OSI Model Concepts Flashcards
Open Systems Interconnection (OSI) reference model
The International Organization for Standardization (ISO) developed the Open Systems Interconnection (OSI) reference model to promote understanding of how components in a network system work. It does this by separating the functions of hardware and software components into seven discrete layers. Each layer performs a different group of tasks required for network communication.
Although not all network systems implement layers using this precise structure, they all implement each task in some way. The OSI model is not a standard or a specification; it serves as a functional guideline for designing network protocols, software, and appliances and for troubleshooting networks.
network protocol
A network protocol is a set of rules for exchanging data in a structured format. A network protocol has two principal functions:
Addressing—Describing where data messages should go. At each OSI model layer, there are different mechanisms for identifying nodes and rules for how they can send and receive messages.
Encapsulation—Describing how data messages should be packaged for transmission. Encapsulation is like an envelope for a letter, with the distinction that each layer requires its own envelope. At each layer, the protocol adds fields in a header to whatever payload data it receives from an application or other protocol.
Same Layer Interaction
communication between nodes at the same layer
Adjacent layer Interaction
To transmit or receive a communication, on each node, each layer provides services for the layer above and uses the services of the layer below.
Protocol Data Unit (PDU)
Network packet encapsulating a data payload from an upper layer protocol with header fields used at the current layer.
Layer 1 - The Physical layer
The Physical layer (PHY) of the OSI model is defined as layer 1. The Physical layer is responsible for the transmission and receipt of the signals that represent bits of data. Transmission media can be classified as cabled or wireless:
Cabled—A physical signal conductor is provided between two nodes. Examples include copper or fiber optic cable types. Cabled media can also be described as bounded media.
Wireless—Uses free space between nodes, such as microwave radio. Wireless media can also be described as unbounded media.
The Physical layer specifies the following:
Physical topology—The layout of nodes and links as established by the transmission media. An area of a larger network is called a segment. A network is typically divided into segments to cope with the physical restrictions of the network media used, to improve performance, or to improve security. At the Physical layer, a segment is where all the nodes share access to the same media.
Physical interface—Mechanical specifications for the network medium. For cabled media, this means the construction of the cable, the interface/connector form factor, and the number and functions of the pins in a connector. For wireless media, it means radio transceiver and antenna specifications.
Signaling—The process of transmitting and receiving encoded data over the network medium. A modulation scheme describes how electrical, light, or radio signals represent bits. Timing and synchronization schemes ensure senders and receivers can identify groups of signals as a chunk or frame of data.
Devices that operate at the Physical layer include the following:
Transceiver—The part of a network interface that sends and receives signals over the network media.
Repeater—A device that amplifies an electronic signal to extend the maximum allowable distance for a media type.
Hub—A multiport repeater, deployed as the central point of connection for nodes.
Media converter—A device that converts one media signaling type to another.
Layer 2 - Data Link Layer
Layer 2 is referred to as the Data Link layer. It is responsible for transferring data between nodes on the same logical segment. At the Data Link layer, a segment is one where all nodes can send traffic to one another using hardware addresses, regardless of whether they share access to the same media. A layer 2 segment might include multiple physical segments. This is referred to as a logical topology.
Local networks do not typically connect hosts directly with point to point or mesh links. To reduce cabling and interface costs, each host is connected to a central node, such as a switch or a wireless access point. The central node provides a forwarding function, receiving the communication from one node and sending it to another. To do this, each node interface must have a Data Link layer address. The addresses of interfaces within the same layer 2 segment are described as local addresses or hardware addresses.
The Data Link layer also performs an encapsulation function. It organizes the stream of bits arriving from the Physical layer into structured units called frames. Each frame contains a Network layer packet as its payload. The Data Link layer adds control information to the payload in the form of header fields. These fields include source and destination hardware addresses, plus a basic error check to test if the frame was received intact.
