(i) Physical layer (Layer 1)
The Physical layer of the OSI model defines the network medium, hardware, and topology used in the network; the maximum speed, bandwidth, and cable lengths are also defined in this layer. It also details the electrical characteristics of the media, such as voltage or current. In wireless networks, it defines the frequencies over which the signals travel. The following are two main components of this layer:
Topology
The physical network topology used may be bus, ring, star, or mesh.
Hardware
The network hardware includes the network media such as cables and
connectors, and their connection details.
Network hubs and repeaters work at the physical layer of the OSI model.
The Physical layer of the OSI model defines the network medium, hardware, and topology used in the network; the maximum speed, bandwidth, and cable lengths are also defined in this layer. It also details the electrical characteristics of the media, such as voltage or current. In wireless networks, it defines the frequencies over which the signals travel. The following are two main components of this layer:
Topology
The physical network topology used may be bus, ring, star, or mesh.
Hardware
The network hardware includes the network media such as cables and
connectors, and their connection details.
Network hubs and repeaters work at the physical layer of the OSI model.
(ii) Data Link layer (Layer 2)
The Data Link layer defines the interface between the physical media and the soft- ware running on the computer. It is responsible for sending and receiving the data frames to and from the Physical layer. This layer performs functions such as packet addressing, error detection, error correction, and hardware addressing. This layer is further divided into the following two sub layers:
Media Access Control (MAC)
The MAC sub layer is defined in the IEEE 802.1 standard. It is responsible for controlling access to network media and for moving the data packets from one network interface to another. The IEEE 802.1 standard defines the MAC address (also called the hardware address) of the network interfaces. A MAC address is hard coded onto every network interface.
Logical Link Control (LLC)
The LLC sub layer is defined in the IEEE 802.2 standard. It is responsible for error detection, error correction, synchronization of data frames, and flow control.
The Data Link layer defines the interface between the physical media and the soft- ware running on the computer. It is responsible for sending and receiving the data frames to and from the Physical layer. This layer performs functions such as packet addressing, error detection, error correction, and hardware addressing. This layer is further divided into the following two sub layers:
Media Access Control (MAC)
The MAC sub layer is defined in the IEEE 802.1 standard. It is responsible for controlling access to network media and for moving the data packets from one network interface to another. The IEEE 802.1 standard defines the MAC address (also called the hardware address) of the network interfaces. A MAC address is hard coded onto every network interface.
Logical Link Control (LLC)
The LLC sub layer is defined in the IEEE 802.2 standard. It is responsible for error detection, error correction, synchronization of data frames, and flow control.
(iii) Network layer (Layer 3)
The Network layer is responsible for end-to-end communications between two computers on different networks. One of the primary functions of this layer is routing, which enables computers to forward traffic to a remote network. This functionality is provided by network protocols. Network protocols perform route selection, which is a process that determines the best path to a destination network.
Unlike the Data Link layer that uses a MAC address to forward packets to a host in a single network, the Network layer uses software-configured, Layer 3 addresses (such as an IP address or an IPX address) to send the packet to its destination network. Other functions of the network layer include packet sequencing, end-to-end error detection, congestion control, and addressing.
The IP and the IPX work at the Network layer of the OSI model. Besides this, routing protocols such as RIP, OSPF, and NLSP also work at this layer.
Network interface cards, switches, bridges, and wireless access points work at the Data Link layer of OSI model.
The Network layer is responsible for end-to-end communications between two computers on different networks. One of the primary functions of this layer is routing, which enables computers to forward traffic to a remote network. This functionality is provided by network protocols. Network protocols perform route selection, which is a process that determines the best path to a destination network.
Unlike the Data Link layer that uses a MAC address to forward packets to a host in a single network, the Network layer uses software-configured, Layer 3 addresses (such as an IP address or an IPX address) to send the packet to its destination network. Other functions of the network layer include packet sequencing, end-to-end error detection, congestion control, and addressing.
The IP and the IPX work at the Network layer of the OSI model. Besides this, routing protocols such as RIP, OSPF, and NLSP also work at this layer.
Network interface cards, switches, bridges, and wireless access points work at the Data Link layer of OSI model.
(iv) Transport layer (Layer 4)
The Transport layer works with the Network layer to provide guaranteed delivery of data packets in order to acknowledge that data is received at the destination. It performs segmentation of data by breaking it down into manageable packets. End-to-end error detection ensures that the data is received without damage. Flow control ensures that transmission speed is regulated in order to avoid dropped packets.
Both connectionless and connection-oriented protocols work at the Transport layer. UDP is a connectionless protocol, while TCP is a connection-oriented protocol.
The Transport layer works with the Network layer to provide guaranteed delivery of data packets in order to acknowledge that data is received at the destination. It performs segmentation of data by breaking it down into manageable packets. End-to-end error detection ensures that the data is received without damage. Flow control ensures that transmission speed is regulated in order to avoid dropped packets.
Both connectionless and connection-oriented protocols work at the Transport layer. UDP is a connectionless protocol, while TCP is a connection-oriented protocol.
(v) Session layer (Layer 5)
The Session layer provides several functions to regulate the communications session between two computers on the network. It is responsible for setting up and terminating a session as well as for controlling the dialog between applications on two computers.
(vi) Presentation layer (Layer 6)
The Presentation layer is responsible for translating syntax or format of data so that the receiving computer can understand it. The translating syntax also provides functions such as compression/decompression, encoding/decoding, and encryption/decryption. Some of the common data formats working at this layer include the following:
• Graphic file formats such as JPEG, TIFF, or GIF
•Text and data file formats such as American Standard Code for Information Interchange (ASCII) and Extended Binary Coded Decimal Interchange Code (EBCDIC)
• Sound and Video formats such as MPEG, AVI, QuickTime Video, or MIDI files
(vii) Application layer (Layer 7)
The Application layer is responsible for accepting requests from users and applications and passing them on to the lower layers of the OSI model. In other words, it provides an interface between the applications running on the computer and network protocols. Applications (such as file transfers, email, FTP, or Telnet) use the services provided by Application layer protocols, which in turn use the lower layer protocols to communicate over the network.
The Session layer provides several functions to regulate the communications session between two computers on the network. It is responsible for setting up and terminating a session as well as for controlling the dialog between applications on two computers.
(vi) Presentation layer (Layer 6)
The Presentation layer is responsible for translating syntax or format of data so that the receiving computer can understand it. The translating syntax also provides functions such as compression/decompression, encoding/decoding, and encryption/decryption. Some of the common data formats working at this layer include the following:
• Graphic file formats such as JPEG, TIFF, or GIF
•Text and data file formats such as American Standard Code for Information Interchange (ASCII) and Extended Binary Coded Decimal Interchange Code (EBCDIC)
• Sound and Video formats such as MPEG, AVI, QuickTime Video, or MIDI files
(vii) Application layer (Layer 7)
The Application layer is responsible for accepting requests from users and applications and passing them on to the lower layers of the OSI model. In other words, it provides an interface between the applications running on the computer and network protocols. Applications (such as file transfers, email, FTP, or Telnet) use the services provided by Application layer protocols, which in turn use the lower layer protocols to communicate over the network.
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