SEG 3255 Communication and Networking

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SEG 3255Communicationand Networking

• Winter 2008 course notes
• A. Williams

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What is a Network?

• A set of processing nodes connected by
  communication links.
• Many topologies possible
• Many types of communication media
• twisted (copper) pair
• coaxial (copper) cable
• radio
• infrared
• fiber optic cable
• satellites

bus
ring
mesh
star
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Types of Communication Links

• Point-to-point
• Shared / broadcast

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Circuit Switching

• All resources (e.g. communication links) needed
  by a call are dedicated to that call for its
  duration.
• Example a voice telephone call
• Call from A to F blocks calling from B to E.
• Resource reservation resources are always
  available when needed by a call, providing a
  guaranteed quality of service.

F
A
E
B
C
D
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Packet Switching

• Data entering network is divided into small
  chunks called packets.
• Packets traversing the network share network
  resources with other packets.
• Demand for resources may exceed resources
  available
• Contention two packets arrive simultaneously at
  D destined for E or F
• Queuing (waiting) for resources.
• Statistical sharing of resources.

1
F
A
1
E
2
1
2
2
B
C
D
2
1
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Why resource sharing?

• To save/make money!
• Example 1 Mbit/sec link each user requires 100
  Kbits/sec when transmitting each user has data
  to send only 10 of the time.
• Circuit switching give each caller 100
  Kbits/sec capacity. 10 callers can be supported.
• Packet switching with 35 calls in progress, the
  probability that 10 or more callers are
  simultaneously active is less than 0.0004. Many
  more callers can be supported with only a small
  probability of contention.
• If users are bursty, then packet switching is
  advantageous.

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Elements of a Network
Three networks forming an internetwork
C
B
D
A

• Communication links
• Buffers to hold packets when contention for
  communications link.
• Network set of nodes (hosts, routers, gateways)
  within a single administrative domain (e.g.
  university department, company).

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Elements of a network (2)
C
B
D
A

• Internetwork a collection of interconnected
  networks
• Active network elements hardware running
  protocols
• Host hardware running applications which use
  network (e.g. A).
• Router hardware (often without application
  level functions) routing packets from input line
  to output line (e.g. C).
• Gateway a router connected directly to two or
  more networks (e.g. B and D).

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Protocols

• Rules by which active network elements
  communicate with each other is a protocol
• Protocols define the formats and timing of
  messages exchanged, and actions taken on receipt
  of messages for peer entities
• Protocols in everyday life
• Rules by which two or more people communicate to
  provide a service, or to get something done
• Example traffic lights guiding traffic flow
• Example military precedence for entering a
  vehicle

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Layered Architecture

• Architecture of a complex system can be
  simplified by layering.
• Layer N relies on services of layerN 1 to
  provide a service to layerN 1
• Service required from lower layer is independent
  of how that service is implemented
• Interfaces define how services are requested
• Benefits
• Information/complexity hiding
• Layer N change doesnt affect other layers

interface
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Layering and Protocols

• The network consists of geographically
  distributed hardware and software components
• A distributed, layered view
• Principal challenge how to provide services
  when
• resources and information needed are distributed
• communication via unreliable medium

A
B
C
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Layering and Protocols

• Peer entities (processes) in layer N provide
  service by communicating (via messages) with each
  other using the communication service provided by
  layer N 1
• logical versus physical communication

A
B
C
Logical communication Layer 4 to Layer 4
Actual communication path
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Generic Layer Issues (1)

• Error control making a channel more reliable,
  and handling lost or out of sequence messages.
• Flow control avoid flooding a slower peer
  entity.
• Resource allocation mediating contention for
  physical (e.g. buffers) or logical (e.g. data
  structures) resources
• Fragmentation dividing chunks of data into
  smaller pieces, and subsequent reassembly
• Multiplexing combining several higher layer
  sessions

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Generic Layer Issues (2)

• Connection setup initiating logical
  communication with peer entity
• Addressing / naming managing identifiers
• Compression reducing data rate
• Encryption provide data security
• Timer management bookkeeping and error recovery

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OSI Reference Model

• OSI (Open Systems Interconnect) reference model
  adopted in 1984
• Model consists of a 7 layer stack

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Functions of the OSI Layers

• Physical
• The bits that are transmitted over the
  communication media.
• Deals with network hardware, bit encoding.
• Examples copper, fibre, radio, satellite.
• Data Link
• Activates, maintains, and deactivates the
  physical link between two adjacent nodes.
• Deals with framing, windowing, flow control,
  error detection and recovery.

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Functions of OSI Layers (2)

• Network
• Determines how best to route packets of data from
  source to destination via intermediate network
  nodes.
• Deals with addressing, routing, fragmentation,
  and congestion.
• Transport
• Ensures that data is transmitted reliably between
  source and destination.
• Deals with end to end integrity and quality of
  service.

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Functions of OSI Layers (3)

• Session (often omitted)
• Controls the dialogue between two host
  applications.
• Provides check points and error recovery.
• Reports exceptions to upper layers.
• Presentation (often done by application)
• Resolves data representation differences.
• Performs data compression and encryption.
• Application
• Perform functions to implement network
  applications.
• Examples e-mail, teleconferencing.

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Data Transmission in the OSI model

• Send side layer N takes a protocol data unit
  (PDU) from layer N 1, adds its own fields to
  form a new PDU, and passes it to layer N 1
• Receive side layer N takes the PDU from layer N
  1, strips the layer N fields, and passes it to
  layer N 1
• T transport header e.g. sequence numbers,
  error correction bits, time stamp info
• N network header e.g. source and destination
  addresses
• L link header e.g. error detection bits,
  acknowledgment field

A
A
S
A
S
T
A
S
T
N
A
S
T
N
L2
L1
network
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The Internet Reference Model

• De facto model that was defined after ARPANET was
  up and running.

