MULTIPLE - SECONDARY COMMUNICATION

• The process is a little more involved if there is more than one secondary in the piconet . The primary uses the even - numbered slots but a secondary sends in the next odd - numbered slot if the packet in the previous slot was addressed to it .
• All secondaries listen on even - numbered slots , but only one secondary sends in any odd - numbered slot as shown below:

1. In slot 0 , the primary sends a frame to secondary 1 .
2. In slot 1 , only secondary 1 sends a frame to the primary because the previous frame was addressed to secondary 1 ; other secondaries are silent .
3. In slot 2 , the primary sends a frame to secondary 2 .
4. In slot 3 , only secondary 2 send a frame to primary because the previous frame was addresses to secondary 2 ; other secondaries are silent .
5. The cycle continues .

                  This access method is similar to a poll / select operation with reservation . When the primary selects secondary , it also polls it .

                     The next time slot is reserved for the polled station to send its frame , if the polled secondary has no frame to send , the channel is silent .

                       Each frame is transmitted over a logical channel , called a LINK , between master and a slave . Two types of links can exist:

                                                                      1. Synchronous connection oriented link( SCO )

                                                                      2. Asynchronous connection link( ACL )

Synchronous Connection Oriented (SCO):-

•  The other is the SCO link , for real - time data , such as telephone connections .
• SCO link is used when avoiding latency (delay in data delivery) is more important than integrity .
• SCO link , a physical link is created between the primary and a secondary by reserving specific slots at regular intervals . The basic unit of connection is 2 slots , one foe each direction .
• If a packet is damaged , it is never re transmitted . This type of channel is allocated a fixed slot in each direction . Due to the time - critical nature of SCO links , frames sent over them are never re transmitted . Instead , forward error correction can be used to provide high reliability .
• A slave may have up to three SCO links with its master . Each SCO link can transmit one 64 Mbps PCM audio channel .

Asynchronous Connection - Less(ACL) :-

• The first is the ACL link , which is used for packet - switched data available at irregular intervals .
• These data come from the L2CAP layer on the sending side and are delivered to the L2CAP layer on the receiving side .
• ACL traffic is delivered on a best efforts basis . No guarantees are given .
• Frames can be lost and may have to be re transmitted . A slave may have only one ACL link to its master .