Basics

Open Systems Interconnect Model

  • ISO standard
  • standardizes how computer communicate over a network
  • 7-layered approach to data transmission
    • operations are divided into related groups of actions at each layer
  • a layer serves the layer above it

Example

Sending an email:

  • Computer (sender)
  • Switch
  • Serve (receiver)

Layers (Overview)

Layer Name Includes Devices
7 Application    
6 Presentation    
5 Session    
4 Transport TCP/UDP, Port  
3 Network IP Address Routers
2 Data Link Ethernet MAC Address Switches
1 Physical   Hubs
  • Sender sends email
  • the package goes from layer 7 down to 1
  • and then up from 1 to 7 when arriving at receiver

Describing problems in terms of OSI model:

  • A cable was unplugged: Layer 1 problem
  • A user made a mistake: Layer 8 problem

Accronyms to remember the names of the layers

  • Please Do Not Throw Sausage Pizza Away
  • Please Don’t Need Those Stupid Packets Anyway
  • Please Do Not Teach Students Pointless Acronyms
  • Please Do Not Take Sales People’s Advice
  • Please Do Not Touch Superman’s Private Area

Upper Layers

  • Upper Layers: 7,6,5
    • Application developers

Layer 7: Application Layer

  • provides network services to the application of the user
  • establishes availability of intended communication partners
  • agreement on procedures for error recovery and control of data integrity

Layer 6: Presentation Layer

  • ensures information sent at layer 7 of system 1 is readable by layer 7 of another system
  • translates among multiple data formats using a comon format
  • e.g. computers with different encoding schemes

Layer 5: Session Layer

  • establishes, manages, and terminates sessions between two communicating hosts
  • synchronizes dialog between presentation layers of the two host and manages their data exchange
    • e.g., web servers have many users (many communication processes open at any given time to track)
  • offers efficient data transfer, CoS, and exception reporting of upper layer problems

Lower Layers

  • Lower Layers: 4,3,2,1
    • Network engineers

Layer 4: Transport Layer

  • main characteristics: TCP (preferrable for reliability) or UDP (preferrable for speed) transport used? What is the port number?

Definition:

  • defines services to
    • segment,
    • transfer, and
    • reassemble the datafor individual communications between the end devices
  • breaks down large files into smaller segments taht are less likely to incur transmission problems

Layer 3: Network Layer

  • most important info: the source and destination IP address
  • the layer where routers operate

Definition:

  • provides connectivity and path selection between two host systems (they might be located on geographically separated networks)
  • manages the connectivity of hosts by providing logical addressing
  • most important info: source and destination layer 2 address
    • e.g., the source and destination MAC address if Ethernet is the layer 2 technology
  • the layer where switches operate

Definition:

  • defines
    • how data is formateed for transmission and
    • how access to physical media is controlled
  • typically includes error detection and correction to ensure a reliable delivery of the data

Layer 1: Physical Layer

  • physical components of the network, e.g., the cables being used

Definition:

  • enables bit transmission between end devices
  • defines specifications needed for
    • activating,
    • maintaining, and
    • deactivating the physical link between end devices
  • e.g., voltage levels, physical data rates, maximum transmission distances, physical connectors etc.