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