Information Systems Security


Digital communication between various computers is only possible by use of protocols, which are a set of standard rule that determine how data is to be transmitted in a network modern. With the need to establish a universal and interoperable communication platform, the International Organization for Standardization (OSI) established the Open Systems Interconnect (OSI) Reference Model; that was developed with the aim of establishing a universal digital communications platform. The layered approach deployed in OSI model plays an integral role in facilitating the implementation of security of network communications. In addition, the OSI reference model serves as a theoretical model that explains how network protocols are required to function. An important aspect of the OSI model is that its functionality significantly depends on the layered approach with each layer having a specific task in order to facilitate digital data exchange in a network. This essay outlines the specific tasks of the layers of the OSI model in facilitating data communication between computers (Christos, 2007).

Communication between the adjacent layers in the OSI reference model is facilitated through encapsulation, which refers to the adding of header to the data as it travels down from the topmost layer to the bottom most layer. On the receiving end, de-encapsulation takes place and entails the removal of header information from the data as it moves from the bottommost layers to the topmost layer. The OSI reference model is divided into seven layers, with each having a distinct functionality to facilitate data exchange in a network (Christos, 2007). The following diagram lists the OSI layers and their respective data names.

Layer name Layer number Data names
Application Layer 7 Data stream
Presentation Layer 6 Data stream
Session Layer 5 Data stream
Transport Layer 4 Segment/ Datagram
Network Layer 3 Packet
Data Link Layer 2 Frame
Physical Layer 1 Bits

Source: Behrouz, F. (2001). Data communications and Networking. New York: Mc Graw Hill. Page 56.

The physical layer

This is the first layer of the OSI model and performs the conversion of frames to bits in order to foster data transmission over the network medium. On the receiving end, the physical layer converts the bits into frames so that they are used by the Data Link layer. The physical layer comprises of various device drivers, which determine the protocols to be used in the transmission and reception of data in form of bits. Some of the elements found in the physical layer include interface standards, electrical characteristics of the transmitting medium and a number of protocols. Using the device drivers and interface standards, layer 1 manages much of the throughput rates for the network hardware, ensures there is synchronization between analogue and digital signals, manages the noise levels and the mode of data transmission over the physical hardware, such as digital, analog or use of light pulses. Some of the network devices found in the physical layer includes repeaters, Network Interface Cards, signal amplifiers and hubs; these devices perform most of the hardware-based activities on the communication system (Behrouz, 2001).

The Data Link Layer

The fundamental function of the data link layer is to convert the data packets from the network layer into the appropriate format for transmission. The required format for transmission depends on the implemented hardware and technology in the computer network such as Ethernet, token ring, and Asynchronous Transfer Mode (ATM). The protocols used in the data link layer depend on the implemented technology responsible for transforming the data packets into formatted frames, after which they are delivered into the physical layer for transmission. Some of the protocols found in the data link layer include Reverse Address Resolution Protocol (RARP), Point-to- Point protocol, Address Resolution Protocol and Serial Line Internet Protocol (Frye, 2007). In the data link layer, the hardware source and destination addresses are added to the data frames. The hardware address is usually in the form of Media Access Control (MAC) address that is 48-bit address. Some of the hardware devices found in layer 2 of the OSI model include switches and bridges, which have the capability to handle MAC-based traffic routing

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