Computer Networks. Internet

Whenever there is a need for information to be passed between two computer systems, a communication scheme must be developed to support it. This scheme must include clear definitions of the physical connection, e.g., the wires and voltage levels, the pattern of signals used to synchronize and define the data, methods of encryption and error checking, and so on. Entire books are written on the system of layers that define these communication schemes. The Open Systems Interconnection (OSI) model of networks is a method for classifying the complex components of a communication scheme so that they are easier to describe and understand. There are seven layers to this model ranging from the physical implementation (the wires and such) to the more abstract layer of the applications where the details of the network are hidden. The following is a description of the seven layers:

• Application Layer (layer 7) – At this layer, one application must format the data so that a second application can properly interpret it. Examples include formatting data so that an e-mail agent or a web browser can properly display data to the user.

• Presentation Layer (layer 6) – This layer eliminates compatibility problems caused by serving different types of networks. It acts as a generic interface between the application and the details of the network.

• Session Layer (layer 5) – This layer starts, maintains, and stops the logical connection between two applications.

• Transport Layer (layer 4) – When a block of data is sent from one application to another, mechanisms are needed to provide error checking, the partitioning of large blocks of data into smaller packets, the reassembly of the data packets, and flow control. The transport layer is responsible for these functions.

• Network Layer (layer 3) – Once a packet from the transport layer is placed on the network, it needs to find its way from one physical host to another. This is the responsibility of the network layer. It provides routing, forwarding, sequencing, and logical addressing. Since the delivery of a packet may involve several types of networks, this layer must remain independent of the physical implementation of the network.

• Datalink Layer (layer 2) – The datalink layer handles the bit-level specifications used to deliver a message within a specific type of network. Examples of datalink protocols include the IEEE 802.3 protocol which defines how the bits are organized in an Ethernet frame and the Serial Line Internet Protocol (SLIP) which may be used on a dial-up network. While the network layer uses logical addresses to identify hosts, the datalink layer uses the physical addresses of the hardware.

• Physical Layer (layer 1) – This layer defines how the bits are transmitted. For example, logic ones and zeros may be identified by different voltage levels, the presence or absence of light, or the frequency of a radio signal. This hardware level description of the network is considered the physical layer. Since most protocols are described with respect to the layer on which they function, it is important to have a basic understanding of the OSI layer definitions.

Date: 2015-12-17; view: 843