BRP Domain Transfer

In the 3GPP CS-to-PS domain transfer procedure, the CS bearer of the UE-MGW segment is released after the IP bearer is established. If the UE moves back to the CS domain again, the released CS bearer must be re-established. Such bearer re-establishment contributes extra overload to the domain transfer. To speed up the subsequent switchings, we may not release the CS bearer at the CS-to-PS domain transfer, and postpone the bearer release until the VCC call is complete. If the user moves back from the PS domain to the

CS domain, the bearer re-establishment is eliminated. Same argument applies to the IP bearer re-establishment.

Based on the above intuition, we propose the Bearer Reservation with Preemption (BRP) scheme that speeds up the domain transfer process. The BRP scheme utilizes enhanced Multi-Level Precedence and Pre-emption (eMLPP) service [32] and Multimedia Priority Service (MPS) [33] to provide reservation and preemption of CS and IP bearers.

In BRP, two eMLPP priority levels are defined: the high priority and the low priority.

When there is no available channel at the MSC, a call arrival with high priority can preempt a call with low priority, i.e., the high priority call is established, and the low priority call is force-terminated.

In BRP, a VCC call before domain transfer is set up with high priority. When the UE switches this call from the CS domain to the PS domain, instead of releasing the CS bearer in the UE-MGW segment, this CS bearer is reserved with low priority. When the UE switches the call back to the CS domain, the domain transfer process simply raises the priority level of the reserved CS bearer to high priority. If the reserved CS bearer with low priority is preempted (and the preempted channel is used by an incoming high-priority call), the CS bearer is released. In this case, the VCC call is not terminated because the IP bearer is used. When the call is switched back to the CS domain, the CS bearer needs to be re-established.

4.3.1 CS-to-PS Domain Transfer in the BRP Scheme

Figure 4.5 illustrates the BRP message flow for CS-to-PS domain transfer with IP bearer establishment with the following steps:

UE

MSC MGCF

CS IMS

IP Bearer CS Bearer

CS Bearer

IP Bearer IP Bearer

CS Bearer CS Bearer

S-CSCF

MGW VCC

AS Callee

C*.6 ISUP FAR (low priority)

C*.7 ISUP FAA

Steps C.1-C.5

Steps C.6-C.9

Figure 4.5: CS-to-PS Domain Transfer with IP Bearer Establishment (BRP)

Steps C.1-C.5 Same steps as in Figure 4.4 initiate the establishment of the IP bearer in the UE-MGW segment.

Step C*.6 The MGCF lowers the priority level for the CS bearer, and sends an ISDN User Part (ISUP) Facility Request (FAR) message with the parameter “low priority”

to the MSC.

Step C*.7 According to the priority level indicated in the received ISUP FAR message, the MSC lowers the priority level for the CS bearer, and sends an ISUP Facility Accepted (FAA) message to the MGCF.

Steps C.6-C.9 Same steps as in Figure 4.4 exchange the Session Initiation Protocol (SIP) 200 OK and ACK messages to complete the IP bearer establishment in the UE-MGW segment.

By adding two messages (Steps C*.6 and C*.7), the BRP scheme eliminates eleven messages (Steps C.10-C.18) in Figure 4.4. Therefore, the message exchange cost is reduced by 36%. If the IP bearer has been reserved and not preempted before the domain transfer occurs (not shown in this dissertation), the message exchange cost is reduced by 68%.

Also note that after the transfer, the CS radio link to the UE may be disconnected, but the CS bearer at the MSC is still maintained. This idea is similar to the “always on”

concept of GPRS [4].

4.3.2 PS-to-CS Domain Transfer in the BRP Scheme

After the call has been successfully switched to the PS domain, the UE may decide to switch the call back to the CS domain again. If the reserved CS bearer has not been preempted, the UE does not need to initiate a new call for establishing the CS bearer in the UE-MGW segment. Instead, the UE only needs to raise the priority level of the reserved CS bearer to high priority. Also, unlike the procedure in Figure 4.3, the IP bearer is not released. Therefore, IP bearer needs not be re-established when the call switches back to the PS domain. Figure 4.6 illustrates the BRP message flow for PS-to-CS domain transfer without CS bearer establishment with the following steps:

Step B*.1 The UE sends a Call Management (CM) SERVICE REQUEST message to the MSC to raise the priority level of the CS bearer in the UE-MGW segment.

Step B*.2 The MSC raises the priority level for the CS bearer. Then the MSC sends an ISUP FAR message with the parameter “high priority” to the MGCF.

Steps B*.3 and B*.4 The MGCF raises the CS bearer’s priority, and lowers the IP

UE

CS MGCF MGW

IP Bearer IP Bearer

B*.3 H.248 Move CS Bearer

CS Bearer

S-CSCF

B*.1 CM SERVICE REQUEST (high priority) B*.2 ISUP FAR (high priority)

B*.5 ISUP FAA B*.6 CM SERVICE ACCEPT

IP Bearer CS Bearer

CS Bearer IP Bearer

IMS MSC

VCC

AS Callee

B*.4 H.248 Reply

Figure 4.6: PS-to-CS Domain Transfer without CS Bearer Establishment (BRP) the MGW to switch the UE-MGW segment from the PS bearer to the reserved CS bearer.

Steps B*.5 and B*.6 To complete this priority update, the MGCF sends an ISUP FAA message to the MSC. Then the MSC sends a CM SERVICE ACCEPT message to the UE to indicate successful priority update of the CS bearer. At this point, the UE-MGW segment is switched from the IP bearer to the CS bearer.

In the BRP scheme, six messages (Steps B*.1-B*.6) modify the priorities of the CS and the PS bearers. On the other hand, the 3GPP procedure in Figure 4.3 exchanges twenty-six messages (Steps B.1-B.18) to establish a new CS bearer, and release the old IP bearer. Therefore, the message exchange overhead is reduced by 77%. If the CS bearer has been preempted before the call is switched back to the CS domain (not shown in this dissertation), the message exchange cost is reduced by 15.4%.