Lesson 2.docx

**What is OSI?** The **"OSI Reference Model" --** also known as ***Open Systems Interconnection*** is a universal standard for networking formulated. The **OSI model** was initially developed around the 1970s. It has set the standards of network communications between different network types like...

**What is OSI?** The **"OSI Reference Model" --** also known as ***Open Systems Interconnection*** is a universal standard for networking formulated. The **OSI model** was initially developed around the 1970s. It has set the standards of network communications between different network types like LANs, MANs, WANs and others. The OSI also standardizes the quality of network related hardware devices and soft wares. The reference model was founded and continuously governed by the following organizations: **Brief Introduction on the organizations who have founded the OSI model.** The **ISO** "is a non-governmental worldwide federation of national standards bodies" (techtarget.com). Its goal in relation to computer networking is to ensure that communication and proper networking standards are being met, it also had greatly contributed with the development of network protocol standards. **IEEE** is a type of international organization of professionals whose focus is on the advancement of technology that is beneficial to human society (ieee.org) **ANSI** is a private, non-profit organization that monitors standards for US enterprises, government agencies and even international groups to ensure a safe and good quality products or service. It conducts quality checks from various types of products from bicycle helmets to network related equipment's. (ansi.org) **ITU** is a type of organization classified as a United Nations Agency exclusively dedicated ensuring safety and quality of information and communications technology products and services. It sets standards for computer related hardware and software like routers, e-mail standards, wired and wireless media and other. **The features of OSI model will greatly help you to assess the following:** - How to choose the proper network equipment suited to your needs - How to be able to provide effective and proper network designs - How to choose right design of equipment's that will be compatible with other networks - How to do network related troubleshooting Each individual layer is called a "**stack".** "The layers can be called by its actual layer name or by its number stack in the reference model". **Example:** Layer 7 or Application Layer The bottom part of the "OSI layer" is in charged with the constructing of frames and packet transmission of signals. It also handles the physical network equipment such as network cables and network devices. The middle layer on the other hand is the one in charge with coordination of and communication between nodes. It ensures that network communication session will be uninterruptable or without errors. The Top layers are the ones responsible for direct communication with software applications and data presentation, like encryption, data management and operating systems functions. Together, the set of layers is called a stack. **1. Physical Layer --** is located at the bottom part of the OSI layer. It is also known as "**Layer 1"**. The hardware used within this stack are responsible for initiating, transmission, and detection of "voltage" crucial with data transfer and receiving process. Network signals can either be "analog" or "digital" (Jill West, 2018)**.** **The physical layer encompasses the following responsibilities:** **There are two types of network signals, Digital and Analog.** **Analog signal --** is classified as a continuous signal with varying wave length patterns. It is measured by a positive and negative voltage levels. ***Examples:*** *Natural Human Voice, Light waves, radio* and *telephone signals.* **Digital signal -** also uses a uses voltage levels but the difference is that digital can only generate binary of ones and zeros. It is basically a preferred method used on most LANs and high-speed WANs.  **There are two types of signal interference namely, electromagnetic interference and radiofrequency interference.** - An **Electromagnetic interference (EMI)** -- occurs when magnetic force fields have been accidentally yielded by electrical devices such as fans, elevator motors, portable heaters, and air-conditioning units. - **Radio frequency interference (RFI)** -- occurs there is a disturbance with electrical devices that emits radio waves such as radio, television, substandard built computers or even TV equipment. **2. Data Link Layer --** is the second layer on the OSI and also located at the bottom part. This layer is also accountable for dealing with data transmission errors. It ensures a regular flow of data and its movement in and out of the physical layer. The data link layer is mostly implemented in the network adapters and network devices like routers. The Data link layer also checks for data transmission state if it will be duplicated, incorrect or just partially receive. If an error occurs, the data link will request for a re transmission of data. It is able to check for errors because of the use cyclic redundancy check. A ***cyclic redundancy check*** (CRC) is type of an error-detection method. The "***Logical link control*** (LLC)" and "***Media access control*** (MAC)" are the two important sublayers of this layer. **The LLC sublayer** has the responsibility for monitoring and managing flow control, error control, frames synchronization and avoiding network traffics **The MAC sublayer** is the one who checks on the "logical address of a network device". A MAC address is a unique coded hexadecimal number like 