MULTIPLE ACCESS

MULTIPLE ACCESS

1. Data Link Layer

   • Upper Sublayer --> data link control Lower Sublayer --> resolving access to the shared media

2. Random access protocols --> No station is superior to another station -->Possibility for access conflict (collision)

   • MA (multiple access)
     • CSMA (carrier sense multiple access)
       • CSMA/CA (Collision Avoidance)
         • to avoid collisions on wireless network
           • strategies
             • Interframe space
               • station needs to wait time equal to the contention time
             • contection window ->an amount of the divided into slots
               • if channel busy, it stops the timer and restart it when the channel become idle
             • Acknowledgement
       • CSMA/CD (Collision Detection)
         • medium send frame,if transformation successful station is finished.if there collision ,frame send again.
       • Each station first listen before sending -->Reduce possibility of collision not eliminate it
         • Persistence Strategy -->procedure for a station that sense a busy medium
           • methods
             • Nonpersistence
               • sense the line , if the line idle station send immediately otherwise waits
               • reduce the chance of collision
             • 1 Persistence
               • if the line idle station send immediately( with probability of 1)
               • increase the chance of collision
             • P-Persistence
               • if the line idle 1.with probability p,the station sends its frame 2.with probability q=1-p, the station waits for the beginning of the next slot and checks the line again.
     • -->ALOHA allows multiple access to shared the medium -->Potential of collision -->When a station sends data,another station may attempt to do so at the same time

3. Controlled-access protocols

   • -->The stations consult one another to find which stations has right to send. -->A station cannot send unless it has been authorized by other stations.
   • methods
     • Reservation
       • A station needs to make reservation before sending data
     • Polling
       • one device designated as a primary station and other devices are secondary stations
       • primary devices controls the link and secondary devices follow its instruction
         • Select Mode -> used when the primary device has something to send
         • Poll Mode -> used by the primary device to solicit transmission from the secondary device
     • token passing
       • station organized in a logical ring.Each station has a predecessor (station before the station in the ring) and successor (station after the station the ring)
       • token-->a special packet -->circulate around the ring
       • the station captures the token and sends one or more frames and finally releases the token to be used by the successor station

4. Channelization protocols

   • a multiple-access method in which available bandwidth of the link is shared in time,frequency or code.
   • protocols
     • FDMA (Frequency-Division Multiple Access)
       • -->available bandwidth is divided into frequency band -->Each station allocated a band to send its data --> used in cellular telephone and satellite -->used in data link layer
     • TDMA (Time-Division Multiple Access)
       • -->station share the bandwidth -->Each station allocated a time slot during send data -->Problem=achieving synchronization between different stations -->used in data link layer
     • CDMA (Code-Division Multiple Access)
       • -->no bandwidth sharing -->All station can send data simultaneously -->one channel carries all transmission simultaneously
         • based on coding theory.Each station assigned a code/sequence(chips).Sequenced selected.
           • Rules (send a 0 bit -> encodes -1) (send a bit 1 -> encodes +1)(idle ->no signal,interpreted as a 0)

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