2.2 OSI Model and Data Encapsulation

OSI Model Facts

The Open Systems Interconnection (OSI) model is a theoretical way to classify and explain the complex process of sending data on a network. The OSI model is the most widely used method for network communications. Remember that it is only a theoretical model that defines standards for programmers and network administrators. It is not a model of actual physical layers.

OSI Model Benefits

The OSI model:

• Provides a common language and reference point for network professionals.
• Divides networking tasks into logical layers for easier comprehension.
• Allows specialization of features at different levels.
• Aids in troubleshooting.
• Promotes standards of interoperability between networks and devices.
• Provides modularity in networking features. It allows developers to change features without changing the entire approach.

OSI Model Limitations

The OSI model has the following limitations:

• OSI layers are theoretical and do not perform real functions.
• Industry implementations rarely have a layer-to-layer correspondence with the OSI layers.
• A variety of protocols are used within the OSI model to send or receive the overall message. This can sometimes complicate the overall process.
• A particular protocol implementation may not represent every OSI layer. A protocol may be used by multiple layers.

Mnemonic Device for Layers

To help remember the layer names of the OSI model, try the using the bottom to top or top to bottom mnemonic device in the following table.

Layer	Name	Mnemonic (Bottom to Top)	Mnemonic (Top to Bottom)
Layer 7	Application	Away	All
Layer 6	Presentation	Pizza	People
Layer 5	Session	Sausage	Seem
Layer 4	Transport	Throw	To
Layer 3	Network	Not	Need
Layer 2	Data Link	Do	Data
Layer 1	Physical	Please	Processing

OSI Layers Facts

OSI Model Layer Functions

Layer	Description and Keywords
Application (Layer 7)	The Application layer integrates network functionality into the host operating system and enables communication between network clients and services.\

The Application layer does not include specific applications that provide services, but rather provides the capability for services to operate on the network.
Most Application layer protocols operate at multiple layers down to the Session and even Transport layers. However, these protocols are classified as Application layer protocols because they start at the Application layer (the Application layer is the highest layer they operate in). Services typically associated with the Application layer include:
Hypertext Transfer Protocol (HTTP)
Telnet
File Transfer Protocol (FTP)
Trivial File Transfer Protocol (TFTP)
Simple Network Management Protocol (SNMP)\ | | Presentation (Layer 6) | The Presentation layer formats (presents) data in a form compatible for receipt by the Application layer or the destination system. Specifically, the Presentation layer:\

Formats and translates data between systems.
Negotiates data transfer syntax between systems. It converts character sets to the correct format.
Encapsulates data into message envelopes. It encrypts and compresses the data.
Restores data through decryption and decompression. | | Session (Layer 5) | The Session layer manages the sessions in which data are transferred. A session refers to each client connection. Session layer functions include:\

Management of multiple sessions. A server can concurrently maintain thousands of sessions.
Assignment of a session ID number to each session to keep data streams separate.
The setup, maintenance, and teardown of communication sessions.\ | | Transport (Layer 4) | The Transport layer provides a transition between the upper and lower layers of the OSI model. The transition makes the upper and lower layers transparent to each other. Transport layer functions include:

End-to-end flow control. Port and socket number assignment. Segmentation, sequencing, and combination. Connection services, either reliable (connection-oriented) or unreliable (connectionless) delivery of data.

A data segment is a unit of data at the Transport layer. | | Network (Layer 3) | The Network layer describes how data is routed across networks and on to the destination. Network layer:

Identifies hosts and networks by using logical addresses. Maintains a list of known networks and neighboring routers. Determines the next network point where data should be sent.

To select the optimal path for data, routers use a routing protocol that takes various factors into account, such as the number of hops in the path, link speed, and link reliability. A packet is a data segment at the Network layer. | | Data Link (Layer 2) | Logical Link Control (LLC) | The Data Link layer defines the rules and procedures for hosts as they access the Physical layer. These rules and procedures define:

How physical network devices are identified on the network by defining a unique hardware address (physical or MAC address). How and when devices have access to the LAN and can transmit on the network medium (media access control and logical topology). How to verify that the data received from the Physical layer is error free (using parity and cyclic redundancy check (CRC)). How devices control the rate of data transmission between hosts (flow control).

The Data Link layer has two sublayers:

The LLC is the upper sublayer. It: Is an interface between the MAC sublayer and the Network layer. Provides flow control and transmission for analog and/or digital streams of over a shared link for the logical link. The MAC layer is the lower sublayer. The MAC sublayer:

Controls the hardware. Provides flow control and transmission for analog and/or digital streams of over a shared link.

Frames are data segments at the Data Link layer. Switches, bridges and NICs, and WAPs function in Layer 2. | | Media Access Control (MAC) | | Physical (Layer 1) | The Physical layer of the OSI model sets standards for sending and receiving electrical signals between devices. Protocols at the Physical layer identify:

Conversion of digital data (bits) to electric pulses, radio waves, or pulses of light. Specifications for cables and connectors. The physical topology.

Bits are data segments at the Physical layer. NICs, repeaters, hubs, WAPs, and modems function in Layer 1. |

TCP/IP Model Layers

The TCP/IP model incorporates the general concepts and structure of the OSI model. The layers of the TCP/IP model are as follows:

Layer	Description
Application	The Application layer corresponds to the Session, Presentation, and Application layers of the OSI model. Protocols associated with the Application layer include:

FTP HTTP Telnet Simple Mail Transfer Protocol (SMTP) Domain Name System (DNS) SNMP | | Transport | The Transport layer matches the Transport of the OSI model. This layer is responsible for:

Error checking and reliable packet delivery. Breaking the data stream into segments. Assigning sequence numbers so the packets can be reassembled correctly on the remote side after transport.

Protocols associated with the Transport layer include:

Transport Control Protocol (TCP) User Datagram Protocol (UDP) | | Internetwork | The Internetwork layer is comparable to the Network layer of the OSI model. It is responsible for moving packets through a network. This involves:

Addressing hosts. Making routing decisions to identify how the packet traverses the network.

Protocols associated with the Internetwork layer include:

Address Resolution Protocol (ARP) Internet Control Message Protocol (ICMP) Internet Group Management Protocol (IGMP) | | Network | The Network layer, (sometime called the Link Layer) corresponds to the Physical and Data Link layers of the OSI model. It is responsible for describing the physical layout of the network and formatting messages on the transmission medium. |

tip

The TCP/IP model focuses specifically on the functions in the Internet layer and Transport layers. All other functions of the traditional OSI model are encompassed in the first and fourth layers.

Model Comparison

The following graphic shows how the two networking models map to each other: