Introduction to Communications, Standards, and Protocols

1
Introduction to Communications, Standards, and
Protocols

• CIS-4445 Data Communications
• Chapter 1

2
CHAPTER 1 HOMEWORK
Review Questions 1, 6, 8, 11, 15
Exercises 1, 4, 6, 7, 8, 14
3
Introduction

• Why Study Communications
• Computer Networks
• Standards
• Open Systems and the OSI Model
• The Future of Data Communications

4
Some Modern Types of Communication

• Radio TV (land-based radio transmissions, and
  cable)
• Satellite (land-to-space radio transmissions)
• Telephones (along cable)
• Computers (radio, cable, satellite)
• Local Area Networks (LANs)
• Wide Area Networks (WANs), e.g. the Internet
• email
• file transfer

5
Some Modern Types of Communication

• FAX
• Voice and video communications (e.g. video
  phones)
• Teleconferencing
• Cell phones (radio)

6
Some of the Issues Which We Must Consider

• How to connect two things together so that they
  can communicate? E.g. wire, cable, optic fibre,
  radio waves.
• Planning for the expected usage. Balancing cost
  v/s features.
• Rules (or protocols) for the communication.
• Efficiency
• Security
• Compatibility with existing systems

7
Introduction

• Why Study Communications
• Computer Networks
• Standards
• Open Systems and the OSI Model
• The Future of Data Communications

8
Computer Networks

• A Network is a system connecting devices such as
  PC's, printers, disk drives, etc. Reasons for
  doing this include
• Sharing files (via a PC called a file server)
• Sharing an "expensive" device such as a laser
  printer
• Allowing other machines to perform some
  processing for you.
• LAN (Local Area Network)
• Small geographical area (one or a small group of
  buildings)

9
Computer Networks

• WAN (Wide Area Network)
• Large geographical area (e.g. a town, country,
  continent or the world).
• Either way, devices must be connected so that
  more than one user can use them.
• Must be able to handle conflicts (e.g. two people
  wanting to print on a single printer at the same
  time).
• Must allow information to be sent with little or
  no delay.
• Connection strategy used to accomplish this is a
  Network Topology.

10
Network Topologies

• Several different topologies to consider
• Fully-connected topology
• Common Bus topology (or Bus topology)
• Star topology
• Ring topology
• Combined topology

11
Fully-Connected Topology

• Direct connection between each pair of devices
• Simple communication no competition for the
  common lines.
• Expensive
• Under-utilized lines are wasted
• Better to allow devices to share lines

12
Fully-Connected Topology
13
Common Bus Topology (or Bus topology)

• All devices share the same single bus (line)
• Each device "listens" to what's happening on the
  line
• If data destined for it arrives, it reads the
  data
• To send data it first waits until the line is
  quiet
• Devices may transmit simultaneously, resulting in
  a collision
• Advantage - Easy to add/remove device

14
Common Bus Topology (or Bus topology)

• CSMA/CD (Carrier Sense, Multiple Access with
  Collision Detection)
• Devices listen while sending data
• If detect collision
• stop transmitting
• wait random amount of time
• try transmit again
• Other methods exist to avoid/detect collisions
• E.g. Ethernet bus using copper cable or optical
  fibre

15
Common Bus Topology
Server
Printer
Workstation
Workstation
Workstation
Mainframe
Workstation
16
Star topology

• All data sent to central computer first.
• Central computer then sends it to correct
  destination.
• Advantage
• Centralized control of all transmissions
• Disadvantage
• If central computer is down, no communication can
  occur.
• More difficult to add/remove devices
• E.g. Mainframe with many terminals attached to
  it.

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Star Topology
Printer
User
Communications Computer
Server
18
Ring topology

• Devices connected in one or more "circles"
• Device can only communicate with immediate
  neighbors
• Unidirectional data travels in only one
  direction
• Each device communicates with one neighbor only
• Bi-directional data can travel in either
  direction
• Each device communicates with both neighbors

19
Ring topology

• Disadvantage
• Data may need to travel past many devices to
  reach its destination
• Cannot add/remove devices easily
• Communication limited if one machines goes down
• E.g. IBM's Token Ring network.
• Device must have the token before it can send
  data
• When finished sending, it passes the token to the
  next device in the ring.

