The OSI Model

The Open Systems Interconnection (OSI) reference model, developed by the International Organization for Standardization (ISO) in 1977, is a conceptual framework that can be used to get a better understanding of the complex interactions taking place among the various devices in a network. The purpose of the OSI model is to demonstrate how the network entities should work together for communication.

The OSI model organizes network communication through seven different layers. Each layer has a group of protocols associated with it. Protocols can be defined as rules that every computer should follow in order to communicate within a network. A protocol stack is formed when a communication needs more than one protocol to complete the process. A commonly used protocol stack is TCP/IP, which is used for communication over the Internet. Computers in a network are connected using a specific protocol stack, without which communication between them is not possible.

To understand how the computers in a network communicate with each other, we should know that the data first travels vertically within the layers, and then it goes from the uppermost layer to the lowest layer, and vice versa. When it reaches the lowest layer, i.e., the Physical layer, it travels horizontally through the network cable to the physical layer of the other computer.

When a signal is sent through a network, it first travels within the OSI layers of the sending computer. Each layer adds some information in the form of a header to the data as it passes through it. This information is read and understood by the corresponding layer of the receiving computer and then removed from the message. When it reaches the uppermost layer, the last header is removed, and the message is converted to ready-to-use form.

The OSI model organizes communication services into seven groups called layers. The layers are as follows:

Layer 1, The Physical Layer: The physical layer is responsible for packaging and transmitting data on the physical media. This layer conveys the bit stream through a network at the electrical and mechanical levels. It deals with physical network devices such as connectors, hubs, network interface cards and their types, and the type of physical topology used in the network. The physical layer defines the physical medium requirements that may be different for different physical media.

The network component that operates at this level is a repeater. The function of the repeater is to send the signals, without filtering or interpreting them, from one port to the other ports to which it is connected. It increases the strength of signals that become weak because of traveling over long distances. A repeater requires no information from the upper level of the OSI model, as it only passes data bits without determining whether or not the data is in the correct format.

Layer 2, The Data-Link Layer: The data-link layer is responsible for error-free transfer of data frames. This layer provides synchronization for the physical layer. Data, received by this layer in the form of packets from the network layer, is transmitted to the physical layer in the form of frames. The data link layer adds to it information such as the frame type, routing, and segmentation information as a header.

This layer ensures that the data frames are sent correctly on the network. It performs a Cyclic Redundancy Check (CRC) that checks for the lost and damaged frames, and then a request is sent for these frames to be sent again.

The data link layer also checks for the frame destination in a broadcast network and ensures through the destination ID that the signal is sent to the computer to which it is destined.

The data link layer can further be divided into two sub layers, namely Logical Link Control and Media Access Control.

Logical Link Control: The function of the LLC or Logical Link Control sub layer is to establish and maintain links between the network devices. It provides some Service Access points that are used by other computers to send their signal from the LLC to the other layer on the OSI.

Media Access Control (MAC): This sub layer is responsible for controlling the traffic on the communication channel used by different network devices. This layer communicates with the network interface cards of the computer and provides for shared access of the communication channel. Every NIC comes with a unique 12 digit hexadecimal number, known as the MAC address, which is used to establish a logical link connection between the computers in the network. The network component that operates at this layer is bridge. A bridge is an interconnectivity device that connects two local area networks (LANs) or two segments of the same LAN using the same communication protocols, and provides address filtering between them. This device can be used to divide busy networks into segments and reduce network traffic. A bridge broadcasts data packets to all the possible destinations within a specific segment.

Layer 3, The Network Layer: The network layer controls the operation of the subnet. It determines the physical path that data takes on the basis of network conditions, priority of service, and other factors. The network layer is responsible for routing and forwarding data packets.

The protocols and devices at this layer receive the packets from other devices, determine their final destination, and choose the most appropriate route to send the packets in order to avoid network congestion. As opposed to the data link layer, which deals with the logical physical addresses of the devices, the network layer deals with logical addresses that are independent of the physical hardware. The function of the network layer is to translate the logical network address into physical machine address, and also to divide large data packets into smaller packets to facilitate the transmission of data to the data link layer.

This layer also performs error handling and diagnosing of the host or other network devices. It also deals with route selection, gateway services, network addressing, message and packet switching, and network and packet flow control.

The network device that operates at this layer is a router. A router is a device that routes data packets among computers in different networks. It is used to connect multiple networks. It determines the path to be taken by each data packet to its destination computer. Router maintains a routing table of the available routes and their conditions. By using this information, along with distance and cost algorithms, the router determines the best path to be taken by the data packets to the destination computer. A router connects dissimilar networks, such as Ethernet, FDDI, and Token Ring, and routes data packets among them.

Layer 4, The Transport Layer: The transport layer ensures that messages are delivered in the order in which they are sent, and that there is no loss or duplication. It also ensures complete data transfer. It keeps track of the data that comes from other layers, divides the large amount of data into smaller packets to facilitate transmission, and then sends them to the lower layers. When the data reaches the destination computer, the transport layer reassembles the data and again sends it to the upper layers.

The protocols at this layer are responsible for efficient and reliable communication between various devices of the network. This layer detects lost transmissions and manages the speed of the data to ensure that the network does not become overcrowded with traffic. It also sends an acknowledgement to the sender when the message is received.

Another important function of the transport layer is to provide connection services to the upper layer protocols and applications. Depending on the protocol suites used in the network, these services may be either connection-oriented or connectionless. A connection-oriented service requires a logical connection to be established among the devices to send messages, whereas a connectionless service does not wait for a connection to be established. Instead, it starts sending the message as soon as it has some message to be sent. Some protocol suites, such as the TCP/IP protocol suite, provide for both connection-oriented and connectionless services.

Layer 5, The Session Layer: The session layer establishes a communication session between the processes running on different communication entities in a network and can support a message-mode data transfer. It deals with session and connection coordination.

There are three types of communication that can take place between a sender and receiver: Simplex, Half duplex, and Full duplex. In the Simplex mode, there is one-way transfer of data; and in the Half duplex mode, the data is transferred only in one direction at a time. The receiver has to wait until the sender completes the data transfer, and only then it can send the data. In the Full duplex mode, the transfer of data is unidirectional, and both devices can send and receive the data simultaneously.

A communication session consists of four different phases: Connection establishment, Data transfer, Connection release, and Error correction. The session layer controls the flow of traffic by determining whether the data can flow in a single direction or in both the directions at a time.

The session layer also provides tokens to the sender and receiver because some protocols do not allow the simultaneous flow of data in both the directions. The session layer ensures that only the side that is holding the token can send the data at a given time.

Another important function of the session layer is data synchronization. It ensures that if a network failure occurs, not all the data should be required to be sent. For this, the session layer provides some checkpoints, which ensures that only the data lost after the last checkpoint is to be sent over the network.

Layer 6, The Presentation Layer: The presentation layer serves as the data translator for a network. It is usually a part of an operating system and converts incoming and outgoing data from one presentation format to another. This layer is also known as syntax layer.

In the sending computer, the presentation layer encodes the data in a format that is independent of the type of devices or computers. This helps in transferring the data over different types of networks or computers. The presentation layer also encrypts the data for secure transmission. It uses two formats for data encryption, the public key and the private key. To minimize the traffic load on the network, this layer also compresses the data.

On the receiving computer, the presentation layer decodes the data according to the type of device. It also decodes and uncompresses the data and makes it ready to be used by the application layer.

Layer 7, The Application Layer: The application layer serves as a window for users and application processes to access network services. It handles issues such as network transparency, resource allocation, etc. This layer is not an application in itself. However, some applications may perform application layer functions.

The application layer provides an interface to the applications on a computer to communicate with the network. It also provides network connectivity to services such as transfer of files, telnet, e-mail, FTP, and other network-related services.

Each communicating entity is equipped with these seven layers. When a message is sent between two communicating entities, data flows down through each layer in the sending entity and flows up through each layer in the receiving entity.

As the message travels down the sending entity layer stack, each layer it passes through (physical layer is an exception) adds a header. These headers contain pieces of control information that are read and processed by the corresponding layer on the receiving entity layer stack. As the message travels up the stack of the receiving entity, each layer strips the header added by its peer layer.

For example, two networked applications, based on Windows and Macintosh operating systems, are communicating with each other. At layer 7, the Windows application requests data from the Macintosh application. The request is sent to the Windows application's layer 6. This layer receives the request as a data packet. It then adds some information, known as a header, and passes the packet down to layer 5. The process continues until the request reaches the physical layer.

Note: Physical layer does not add a header.

The physical layer sends this request packet to the Macintosh application through the network transmission media. When the Macintosh application receives this packet, the header that was added at the data-link layer of Windows application is stripped at the data-link layer of the Macintosh application. The Macintosh data-link layer performs the tasks requested in the header and passes the request packet to the next higher layer, i.e., the network layer. The process is repeated until the Macintosh application's application layer receives the packet and interprets the request.

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