Reference Models The two important network reference models are briefly discussed below; OSI Reference Model and TCP/IP Reference Model ...
The two important network reference models are briefly discussed below; OSI Reference Model and TCP/IP Reference Model
A. OSI (Open Systems Interconnection) Reference Model: The OSI model is based on a proposal developed by the ISO as a 1st step towards international standardization of the protocols used in the different layers. It is also called the ISO (International Standard Organization) OSI Reference model as it deals with connecting open systems i.e. systems which are open for communication with other systems (Wetherall, 2018). There are seven layers in OSI model as explained below;
i.Layer 1 (Physical): The physical layer transmits raw bits through a communication channel. It simply provides the hardware to send and receive data on a carrier, including cables, cards and physical aspects. Protocols with physical layer components are fast Ethernet, RS232 and ATM. e.g.; Ethernet, RJ45, and so on.
ii. Layer 2 (Data Link): It provides knowledge and management of the transmission protocol and handles physical layer, flow control and frame synchronization errors. The layer of the data link is divided into two sub-layers: Media Access Control (MAC) and Logical Link Control (LLC). The sub-layer of the MAC controls how a network computer accesses the data and allows it to be transmitted. The LLC layer controls the synchronization of frames, flow control and error control. Examples, IEEE 802.5 / 802.2, IEEE 802.3 / 802.2, and so on.
iii. Layer 3 (Network): The network layer provides switching and routing technologies for the transmission of data from node to node by creating logical paths known as virtual circuits. This layer functions as well as addressing, internet work, error handling, congestion control and packet sequencing are routing and forwarding. e.g.; AppleTalk, IP, IPX, etc.
iv. Layer 4 (Transport): Transport layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. Examples include SPX, TCP, UDP, etc.
v. Layer 5 (Session): This layer maintains connections between applications, manages and terminates them. The session layer sets up conversations, exchanges and dialogs between the applications at each end, coordinates and ends. It competes with coordination of sessions and connections. Examples are NFS, names of NetBIOS, RPC, SQL, etc.
vi. Layer 6 (Presentation): This layer independently translates from application to network format and vice versa from differences in data representation (e.g. encryption). The presentation layer transforms the data into a form that can be accepted by the application layer. This layer formats and encrypts data to be sent over a network, so that compatibility issues are free. The syntax layer is sometimes called. Examples, encryption, MPEG, MIDI, ASCII, TIFF, GIF, etc.
vii. Layer 7 (Application): Application layer supports processes for applications and end users. Communication partners are recognized, service quality is identified, user authentication and privacy are taken into account and any data syntax constraints are identified. Everything in this layer is specific to the application. This layer provides file transfer, e-mail and other network software services with application services. Telnet and FTP are entirely applications at the application level. Examples, WWW browsers, NFS, Telnet, HTTP, FTP, etc.
B. TCP/IP Reference Model: Transmission Control Protocol/Internet Protocol (TCP/IP) is one of the most important protocol of networking. It is a set of networking protocols that allows two or more computers to communicate. It was developed by ARPANET (Advanced/ American Research Project Agency Network) and is managed by IANA (Internet Assign Number/ Network Authority). The various layers of TCP/IP is explained below;
i. Link Layer: Link layer is the combination of Data Link layer and Physical Layer of OSI model. It looks out for hardware addressing and the protocols present in this layer allows for the physical transmission of data. It is not really a layer at all, but rather an interface between hosts and transmission links.
ii. Internet Layer: This layer holds the functions of OSI’s Network layer. It defines and delivers official packet format and protocol called Internet Protocol (IP) along with companion protocol called ICMP (Internet Control Message Protocol) which helps it function. The Internet Protocol has two versions IPV4 and IPV6. IPV4 is currently most used by websites whereas IPV6 is growing and enhancing as the number of IPV4 address are limited in number when comparing the numbers of users.
iii. Transport Layer: This layer is designed to allow peer entities on the source and destination hosts to carry on conversation, just as in the OSI transport layer. The two main protocols are defined in this layer are; TCP (Transmission Control Protocol) which serves reliable and error free connection between the systems in the internet. It segments and sequences the data. And the other protocol is UDP (User Datagram Protocol) which is unreliable, connectionless and do not want TCP's sequencing and wish to serve their own. It is mainly used as transmitting speech or video.
iv. Application Layer: This layer includes all the higher level protocols and also performs the functions of three layers of OSI model i.e. Application Presentation and Session Layer. Some important protocols included in this layer are: HTTP, HTTPS, FTP, Telnet, SSH, SMTP, SNMP, NTP, DNS, RTP, etc. The Doman Name System (DNS) is used for mapping host names onto their network addresses, HTTP (Hypertext transfer protocol) used for fetching pages on World Wide Web. HTTPS means HTTP Secured, it is the combination of HTTP with SSL (Secured Socket Layer). SSL provides efficiency while browsing sensitive authentication, sign in and during bank transactions. SSH (Secured Shell) uses a username/ password authentication to establish a secured connection over an unsecured network. SSH is more about network tunneling while SSL is more about certificates. Likewise, RTP (Real-time transport protocol) is used for delivering real-time media such as movies or voices.
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