Communications Models Ch. 2.pptx

Communicatio ns Models Before explaining the models used in data communications, it might be useful to explain what models are and what purpose they serve Modeling We use models to simplify explanation when the real object is too complex to visualize in human terms and to represent objects that...

Communicatio ns Models Before explaining the models used in data communications, it might be useful to explain what models are and what purpose they serve Modeling We use models to simplify explanation when the real object is too complex to visualize in human terms and to represent objects that we cannot physically capture We first consider the International Organization for Standardization Open Systems Interconnection model, which is a model of the functionality required to communicate from the source end user to the destination end user To understand data communications, it is important that you grasp what these models represent, as almost all discussions of protocols and standards are based on these models The OSI model describes these functions and generally specifies the order in which they take place in transmission ISO OSI Model The OSI model is complex and understanding its functions is crucial to understanding data communications The OSI model goes from the concrete to the abstract Mail Analogy Conversely, when you filled out the PO, you did not consider how many places the envelope that contained it would be picked up at and delivered to on its way to the end user Mail Analogy This analogy bears a close similarity to the OSI model for end-user-to-end-user communications, where the original sender is one end user and the person who will read the purchase order is the end user at the receiving end One critical question should be addressed here: how did the sending user know that the letter was received? The Open Systems Interconnection reference model is an attempt to standardize the functionality of end-userto-end-user computer communications OSI Model Some communications systems built today are OSI compliant In the OSI model, the line of demarcation between data communications and data processing exists at the border between the Transport and Session layers The Physical layer provides the mechanical and electrical means of connecting to the medium, be it copper, fiber optic, or wireless Physical Layer Some examples of the Physical layer include Electronic Industries Alliance /Telecommunications Industry Association 232, EIA/TIA 485, or the LAN network interface card The Physical layer provides line termination and/or impedance matching as well as synchronization of data For every connection or task requiring communications, there will be a set of SAPs: the source service access point and the destination access service point Service Access Points or the User Datagram Protocol SAPs are nothing more than addresses in memory assigned by whatever program is controlling the communications The Network layer performs enduser-to-end-user routing Network Layer Layer 3 is almost totally focused on routing functions The Internet Protocol and Internet Packet Exchange , along with the OSI Internet protocol, are all concerned with routing and the path that packets take from end user to end user The Transport layer is responsible for reliable end-user-to-end-user communications Transport Layer The most common Transport protocols are TCP, which is connection-oriented, and UDP, which is connectionless TCP is not OSI compliant, yet it has Layer 4 functionality and is the de facto standard for data communications today The Session layer is concerned with the jobs at hand and with scheduling jobs from the Applications layer through the Presentation layer Session Layer It allocates system resources as required and communicates commands to the Transport layer The Session layer establishes virtual connections and closes them when the communication tasks are complete The Presentation layer ensures that the communications data has the correct syntax for the Applications layer Presentati on Layer The Presentation layer ensures that communications are performed in a common language; the encryption and decryption of the data is usually performed at the Presentation layer This layer also ensures that all the data typing and formatting will interface with the Application and Session layers Application layer functions are extremely high level compared to the bit functions at the Physical layer Applicatio n Layer The Application layer is much like an operating system using a graphical user interface The Application layer takes requests and gives output to the user in the user’s required form The Tortuous Path The Tortuous Path ∙The Network layer ensures that both the receive and send IP addresses are in the proper locations in the protocol header at the sending end, and that this is the correct station at the receiving end Originating at about the same time as the OSI ISO model, the Internet model defines data communications in only five layers, as figure 2-5 illustrates The Internet Model The Internet model is the most widely used protocol model for all data communications The Data Link and Physical layers generally follow the IEEE 802 model , but the Network layer always uses the IP format, which is currently defined as either version 4 or version The TCP/IP suite, as currently delivered, provides a number of standard applications , in addition to the Layer 3 and 4 functions The IEEE 802 Model ∙ IEEE 802 was established as a local area network specification ∙ This standard divides the OSI Data Link layer into two distinct sublayers: Media Access Control and Logical Link Control ∙ IEEE 802 also defines how different standard networks are to be physically connected: what form of media access they should use and how the user will interface data to the Data Link layer Logical Link Control ∙ The number 802 is the designation for the main IEEE committee for LAN standardization and specification ∙ An example is 802.3, the standard for a Carrier Sense Multiple Access with Collision Detection network and a formalization of Ethernet IEEE 802 CURRENTLY SPECIFIES SIX DIFFERENT LAN TECHNOLOGIES, MANY OF WHICH WILL BE DISCUSSED AT SOME LENGTH IN LATER CHAPTERS: IEEE 802.3: THIS STANDARD DEFINES THE CSMA/CD NETWORK AND DESCRIBES SIX LAN TYPES: 10BASE5, 10BASE2, 10BASE-T, 100BASE-T, 1000BASE-T, AND 10000BASE-T IEEE 802.4: THIS STANDARD DEFINES THE TOKEN-PASSING BUS, A BUS TOPOLOGY THAT USES A TOKEN-PASSING ACCESS THE RATIONALE BEHIND THE IEEE 802 MODEL AND ITS SPECIFICATIONS IS THIS: IF YOU MEET THE EXTERNAL INTERFACE REQUIREMENTS OF THE LLC LAYER, THEN YOUR COMMUNICATIONS WILL WORK REGARDLESS OF THE UNDERLYING MAC TECHNOLOGY BEING USED The IEEE 802 Model One other set of communication models remains to be discussed before we move on to the details of each model These are application models ; they dictate how communications and networking software applications are applied Applicatio n Models We will look at four types of application models: one-tier, two-tier, three-tier, and N-tier User Services are composed of the graphical user interface and presentation logics and are sometimes referred to by those names The majority of databases of any size are managed by relational database management systems , such as Access, Oracle, or SQL Server The Business Services are an extremely important part of the system; they supply the organization, as well as the boundaries, procedures, and rules Client-server systemsrefers were to “Client-server” An example in ofthe a one-tier model is established 1980s, whenfor the functional mo del a stand-alone PC running a checkpersonal computers were first balancing program in which check the relationship networked to improve data is stored in a no personal between the de file a nd performance over simple database, such as Access sharing the sha re d reso urce Application Models In control systems, the publishersubscriber approach is a better method for exchanging data PublisherSubscribe r Using the publisher-subscriber method, a client application communicates a request to a server and is put on a list to receive a response when the value changes; the subscriber expects to receive a response within a designated period This request can be for a one-time receipt of data, or for data at regular intervals, or for data only when values change The subscriber application is provided by the publisher Summary