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OSI Model - Page 1

During the past two decades there has been a tremendous increase in the numbers and sizes of networks. Many of the networks, however, were built using different implementations of hardware and software. As a result, many of the networks were incompatible and it became difficult for networks using different specifications to communicate with each other. To address this problem, the International Organization for Standardization (ISO) researched many network schemes. The ISO recognized that there was a need to create a network model that would help network builders implement networks that could communicate and work together (interoperability) and therefore, released the Open System InterConnection (OSI) reference model in 1984.

This Study Guide explains how standards ensure greater compatibility and interoperability between various types of network technologies. Here you will learn how the OSI reference model networking scheme supports networking standards. In addition, you will see how information or data makes its way from application programs (such as spreadsheets) through a network medium (such as wires) to other application programs located on other computers on a network. As you work through this chapter, you will learn about the basic functions that occur at each layer of the OSI model, which will serve as a foundation as you begin to design, build and troubleshoot networks.

The most basic level of computer information consists of binary digits, or bits (0s and 1s). Computers that send one or two bits of information, however, would not be very useful, so other groupings - bytes, kilobytes, megabytes, and gigabytes - are necessary. In order for computers to send information through a network, all communications on a network originate at a source, then travel to a destination.


As illustrated in the Figure, the information that travels on a network is referred to as data, packet, or data packet. A data packet is a logically grouped unit of information that moves between computer systems. It includes the source information along with other elements that are necessary in order to make communication possible and reliable with the destination device. The source address in a packet specifies the identity of the computer that sends the packet. The destination address specifies the identity of the computer that finally receives the packet. If one computer (host A) wants to send data to another computer (host B), the data must first be packaged by a process called encapsulation.

In order for data packets to travel from a source to a destination on a network, it is important that all the devices on the network speak the same language or protocol. A Protocol is a set of rules that make communication on a network more efficient. Some common examples are as follows:


One technical definition of a data communications protocol is: a set of rules, or an agreement, that determines the format and transmission of data. Layer n on one computer communicates with Layer n on another computer. The rules and conventions used in this communication are collectively known as the Layer n protocol.

The early development of LANs, MANs, and WANs was chaotic in many ways. The early 1980's saw tremendous increases in the numbers and sizes of networks. As companies realized the money they could save and the productivity they could gain by using networking technology, they added networks and expanded existing networks almost as rapidly as new network technologies and products could be introduced.

By the mid-1980's, these companies began to experience growing pains from all the expansions they had made. It became harder for networks that used different specifications and implementations to communicate with each other. They realized that they needed to move away from proprietary networking systems.

Proprietary systems are privately developed, owned, and controlled. In the computer industry, proprietary is the opposite of open. Proprietary means that one or a small group of companies controls all usage of the technology. Open means that free usage of the technology is available to the public.

To address the problem of networks being incompatible and unable to communicate with each other, the International Organization for Standardization (ISO) researched network schemes like DECNET, SNA, and TCP/IP in order to find a set of rules. As a result of this research, the ISO created a network model that would help vendors create networks that would be compatible with, and operate with, other networks.

The process of breaking down complex communications into smaller discrete tasks could be compared to the process of building an automobile. When taken as a whole, the design, manufacture, and assembly of an automobile is a highly complex process. It's unlikely that one single person would know how to perform all the required tasks to build a car from scratch. This is why mechanical engineers design the car, manufacturing engineers design the molds to make the parts, and assembly technicians each assemble a part of the car.

The OSI reference model (Note: Do not confuse with ISO.), released in 1984, was the descriptive scheme they created. It provided vendors with a set of standards that ensured greater compatibility and interoperability between the various types of network technologies that were produced by the many companies around the world.

The OSI reference model is the primary model for network communications. Although there are other models in existence, most network vendors, today, relate their products to the OSI reference model, especially when they want to educate users on the use of their products. They consider it the best tool available for teaching people about sending and receiving data on a network.

The OSI reference model allows you to view the network functions that occur at each layer. More importantly, the OSI reference model is a framework that you can use to understand how information travels throughout a network. In addition, you can use the OSI reference model to visualize how information, or data packets, travels from application programs (e.g. spreadsheets, documents, etc.), through a network medium (e.g. wires, etc.), to another application program that is located in another computer on a network, even if the sender and receiver have different types of network media.

