Delayed ACK in dual-mode call handover

Abstract

Handover of a call to a dual-mode phone from cellular to Wi-Fi. When handing over a call mediated by a mobility controller to a dual-mode phone and switching the call from a cellular to a Wi-Fi call, the mobility controller initates a Wi-Fi connection to the dual mode phone. When the Wi-Fi connection is established, and with the cellular connection through the mobility controller still in place, the mobility controller starts a timer with a predetermined value and the dual-mode phone initiates release of the cellular connection. When the timer expires, the mobility controller switches the call from the cellular connection to the Wi-Fi connection.

Description

BACKGROUND OF THE INVENTION

The present invention relates to call handover in dual-mode phones.

A dual-mode phone, also called an endpoint, can accept voice/video calls from a Wi-Fi network or a cellular network. In an ideal world, the user of the dual-mode phone should be able to transition seamlessly and transparently from the cellular network to the Wi-Fi network.

In the real world, the Wi-Fi connection for the dual-mode phone is managed by a mobility controller using well known SIP and RTP protocols. SIP (Session initiation protocol) is defined for example in RFC 3261 from the IETF SIP Working Group. SIP provides for call signaling and call setup for voice over IP (VoIP) devices. The actual voice and/or video packet streams usually operate under RTP (Real-time Transport Protocol) defined for example in IETF RFC 3550.

In operation, an endpoint such as a dual-mode phone can accept or originate calls on either a cellular network or a Wi-Fi network, When the mobile endpoint is on a call using the cellular network and moves into range of the Wi-Fi network, it is desirable to switch the call to the Wi-Fi network. Similarly, when the mobile endpoint is on a call using the Wi-Fi network and moves out of range of the Wi-Fi network, it is desirable to hand the call off to the cellular network. The problem is to manage these transitions minimizing delay, dropped packets, and duplications of packets during connection handover.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention in which:

FIG. 1 shows a dual-mode phone in a network, and

FIG. 2 shows a diagram of call switching.

DETAILED DESCRIPTION

Embodiments of the invention relate to dual-mode phones, and more particularly, to call handover in dual-mode phones. In an embodiment of the invention, a controller when switching a dual-mode phone from a cellular call to a Wi-Fi connection, establishes the Wi-Fi connection with the dual-mode phone, starts a timer, and begins teardown of the cellular leg of the call. When the timer expires, the controller begins sending voice traffic over the Wi-Fi connection to the dual-mode phone.

FIG. 1. shows an embodiment of the invention including a dual-mode phone. Enterprise SIP PBX communicates 210 with public switched telephone network (PSTN) 200. SIP PBX 100 also communicates with fixed mobility controller (FMC) 120. Mobility controller 120 bridges the worlds of SIP PBX 100 and Wi-Fi network controller 130. SIP PBX 100 and fixed mobility controller 120 communicate for example using protocols established by RFC 3261, RFC 2833, and RFC 3631, incorporated herein by reference. Voice streams are typically encoded using the RTP protocols defined by RFC 3550, incorporated herein by reference.

As understood in the art, such a hardware platform as shown in FIG. 1 for hosting mobility controller 120 includes a central processing unit (CPU) 122 connected to memory hierarchy 124, and to one or more interfaces 126. In one embodiment, CPU 122 is a MIPS-style CPU such as those from Raza Microelectronics or Cavium Networks, although CPUs from other companies such as Intel, AMD, Freescale, or the like may also be used. Memory hierarchy 124 includes read-only memory for system initialization, high-speed read-write memory such as DRAM, and bulk memory such as a hard disk or compact flash storage for files. Interfaces 126 include wired and wireless Ethernet interfaces as are known to the art. Controller 120 typically operates under the control of an operating system such as Linux, hosting software processes which provide device functionality. Additional hardware may be provided to provide for high-speed packet handling, crypto processing, and the like. While FIG. 1 shows mobility controller 120 and Wi-Fi-network controller 130 as separate entities, it should be noted that they may also be software processes running on a shared hardware platform. As an example, a single hardware platform based on a multi-core MIPS processor such as those from Raza Microelectronics or Cavium Networks could host the functions of Wi-Fi network controller 130 along with mobility controller 120 and additional functions such as intrusion detection and/or firewalls.

Dual-mode phone 300 has first radio subsystem 310 for Wi-Fi communications such as to Wi-Fi network controller 130. Dual-mode phone 300 also has second radio subsystem 320 for communicating with cellular network 230 and through public switched telephone network 200. As is common with many such devices, dual-mode phone 300 is a digital device operating under control of CPU 330 through programs and data stored in memory hierarchy 340. Other aspects of dual-mode phone 300 such as speakers, microphones, power and display, keyboards and the like are understood by those in the art, and are not shown.

In operation, dual-mode phone 300 may operate as a cellular phone, connecting to cellular network 230 and public switched telephone network 200 through second radio subsystem 320. Or, it may operate as a Wi-Fi phone, connecting to public switched telephone network 200 through controller 120 and SIP PBX 100.

