OSI Model- Cont

What happen when a message travels from device A to Device B?

o    As the message travels from A to B , it may pas through many intermediate "Nodes" o    These nodes usually involve only the first three layers of the OSI Model o    In  developing  the  OSI  model,  designers  identified  which  networking  functions  had related uses and collected those functions into discrete groups that became the layers o    Each layer defines a family of functions distinct from other layers o    By  defining  and  localizing  functionality  in  this  fashion  ,  the  designers  created  an architecture that is both comprehensive and flexible o    The OSI model allows complete transparency b/w otherwise incompatible systems Peer-to-Peer Processes o    Within  a  single  machine,  each  layer  provides  services  to  the  layer  above  it  and  all upon the services from the layer below it. o    For example Layer 3 o    Between  machines,  layer  x  on  one  machine communicates with layer x on the other machine. o    The communication is governed by Protocols o    The processes on each m/c that communicate at a given layer are called Peer -to peer processes Headers and trailers o    Control data added to a data parcel o    Sender appends header and passes it to the lower layer o    Receiver removes header and passes it to upper layer  o    Headers are added at layer 6,5,4,3,2. Trailer is added at layer 2 Passing of data and network information down through the layers of sending machine AND Back up through the layers of the receiving machine is made possible by an INTERFACE o    Each interface defines what information and services a layer must provide for the layer above it o    Interface provides MODULARITY o    Each layer works as a separate module o    Any modification or replacements can be made without changes in surrounding layers o    Organization of Layers    Network Support Layers -Deals with the Physical aspect of moving data from one device to another -Layers 1, 2, 3    User Support Layers -Allows interoperability among unrelated software systems -Layers 5, 6, 7 Organization of Layers o    Layer 4 -Ensures end-to-end reliable transmission o    Upper OSI Layers always implemented in Software o    Lower Layers are a combination of software and hardware o    Physical layer is mostly Hardware