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OPEN SYSTEMS INTERCONNECT (OSI)
The OSI (Open Systems Interconnect) model, was created by the ISO (International Standards Organization). OSI Model follows what happens to the user's file as it passes through the different OSI Model layers starting at the Application Layer and working down the Physical Layer. These are the 7 Layers of the OSI model:
Open Systems Interconnection Model
Network services to application programs. Provides network service to application for application programs such as file transfers and electronic mails. The flowchart shown in Fig. 1.illostrates a basic PC logic flowchart. The Keyboard & Application are shown as inputs to the CPU (requesting access to the hard disk). The Keyboard requests accesses through user inquiries (such as "DIR" commands) and the Application seeks access through "File Openings" and "Saves". The CPU, through the Disk Operating System, sends and receives data from the local hard disk ("C:" in this example).
Fig. 1. Basic PC Logical Flowchart A PC(workstation) connected to a network has a software "Network Redirector", the name depends on the network, Terminate and Stay Resident (TSR) between the CPU and PC operating system e.g. PC Operating System (OS) (as in Fig 2.) which presents the network hard disk as another local hard disk to the CPU e.g. "G:", "F:", "E:" etc. All requests by the CPU are intercepted by this software and checks to see if either a local or a network drive is requested. If a local drive is requested, the request is passed on to PC OS. However, if a network drive is requested, the request is then passed on to the network operating system (NOS).
Fig. 2. Simple Network Redirection See the Application layer of the OSI Model, as shown in Fig. 3.
Fig. 3. PC Workstation with Network Aware Software The application layer contains a variety of protocols that are
needed network communication. top
Data representation and interpretation. This layer uses a set form of translations that allows the data to be interpreted properly. Presentation layer can add data encryption for security reasons. The presentation layer is concerned with the syntax and semantics of the information transmitted. When the Network Redirector sends CPU operating system native code to the network operating system: the coding and format of the data is not recognizable by the network operating system. The data consists of file transfers and network calls by network aware programs. These items are represented as character strings, integers, floating point numbers, and data structures composed of several simpler items. Different computers have different codes for representing character strings, integers and so on. In order to make it possible for computers with different representation 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 job of managing these abstract data structures and converting from the representation used inside the computer to the network standard representation is handled by the presentation layer. The Presentation layer presents data to and from the terminal using special control characters to control the screen display. The Presentation Layer also controls security at the file level: this provides both file locking and user security.
Fig. 4.Presentation Layer
Data compression can also be used here to reduce the
number of bits that have to be transmitted and cryptography is frequently
required for privacy and authentication. top
Inter host communication: This layer allows users on different machines to establish sessions between them that is it manages the communications between the workstation and the network by directing the information to the correct destination, and identifying the source to the destination, the type of information as data or control, manages the initial start-up of a session, and the orderly closing of a session and manages Log on procedures and Password recognition an in Fig. 5. Like the transport layer a session allows ordinary data transport, but it also provides some enhanced services useful in a some applications, such as allowing a user to log into a remote time-sharing system or to transfer a file between two machines, manage dialogue control.
Fig. 5. Session Layer top
End-to-end connection reliability. The basic function of the transport layer, is to accept data from the session layer, split it up into smaller units of data segments pass these to the network layer, and ensure that the pieces all arrive correctly at the other end and combines incoming segments into a contiguous file. All this must be done efficiently.
Fig. 6. Transport Layer TCP establishes connections
between two hosts on the network through 'sockets' which are determined by
the IP address and port number. TCP keeps track of the packet delivery
order and the packets that must be resent. Maintaining this information
for each connection makes TCP a stateful protocol. UDP on the other hand
provides a low overhead transmission service, but with less error
checking. NFS is built on top of UDP because of its speed and
statelessness. Statelessness simplifies the crash recovery. top
Address and determining the best path. The Network layer is concerned with the path through the network (controlling the operation of the subnet). This includes routing of packets to destination, switching, and controlling the flow of information between hosts. Routes could be based on static tables or dynamic determined anew for each packet, to reflect the current network load. The Network layer converts the segments into smaller datagrams the network can handle: network hardware source and destination addresses are also added. The Network layer does not guarantee that the datagram will reach its destination. The transport layer also controls the traffic density.
Fig. 7. Network Layer top
Media access. The Data Link layer is a firmware layer of the network interface card by taking a raw transmission facility and transform it into a line that appears free of transmission errors in the network layer and it works at the bit level, and adds start / stop flags and bit error checking (CRC or parity),error checking is at the bit level only and packets with errors are discarded and a request for re-transmission is sent out, to the packet frame as it puts the datagrams into packets (frames of bits: 1s & 0s) for transmission, and assembles received packets into datagrams. The Data Link layer is primarily concerned with bit sequence.
Fig. 8. Data Link Layer Transmission of data is accomplished by having the sender break the input
data up into data frames of a few hundred bytes, transmit the frames
sequentially, and process the acknowledgment frames sent back by the receiver
and creates
and recognize frame boundaries by attaching special
bit patterns to the beginning and end of the frame carefully to avoid
confusion. top
Transmission of binary data. The Physical layer concerned with the transmission of raw bits over a communication channel. When transmitting data over the network it has to be ensured that when one side sends a 1 bit, it is received by the other side as a 1 bit, not as a 0 bit depending on:
The physical layer also manages the network card's hardware interface to the network.
Fig. 9. Physical Layer top
Normally each layer adds a Header (which contain information that specifically addresses layer-to-layer communication) and a Trailer to its Data, which consists of the next higher layer's Header, Trailer and Data as it moves through the layers. As illustrated below the Transport Header (TH) contains information that only the Transport layer sees. All other layers below the Transport layer pass the Transport Header as part of their Data.
PDU - Protocol Data Unit (a fancy name for Layer Frame) top
OSI Model Functional Drawing
OSI Model Functional Drawing top
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