Networking Model

Layered Tasks

             We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.

Sender, Receiver, and Carrier

Network model ( OSI MODEL)

Networking Layer

                   The OSI model is based on a proposal developed by the international standards organization (Iso) As a first step toward international standardization of the protocol used in the various layer.it was revised in 1995. the model is called iso OSI (open systems interconnection ) reference model because it deals with connecting open system that is, systems that are open for communication with other systems.

Open Systems Interconnection

           The OSI network model is a layered framework for the design of network model systems that allow communication between all types of computer systems. It consists of seven separate but related layers, each of which defines a part of the process of moving information across a network model.

            The purpose of the OSI network model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software.

             The OSI network model is not a protocol; it is a network model for understanding and designing a network model architecture that is flexible, robust, and interoperable.
             The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be briefly summarized as follows:

1. A layer should be created where a different abstraction is needed.

2. Each layer should perform a well-defined function.

3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols.

4. The layer boundaries should be chosen to minimize the information flow across the interfaces.

5.  The number of layers should be large enough that .distinct functions need not be thrown together in the same layer out of necessary and small enough that the architecture does not become unwieldy.

                               Seven Layer Model

1.     Physical Layer

2.     Data Link Layer

3.     Network Layer

4.     Transport Layer

5.     Session Layer

6.     Presentation Layer

7.     Application Layer

Application Layer

                     The application layer contains a variety of protocols that are commonly needed by users. One widely used application protocol is HTTP which is the basis for the world wide Web. When a browser wants a web page, it sends the name of the page it wants to the server hosting the page using HTTP. the server then sends the page back. Other application protocols are used for file transfer, electronic mail, and network news.
             Offers direct access for Applications to use network resources. Represented in software API’s. ODBC drivers offer application layer interfaces.

Presentation Layer

                    Unlike the lower layers, which are mostly concerned with moving bits around, the presentation layer is concerned with the syntax and semantic of the information transmitted. In order to make it possible for computers with different internal data representations to communicate, the data structures to be exchanged can be defined in an abstract way, along with a standard encoding to be used " on the wire" the presentation layer manages these abstract data structures and allows higher- level data structures to be defined and exchanged.      
                  Determines the format used to exchange data among networked computers Sometimes called the network’s translator.

           Changes the format from an application layer format to an intermediate format that both computers recognize

Session Layer

              The session layer allows users on different machines to establish sessions between them. Sessions offer various services, including dialog control token management and synchronization.             
         Enables two applications on different computers to establish, use, and end a connection called a session. Provides name recognition and the functions needed to communicate over the network. Provides synchronization between user tasks by placing checkpoints in the data stream. Provides dialog between the communicating processes for regulating who transmits and who receives

Transport Layer

              The basic function of the transport layer is to accept data from above it, split it up into smaller units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. Furthermore, all this must be done efficiently and in a way that isolates the upper layers from the inevitable changes in the hardware technology over the course of time. 

The transport layer also determines what type of service to provide to the session layer, and, ultimately, to the user of the network. The most popular type of transport connection is an error-free point-to-point channel that delivers messages or bytes in the order in which they were sent. However, other possible kinds of transport service exist, such as the transporting of isolated messages with no guarantee about the order of delivery, and the broadcasting of messages to multiple destinations. The type of service is determined when the connection is established. 

                          The transport layer is a true end-to-End layer; it carries data all the way from the source to the destination. In other words, a program on the source machine carries on a conversation with a similar program on the destination machine, using the message headers and control messages. In the lower layers, each protocol is a machine and its immediate neighbors, and not between the ultimate source and destination machines, which may be separated by many routers. The difference between layers 1 through 3, which are chained, and layers 4 through 7, which are end-to-end.            

                  Ensures the messages are transferred error free, in sequence, and with no losses or duplication. Divides messages up into packets and reassembles the packets on the other side. Sends acknowledgment of receipt.

Network Layer

                The network layer controls the operation of the subnet. A key design issue is determining how packets are routed from source to destination. Routes can be based on static tables that are " wired into " the network and rarely changed, or more often they can be updated automatically to avoid failed components. They can also be determined at the start of each conversation, for example, a terminal session, such as a login to a remote machine. Finally, they can be highly dynamic, being determined anew each packet reflect the current network load.

