Data communications system components

 

OSI

·         OSI stands for Open Systems Interconnection

·         Created by International Standards Organization (ISO)

·         Was created as a framework and reference model to explain how different networking technologies work together and interact

·         Each layer has specific functions it is responsible for

·         All layers work together in the correct order to move data around a network

 Physical Layer 

The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to Occur.

 The physical layer is also concerned with the following:

 o Physical characteristics of interfaces and medium. The physical layer defines the characteristics of the interface between the devices and the transmission medium. It also defines the type of transmission medium. 

 o Representation of bits. The physical layer data consists of a stream of bits (sequence of Os or 1s) with no interpretation. To be transmitted, bits must be encoded into signals--electrical or optical. The physical layer defines the type of encoding (how Os and Is are changed to signals).

Data rate. The transmission rate-the number of bits sent each second-is also defined by the physical layer. In other words, the physical layer defines the duration of a bit, which is how long it lasts. 

 o Synchronization of bits. The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized. 

 o Line configuration. The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration, a link is shared among several devices. 

 o Physical topology. The physical topology defines how devices are connected to make a network. Devices can be connected by using a mesh topology (every device is connected to every other device), a star topology (devices are connected through a central device), a ring topology (each device is connected to the next, forming a ring), a bus topology (every device is on a common link), or a hybrid topology (this is a combination of two or more topologies).

 o Transmission mode. The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex. In simplex mode, only one device can send; the other can only receive. The simplex mode is a one-way communication. In the half-duplex mode, two devices can send and receive, but not at the same time. In a full-duplex (or simply duplex) mode, two devices can send and receive at the same time. 

 Data Link Layer 

 The datalink layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer). 

 Other responsibilities of the datalink layer include the following:

Framing. The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

Physical addressing. If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the network to the next one.

Flow control. If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and re-transmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame.

Access control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

 

 

 Network Layer-

 

The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links).

• Logical addressing. 
• Routing. 

Transport Layer

 

The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on a host.

•     Service-point addressing.
•     Segmentation and reassembly.
•     Connection control.
•     Flow control.
•     Error control.  

Session Layer

•    Dialog control. 
•    Synchronization.  

 

Presentation Layer

 

•     Translation.
•     Encryption. 
•     Compression

 

Application Layer

 

•     Network virtual terminal.
•     File transfer, access, and management. 
•     Mail services. 
•     Directory services.