OSI model

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History of the OSI Model:[edit]

The work on the Open Standard Interface (OSI) reference model was started in 1977 by the International Organization for Standards. It was then decided that OSI will have two major components - a 7-layer model and a set of specific protocols. The various issues on OSI design have evolved from a networking model called CYCLADES. This also influenced the design of Internet architecture then. Since the inception of the OSI reference model, the working of Internet technology has become very smooth.

OSI Model[edit]

The OSI model is made up of seven layers which are presented as a stack. Data which is passed over the network moves through each layer.

The seven layers of the OSI model are:

  • Application Layer - layer 7
  • Presentation Layer - layer 6
  • Session Layer - layer 5
  • Transport Layer - layer 4
  • Network Layer - layer 3
  • Data-Link Layer - layer 2
  • Physical Layer - layer 1

Each layer of the OSI model has its own unique functions. The process of sending data is typically started at the Application layer, is sent through the stack to the Physical layer, and then over the network to the recipient. Data is received at the Physical layer, and the data packet is then passed up the stack to the Application layer.

Different protocols operate at the different layers of the OSI model. Each layer of the OSI model has its own protocols. TCP and IP are collectively called the protocol stack or the network/transport protocols. This is due to the protocols operating at the Network and Transport layers to make it possible for computers to communicate. A protocol stack, r stack, is a group of protocols which are arranged in layers to enable communication. In the protocol stack, each layer provides services to the layer above it; and each layer also receives services from the layer beneath it. For two computers to partake in communications, each computer has to be running the same protocol stack. They can however have different operating systems.

The Physical Layer (layer 1)[edit]

The first layer in the OSI model is the Physical layer which transmits raw bit streams over a physical medium. The Physical layer deals with establishing a physical connection between computers to enable communication. The physical layer is hardware specific and deals with the actual physical connection between the computer and the network medium. All devices that function at the Physical layer handle signalling. Data handled at the layer are in bits (1s and 0s). The 1s and 0s are in represented by pulses of light or electricity.

The details on the actual physical connection defined at this layer include:

Physical topologies of the network.
Network connection types and how cable is attached to the Network Interface Card (NIC).
Data encoding: This relates to the analogue and digital signalling methods utilized to encode data in the signals.
Bit synchronization

The specifications of the Physical layer include:

  • Physical layout of the network
  • Voltage changes and the timing of voltage changes.
  • Data rates
  • Maximum transmission distances
  • Physical connectors to transmission mediums

The issues normally clarified at the Physical Layer include:

  • Whether data is transmitted synchronously or asynchronously.
  • Whether the analogue or digital signalling method is used.
  • Whether baseband or broadband signalling is used

The Data-link Layer (layer 2)[edit]

The Data-link layer of the OSI model enables the movement of data over a link from one device to another, by defining the interface between the network medium and the software on the computer. The Data-link layer maintains the data link between two computers to enable communications.

The responsibilities of the Data-link layer include:

  • Packet addressing
  • Media access control
  • Format the frame used to encapsulate data
  • Error notification on the Physical layer
  • Managing of error messaging specific to the delivery of packets.

Ensures that frames are transmitted from one computer to another computer with no errors. It establishes error-free connections between two devices.

Layer 2 manages the ordering of bits, packets, to and from data segments. The ensuing result is called frames. Frames contain data that is already arranged in an orderly manner. The Data-link layer receives packets from the Network layer and structures these packets into frames. The frames are then moved to the Physical layer for sending. A cyclic redundancy check (CRC) is added to the data frame. The CRC detects damaged frames. The computer at the receiving end can request the cyclic redundancy check (CRC) so that it can verify that the frame is not damaged. The Data-link layer can determine when a frame is lost. It also requests any lost frames to be retransmitted. By performing these tasks, the Data-link layer makes it possible for data bits to be transmitted in an organized manner.

The Data-link layer is divided into the following two sub layers:

  • Logical Link Control (LLC) sub layer: The LLC sub layer provides and maintains the logical links used for communication between the devices.

The functions at the LLC sublayer of the Data-link layer include the following:

  1. Error checking
  2. Frame synchronization
  3. Flow control

Media Access Control (MAC) sublayer: The MAC sublayer of the Data-link layer controls the transmission of packets from one network interface card (NIC) to another over a shared media channel. A NIC has a unique MAC address, or physical address. This address identifies the particular NIC on the network. To ensure that these addresses are unique, the MAC addresses are usually permanently burned in the memory of the NIC. The MAC sublayer handles media access control which essentially prevents data collisions. It provides for the allocation of network access to computers, and more importantly, it prevents computers from transmitting data simultaneously.

The common media access control methods are listed below.

  • Token Passing; utilized in Token Ring and FDDI networks
  • Carrier Sense Multiple Access/Collision Detection (CSMA/CD); utilized in Ethernet networks.
  • Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA); utilized in AppleTalk networks.

