- Layers of OSI

Sim-Ex™ Tutorial for Network+

2.0 Protocols and standards

  1. Layers of OSI
  2. MAC Address
  3. Subnetting
  4. Public and Private Networks
  5. TCP/IP Protocols
  6. Network Utilities

2.1 Layers of OSI:

The Seven layers of OSI model are:

  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical

Application layer: This layer provides a means for the user to access information on the network through an application. Many user applications that need to communicate over the network interact with the Application layer protocol directly. The user applications are not part of OSI Application layer, use the networking services offered by the networking protocol suite. Application layer functions typically include identifying communication partners, and determining availability of required resources. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

Presentation layer: Presentation layer converts local host computer data representations into a standard network format for transmission on the network. On the receiving side, it changes the network format into the appropriate host computer's format so that data can be utilized independent of the host computer. ASCII and EBCDIC conversions, cryptography, and the like are handled here.

Examples of Presentation layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes.

Presentation layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding.

Session layer: The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and 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).

Transport layer: Transport layer is responsible for providing reliable service between the hosts. Upper layer datagrams are broken down into manageable datagrams and then appropriate header information (such as sequence number, port number, etc.) is added to the datagram before passing it on to the Network layer. Two frequently used transport protocols are the TCP (Transmission Control Protocol) and the UDP (User Datagram Protocol).

Important features of Transport layer:

  • Transport layer ensures reliable service.
  • Breaks the message (from sessions layer) into smaller datagrams, and appends appropriate unit header information.
  • Responsible for communicating with the Session layer

Important features of TCP/UDP:

  • TCP/IP widely used protocol for Transport/Network layers
  • TCP: (Transport Control Protocol) TCP ensures that a packet has reached its intended destination by using an acknowledgement. If not, it retransmits the lost messages. Hence, TCP is called a connection oriented protocol.
  • UDP (Universal Data gram Protocol): UDP 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 reached the intended partner(s). Hence, UDP is often called connectionless protocol.
  • Application programs that do not need connection-oriented protocol generally use UDP.

Network layer: Network layer is responsible for the routing of packets through the entire network. The layer uses logical addressing for this purpose. Note that the physical address (like MAC address) keeps changing from hop to hop when a packet travels from source to destination. As a result, an address that doesn't change is required to ensure continuity between hops. This is nothing but logical address. For IP networks, IP address is the logical address; and for Novell network, IPX address is the logical address, and so on. This layer also provides for congestion control, and accounting information for the network. IP (Internet Protocol) is an example of a network layer protocol.

Data link layer: Data link layer provides delivery of information frames between communicating partners. This layer is responsible for flow regulation, error detection and correction, and framing of bits for transmission. The network data frame is made up of checksum, source address, destination address, and the data itself. The largest frame size that can be sent is known as the maximum transmission Unit (MTU).

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Important features of Data link layer:

  • Assembles bits into frames, making them ready for transmission over the network.
  • Provides error detection, and correction to transmitted frames. If the checksum is not correct, it asks for retransmission. (Send a control message).
  • Consists of two sub layers:
    • Logical Link Control (LLC): Defines how data is transferred over the cable and provides data link service to the higher layers.
    • Medium Access Control (MAC): Controls media access by regulating the communicating nodes using pre-defined set of rules. (i.e. Token passing, Ethernet [CSMA/CD] all have MAC sub-layer protocol).

Different Data link layer protocols define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to logical addressing) defines how devices are addressed at the data link layer. The protocols used in Data link layer are SLIP, PPP, and CSLP.

Physical layer: This is the bottom-most layer of the OSI model. The 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 (wireless) signals. Physical layer specifications define characteristics such as media, data rates, maximum transmission distances, and physical connectors.

Frequently used Physical layer protocols:

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Some of the important standards that deal with physical layer specifications are:

RS-232(for serial communication lines), X.21, EIA 232, and G730.

Physical layer and Data link layer implementations can be categorized as either LAN or WAN specifications.

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