In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function. This separation of networking functions is called layering. Dividing the network into these seven layers provides the following advantages:


The problem of moving information between computers is divided into seven smaller and more manageable problems in the OSI reference model. Each of the seven smaller problems is represented by its own layer in the model. The seven layers of the OSI reference model are:

Layer 7: Application Layer
Layer 6: Presentation Layer
Layer 5: Session Layer
Layer 4: Transport Layer
Layer 3: Network Layer
Layer 2: Data Link Layer
Layer 1: Physical Layer

To help remember them in the correct order a common mnemonic is often used from 7 to 1 (top to bottom):

For those of you who like the Domino's or Pappa John's pies, try from 1 to 7 (bottom to top):

During the course of these Study Guides, you will work your way through the OSI model, layer by layer. By working through the layers of the OSI reference model, you will understand how data packet units travel through a network and what devices operate at each layer as data packets travel through them. As a result, you will understand how to troubleshoot network problems as they may occur during data packet flow.

Each individual OSI layer has a set of functions that it must perform in order for data packets to travel from a source to a destination on a network. Below is a brief description of each layer in the OSI reference model as shown in the Figures.

Layer 7: The Application Layer
The application layer is the OSI layer that is closest to the user; it provides network services to the user's applications. It differs from the other layers in that it does not provide services to any other OSI layer, but rather, only to applications outside the OSI model. Examples of such applications are spreadsheet programs, word processing programs, and bank terminal programs. The application layer establishes the availability of intended communication partners, synchronizes and establishes agreement on procedures for error recovery and control of data integrity. If you want to remember Layer 7 in as few words as possible, think of browsers.
Protocols that operate on this layer include: TELNET, HTTP, FTP, TFTP, SMTP, NTP, SNMP, EDI.


Layer 6: The Presentation Layer
The presentation layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system. If necessary, the presentation layer translates between multiple data formats by using a common format. If you want to think of Layer 6 in as few words as possible, think of a common data format. Also think of encryption and compression as types of formats.
Specifications defined at this layer include: GIF, TIFF, JPEG, MPEG, MIME, ZIP, and ASCII.


Layer 5: The Session Layer
As its name implies, the session layer establishes, manages, and terminates sessions between two communicating hosts. The session layer provides its services to the presentation layer. It also synchronizes dialogue between the two hosts' presentation layers and manages their data exchange. In addition to session regulation, the session layer offers provisions for efficient data transfer, class of service, and exception reporting of session layer, presentation layer, and application layer problems. If you want to remember Layer 5 in as few words as possible, think of dialogues and conversations.
Protocols/API's that operate on this layer include: RPC, SQL, NETBIOS.


Layer 4: The Transport Layer
The transport layer segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system. The boundary between the transport layer and the session layer can be thought of as the boundary between application protocols and data-flow protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower four layers are concerned with data transport issues.

The transport layer attempts to provide a data transport service that shields the upper layers from transport implementation details. Specifically, issues such as how reliable transport between two hosts is accomplished is the concern of the transport layer. In providing communication service, the transport layer establishes, maintains, and properly terminates virtual circuits. In providing reliable service, transport error detection-and-recovery and information flow control are used. If you want to remember Layer 4 in as few words as possible, think of quality of service, and reliability.

Protocols that operate on this layer: TCP, UDP, NETBEUI, SPX.
These protocols are either connectionless or connection-oriented.

Connection-oriented means that a connection (a virtual link) must be established before data can be exchanged. This can guarantee that data will arrive, and in the same order it was sent. It guarantees delivery by sending acknowledgements back to the source when messages are received. It is a form of flow control, and is called "Windowing". TCP is an example of an connection-oriented transport protocol.

A common example of connection-oriented communication is a telephone call: you call, the 'destination' picks up the phone and acknowledges and you start talking (sending data). When a message or a piece of it doesn't arrive, you say: "What!?" and the sender will retransmit the data.

Connectionless is the opposite of connection-oriented; the sender does not establish a connection before it sends data, it just sends without guaranteeing delivery. UDP is an example of an connectionless transport protocol. A common example is regular mail versus registered/return receipt mail. Regular mail is UDP and registered mail is TCP.


Layer 3: The Network Layer
The network layer is a complex layer that provides connectivity and path selection between two host systems that may be located on geographically separated networks. If you want to remember Layer 3 in as few words as possible, think of path selection, routing, and addressing.
Also this layer is responsible for logical addressing (also known as network addressing or Layer 3 addressing) - for example, IP addresses. Examples of protocols defined at this layer: IP, IPX, AppleTalk, ICMP, RIP, OSPF, BGP, IGRP, and EIGRP. Devices that operate on this layer: Routers, Layer 3 Switches.

