1.0 Networking Concepts


1.1 Compare the layers of the OSI and TCP/IP models.

  • OSI model:

    The Open System Interconnect (OSI) reference model was developed by the International Standards Organization (ISO), to define a networking framework for implementing protocols necessary to transfer application data from one computer to a similar application on another computer. It consists of seven conceptual layers, each designed to address different network functions.

    Each network function is assigned to one or more adjacent layers and is somewhat independent of the other layers. Control is passed beginning with the application layer, proceeding through each layer until it reaches the physical layer. From there, it is passed across the network to the intended recipient and back up through the levels there. One layer does not need to know how an adjacent layer is executed, it simply needs to know how to communicate with it.


    Additional Reading
    7 Layers of OSI


    • Layer 1 - Physical

      The Physical Layer involves the actual transmission of signals over a medium from one computer to another. It defines the electrical and physical specifications for networking devices. This includes signal timing, voltage levels, cable specifications, the layout of pins, and maximum transmission length. Hubs, repeaters, and network adapters operate at this layer.


    • Layer 2 - Data link

      The Data Link Layer provides for the reliable transfer of data between network entities, including the physical addressing, network topology, error notification and correction, and flow control.

      Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer. Network topology consists of the data link layer specifications that often define how devices are to be physically connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.

      The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sublayers:

      • The Logical Link Control sublayer is primarily concerned with multiplexing protocols to be sent over Media Access Control (MAC) sublayer. The LLC does this by splitting up the data to be sent into smaller frames and adding descriptive information to these frames, called headers. The amount and type of information added is determined by the protocol used.
      • The Media Access Control (MAC) sublayer includes the MAC address. The MAC address provides this sublayer with a unique identifier so that each network access point can communicate with the network. The MAC sublayer is also responsible for the actual access to the network cable, or communication medium.

    • Layer 3 - Network
      The Network Layer provides the functional and procedural means of transferring data while maintaining the quality of service requested by the transport layer. The network layer performs network routing functions, fragmentation and reassembly, and reports delivery errors. Routers operate at this layer.

    • Layer 4 - Transport
      The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state- and connection-oriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred.

    • Layer 5 - Session
      The Session Layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures.

    • Layer 6 - Presentation
      The Presentation Layer establishes context between application-layer entities and transforms data into a form that the application accepts. This layer formats and encrypts data to be sent across a network.

    • Layer 7 - Application
      The Application Layer is the OSI layer closest to the end user. Both the OSI application layer and the user interact directly with the software application. Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication.

  • TCP/IP model:

    The TCP/IP model is a descriptive framework for the Internet Protocol Suite of computer network protocols. Also called the Internet Model or DoD Model, the TCP/IP Model consists of four layers that define the general design guidelines and implementations of specific networking protocols to enable computers to communicate over a network.


    • Network Interface Layer

      Layer 1 combines the Physical and Data layers and routes the data between devices on the same network. It also manages the exchange of data between the network and other devices.

      Also known as the Link Layer, this layer defines the networking methods within the scope of the local network link on which hosts communicate without intervening routers. This layer describes the protocols used to describe the local network topology and the interfaces needed to affect transmission of Internet Layer datagrams to next-neighbor hosts. (cf. the OSI Data Link Layer).


    • Internet Layer

      Layer 2 corresponds to the Network layer. The Internet Protocol (IP) uses the IP address, consisting of a Network Identifier and a Host Identifier, to determine the address of the device it is communicating with.

      The Internet Layer has the task of exchanging datagrams across network boundaries. It is therefore also referred to as the layer that establishes internetworking, indeed, it defines and establishes the Internet. 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.


    • Transport Layer

      Layer 3 corresponds to the OSI Transport layer, this is the part of the protocol stack where the Transport Control Protocol (TCP) can be found. TCP works by asking another device on the network if it is willing to accept information from the local device.

      The Transport Layer constitutes the networking regime between two network hosts, either on the local network or on remote networks separated by routers. The Transport Layer provides a uniform networking interface that hides the actual topology (layout) of the underlying network connections. This is where flow-control, error-correction, and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections between Internet hosts.


    • Application Layer

      Layer 4 combines the Session, Presentation and Application layers of the OSI model. Protocols for specific functions such as e-mail (Simple Mail Transfer Protocol, SMTP) and file transfer (File Transfer Protocol, FTP) reside at this level.

      The Application layer is the layer in which applications create user data and communicate this data to other processes or applications on another or the same host. The communications partners are often called peers. This is where the "higher level" protocols such as SMTP, FTP, SSH, HTTP, etc. operate.


      • (Also described as: Link Layer, Internet Layer, Transport Layer, Application Layer)

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