Internetworking present challenges - interoperating between products from different manufacturers requires consistent standards. Network reference models were developed to address these challenges. A network reference model serves as a blueprint, detailing how communication between network devices should occur. The Open Systems Interconnect (OSI) and Department of Defense (DoD) models are the most widely recognized reference models.
What is OSI Model?
The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO), and formalized in 1984. It provided the first framework governing how information should be sent across a network. The OSI model consists of seven layers, each corresponding to a specific network function:
Layer 7. Application
Layer 6. Presentation
Layer 5. Session
Layer 4. Transport
Layer 3. Network
Layer 2. Data-link
Layer 1. Physical
The top three layers of the OSI model are often referred to as the upper layers: Layer 7. Application layer, Layer 6. Presentation layer and Layer 5. Session layer.
Protocols that operate at these layers manage application-level functions, and are generally implemented in software. The function of the upper layers of the OSI model can be difficult to visualize. Upper layer protocols do not always fit perfectly within a layer, and often function across multiple layers.
The bottom four layers of the OSI model are often referred to as the lower layers: Layer 4. Transport layer, Layer 3. Network layer, Layer 2. Data-Link layer and Layer 1. Physical layer.
Protocols that operate at these layers control the end-to-end transport of data between devices, and are implemented in both software and hardware.
Layer 7 - Application
The Application layer provides the interface between the user application and the network. A web browser and an email client are examples of user applications. The user application itself does not reside at the Application layer - the protocol does. The user interacts with the application, which in turn interacts with the application protocol. Examples of Application layer protocols include: FTP via an FTP client, HTTP via a web browser, POP3 and SMTP via an email client, Telnet.
The Application layer provides a variety of functions:
•Identifies communication partners
•Determines resource availability
The Application layer interacts with the Presentation layer below it. As it is the top-most layer, it does not interact with any layers above it.
Layer 6 - Presentation
The Presentation layer controls the formatting and syntax of user data for the application layer. This ensures that data from the sending application can be understood by the receiving application. Standards have been developed for the formatting of data types, such as text, images, audio, and video. Examples of Presentation layer formats include: Text (RTF, ASCII, EBCDIC), Images (GIF, JPG, TIF), Audio (MIDI, MP3, WAV), Movies (MPEG, AVI, MOV).
If two devices do not support the same format or syntax, the Presentation layer can provide conversion or translation services to facilitate communication. Additionally, the Presentation layer can perform encryption and compression of data, as required. However, these functions can also be performed at lower layers as well. For example, the Network layer can perform encryption, using IPSec.
Layer 5 - Session
The Session layer is responsible for establishing, maintaining and ultimately terminating sessions between devices. If a session is broken,this layer can attempt to recover the session. Sessions communication falls under one of three categories:
• Full-Duplex – simultaneous two-way communication
• Half-Duplex – two-way communication, but not simultaneous
• Simplex – one-way communication
Many modern protocol suites, such as TCP/IP, do not implement Session layer protocols. Connection management is often controlled by lower layers,such as the Transport layer. The lack of true Session layer protocols can present challenges for high availability and failover. Reliance on lower-layer protocols for session management offers less flexibility than a strict adherence to the OSI model.
Layer 4 - Transport
The Transport layer does not actually send data, despite its name. Instead, this layer is responsible for the reliable transfer of data, by ensuring that data arrives at its destination error-free and in order. Transport layer communication falls under two categories:
• Connection-oriented – requires that a connection with specific agreed-upon parameters be established before data is sent.
• Connectionless – requires no connection before data is sent.
Connection-oriented protocols provide several important services:
• Segmentation and sequencing – data is segmented into smaller pieces for transport. Each segment is assigned a sequence number, so that the receiving device can reassemble the data on arrival.
• Connection establishment – connections are established, maintained and ultimately terminated between devices.
• Acknowledgments – receipt of data is confirmed through the use of acknowledgments. Otherwise, data is retransmitted, guaranteeing delivery.
• Flow control (or windowing) – data transfer rate is negotiated to prevent congestion.
The TCP/IP protocol suite incorporates two Transport layer protocols:
• Transmission Control Protocol (TCP) – connection-oriented
• User Datagram Protocol (UDP) - connectionless
Layer 3 - Network
The Network layer controls internetwork communication, and has two key responsibilities:
• Logical addressing – provides a unique address that identifies both the host, and the network that host exists on.
• Routing – determines the best path to a particular destination network, and then routes data accordingly.
Two of the most common Network layer protocols are:
• Internet Protocol (IP)
• Novell’s Internetwork Packet Exchange (IPX).
IPX is almost entirely deprecated.
Layer 2 - Data Link
While the Network layer is concerned with transporting data between networks, the Data-Link layer (Layer 2) is responsible for transporting data within a network. The Data-Link layer consists of two sublayers:
• Logical Link Control (LLC) sublayer
• Media Access Control (MAC) sublayer
The LLC sublayer serves as the intermediary between the physical link and all higher layer protocols. It ensures that protocols like IP can function regardless of what type of physical technology is being used.
Additionally, the LLC sublayer can perform flow-control and errorchecking, though such functions are often provided by Transport layer protocols, such as TCP.
The MAC sublayer controls access to the physical medium, serving as mediator if multiple devices are competing for the same physical link. Datalink layer technologies have various methods of accomplishing this -Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD), and Token Ring utilizes a token.
The Data-link layer packages the higher-layer data into frames, so that the data can be put onto the physical wire. This packaging process is referred to as framing or encapsulation.
The encapsulation type will vary depending on the underlying technology.
Common Data-link layer technologies include following:
• Ethernet – the most common LAN data-link technology
• Token Ring – almost entirely deprecated
• FDDI (Fiber Distributed Data Interface)
• 802.11 Wireless
• ATM (Asynchronous Transfer Mode)
The data-link frame contains the source and destination hardware (or physical) address. Hardware addresses uniquely identify a host within a network, and are often hardcoded onto physical network interfaces. However, hardware addresses contain no mechanism for differentiating one network from another, and can only identify a host within a network. The most common hardware address is the Ethernet MAC address.
Layer 1 - Physical
The Physical layer controls the signaling and transferring of raw bits onto the physical medium. The Physical layer is closely related to the Data-link layer, as many technologies (such as Ethernet) contain both datalink and physical functions. The Physical layer provides specifications for a variety of hardware:
• Connectors and transceivers
• Network interface cards (NICs)
• Wireless radios
Physical-layer devices and topologies are covered extensively in other guides.
Remembering the OSI Model 7 layers – 8 mnemonic tricks
If you need to memorize the layers for a college or certification test, here are a few sentences to help remember them in order. The first letter of each word is the same as a layer of the OSI model.
From Application to Physical (top down):
All People Seem To Need Data Processing
All Pros Search Top Notch Donut Places
A Penguin Said That Nobody Drinks Pepsi
A Priest Saw Two Nuns Doing Pushups
From Physical to Application (bottom up):
Please Do Not Throw Sausage Pizza Away
Pew! Dead Ninja Turtles Smell Particularly Awful
People Don’t Need To See Paula Abdul
Pete Doesn’t Need To Sell Pickles Anymore