OSI - OPEN SYSTEM INTERCONNECTION

• The Application, Presentation and Session layers of the OSI model map directly to the Application layer of the TCP/IP Model.
• The Transport Layer of the OSI maps directly to the Transport layer of the TCP/IP Model.
• The Network Layer of the OSI maps directly to the Internet Layer of the TCP/IP Model.
• The Data Link and Physical Layers of the OSI map directly to the Network Access Layer of the TCP/IP Model.
• Network Models help software vendors create products that are interoperable.
OSI – Layer 7 – Application
• End users themselves interact with the network.
• Authentication services (not encryption, which runs at layer 6).
• Ensures that remote communications partner is available.
• That the needed communication partner is available.
• Both ends of the communication agree on procedures including data integrity, privacy and error recovery.
• Anything that requires the end user to enter a request is an application layer protocol, including:
o Telnet
o HTTP
o FTP
o SNMP
OSI – Layer 6 – Presentation
• Can be described with a question, “How should this data be presented?”
• Encryption occurs at this layer.
• Primary tasks the presentation layer is concerned with:
o Compatibility with the Operating System.
o Proper encapsulation of data for network transmission.
o Data Formatting.
o Data Encryption and decryption, compression and translation.
• Some file types which are used at the presentation layer include:
o JPEG
o ASCII
o GIF
o MPEG
• Presents data to the Application layer and is responsible for data translation and code formatting. It is essentially a translator and provides coding and conversion functions.
OSI – Layer 5 – Session
• Layer 5 is the “manager” of the two-way communication between two remote hosts.
• Handles the creation, maintenance and teardown of communications between those two hosts.
• This overall communication itself is referred to as a session.
OSI – Layer 4 – Transport
• The Transport Layers purpose is to establish a logical end-to-end connection between two systems, segment data received from the upper layers of the OSI model, and to make sure data gets to the destination in the correct order and free of errors.
• There are two methods for transporting data at the Transport Layer:
o Connection-orientated, referring to TCP.
o Connectionless, referring to UDP.
OSI – Layer 3 – Network
• IP runs at this layer.
• Sometimes called “The routing layer”.
• Routing is a two-question process:
o What valid path exists from the local router to a given destination.
o What is the best path to take to get there.
OSI – Layer 2 – Data Link
• Devices which operate at the Data Link Layer:
o Switches.
o Wireless Access Points.
o Cable and DSL modems.
• Ethernet.
• High Data Link Control (HDLC).
• Point-to-Point Protocol (PPP).
• Frame Relay.
• Provides error detection, but not error recovery using Frame Check Sequence (FCS).
• Media Access Control (MAC) addresses are used at this layer.
OSI – Layer 1 – Physical
• The Physical layer does two things, it sends and receives bits. 100101011101010101101
• The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems.