Additional rationales for the OSI model are given and most significant among them is that it was designed to overcome interoperability issues with networking products from disparate vendors (Ciccarelli et al., 2008; Davis, 2008). Other reasons given for creating the OSI model include: ¢ to clarify general functions of internetworking (Rivard & Roherty, 2008) ¢ to divide the complexity of networking into smaller more manageable bites (Rivard & Roherty, 2008) ¢ to simplify troubleshooting (Ciccarelli et al., 2008) ¢ to allow developers to change and improve components of a networking stack without altering the function of other layers (and the need to re-write them) (Rivard & Roherty, 2008)
A key to the understanding the OSI model is the idea of data passing down through the OSI model layers in one computer, then across a network media and then up through the OSI model layers in a second computer (Hill, 2002). As data flows down through the OSI model layers, each layer that has an applicable protocol, labels the data packet with a header and a trailer specific to the layer this is known as encapsulation (Hill, 2002). On the recipient computer, the data header and trailer is removed after its values are utilized (or de-encapsulated) as the data packet travels up the corresponding layer of the OSI model (Ciccarelli et al., 2008; Hill, 2002). Each layer prepares the data for the adjacent layer before it is forwarded by adding header and trailer data to the data. Layer responsibilities and definitions:
(7) Application: User programs communicate with APIs in the application layer which allows user programs to contain very little networking code themselves (Hill, 2002). The application layer provides access to the network protocol stack. DNS operates at the application layer. Other important examples of protocols operating at this layer are: http, POP3 (email protocol), SMTP (email transport protocol), IMAP (internet message access protocol), FTP and Telnet includes network printing and database services. Layer devices include: hosts, servers, PCs and laptops, cellular phones [pic](Ciccarelli et al., 2008; Hill, 2002; Messer, 2010). (6) Presentation: This layer ensures that format of data input from the application layer is in a form suitable for transport over the network, and that it can be interpreted by the receiving node. Translates character sets such as ASCII, EBCDIC and Unicode for compatibility with remote nodes. This layer compresses and encrypts data as needed [pic](Ciccarelli et al., 2008; doogie966, 2011; Hill, 2002; Messer, 2010).
(5) Session: Sets up, maintains and breaks-down connections between network endpoint applications; negotiates whether transmission will be simplex, half-duplex, or full duplex. Simplex is similar to a broadcast in that it is one way; half-duplex is two way transmission, but only one direction at a time; full duplex is two-way communication at the same time using different channels to manage send and receive traffic [pic](Ciccarelli et al., 2008; Hill, 2002; Messer, 2010; Rivard & Roherty, 2008). Remote procedure calls originate from the session layer (doogie966, 2011). (4) Transport: Provides reliable transport using connection-oriented services (transmission control protocol -TCP) and connection-less oriented services (user datagram protocol, or UDP). Important functions provided by this layer are flow control, error detection and error recovery. Flow control starts before data is actually sent. The transmitting node sends the receiving node a message indicating that it is about to transmit data to the receiving node.
The receiving node acknowledges the receipt of the notice from the sender then data is transmitted. Flow control utilizes TCP protocol to ensure delivery of data. TCP is associated with connection-oriented service where communication is assured. Flow control also manages data transmission speed through a windowing process. A TCP window is that specific amount of data that can be sent before an acknowledgement is required by the receiving node. This prevents the receiving device from being overwhelmed by data volume that is cant process fast enough. Connection-less transmissions (UDP) offers no error detection or recovery mechanism and is faster without the overhead of recovery requirements [pic](Ciccarelli et al., 2008; Hill, 2002; Messer, 2010; Rivard & Roherty, 2008). (3) Network: Responsible logical addressing using IP protocol, or path determination (routing) and packet (datagrams) delivery [pic](Ciccarelli et al., 2008; Hill, 2002; Messer, 2010; Rivard & Roherty, 2008).
When traffic arrives at the network layer, header/trailer information is added or taken away that includes the logical addressing data or IP. IP addressing allows a router to route packets to remote nodes IP. There are two ways routing can be processed one is source routing. In source routing, IP data for every router on the path to the destination is included in the data packet. The routing method calls for the source and destination IP information to be contained within the packet. Once on the network, routers use the source and destination addresses to forward the packet to the next router, or hop, on the path to the destination (Ciccarelli et al., 2008). (2) Data Link: Consists of Logical Link Control Layer and the Media Access Control Layer: MAC layer Responsible for physical addressing or for getting traffic onto the network using the MAC protocol, this layer is responsible for NIC (L1&L2) control.
Switches and bridges operate at this layer, since they are using the MAC physical address of the sender/receiver devices to determine which port on a switch to use, or whether to let traffic pass (bridge). Switches also operate at layers 3 and 4 [pic](Ciccarelli et al., 2008; Hill, 2002; Messer, 2010). Media access control (MAC Address) addressing is managed and added/removed from traffic at this layer. Logical addresses (IP) are not considered at this layer. LLC sublayer bridges the MAC sublayer to the upper layer protocols. 1) Physical Responsible for physical devices such as NICs(L1 &2), cabling, fiber, coax, wireless, hubs, connectors (e.g. RJ-45, BNC and for implementing standards for physical configuration of connectors such as TIA/ETA (RJ-45). Also, the physical layer manages signaling and voltages required to transmit data [pic](Ciccarelli et al., 2008; Hill, 2002; Martin, Not Given; Messer, 2010).
Ciccarelli, P., Faulkner, C., FitzGerald, J., Dennis, A., Groth, D., Skandier, T., & Miller, F. (2008). Networking Basics. Hoboken, NJ: John Wiley & Sons, Inc. Davis, D. (2008). Cisco Administration 101: Understand the OSI model to become a better Cisco troubleshooter. Retrieved 2/13/2013, 2013, from http://www.techrepublic.com/blog/networking/cisco-administration-101-understand-the-osi-model-to-become-a-better-cisco-troubleshooter/556 doogie966. (2011). OSI protocols and devices. from http://quizlet.com/6089965/osi-layer-protocols-devices-flash-cards/ Hill, B. (2002). Cisco: The Complete Reference: McGraw-Hill/Osborne Media. Martin, U. o. T. a. (Not Given). EIA/TIA 568A & 568B Standard. Retrieved February 16, 2013, from http://www.utm.edu/staff/leeb/568/568.htm Messer, J. (2010). The OSI Model CompTIA Network+ N10-004: 4.1. Retrieved February 14, 2013, from https://www.youtube.com/watch?feature=endscreen&v=W438koUR04o&NR=1 Rivard, E., & Roherty, J. (2008). CCNA Flash Cards & Practice Pack (Third ed.): Cisco Systems, Inc.