Chapter 1

• An Overview of Computer Networks

  
• Five components of data communication

  • Data Representation
  • Data Flow
  • Components
  • Distributed Processing
  • Network Criteria (performance, reliability, and security)
• Data flow


  • Simplex
  • Half-duplex
  • Full-duplex
• Physical Structures

  • Type of connections
  • Topologies

  
• Categories of Networks

  • LAN
  • MAN
  • WAN
  • WLAN
• Interconnection of Networks: Internet
• Point to point


  A dedicated link is provided between two devices
• Multipoint


  More than two specific devices share a single link
• MESH Topology

  • Every device has a dedicated point-to-point link to every other devices
  • Dedicated link carries traffic only between the two devices it connects
  • A fully connected mesh network has n(n-1)/2 physical channels to link n devices
  • Every device on the network must have n-1 input/output (I/O) ports

  
• MESH Topology Advantages


  • Less traffic, robust, secure, easy to maintain
• MESH Topology Disadvantages

  • Need more resource (cable and ports), expensive
• STAR Topology

  • Each device has a dedicated point-to-point link only to a central controller, usually called a hub
  • No direct traffic and link between devices

  
• STAR Topology Advantages

  • Less expensive, Easy to install and reconfigure, Robustness
• STAR Topology Disadvantages

  • Single point of failure
• BUS Topology

  • A multipoint topology
  • All devices are linked through a backbone cable
  • Nodes are connected to the bus cable by drop lines and taps
• BUS Topology Advantage

  • Ease of installation
• BUS Topology Disadvantages

  • Difficult reconnection and fault isolation
  • Broken or fault of the bus cable stops all transmission
• RING Topology

  • Each device is dedicated point-to-point connection only with the two devices on either side of it
  • A signal is passed along the ring in the direction, from device to device, until it reaches its destination
  • Each device in the ring incorporates a repeater

  
• RING Topology Advantages

  • Relatively easy to install and reconfigure
  • Fault isolation is simplified
• RING Topology Disadvantages

  • Unidirectional traffic
• Tree Topology Advantages

  • Point-to-point wiring for individual segments
  • Supported by several hardware and software venders
• Tree Topology Disadvantages

  • Overall length of each segment is limited by the type of cabling used
  • If the backbone line breaks, the entire segment goes down
  • More difficult to configure and wire than other topologies
• Tree topologies integrate multiple topologies together
• A hybrid topology: a star backbone with three bus networks
• An isolated LAN connecting 12 computers to a hub in a closet
• Categories of Networks

  • Local Area Network (LAN)
  • Wireless Local Area Network (WLAN)
  • Metropolitan Area Network (MAN)
  • Wide Area Network (WAN)
• WANs: a switched WAN and a point-to-point WAN
• A heterogeneous network made of four WANs and two LANs
• Standards Organizations


  • International Organization for Standardization (ISO)
  • International Telecommunication Union - Telecommunication Standards (ITU-T)
  • American National Standards Institute (ANSI)
  • Institute of Electrical and Electronics Engineers (IEEE)
  • Electronic Industries Association (EIA)
• Protocols

  • Syntax: format of the data
  • Semantics: meaning of each section
  • Timing: when data should be sent and how fast
• Standards

  • De facto: by fact (not approved as a standard)
  • De jure: by Law (approved)
• OSI Model
• Network Models

  • A network model is a layered architecture
  • Task broken into subtasks
  • Implemented separately in layers in stack
  • Functions need in both systems
  • Peer layers communicate
  • Protocol: A set of rules that governs data communication, it represents an agreement between the communicating devices

  
• Tasks involved in sending a letter
• ISO is the organization, OSI is the model
• Layered Architecture


  • Peer-to-Peer Processes
  • Encapsulation
• Seven layers of the OSI model

  • Layer 7. Application
  • Layer 6. Presentation
  • Layer 5. Session
  • Layer 4. Transport
  • Layer 3. Network
  • Layer 2. Data Link
  • Layer 1. Physical
• Layered Architecture

  • Each layer performs a subset of the required communication functions
  • Each layer relies on the next lower layer to perform more primitive functions
  • Each layer provides services to the next higher layer
  • Changes in one layer should not require changes in other layers
  • The processes on each machine at a given layer are called peer-to-peer process
• Communication in the OSI model

  1. Communication must move downward through the layers on the sending device, over the communication channel, and upward to the receiving device

  2. Each layer in the sending device adds its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it

  3. At the receiving device, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it
• Peer-to-Peer Process


  • The passing of the data and network information down through the layers of the sending device and backup through the layers of the receiving device is made possible by interface between each pair of adjacent layers
  • Interface defines what information and services a layer must provide for the layer above it