Introduction
Nowadays civilization can’t imagine its life without technology: any business won’t survive without an external storage and a proper software; any person won’t be able to achieve a success without a computer. However, to organize and grow any business or an organization, a network is required. A purpose of a network is the connection of several devices and the traffic flow over this connection. As anything, networking has its own advantages and disadvantages.
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One of the main advantages of networking is sharing with other computers within the current network: either the information (files, etc.), or devices (printers, scanners, etc.). Other prone of Networking is that software license costs can be reduces because only one licensed software can be installed to all computers. Otherwise, any standalone machine requires a license. Another very important advantage is that data can be easily backed up because it stored at a server database, while on a standalone computer it can be just lost. Besides all of the above, networking eases communication between people in the organization by using internet or some internal instant massagers or task setting programs.
However, there are some disadvantages too. At first, purchasing media cables and all networking equipment, including servers, is very expensive. At second, any network requires a qualified network administrator. If a network is big, it becomes more complicated to manage it because it requires a whole department of IT personnel who needs to be trained periodically. Also, there is always a risk of viruses or hacking that can harm the whole network. Security procedures, like a firewall, are needed to prevent it.
In the current term project I will review Networking Standards and OSI layers.
Networking Standards (OSI Layers)
In order for the any network to function some kind of a standard was needed. For this reason, in 1979 the International Organization for Standardization (ISO) invented and implemented the Open Systems Interconnect OSI Reference Model to “define and categorize the functions that all networks, regardless of vendor, provide”. (Mark L. Van). In other words, it is a standard model for communication of two end users of any network. Along with this model, communication rules and protocols were created to make this model work properly.
OSI Model consists of seven layers, which I will talk about in details in the current project. Each layer has its own purpose and performs as a separated function. There are several advantages of breaking the Model into smaller pieces. First of all, network problems can be easier be solved by identifying the layer where the issue accrued. Also, new protocols and other network services are easier to add to a layered architecture.
As I mentioned above, the OSI Model has seven layers. There are:
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Layer 7: Application
The application layer is the top layer in the model and it is responsible for applications used by the end user. For example, resource sharing, remote file and printer access, email, network management. Also, this layer provides application access security checking and information validation.
According to Tan Ten Hong and his book “Comparing OSI and TCP/IP.”, the application layer has next functions:
File Transfer, Access and Management (FTAM): Provides handling services in the network, including the movement of files between different systems, reading, writing and deletion of remote files, and management of remote file storage.
Virtual Terminal (VT): Provides services to access applications in different remote computer systems through stimulating a real terminal.
Electronic Mail and Messaging Handling (MHS): Facilitates the electronic exchange of documents.
Directory Services (DS): Provides services with the ability to match names with addressing information.
Common management Information Protocol (CMIP): Provides services for network management.
There are a large number of application layer protocols that are familiar to the common Internet user, including the File Transfer Protocol (FTP), Domain Name Service (DNS), Hypertext Transfer Protocol (HTTP) and Simple Mail Transfer Protocol (SMTP).
Layer 6: Presentation
The Presentation layer is responsible for the way data is presented to the application.
Main functions of the presentation layer are:
Checking syntax and semantics of the information transmitted
Character code translation (i.e. ASCII vs. EBCDIC vs. Unicode)
Provides common communication services such as encryption, text compression, and reformatting
Common formats associated with the Presentation Layer are: Moving Picture Experts Group Audio layer 3 (MP3), Moving picture Experts Group Audio layer4 (MP4), Moving Picture Experts Group (MPEG), Joint Photographic Experts Group (JPEG), Portable Network Graphics (PNG), Tagged Image File Format (TIFF).
Layer 5: Session
Session’s basic purpose is to provide the ability to the presentation entities to organize the communication for multiple communication sessions taking place at the same time.
Main functions of the Session Layer are (Alani):
The session layer is responsible for starting the sessions between the communicating entities. And when the session is over, the session layer is responsible for the release of the communication. Data transfer takes place in between.
Token management.
Session-connection to transport-connection mapping (in connection-oriented transfer only)
Common protocols associated with a Session layer are ADSP, ISNS, Net BIOS, CHAP.
Layer 4: Transport
Transport layer is not considered as an upper layer of the Model like layers 7-5. Transport layer is between upper and lower layers.
There are several functions of a Transport Layer (Alani):
Initiation of the connection between the communicating entities and release the connection when the data transfer is over.
Sequence control of data transferred to guarantee that the data arrives in the same sequence in which it was sent.
