**What is the Open Systems Interconnection (OSI) model?** The OSI model is *a conceptual model created by the ISO that enables diverse communication systems to communicate using standard protocols.* > **What is the Open Systems Interconnection (OSI) model based on?** > The OSI model is based on *the concept of splitting up a communication system into a stack of seven abstract layers.* > > **What are the seven layers of the Open Systems Interconnection (OSI) model in order from lowest to highest?** > The seven layers of the OSI model are: > 1. Physical layer - Transmits raw bit stream over the physical medium. > 2. Data link layer - Defines the format of data on the network. > 3. Network layer - Decides which physical path the data will take. > 4. Transport layer - Transmits data using transmission protocols including TCP and UDP. > 5. Session layer - Maintains connections and is responsible for controlling ports and sessions. > 6. Presentation layer - Ensures that data is in a usable format and is where data encryption occurs. > 7. Application - Human-computer interaction layer, where applications can access network services. ... # Why does the OSI model matter? **Does the modern Internet use the Open Systems Interconnection (OSI) model?** *No*, the modern Internet doesn't use the OSI model. > **What is the Open Systems Interconnection (OSI) model useful for and why?** > The OSI model is useful for *troubleshooting because isolating the problem to a specific layer avoids a lot of unnecessary work.* # 7. The application layer **What does the application layer do?** The application layer *provides the protocols and data manipulation that client software relies on.* > **Are client software applications a part of the application layer?** > *No*, client software applications aren't part of the application layer. > [!example] Examples of client software applications which rely on the application layer > * Web browsers. > * Email clients. > [!example] Examples of application layer protocols > * Hypertext Transport Protocol (HTTP). > * Simple Mail Transfer Protocol (SMTP). # 6. The presentation layer **What does the presentation layer do?** The presentation layer *prepares data so that it can be used by the application layer.* > **What three things does the presentation layer do to data?** > The three things that the presentation layer does to data are: > 1. Translation. > 2. Encryption. > 3. Compression. ... # 5. The session layer **What does the session layer do?** The session layer *ensures that sessions stay open long enough to transfer all the data being exchanged and then closed to avoid wasting resources.* > **What is a session?** > A session is *the time between when communication is opened and closed.* > > **What can the session layer do with large data transfers?** > With large data transfers, the session layer can *set checkpoints to resume from in case the session is interrupted.* # 4. The transport layer **What does the transport layer do?** The transport layer *takes data from the session layer and breaks it up into chunks to send to the network layer.* > **What are the chunks of data from the session layer called?** > The chunks of data from the session layer are called *segments.* > > **What does the receiving device do with segments?** > With segments, the receiving device *reassembles the segments into data that the session layer can consume.* > > **What two types of controls does the transport layer have over the transmission of data?** > The two types of controls that the transport layer has over the transmission of data are: > 1. Flow control - Ensuring the speed of transmission doesn't overwhelm the receiving device. > 2. Error control - Ensuring the data on the receiving device is complete and re-transmitting otherwise. > [!example] Examples of transport layer protocols > * Transmission Control Protocol (TCP). > * User Datagram Protocol (UDP). # 3. The network layer **What does the network layer do?** The network layer *facilitates data transfer between two different networks.* > **Is the network layer necessary if two devices are on the same network?** > *No*, the network layer is unnecessary if two devices are on the same network. > > **What does the network layer do with segments from the transport layer?** > With segments from the transport layer, the network layer *breaks them up into packets on the sender and reassembles them on the receiver.* > > **What is the process of finding the best physical path for data to reach the receiver?** > The process of finding the best physical path for data to reach the receiver is *routing.* > [!example] Examples of network layer protocols > * Internet Protocol (IP). > * Internet Control Message Protocol (ICMP). > * Internet Group Message Protocol (IGMP). > * IPsec suite. # 2. The data link layer **What does the data link layer do?** The data link layer *facilitates data transfer on the same network.* > **What does the data link layer do with packets from the network layer?** > With packets from the network layer, the data link layer *breaks them up into frames.* # 1. The physical layer **What is the physical layer comprised of?** The physical layer is comprised of *the physical equipment involved in data transfer.* > [!example] Examples of physical equipment in the physical layer > * Cables. > * Switches. # How data flows through the OSI model **How does comprehensible information need to be transferred over a network from one device to another?** Comprehensible information needs to be to be transferred over a network from one device to another *by travelling down the seven layers of the OSI model on the sender and up the seven layers on the receiver.* > [!example] Example of how comprehensible information is transferred over a network from one device to another using the OSI model > This is what it looks like whenever you send an email to your friend: > * The email software - You write the email and click the send button. > * Application layer - Since you're sending an email, the SMTP protocol is picked. > * Presentation layer - The raw data of the email is compressed. > * Session layer - A communication session is opened. > * Transportation layer - The compressed data of the email is broken up into segments. > * Network layer - The segments are broken up into packets. > * Data link layer - The packets are broken up into frames. > * Physical layer - The frames are converted to a bitstream and transmitted through a cable. > > On the receiving end: > * Physical layer - Your friend's computer receives a bitstream over Wi-Fi and converts it to frames. > * Data link layer - The frames are assembled into packets. > * Network layer - The packets are assembled into segments. > * Transportation layer - The segments are assembled into the compressed data of the email. > * Session layer - The communication session is closed. > * Presentation layer - The compressed data of the email is decompressed. > * Application layer - The raw data of the email is given to the email software. > * The email software - The email shows up in the inbox of your friend where it can be read.