Communication considerations

1
Communication considerations

• AHHH!! What do those electrical signals mean?
• How can I send a bit, what signal do I use for 0
  and which for 1?
• How do devices make use of the wire?
• How do I derive meaningful information from all
  of these bits
• How are transmission errors discovered and dealt
  with?
• How do packets get from one system to another?

2
Communication Considerations

• How do I send large amounts of data and how do I
  ensure that I receive all of my data?
• How do machines keep track of who there are
  talking to?
• What language is this, how can I the computer
  understand different formats?
• How does a user gain access to the network?
• How do programmers write programs to use the
  network?

3
Our solution the OSI model

• Application
• Presentation
• Session
• Transport
• Network
• Data link
• Physical

4
What is a Protocol?

• Allows entities (i.e. application programs) from
  different systems to communicate
• Shared conventions for communicating information
  are called protocols
• Includes syntax, semantics, and timing

5
Why Use Protocol Architecture?

• Data communications requires complex procedures
• Sender identifies data path/receiver
• Systems negotiate preparedness
• Applications negotiate preparedness
• Translation of file formats
• For all tasks to occur, high level of cooperation
  is required

6
Modular Approach

• Breaks complex tasks into subtasks
• Each module handles specific subset of tasks
• Communication occurs
• between different modules on the same system
• between similar modules on different systems

7
OSI Lower Layers

• Physical
• Data Link
• Network

8
OSI Physical Layer

• Concerned with transmission of unstructured bit
  stream over physical medium
• Deals with accessing the physical medium
• Mechanical characteristics
• Electrical characteristics
• Functional characteristics
• Procedural characteristics

9
OSI Data Link Layer

• Responsible for error-free, reliable transmission
  of data
• Flow control, error correction

10
OSI Network Layer

• Responsible for routing of messages through
  network
• Concerned with type of switching used (circuit v.
  packet)
• Handles routing between networks, as well as
  through packet-switching networks

11
OSI Upper Layers

• Transport
• Session
• Presentation
• Application

12
OSI Transport Layer

• Isolates messages from lower and upper layers
• Breaks down message size
• Monitors quality of communications channel
• Selects most efficient communication service
  necessary for a given transmission

13
OSI Session Layer

• Establishes logical connections between systems
• Manages log-ons, password exchange, log-offs
• Terminates connection at end of session

14
OSI Presentation Layer

• Provides format and code conversion services
• Examples
• File conversion from ASCII to EBDIC
• Invoking character sequences to generate bold,
  italics, etc on a printer

15
OSI Application Layer

• Provides access to network for end-user
• Users capabilities are determined by what items
  are available on this layer

16
OSI in Action Outgoing File Transfer

• Program issues command to Application Layer
• Application passes it to Presentation, which may
  reformat, passes to Session
• Session requests a connection, passes to
  Transport
• Transport breaks file into chunks, passes to
  Network

• Network selects the datas route, passes to Data
  Link
• Data Link adds error-checking info, passes to
  Physical
• Physical transmits data, which includes
  information added by each layer

17
OSI in Action Incoming File Transfer

• Physical receives bits, passes to Data Link
• Data Link checks for errors, passes to Network
• Network verifies routing, passes to Transport
• Transport reassembles data, passes to Session
• Session determines if transfer is complete, may
  end session, passes to Presentation

• Presentation may reformat, perform conversions,
  pass to Application layer
• Application presents results to user (e.g.
  updates FTP program display)