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Question: Explain the functional architecture of OSI in detail.

Subject: Telecom Network Management

Topic: OSI Network Management

Difficulty: Medium

tnm(38) • 1.2k views
modified 7 days ago by gravatar for Sanket Shingote Sanket Shingote ♦♦ 210 written 12 months ago by gravatar for Tabassum Tabassum20

Functional architecture of OSI:

The OSI model is composed of seven ordered layers: physical (layer 1), data link (layer 2), network (layer 3), transport (layer 4), session (layer 5), presentation (layer 6), and application (layer 7).

The OSI Model

1) Physical Layer: The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission media.

The physical layer is also concerned with the following:

  • Physical characteristics of interfaces and media. The physical layer defines the characteristics of the interface between the devices and the transmission media. It also defines the type of transmission media.
  • Representation of bits. The physical layer data consists of a stream of bits (sequence of 0s or 1s) with no interpretation. To be transmitted, bits must be encoded into signals, electrical or optical.

  • Data rate. The number of bits sent each second is also defined by the physical layer.

  • Synchronization of bits. The sender and receiver must not only use the same bit rate but must also be synchronized at the bit level.

  • Line configuration. The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected together through a dedicated link. In a multipoint configuration, a link is shared between several devices.

  • Physical topology. The physical topology defines how devices are connected to make a network. Devices can be connected using a mesh topology, a star topology, a ring topology or a bus topology.

  • Transmission mode. The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex.

2) Data Link Layer: The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer). Other responsibilities of the data link layer include the following:

  • Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

  • Physical addressing. If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame.

  • Flow control. If the rate at which the data is absorbed by the receiver is less than the rate produced at the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver.

  • Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames.

  • Access control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

3) Network Layer: The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks. Other responsibilities of the net-work layer include the following:

  • Logical addressing. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.

  • Routing. When independent networks or links are connected together to create internetworks or a large network, the connecting devices (called routers or snitches) route or switch the packets to their final destination. One of the functions of the network layer is to provide this mechanism.

4) Transport Layer: The transport layer is responsible for process-to-process delivery of the entire message. Other responsibilities of the transport layer include the following:

  • Segmentation and reassembly. A message is divided into transmittable segments, with each segment containing a sequence number. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission.

  • Connection control. The transport layer can be either connectionless or connection-oriented. A connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. A connection-oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred, the connection is terminated.

5) Session Layer: The session layer is the network dialog controller. It establishes, maintains and synchronizes the interaction between communicating systems. Specific responsibilities of the session layer include the following:

  • Synchronization. The session layer allows a process to add checkpoints (synchronization points) into a stream of data.

6) Presentation Layer: The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. Specific responsibilities of the presentation layer include the following:

  • Translation. Different computers use different encoding systems, the presentation layer is responsible for interoperability between these different encoding methods. The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver-dependent format.

  • Encryption. To carry sensitive information a system must be able to assure privacy. Encryption means that the sender transforms the original information to another form and sends the resulting message out over the network. Decryption reverses the original process to transform the message back to its original form.

  • Compression. Data compression reduces the number of bits contained in the information. Data compression becomes particularly important in the transmission of multimedia such as text, audio and video.

7) Application Layer: The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management and other types of distributed information services. Specific services provided by the application layer include the following:

  • Network virtual terminal. A network virtual terminal is a software version of a physical terminal and allows a user to log on to a remote host.

  • File transfer, access, and management (FTAM). This application allows a user to access files in a remote host, to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.

  • E-mail services. This application provides the basis for email forwarding and storage.

  • Directory services. This application provides distributed database sources and access for global information about various objects and services.

modified 12 months ago  • written 12 months ago by gravatar for Tabassum Tabassum20
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