Devices that operate at the Data Link layer include the following:
- Network adapter or network interface card (NIC)—A NIC joins an end system host to network media (cabling or wireless) and enables it to communicate over the network by assembling and disassembling frames.
- Bridge—A bridge is a type of intermediate system that joins physical network segments while minimizing the performance reduction of having more nodes on the same network. A bridge has multiple ports, each of which functions as a network interface.
- Switch—An advanced type of bridge with many ports. A switch creates links between large numbers of nodes more efficiently.
- Wireless access point (AP)—An AP allows nodes with wireless network cards to communicate and creates a bridge between wireless networks and wired ones.
Layer 3 - The Network Layer
Layer 3 is the Network layer. This layer is responsible for moving data around a network of networks, known as an internetwork. While the Data Link layer is capable of forwarding data by using hardware addresses within a single segment, the Network layer moves information around an internetwork by using logical network and host IDs. The networks are often heterogeneous; that is, they use a variety of Physical layer media and Data Link protocols. The main appliance working at layer 3 is the router.
At layer 3, each packet is given a destination network address. Routers are configured with information about how to reach these different logical networks. The packet is forwarded, router by router (or hop by hop), through the internetwork to the target network. Once it has reached the destination network, the hardware address can be used to deliver the packet to the target node.
It is usually important for traffic passing between networks to be filtered. A basic firewall operates at layer 3 to enforce an access control list (ACL). A network ACL is a list of the addresses and types of traffic that are permitted or blocked.
Layer 4 - Transport Layer
The first three layers of the OSI model are primarily concerned with moving frames and datagrams between nodes and networks. At the Transport layer—also known as the end-to-end or host-to-host layer—the content of the packets becomes significant. Any given host on a network will be communicating with many other hosts using many different types of networking data. One of the functions of the Transport layer is to identify each type of network application by assigning it a port number. For example, data requested from an HTTP web application can be identified as port 80, while data sent to an email server can be identified as port 25.
At the Transport layer, on the sending host, data from the upper layers is packaged as a series of layer 4 PDUs, referred to as segments. Each segment is tagged with the application’s port number. The segment is then passed to the Network layer for delivery. Many different hosts could be transmitting multiple HTTP and email packets at the same time. These are multiplexed using the port numbers along with the source and destination network addresses onto the same link.
At the Network and Data Link layers, the port number is ignored—it becomes part of the data payload and is invisible to the routers and switches that implement the addressing and forwarding functions of these layers. At the receiving host, each segment is decapsulated, identified by its port number, and passed to the relevant handler at the Application layer. Put another way, the traffic stream is de-multiplexed.
The Transport layer can also implement reliable data delivery mechanisms, should the application require it. Reliable delivery means that any lost or damaged packets are resent.
Devices working at the Transport layer include multilayer switches—usually working as load balancers—and many types of security appliances, such as more advanced firewalls and intrusion detection systems (IDSs).
The Upper Layers
The upper layers of the OSI model are less clearly associated with distinct real-world protocols. These layers collect various functions that provide useful interfaces between software applications and the Transport layer.
Layer 5—Session
Most application protocols require the exchange of multiple messages between the client and server. This exchange of such a sequence of messages is called a session or dialog. The Session layer (layer 5) represents functions that administer the process of establishing a dialog, managing data transfer, and then ending (or tearing down) the session.
Layer 6—Presentation
The Presentation layer (layer 6) transforms data between the format required for the network and the format required for the application. For example, the Presentation layer is used for character set conversion, such as between American Standard Code for Information Interchange (ASCII) and Unicode.
Layer 7—Application
The Application layer (layer 7) is at the top of the OSI stack. An Application layer protocol doesn’t encapsulate any other protocols or provide services to any protocol. Application layer protocols provide an interface for software programs on network hosts that have established a communications channel through the lower-level protocols to exchange data.
More widely, upper-layer protocols provide most of the services that make a network useful, rather than just functional, including web browsing, email and communications, directory lookup, remote printing, and database services.
