OSI (Open Systems Interconnection) Model


The OSI (Open Systems Interconnection) model is a helpful tool to help frame discussions and troubleshooting of a wide range of technologies and protocols. It provides the basis for understanding Ethernet, the many ways a computer can communicate, understanding the differences between hubs, switches, and routers, and provides a detailed description of how WAN technologies operate. By separating the functions of data communications into 7 groups or layers, the OSI model defines standards for how physical transmission of media is created and sent, methods of determining the correct data flow rate, and provides instructions for when to send and when not to send data. Each layer of the OSI model represents various protocols. Layer 7 (application)- SSH, Telnet, FTP. Layer 6 (presentation)- SNMP, SMTP, HTTP. Layer 5 (session)- NETBIOS, Named Pipes, RPC. Layer 4 (transport)- UDP, TCP. Layer 3 (network)- IP. Layer 2 (data link)- Ethernet. And finally Layer 1 (physical)- Cat-5.
The Transmission Control Protocol (TCP) provides the means for connection-oriented communication in networks that utilizes the TCP/IP protocol suite. It is the most common type of session of a typical network and is designed to check for errors. The TCP/IP model defines 4 layers instead of the 7 of the OSI model to describe designs and implementations of the individual protocols for network communication. TCP is very reliable and guarantees that delivery packets will be uncorrupted by controlling the session via flow control, congestion control, and re-transmission of lost packets. TCP is slower than UDP however due to the additional overhead and processing time it needs. This is due to TCP’s optimization for accurate delivery rather than timely delivery. TCP should be used when no packet loss is a requirement as well as having the data free from errors. Some instances that TCP would be useful are viewing web pages, emails, and networking communication.
TCP is highly needed because having errors in anything we do is not acceptable. For the TCP/IP model, each of the 4 layers represents specific protocols. Application Layer (layer 1)- HTTP, POP3, Telnet, SMTP, DNS, TFTP, FTP. Transport Layer (layer 2)- UDP, RTP, TCP. Internet Layer (layer 3)- ICMP, ARP, RARP, IP. Network Interface Layer (layer 4)- Token Ring, FDDL, Ethernet, RS-232.
Apart from the fact that knowing the OSI and TCP/IP model is crucial for exams and certifications, they provide a detailed description of what happens at every step (every layer) of computer communication. They are also accommodating when troubleshooting issues as one can see what is happening at each level of the process. Although these models are outdated, they provide the general outlook and functionality of the entire communication process and divides each step into a specific layer.
TCP and UDP Differences. Think Like a Computer. Web. 09 Mar 2011. 19 Jan 2017.
Meyers. M. (2015). CompTIA Network Certification Exam Guide: Exam N10-006. New York: McGraw Hill Education.
Why Learning the OSI Model Is Important. MC MCSE. Retrieved from

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