# Critique Of Osi Model-Data Communication-Lecture Notes, Study notes for Data Communication. Agra University

## Data Communication

Description: Data Communication is exchange of data between two devices. In computers data exchange is in form of 0 and 1. This course discuss how computer communicate, what is medium and what are expenses. This handout includes: Critique, Osi, Model, Widespread, Implementations, Empty, Presentation, Apocalype, Standards
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LECTURE # 8
Critique of OSI Model
Reasoning for OSI not getting Widespread
¾Bad 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
–Flow control, error control, addressing is multiple
–Session and Presentation(EMPTY), Network and DL(Full)
Apocalypse of Two Elephants
Physical (Layer 1)
oCoordinates the functions required to transmit a bit stream over a physical medium
oDeals with mechanical and electrical specifications of Tx. Medium and Interface
oAlso defines procedures and functions that physical devices and interfaces need to
perform for TX. To occur (Figure)
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Figure
xFunctions of Physical Layer
oPhysical Characteristics of Interface & Media
¾Defines characteristics of Interface b/w device and Tx Medium
¾Interface is a plug gable connector that joins one or more signal conductors
¾Also defines the type of transmission medium
oRepresentation of Bits/Encoding
¾The physical layer data consists of a stream of bits (sequence of 1’s and 0’s)
¾To be transmitted the bits must be ENCODED into signals: Electrical or
Optical
¾Physical layer decides the type of ENCODING
oData Rate / Transmission Rate
¾Date Rate ( Bits per second) also decided by the Physical Layer
¾So , Physical layer defines the Duration of a Bit
¾Means how long will a bit last
oSynchronization of Bits
¾Sender and Receiver must be synchronized at the bit level
¾Sender and Receiver clocks must be synchronized
¾It is done by Physical layer
oLine Configuration
¾Physical Layer is also concerned with Line Configuration
¾Line Configuration represents the connection of device with the Medium
¾Point-To-Point or Multipoint
oPhysical Topology
¾Mesh, Star, Ring, Bus etc.
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oTransmission Mode
¾Physical Layer also defines the direction of Transmission between the devices
¾Simplex. Half Duplex, Full Duplex
Data Link Layer (Layer 2)
oTransforms physical layer which is raw transmission facility to a reliable link
oResponsible for Node to Node Delivery
oMakes physical layer look error free to the upper layer
Figure
xFunctions of Data Link Layer
oFraming
¾The data link divides the stream of bits from Network layer into manageable
data units called “FRAMES”. This process is known as Framing.
¾Frames need to be transmitted to different systems on a network
¾If 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
oFlow Control
¾Data Link layer imposes Flow Control mechanisms to prevent overwhelming
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oError Control
¾Data link layer adds reliability to physical layer by adding mechanisms to
detect and retransmit lost or damaged frames
¾Also uses a mechanism to prevent duplication of frames
¾Error Control bits are added to the form in the TRAILER
oAccess Control
¾Two or more devices may be connected to a single link
¾Data link protocols are necessary to determine which device will have the
control of the link at a given time
9EXAMPLE
Node with physical address 10 sends a frame to a node with
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
oResponsible for Source-to-Destination delivery
oDL Layer oversees the delivery of data between 2 systems on the same network
oNetwork Layer ensures that each packet gets from its point of origin to its final
destination
¾Node –to Node vs Source to Destination
9If the two systems are connected to the same network, there is no need for
Network layer and node –to node delivery is enough
9If two systems are connected to two different networks, there is often a
need for Source-to destination delivery
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