OSI Model (Layers)

Physical (deals with the "mechanical, electrical, functional, and procedural means" required for transmission of data)
[wiring: 10BaseT, 10Base2, etc.]
Data Link ("provides for the control of the physical layer, and detects and possibly corrects errors that can occur")
[handles data transmission errors] [Ethernet, Token-ring, etc.]
Network (provides the physical routing of the data, determining the path between the machines)
Transport (provides the "transparent transfer of data from a source end open system to a destination end open system")
[establishes, maintains, and terminates communications between two machines; responsible for ensuring that data sent matches data received]
Session (organizes and synchronizes the exchange of data between applications)
[handles application errors]
Presentation (converts applications' data (in ASCII or EBCDIC) into/from a common format)
Application (end-user interface)

Using the OSI Model	Open System Interconnection Model (OSI) developed by the International Standards Organization (ISO)
Layer 1 (The Physical Link)	voltages, physical circuitry, timing, wiring uses EIA-232-D interface standard and V.24 & V.28 standards
Layer 2 (The Data Link)	transmission of characters and their sequencing checks for errors in transmission breaks message into frames holds data in buffers if devices have different speeds subdivided into Media Access Control (MAC) and Logical Link Control (LLC) layers MAC - deals with network access (token-passing / collision sensing) LLC - sends / receives user data messages
Layer 3 (Network Layer)	defines message addressing & routing methods routes communications by most logical path burstiness problem - gaps in communication packet switching Virtual mode - packets are held until all packets are received Datagram mode - user is responsible for reordering packets
Layer 4 (Transport Layer)	establishes protocols and controls provides data security controls sequencing of packets (resides in host) establishes, maintains, and terminates communications between two machines responsible for ensuring that data sent matches data received
Layer 5 (Session Layer)	sets up resources coordinates equipment handles transfer of files recognizes users sets priorities supports creation / termination of sessions
Layer 6 (Presentation Layer)	- handles formatting and display of information
Layer 7 (Application Layer)	- focuses on end-user needs