During call handover, dual-mode phone 300 and mobility controller 120 must deal with two RTP voice streams, one from the cellular side, and the other on the Wi-Fi side. Envisioning these sides as buffers, the cellular side represents a longer, deeper buffer, with more voice data in transit. During call handover, which is mediated by controller 120, care should be taken to minimize cluttered and/or duplicated audio at dual-mode phone 300.

In both scenarios described herein according to the invention, one leg of call 240 comes in through SIP PBX 100. As an example, if a caller dials in to the enterprise, the call comes in 240 to SIP PBX 100. If the destination is dual-mode phone 300, call 240 is handled by controller 120, which routes the call to the Wi-Fi side of dual mode phone 300 if the phone is within Wi-Fi range, or to the cellular side of dual-mode phone 300 if the phone is not within Wi-Fi range.

Handover from Wi-Fi to Cellular. Assume dual-mode phone 300 is in a Wi-Fi call with another phone. Call 240 comes in through SIP PBX 100 through controller 120 and Wi-Fi network controller 130 to Wi-Fi radio subsystem 310. This is shown in FIG. 2 with call leg 2 representing the connection between controller 120 and the Wi-Fi side of dual-mode phone 300, and call leg 1 representing the PSTN connection between controller 120 and the incoming PSTN call 240 through SIP PBX 100. Assume that either dual-mode phone 300 or controller 120 sense that dual-mode phone 300 is moving out of Wi-Fi range, and initiate the handover process with controller 120. Controller 120 begins call setup on the cellular number associated with dual-mode phone 300 by sending INVITE to SIP PBX 100, which uses PSTN 200 to establish a call through cellular network 230 and cellular radio 320 in dual-mode phone 300. This new leg is shown as call leg 3.

When dual-mode phone 300 completes the cellular connection as call leg 3, the Wi-Fi connection is still present. Controller 120 receives SIP notification (2000K) from PBX 100 that the cellular leg has been completed. Controller 120 then switches the connection of FIG. 2 between call leg 1 and Wi-Fi call leg 2 to call leg 1 and cellular leg 3. This switching is performed, as is known to the art, by changing the network address translation (NAT) bindings associated with the RTP data streams for the call legs, directing RTP data from leg 1 to leg 3, and data from leg 3 to leg 1. The Wi-Fi connection may then be terminated; controller 120 sends back a SIP ACK message to PBX 100, and then disconnects Wi-Fi leg 2 of the connection by sending the SIP BYE message to the Wi-Fi side of dual-mode phone 300 through Wi-Fi controller 130.

Handover from Cellular to Wi-Fi. Assume dual-mode phone 300 is in a cellular call with another phone. Call 240 comes in through SIP PBX 100 through controller 120 (leg 1 of FIG. 2), back through SIP PBX 100 (leg 2), and connects 210 to PSTN 200, 220, to cellular network 230, through to cellular radio subsystem 310 of dual-mode phone 300. When dual-mode phone 300 moves in range of Wi-Fi network 130, controller 120 is notified, and initiates the transition of the call to the Wi-Fi network by initiating a Wi-Fi (SIP) connection through Wi-Fi controller 130 to dual-mode phone 300.

When a Wi-F SIP connection (leg 3 of FIG. 2) is established between dual-mode phone 300 and controller 120, controller 120 receives a SIP notification (2000K) from dual-mode phone 300. At this stage, both Wi-Fi and cellular connections are active to dual-mode phone 300. Once controller 120 receives the SIP 200 OK message from dual-mode phone through Wi-Fi controller 130, controller 120 starts the timer 128 for a predetermined period and starts the release procedure for cellular call leg 2. When timer 128 expires, allowing data buffered in the cellular network to run out, controller 120 switches the call through to the Wi-Fi leg and sends the SIP ACK message to dual-mode phone 300 via WI-FI controller 130. This switching is performed as before by changing the NAT bindings of the RTP data streams associated with the call legs.

While the invention has been described in terms of several embodiments, the invention should not be limited to only those embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is this to be regarded as illustrative rather than limiting.

Claims

1. A method of transferring a call from the cellular side to the Wi-Fi side of a dual-mode phone, the call mediated by a controller, comprising: receiving, in the controller, an indication that the dual-mode phone is within Wi-Fi range,the controller making a Wi-Fi connection to the dual-mode phone, the dual-mode phone having a cellular connection mediated by the controller,the controller starting a timer for a predetermined time when the Wi-Fi connection to the dual-mode phone is made, andconnecting the call to the Wi-Fi connection when the timer expires.

2. The method of claim 1 where the step of making a Wi-Fi connection further comprises: the controller initiating the connection through a Wi-Fi network, andthe controller receiving a notification the Wi-Fi connection has completed.

3. The method of claim 1 where the step of connecting the call to the Wi-Fi connection further comprises: changing NAT bindings of the RTP data streams associated with the call and the Wi-Fi connection.

4. The method of claim 1 where the call is directed to the controller by a SIP PBX.

5. The method of claim 1 where the dual-mode phone initiates the release procedure for the cellular connection when the Wi-Fi connection from the controller is made.