Physical Layer
The physical layer is responsible for the transmission and reception of unstructured raw data between a
device—such as a network interface controller, Ethernet hub, or network switch—and a physical transmission
medium. It converts digital bits into electrical, radio, or optical signals (analog signals).
Layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data
rates, maximum transmission distances, modulation schemes, channel access methods, and physical connectors. This
includes the layout of pins, voltages, line impedance, cable specifications, signal timing, and frequency for
wireless devices.
Bit rate control is handled at the physical layer and may define transmission modes such as simplex, half
duplex, and full duplex. The components of a physical layer can be described in terms of the network topology.
Physical layer specifications are included in standards like Bluetooth, Ethernet, and USB. A lesser-known
example is the CAN standard.
The physical layer also specifies how encoding occurs over a physical signal, such as electrical voltage or a
light pulse. For example, a 1 bit might be represented on a copper wire by a transition from 0 volts to 5 volts,
while a 0 bit might be represented by the reverse. Common issues at this layer include incorrect media
termination, electromagnetic interference (EMI), noise scrambling, and misconfigured or faulty NICs and hubs.
Data Link Layer
The data link layer provides node-to-node data transfer—a link between two directly connected nodes. It detects
and may correct errors that occur in the physical layer. This layer defines protocols to establish and terminate
connections between physically connected devices, as well as manage flow control between them.
IEEE 802 divides the data link layer into two sublayers:
-
Medium Access Control (MAC) layer – Controls how devices in a network gain access to the
transmission medium and permission to send data.
-
Logical Link Control (LLC) layer – Identifies and encapsulates network layer protocols, and
manages error checking and frame synchronization.
The MAC and LLC layers are used in IEEE 802 networks such as 802.3 Ethernet, 802.11 Wi-Fi, and 802.15.4 Zigbee.
The Point-to-Point Protocol (PPP) is a data link layer protocol that operates over various physical layers,
including synchronous and asynchronous serial lines.
The ITU-T G.hn standard, which enables high-speed local area networking over existing wiring (power lines, phone
lines, and coaxial cables), includes a complete data link layer with error correction and flow control via a
selective-repeat sliding-window protocol.
Security at this layer, including authenticated encryption, can be implemented using MACsec.
Network Layer
The network layer provides the functional and procedural means of transferring packets from one node to another
across different networks
. A network is a medium to which many nodes can be connected, where each node has an
address and can transfer messages to other nodes simply by specifying the message content and destination address. The network
determines the route, possibly through intermediate nodes.
If a message is too large to be transmitted directly over the data link layer, the network layer may fragment
the message at the source node, transmit the fragments independently, and reassemble them at the destination. It may report
delivery errors, but is not required to do so.
Message delivery at the network layer is not inherently reliable. While some network layer protocols offer
reliable delivery, it is not a mandatory feature of the layer.
Several layer-management protocols defined in ISO 7498/4 belong to the network layer. These include routing
protocols, multicast group management, network-layer information and error handling, and address assignment. It is the
function of the payload—not the transport protocol—that determines their classification at this layer.
Security at the network layer, including authenticated encryption, can be implemented using
IPsec.
Transport Layer
The transport layer provides the functional and procedural means of transferring variable-length data sequences
from a source to a destination host, while maintaining quality of service. It controls the reliability of a link
through flow control, segmentation and reassembly, and error control.
Some transport layer protocols offer connection-oriented communication with reliable delivery, while others
provide connectionless service without delivery guarantees. This layer also enables multiplexing of data from
multiple applications on the same host.
The transport layer receives data from the session layer and segments it into smaller units if necessary. Each
segment is assigned a transport header containing control information such as source and destination port
numbers, sequence numbers, and error-checking data. These segments are then passed to the network layer for
transmission.
On the receiving end, the transport layer reassembles the segments into the original data stream and forwards it
to the session layer. It also manages flow control and error recovery to ensure accurate and ordered delivery.
Common protocols at this layer include Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and
Stream Control Transmission Protocol (SCTP).
Session Layer
The session layer controls the dialogues (connections) between computers. It establishes, manages, and
terminates connections between local and remote applications. This layer supports full-duplex, half-duplex, and
simplex communication modes, and handles procedures such as checkpointing, adjournment, termination, and
restart. It is commonly used in remote procedure calls.
The session layer offers three modes of communication:
- Simplex – Communication is unidirectional. One party sends information, and the other only
receives.
- Half-duplex – Both parties can communicate, but only one can send at a time while the other
waits.
- Full-duplex – Both parties can communicate simultaneously.
This layer is responsible for managing and controlling the dialog between devices. It establishes, maintains,
and terminates connections between applications. It also synchronizes data exchange by inserting checkpoints
into the data stream, allowing recovery if a connection is lost or interrupted.
Protocols that operate at the session layer include NetBIOS, SAP, PPTP, and RPC.
Presentation Layer
The presentation layer is responsible for the delivery and formatting of information to the application layer
for further processing or display. Often referred to as the syntax layer, it ensures that data is in a usable
format and handles data encryption. This layer may include compression, decompression, encryption, and
decryption algorithms.
It translates data between the application layer and the network format. This includes converting character
encoding (e.g., ASCII to EBCDIC), serializing complex data structures into byte streams, and compressing data to
reduce transmission size.
Common protocols and formats that operate at the presentation layer include:
- Multipurpose Internet Mail Extensions (MIME)
- Secure Sockets Layer (SSL) / Transport Layer Security (TLS)
- Character and image formats such as ASCII, EBCDIC, JPEG, GIF, TIFF, and PNG
Application Layer
The application layer is the topmost layer of the OSI model and serves as the interface between the user and the
network. It provides network services directly to end-user applications and is responsible for identifying
communication partners, determining resource availability, and synchronizing communication.
This layer encompasses a variety of protocols that facilitate different types of network communication. Common
application layer protocols include:
- HTTP (Hypertext Transfer Protocol) – Transmits web pages over the internet.
- FTP (File Transfer Protocol) – Transfers files between computers on a network.
- SMTP (Simple Mail Transfer Protocol) – Sends email messages.
- DNS (Domain Name System) – Resolves domain names to IP addresses.
- Telnet – Enables remote login to another computer over a network.
The application layer also provides services such as file transfers, email, and network resource sharing. It
enables users to interact with software applications that utilize network resources, making it an essential
component of modern networking.
Citations and Credits
Information about the OSI Model layers was sourced from the following websites:
Images were sourced from the following websites:
