Understanding the Osi Model

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 has two sub layers: MAC and LLC. MAC stands for Media Access Control. MAC addressing is physical addressing, which has a 48-bit address deputed to a device’s network interface card. MAC addresses are usually written in hexadecimal notation. Of the 48-bit address, the first 24 bits are commonly known as the vendor code. Vendor codes determine the manufacturer of a network device. This determination is based on the first half of the device’s MAC address. In the last 24 bits each vendor is accountable for using their own unique values, so that no two MAC addresses should have the same value. Layer 2 devices consider a network as a logical topology. Logical topology regards bus and ring topologies. When a network is connected with several devices it has allowance to transmit on the media. If not, multiple devices may transfer at the same time and hinder one another’s transmission. Another sub layer to the data link layer is LLC. LLC stands for Logical Link Control. Connection services are used for when a device on a network receives a message from a different device on the network. The receiving device can give feedback to the sender in the form of a message. The two functions provided by messages are flow control and error control. Flow control restricts how much data senders are able to send simultaneously. This is so the receiver is not overwhelmed with more information than it can handle. Error control permits the data receiver to give the sender acknowledgement of the data frame, whether it was received, or if the data was corrupted when received. The network layer, layer 3, is first and foremost concerned with forwarding data based on logical addresses. There are many routed protocols that have their own logical addressing design. Though the most commonly distributed routed protocol is Internet Protocol. At layer 3, switching is making decisions about how data should be forwarded. Packet switching, circuit switching, and message switching are all switching methods. Packet switching has a data stream that is divided into individual packets. Each individual packet contains a Layer 3 header that has a source and a Layer 3 destination address. Circuit switching has a vigorously devoted communication link among two parties in order for them to communicate. A simpler explanation of circuit switching is when two lines open and close at the same time. Message switching is different than packet switching and circuit switching. The data stream in message switching is divided into messages, where the individual messages are tagged with a destination address. On the way to their destination the messages travel from one network device to another. Message switching within a network is sometimes referred to as a store-and-forward network, for the simple fact that some devices may temporarily store messages before forwarding them. Layer 4, the transport layer, serves as a divider between the upper and lower layers. There are two transport layer protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP is a connection based transport protocol that provides dependable transports; if a segment is dropped the sender can distinguish it and retransmit it. Windowing is used in TCP connections when one or more segments are sent at the same time. The receiver will acknowledge the receipt of all segments in the window with a single accommodation. The UDP is not a connection based protocol. These connectionless protocols do not provide reliable transports; if a segment drops the sender will not be aware of this. Therefore, no retransmission will arise. Internet Control Message Protocol (ICMP) is another transport layer protocol that may be experienced in this layer. Utilities like ping and traceroute are used with ICMP. The fifth layer, the session layer, is accountable for setting up, maintaining, and tearing down of a session. A session is like a dialogue that should be treated independently from other sessions to avoid crossfire of data from other dialogues. When setting up a session the user’s credentials should be checked, numbers should be assigned to the sessions’ communication flow, and figuring which services are going to be needed during the session and which devices should start the transmission of data. To maintain a session one must transfer data, restoration of a disconnected session, and to allow receipt of data. An example of tearing down a session would be when two networkers are communicating through dial-up and one loses connection. The loss of connection tears down the session.

The presentation layer is the sixth layer. The presentation layer is responsible for formatting of data being exchanged and securing that data with encryption (Wallace, CompTIA Network + N10-005, 2012). The presentation layer handles data formatting with the same techniques as formatting text. Data formatting consists in the presentation layer consists of text, graphics, and even multimedia. Encryption is used to disguise personal data so a malicious user cannot intercept the personal data such as social security numbers or confidential documents. The application layer is the final layer, layer seven. This layer supplies application services within a network. End-user applications, such as Microsoft Word, do not dwell within this layer. E-mail, however, is an application layer service that inhabits the seventh layer. Another basic of layer seven is advertising services that are available. Some of these services send out advertisements to publicize their services to other networked devices. In all, the OSI Model is beneficiary in the aspect that it organizes thoughts of a network and gives network explorers a computer networking language. The main purpose for the OSI Model is to easily comprehend the computer communication of networking.
References

Mueller, S., Soper, M. E., & Prowse, D. L. (2011). CompTIA A+ 220-701 and 220-702. In S. Mueller, M. E. Soper, & D. L. Prowse, CompTIA A+ 220-701 and 220-702 (p. 14). Indianapolis: Pearson Education, Inc.
Simoneau, P. (2012, December 1). Global Knowledge Training LLC. Retrieved from Global Knowledge Web site: http://faculty.spokanefalls.edu/Rudlock/files/WP_Simoneau_OSIModel.pdf
Wallace, K. (2012). CompTIA Network + N10-005. In K. Wallace, CompTIA Network + N10-005 (p. 45). Indianapolis: Pearson Education, Inc.
Wallace, K. (2012). CompTIA Network + N10-005 Authorized Cert Guide. In K. Wallace, CompTIA Network + N10-005 Authorized Cert Guide (p. 35). Indianapolis: Pearson Education, Inc.

References: Mueller, S., Soper, M. E., & Prowse, D. L. (2011). CompTIA A+ 220-701 and 220-702. In S. Mueller, M. E. Soper, & D. L. Prowse, CompTIA A+ 220-701 and 220-702 (p. 14). Indianapolis: Pearson Education, Inc. Simoneau, P. (2012, December 1). Global Knowledge Training LLC. Retrieved from Global Knowledge Web site: http://faculty.spokanefalls.edu/Rudlock/files/WP_Simoneau_OSIModel.pdf Wallace, K. (2012). CompTIA Network + N10-005. In K. Wallace, CompTIA Network + N10-005 (p. 45). Indianapolis: Pearson Education, Inc. Wallace, K. (2012). CompTIA Network + N10-005 Authorized Cert Guide. In K. Wallace, CompTIA Network + N10-005 Authorized Cert Guide (p. 35). Indianapolis: Pearson Education, Inc.