Computer-Network-No8-Cont-Mediam-Access-Control-from-APCOMS- by harshi446


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                                                                                                                                                         Faisal Amjad CPS 422

                               CPS 422                                                               Categories of Multiple Access Protocols
                           Computer Networks                                                                             Multiple Access Protocols

                                 DATA LINK LAYER                                                     Random Access           Controlled Access       Channelization

              MEDIUM ACCESS CONTROL                                                                    Multiple Access          Reservation           FDMA

                                                                                                       CSMA                     Polling               TDMA

                                                                                                       CSMA / CD                Token Passing         CDMA

                                                                                                       CSMA / CA

                                                                              Faisal Amjad CPS 422

                                                                              Faisal Amjad CPS 422                                                       Faisal Amjad CPS 422

                                                                                                     Reservation Protocol
                                                                                                     Stations need to make reservation before
                                                                                                     sending data
                                                                                                     Time is divided into intervals or slots
                            Reservation Protocols                                                    Reservation always precedes data transmission
                                                                                                     For N stations, there are N Reservation mini-
                                                                                                     Each mini-slot belongs to a station
                                                                                                     When a station has to send data it makes a
                                                                                                     reservation in its own mini-slot
                                                                                                     After the reservation mini slots have finished,
                                                                                                     stations that have made reservations can send
                                                                                                     data sequentially

                                                                              Faisal Amjad CPS 422                                                       Faisal Amjad CPS 422

           Reservation Access Method
          Reservation                                             Reservation
             Slot                                                    Slot

         1 2 3 4 5                                              1 2 3 4 5
                              Data        Data        Data                             Data
         1 0 1 0 1                                              0 1 0 0 0
                            station 1   station 3   station 5                        station 2

           Station 3’s                                          Station 2’s
            mini-slot                                            mini-slot



                                                                   Faisal Amjad CPS 422                                                               Faisal Amjad CPS 422

           Polling                                                                            SELECT
                                                                                                                                         A            B

            Polling works where link is in un-balanced mode
            i.e. with stations designated as primary and
            All data exchanges are made through the                                                                        SEL

            primary station even if destination is a                                                                       ACK

            secondary                                                                                                     DATA

            Primary is always the initiator of a session as it                                                             ACK
            controls the link
            Two modes Select and Poll
                                                                                                      Time                                           Time

                                                                                          • Select mode is used when Primary has data to send

                                                                   Faisal Amjad CPS 422                                                               Faisal Amjad CPS 422
                                                     A             B

                                                                                                             TOKEN PASSING





                    Time                                          Time

         • Poll mode is used when Primary Solicits data from secondary stations

                                                                   Faisal Amjad CPS 422                                                               Faisal Amjad CPS 422
           TOKEN PASSING                                                                  Token Passing Network
            Token is a special Frame which gives the holder
            station the “Right to Transmit”
            All stations are organized in a ring                                                        A            B                   C            D

            Frames are passed from the Predecessor to the
            successor after a specified time interval
            When there is no data to be sent the token                                                             Circulating Token
            circulates around the ring
            Whenever a station has data to send, it waits
            for a token to arrive
            Station then captures the token and keeps
            transmitting data until allocated time for                                                  H            G                   F            E

            keeping the token expires                                                     Successor of station G                       Predecessor of station G

            After the specified time the token must be
            passed on to the successor


                                                                Faisal Amjad CPS 422

         Token Passing Procedure                        Start

                                                    Wait for Token

                                                    Capture Token

             Allocated time
                 expired      Send frame            Data to Send

                     Yes                                     No

                              Release Token



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