* l l Controls congestion if too many packets are on the network Translates  logical  addresses  or  names  into physical addresses Has  accounting  functions  to  count  packets  orbits sent by users to produce billing information This layer acts in our postal service analogy, like the regional  mail  distribution  centers  throughout  the country. The trucks are directed to the centers and are routed along the best path to their final destinations. Layer 4—The transport layer  ensures data units are delivered error-free, in sequence, with no losses or duplications.  It  relieves  higher  layer  protocols  from  any concern with the transportation of data between them, as  follows: l l l l l Message  segmentation—accepts  data  from  the session layer, splits it up into smaller units, and passes the units down to the network layer Establishes  and  deletes  host-to-host  connections across the network Multiplexes  several  message  streams  onto  one channel  and  keeps  track  of  which  message belongs to which connection Provides  reliable  end-to-end  delivery  with acknowledgment Provides end-to-end flow control and window management The transport layer functions are provided by the mail truck dispatcher, who takes over if there is a wreck out in the system. If the network goes down, the transport layer software will look for alternate routes or perhaps save the transmitted data until the network connection is reestablished. Layer 5—The session layer  allows  users  on different machines to establish sessions between them. It performs the functions that enable two applications to communicate across the network, performing security, name recognition, logging, administration, and other similar functions. Unlike the network layer, this layer is dealing with the programs in each machine to establish conversations between them, as follows: Allows two applications processes on different machines to establish, use, and terminate a connection (or session) Performs synchronization between end-user tasks by placing checkpoints in the data stream so if the network fails, only the data after the last checkpoint has to be retransmitted Provides dialogue control (who speaks, when, how long, and so on) The session layer in our postal agency recognizes different zip codes and reroutes letters. Layer 6—The presentation layer formats data to be presented to the application layer. It can be viewed as the translator for the network. This layer provides a common representation for data that can be used between  the  application  processes.  The  presentation layer relieves the applications from being concerned with   data   representation,   providing   syntax independence, as follows: l l l Encodes  data  in  a  standard  way  (integers, floating point, ASCII, and so on) Provides  data  compression  to  reduce  the  number of bits that have to be transmitted Provides  data  encryption  for  privacy  and authentication This layer functions like a translator who translates a letter from French into English. Layer 7—The application layer serves as the window for the application process to access the OSI environment.  This  layer  represents  the  services  that directly  support  users  and  application  tasks.  It  contains a  variety  of  commonly  needed  protocols  for  the following: Network virtual terminals File transfers Remote file access Electronic mail Network management In our analogy, the application layer is the person who writes  or  reads  the  letter. CABLING A  data  communications  network  must  have  cabling to allow individual computers and other peripherals to talk to one another and share resources. And wouldn’t it be easier if there were only one type available? There 1-15