CISSP OSI Model
The Open Systems Interconnection or OSI model is essentially a reference system that lays out how computers communicate with each other over a network. The model is used to describe each component in data communication so that rules and standards can be established regarding applications and network infrastructure. The OSI model contains seven layers that conceptually stack from the bottom to the top.
| Layer | OSI Model | Details |
| 7 | Application | Layer 7 is the layer most people are familiar with because it communicates directly with the user. An application that runs on a device might communicate with other OSI layers, but the interface runs on layer 7. For instance, an email client that transfers messages between client and server runs on layer 7. HTTP, SMTP, DNS |
| 6 | Presentation | The Presentation Layer represents the area that is independent of data representation at the application layer. In general, it represents the preparation or translation of application format to network format, or from network formatting to application format. Compression, Encryption, Character Encoding, File Formats |
| 5 | Session | This is the layer responsible for opening and closing communication between the two devices. The time between when the communication is opened and closed is known as the session. Managing dialog, SQL, RPC |
| 4 | Transport | The transport layer manages the delivery and error checking of data packets. It regulates the size, sequencing, and ultimately the transfer of data between systems and hosts. Segments, TCP, UDP, TLS, SSL, SCTP, DCCP |
| 3 | Network | The network layer is responsible for breaking up the data on the sender’s device and reassembling it on the recipient’s device when the transmission is across two different networks. When communicating within the same network, the network layer is unnecessary. Datagrams/Packets, Routers, Layer 3 Switches, IPSec |
| 2 | Data Link | The data link layer is very similar to the network layer, except the data link layer facilitates data transfer between two devices on the same network. The data link layer takes packets from the network layer and breaks them into smaller pieces called frames. Frames, Hubs, Switches, ATM, Frame-Relay, PPTP, L2TP |
| 1 | Physical | At the bottom of our OSI model we have the Physical Layer, which represents the electrical and physical representation of the system. This can include everything from the cable type, radio frequency link (as in a Wi-Fi network), as well as the layout of pins, voltages, and other physical requirements. Bits, Cables, Hardware signals |
TCP/IP Model
We have just covered the OSI model which was developed to describe the functions of Network communications by dividing the communication procedure into smaller and simpler components. The TCP/IP model is a concise version of the OSI model. It contains four layers, unlike the seven layers in the OSI model. TCP/IP is the conceptual model and set of communications protocols used on the Internet and similar computer networks. It is commonly known as TCP/IP because the foundation protocols in the suite are the Transmission Control Protocol (TCP) and the Internet Protocol (IP). This model is divided into 4 layers:
- Application layer is where applications or processes create user data and communicate the data to other applications. The application layer is the OSI layer, which is closest to the end-user. The applications make use of the services provided by the underlying lower layers, especially the transport layer which provides reliable or unreliable pipes to other processes. TCP/IP application layer protocols provide services to the application software running on a Computer Like a Web browser. The application layer does not define the application itself, but it defines services that applications need. For example, application protocol HTTP defines how web browsers can pull the contents of a web page from a web server. In short, the application layer provides.
- Transport layer defines protocols for setting up the level of transmission service for applications. This layer is responsible for reliable transmission of data and the error-free delivery of packets. The transport layer performs host-to-host communications on either the same or different hosts and on either the local network or remote networks separated by routers.
- Internet layer, also known as the network layer, controls the flow and routing of traffic to ensure data is sent speedily and accurately. This layer is also responsible for reassembling the data packet at its destination. This layer defines the addressing and routing structures used for the TCP/IP protocol suite. The primary protocol in this scope is the Internet Protocol, which defines IP addresses. Its function in routing is to transport datagrams to the next IP router that has the connectivity to a network closer to the final data destination.
- Data Link layer/ Network Access Layer defines the networking methods within the scope of the local network link on which hosts communicate without intervening routers. This layer includes the protocols used to describe the local network topology and the interfaces needed to effect transmission of Internet layer datagrams to next-neighbor hosts. This also covers ethernet cables, wireless networks, network interface cards, device drivers in your computer, and so on.
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