OSI Model and TCP/IP Suite

• There are two networking models: OSI Model and TCP/IP Suite.
• TCP/IP Suite is a networking model, often referred to as TCP/IP.
• Networking models categorize and provide a structure for network protocols and standards.
• Network protocols are a set of rules defining how network devices and software operate.
• Protocols dictate the logical rules for device communication, separate from physical standards.
• The OSI Model aimed to standardize Network Communications.
• Though not currently in use, it significantly influenced network engineering thinking and is still referenced.

OSI Model

• Stands for 'Open Systems Interconnection'.
• It conceptually categorizes and standardizes network functions.
• It was created by the 'International Organization for Standardization' (ISO).
• Functions are divided into 7 layers that work together.

Layers – Explanations

• Layer 7 (Application Layer) is closest to the end-user and interacts with software applications like web browsers (e.g., Chrome, Firefox).
• HTTP and HTTPS are Layer 7 Protocols, with HTTPS indicating secure communication, as seen on websites like Cisco.com.
• Functions of Layer 7 include identifying communication partners and synchronizing communication.

OSI Model - Application Layer

• Two computers, each represented with an OSI model, illustrate data transfer.

• A software application interacts with a web browser at Layer 7, sending data to another computer.

• Data is transferred from Layer 7 down to Layer 1 in the sending computer and back up in the receiving computer.

• At each layer, additional data is added, a process called Encapsulation.

• This added information encapsulates the original data as it passes through each layer.

• Once the electrical signals reach the physical layer, the receiving computer performs the reverse process, called De-Encapsulation, removing the extra information layer by layer.

• Communication between Application Layers on two systems/computers is called Same-Layer Interaction.

• It’s the process that identifies partners for communication and synchronizes it.

• Layer 6 (Presentation Layer) translates data from "application format", understandable by applications, into a format suitable for network transmission.

• Encryption and decryption are functions of the Presentation Layer, ensuring only the intended recipient can read transmitted data.

• Layer 5 (Session Layer) manages dialogues or sessions between hosts, establishing, managing, and terminating connections.

• This layer is used between a local application and a remote application, such as a web browser and YouTube.

• YouTube's servers manage numerous user sessions, which is the role of the session layer in the OSI Model.

• The upper layers (application, presentation, session) are primarily the concern of application developers rather than network engineers, who use them to connect their applications to the network.

• Layer 4 (Transport Layer) prepares data for transmission over the network and receives a header.

• It segments large data into smaller pieces called segments for easier transmission.

• These segments minimize potential transmission problems, ensuring that if one fails, the entire transmission isn't lost.

• The transport layer represents host-to-host communication, known as end-to-end communication.

• Data entering the transport layer and receiving a header becomes a segment; segmented data receives a header for each segment.

• Layer 3 (Network Layer) adds another header to the data, creating connectivity between end hosts across different networks.

• It provides logical addressing (IP addressing) and path selection between source and destination, especially important in large networks like the internet.

• Routers operate at Layer 3.

• The segments from the transport layer are now referred to as packets at the network layer.

• Layer 2 (Data Link Layer) facilitates node-to-node connections and data transfer and is adjacent to Layer 1.

• Data link layer defines data formatting for transmission medium.

• Detects and corrects errors in the physical layer, using addresses for operation.

• Switches operate at Layer 2, using destination addresses to forward data.

Data encapsulation and transmission

• An L2 Trailer and L2 Header are added, and the data is now called a Frame.
• The data becomes ready for cable transmission, either as electrical signals or wireless signals (Wi-Fi).
• Layer 1 (Physical Model) defines the characteristics of the connection (cable or Wi-Fi) used for data transmission, including voltage levels, maximum transmission distances, physical connectors, and cable specifications. Digital bits are converted to electrical signals or radio signals, depending on the connection type.
• The complete frame is sent from the local device via Ethernet cable.
• Upon reaching the remote device, the reverse process of encapsulation, called de-encapsulation, begins.

Units of data transfer

• Segment: Term for layer 4 Protocol Data Unit (PDU).
• Packet: Term for layer 3 PDU.
• Bit: In layer 1 term for PDU.

TCP/IP Suite

• Like the OSI model, it is a set of communication protocols used on the internet and other networks.
• It is known as TCP/IP because TCP and IP are the two core protocols in this model.
• TCP/IP Suite has a similar structure to the OSI Model but with fewer layers. While the TCP/IP model is more widely implimented, the TCP/IP is still very useful.