“Rejoice always, pray without ceasing, give thanks in all circumstances; for this is the will of God in Christ Jesus for you.”
Sources: Open For More Detail
(1): AWS: What is OSI Model?
(2): CCNA Official Cert Guide, Vol.1 – Wendell Odom (2020) — ISBN-13: 978-0-13-579273-5
Purpose: Open For More Detail
The purpose of this post is to provide an overview of the OSI model for Networking.
The secondary purpose of this post is to act as a general landing page.
The OSI Model
OSI stands for Open Systems Interconnection.
The Open Systems Interconnection model was developed by the: International Organization for Standardization (ISO), International Telegraph & Telephone Consultative Committee (CCITT) and others; in the 1970’s & 1980s.
The OSI Model is a framework that divides network communication & interoperability into seven–distinct–layers.
A goal of the OSI Model is to act as a universal language/framework, so various technologies can communicate with standard protocols & rules. Without these standards, different manufactures may develop products that are incompatible with other network devices.
There are several alternatives to the OSI model. For instance, the TCP/IP model competes & dominates against the OSI model.
In real life, technicians utilize similar terminology when explaining concepts found in both TCP/IP & OSI models. In fact, the first four layers (1) physical, (2) data link, (3) network, and (4) transport are functionally similar for OSI & TCP/IP model.
1 Thessalonians 5:16-18 ( HCSB)
“Rejoice always!
Pray constantly.
Give thanks in everything,
for this is God’s will for you in Christ Jesus.”
The Seven Layers
The OSI Model consists of seven layers.
More Information Here:
Each of these layers, generally consists of a set of functions, that together make up a bigger system of communication.
Data is transmitted bi-directionally, as each layer communicates with the layers above & below. However, these layers are also “only aware of the interfaces to communicate with the layer above and below it.”
When a protocol from a higher layer, uses the lower-level layer to perform a particular service, that interaction is called adjacent-layer interaction.
These seven layers are: (1) Physical, (2) Data-Link, (3) Network, (4) Transport , (5) Session, (6) Presentation, & (7) Application.
Layers 5-7 comprise the software layers; where data transmissions from software applications (Operating Systems, Web Browsers, Email) occur.
IBM considers Layer 4 to be the “heart of OSI”. Layer 4 handles all data communication between the network & the various systems.
Layers 1-3 comprise the hardware layers; where network data moving through physical components is processed.
More Detail Below
(1) The Physical Layer:
The physical layer is comprised of the physical network components that transmit raw data with physical signals. This raw data is converted from a physical signal to bits (0’s & 1’s).
This first step is crucial, in allowing the transmission to be communicated in a way other devices in the network can understand.
The physical layer works with physical signals that include: electrical voltage, radio, and light pluses. The medium can be: fiber-optic cables, copper cables, and air.
The physical layer handles: (1) Bit rate control, (2) Bit synchronization, (3) Transmission mode, (4) Physical topologies.
A Network-Interface Card (NIC) allows the hardware on your computer to to implement physical layer protocols. The NIC is important to the data-link layer, as well.
Physical Layer Protocols*
Ethernet, 802.11 (Wi-Fi)
* Many of the physical layer protocols are likely shared with the data-link layer.
(2) The Data-Link Layer:
The data-link layer manages the transfer of error-free data between two interacting devices on the same network. The data-link layer builds upon the connection that the physical layer has already established.
The data-link layer takes these digital signals & encapsulates them into data frames. The Network Interface Card (NIC) can the physical signal & translates it into a logical format at the data-link layer. This data-link layer will provide the rules the physical layer will use.
The data-link layer also receives packets from the network layer. It then divides these network packets into data frames. These data frames are then transmitted to the host. The data frames encapsulate the data with both a header & a trailer.
Encapsulation in networking means to provide headers around the data being transmitted or received. However, in the case of the data-link layer, a header & trailer (footer) is provided in the encaspulation service. Therefore, the data-link layer provides services to the network layer.
The data-link layer can be divided further into two subparts: (1) logical link control layer & (2) media access control layer.
Logical Link Control (LLC) sub-layer:
The LLC sublayer acts an interface between the MAC sublayer & the network layer. The LLC sublayer handles: “flow control, synchronization, and multiplexing“.
Media Access Control (MAC) sub-layer:
The MAC sub-layer controls how devices across various network mediums access & transmit data.
For instance: when multiple devices are sharing a single communication channel, the media access control sub-layer will determine which device will have control–in that particular instance.
