Data-Plane and Control-Plane Functions in Base Stations and Mobile Stations

Figure below shows the user data processing path at the BS and MS. As shown in the figure, the user data traverses the path from network layer to physical layer and vice versa. In the transmitter side, a network layer packet is processed by the convergence sub-layer, the ARQ function (if enabled), the fragmentation/packing function, and the MAC PDU formation function, to form the MAC PDU to be sent to the physical layer for processing. In the receiver side, a physical layer SDU is processed by MAC PDU formation function, the fragmentation/packing function, the ARQ function (if enabled), and the convergence sub-layer function, to form the network layer packets. The control primitives between the MAC CPS functions and between the MAC CPS and PHY that are related to the processing of user traffic data are also shown below

Signal flow graph in data- and control-planes

The control-plane signaling and processing flow graph at the BS and the MS. In the transmitter side, the flow of control primitives from control-plane functions to data-plane functions and processing of control-plane signals by data-plane functions in order to construct MAC management messages and MAC header/sub-headers, to be transmitted over the air interface, are illustrated. In the receiver side, the arrows show the processing of the MAC control messages through data-plane functions and the reception of the corresponding control-plane signals by control-plane functions. The dotted arrows show the control primitives between MAC CPS functions and between MAC CPS and physical layer functions that are related to the processing of control-plane signaling. The control primitives to/from M-SAP/C-SAP define the network related functionalities, such as inter-BS interference management, inter/intra RAT mobility management, etc., as well as management-related functionalities, such as location management, system configuration, etc. 

Base Station Initiated Handoff | WiMAX HANDOFF CONTROL

The serving BS may decide to no longer manage an MS and initiate handoff for it. This occurs generally when the serving BS can no longer provide the required QoS or when it detects that the MS is moving out of its coverage area. Although the causes of a BS-initiated handoff are similar to the causes of an MS-initiated handoff, it is useful to let the BS decide to centralize the handoff procedure. In fact, the MSs are generally tiny equipments with limited power and computing resources; therefore, it is important to implement the handoff process at the BS level. 

The serving BS continues broadcasting the MOB_NBR-ADV message for the served MSs, but it orders the MS that needs to perform handoff via a MOB_BSHO-REQ message to start scanning the neighboring BSs. The MOB_BSHO-REQ message transmitted on the basic connection defines a list of recommended target BSs along with service level predictions and channel details. Upon receiving that message, the BS starts the scanning procedure and sends back a MOB_BSHO-RSP message to the serving BS indicating a list of recommended BSs. 

The rest of the handoff process is similar to the MS-initiated handoff case. In fact, the MS waits for the list of the target BSs and then sends a HO-IND message to its serving BS. Upon receiving the fast ranging IE, the MS sends the RNG-REQ ranging request message to the target BS to register with it. The BS-initiated handoff process, described earlier, is depicted by the flow chart in Figure 1.

Figure 1: The BS initiated handoff at the MS level.