| The ISO OSI model are explained below. The | | | | UDP (Universal Data gram Protocol): UDP |
| seven layers of the OSI model are: | | | | simply transmits packets over the internet. |
| | | | It does not wait for an acknowledgement. It |
| | | | is the responsibility of upper layer |
| | | | protocols to ensure that the information had |
| Application | | | | reached the intended partner(s). Hence, UDP |
| | | | is often called connectionless protocol. |
| Presentation | | | | |
| | | | |
| Session | | | | |
| | | | Application programs that do not need |
| Transport | | | | connection-oriented protocol generally use |
| | | | UDP. |
| Network | | | | |
| | | | 5. Network layer: Network layer is |
| Antiqua">Data Link | | | | responsible for the routing of packets |
| | | | through the entire network. The layer uses |
| Physical | | | | logical addressing for this purpose. Note |
| | | | that the physical address (like MAC address) |
| 1 Application layer: This layer provides a | | | | keeps changing from hop to hop when a packet |
| means for the user to access information on | | | | travels from source to destination. As a |
| the network through an application. Many user | | | | result, an address that doesn't change is |
| applications that need to communicate over | | | | required to ensure continuity between hops. |
| the network interact with the Application | | | | This is nothing but logical address. For IP |
| layer protocol directly. The user | | | | networks, IP address is the logical address; |
| applications are not part of OSI Application | | | | and for Novell network, IPX address is the |
| layer, use the networking services offered by | | | | logical address, and so on. This layer also |
| the networking protocol suite. Application | | | | provides for congestion control, and |
| layer functions typically include identifying | | | | accounting information for the network. IP |
| communication partners, and determining | | | | (Internet Protocol) is an example of a |
| availability of required resources. Some | | | | network layer protocol. 6. Data link layer: |
| examples of application layer implementations | | | | Data link layer provides delivery of |
| include Telnet, File Transfer Protocol (FTP), | | | | information frames between communicating |
| and Simple Mail Transfer Protocol (SMTP). 2 | | | | partners. This layer is responsible for flow |
| Presentation layer: Presentation layer | | | | regulation, error detection and correction, |
| converts local host computer data | | | | and framing of bits for transmission. The |
| representations into a standard network | | | | network data frame is made up of checksum, |
| format for transmission on the network. On | | | | source address, destination address, and the |
| the receiving side, it changes the network | | | | data itself. The largest frame size that can |
| format into the appropriate host computer's | | | | be sent is known as the maximum transmission |
| format so that data can be utilized | | | | Unit (MTU).Important features of Data link |
| independent of the host computer. ASCII and | | | | layer: |
| EBCDIC conversions, cryptography, and the | | | | |
| like are handled here. | | | | Assembles bits into frames, making them |
| | | | ready for transmission over the network. |
| Examples of Presentation layer coding and | | | | |
| conversion schemes include common data | | | | Provides error detection, and correction to |
| representation formats, conversion of | | | | transmitted frames. If the checksum is not |
| character representation formats, common data | | | | correct, it asks for retransmission. (Send a |
| compression schemes, and common data | | | | control message). |
| encryption schemes. Presentation layer | | | | |
| implementations are not typically associated | | | | Consists of two sub layers: |
| with a particular protocol stack. Some | | | | |
| well-known standards for video include | | | | Logical Link Control (LLC): Defines how data |
| QuickTime and Motion Picture Experts Group | | | | is transferred over the cable and provides |
| (MPEG). QuickTime is an Apple Computer | | | | data link service to the higher layers. |
| specification for video and audio, and MPEG | | | | |
| is a standard for video compression and | | | | Medium Access Control (MAC): Controls media |
| coding.3. Session layer: The session layer | | | | access by regulating the communicating nodes |
| establishes, manages, and terminates | | | | using pre-defined set of rules. (i.e. Token |
| communication sessions. Communication | | | | passing, Ethernet [CSMA/CD] all have MAC |
| sessions consist of service requests and | | | | sub-layer protocol). |
| service responses that occur between | | | | |
| applications located in different network | | | | |
| devices. These requests and responses are | | | | |
| coordinated by protocols implemented at the | | | | |
| session layer. Some examples of session-layer | | | | |
| implementations include AppleTalk's Zone | | | | |
| Information Protocol (ZIP), and Decent Phase | | | | |
| Session Control Protocol (SCP).4.Transport | | | | |
| layer: Transport layer is responsible for | | | | |
| providing reliable service between the hosts. | | | | Different Data link layer protocols define |
| Upper layer datagrams are broken down into | | | | different network and protocol |
| manageable datagrams and then appropriate | | | | characteristics, including physical |
| header information (such as sequence number, | | | | addressing, network topology, error |
| port number, etc.) is added to the datagram | | | | notification, sequencing of frames, and flow |
| before passing it on to the Network layer. | | | | control. Physical addressing (as opposed to |
| Two frequently used transport protocols are | | | | logical addressing) defines how devices are |
| the TCP (Transmission Control Protocol) and | | | | addressed at the data link layer. The |
| the UDP (User Datagram Protocol). Important | | | | protocols used in Data link layer are SLIP, |
| features of Transport layer: | | | | PPP, and CSLP.7. Physical layer: This is the |
| | | | bottom-most layer of the OSI model. The |
| Transport layer ensures reliable service. | | | | Physical layer handles the bit-level |
| | | | communications across the physical medium. |
| | | | The physical medium could be made up of wired |
| | | | electrical signals, or light, or radio |
| Breaks the message (from sessions layer) | | | | (wireless) signals. Physical layer |
| into smaller datagrams, and appends | | | | specifications define characteristics such as |
| appropriate unit header information. | | | | media, data rates, maximum transmission |
| | | | distances, and physical connectors. |
| | | | |
| | | | Some of the important standards that deal |
| Responsible for communicating with the | | | | with physical layer specifications are: |
| Session layer | | | | |
| | | | RS-232(for serial communication lines), |
| Important features of TCP/UDP: | | | | X.21, EIA 232, and G730. |
| | | | |
| TCP/IP widely used protocol for Transport | | | | Physical layer and Data link layer |
| Network layers | | | | implementations can be categorized as either |
| | | | LAN or WAN specifications. Vijayanand has |
| TCP: (Transport Control Protocol) TCP | | | | done his post graduation (M.E.) in |
| ensures that a packet has reached its | | | | Communication Systems. He has several years |
| intended destination by using an | | | | of experience in electronic assembly and |
| acknowledgement. If not, it retransmits the | | | | design, rf testing, and networking. He is a |
| lost messages. Hence, TCP is called a | | | | Cisco Certified Networking Professional, and |
| connection oriented protocol. | | | | an MCSE (Microsoft Certified Systems |
| | | | Engineer). He is a director of Anand Software |
| | | | and Training Pvt. Ltd. Complete tutorial on |
| | | | Networking is available at SimulationExams. |