OSI
TCP/IP
Application
Presentation
Session
Application
Transport
Transport
Network
Internet
Data Link
Data Link
Physical
Physical
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Disadvantages of layering

• Layering has many conceptual advantages, but
  fanatical adherence to layering is problematic.
• Layer N may duplicate lower layer functionality
• Example error recovery on both a hop by hop and
  end to end basis.
• Different layers may need the same information.
• Example time stamps
• Layer N may need to know non-adjacent layer
  information.
• Example choosing packets to drop if congested
• OSI session layer has not proven to be
  particularly useful.

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Standards Bodies

• Formal accredited standards bodies produce
  national and international standards.
• National standards bodies
• Industry Canada
• National Research Council (Canada) (NRC-CNRC)
• Canadian Standards Association (CSA)
• American National Standards Institute (ANSI)
• US National Institute of Standards and Technology
  (NIST)
• International standards bodies
• International Organization for Standardization
  (ISO)
• International Telecommunications Union (ITU)
• Non-governmental organizations

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Standards Bodies

• ISO (www.iso.ch)
• Non-treaty agency of the United Nations.
• Collaborates standards development for
  information technology.
• ITU (www.itu.int) ITU-T telecom sector of ITU
• UN treaty agency that sets telecommunications
  standards.
• ANSI (www.ansi.org)
• The US national standards body.
• Coordinates and accredits standards development
  across the US.
• IEEE (www.ieee.org)
• US based international professional organization.
• Develops standards and submits to ANSI for
  approval.

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Standards Bodies

• Telcordia (www.telcordia.com)
• Coordinates and develops standards for US
  telephone service
• ETSI (www.etsi.org)
• European Telecomunication Standards Institute
• Similar to Telcordia, but for Europe
• IAB / IETF / IRTF
• Internet Architecture Board (www.iab.org)
• Internet Engineering Task Force (www.ietf.org)
• Internet Research Task Force (www.irtf.org)
• Object Management Group (OMG) (www.omg.org)
• Consists of many companies
• Develops/co-ordinates CORBA/IDL, UML standards
• WWW consortium (www.w3.org)
• Develops/co-ordinates standards such as HTTP,
  HTML, XML,

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Protocol Description Techniques

• In order to ensure that protocol implementations
  are interoperable, there should be an unambiguous
  definition for a protocol.
• The protocol operation is typically described
  using one or more of the following techniques
• State diagram
• State transition table
• Standardized communications formal description
  technique (FDT)
• Specification and Description Language (SDL)
• Message sequence charts (MSCs)
• Unified Modelling Language (UML)
• OSI Service primitives

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Message Sequence Charts (MSCs)

• An exchange of messages over time is illustrated
  in order to describe an aspect of the protocol
  operation (a scenario).
• Since there are typically an infinite set of
  allowable message sequences, a set of MSCs cannot
  completely describe a protocol.
• MSC format has been standardized by ITU-T
  (standard Z.120, revised 2000)

Phone
Switch
lift receiver
time
dial tone
hang up
dial tone off
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OSI Service Primitives

• Four classes defined in the OSI model
• Request An entity wants the service to do some
  work
• Indication An entity is informed about an event
• Response An entity wants to respond to an event
• Confirm An entity receives confirmation of a
  previous request

Layer N 1
Layer N 1
1 request
3 response
2 indication
4 confirm
Layer N
Layer N
peer protocol
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OSI Service Primitives (2)

• A set of service primitives that could describe a
  simple connection-based protocol
• CONNECT.request Request a connection to be
  established
• CONNECT.indication Signal the called party of an
  incoming request
• CONNECT.response Used by the called party to
  accept or reject a call
• CONNECT.confirm Tells the caller whether the call
  was accepted.
• DATA.request Request that data be sent
• DATA.indication Signal the arrival of data
• DISCONNECT.request Request that the connection be
  released
• DISCONNECT.indication Signal that the connection
  has been released

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The Internet

• Grew out of the US defense department Advanced
  Projects Research Agency network (ARPANET)
• As other networks were connected to the ARPANET,
  notably the National Science Foundation network
  (NSFNET), the resulting internetwork has become
  known as the Internet
• Foundation is the TCP/IP (1983) protocols for
  communication

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The Internet

• What does it mean to be on the Internet?
• machine runs TCP/IP protocol stack
• machine has IP address
• machine can send IP packets to other internet
  hosts (connected to IP router)
• Four classic (1980s) Internet applications
• electronic mail
• Usenet news
• remote login
• file transfer

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TCP/IP layers (2)

• Internet Layer
• Official packet format and protocol Internet
  Protocol (IP).
• Layer function is to deliver IP packets to their
  destination.
• IP Version 4 in use, version 6 is being phased in
  (slowly)

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TCP/IP layers (2)

• Transport Layer
• Designed to allow source to destination
  conversation.
• Transmission Control Protocol (TCP) provides a
  reliable, connection-oriented service
• User Datagram Protocol (UDP) is an unreliable,
  connectionless protocol, used where prompt
  delivery is the chief concern

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TCP/IP layers (3)

• Application Layer
• file transfer (FTP)
• e-mail (SMTP)
• virtual terminal (TELNET)
• news (NNTP)
• World Wide Web (HTTP)