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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

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)

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

Figure

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

EXAMPLE

• 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

NETWORK LAYER

• 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

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

Summary

• The OSI Model
• Function of Layers

Reading Sections

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