20-04-2013, 02:50 PM
7 Layer OSI Model
Layer OSI.ppt (Size: 263 KB / Downloads: 138)
History
Rapid growth of computer networks caused compatibility problems
ISO recognized the problem and released the OSI model in 1984
OSI stands for Open Systems Interconnection and consists of 7 Layers
The use of layers is designed to reduce complexity and make standardization easier
Flat Addressing
Flat addressing schemes do not provide anything other than a unique identifier. They provide no real information about where the object being addressed resides.
Example: SSN# (may provide insight to where the person was born, but not to where they are now)
Hierarchical Addressing
Hierarchical addressing schemes provide layers or a hierarchy to the address that provide information about where the addressed object exists within the hierarchy.
Example: phone numbers (area code, local prefix, and four digit number unique to that area code/prefix combination).
Talking to Everyone
Special kinds of addresses exist at both layer #2 and #3 called broadcast addresses
Typically network devices are interested in only traffic addressed directly for them and any traffic addressed with the destination address set to broadcast
If they are paying attention to other traffic, they are said to be in promiscuous mode
Layer 4: The Transport Layer
PDU: Segment
Service Point Address (more often called a port) used to track multiple sessions between the same systems. SPA’s are used to allow a node to offer more than one service (i.e. it could offer both mail and web services)
This layer is why you have to specify TCP or UDP when dealing with TCP/IP
Network Design & Admin Issues
Examining network protocols and how they relate to the OSI model help aide network administers design networks and help admins troubleshoot strange behavior.
If you don’t understand what mechanisms your network is using to communicate, you are more likely to introduce new problems while trying to fix old ones.