20
Ring Topology
21
Combined topology

• Many combinations of the above topologies are
  possible
• e.g. common bus as a back-bone connecting major
  devices such departments
• Sub-networks branch out from the backbone
• A bridge connects LANs together
• e.g. most larger networks

22
Combined Topology
Server
Workstation
Local Area Network
BRIDGE
BRIDGE
Local Area Network
Workstation
Mainframe
Workstation
Printer
23
Introduction

• Why Study Communications
• Computer Networks
• Standards
• Open Systems and the OSI Model
• The Future of Data Communications

24
Standards

• Why are they necessary?
• There are many different types of computer
• Incompatibilities arise if everyone does not
  follow accepted guidelines
• Protocol agreement on how to act in various
  situations
• Protocols must be accepted and used by everyone
  otherwise they are useless
• Many protocols/standards have been developed
• What types are there?
• De-facto standards (exist because they are widely
  used)
• Those adopted by an international Standards
  Agency

25
Standards

• Who makes the standards anyway?
• ANSI (e.g. ASCII, optical-fibre standard FDDI)
• CCITT (e.g. network telephone communications
  like X.25 and X.400 (email))
• EIA (e.g. RS-232 for modems, printers etc.)
• IEEE (e.g. bus and token ring LAN standards)
• ISO (e.g. the layered OSI model)
• NIST (e.g. data encryption standard DES)
• IBM (e.g. EBCDIC, SNA protocol)
• etc. etc. etc.

26
Introduction

• Why Study Communications
• Computer Networks
• Standards
• Open Systems and the OSI Model
• The Future of Data Communications

27
The OSI Model

• OSI Open Systems Interconnect
• Idea is that any two computers can communicated
  as long as they are connected somehow
• Layered approach (7 layers)
• Each layer has specific function
• A Layer communicates only with the layers
  immediately above and below it
• Higher layers deal with applications, services,
  etc
• Lower layers deal with actual transmission of
  data

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The OSI Model

• The layers are abstract
• Hide implementation details from the other layers
• Any layer's implementation can be changed without
  affecting the other layers
• Example one manager sending communication to
  another manager via secretaries, fax, telephone,
  etc.

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The OSI Model - 7 Layers
Logical Communications between layers
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Physical transmission of data
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The OSI Model - Overview
Application

• Application Layer
• Not the application programs themselves
• Called the application layer because it contains
  network applications that provide network
  services to the user
• Provides services such as email, file transfer,
  remote job entry, and resource allocation
• On differing terminals, defines protocols for
  screen display and editing

Presentation
Session
Transport
Network
Data Link
Physical
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Application Layer

• Communicates with user and application programs
  in the form of network services
• Email protocol defines how the electronic mail
  system works.
• FTP (File Transfer Protocol) defines how files
  can be transferred over networks
• Virtual Terminal protocol how to connect a
  terminal to a remote computer via a network (as
  if it's a local machine).
• How information is displayed
• How key presses are interpreted.
• Distributed Systems defines how many devices may
  run the same software and access shared resources

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The OSI Model - Overview
Application

• Presentation Layer
• Responsible for presenting data in a format its
  user can understand
• May mean converting data to/from the correct
  formats (e.g. different ways of storing
  characters (ASCII to EBCDIC conversion) or
  numbers.)
• Provides Security measures - Encryption/Decryption
  
• Provides data compression

Presentation
Session
Transport
Network
Data Link
Physical
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Presentation Layer

• Responsible for making meaningful information
  from the data received
• Format Conversion of Information
• e.g. Sender uses ASCII, receiver uses EBCDIC.
• e.g. Sender uses 16-bit integers, receiver uses
  32-bit.
• Data Compression - can be very unique to the type
  of information
• Reduce the number of bits which must be
  transmitted.
• Cost savings (time, money)
• e.g. Look for patterns.
• e.g. Represent frequently-occurring characters
  using fewer bits (e.g. Huffman coding)
• Security
• Encryption/decryption of data

34
The OSI Model - Overview
Application

• Session Layer
• Responsible for establishing a session or logical
  connection between two different computers
• Handles error recovery - what happens if network
  goes down while you're using it to send a large
  file?
• Synchronization between each end on who can talk
  and when
• Bracket operations that must appear to the user
  as a single transaction