Network layer addresses:
Also known as Layer 3 or Logical addresses. These type of addresses are protocol-dependent, for example if the network protocol is IP, IP addressing will be used which is made up of a network part and a host part and needs a subnet mask to determine the boundaries of these parts.
An example of an IP address is: and a subnet mask:


Layer 2: The Data Link Layer
The data link layer provides reliable transit of data across a physical link. In so doing, the data link layer is concerned with physical (as opposed to logical) addressing, network topology, network access, error notification, ordered delivery of frames, and flow control. Provides error checking by adding a CRC to the frame, and flow control. If you want to remember Layer 2 in as few words as possible, think of frames and media access control.
Devices that operate on this layer: Switches and Bridges.

IEEE 802 Data Link sub layers
Around the same time the OSI model was developed, (February 1980 - hence 802) the IEEE developed the 802-standards such as 802.5 Token Ring and 802.11 for wireless networks. Both organizations exchanged information during the development which resulted in two compatible standards. The IEEE 802 standards define physical network components such as cabling and network interfaces, and correspond to the Data Link and/or Physical layer of the OSI model. The IEEE refined the standards and divided the Data Link layer into two sub layers.

The LLC and the MAC sub layer
LLC sublayer

LLC is short for Logical Link Control. The Logical Link Control is the upper sublayer of the Data Link layer. LLC masks the underlying network technology by hiding their differences hence providing a single interface to the network layer. The LLC sublayer uses Source Service Access Points (SSAPs) and Destination Service Access Points (DSAPs) to help the lower layers communicate to the Network layer protocols acting as an intermediate between the different network protocols (IPX, TCP/IP, etc.) and the different network types (Ethernet, Token Ring, etc.) This layer is also responsible for frames sequencing and acknowledgements.
The LLC sublayer is defined in the IEEE standard 802.2.

MAC sublayer
The Media Access Control layer takes care of physical addressing and allows upper layers access to the physical media, handles frame addressing, error checking. This layer controls and communicates directly with the physical network media through the network interface card. It converts the frames into bits to pass them on to the Physical layer who puts them on the wire (and vice versa).

IEEE LAN standards such as 802.3, 802.4, 802.5 and 802.10 define standards for the MAC sublayer as well as the Physical layer.
Other standards on this layer include: X.25 and Frame Relay.

Data Link layer addresses
Also known as layer 2 addresses, BIAs (Burned-in Address), physical address and most commonly referred to as MAC address. This is a fixed address programmed into a NIC or a router interface for example.
00-10-E3-42-A8-BC is an example of a MAC address. The first 6 hexadecimal digits (3 bytes) specify the vendor/manufacturer of the NIC, the other 6 digits (3 bytes) define the host.
The layer 2 broadcast address is FF-FF-FF-FF-FF-FF.


Layer 1: The Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes are defined by physical layer specifications. If you want to remember Layer 1 in as few words as possible, think of signals and media.
Devices that operate on this layer: HUBs/concentrators, repeaters, NICs, and LAN and WAN interfaces such as RS-232, OC-3, HDLC, BRI, V.24, V.35, X.25 and Frame Relay.


TCP/IP stack vs. the DoD Model
TCP/IP operation is defined in its own model: the DoD model. DoD is short for Department of Defense, who designed TCP/IP for ArpaNet. Although they are similar, in contrary to the 7-layer OSI model the DoD model has 4 layers. Each DoD layer and its functions corresponds to 1 or more OSI layers and their functions, which is represented in the image below:


For the CCNA exam you don't need to know the DoD model in detail, but if you know the OSI model and the related DoD layers you can easily identify the layer at which a certain protocol or standard is specified, for example:
Process/Application: Telnet, FTP, SMTP, HTTP, SNMP, etc.
Host To Host: TCP UDP.
Internet: IP, ICMP, ARP, RARP, BootP, etc.
Network Access: Ethernet, Fast Ethernet, Token Ring, FDDI, etc.

The Encapsulation Process
Encapsulation wraps data with the necessary protocol information before network transit. Therefore, as the data packet moves down through the layers of the OSI model, it receives headers, trailers, and other information. NOTE: The word "header" means that address information has been added.

