The 7 Layers of the OSI Model


The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

Application (Layer 7)	Collection of miscellaneous protocols for high level applications Electronic mail, file transfer, connecting remote terminals, etc. E.g. SMTP, FTP, Telnet, HTTP, etc. This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation (Layer 6)	Just theory! Very few applications use it. Concerned with the semantics of the bits. Define records and fields in them. Sender can tell the receiver of the format. Makes machines with different internal representations to communicate. If implemented, the best layer for cryptography. This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Session (Layer 5)	Just theory! Very few applications use it. Enhanced version of transport layer. Dialog control, synchronization facilities. Rarely supported (Internet suite does not). This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Transport (Layer 4)	Network layer does not deal with lost messages. Transport layer ensures reliable service. Breaks the message (from sessions layer) into smaller packets, assigns sequence number and sends them. Reliable transport connections are built on top of X.25 or IP. In case IP, lost packets arriving out of order must be reordered. TCP : (Transport Control Protocol) Internet transport protocol. TCP/IP Widely used for network/transport layer (UNIX). UDP (Universal Datagram Protocol) : Internet connectionless transport layer protocol. Application programs that do not need connection-oriented protocol generally use UDP. This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Network (Layer 3)	Concerned with the transmission of packets. Choose the best path to send a packet ( routing ). It may be complex in a large network (e.g. Internet). Shortest (distance) route vs. route with least delay. Static (long term average) vs. dynamic (current load) routing. Two protocols are most widely used. X.25 Connection Oriented Public networks, telephone, European PTT Send a call request at the outset to the destination If destination accepts the connection, it sends an connection identifier IP (Internet Protocol) Connectionless Part of Internet protocol suite. An IP packet can be sent without a connection being established. Each packet is routed to its destination independently. This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Data Link (Layer 2)	Handles errors in the physical layer. Groups bits into frames and ensures their correct delivery. Adds some bits at the beginning and end of each frame plus the checksum. Receiver verifies the checksum. If the checksum is not correct, it asks for retransmission. (send a control message). Consists of two sublayers: Logical Link Control (LLC) defines how data is transferred over the cable and provides data link service to the higher layers. Medium Access Control (MAC) defines who can use the network when multiple computers are trying to access it simultaneously (i.e. Token passing, Ethernet [CSMA/CD]). At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

Physical (Layer 1)	Concerned with the transmission of bits. How many volts for 0, how many for 1? Number of bits of second to be transmitted. Two way or one-way transmission Standardized protocol dealing with electrical, mechanical and signaling interfaces. Many standards have been developed, e.g. RS-232 (for serial communication lines). Example: X.21 This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

CREDITS: The 7 Layers of OSI graphic: The Abdus Salam International Centre for Theoretical Physics. Definitions and definition links were obtained, in part, from: Webopedia Online Computer Dictionary for Internet Terms and Technical Support