Lesson 2: Network Models
Comp 277: Data Communications and Networking
A network is a combination of hardware and software that sends data from one location to another.
2.1 Layers of Network
Sender Receiver
2.2 The OSI Model (Open Systems Interconnection)
Before the 1990's, the OSI dominated the data communications, however, the TCP/IP overtook because it was extensively tested on the Internet; the OSI model was never fully implemented.
In 1947, the International Standards Organization (ISO) was a multinational body dedicated to worldwide agreement on international standards; OSI was the standard introduced in 1970.
The purpose of the OSI model is to show to facilitate communication between different systems (open systems).
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Lesson 2: Network Models
Comp 277: Data Communications and Networking
The seven layer of the OSI Model
- Physical Layer
The physical layer coordinates the functions required to carry a bit stream over a physical medium. It is also responsible for the following:
-physical characteristics of interfaces and medium
-representation of bits (no interpretation)
-data or transmission rate – number of bits sent per second and how long it will last
-synchronization – sender and receiver clock must be synchronized
-line configuration (connection of devices to media)
-physical topology – how devices are connected to make a network (star, mesh, ring, etc.)
-transmission mode (simplex, duplex, half-duplex)
- Data Link Layer
The data link layer transforms the physical layer (a raw transmission facility) to a reliable link. It is also responsible for:
-framing (dividing the stream of bits received from the network layer into manageable data units called frames
-physical addressing-if the frames are for different systems on the network, the data link adds a header to the frame to define the sender and/or the receiver of the frame; if the frame is for outside the network, the receiver address is the address of the device that connects the network to the nest one
-flow control – if the rate at which data are absorbed by the receiver is less that the rate at which data are produced by the sender, the data link layer imposes data flow control to avoid overwhelming the receiver.
-error control – adds reliability to the physical layer; detect and retransmit damaged or lost frames; recognizes duplicate frames – this is done by adding a trailer at the end of the frame
-access control
- Network Layer
The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple links. This layer is responsible if systems are connected on different network, otherwise only data link is required. This layer is also responsible for:
-logical addressing (distinguishing the source and destination systems)
-routing – routes or switches the packets to their final destination
- Transport Layer
The transport layer is responsible for process-to-process delivery of the entire message;
-it ensures that the whole message arrives intact and in order addressing both error control and flow control at the source-to-destination level.
-service point addressing – delivery not only from one computer to the next but also from a specific process (running program) on the other; includes service point address (or port address).
-segmentation and reassembly – reassemble the message correctly upon arriving at the destination connection control.
-flow control – end-to-end control as opposed to single link control.
-Error control – process-to-process rather than across a single link; error correction is usually achieved through retransmission.
- Session Layer
Session layer is the network dialog controller.
-It establishes, maintains, and synchronizes the interaction among communicating systems.
-Dialog control allows two processes to take place in either half-duplex or full-duplex.
-Provides synchronization.
- Presentation Layer
Presentation layer is the syntax and semantics of the information. It is also responsible for:
-translation – changes the information from its sender-dependent format into common format and vice versa.
-Encryption – sender transforms the original information to another form and resends the message out over the network (opposite is decryption).
-Compression – reduces the number of bits in the information; important for transmission of multimedia such as text, audio and video.
- Application Layer
Application layer enables the user whether human of software to access the network, provide user interface. Specific services provided by application layer are:
-network virtual terminal –software version of a physical terminal which allows that user to log on to a remote host.
-File transfer, access and management – to access files in remote host to retrieve files to manage or to control files.
-Mail services – basis for email forwarding storage.
-Directory Services – distributed database sources to access for global information about various objects and entries.
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