Call Processing and Soft Handoff Mechanism | Solutions and Recommendations

Figure 1 depicts interactions among various network elements for ingress data (i.e. data flowing from the mobile device to the edge router. Figure 2 depicts the interactions for the egress data flowing in the reverse direction. These figures are OO (object-oriented) style sequence diagrams for call flow in an UML (Universal Modeling Language) like notation.

Figure 1: Data transfer from the mobile device to an edge router

 

Figure 2: Soft hand off and data transfer from an edge router to the mobile

 

In both the scenarios described here, the first step is designation of one of the routers accessible to the mobile device as the primary router. In case of mobile originated calls, the mobile informs the WR from whom the strongest signal is received to take the responsibility as prime WR. On the other hand, in case of mobile terminated calls, the edge router determines the prime router based on the mobile location after locating and successfully paging the mobile. Once the primary router is determined, it initiates the process for setting MPLS tunnels between itself and the edge router as well as the secondary routers. The key innovation here is to employ these MPLS tunnels to emulate the BS-BSC and BSC-MSC (mobile switching center) A3/A7 interfaces in the legacy wireless systems. Distribution of call flow control this way among various routers this way results in enormous cost savings for the customers due to effective utilization network links by reduction of the call control and data paths. Figures 1 and 2 depict the call flows after creation of MPLS tunnels.

In case of ingress data, the radio frames transmitted by the device are received by all active routers including a primary router and a number of secondary routers as shown in Figure 1. The secondary routers simply forward the radio frames to the primary router. The SDU of the primary WR selects the best one among all such frames including the one directly received, inserts that into an IP packet, and transmits it to a back-haul network via an edge router for onward transmission to the other party.

The flow in case of egress data is naturally in the opposite direction as shown in Figure 2. The core network hands over the IP packets to an edge router for onward transmission to the primary router through a pre-established or dynamic MPLS path. The primary WR segments the packets into radio frames and multicasts them to all the secondary WRs in the active set via dynamically configured Label Switch Paths (LSPs). The primary and secondary WRs then transmit each one of these received radio frames after different amounts of delay offset to the mobile device so that the replicas of individual frames from different WRs arrive simultaneously at the destination. These synchronous radio frames received from different WRs are analyzed by the mobile device to not only obtain the best (correct) radio frame, but also assess the power levels of the frames. As the mobile device moves away from the primary WR, the power level of the radio frames from the mobile at the primary as well as that of the frames from the primary at the mobile drop. When the power level drops below a pre-configured threshold, the mobile device sends a control message to the primary WR indicating the power level of the prospective primary. The new primary WR could be one of the previous secondary wireless routers in an active set for the call. To achieve micro mobility, the current primary WR would signal the new primary an indication of the handover of its responsibility as primary WR. It would also supply to the latter the list of active WRs in the same control message. After receiving this message, the new primary WR receiving the strongest signals first confirms to the current primary that it is ready to take control of the traffic distribution, and then establishes multicast MPLS paths (LSPs) for the secondary routers in the active list. The LSPs provide synchronized framing for distribution and selection between neighbors of wireless traffic and fast rerouting for soft handoff using RSVP.