The Open Systems Interconnect (OSI) model has seven layers, recommendation X.200 describes seven layers, labeled 1 to 7. Layer 1 is the lower layer in this model.
| OSI Model | ||||
|---|---|---|---|---|
| Data unit | Layer | Function | ||
| Host layers | Data | 7. Application | Network process to application | |
| 6. Presentation | Data representation, encryption and decryption, convert machine dependent data to machine independent data | |||
| 5. Session | Interhost communication, managing sessions between applications | |||
| Segments | 4. Transport | Reliable delivery of packets between points on a network. | ||
| Media layers | Packet/Datagram | 3. Network | Addressing, routing and (not necessarily reliable) delivery of datagrams between points on a network. | |
| Bit/Frame | 2. Data link | A reliable direct point-to-point data connection. | ||
| Bit | 1. Physical | A (not necessarily reliable) direct point-to-point data connection. | ||
At each level (N), two entities (layer N peers) exchange protocol data units (PDUs) by means of a layer-N protocol. A service data unit (SDU) is the payload of a PDU, transmitted unchanged to a peer.
1. PHYSICAL LAYER
The physical layer has the following major functions:
- it defines the electrical and physical specifications of the data connection. It defines the relationship between a device and a physical transmission medium (e.g., a copper or fiber optical cable). This includes the layout of pins, voltages, line impedance, cable specifications, signal timing, hubs, repeaters, network adapters, host bus adapters (HBA used instorage area networks) and more.
- it defines the protocol to establish and terminate a connection between two directly connected nodes over a communications medium.
- it may define the protocol for flow control.
- it defines a protocol for the provision of a (not necessarily reliable) connection between two directly connected nodes, and the modulation or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over the physical communications channel. This channel can involve physical cabling (such as copper and optical fiber) or a wireless radio link.
2. DATA LINK LAYER
The data link layer provides a reliable link between two directly connected nodes, by detecting and possibly correcting errors that may occur in the physical layer. The data link layer is divided into two sublayers:
- Media Access Control (MAC) layer - responsible for controlling how computers in the network gain access to data and permission to transmit it.
- Logical Link Control (LLC) layer - control error checking and packet synchronization.
The Point-to-Point Protocol (PPP) is an example of a data link layer in the TCP/IP protocol stack. The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete data link layer that provides both error correction and flow control by means of a selective-repeat sliding-window protocol.
3. NETWORK LAYER
The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:
- Routing: routes frames among networks.
- Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.
- Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
- Logical-physical address mapping: translates logical addresses, or names, into physical addresses.
- Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.
The network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address.
4. TRANSPORT LAYER
The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers. The transport layer provides:
- Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
- Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
- Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.
- Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).
OSI defines five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the fewest features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0-4 classes are shown in the following table:
Feature name TP0 TP1 TP2 TP3 TP4 Connection-oriented network Yes Yes Yes Yes Yes Connectionless network No No No No Yes Concatenation and separation No Yes Yes Yes Yes Segmentation and reassembly Yes Yes Yes Yes Yes Error recovery No Yes Yes Yes Yes Reinitiate connectiona No Yes No Yes No Multiplexing / demultiplexing over single virtual circuit No No Yes Yes Yes Explicit flow control No No Yes Yes Yes Retransmission on timeout No No No No Yes Reliable transport service No Yes No Yes Yes a If an excessive number of PDUs are unacknowledged.
An easy way to visualize the transport layer is to compare it with a post office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP network, or end-to-end encryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a network-layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames inside transport packet. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the transport layer, the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite are commonly categorized as layer-4 protocols within OSI.
5. SESSION LAYER
The session layer controls the dialogues (connections) between computers, allows session establishment between processes running on different stations. It provides:
- Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.
- Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.
The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls.
6. PRESENTATION LAYER
The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station. The presentation layer provides:
- Character code translation: for example, ASCII to EBCDIC.
- Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.
- Data compression: reduces the number of bits that need to be transmitted on the network.
- Data encryption: encrypt data for security purposes. For example, password encryption.
The original presentation structure used the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to anASCII-coded file, or serialization of objects and other data structures from and to XML.
7. APPLICATION LAYER
The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:
The data link layer provides a reliable link between two directly connected nodes, by detecting and possibly correcting errors that may occur in the physical layer. The data link layer is divided into two sublayers:
- Media Access Control (MAC) layer - responsible for controlling how computers in the network gain access to data and permission to transmit it.
- Logical Link Control (LLC) layer - control error checking and packet synchronization.
3. NETWORK LAYER
The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:
- Routing: routes frames among networks.
- Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.
- Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
- Logical-physical address mapping: translates logical addresses, or names, into physical addresses.
- Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.
The network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address.
4. TRANSPORT LAYER
The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers. The transport layer provides:
- Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
- Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
- Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.
- Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).
OSI defines five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the fewest features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0-4 classes are shown in the following table:
| Feature name | TP0 | TP1 | TP2 | TP3 | TP4 |
|---|---|---|---|---|---|
| Connection-oriented network | Yes | Yes | Yes | Yes | Yes |
| Connectionless network | No | No | No | No | Yes |
| Concatenation and separation | No | Yes | Yes | Yes | Yes |
| Segmentation and reassembly | Yes | Yes | Yes | Yes | Yes |
| Error recovery | No | Yes | Yes | Yes | Yes |
| Reinitiate connectiona | No | Yes | No | Yes | No |
| Multiplexing / demultiplexing over single virtual circuit | No | No | Yes | Yes | Yes |
| Explicit flow control | No | No | Yes | Yes | Yes |
| Retransmission on timeout | No | No | No | No | Yes |
| Reliable transport service | No | Yes | No | Yes | Yes |
| a If an excessive number of PDUs are unacknowledged. | |||||
An easy way to visualize the transport layer is to compare it with a post office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP network, or end-to-end encryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a network-layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames inside transport packet. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the transport layer, the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite are commonly categorized as layer-4 protocols within OSI.
5. SESSION LAYER
The session layer controls the dialogues (connections) between computers, allows session establishment between processes running on different stations. It provides:
- Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.
- Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.
The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls.
The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station. The presentation layer provides:
- Character code translation: for example, ASCII to EBCDIC.
- Data conversion: bit order, CR-CR/LF, integer-floating point, and so on.
- Data compression: reduces the number of bits that need to be transmitted on the network.
- Data encryption: encrypt data for security purposes. For example, password encryption.
The original presentation structure used the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to anASCII-coded file, or serialization of objects and other data structures from and to XML.
7. APPLICATION LAYER
The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:
- Resource sharing and device redirection
- Remote file access
- Remote printer access
- Inter-process communication
- Network management
- Directory services
- Electronic messaging (such as mail)
- Network virtual terminals
In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application-layer implementations include:
- On OSI stack:
- FTAM File Transfer and Access Management Protocol
- X.400 Mail
- Common Management Information Protocol (CMIP)
- On TCP/IP stack:
In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application-layer implementations include:
- On OSI stack:
- FTAM File Transfer and Access Management Protocol
- X.400 Mail
- Common Management Information Protocol (CMIP)
- On TCP/IP stack:
- Hypertext Transfer Protocol (HTTP),
- File Transfer Protocol (FTP),
- Simple Mail Transfer Protocol (SMTP)
- Simple Network Management Protocol (SNMP)
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