Critique Of Osi Model-Data Communication-Lecture Notes, Study notes for Data Communication Systems and Computer Networks. Agra University
dharmaketu4 August 2012

Critique Of Osi Model-Data Communication-Lecture Notes, Study notes for Data Communication Systems and Computer Networks. Agra University

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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: Cri...
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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

 Data Link Layer (Layer 2)

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


 Functions of Data Link Layer

o Framing

 The data link divides the stream of bits from Network layer into manageable data units called “FRAMES”. This process is known as Framing.

o 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

o Flow Control  Data Link layer imposes Flow Control mechanisms to prevent overwhelming

the receiver

o 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

o 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


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


 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

o 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

o 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

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