Chapter 1: Internetworking

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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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Internetworking Models

• Most networks are designed as a stack of layers,
  each one built upon the one below it. Why?

Host 1
Host 2
Layer 3 protocol
Layer 3
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
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• 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
Jaime bien les lapins
Message
Chinese French
Information for the remote translator
Urdu English
Translator
Translator
L Dutch
L Ducth
Ik vind konijnen leuk
Ik vind konijnen leuk
use Dutch
Secretary
Secretary
Information for the remote secretary
Fax
Fax
L Ducth
L Ducth
Ik vind konijnen leuk
Ik vind konijnen leuk
use fax
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Open Systems Interconnection (OSI) Reference Model
Application

• Provides user interface
• Initiates services

Presentation

• Transfer data into standard format before
  transmission

The upper levels
Session

• 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

The lower levels
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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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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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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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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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
receiver
Synchronize
Negotiate connection
Synchronize
Acknowledge
Connection Establish
Virtual Circuit
Data Transfer
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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
receiver
Window size of 3
Window size of 1
receive 1
send 1
send 1
ack. 2
send 2
send 2
receive 2
send 3
ack. 3
ack. 4
send 3
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
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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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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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Example568B 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 hubby
  itselfis a device that does not segment the
  network

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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