Segmentation: data is transformed into segments at the sender and reconstructed at the recipient.
Error detection in segments and attempt to recover these errors to their original error-free form.
Speaking about the layer 4 it is necessary to mention that data can be transferred through reliable (TCP) and unreliable (UDP) protocols. TCP provides data segmentation, flow control, windowing and acknowledgement, error checking and error correction. While UDP does not provide error correction or flow control. However, UDP allows huge amount of data to get transferred very fast in extremely high speed.
Layer 3: Network
A main purpose of the Layer 3 is routing of the network traffic. It is very important because it is routing data from one network to another by making logical decisions and controlling the subnet.
Besides routing, main function of the Layer 3 are (Tan Ten Hong):
provides logical addressing (IP Address)
Network connection and multiplexing: provides network connections between transport-layer entities by employing the data-link connections available.
Segmentation (Fragmentation) and blocking: this layer makes fragments of data smaller to facilitate the transfer.
Error detection and recovery
Sequencing and flow control
Creates Packets which is sent through the routers
The most common network layer protocol is IP, but other commonly used protocols include the Internet Control Message Protocol (ICMP) and Internet Group Message Protocol (IGMP).
Layer 2: Data Link
The main task of the data link layer is to take a raw transmission and transform it into a line that appears free of transmission errors in the network layer. It accomplishes this task by having the sender break the input data up into data frames, transmit the frames sequentially, and process the acknowledgment frames sent back by the receiver (Sequeira).
Data-link layer has many complex functions as compared to other layers. Datalink layer provides different type of functions for connection-oriented and connectionless communications.
Functions for connected and wireless communications of this Layer are (Alani):
Control of data-circuit interconnection
Data Encapsulation
Identification and parameter exchange
Creation of Frames, MAC address
Error detection using CRC(Cyclical Redundancy Check)
The most familiar standards used at the data link layer include IEEE 802.3 (Ethernet) Media Access Control (MAC) and Logical Link Control (LLC) sublayers. The LLC acts as an interface between the physical layer and the MAC sublayer, and the MAC sublayer provides the ability for multiple terminals (computers) to communicate over the same physical medium. Other standards examples include Asynchronous Transfer Mode (ATM), High-Level Data Link Control (HDLC), Frame Relay and the Point to Point Protocol (PPP).
Layer 1: Physical
Layer 1 of the OSI model is named the physical layer because it is responsible for the transmission and reception of wire level data. Basically, it is responsible for moving bits over the medium.
Physical layer protocols vary depending on the type of the physical medium and the type of the signal carried on it. The signal can be an electrical voltage carried over a cable, a light signal carried through a fiber link, or even an electromagnetic signal carried in the air on in the outer space.
Conclusion
Understanding OSI Model is very important if the one want to succeed in Networking and Telecommunications. Main points that are necessary to know about it is that the model consists of the 7 layers and each of them are essential in transferring data from the Source to the Destination.
In the conclusion I want to point out some advantages and disadvantages of a wired and a wireless connection.
The main advantage of the wired connection is that it is secured. It means that chances are really high of receiving the same data that was sent from the source. Also, it provides data segmentation, flow control, windowing and acknowledgement, error checking and error correction. Besides that, a transfer speed is usually faster that wireless. Wired networks are widely used in business and educational institutions. The main disadvantage is that the needs to set at one place and movements with a connected device are limited. Also, lots of media are needed to set it up, which can be quite expensive.
The main advantage of the wireless connection is that it can be accessed from any place within the range. Also less equipment is needed which makes it less complicated to set up. However, upgrading a wireless network can be difficult and expensive. The huge minus is that wireless connection is not secured as a wired one. . It does not provide error correction or flow control. Also, both bandwidth and information can sometimes be accessed.
Bibliography
Alani, M. M. (2016, August 6). Guide to OSI and TCP/IP Models. Retrieved from http://www.springer.com/978-3-319-05151-2
Mitchell, B. (n.d.). Basic Network Design – The OSI Model. Retrieved from http://compnetworking.about.com/library/weekly/aa052800a.htm
Pearson IT. (n.d.). OSI and TCP/IP Model Layers |. Retrieved from http://www.pearsonitcertification.com/articles/article.aspx?p=1804869
Rami, F. (2011, April 09). Computer Networks: The OSI Reference Model. Retrieved from http://www.rad.com/networks/1994/osi/intro.htm
Tan, H. T. (2011, July 31). Comparing OSI and TCP/IP. Retrieved from http://members.tripodasia.com.sg/osi/home.htm
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