The MAC layer is oriented to the connections. Each connection corresponds to a service flow. The service flow defines the QoS parameters of PDU transmitted over the connection.
The design core of the MAC protocol is the concept of service flow architecture on the connection.
The service flow provides an uplink/downlink QoS management mechanism. An MS is based on a connection to request the bandwidth. Actually, the MS is associated with the service flow. Each connection is identified by a 16-digit CID.
The basic three pairs of management connections are Basic management connection (emergent time and short MAC layer management message), Primary management connection (carrying longer MAC management messages of some delays), and Secondary management connection (tolerate the messages of delay based on the DHCP, TFTP, and SNMP).
Different connections have different QoS levels. The uplink CID and downlink CID of a connection are the same.