microsoft powerpoint - 2g_gprs_3g

Handoff forwardIt is possible that the terminal will move from system B to a third system CThis produces two possibilities in Figures 49 and 410In Figure 49 MSC-B and MSC-C perform a handoff forward procedure the one that moved the call from system A to system BSystem B provides a path from MSC-A to MSC-CThe situation can continue adding more and more MSCsto the chain up to a limit established by the anchor systemFigure 49 Call path after handoff forward to

system C

Handoff to thirdAn alternative occurs when there is a direct connection between systems A and CIS-41 includes a protocol referred to as handoff to third which establishes a direct link between MSC-A and MSC-C and release the link between A and B

Figure 410 If there are circuits connecting MSC-Aand MSC-C the system performs handoff to third

Handoff ProtocolsThere are two phases to every handoff procedure

Location phasethe serving MSC collects measurement reports from cells in the neighborhood of the cell presently occupied by a terminalWhen measurements are required from one or more cells in a system adjacent to the serving system the adjacent system becomes a candidate systemThe serving MSC and a candidate MSC exchange handoff measurement request messages

A HANDOFF MEASUREMENT REQUEST INVOKE message transmitted by the serving MSC includes

information about the terminal (station class mark SCM indicates the capabilities of the terminal)information about the serving base station (SAT and a base station identifier) andinformation about the radio channel carrying the call (channel number)

Based on the identity of the serving base station the candidate MSC selects one or more candidate cells and transmits a HANDOFF MEASUREMENT REQUEST RESULT message to the serving MSC

The HANDOFF MEASUREMENT REQUEST RESULT message contains identities of candidate cells and associated signal strength measurementsThe serving MSC selects a target cell for the handoffIf the target cell is served by a candidate MSC this MSC becomes the target MSC for the handoffThe handoff procedure then moves from the location phase to the handoff phase

Handoff phasethe serving MSC determines the type of handoff to initiate (forward back or handoff to third)

Handoff Forward ProtocolThe serving MSC sends a FACILITIES DIRECTIVE INVOKE message to the target MSCThis message contains

information about the terminal (SCM MIN ESN)information about the call

billing ID (established by the anchor MSC at the beginning of the call)inter-MSC circuit (voice trunk that will carry the call from the serving MSC to the target MSC)inter-switch count (the total number of MSCs through which the call will pass after the handoff)

information about the call status (serving cell serving channel) andtarget cell identifier (based on measurement reports from the get MSC)

If the target MSC accepts the handoff it selects a channel to handle the call in the new cell and then sends a FACILITIES DIRECTIVE RESULT message to the serving MSCThis message contains information about the new channel

channel number SAT and transmit power level (VMAC)On receiving this message the serving MSC sends an AMPS HANDOFF message to the terminal through the serving cellWhen the target base station detects the SAT it sends a messageto the target MSC which completes the handoff forward operation by sending a MOBILE ON CHANNEL INVOKE message to the prior serving MSC