| Internetworking |

Submitted By: Courtney White
Submission Date: 11/20/2012
Submitted To: Professor Conn
Assignment 4

An internetwork is an assortment of distinct networks that functions as a single large network. Internetworking denotes to the industry, products, and procedures that meet the challenge of crafting and governing internetworks. (Corner & Stevens, 1952) The first networks were time-sharing networks that used mainframes and attached terminals. Local-area networks progressed around the PC insurrection. (Davidson & Muller, 1992) LANs allowed numerous operators in a somewhat trifling area to interchange files and messages, as well as access shared resources. Wide-area networks interconnect LANs with geographically disseminated users to create connectivity. (Davidson & Muller, 1992)
Though it is quite simple to visualize internetwork, implementing a practical internetwork is no unpretentious job. Many encounters must be tackled, especially in the areas of connectivity, reliability, network management, and flexibility. Each area is plays a key role in inaugurating a proficient and operative internetwork. The test when connecting various systems is to support communication among disparate technologies. Different sites, for example, may use different types of media operating at varying speeds, or may even include different types of systems that need to communicate. Because establishments depend comprehensively on data communication, internetworks must deliver a certain level of steadfastness. This is a volatile world; so many large internetworks include redundancy to allow for communication even when problems occur.
The Open Systems Interconnection reference model defines in what way information from a software application in one computer travels through a network medium to a software application in another computer. (Eady, 2004)The OSI reference model is a conceptual model composed of seven layers, each postulating specific network tasks. The OSI model divides the tasks involved with moving material between networked computers into seven reduced, more manageable task groups. (Corner & Stevens, 1952)A task is then apportioned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented freely. This allows the resolutions presented by one layer to be updated without adversely affecting the other layers. The seven layers of the Open Systems Interconnection (OSI) reference model are: * Layer 7-Application * Layer 6-Presentation * Layer 5-Session * Layer 4-Transport * Layer 3-Network * Layer 2-Data link * Layer 1-Physical (Eady, 2004)
The physical layer describes the electrical, mechanical, procedural, and functional specifications for initiating and deactivating the physical link between communicating network systems. (Eady, 2004)Physical layer specifications define physiognomies such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. Physical layer implementations can be categorized as either LAN or WAN specifications.
The data link layer provides dependable transit of data across a physical network link. Dissimilar data link layer specifications describe altered network and protocol characteristics. Physical addressing outlines how devices are addressed at the data link layer. (Eady, 2004)Network topology contains the data link layer specifications that often describe how devices are to be physically connected. (Corner & Stevens, 1952)Finally, flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.
The Logical Link Control sub layer of the data link layer manages communications between devices over a single link of a network. (Corner & Stevens, 1952)LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services used by higher-layer protocols. (Eady, 2004)IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share a single physical data link. (Eady, 2004)The Media Access Control (MAC) sub layer of the data link layer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one another at the data link layer.
The network layer describes the network address, which varies from the MAC address. (Eady, 2004)Some network layer implementations describe network addresses in a way that route selection can be determined systematically by associating the source network address with the destination network address and applying the subnet mask. (Corner & Stevens, 1952)Because this layer defines the logical network layout, routers can use this layer to determine how to forward packets. Much of the design and configuration work for internetworks happens at Layer 3, the network layer.
The transport layer admits data from the session layer and sections the data for conveyance across the network. (Eady, 2004)The transport layer is accountable for making sure that the data is distributed error-free. Flow control generally occurs at the transport layer. Flow control oversees the data transmission between devices so that the communicating device does not send more data than the receiving device can process. Multiplexing enables data from several applications to be conveyed onto a single physical link. (Corner & Stevens, 1952) Virtual circuits are recognized, upheld, and terminated by the transport layer. Error checking encompasses creating various instruments for detecting transmission errors, while error recovery involves acting, such as requesting that data be retransmitted, to resolve any errors that occur. (Davidson & Muller, 1992)The transport protocols used on the Internet are TCP and UDP.
The session layer creates, oversees, and dismisses communication sessions. Communication sessions consist of service requests and service replies that occur between applications located in different network devices. (Eady, 2004) These requests and responses are coordinated by protocols implemented at the session layer. An example of session-layer implementations includes Zone Information Protocol (ZIP) which is the AppleTalk protocol that coordinates the name binding process. (Eady, 2004)
The presentation layer provides a diversity of coding and conversion functions that are applied to application layer data. (Eady, 2004) These functions safeguard the information sent from the application layer of one system, and ensure that it would be readable by the application layer of another system. Some examples of presentation layer coding and conversion schemes are: common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes. (Eady, 2004)
The use of normal image, sound, and video formats allow the swapping of application data between different types of computer systems. Conversion schemes are used to interchange information with systems by using different text and data representations. Standard data compression schemes enable data that is compressed at the source device to be properly decompressed at the destination. Standard data encryption schemes enable data encrypted at the source device to be properly deciphered at the destination. Presentation layer implementations are not connected with a specific protocol stack. Some well-known standards for video include QuickTime and Motion Picture Experts Group. QuickTime is an Apple Computer specification for video and audio, and MPEG is a typical for video compression and coding.
The application layer is the OSI layer adjoining the end user, which means that both the OSI application layer and the user interrelate directly with the software application. (Eady, 2004)This layer intermingles with software applications that implement a communicating component. Application layer functions include classifying communication partners and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.
The OSI model provides a theoretical structure for communication between computers, but the model itself is not a method of communication. (Eady, 2004)Actual communication is made probable by using communication protocols. In the context of data networking, a protocol is a formal set of rules and conventions that governs how computers exchange material over a network medium. A protocol implements the functions of one or more of the OSI layers.

A wide variety of communication protocols exist. Some of these protocols include LAN protocols, WAN protocols, network protocols, and routing protocols. LAN protocols function at the physical and data link layers of the OSI model. (Davidson & Muller, 1992)WAN protocols operate at the lowest three layers of the OSI model. Routing protocols are network layer protocols that are accountable for switching information between routers so that the routers can select the proper path for network traffic. Network protocols are the various upper-layer protocols that exist in a given protocol suite. Many protocols rely on others for operation. (Davidson & Muller, 1992)
The seven OSI layers use numerous forms of control information to communicate with their peer layers in other computer systems. (Eady, 2004)This control information comprises of detailed demands and directions that are switched between peer OSI layers. Control information typically takes one of the two forms: headers and trailers. Headers are prepended to data that has been passed down from upper layers. Trailers are attached to data that has been passed down from upper layers. An OSI layer is not obligatory to attach a header or a trailer to data from upper layers. (Eady, 2004)
Headers, trailers, and data are comparative concepts, contingent on the layer that analyzes the information unit. (Eady, 2004) At the network layer, an information unit comprises of a Layer 3 header and data. At the data link layer, however, all the information passed down by the network layer is treated as data. (Corner & Stevens, 1952)The data portion of an information unit at a given OSI layer potentially can contain headers, trailers, and data from all the higher layers. This is known as encapsulation. (Corner & Stevens, 1952)

Bibliography

Corner, D., & Stevens, D. L. (1952). Internetworking with TCP/IP: Design, implementation, and internals. indianapolis: Prentice Hall.
Davidson, R. P., & Muller, N. J. (1992). Internetworking LANs: . Artech House.
Eady, F. (2004). Networking and Internetworking with Microcontrollers. Elsevier Inc.