OSI Model

The Open Systems Interconnection (OSI) model is a conceptual model that defines standards of the communication between computing systems without regard to its underlying internal structure and technology. Its major aim helps diverse communication systems to be able to connect and exchange data to each other with standard communication protocols. The model divides a communication system into 7 abstraction layers. Each layer provides services to the layer above it and is served by the layer below it. The application layer is the one that does not have a layer above. Instead, it is an entry for application processes that gain access to network services.

No.	Layer Name	Protocol Data Unit	Group
7	Application	Data	Upper Layers
6	Presentation
5	Session
4	Transport	Segment	Lower Layers
3	Network	Packet
2	Data Link	Frame
1	Physical	Bit

• Layer 7
• Layer 6
• Layer 5
• Layer 4
• Layer 3
• Layer 2
• Layer 1

Application Layer

The application layer is the highest layer in the Reference Model of Open Systems Interconnection. It provides a sole mean for the application processes to obtain access to the as well as access to open system interconnection environment. This layer also provides the processes access security checking and information validation.

The Application layer provides the following functions:

• File Transfer, Access and Management: provides handling services in the network. This includes the movement of files between different systems, reading, writing and deletion of remote files, and management of remote file storage.
• Virtual Terminal: provides services to access applications in different remote computer systems through stimulating a real terminal.
• Electronic Mail and Messaging Handling: provide an easy mean for the electronic exchange of documents.
• Directory Services: provides services with the ability to match names with addressing information.
• Common Management Information Protocol: provides services for network management.

Presentation Layer

The presentation layer is the second highest layer next to the application layer. It is responsible for the format of the data that is transferred during network communications. This layer is also about the syntax and semantics of the information transmitted. It converts the outgoing messages into a generic format for the transmission. It converts the incoming messages from the generic form to a format that is understandable to the receiving application. Each computer system has a different code scheme for representing data. Furthermore, the presentation layer makes it possible for communication between computer systems with different representation. It provides common communication services such as encryption, text compression, and reformatting. In addition, it is also concerned with other aspects of information representation including data compression and cryptography. Data compression reduces the number of bits that have to be transmitted. Cryptography is required for information security.

Session Layer

The session layer is the layer 5 immediately below the presentaion layer. It allows two application processes to hold ongoing communications called a session across a network. The applications on either end of the session can exchange data to another for as long as the session lasts. The session layer establishes session, exchange data or message, and closes when the session is no longer needed or inactive. It also monitors session identification so that only process with valid session identification can participate in the communication and security services to control access to session information. In addition, a session can allow a user to log into a remote time-sharing system or transfer a file between two processes. The session layer has the option to provide one-or-two-way communication called dialogue control.

Transport Layer

Transport Layer is the layer 4. It provides means to transfer data sequences from a source to a destination host. It controls the reliability of a given link through flow control, segmentation, and error control. It can keep track of the segments and retransmit those that fail delivery. The transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred. The transport layer creates segments out of the message received from the application layer. Segmentation is the process of dividing a long message into smaller messages.

Network Layer

The network layer is the layer 3. It controls traffices between LAN, VLANs, WLAN, and WAN, and provides routing, congestion control and accounting. Message delivery at this layer is not necessarily guaranteed to be reliable. The layer may provide reliable message delivery, but it is not required to do so. The main protocols at this layer include routing protocols, multicast group management, network-layer information and error, and network-layer address assignment. Furthermore, the layer provides the functional and procedural means of transferring variable length data sequences called packets from one host to another in another network.

Data Link Layer

The data link is the layer 2. It takes a raw transmission and transforms it into a line transmission without errors in the network layer. It accomplishes this task by having the sender break the input data up into data frames, transmit the frames sequentially, and process the acknowledgment frames sent back by the receiver. The protocol packages the data into frames that contain source and destination physical addresses. Ethernet, Token Ring, and ARCnet are examples of LAN data link protocols, and Point-to-Point Protocol (PPP) or Serial Line Internet Protocol (SLIP) are examples of WAN data link. IEEE 802 separates the data link layer into two sublayers including Medium access control (MAC) layer and Logical link control (LLC). The MAC layer is responsible for controlling how devices in a network gain access to a medium and permission to transmit data. The LLC layer is responsible for identifying and encapsulating network layer protocols, and controls error checking and frame synchronization.

List of Active 802 Standards

No.	Standars	Description
1	802.1	Higher Layer LAN Protocols Working Group
2	802.3	Ethernet Working Group
3	802.11	Wireless LAN Working Group
4	802.15	Wireless Personal Area Network (WPAN) Working Group
5	802.18	Radio Regulatory TAG
6	802.19	Wireless Coexistence Working Group
7	802.24	Vertical Applications TAG

Physical Layer

The physical is the lowest at the bottom of the OSI model. It is responsible for the transmission and reception of unstructured raw data between a device and a physical transmission medium. It converts the digital bits into electrical, radio, or optical signals depending on the medium type. It also specify characteristics for transmission such as voltage levels, the timing of voltage changes, physical data rates, maximum transmission distances, modulation scheme, channel access method and physical connectors. In addition, it defines the layout of pins, voltages, line impedance, cable specifications, signal timing and frequency for wireless devices. The physical also controls bit rate control and may define transmission mode as simplex, half duplex, and full duplex.