The Network Layer (layer 3)[edit]

The Network layer of the OSI model is responsible for moving packets between devices, by providing end-to-end communications between computers that exist on different network. One of the main functions performed at the Network layer is routing. Routing enables packets to be moved among computers which are more than one link from one another.

The functions performed at the Network layer of the OSI model are listed below:

  • Traffic direction to the end destination
  • Addressing; logical network addresses and services addresses
  • Routing functions; route discovery and route selection
  • Packet switching
  • Packet sequence control
  • End-to-end error detection, from the data sender to the receiver of data
  • Congestion control
  • Network layer flow control and Network layer error control
  • Gateway services

The Transport Layer (layer 4)[edit]

The Transport layer is responsible for transporting data in a sequential manner, and with no data loses. The Transport layer divides large messages into smaller data packets so that it can be transmitted to the destination computer. It also reassembles packets into messages for it to be presented to the Network layer.

The important functions performed at the Transport layer to enable network communication are listed below:

  • Guaranteed data delivery
  • Name resolution
  • Flow control
  • Error detection
  • Error recovery

The Transport layer at each computer verifies that the application transmitting the data is actually allowed to access the network. It also verifies that each end of the network connection can start the data transfer process. The transport protocols running on each host partaking in communication monitors the data transfer process, and monitors for errors as well.

The common Transport protocols utilized at this layer are:

Transmission Control Protocol (TCP): TCP is a connection-orientation protocol that offers greater reliability when it comes to transporting data than what UDP, the other TCP/IP protocol which works at this layer provides. With TCP, the application which sends the data receives acknowledgement or verification that the data was actually received.

User Datagram Protocol (UDP): UDP is a connectionless protocol that does not provide reliable data transport. No acknowledgements are transmitted.

The Session Layer (layer 5)[edit]

The Session layer of the OSI model enables communication sessions to be established between processes or applications running on two different computers. A process is a specific task that is associated with a particular application. Applications can simultaneously run numerous processes. The Session layer utilizes the virtual circuits created by the Transport layer to establish communication sessions.

The important functions performed at Session layer to establish, maintain and terminate communication sessions are summarized below:

  • Establishes, terminates, and monitors communication sessions between applications.
  • Name lookup and security functions.
  • Placement the header information in a packet which determines the point where a message starts and the point where a message ends.
  • Data synchronization. The layer performs synchronization between the Session layer of the data sender and the Session layer of the receiver of the data.
  • Controls whether the communication or messages being exchanged in a session are transmitted as full duplex messages or half duplex messages.
  1.          Full duplex: Information is transmitted simultaneously, and in both directions.
  2.          Half duplex: Information is transmitted in both directions, and flows in one direction at a time.

The Presentation Layer (layer 6)[edit]

At the Presentation layer of the OSI model, the data being transmitted is translated. The layer is responsible for translating data between the formats which the network requires and the formats which the computer is anticipating. The presentation layer translates the formats of each computer to a common transfer format which can be interpreted by each computer.

The functions performed at the Presentation layer of the OSI are:

  • Protocol conversion.
  • Data translation.
  • Data encryption and decryption.
  • Data compression.
  • Character set conversion.
  • Interpretation of graphics commands.

Data is translated at the Presentation layer when it is transmitted from the sender to the receiver. The application of the sender moves the data to the Presentation layer. The Presentation layer translates the data to a common format which can be read by both computers. When the data is received, the Presentation layer translates the data to a format which the application can read.

Gateway services also function at the Presentation layer. A gateway can be defined as a connection point between networks which run different systems and applications. Gateways are typically deployed through software. An example is Gateway Services for NetWare (GSNW).

OSI Layers Protocols[edit]

OSI Layer                                                              Protocols
Application Layer                                                    DNS, FTP, TFTP, BOOTP, SNMP, SMTP
Presentation Layer                                                 SMB, NCP
Sessions Layer                                                      NETBIOS
Transport Layer                                                      TCP, ARP, RARP, SPX, NWLINK, NETBIOS
Network Layer                                                        IP, ARP, RARP, ICMP, IGMP
Data link Layer                                                       LLC
Physical Layer                                                       LLC

Advantages of OSI Model:[edit]

• Different operating systems using OSI model can communicate with each other.
• Developers can change features of one layer without changing the code simplifying the troubleshooting.
• Error-checking schemes determine whether transmitted data has become corrupt or otherwise damaged while traveling from the source to the destination. Error checking is implemented at several of the OSI layers. One common error-checking scheme is the cyclic redundancy check (CRC), which detects and discards corrupted data. Error-correction functions (such as data retransmission) are left to higher-layer protocols.

See Also[edit]



Networking foundations By Patrick Ciccarelli, Christina Faulkner


1)OSI model
2)OSI Layers