OsI Model

Application (Layer 7)

The Application layer integrates network functionality into the host operating system and enables network 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, they are classified as Application layer protocols because they start at the Application layer (the Application layer is the highest layer where they operate).

Data Link (Layer 2)-Logical Link Control (LLC) Media Access Control (MAC)

The Data Link layer defines the rules and procedures for hosts as they access the Physical layer. These rules and procedures specify or define: • How hosts on the network are identified (physical or MAC address). • How and when devices can transmit on the network medium (media access control and logical topology). • How to verify that the data received from the Physical layer are error free (parity and CRC). • How devices control the rate of data transmission between hosts (flow control). Data at the Data Link layer are referred to as frames.

A frame

is a structured package for moving data that includes not only the raw data, or " payload, " but also the sender ' s and receiver ' s network addresses, and error checking and control information.

Reassembly

is the process of reconstructing the segmented data units.

In the second layer of the OSI model, the Data Link layer

protocols divide data they receive from the Network layer into distinct frames that can then be transmitted by the Physical layer.

The primary function of protocols at the Network layer

the third layer in the OSI model, is to translate network addresses into their physical counterparts and decide how to route data from the sender to the receiver. Addressing is a system for assigning unique identification numbers to devices on a network.

Using the OSI model to discuss networking concepts has the following advantages:

• 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 (developers can change features without changing the entire approach)

Network (Layer 3)

The Network layer describes how data are routed across networks and on to the destination. Network layer functions include: • Identifying hosts and networks by using logical addresses. • Maintaining a list of known networks and neighboring routers. • Determining the next network point to which data should be sent. Routers use a routing protocol that takes into account various factors, such as the number of hops in the path, link speed, and link reliability, to select the optimal path for data. Data at the Network layer are referred to as packets.

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: • How digital data (bits) are converted to electric pulses, radio waves, or pulses of light. • Specifications for cables and connectors. • The physical topology. Data at the Physical layer are referred to as bits.

Presentation (Layer 6)

The Presentation layer formats or "presents" data into a compatible form for receipt by the Application layer or the destination system. Specifically, the Presentation layer ensures: • Formatting and translation of data between systems. • Negotiation of data transfer syntax between systems by converting character sets to the correct format. • Encapsulation of data into message envelopes by encryption and compression. • Restoration of data by decryption and decompression

Session (Layer 5)

The Session layer manages the sessions in which data are transferred. Session layer functions include: • Management of multiple sessions (each client connection is called a session). A server can concurrently maintain thousands of sessions. • Assignment of a session ID number to each session to keep data streams separate. • The setting up, maintenance, and tearing down of communication sessions.

Transport (Layer 4)

The Transport layer provides a transition between the upper and lower layers of the OSI model, making the upper and lower layers transparent from each other. Transport layer functions include: • End-to-end flow control. • Port and socket numbers. • Segmentation, sequencing, and combination. • Connection services, either reliable (connection-oriented) or unreliable (connectionless) delivery of data. Data at the Transport layer are referred to as segments.

In fragmentation

a Network layer protocol (such as IP) subdivides the segments it receives from the Transport layer into smaller packets.

to route means to

intelligently direct data based on addressing, patterns of usage, and availability.

Sequencing

is a method of identifying segments that belong to the same group of subdivided data.