0004AC8428DE usually located into a chip or board on the "network interface" of the devices. (0004AC8428DE could also be formatted as 00-04-AC-84-28-DE or as 00:04:AC:84:28:DE). MAC addresses are not some randomly arranged hexadecimal numbers they follow a pattern too. The first half part of the address is assigned to a specific network vendor, to distinguish who manufactured the network device or its interface, and while the second half, typically supplied by the vendor, is unique to the interface or the device. - Layer purpose: "**transforms bits and format into frames"** - **Frame**: refers to "unit of data on a network" **3. Network Layer --** is the third layer or the layer 3 on the OSI. This layer is assigned with the regulation and monitoring of passage of packets on the routes on a network. - Layer purpose: "**regulates and controls passage of packets along routes on the network"** - Contains physical routes: cable and wireless pat - Contains Logical routes: software paths The Network layer also acts a traffic director. It sets an itinerary for packets and looks for efficient different paths for packets. The Network layer uses a a metrics in gathering information about the location of different network and node, this process is known as discovery. A packet is a discrete unit of data that is formatted as a signal for transmission over a network. **4. Transport Layer --** is the fourth layer or the layer 4 on the OSI. It is located on the middle part of the "OSI reference model". It is a responsible for delivering data from the sending node to its destination node. It ensures that data is received by the target node. - Layer purpose: **reliable data transmission** - Fragments messages into smaller units **5. Session Layer-** also known as Layer 5 in the OSI layer. Session layer is considered to be responsible for building and maintaining of proper communication between nodes. This layer can also identify the duration of transmission and recovery from transmission errors of the node. for example, it establishes which node transmits first. The Session layer also determines how long a node can transmit and how to recover from transmission errors. If a transmission session is inadvertently broken at a lower layer, the Session layer attempts to reestablish the communication. **A session layer has two ways of sending data** - "**Two-way alternate mode (TWA)"** or also known as half-duplex is the one who controls the node that is being set up separately to send and receive. In a half-duplex setting, there is only one lane for the data to send and receive, meaning the send and receive process cannot be done simultaneously. - Analogize: use of a Walkie Talkie - "**Two-way simultaneous (TWS**)" also known as full duplex, in which nodes can be configured to transmit data and receive data simultaneously. The data can simultaneously process the action of sending and receiving at the same time. This process is more efficient that the TWA. - Used in full-duplex communications **6. Presentation Layer --** this is also known as Layer 6 in the OSI reference model. This layer manages all the system software applications and operating systems processes. - Primary purpose: **it's governs the operating systems and monitors its task like data formatting, syntax checking, etc.** - "EBCDIC (Extended Binary Coded Decimal Interchange Code)" is an "8-bit coding method for 256-character set" that is mainly used by IBM computers and other older computers. - "ASCII (American Standard Code for Information Interchange)" is an "8-bit character coding method" that supports "128 characters" mainly used by computers with operating systems running UNIX/Linux, Windows 7 or Mac OS X **7. Application Layer --** is the top layer of the "OSI model". This layer manages the computer user's most direct access to "user applications and network services". This layer also manages the process of remote access control of files and printers; it also manages message handling for emails, file transfer and management. - Layer purpose: "**Govern the user's most direct access to applications and network services"** **How Communication between Stacks happens** - Let's look up at the sample analogy of two computers. - The OSI model regulates standards for communicating on a LAN and event multiple LANs. It also manages the process of internetworking between different types of networks, example: WANs and LANs afterwards it constructs a message at the client workstation. The Message is created at Application layer then travels all the down to the Physical Layer. The data is being added each time from the top layer down to the physical layer. **Sample Scenario of how the OSI Model is imposed** **Setting**: local server located at the MIS office was being accessed by a workstation from CICT department 1. "Redirector at Application layer locates shared drive" 2. "Presentation layer ensures data format is ASCII code" 3. "Session layer establishes and maintains link" 4. "Transport layer monitors transmission/reception errors" 5. "Network layer routes packet along shortest path" 6. "Data Link layer formats frames, verifies address" 7. "Physical layer converts data to electrical signal"  **ADDITIONAL NOTES:** - The 7-layer OSI model is the "foundation of LAN and WAN communications" - Bottom layers: are in charge with data connectivity, frame formation, encoding, signal transmission and conversion of digital to analog signal and analog to digital signal - Middle layers: establish and maintain sessions (network traffic control) - Upper layers: presentation of data, data encryption (visible applications and security) - Information is transported over LANs by using a LAN transmission or access method.