osi model

OSI MODEL

  • Model created to understand how data flows in a network

OSI vs TCP/IP




The Open Systems Interconnection (OSI) Reference Model

  • Is a conceptual framework or reference model that used to understand how network communication functions




  • Open Systems Interconnection (OSI) Model was published by International Organization for Standardization(ISO)in 1984 as standard ISO 7498

  • Protocol:

    •  A set of rules that governs data communication

    • It represents an agreement between thecommunicating devices




  • Divided into seven distinct layers, each layer has a specific set of functions and interacts with the layers above and below it.


  • The numbering here depends on if you are the sender or receiver. The sender is 7→1 and the receiver is 1→7

  • WE WILL FOCUS ON SENDER SIDE AS A REFERENCE POINT THEREFORE WE START WITH THE PHYSICAL LAYER

  • .

  • UPPER LAYER

    • The Application, Presentation and Session layers are also known as the Upper Layers.

  • LOWER LAYER

    • The Transport, Network, Data Link, and Physical are also known as the Lower Layers.



PNEUMONIC DEVICE WE CAN USE

  • “Please Do Not Throw Sausage Pizza Away.”

  • OR

  • P-DNT-SPA (Whoa! A P-shaped donut taking a spa )




Continuation of OSI Reference Model

  • The OSI model does not directly correspond to the actual implementation of network protocols but serves as a reference model for understanding how network communication works



  • The most referenced model in modern networking is the TCP/IP model, which has fewer layers and aligns more closely with the protocols used on the internet.

  • However, the OSI model remains a valuable conceptual framework for understanding how network communication, concepts and principles work.






The Open Systems Interconnection (OSI)

  • Defines seven (7) layers or levels in a complete communication system

  • Establishes a set of common rules that computers of disparate origins can exchange information

  • The layers represent segmentation of software responsibilities

  • Supporting software is embedded in each node to provide interface between layers



ADVANTAGES OF ADOPTING A LAYERED MODEL

  • Change - minimize impact on other layers

  • Design - specialize on one area

  • Learning - reduce complexity

  • Troubleshooting – facilitated by knowing purpose of each layer

  • Standards – guidelines * for interoperability

    • guideline and framework—not a rigid standard

OSI Layer Functions

Physical Layer

  • Transports Bits

  • Physical Layer: This is the lowest layer and deals with the physical transmission of data over the network medium. It defines characteristics such as the type of cables, connectors, and electrical voltages used for data transmission. The physical layer is responsible for the movements of individual bits from one hop (node) to the next.

    • The only data is digital

  • Function

    • Physical characteristics of interfaces and media

    • Representation & Synchronization of bits

    • Data rate

    • Line configuration (point-to-point or multipoint)

    • Physical topology (mesh, star, ring or bus)

    • Transmission mode (simplex, half-duplex or duplex)

    • Maximum transmission distances, voltage level



(Slide that isnt in canvas)

Physical Layer

•Physical characteristics if the interface and medium- The Physical layer defines the characteristics of the interface between the device and the transmission medium. It also defines the type of transmission medium.

• Representation of bits- The Physical layer data consists of a stream of bits with no interpretation. To be transmitted the bits must be encoded into signals- electrical or optical. The Physical Layer defines the type of encoding.

• Data Rate- The transmission rate- the number of bits sent each second- is also defined by the physical layer. The physical laye defines the duration of a bit, which is hw long it last.

• Synchronization of bits- The sender and receiver not inly must use the same bit rate but also must be synchronized at the bit level. The sender and the receiver clocks must be synchronized.

• Line Configuration- The physical layer is concerned with the connection of deivces to the media. In a point to point configuration, two device are connected through a dedicated link. In a multipoint configuration a link is shared among multile device.

• Physical Topology- The physical topology defines how devices ar connected to make a network.

Transmission mode- The physical layer also defines the direction of transmission between two devices:

simplex, half duplex and full duplex.



Data Link Layer

  • Transports frames

  • Data Link Layer focuses on the reliable transmission of data frames between two directly connected nodes. It also handles error detection and correction. This layer is responsible for addresses and organizing data into frames. The data link layer is responsible for moving frames from one hop (node) to the next.

  • Function

    • Framing

    • Physical addressing

    • Flow control

    • Error control

    • Access control




The Data Link layer consists of two sublayers:

1. LLC (Logical Link Control) sublayer

2. MAC (Media Access Control) sublayer



  • The role of the LLC sublayer is to control data flow among various applications and services, as well as provide acknowledgment and error notification mechanisms.



  • MAC sublayers, control access to the physical media for transport. It is also responsible for the physical addressing of frames.

  • Mac sublayers, determines hardware addresses




(Missing slide)

Data Link layer

* Framing: The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

• Physical addressing: If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame. If the frame is intended for a system outside the sender's network, the receiver address is the address of the device that connects the  network to the next one.Header makes sure the message goes to right place

* Flow control- If the rate at which the data are absorbed by receiver is less than the rate at which the data are produced in the sender, the data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

• Error Control- The data link layer adds reliability to the physical layer by adding mechanism to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames. Error control is normally achieved through a trailer added to the end of the frame. Trailer makes sure the message stays intact

• Access Control- When two or more devices are connected to the same link data link layer protocols are necessary to determine which device has control over the link at any given time.



Network Layer

  • Delivers Packets

  • The Network Layer is responsible for routing data between different networks. It handles logical addressing (e.g., IP addresses) and determines the best path for data to travel from the source to the destination across multiple networks. Routers and Layer 3 switches are examples of devices that operate at this layer.

