The OSI Model

1. Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. An open system is a model that allows any two different systems to communicate regardless of their underlying architecture.

2. The OSI model is a layered framework for the design of network systems that allows for communication across all types of computer systems. It consists of seven separate but related layers, each of which defines a segment of the process of moving information across a network.
   • Allows complete transparency between otherwise incompatible systems.

   • Within a single machine, each layer calls upon the services of the layer just below it.

   • Between machines, layer x on one machine communicates with layer x on another machine. The processes on each machine that communicate at a given layer are called peer-to-peer processes.

   • The passing of the data and network information down through the layers of the sending machine and back up through the layers of the receiving machine is made possible by an interface between each pair of adjacent layers.

   • The seven layers can be thought of as belonging to three subgroups.
     • Layers 1, 2, and 3 -- physical, data link, and network -- are the network support layers; they deal with the physical aspects of moving data from one device to another.

     • Layer 4, the transport layer, ensures end-to-end reliable data transmission while layer 2 ensures reliable data transmission on a single link.

     • Layers 5, 6, and 7 -- session, presentation, and application -- can be thought of as the user support layers; they allow interoperability among unrelated software systems.

3. Functions of the layers
   • The physical layer coordinates the functions required to transmit a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium.
     • Physical characateristics of interfaces and media

     • Representation of bits (encoding)

     • Data rate (transmission rate)

     • Synchronization of bits

     • Line configuration (point-to-point or multipoint)

     • Physical topology

     • Transmission mode (simplex, half-duplex, full-duplex)

   • The data link layer transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for node-to-node delivery.
     • Framing

     • Physical addressing

     • Flow control

     • Error control

     • Access control

   • The network layer is responsible for the source-to-destination delivery of a packet possibly across multiple networks (links).
     • Logical addressing

     • Routing

   • The transport layer is responsible for source-to-destination (end-to-end) delivery of the entire message. For added security, the transport layer may create a connection between the two end ports. By confining transmission of all packets to a single pathway, the transport layer has more control over sequencing, flow, and error detection and correction.
     • Service point addressing -- the network layer gets each packet to the correct computer; the transport layer gets the entire message to the correct process on that computer.

     • Segmentation and reassembly

     • Connection control

     • Flow control

     • Error control

   • The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction between communicating systems.
     • Dialog control

     • Synchronization

   • The presentation layeris concerned with the syntax and semantics of the information exchanged between two systems.
     • Translation

     • Encryption

     • Compression

   • The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer,shared database management, and other types of distributed information services.
     • Network virtual terminal

     • File transfer, access, and management (FTAM)

     • Mail services

     • Directory services

4. TCP/IP (Transmission Control Protocol/Internetworking Protocol) Protocol Suite
   • Developed prior to the OSI model

   • Five layers -- physical, data link, network, transport, and application (represents the top three layers of the OSI model).