CS601 - Data Communication - Lecture Handout 08

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Related Content: CS601 - VU Lectures, Handouts, PPT Slides, Assignments, Quizzes, Papers & Books of Data Communication

Critique of OSI Model

Reasoning for OSI not getting Widespread

  • Bad Timing(slide) (Apocalypse of Two Elephants)
    • David Clarke of MIT
    • If standards are written too early: subject is badly understood and bad standards
    • If standards are written too late so many companies may have already made investments in doing the same thing with different other ways
  • Bad Technology
    • Flow control, error control, addressing is multiple
    • Session and Presentation(EMPTY), Network and DL(Full)
  • Bad Implementations

Apocalypse of Two Elephants

Apocalypse of Two Elephants

Physical (Layer 1)

  • Coordinates the functions required to transmit a bit stream over a physical medium
  • Deals with mechanical and electrical specifications of Tx. Medium and Interface
  • Also defines procedures and functions that physical devices and interfaces need to perform for TX. To occur (Figure)



Functions of Physical Layer

  • Physical Characteristics of Interface & Media
    • Defines characteristics of Interface b/w device and Tx Medium
    • Interface is a plug gable connector that joins one or more signal conductors
    • Also defines the type of transmission medium
  • Representation of Bits/Encoding
    • The physical layer data consists of a stream of bits (sequence of 1’s and 0’s)
    • To be transmitted the bits must be ENCODED into signals: Electrical or Optical
    • Physical layer decides the type of ENCODING
  • Data Rate / Transmission Rate
    • Date Rate ( Bits per second) also decided by the Physical Layer
    • So , Physical layer defines the Duration of a Bit
    • Means how long will a bit last
  • Synchronization of Bits
    • Sender and Receiver must be synchronized at the bit level
    • Sender and Receiver clocks must be synchronized
    • It is done by Physical layer
  • Line Configuration
    • Physical Layer is also concerned with Line Configuration
    • Line Configuration represents the connection of device with the Medium
    • Point-To-Point or Multipoint
  • Physical Topology
    • Mesh, Star, Ring, Bus etc.
  • Transmission Mode
    • Physical Layer also defines the direction of Transmission between the devices
    • Simplex. Half Duplex, Full Duplex

Data Link Layer (Layer 2)

  • Transforms physical layer which is raw transmission facility to a reliable link
  • Responsible for Node to Node Delivery
  • Makes physical layer look error free to the upper layer



Functions of Data Link Layer

  • Framing
    • The data link divides the stream of bits from Network layer into manageable data units called “FRAMES”. This process is known as Framing.
  • Physical Addressing
    • Frames need to be transmitted to different systems on a network
    • Data Link layer adds a HEADER to Frame
    • Header defines the physical address of sender(Source address) and/or receiver address (Destination address)
    • If frame is intended for a device outside the network, the receiver address is the address of the device that connects one network to the other
  • Flow Control
    • Data Link layer imposes Flow Control mechanisms to prevent overwhelming the receiver
  • Error Control
    • Data link layer adds reliability to physical layer by adding mechanisms to detect and retransmit lost or damaged frames
    • Also uses a mechanism to prevent duplication of frames
    • Error Control bits are added to the form in the TRAILER
  • Access Control
    • Two or more devices may be connected to a single link
    • Data link protocols are necessary to determine which device will have the control of the link at a given time


  • Node with physical address 10 sends a frame to a node with physical address 87
  • Two nodes are connected by a link.
  • At the DL level, this frame contains physical address in the Header, This is the only address needed at this level
  • Rest of header contains other info as needed
  • Trailer contains extra bits needed for error detection



  • Responsible for Source-to-Destination delivery
  • DL Layer oversees the delivery of data between 2 systems on the same network
  • Network Layer ensures that each packet gets from its point of origin to its final destination


  • Node –to Node vs Source to Destination
    • If the two systems are connected to the same network, there is no need for Network layer and node –to node delivery is enough
    • If two systems are connected to two different networks, there is often a need for Source-to destination delivery

Node –to Node vs Source to Destination

Function of Network Layer

  • Logical Addressing
    • Physical addressing implemented by Data link layer handles addressing problem locally
    • If a packet is going from one network to another, we need another addressing system to help distinguish source & destination systems
    • Network layer adds Header to the data coming from upper layers that among other things include LOGICAL ADDRESS of the sender and receiver
  • Routing
    • When independent networks or links are connected together to create an “internetwork”, the internetworking devices route packets to their final destination
    • Routers are those internetworking devices
    • One of the functions of Network layer is to define this route

Example Network Layer

  • We want to send data from a node with network address ‘A’ and physical address 10, located on one LAN to
  • A node with network address P and physical address 95 located on another LAN
  • Because the two nodes are present on two different networks, we cannot use physical address only
  • We need a Network address that can pass us from the Network boundaries
  • The packet therefore contains the logical address which remains the same from source to destination
  • The physical address will change when packet moves from one network to the other
  • The box with R is a Router


  • The OSI Model
  • Function of Layers

Reading Sections

  • Section 3.1,3.2, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan