Lecture 2: OSI Model

  • OSI Stands for OPEN SYSTEM INTERCONNECTION

  • Initially communication was done between same manufacture devices

  • The OSI model is the primary architectural model for networks. It describe how data and network information are communicated from an application on one computer through the network media to an application on another computer. The OSI reference model breaks this approach into layer

  • The OSI isn’t a physical model. Rather, it’s a set of guidelines that application developers can use to create and implement applications that run on a network.

The OSI has seven different layer divided into two groups
  • The top three layers define how the application within the end stations will communicate with each other and with users

  • The Bottom four layers define how data is transmitted end to end.

UPPER LAYERS
LOWER LAYERS
END to END Seven Layers
  • Each layer in the OSI Model adds its own information to the front of the data it receives from the layer above it.

  • This information in front of the data is called a header and contains information specific to the protocol operating at that layer.

  • The process of adding the header is called ENCAPSULATION.

APPLICATION LAYER

The OSI model defines the application layer as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human-recognizable format and to interface with the presentation layer below it.
Examples of applications that utilize the network are:
  • Telnet

  • FTP

  • SMTP

  • HTTP

  • TFTP

  • NTP

  • SNMP

PRESENTATION LAYER

The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.
Presentation layer handles
  • Translation

  • Compression

  • Encryption

SESSION LAYER

  • The session layer is responsible for setting UP, managing and then tearing down sessions between Presentation layer entities.

  • Session layer keeps different applications data separate from other application data

TRANSPORT LAYER

  • This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

  • Sequences data packets, and requests retransmission of missing packets. It also repackages messages for more efficient transmission over the network.

    • The most common example of Transport layer are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)

  • Transmission Control Protocol (TCP)

    • Connection Oriented

    • Reliable

    • 3 Way Handshaking

  • User Datagram Protocol (UDP)

    • Connectionless

    • Unreliable

    • Not negotiate 3 way handshaking

    • Faster than TCP

3 Way Handshaking
Reliable Data Transport

Error detection

Sequence numbers and acknowledgments cover discarding duplicate packets, retransmission of lost packets, and ordered-data transfer

Flow Control

  • TCP uses an end-to-end Flow Control protocol to avoid having the sender send data too fast for the TCP receiver to receive and process it reliably.

  • TCP uses a Sliding Window flow control protocol


WINDOWING
NETWORK LAYER
  • Network Layer deals with the function of path selection and logical addressing

    • The protocols use are:
    • IP (internet protocol) in TCP/IP

    • IPX in Novell

    • DDR in AppleTalk

  • Every network and host has IP address.

    • IP define three different network classes:
    • Class A 1-126

    • Class B 128-191

    • Class C 192-223

  • Routers, or other layer-3 devices, are specified at the Network layer and provide routing services in an internetwork.

DATA LINK LAYER
  • Data Link Layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses and will translate messages from the Network layer into bits for the Physical layer to transmit

    • The Data-Link layer contains two sub layers:
    • Logical Link Control (LLC)

    • Media Access Control (MAC)

Logical Link Control (LLC) 802.2

The LLC functions include:
  • Managing frames to upper and lower layers

  • Error Control

  • Flow control

    • Managing frames to upper and lower layers
    • LLC allowing multiple higher-layer protocols to access one or more lower-layer networks. For example, a server may have multiple network interface cards The LLC will forward packets from upper-layer protocols to the appropriate network interface. This scheme allows upper-layer protocols to operate without specific knowledge of the lower-layer network in use.

    Flow Control
    • Another communications control defined on the LLC sub layer is flow control. The Transport layer of the OSI model actually manages the mechanisms used to control the flow of data between two hosts. The Data Link layer defines the data values used in the flow control signaling between two transmitting hosts.

    • Flow controls are used in the data link layer to control flow between devices that are directly connected. In contrast, TCP controls flow between devices that may be connected across a multi-hop routed network.

    Error Detection
    • Data link layers also include mechanisms to detect transmission errors.

    • For a receiver to detect transmission error, the sender must add redundant information (in the form of bits) as an error detection code to the frame sent

    • The simplest error detection code is the Parity bit.

Media Access Control (MAC) 802.3
  • The MAC sub layer provides addressing and channel access control mechanisms that make it possible for several terminal or network nodes to communicate within a multiple access network that incorporates a shared medium, e.g. Ethernet.

  • MAC, Media Access Control, address is a globally unique identifier assigned to network devices, and therefore it is often referred to as hardware or physical address.

  • MAC addresses are 6-byte (48-bits) in length, and are written in MM:MM:MM:SS:SS:SS format.

  • The first 3-bytes are ID number of the manufacturer.

  • The second 3-bytes are serial number assigned by the manufacturer.

PHYSICAL LAYER
  • The OSI Model's layer 1, the Physical Layer, outlines the characteristics of the physical medium between networked devices

  • Transmission and receipt of data from the physical medium is managed at this layer

  • On this layer frame is converted into bits.

FIVE STEP OF ENCAPSULATION:
  1. User information is converted into DATA.

  2. Data is converted into SEGMENTS for transport across the network.

  3. Segments are converted into PACKETS for transport across the network.

  4. Packets are converted into FRAMES and the Data Link header is added.

  5. The data in the frames is converted into BITS for transmission over the physical media.

OSI vs TCP/IP MODEL
  • TCP/IP was developed by the Department of Defense (DOD)

  • Open Systems Interconnection was developed by the International Organization for Standardization (ISO)

  • Both the TCP/IP and OSI model work in a very similar fashion

  • The most apparent difference is the number of layers. TCP/IP is a four-layered structure, while OSI is a seven-layered model

Subscribe to: Posts (Atom)

Total Pageviews