Understanding the Osi Model

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  • Topic: OSI model, Internet Protocol Suite, Data Link Layer
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Understanding the OSI Model

November 26, 2012
Understanding the OSI Model
The Open Systems Interconnection (OSI) Model is a reference tool for understanding data communications between any two networked systems (Simoneau, 2012). The OSI Model dissects the communication process down to seven layers, where each layer performs specific functions. It helps break a network down to controllable pieces; without this OSI Model networks would be difficult to comprehend. The OSI Models’ seven-layers consist of: physical, data link, network, transport, session, presentation, and application. Within the lower layers (layers 1-3) are based upon mostly hardware, where as the upper layers (layers 5-7) rely typically on software. Between the upper and lower layers resides layer 4, which acts as a divider.

The physical layer concentrates on the transmission of data on the network. One of its fundamentals is how bits are represented on the medium. Binary expression is a stream of 1s and 0s that represent data on a computer network. These streams of 1s and 0s can also be represented through the copper wiring in electrical voltage or the light that is carried through fiber-optic cabling. There are two types of approach: current state modulation and state transition modulation. Current state modulation is the presence or absence of voltage or light that can represent a binary 1 or 0. The other approach, state transition modulation, also represents binary data; this modulation is the transition between voltage or the presence of light that helps indicate a binary value. Layer 1, the physical layer, obviously uses physical topology. Within the physical topology are bus, ring, and star topologies. Asynchronous and synchronous are two basic approaches to bit synchronization. Synchronizing bits is used for two networked devices to communicate effectively in the physical layer. This is so both networked devices can agree upon when one bit stops and another starts. With the asynchronous approach a sender signals that it is about to start transmitting. This is by sending a start bit to the receiver. The receiver then picks up the start bit and starts its own internal clock to measure the subsequent bits. Once the sender transmits its data, it then sends a stop bit to signify that it has finished its transmission. With synchronous approach the networks synchronize the internal clocks of the sender and receiver to guarantee they agree with when the bits begin and end. An external clock, provided by a service provider, is used to make this synchronization happen. This is indited by both sender and receiver.

Also with the physical layer, layer 1, is bandwidth usage. Other than bits and bytes and their multiples, probably the second most significant concept to understand about computer measurements is bandwidth, also known as data transfer rate (Mueller, Soper, & Prowse, 2011). Bandwidth usage includes two basic fundamental approaches: broadband and baseband. Broadband technologies divide the bandwidth available on a medium (for example, copper or fiber-optic cabling) into different channels (Wallace, 2012). After dividing the available bandwidth, different communication streams will then convey over the various channels. An example of this is Frequency-Division Multiplexing, or FDM, used by a cable modem that uses specific ranges of frequencies on the cable that comes into a home from a local company to carry incoming data, outgoing data, and other frequency ranges for television stations. Unlike broadband, baseband uses all available frequencies on a medium to transmit data. An example of networking technology using baseband is Ethernet.

Following layer 1 is the data link layer, layer 2. The data link layer pertains with packaging data into frames and sending those frames on to the network, performing error detection and correction, identifies network devices with different addresses, and managing flow control. The data link layer...
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