The Application, Presentation and Session layer are known as the Upper Layer and are implemented in software. The Transport and Network layer are mainly concerned with protocols for delivery and routing of packets to a destination and are implemented in software as well. The Data Link is implemented in hard- and software and the Physical layer is implemented in hardware only, hence its name. These last two layers define LAN and WAN specifications.

A more detailed description of each layer follows below, but here's what basically happens when data passes from Host A to Host B:
1. the Application, Presentation and Session layers take user input and converts it into data,
2. the Transport layer adds a segment header converting the data into segments,
3. the Network layer adds a network header and converts the segments into packets or datagrams,
4. the Data Link layer adds a frame header and trailer converting the packets into frames,
5. the MAC sublayer layer converts the frames into a bits which the Physical layer can put on the wire.

The steps are known as the 5 steps of data encapsulation.
When the bits stream arrives at the destination, the Physical layer takes it of the wire and converts it into frames, each layer will remove their corresponding header while the data flows up the OSI model until it is converted back to data and presented to the user, this is known as decapsulation.
To see how encapsulation occurs, lets examine the manner in which data travels through the layers as illustrated in the Figure .


Once the data is sent from the source, as depicted in the Figure, it travels through the application layer down through the other layers. As you can see, the packaging and flow of the data that is exchanged goes through changes as the networks perform their services for end-users. As illustrated in the Figures, networks must perform the following five conversion steps in order to encapsulate data:


Build the data
As a user sends an e-mail message, its alphanumeric characters are converted to data that can travel across the internetwork.

Package the data for end-to-end transport
The data is packaged for internetwork transport. By using segments, the transport function ensures that the message hosts at both ends of the e-mail system can reliably communicate.

Append (add) the network address to the header
The data is put into a packet or datagram that contains a network header with source and destination logical addresses. These addresses help network devices send the packets across the network along a chosen path.

Append (add) the local address to the data link header
Each network device must put the packet into a frame. The frame allows connection to the next directly-connected network device on the link. Each device in the chosen network path requires framing in order for it to connect to the next device.

Convert to bits for transmission
The frame must be converted into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire). A clocking function enables the devices to distinguish these bits as they travel across the medium. The medium on the physical internetwork can vary along the path used. For example, the e-mail message can originate on a LAN, cross a campus backbone, and go out a WAN link until it reaches its destination on another remote LAN. Headers and trailers are added as data moves down through the layers of the OSI model.

To help remember them in the correct order, I use this mnemonic:

Taken from my college days when my Irish friend was given our money to go buy beer.
Often he did not return with our drinks. So sad!

This mnemonic represents:
Data at the top 3 Layers
Segments at the Transport/4th Layer
Packets or Datagrams at the Network/3rd Layer
Frames at the Data Link/2nd Layer, and
Bits at the Phisical/1st Layer

In order for data packets to travel from the source to the destination, each layer of the OSI model at the source must communicate with its peer layer at the destination. This form of communication is referred to as Peer-to-Peer Communications. During this process, each layer's protocol exchanges information, called protocol data units (PDUs), between peer layers. Each layer of communication, on the source computer, communicates with a layer-specific PDU, and with its peer layer on the destination computer as illustrated in the Figure.


Data packets on a network originate at a source and then travel to a destination. Each layer depends on the service function of the OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field; then it adds whatever headers and trailers the layer needs to perform its function. Next, as the data moves down through the layers of the OSI model, additional headers and trailers are added. After Layers 7, 6, and 5 have added their information, Layer 4 adds more information. This grouping of data, the Layer 4 PDU, is called a segment.

The network layer, for example, provides a service to the transport layer, and the transport layer presents data to the internetwork subsystem. The network layer has the task of moving the data through the internetwork. It accomplishes this task by encapsulating the data and attaching a header creating a packet (the Layer 3 PDU). The header contains information required to complete the transfer, such as source and destination logical addresses.

The data link layer provides a service to the network layer. It encapsulates the network layer information in a frame (the Layer 2 PDU); the frame header contains information (e.g. physical addresses) required to complete the data link functions. The data link layer provides a service to the network layer by encapsulating the network layer information in a frame.

The physical layer also provides a service to the data link layer. The physical layer encodes the data link frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.

Data Encapsulation Summary
What basically happens when data passes from Host A to Host B? The steps are known as the 5 steps of data encapsulation. When the bits stream arrives at the destination, the Physical layer takes it of the wire and converts it into frames, each layer will remove their corresponding header while the dataflow up the OSI model until it is converted back to data and presented to the user, this is known as de-capsulation.
OSI layers reference table:

OSI Table