OSI Model-Cont

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) o    Coordinates the functions required to transmit a bit stream over a physical medium o    Deals with mechanical and electrical specifications of Tx. Medium and Interface o    Also  defines  procedures  and  functions  that  physical  devices  and  interfaces  need  to perform for TX. To occur  (Figure) •     Functions of Physical Layer o    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 o    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 o    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 o    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 o    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 o    Physical Topology    Mesh, Star, Ring, Bus etc. o    Transmission Mode    Physical Layer also defines the direction of Transmission between the devices    Simplex. Half Duplex, Full Duplex