Additionally, the data-link layer ensures data integrity by implementing error control mechanisms that detect damaged or lost frames. These damaged or lost frames may be retransmitted.
Finally, the data link layer can put limit the amount of data transmitted by the sender before receiving an acknowledgment of delivery by the recipient. This flow control mechanism helps to prevent data corruption.
Data Link Protocols
Ethernet, 802.11 (Wi-Fi)
(3) The Network Layer:
The network layer can be known as the Internet layer. This alias holds it’s weight due to the Internet Protocol (IP) the network layer focuses on.
The network layer routes, addresses, and forwards data across different networks. Devices on the same network, don’t need to rely on the network layer, but instead the data-link layer.
The network layer will determine an optimal route for data to travel between nodes on different networks. These nodes will each have a unique IPv4 or IPv6 address.
Additionally, the network layer can break up LARGE segments (from layer 4) into packets. When acting as a receiver, devices on the network layer will reassemble packets back into larger segments.
It is worth noting that the network layer is not required to report errors or offer reliable message delivery.
Network Layer Protocols
Internet Protocol version 4 (IPv4), Internet Protocol Version 6 (IPv6), Internet Control Message Protocol (ICMP)
(4) The Transport Layer:
The transport layer is considered the “heart of OSI” by IBM. This layer transmits “end-to-end” data, ensuring the “data isn’t lost, misconfigured, or corrupted.”
The transport layer attaches a transport layer header which includes a service point/port address.
The transport layer supports multiplexing, which allows “multiple network applications to use the same connection simultaneously.”
Connectionless services are typically faster & less reliable than connection-oriented services. This is because conectionless services lack the connection-oriented three-part process, described below.
Connectionless services focus only on data transfer. The receiver does not need to confirm the receipt. This accelerates data transfer.
In contrast to connectionless services, Connection-Oriented Services operate with a three-part process consisting of: (1) connection establishment, data transfer/termination, (3) transmission of an delivery acknowledgement receipt.
This three-part process ensures connection-oriented services are slower but more reliable.
Common Protocols for Layer 4: Transport
Transmission Control Protocol (TCP), User Datagram Protocol (UDP)
(5) The Session Layer:
The session layer is responsible for session management. Which means, the session layer begins, manages, & ends connections between interacting network devices. The session layer ensures resources are not “overused nor underutilized“.
The session layer keeps the session open, long enough to transmit data, and then closes the session. Closing the session preserves the network’s resources.
The session layer also handles recovery points, which act as checkpoints. These check points allow a session to be resumed from a specific recovery point, if that session gets interrupted.
The session layer establishes protocols for connecting & disconnecting sessions with related data streams like audio & video in a web conference.
Session Layer Protocols
Network File System (NFS), Server Message Block (SMB)
(6) The Presentation Layer:
The presentation layer alias is the syntax layer, due to the role the presentation layer plays in converting data–like images, & graphics–into a format that the application layer can transmit. The presentation layer is concerned with the syntax of the data.
The presentation layer helps with “data translation, compression, and encryption” which prepares data for the application layer.
During the encapsulation process, data is translated into a format that the application layer specifies.
Additionally, it is the presentation layer that encrypts data to prepare it for transmission. The presentation layer decrypts data, if that data is being received.
Data is compressed by the presentation layer to reduce the size when that data is being transmitted. If the data needs to be utilized, the presentation layer can decompress it.
Presentation Layer Protocols
Sockets layer/transport layer security (SSL/TLS), JPEG protocols, MPEG protocols, JavaScript Object Notation (JSON), Comma Separated Values (CSV).
(7) The Application Layer:
The application layer helps “convert computer-readable network data into user-readable responses.” In reverse, the application layer will take the data people can read and transmit it to the presentation layer, which formats the data for the computer.
The application layer is closest to the end-user, us or you.
The application layer helps API endpoints communicate with the other layers of the OSI model. These process helps software applications communicate–by sending & receiving data–with the network.
The application layer ensures that the interfaces needed for communication exist & that the receiving device is able to accept the data.
The application layer is important for remote access & directory services.
Application Layer Protocols
Various protocols in the application layer include: Hypertext Markup Language (HTML), HyperText Transfer Protocol (HTTP), HyperText Transfer Protocol Secure (HTTPS), File Transfer Protocol (FTP), DNS, Post Office Protocol version 3 (POP3), & Simple Mail Transfer Protocol (SMTP).
1 Thessalonians 5:16-18 ( NLT)
“Always be joyful. Never stop praying. Be thankful in all circumstances, for this is God’s will for you who belong to Christ Jesus.”
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