Chapter 1

• Objectives, In this chapter we will
  • Look for a grand overview of data communications and networking
  • Define a number of terms
  • Discuss Data Communications
  • Discuss Data Communications Networking
  • Discuss Protocols
  • Discuss Protocol Architecture
  • Discuss Standards

• Some statements:
  • In the 60's telephone (voice ) communications networks
  • In the 60's computer ( data ) communications networks
  • In the 70's these merged.
  • "There is no fundamental difference between data processing and data communications"
  • "There are no fundamental differences among data, voice and video communications"
  • "The distinction among single-processor computer, multiprocessor computer, local network, metropolitan network and long-haul network has blurred"
• A Communications Model.
  • 
  • A Source generates data to be transmitted.
    • Telephone
    • Networked camera
    • Computer
  • A Transmitter encodes the data to be data into a transmittable form
    • Modem
    • Network Card (coax based)
    • Network Card (optical)
  • A Transmissions System transports the encoded message
    • This may be very complex.
  • A Receiver accepts the signal from the transmission system and decodes it into data
  • A Destination Takes the data.

• Tasks associated with networks:
  • Transmission System Utilization - the percentage of the transmission system being used, must be maximized.
    • Multiple users are supported simultaneously through the use of multiplexing.
    • Congestion must be controlled to satisfy demand
  • Signal Generation or encoding the data into a form that can be sent over the transmission system.
    • A device that interfaces with the transmission system must do this.
  • Transmissions must be synchronized
  • Errors must be detected and possibly corrected or at least fixed.
  • The address or location of the destination must be determined.
  • and a route or path to that address must be found
  • Disrupted transmissions should be recoverable
  • Communications should be secure
  • Facilities need to be in place to allow for Network Management

• Data Communications (Chapters 3- 8) deals with turning data into a signal, placing that signal onto the transmission system, sending the data through the transmission system and removing it from the transmission system.
  • A new look at our model.
    1. A message (in text) is typed in at the source. m
    2. This information is encoded in binary data g(t)
    3. The transmitter turns this data into a signal s(t)
    4. The signal (s(t)) is place in the transmission system.
    5. On the other end a signal r(t) is received, we hope that r(t) = s(t), but it may not be.
    6. The signal r(t) is turned into data g'(t). In some systems it is the responsibility of the receiving system to check to see if g'(t) =g(t)
    7. The data g'(t) is turned back into a message m'
  • With email, file transfer and such, m must be the same as m'
  • With voice and video, m' must be close to m, but they need not be identical.
  • The message can possibly be transformed text->digital data -> analog signal -> digital data -> text.
  • Other examples are possible (video, voice, telemetry data, ...)
• Data Communication Networking
  • Direct point to point connections (like a parallel printer connection) are rare
  • Most communications involve a network.
  • WAN - wide area network
    • Long distances 10s to 1000s of kilometers
    • Crossing of public right-of-ways
    • Transmission media and hardware are owned by communications companies
    • No concern with data content, only data movement.
    • Typically
      • Circuit switched.
        • A single path is established and all data flows through it.
        • 
        • This is called a dedicated circuit.
        • The phone system.
      • Packet switched.
        • Packets are sent independently
        • 
        • Each packet may take an independent route
        • This is common in computer networks.
    • In addition
      • Frame Relay
        • This is a modified version of packet switched networks.
        • An attempt is made to reduce overhead in the system.
      • ATM
        • Evolved from circuit switching
        • Packets are of fixed size
        • Still a circuit switching network
        • Quality of service and bandwidth addressed
      • ISDN
        • Integrated Services Digital Network
        • Designed to replace telephone networks
  • LAN - Local Area Network
    • Scope- building, floor, or campus
    • Much faster than an WAN
    • The organization usually owns everything
    • Traditionally broadcast, but switching is here too
    • Ethernet, ATM, Fibre Channel

• Protocols and Protocol Architecture
  • We are familiar with application protocols (FTP, HTTP, ...)
  • These often assume a valid network
  • A protocol is a set of rules governing the exchange of data between two entities.
  • At the network level a protocol includes:
    • Syntax - data formats and signal levels
    • Semantics - control information
    • Timing - speed matching and sequencing
  • In network communications, there are many segments that can change:
    • Underlying transmission system (physical network)
    • Protocol running on the local network
    • Connections to remote network
  • It would be bad to be forced to rewrite each application for each variation.
    • ftp without a network
    • ftp for ethernet over coax, twisted pair, fiber, 10, 100, 1000 MB.
    • ftp for atm 25mb, 155mb, 600mb, ...
    • ie each protocol at every level.
  • For that reason, we have developed a hierarchy of protocols or a protocol architectures.
  • These are generally a layered approach.
  • Separate sub-tasks are implemented as a layer.
  • High level applications rely on the underlying levels and only deal with their own protocol.
    • User authentication
    • File exchange
    • ...
  • An example three layer model
    • 
    • Network Access Layer
      • Concerned with exchanging data between computer and network
      • Network addresses
      • Network services (quality, bandwidth)
      • This is a network dependent layer
    • Transport Layer
      • Concerned with the message.
      • All data in order.
      • All data present.
      • No knowledge of the contents of the message or of the function of the network.
    • Application Layer
      • The logic of the application

    • There are two addresses involved here.
      • Each machine has its own address
      • Applications on machines have an address too
        • Multiple applications can be running on a single machine
          • Web server
          • ftp server
          • Tenet server
          • Client for each of these
        • These are called service access points (SAP)
        • Also known as ports.
        • Pigeon hole mail box system.
    • At each layer, data is encapsulated into a packet.
      • Also called a protocol data unit (PDU)
      • A packet contains a payload
      • Also header and trailer information
      • Transport Layer Header (Examples)
        • Destination SAP
        • Sequence Number
        • Error Detection Code
      • Network Layer Header (EXAMPLES)
        • Destination Computer Address
        • Source Computer Address
        • Priority Request Information
      • Packets may be broken down further at lower levels.
      • 

  • There are two protocol architectures that we will study:
    • The real world uses the TCP/IP model
      • Developed by the DOD through DARPA (Defense Advanced Research Projects Agency)
      • Consists of five levels
        • Application Layer
        • Host to Host (Transport) Layer (TCP)
        • Internet Layer (IP)
          • Routing data between networks
          • Router (or gateway) a connection between two networks
        • Network Access Layer
        • Physical Layer
          • The interface between the computer and the network.
      • No formal definition of the levels
      • We will look at the pieces all semester, but in chapters 15 and 16
    • The theoretical world uses the OSI model
      • Open Systems Interconnection model.
      • Developed by the International Organization for Standardization (ISO)
      • Studied thoroughly, Mostly Academic these days.
      • Seven Layers
        • Application
        • Presentation
          • Mechanism for data representation
        • Session
          • Establishes, manages and terminates sessions
        • Transport
        • Network
        • Data Link
        • Physical
    • We will look at these architectures in chapter 2.
    • 

• Standards
  • As you can guess, without standards, this whole thing would fall apart.
  • Communications companies have long accepted standards
  • Computer companies have not.
  • Standards
    • Assures that there will be a large marked for a product
    • Allow multiple vendors to participate, give the consumer more flexibility
    • Freeze technology at the standard.
    • Inhibit new growth. Why develop new ideas when they must be shared (standards) and other companies will benefit from your work.