  • The network layer is responsible for the delivery of individual packets from the source host to the destination host.(Transmits Packets) 

    • Responsible from the delivery of packets from the original source to the final destination(Best Path Determination)

    • Routed protocols are used to encapsulate data into packets.

    • Routing protocols are used to determine the best route

      • ex. OSPF,IGRP/EIGRP, RIP and BGP

  • It remembers the address of the SD and DH. it could be IP address




Network layer

• Logical Addressing: The addressing implemented by the data link layer handles the addressing problem locally.

• If a packet passes the network boundary, we need another addressing system to help distinguishing the source and destination systems. The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical address of the sender and receiver.

• Routing- When independent networks or links are connected to create internetworks or a large network, the connecting devices route or switch the packet to their final destination. Hop-to-hop delivery.




ROUTING PROTOCOLS

Routing protocols determine how your data gets to its destination and helps to make that routing process as smooth as possible



All routing protocols can be classified into the following:

1. Distance Vector or Link State Protocols

2. Interior Gateway Protocols (IGP) or Exterior Gateway Protocols (EGP)

3. Classful or Classless Protocols 



Router protocols include:

  • Routing Information Protocol (RIP)

  • Interior Gateway Protocol (IGRP)

  • Open Shortest Path First (OSPF)

  • Exterior Gateway Protocol (EGP)

  • Enhanced Interior Gateway Routing Protocol (EIGRP)

  • Border Gateway Protocol (BGP)

  • Intermediate System-to-Intermediate System (IS-IS)








  • Example: IP (Internet Protocol), ICMP (Internet Control Message Protocol).

  • Application: The network layer is responsible for routing data packets between different networks. It uses logical addressing (such as IP addresses) to determine the best path for data to travel.



The Internet Control Message Protocol (ICMP) is a network laye protocol used by network devices to diagnose network communication issues. ICMP is mainly used to determine whether or not data is reaching its intended destination in a timely manner. Commonly, the ICMP protocol is used on network devices, such as routers.








Difference between Unmanaged(Check 2nd layer) and managed switch:

  • IP Address and Routing protocols need to be configured in managed switch









  • No one uses hub anymore since it had limited space

  • Switch vs router

    • Switch is to transfer info within a network

    • Router ROUTES the packets between networks via the switches

Transport Layer 

  • Transmits SEGMENTS

  • The Transport Layer ensures end-to- end communication between devices. It manages flow control, error detection, and data segmentation.

  • The main purpose of this layers are;

    • making sure that the data is delivered error-free and in the correct sequence.

    • Establishes, maintains, and terminates virtual circuits.

    • Provides error detection and recovery.

    • Provides Flow Control and Windowing.

    • Segmentation and reassembly

Transmits Segments.



Example: TCP (Transmission Control Protocol),

UDP (User Datagram Protocol).













Data coming from the upper layers have port addresses j and k (j is the address of the sending process, and k is the address of the receiving process). Since the data size is larger than the network layer can handle, the data are split into two packets, each packet retaining the port addresses (j and k). Then in the network layer, network addresses (A and P) are added to each packet.





Examples of Transport Layer Protocols




Transmission Control Protocol

It is a connection-oriented protocol that helps in the exchange of messages between different devices over a network.

  • TCP keeps track of the segments being transmitted or received by assigning numbers to every single one of them.

  • Flow control limits the rate at which a sender transfers data. This is done to ensure reliable delivery.

  • TCP implements an error control mechanism for reliable data transfer.

  • TCP takes into account the level of congestion in the network.




Advantages of TCP

  • It is reliable for maintaining a connection between Sender and Receiver.

  • It is responsible for sending data in a particular sequence.

  • It allows and supports many routing protocols.

Disadvantages of TCP

  • It is slower than UDP and it takes more bandwidth.

  • Slower, the speed of data based on the speed of the receiver

  • It does not have a multicast or broadcast category.

  • It does not load the whole page if a single data of the page is missing.



User Datagram Protocol

A connectionless protocol. meaning there is no need to establish a connection before data transfer. The UDP helps to establish low-latency and loss-tolerating connections establish over the network. The UDP enables process-to-process communication.

  • Used for simple request-response communication when the size of data is less and hence there is lesser concern about flow and error control.

  • UDP is used for some routing update protocols like RIP(Routing Information Protocol).

  • Normally used for real-time applications which cannot tolerate uneven delays between sections of a received message.




Advantages of UDP

  • It does not require any connection for sending or receiving data.

  • Broadcast and Multicast are available in UDP.

  • UDP can operate on a large range of networks.

  • UDP has live and real-time data.

  • UDP can deliver data if all the components of the data are not complete.

Disadvantages of UDP

  • We cannot have any way to acknowledge the successful transfer of data.

  • UDP cannot have the mechanism to track the sequence of data.

  • UDP is connectionless, and due to this, it is unreliable to transfer data.

  • In case of a Collision, UDP packets are dropped by Routers in comparison to TCP.

  • UDP can drop packets in case of detection of errors.









Session Layer

  • Session Layer is responsible for establishing, maintaining, and terminating communication sessions between devices.

  • It manages session synchronization and handles issues like session checkpoints and recovery in case of network failures.



  • Transmit Data



Ex:

  • NFS(Network File System)

  • SQL(Structured Query Language)









Difference between session and transport

  • Session needs checkpoints (helps when its lots of info)

  • Need the checksum

Diff between presentation and Session

  • Presentation has semantics and syntax

  • By layer 6 it is understandable to us





Encapsulation Pill Photo

  • The Source and destination can actually be on either the header or trailer side





CHECKSUM ORDERS THE DATA AND IS A TYPE OF SYNCHRONIZATION