Topic 2: Network Protocols and Communications

• Communication begins with a message or information that must be sent from a source to a destination.
• A protocol is an agreement or rules to govern a way of communicating.
• A protocol contains

  • An identified sender and receiver
  • Agreed upon method of communicating
  • Common language and grammar
  • Speed and timing of delivery
  • Confirmation or acknowledgement requirements
• Computer communication requires the interaction of:

  • Message Encoding
  • Encoding
  • Decoding
  • Message Formatting and Encapsulation
  • Specific formatting and structure
  • Encapsulation
  • De-encapsulation
  • Message Size
  • Segmentation: breaking a message into smaller pieces
  • Message Timing
  • Access method
  • Flow control
  • Response timeout
  • Message Delivery Options
  • Unicast
  • Multicast
  • Broadcast
• A protocol suite is a group of inter-related protocols working together to perform a communication function.
• The protocol suite interaction can be visualized as layers in a stack where the lower layers of the stack are concerned with moving data over the network and providing services to the upper layers while the upper layers are focused on the content of the message being sent.
• The Transmission Control Protocol/Internet Protocol (TCP/IP Protocol) Model

  • Application
  • Represents the data to the user, plus encoding and dialog control
  • Transport
  • Supports communication between diverse devices across diverse networks
  • Internet
  • Determines the best path through the network
  • Network access
  • Controls the hardware devices and media that make up the network.
• Interaction of protocols

  • Application protocols
  • Hypertext Transfer Protocol (HTTP) governs the way a web server and client interact
  • HTTP defines the content and formatting of requests that are exchanged
  • HTTP relies on other protocols, like TCP, to send messages between client and server
  • Transport Protocols
  • Transmission Control Protocol (TCP) manages the individual conversations between servers and clients
  • Data-link management protocols take the packets from IP and format them to be transmitted over the media
  • The standards and protocols for the physical media govern how signals are sent
• The interaction between various protocols can be visualised using a layered model
• The communications functions are partitioned into a hierarchical set of layers where each layer performs a related subset of the functions required to communicate.
• A layer in the layered model provides services to the next higher layer and uses services of the next lower layer.
• Benefits of Using a Layered Model:

  • Facilitates modular engineering, allowing changes in one layer without affecting other layers above and below
  • Ensures interoperable technology, thus fosters competition because products from different vendors can work together
  • Reduces complexity
  • Standardizes interfaces
  • Accelerates evolution
  • Simplifies teaching and learning
• The Open Systems Interconnection (OSI) Model , researched and developed by the International Organisation for Standardisation (ISO), is a model that provides a common reference for maintaining consistency within all types of network protocols and services, not intended to be an implementation specification
• There are 7 layers in the OSI model (from lowest to highest):

  • Physical
  • Data Link
  • Network
  • Transport
  • Session
  • Presentation
  • Application
• The lower 4 layers in the OSI model (physical, data link, network, transport) are called the data flow layers as the control the physical delivery of messages over the network.
• The top 3 layers in the OSI model (session, presentation, application) are called the application layers as they deal with the user interface, data formatting and the application access.
• Comparing the OSI and TCP/IP models:

  A) Application

  B) Presentation

  C) Session

  D) Transport

  E) Network

  F) Data Link

  G) Physcial

  H) Application

  I) Transport

  J) Internet

  K) Network Access

  
• Data is segmented into multiple pieces, marked with information during encapsulation, before getting sent onto the media.
• There are many end devices sending data at any one time, so the multiple pieces are interleaved onto the media.
• Data encapsulation is the process of adding a header to wrap the data that flows down the OSI model. Each of the data flow layers add its own header to the data received from above
• A protocol data unit (PDU) is the form that a piece of data takes at any layer in the OSI model.

  • Application layers - Data
  • Transport - Segment
  • Network - Packet
  • Data Link - Frame
  • Physical - Bits
• Protocol Encapsulation

  • The application creates the data for transmission and passes it down the layers
  • At Layer 4, the data is segmented. For each segment, a transport header is added (now called a segment) and passed down to Layer 3
  • At Layer 3, the segment is encapsulated with a network header (now called a packet) and passed down to Layer 2
  • At Layer 2, the packet is encapsulated with a header and trailer at both ends (now called a frame). It is then encoded into bits and transmitted over the Layer 1 media.
• Protocol De-encapsulation

  • At the destination, the de-encapsulation occurs
  • The PDU moves up the layers and in the process, each layer removes off the respective headers
  • Eventually the data reaches the Application Layer where it is displayed to the user
• • Physical addresses (MAC addresses) are fixed unique identifiers of network interfaces and represented in 6 hexadecimal pairs
  • Logical network addresses (IP addresses) are assigned to network interfaces and represented in dotted decimal format.
• While communication within the same network, the PDU is addressed to the destination network address as well as the destination physical address.
• Default gateway: The IP address of an interface on a router that is on the same network as the sending host