Uniques by CatheyMc

Network Structure
IP Addressing
Home Networking
ASCII Character Chart
UNIX Basic Commands

FIREWALL.CX OSI MODELc
Elementary Description: Your typed data gets converted a) into segments, then, b) segments into datagrams, 
c)  datagrams into packets, d) packets into frames, and then, e) frames are sent down to the receiving computer.

OSI MODEL

  [the layers or protocol stack]   PDNTSPA
Physical  Data-Link Network  Transport  Session  Presentation  Application, 1978, finalized 1980s

P

SHORT: PHYSICAL LAYER  Takes care of mechanical, timing, & electrical through the link.

 ·         Defines the electrical, mechanical, procedural and functional specification for activating, maintaining and deactivating the physical link between end systems. Networking hardware

·         Wires, connectors, voltages, data rates.  Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors and other similar attributes are defined by physical layer specifications.

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Sometimes called The Network Interface Layer. Ethernet, token ring, wireless, etc.WAN protocol, PPP, X.25 PDUs are called datagrams or generic term "packets".  Includes the IEEE 802 family of standards: search 802 at www.ieee.org

IP packets have two address fields: one at the Network Layer and one at the Data Link Layer. "Broadcasts" at Layer 2 (data Link Layer on the OSI Model)  are not forwarded by routers
 

How is the broadcast address of a network expressed in the “language” of both the Data Link Layer and the Network Layer?  Answer: 0xFF-FF-FF-FF-FF-FF at the Data Link Layer and 255.255.255.255 at the Network Layer.²

D

SHORT: DATA LINK LAYER Transmits the data. Frames and media access control.

·         Physical addressing, network topology, error notification, flow control.  Provides reliable transfer of data across media, physical link.  Concerned with physical addressing, network topology, network access, error notification, ordered delivery of frames and flow control.

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N

  SHORT:  Path selection, routing and addressing.

·         A complex layer that provides connectivity and path selection between two end systems

·         Domain of routing 

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T

   SHORT THOUGHT:  QOS and reliability

·         Concerned with transportation issues between hosts, segments data from the sending host’s system and reassembles the data into a data stream on the receiving host’s system.

·         Data transport reliability

·         Establish, maintain, terminate virtual circuits

·         Fault detection and recovery

·         Information flow control, attempts to provide a data transport service that shields the upper layers from transport implementation details.  Issues such as how reliable transport between two hosts is accomplished is the concern of the transport layer.  SHORT THOUGHT:  QOS and reliability.

·         Concerned with transportation issues between hosts, segments data from the sending host’s system and reassembles the data into a data stream on the receiving host’s system.

·         Data transport reliability

·         Establish, maintain, terminate virtual circuits

·         Fault detection and recovery

·         Information flow control, attempts to provide a data transport service that shields the upper layers from transport implementation details.  Issues such as how reliable transport between two hosts is accomplished is the concern of the transport layer.  SHORT THOUGHT:  QOS and reliability.

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   SHORT THOUGHT:  Dialogues and conversations.

·         Establishes, manages and terminates sessions between two communicating hosts. 

·         Provides its services to the Presentation Layer.

·         Also synchronizes dialogue between the two hosts’ Presentation Layers and manages their data exchange.

·         Offers provisions for efficient data transfer, class of service and exception reporting of Session Layer, Presentation Layer and Application Layer problems. 

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P

SHORT THOUGHT: A common data format

·          Insure data is readable by receiving system.  Insures that the information that the application layer of one system sends out is readable by the application layer of another system.  If necessary, it translates between multiple data formats by using a common format.  .

·         Format of data

·         Data structures

Negotiates data transfer syntax for application layer

A

  SHORT THOUGHT:  Browsers

• Provide network services to application processes ( such as electronic mail, file transfer and terminal emulation.)

• The layer closest to the user

• Does not provide services to any other layer, only to applications outside the OSI layer

• Establishes the availability of intended communication partners, synchronizes and establishes agreement on procedures for error recovery and control of date integrity.