Presentation
Session
Transport
Network
Data Link
Physical
35
Session Layer

• Must be able to establish and maintain a
  connection between two end-users
• Defines if a session can use more than one
  transport connection (e.g. after a network
  failure)
• Defines if multiple sessions can make use of the
  same transport connection

36
Session Layer

• Dialog Management ("who talks when")
• Full-duplex both directions simultaneously
• Half-duplex both directions but take turns
  (using a token)
• Who gets the token first?
• Synchronization Points
• Used to avoid resending large amount of data
  again if error occurs.
• Data divided into Dialog Units (DUs)
• Dialog units must be ACK'd once they arrive
  safely at their destination.
• DUs define Synchronization Points -
  retransmission after error starts from the most
  recent sync point.
• Minor sync points within Major DUs
• Activities - DUs can be grouped as an Activity.

37
The OSI Model - Overview
Application

• Transport Layer
• The lowest of the layers to deal primarily with
  end-to-end communications
• Determines which of the available networks to use
  for communication
• Dedicated connection
• Packet
• etc
• Consider cost, speed, type of data

Presentation
Session
Transport
Network
Data Link
Physical
38
Transport Layer

• Provides a reliable and efficient network
  connection (or transport connection).
• Relies on lower 3 levels to control network
  operations.
• Allows upper 3 layers to function independent of
  the type of network.

39
Transport Layer

• Multiplexing
• Downward Multiplexing
• Can establish more than one physical connection
  to the network.
• Divide the data to be sent between the available
  connections.
• Upward Multiplexing
• More than one "user" could share the same
  physical connection.
• e.g. Downloading 2 files from one machine with a
  single network card.

40
Transport Layer

• Buffering
• Data received from Session layer is divided into
  TPDUs (Transport Protocol Data Units).
• TPDUs sent to destination, where they are
  reassembled.
• TPDUs must be acknowledged (ACK) by the receiver.
  
• Sender buffers (i.e. stores) TPDUs until ACK
  obtained.
• If no ACK arrives within a certain time, the TPDU
  is retransmitted.
• Receiver buffers TPDUs until its own session
  layer can accept them.
• If TPDU arrives and there's no space in buffer,
  TPDU will be ignored and sender will have to
  retransmit it.

41
Transport Layer

• Connection Management Protocol
• How to establish a connection with the other
  side.
• The receiver must agree to the terms of the
  communication "proposed" by the sender.
• 2-Way Handshake
• A sends TPDU to B to request connection
• B sends TPDU saying "OK"
• Connection is established.

42
Transport Layer

• Problem if either response is delayed
• 3-Way Handshake
• A sends TPDU requesting connection. TPDU contains
  sequence number 'x'.
• B sends TPDU saying "OK" to sequence number.
  Sends its own sequence number 'y'.
• A starts transmitting. First TPDU sent contains
  ACK to B's ACK as well as sequence numbers 'x'
  and 'y'.
• B establishes connection when this ACK arrives.

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The OSI Model - Overview
Application

• Network Layer
• Determines the best routes between the two points
  in the network - consider speed, cost, security,
  traffic etc.
• Should all the data go along the same route or
  should parts be transferred independently?
• Billing/Accounting information (who pays, how
  much based on time of day etc.)

Presentation
Session
Transport
Network
Data Link
Physical
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Network Layer

• Transport Layer depends on the Network Layer to
  establish end-to-end communications
• Finds the best route between two points
• Considers cost, speed, availability
• Tries to find the quickest and cheapest route.
• Algorithms exist for this task e.g. Shortest
  Path algorithm
• For datagram circuit switching, must consider
  these things for each packet sent.
• For virtual circuit, only consider these when the
  connection is initially established.
• Accounting Billing
• Who must pay for the use of the network?
• Keeps stats on usage.

45
The OSI Model - Overview

• Data Link Layer
• Supervises the reliable transfer of data between
  two directly-connected nodes in the network
• Error detection and correction - request
  retransmission if error occurs
• Determines amount of information sent (frame
  size)
• Determines format of frames.
• Sends error-free frames to Network Layer

Application
Presentation
Session
Transport
Network
Error Detection/Correction
Data Link
Contention
CSMA/CD
Physical
Collision Detection
Token Passing
46
Data Link Layer - Contention

• It's when two or more devices try to use the same
  transmission medium at the same time.
• Collision results (data sent is garbled)
• Detecting collisions (Bus networks)
• Nodes listen for collisions while transmitting
• If collision occurs, stops transmitting and
  retries later.
• CSMA/CD
• Reduces collisions - does not eliminate them.

47
Data Link Layer - Contention

• Preventing collisions (e.g. Ring or Bus networks)
  
• Token passing
• A token is a special (unique) string of bits
• Only the node "with the token" may transmit
  anything
• Token is passed on, for example, either
• with a packet sent, or
• when the node has nothing more to send, or
• after it's had the token for more than a certain
  time

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Data Link Layer - Error Detection/Correction

• Errors (flipped bits) can result from
• Bad connections
• Faulty lines
• Electrical interference
• Error detection is determining whether an error
  has occurred.
• Error correction is setting "damaged" bits to
  their correct state.
• Parity bit (one simple solution)
• Can detect whether a single bit is in error
• Even parity number of 1's (including the parity
  bit) is even.
• I.e. if frame has odd number of 1's, parity bit
  is 1, else it's 0.
• Problem if more than one bit is wrong it may not
  work.

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The OSI Model - Overview

• Physical Layer
• Handles the physical (e.g. electrical or optical)
  aspects
• Sends frames received from Data Link Layer on
  communication medium without regard to their
  meaning or format
• Retrieves frames without regard to meaning or
  format, passing them to Data Link Layer for
  analysis

Application
Presentation
Session
Transport
Network
Data Link
Circuit Switching
Packet Switching - datagram - virtual
circuit
Message Switching
Physical
50
Physical Layer

• Transmission Media
• Determines how signals are sent
• Twisted wire pairs
• Coaxial cable
• Optical fiber
• Satellites
• Microwave and Radio towers
• Considers properties such as
• Analogue v/s digital transmission
• Bandwidth
• Signal-to-noise ratios

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Physical Layer

• Connection Strategy
• How to connect two nodes
• Not which of the available routes is best (which
  is the job of the network layer
• Various strategies
• Circuit Switching
• Message Switching
• Packet Switching
• Datagram packet switching
• Virtual Circuit packet switching

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Circuit Switching

• Before communication starts
• Route is determined (consider cost, speed,
  availability etc.)
• Connection is made when receiver "accepts" a
  request to connect
• Data cannot be sent until connection is
  established
• Connection is dedicated to the communication
  (cannot be shared)
• Connection is maintained until communication ends
• Intermediate nodes act as a switch - doesn't
  store the message
• Best for transmission if
• Connection is continuous
• Cannot tolerate large delays
• Example older telephone networks

53
Message Switching

• The destination's address is "attached" to the
  message (i.e. the data) which must be sent.
• Entire message is passed from node to node as a
  "single unit".
• Each node stores entire message while it decides
  which node to send it to next.
• Different messages between two nodes can travel
  over different routes - no dedicated single
  route.
• Disadvantages
• Message may be too large to hold at a node.
• If a node fails, the entire message may be lost.
• Example e-mail ??

54
Packet Switching

• Data to be sent is "divided into packets".
• Packets are labeled with the destination's
  address and a sequence number.
• Packets are
• Sent one at a time.
• Stored at intermediate nodes and forwarded when
  the next node in the path has been determined by
  that node.
• Original data put back together at destination
  using the sequence numbers.
• Fixed packet size - buffering at intermediate
  nodes is possible.

55
Packet Switching - Datagrams

• Each packet is transmitted separately - may take
  different routes to the destination
• Adapts to changing conditions
• Packets may not arrive in order
• Routing each packet adds overhead at each node.

56
Packet Switching - Virtual Circuits

• A route (virtual circuit) is established before
  sending any data.
• Packets sent and arrive in order along the same
  path.
• Route is not dedicated - more than one virtual
  circuit may share the same connection.
• No complex routing decisions after virtual
  circuit set up.
• Does not adapt to changing conditions.

57
Introduction

• Why Study Communications
• Computer Networks
• Standards
• Open Systems and the OSI Model
• The Future of Data Communications

58
The Future of Data Communications

• Electronic telephone directories
• Portable telephones
• Electronic mail
• All-digital telephone system
• Electronic media access
• Videoconferencing
• Three-dimensional imaging
• Electronic locators
• Voice recognition/communications
• Mind communications

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End Chapter 1