1 The OSI Model

The Open Systems Interconnection (OSI) model was first introduced in the late 1970’s. An open system is a set of protocols that allows any 2 different systems to communicate regardless of their underlying architecture.

OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable.

Communication is governed by an agreed-upon series of rules and convensions called protocols.

1.1 Encapsulation

The data portion of a packet at level (N-1) carries the whole packet (data and header and maybe trailer) from level N. The concept is called encapsulation. Level (N-1) is not aware of which part of the encapsulated packet is data and which part is the header or trailer. For level (N-1), the whole packet coming from level N is treated as one integral unit.

1.2 Layers in OSI Model

1.2.1 Physical Layer

This layer coordinates the functions required to carry a bit stream over a physical medium.

Physical layer concerned with the followings:

  • Physical characteristics of interface and medium
  • Representation of bits
  • Data rate / Transmission rate ~> The number of bits sent each second. In short, physical layer defines the duration of a bit, which is how long it lasts.
  • Synchronization of bits ~> The sender and the receiver clocks must be synchronized at the bit level.
  • Line configuration ~> This layer is concerned with the connection of devices to the media.
    • In a point-to-point configuration ~> 2 devices are connected through a dedicated link.
    • In a multi-point configuration ~> A link is shared among several devices.
  • Physical topology ~> This layer also concerned with how devices are connected to make a network.
  • Transmission mode ~> This layer also defines the direction of transmission between 2 devices:
    • Simplex ~> Only one device can send; the other can only receive. It is a one-way communication. For example, radio broadcasting, television broadcasting, monitor output, etc.
    • Half-duplex ~> 2 devices can send and receive, but not at the same time. For example, walkie-talkie, USB, browsing the internet, etc.
    • Full-duplex ~> 2 devices can send and receive at the same time. For example, video calls, audio calls, live chats, etc.

1.2.3 Network Layer

The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). If 2 systems are connected to the same link, there is usually no need for a network layer. However, if 2 devices systems are attached to different networks (links) with connecting device between the networks (links), there is often a need for the network layer to accomplish source-to-destination delivery.

Other responsibilities of the network layer includes:

  • Logical addressing ~> The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems. The network layer adds a header to the packet coming from the upper layer.
  • Routing

The network layer is responsible for source-to-destination delivery.

1.2.4 Transport Layer

Transport layer is responsible for process-to-process delivery.

A process is an application program running on a host. Whereas the network layer oversees source-to-destination delivery of individual packets, it doesn’t recognize any relationship between those packets. It treats each one independently, as though each piece belonged to a separate message, whether or not it does. On the other hand, transport layer ensures that the whole message arrives intact and in order.

Other responsibilities of transport layer:

  • Service-point addressing ~> Source-to-destination not only means delivery from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. The transport layer header must therefore include a type of address called a service-point address (also called port address). So we can say,
    • Network layer ~> Correct computer.
    • Transport layer ~> Correct process on that computer
  • 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 (UDP) or connection oriented (TCP).
  • Flow control ~> Flow control at this layer is performed end-to-end rather than across a single link.

1.2.5 Session Layer

The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.

The session layer is responsible for dialog control and synchronization.

Responsibilities of the session layer includes:

  • Dialog control ~> This layer allows 2 systems to enter into a dialog. It allows the communication between two processes to take place in either half duplex (one way at a time) or full duplex (two ways at a time) mode.
  • Synchronization ~> This layer allows a process to add checkpoints (synchronization points) to a stream of data.

Let’s say, we are sending a file of 2000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently. Now if a crash happens during transmission of page 552, the only pages that need to be resent after system recovery are pages 501 to 552.

1.2.6 Presentation Layer

This layer is concerned with the syntax and semantics of the info exchanged between 2 systems.

Other responsibilities are:

  • Translation ~> The processes (running programs) in 2 systems are usually exchanging info in the form of character string, number, and so on. The info must be changed to bit stream, before being transmitted. As different computers use different encoding systems, this layer is responsible for interoperability between these different encoding methods.
    • The presentation layer at the sender changes the info 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 ~> Encryption and decryption of data is done in this layer
  • Compression ~> Data compression reduces the number of bits contained in the info. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video.

1.2.7 Application Layer

This layer provides user interfaces and support for services such as email, remote file access and transfer, shared database management and other types of distributed info services.

The above figure shows only 3 application services.

  • X.400 ~> Message-handling services
  • X.500 ~> Directory services
  • FTAM ~> File transfer, access, and management services

Services provided by the application layer are:

  • Network virtual terminal ~> A software version of a physical terminal, and it allows a user to log on a remote host. To do so, the application creates a software emulation of a terminal at the remote host. The user’s computer talks to the software terminal which, in turn, talks to the host, and vice versa. The remote host believes it is a communicating with one of its own terminals and allows the user to log on.
  • FTAM ~> This layer allows a user to access files in a remote host (to make changes/ read data), to retrieve files from a remote computer for use in the local computer, and to manage or control files in a remote computer locally.
  • Mail services ~> This application provides the basis for e-mail forwarding and storage.
  • Directory service ~> This application provides distributed database sources and access for global info about various objects and services.

2 Questions

  1. What do you mean by OSI model?

  2. OSI model is a protocol. [T/F]

  3. During encapsulation, payloads are coming from layer (N-1) to layer N. [T/F]

  4. What do you mean by ‘De-encapsulation’?

  5. What do you mean ‘Physical addressing’? Which network device assign physical addressing?

  6. ________ is responsible to identify the duplicate frames.

  1. Physical layer header
  2. Data link layer header
  3. Data link layer trailer
  4. Network layer header
  5. Network layer trailer
  1. USB is an example of _________.
  1. Simplex
  2. Half-duplex
  3. Full-duplex
  4. None of the above

  1. What do you mean by ‘Transmission rate’?

  2. _______ layer is responsible for logical addressing?

  1. Physical
  2. Network
  3. Data link
  4. Transport

  1. With the help of a schematic diagram, describe how a date link layer transmits data.

  2. PDUs of transport, data link, and network layers are ________, ________, and _________ respectively.

  1. segment, packet, frame
  2. frame, packet, segment
  3. segment, frame, packet
  4. frame, packet, segment

  1. Application service X.400 is used for ___________.

  2. Changing character strings and numbers to bit stream is done in __________ layer.

  1. Physical
  2. Data link
  3. Transport
  4. Presentation
  1. Synchronization points are added to data stream in ________ layer.
  1. Application
  2. Presentation
  3. Session
  4. Transport
  1. We want to send a file of 450 pages and place a checkpoint with 45-page unit. If an error occurs during transmission of page 137, then re-transmission segment will start from ________ page. [Page count starts from 1]
  1. 135
  2. 136
  3. 137
  4. 1

  1. How translation is done in presentation layer?

  2. Transport layer provides ________ connectivity.

  1. node-to-note
  2. source-to-destination
  3. process-to-process
  4. hop-to-hop