Between which pair of Open System Interconnection (OSI) Reference Model layers are routers used as a communications device?
Correct Answer: C
According to the CISSP CBK Official Study Guide1, routers are used as a communications device between the network and session layers of the Open System Interconnection (OSI) Reference Model. The OSI Reference Model is a conceptual framework or a standard that defines the functions and processes of the communication or the networking system, which consists of seven layers, which are:
* Physical: The layer that defines the physical or the tangible aspects of the communication or the networking system, such as the cables, connectors, or signals that transmit or receive the data or the information.
* Data-Link: The layer that defines the logical or the intangible aspects of the communication or the networking system, such as the frames, addresses, or protocols that organize or control the data or the information.
* Network: The layer that defines the routing or the forwarding aspects of the communication or the networking system, such as the packets, addresses, or protocols that determine or direct the path or the route of the data or the information.
* Transport: The layer that defines the reliability or the quality aspects of the communication or the networking system, such as the segments, ports, or protocols that ensure or verify the delivery or the transmission of the data or the information.
* Session: The layer that defines the connection or the interaction aspects of the communication or the networking system, such as the dialogs, tokens, or protocols that establish or maintain the session or the communication between the systems or the networks.
* Presentation: The layer that defines the format or the representation aspects of the communication or the networking system, such as the encryption, compression, or translation of the data or the information.
* Application: The layer that defines the functionality or the service aspects of the communication or the networking system, such as the applications, programs, or protocols that provide or support the functions or the services of the system or the network.
Routers are used as a communications device between the network and session layers of the OSI Reference Model, as they perform the functions or the tasks that are related to the network and session layers, such as:
* Routing or forwarding the packets or the data between the systems or the networks, based on the network addresses or the protocols, such as the Internet Protocol (IP) addresses or the Internet Protocol (IP) protocols, which are defined or specified at the network layer of the OSI Reference Model.
* Establishing or maintaining the sessions or the communications between the systems or the networks, based on the dialogs, tokens, or protocols, such as the Border Gateway Protocol (BGP) or the Open Shortest Path First (OSPF) protocols, which are defined or specified at the session layer of the OSI Reference Model.
Transport and session is not the pair of OSI Reference Model layers between which routers are used as a communications device, as routers do not perform the functions or the tasks that are related to the transport layer of the OSI Reference Model, such as ensuring or verifying the delivery or the transmission of the data or the information, based on the segments, ports, or protocols, such as the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP) protocols, which are defined or specified at the transport layer of the OSI Reference Model. Data-Link and transport is not the pair of OSI Reference Model layers between which routers are used as a communications device, as routers do not perform the functions or the tasks that are related to the data-link layer of the OSI Reference Model, such as organizing or controlling the data or the information, based on the frames, addresses, or protocols, such as the Media Access Control (MAC) addresses or the Ethernet protocols, which are defined or specified at the data-link layer of the OSI Reference Model. Physical and data-link is not the pair of OSI Reference Model layers between which routers are used as a communications device, as routers do not perform the functions or the tasks that are related to the physical layer of the OSI Reference Model, such as transmitting or receiving the data or the information, based on the cables, connectors, or signals, which are defined or specified at the physical layer of the OSI Reference Model. References: 1