• Include protocols specifying how to represent
data (MPEG, JPEG, PIC, WAV)
• Responsible for data translation, formatting,
encryption, compression.
• We need these services because different
computers use different internal representation
for data (integers and characters)
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The Session Layer (Layer 5)
• Enables two applications on the network to have
an ongoing conversation
• Provide following services
–
–
–
–
Communication setup and teardown
Control for data exchange
Data synchronization definition
Failure recovery
• Examples:
– Structured Query Language (SQL)
– X Windows
– AppleTalk Session Protocol (ASP)
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IST 228\Ch1\Internetworking
The Transport Layer (Layer 4)
• Provides
– end-to-end error free data transport services
– establish a logical connection
– data segmentation into maximum transmission unit
size
– messaging service for session layer
• Protocols in this layer can be
– connection-oriented : require an acknowledgment of
the receipt of data packets.
– connectionless : do not require an acknowledgment of
the receipt of data packets.
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• Connection-oriented protocols:
sender
Synchronize
Negotiate connection
Synchronize
Acknowledge
Virtual Circuit
Connection Establish
Data Transfer
receiver
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• Flow Control
• The segments delivered back to the sender upon their
reception
• Any segment not acknowledged are retransmitted.
• Segments are sequence back into their proper order upon
arrival at their destination
• Manageable data flow is maintained in order to avoid
congestion
sender
receiver
Buffer full
GO
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• Windowing: The quantity of data segment
(in bytes) is sent without receiving an
acknowledgment (ack) is called a window.
sender
receiver
sender
Window
size of 3
Window
size of 1
send 1
receive 1
ack. 2
send 2
receive 2
send 1
send 2
send 3
ack. 3
send 3
receiver
ack. 4
send 4
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• Acknowledgments:
sender
receiver
Positive Acknowledgment
with retransmission
1 2 3 4 5 6
1 2 3 4 5 6
send 1
send 2
send 3
ack. 4
send 4
send 5
Connection lost!
send 6
ack. 5
send 5
ack. 7
IST 228\Ch1\Internetworking
The Network Layer (Layer 3)
• Provides services
– to manage devices addressing
– to tracks the location of devices on the
network
– to determine the best way to move data on
the network
• The network layer must transport traffic
between devices that are not directly
connected.
• Routers are specified at this layer.
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The Data Link (Layer 2)
• Services
– Identification of the source and destination nodes via
their physical address (Media Access Control (MAC)
address)
– Definition of how data is packaged for transport as
frames
– Error detection
– Flow control of information sent across the link
• Has two sublayers:
– Media Access Control (MAC) 802.3
– Logical Link Control (LLC) 802.2
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IST 228\Ch1\Internetworking
The Physical Layer (Layer 1)
• This layer communicates directly with the
various types of actual communication
media
• Services
– definition of the physical characteristics of the
network hardware, including cable and
connector
– Encoding
– Transmission of signals on the wire
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Example:
568B twisted pair wiring scheme
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Layer 1 Network Devices: Repeaters
• The number of nodes on a network and the length of
cable used influence the quality of communication on the
network
• Attenuation
– Natural degradation of a transmitted signal over distance
• Repeaters work against attenuation by repeating signals
that they receive on a network
• Why are repeaters Layer 1 devices?
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Layer 1 Network Devices: Hubs
• Generic connection device used to tie several
networking cables together to create a link
between different stations on a network
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• Hubs that are plugged into electric power are
called active hubs
• A hub that merely connects different cables
on a network and provides no signal
regeneration is called a passive hub and is
not a repeater
• “Hub” is a generic term applied to many
different network-connection devices
• If a hub in some way segments or subdivides
the traffic on a network, it is an intelligent, or
switching, hub
• For the purpose of the CCNS exam, the term
hub—by itself—is a device that does not
segment the network
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IST 228\Ch1\Internetworking
Network Segmentation
• Segmentation
– Process of breaking a network into smaller broadcast or
collision domains
• Ethernet network, which are characterized by IEEE
802.3 standard, define the use of a Carrier Sense
Multiple Access with Collision Detection
(CSMA/CD) access method
– Backoff algorithm : Mathematical calculation
performed by computers after a collision occurs on a
CSMA/CD network
– Backoff period : Random time interval used after a
collision has been detected on an Ethernet network
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Network Segmentation via Bridges
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Layer 2 Devices: Bridges
• Operate at the Data Link layer of the OSI
model
• Filters traffic between network segments
by examining the destination MAC address
– Based on this destination MAC address, the
bridge either forwards or discards the frame
– When a client sends a broadcast frame to
the entire network, the bridge will always
forward the frame
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• Transparent Bridges : Also called learning
bridges because they build a table of MAC
addresses as they receive frames
– This means that they “learn” which addresses
are on which segments
– Ethernet networks mainly use transparent
bridges
• Source-routing bridges : Rely on the
source of the frame transmission to
provide the routing information
– Usually employed by Token Ring networks
• Translation bridges : Can connect
networks with different architectures
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Layer 2 Devices: Switches
• Increase network performance by reducing the
number of packets transmitted to the rest of the
network
• Like bridges, operate at the Data Link layer of
the OSI model
• In an Ethernet network, computers are usually
connected directly to a switch
• Virtual circuit
– Private connections between two points created by a
switch that allows the two points to use the entire
available bandwidth between those two points without
contention
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