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Cheat Sheet

CCNA All-in-One Certification For Dummies

The CCNA exam requires mastery of the OSI reference model. The model is designed in seven functional layers. Each layer has a precise mission, and each layer works fairly independently of the upper and lower layers. Upper layers use the services provided by lower layers, but the internal workings of each layer are not visible to other layers.

OSI for CCNA Layer 7: Application

The CCNA exam requires knowledge of the OSI reference model's application layer. This layer represents the various network applications such as e-mail reader, Web browser, Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Network File System (NFS).

  • The application layer provides a user interface and processes network data.

  • The application layer on the sending host produces the network data to be transmitted from the sender host.

  • The application layer on the receiving host consumes the network data produced and transmitted by the sender host.

OSI for CCNA Layer 6: Presentation

As you prepare for the CCNA exam, the presentation layer is mostly concerned with data format. It converts the data between different formats so that both the sender and the receiver can use heterogeneous data. For example, mail messages contain various data formats: text, application attachments, video, audio, and graphical signature.

  • The presentation layer on the sending host receives the data payload from the application layer.

  • The presentation layer on the sending host converts the data into a format that is easily transportable over the network.

  • The presentation layer on the receiving host converts the data from the network format back to its native format that can be easily interpreted, used, and displayed by the application layer above.

OSI for CCNA Layer 5: Session

As a certified CCNA, you'll work in communications between applications. Some applications need to open logical communication channels between the computer hosts. Logical communication channels (sessions) maintain data about the communication established between the network application running on the sending host and the network application running on the receiving host. The session layer does the following:

  • Opens and maintains logical communication channels between network applications running on the sending host and network applications running on the receiving host.

  • Handles authentication: Some network applications use authentication mechanisms before they open a logical communication channel (session) with a remote host.

OSI for CCNA Layer 4: Transport

On the CCNA exam, you'll work with hosts. The transport layer manages the transport of data between two hosts communicating over a network. In a nutshell, the transport layer does the following:

  • Slices up the data to be transmitted into small chunks called data segments that can be easily sent over the network medium.

  • Reassembles the data in order on the receiving host:

    • Some transport protocols send the segments in order.

    • Other protocols, such as Transmission Control Protocol (TCP), send the data segments in any order and reassemble them in the correct order on the receiving host.

  • Provides some error-detection mechanisms.

OSI for CCNA Layer 3: Network

The CCNA exam covers data packets; the network layer routes data packets across networks that link the sending and the receiving host. In a nutshell, the network layer does the following:

  • Chooses the best route to send packets between hosts.

  • Assigns logical addresses to all devices in the network to be able to identify each source host and each destination host, as well as each network through which packets need to be routed.

    Logical addresses are assigned at the network protocol level. Physical addresses are assigned on a physical device, such as a network card.

  • Receives each data segment from the transport layer on the sending host and wraps it up in a data packet along with routing data. The packet is sent down to the data link layer to send it over the network physical medium.

  • On the receiving host, the network layer unwraps the packet received to extract the data segment and sends it up to the transport layer.

Several protocols operate at the network layer, such as IP, IPX, AppleTalk, and SNA, but the CCNA test is only concerned with IP.

OSI for CCNA Layer 2: Data Link

The CCNA exam has extensive coverage of data routing. Data routing is handled by the data link layer of the OSI. The data link layer does the following:

  • Transmits the data on the physical medium.

  • Routes the data locally on the physical network medium. The data link layer uses physical addresses assigned to each physical network device in the local network to route data from one physical device to another.

  • Receives each packet from the network layer on the sending host and wraps it up in a data frame along with local routing data.

  • Sends each data frame down to the physical layer to code an electrical or optical signal to transmit the data frame over a wire or over the air (wireless transmission).

  • On the receiving host, the data link layer unwraps the data frame received to extract the packet and sends it up to the network layer.

Cisco switches are Layer 2 (data link layer) devices.

OSI for CCNA Layer 1: Physical

The CCNA exam has extensive coverage of hardware. That's the physical layer. The physical layer provides the electrical, optical, or over-the-air connection between the sending host device and the receiving host device. This typically involves copper or fiber-optic cabling, or wireless radio connections, patch panels, signal repeaters, submarine cables, or satellites.

CCNA certification does not require you to be a space science expert. However, you do need to understand that data is always converted into bits that can be transmitted over a medium using electrical current or optical signals that simulate a 1 (signal) or a 0 (no signal).

In a nutshell, the physical layer defines mechanical, electrical, optical, radio, procedural, and functional standards to enable the transmission of data-link (Layer 2) frames over a certain transmission medium.

These standards define how a physical link is built, activated, maintained, and deactivated to enable transmissions between DTE (data terminal equipment) and DCE (data communications equipment).

DTEs are host devices. DCEs are network devices, that is, any device that stands between two host devices.

Most hubs are amplifying the electrical signal; therefore, they are really repeaters with several ports. Hubs and repeaters are Layer 1 (physical layer) devices.

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