Patent Application
20060040666
The present disclosure relates generally to wireless communications, and more particularly to in-call hand-off among different networks, for example, between a wireless local area network (WLAN) and a cellular communication network, communication networks and devices and corresponding methods.
While wireless local area networks (LANs) are used primarily for packet data communications, the idea of carrying voice packets through packet data networks has been explored extensively and is known generally. Wireless LANs are often established for connectivity in environments where signal strength and the quality of cellular communication network coverage are not optimal, for example, within buildings and in underground complexes. Recently, the growing proliferation of wireless LANs has raised interest in providing wireless communication devices that can communicate in both conventional wireless cellular networks and in wireless LANs.
While the idea of communicating in wireless LANS and cellular communication networks is known generally, seamlessly interconnecting wireless local area networks (LANs) and cellular networks poses significant challenges. In voice call communications, for example, interoperability between LANs and cellular communication networks requires capabilities like roaming and handoff that are presently available only in cellular communication networks.
Wireless LANs, many of which operate pursuant to one of the IEEE 802.xx protocols on unlicensed radio spectrum, and cellular networks transmit data and access networks fundamentally differently. Handoff in cellular networks is initiated when base station signal quality drops below a threshold. In cellular networks, where forward and reverse link transmissions are on different frequencies, signal quality measurements are performed easily at the physical layer by directly measuring signal strength or by measuring signal to noise ratio (SNR) information. In a wireless LAN, where network devices share a common frequency, the transmission medium is allocated to only one transmitter at a time for varying durations through a random access procedure. In wireless LANS, signal strength measurements made by a mobile station (MS) will vary depending on the proximity of the entity transmitting within the WLAN. This makes it difficult to measure the quality of the link between the access point (AP) and the wireless communication device using physical layer processes, e.g., signal strength, SNR etc. typically used in cellular networks. These and other differences make for difficulty determining when to handoff from a wireless local area network to other networks like cellular networks.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below.
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The exemplary system of networks 200 also includes a cellular communication network 120 comprising a base station controller (BSC) 122 communicably coupled to one or more cellular base transceiver stations (BTS) 124. The cellular BSC 122 is also communicably coupled to a mobile switching center 126, which is communicably coupled to a public switched telephone network. The cellular BSC 122 is also communicably coupled to a packet data serving node (PDSN), which is communicably coupled to a packet network. Those having ordinary skill in the art are familiar with these and other aspects of cellular communication networks.
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In the IEEE 802.11 wireless standard among other protocols, no part of the physical layer header identifies the originating device. Thus the origin of a particular frame can be determined at the medium access control (MAC) layer only if the frame is received correctly at the physical layer. Since it is not possible to determine the origin of frames received incorrectly, link quality, for example, the frame error rate between the wireless communication device and the access point, cannot be determined autonomously. In other words, in networks where the physical layer messaging information does not identify the source of the message, the total number of frames sent is known only by the source from where the frames originated unless the recipient device receives all frames without error.
To determine uplink frame error, for example, in 802.11 and like applications where physical layer header information does not identify the device originating the frames, it is necessary for the wireless communication device to indicate the number of frames that it sent to the access point. The access point may then determine the frame error rate based on the number of frames received correctly from the wireless communication device relative to the total number of frame sent by the wireless communication device over the tracking period or number of frames tracked, as indicated by the frame tracking variable.
Similarly, to determine the downlink frame error in 802.11 and like applications, it is necessary for the access point to indicate the total number of frames that it sent, i.e., to all devices, during the tracking period or during the number of frames tracked. The recipient device determines the downlink frame error rate based on the number of frames received correctly relative to the total number of frames sent.
In another embodiment, the request 222 from the wireless communication device includes, alternatively, information necessary for the access point to compute the uplink frame error. According to this alternative embodiment, the access point provides the uplink frame error information in the response at 224 instead of or in addition to any information that may be required for the wireless communication device to determine the downlink frame error.
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In some embodiments, the wireless communication device, also referred to as a mobile station (MS), obtains link frame error information periodically, which maybe on a regular or an irregular basis. In other embodiments frame error information is obtained only when the signal quality measured at the physical layer satisfies a condition, for example, when a characteristic of the signal drops below a threshold. For example, the wireless communication device may enter a frame error monitoring mode where frame error information is obtained periodically whenever a signal quality condition measured at the physical layer is satisfied. Limiting the time during which frame error information is obtained reduces error report signaling between the wireless LAN and wireless communication device and also reduces power consumption in the wireless communication device.
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In some embodiments, alternatively, the communication from the MS to the AP at 302 is a request for information required for the MS to compute the downlink frame error. Such a message may include the total number of frames “t”, and possibly the number of frames received in error “n” by the MS. In yet another embodiment, the communication from the MS to the AP at 302 is provides information sufficient for the AP to compute the uplink frame error. The latter exemplary message may include the total number of frames “t”, and the number of frames sent by the MS to the AP, “m”.
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At block 310, the MS determines the frame error for the downlink between the access point and the wireless communication device, for example, by computing a ratio of the number of frames received correctly by the MS from the AP relative to the total number of frames sent by the AP. In one embodiment, the wireless communication device makes the handoff determination by comparing the uplink and/or downlink frame error information to threshold information. Alternatively, the handoff determination or decision may be made on any other basis. In
While the present disclosure and what are presently considered to be the best modes thereof have been described in a manner establishing possession by the inventors and enabling those of ordinary skill in the art to make and use the same, it will be understood and appreciated that there are many equivalents to the exemplary embodiments disclosed herein and that modifications and variations may be made thereto without departing from the scope and spirit of the inventions, which are to be limited not by the exemplary embodiments but by the appended claims.