                  If too many packets are present in the subnet at the same time, they will get in one another's way, forming bottlenecks. Handling congestion is also a responsibility of the network layer, in conjunction with higher players that adapt the load they place on the network. More generally, the quality of service provided is also a network layer issue.

                 When a packet has to travel from one network to another to get to its destination, many problems can arise. The addressing used by the second network may be different from that used by the first one. The second one may not accept the packet at all because it is too large. The protocols may differ, and so on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected. 

                  In broadcast networks, the routing problem is simple, so the network layer is often thin or even nonexistent.

                     Responsible for addressing. Changes logical addresses into physical addresses and back. Determines the path the data should take based on network conditions. Manages traffic problems on the networks. If the destination can’t receive packets as large as the transport layer provides the network layer will further break up and reassemble the packets.

Data-Link Layer

                   The main test is data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors. It does so by masking the real errors so the network layer does not see them. It accomplishes this task by having the sender break up the input data into data frames and transmit the frames sequentially. If the service is reliable, the receiver confirms correct receipt of each frame by sending back an acknowledgment frame.

                 Another issue that arises in the data link layer is how to keep a fast transmitter from drowning a slow receiver in data. Some traffic regulation mechanism may be needed to let the transmitter know when the receiver can accept more data.

                Broadcast networks have an additional issue in the data link layer. How to control access to the shared channel. A special sublayer of the data link layer. The medium access control sublayer deals with this problem.

Broken up into two sub layers

1.     Media Access Control (MAC)

Media Access Protocols

Physical Addressing

2.     Logical Link Control (LLC)

Frame Synchronization

Flow Control

Error Checking

Physical Layer

                         The physical layer is concerned with transmitting raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit it is received by the other side as a 1 bit, not as a 0 bit. Typical questions here are what electrical signals should be used to represent a 1and a 0, how many nanoseconds a bit lasts, whether transmission may proceed simultaneously in both directions, how the initial connection is established, how it is torn down when both sides are finished, how many pins the network connector has, and what each pin is used for . these design issues largely deal with mechanical, electrical, and timing interfaces, as well as the physical transmission medium, which lies below the physical layer.
´ Defines

Electrical Properties

Transmission Media

               Transmission Devices

Physical Topologies

Data signaling

Data synchronization

Data Bandwidth

                

               Each layer communicates directly with its opposing layer on the other computer. There is no system that currently uses the full OSI model. The closest is the IBM networking model for the AS400

Server

All   layers

Workstation

All   layers

Router

Physical Layer

Data Link Layer

Network Layer

     
      Switch

Physical Layer

Data Link Layer

Hub
      Physical layer only

The TCP/IP Reference Model

                          Let us now turn from the OSI reference model to the reference model used in the grandparent of all wide area computer networks, the ARPANET, and its successor, the worldwide internet. Although we will give a brief history of the ARPANET later, it is useful to mention a few key aspects of it now. The ARPANET was a research network sponsored by the DoD(u.s.department of defense). It eventually connected hundreds of universities and government installation, using leased telephone lines. When satellite and radio networks were added later, the existing protocols had trouble interworking with them, so a new reference architecture was needed. Thus, from nearly the beginning, the ability to connect multiple networks in a seamless way was one of the major design goals. This architecture later became known as the TCP/ IP reference model, after its two primary protocols. It was first described by Cerf and Kahn, and later refined and defined as a standard in the internet community. The design philosophy behind the model is discussed by Clark.

                  Given the DoD' s worry that some of its previous hosts, routers, and internet work gateways might get blown to pieces at a moment's notice by an attack from the Soviet Union, another major goal was that the network be able to survive loss of subnet hardware, without existing conversations being broken off. In other words, the DoD wanted connections to remain intact as long as the source and destination machines were functioning, even if some of the machines or transmission lines in between were suddenly put of operation. Furthermore, since an application with divergent requirements were envisioned, ranging from transferring files to real- time speech transmission, a flexible architecture was needed.