Patent Application
20080080462
A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, preferred embodiments of the present invention are described.
According to the present invention, a mobile device in communication with a first wireless system will begin communication with a second wireless system prior to or immediately after losing communication with the first wireless system. The mobile device may begin communication with the second wireless system, such as a wireless access point (“WAP”), upon recognition that the signal strength of the first wireless system is deteriorating or losing strength. The handoff is handled quickly and substantially without interruption or loss of data packets, thereby, providing substantially seamless and continuous network connectivity during the handoff process. The present invention is thus particularly useful in improving the performance of delay sensitive applications, such as Voice-over-IP (VoIP) and streaming multimedia applications. The present invention may operate on widely available IEEE 802.11 wireless NIC cards with no customized support from the underlying operating system. However, the present invention is not limited to currently available wireless NIC cards and may optionally operate using customized support from the underlying operating system.
Background scanning is utilized to discover the presence of wireless systems in the surrounding network topology. The mobile device detects the presence of wireless systems in its vicinity by periodically scanning for radio signals from wireless systems. A wireless NIC card or the like is used to detect Wi-Fi signals indicative of a WAP within communication range. Other types of radio signals may also be detected that are indicative of other type of wireless systems, such as cellar systems, within the communication range of the mobile device within the scope of the present invention. Information relating to currently available wireless systems including, without limitation, security settings and network information, is maintained by the mobile device.
One problem associated with scanning for radio signals is that such scanning typically results in delays. For instance, during the period of time that the channel is selected for scanning, data cannot be received or sent. Additional delays in the wireless NIC are associated with switching channels. To reduce these delays, the present invention uses a combination of channel prioritization and dynamic prediction to select a wireless system with minimal or no delay.
Communication using a particular wireless system may be desired for a number reasons including, without limitation, the wireless system's security settings, network configuration, communication channel, better signal strength or the load in the associated wireless system. A particular level of security for a wireless system may be desired because the wireless system has the same security level as the currently connected wireless system, a minimum security level, a maximum security level, or a preferred security protocol. A handoff to a wireless system utilizing the same channel as the current wireless system may be desired because of the delay associated with switching channels. A particular channel may also be desired for other reasons including, without limitation, radio interference with other wireless systems, signal strength and existence of multiple wireless systems facilitating easier monitoring of the wireless systems and handoff process. Preferential selection of a particular channel over another channel is known as “channel prioritization.”
The mobile device may also select a new wireless system based on its subnet with respect to the subnet of the current wireless system in communication with the mobile device. According to the present invention, mobile devices are configured to handle both Layer2 and Layer3 handoffs between wireless systems. A Layer2 handoff is a handoff between two wireless systems within the same subnet. The mobile device generally maintains the same IP address on the new wireless system in a Layer2 handoff. A Layer3 handoff is a handoff between two wireless systems having different subnets. The mobile device does not maintain the same IP address on the new wireless system in a Layer3 handoff. Instead, the mobile device would request a new IP address on the new subnet using a technique known in the art, such as the Dynamic Host Configuration Protocol (“DHCP”). Prior to a handoff, the mobile device compares the subnets of the available wireless systems to determine whether handing off the mobile device to an available wireless system will require a new IP address to be allocated. A Layer2 handoff that does not require a IP address allocation is preferred over a Layer3 handoff because of the possibility of delays associated with a Layer3 handoff and the increased possibility of packet loss.
The present invention uses, selective MAC layer packet buffering and background address request messages to remove noticeable delays associated with both Layer2 and Layer3 handoffs in Wi-Fi networks. In one embodiment, a handoff buffer is used for selectively storing packets during the handoff process. Moreover, buffering scheme is used for storing packets when the mobile node performs background monitoring of wireless systems which are within the wireless range of the mobile node. The power saving mode feature available in IEEE 802.11 networks is utilized to buffer packets at the wireless system access point during a handoff and background monitoring of wireless systems to prevent packet loss. Utilizing the power saving mode feature decreases the probability of lost packets during link transitioning and makes the handoff process oblivious to the upper layer applications. However, other buffering techniques are possible within the scope of the invention.
Referring to
At location A, mobile device 12 establishes a connection with wireless system 20. At location B, mobile device 12 detects wireless systems 20 and 22. In this example, mobile device 12 maintains the connection with wireless system 20 at location B. Maintaining a connection with a wireless system may be due to numerous factors including, without limitation, the availability, signal strength, security and subnet configuration of the available wireless systems. At location C, mobile device 12 detects wireless systems 20 and 22. Mobile device 12 disconnects from wireless system 20 and establishes a connection with wireless system 22. This is also known as handing off communication from wireless system 20 to wireless system 22. According to the present invention, the handoff occurs while mobile device 12 is within the communication range of wireless systems 20 and 22. The handoff may occur at location C for any number of reasons including, without limitation, availability, signal strength and security configuration of the available wireless systems. For instance, the handoff may occur because wireless system 22 has a stronger signal than wireless system 20. At location D, mobile device 12 detects wireless system 22. In this example, mobile device 12 maintains the connection with wireless system 22.
Referring to
The handoff between different subnets will now be discussed. At location D, mobile device 12 detects wireless systems 62 and 76. Mobile device 12 hands off communication from wireless system 62 to wireless system 76 at location E. As previously discussed, the handoff may occur for various reasons. For instance, the handoff may occur because wireless system 76 has a stronger signal than wireless system 62. Because wireless systems 62 and 76 are on two different subnets, there may be additional delays associated with this handoff including the time required to associate mobile device 12 with a new IP address. However, our invention supports acquiring IP of different subnets beforehand during the background monitoring phase to decrease the delay of L3 handoff process. Accordingly, this additional delay may be taken into consideration during the handoff. In one embodiment, mobile device 12 will handoff while the signal strength of the wireless system currently in communication with mobile device 12 is still strong enough to remain in communication during this increased handoff period.
Referring to
Referring to
At block 160, an mobile device actively scans one or more radio frequencies (known as “channels”). The number of channels are calculated and marked as active at block 162. At block 164, an active scan is performed on the then-current channel. An active scan may include transmitting a PROBE_REQ message and waiting for a response from a wireless system. A list of available wireless systems 161 is updated at block 166. The mobile device will repeatedly switch channels, scan the then-current channel and update the list of available wireless systems 161, until all channels have been scanned at blocks 164-168. At block 170, the available wireless systems list 161 are sorted based on the RSSI. An IP address for the available wireless system is reserved at block 172. The available wireless systems list 161 is partitioned based on each wireless system's subnet at block 174. At block 176, the available wireless systems list 161 is sorted based on the channel frequency. Wireless system having the highest RSSI is selected from the available wireless systems list 161 at block 178.
At block 179, the mobile device connects and starts traditional operations with a wireless system.
Periodic background monitoring of the wireless systems is shown at block 180. At block 182, a delay between the periodic monitoring occurs. The mobile device instructs the then-connected wireless system to begin buffering by sending a PSM message to the wireless system at block 184. Moreover the mobile node starts inline buffering to prevent packet loss at the mobile end during the process. The mobile device switches the channel and broadcasts a PROB_REQ message in that particular channel and waits for a predetermined time to receive the PROBE_RES message from the wireless access points on that channel before switching to a different channel at blocks 186 and 188. The RSSI is measured and the wireless system RSSI trend is updated using EWMA technique at block 190. At block 192, the mobile device switches back to the original channel where it is associated to the wireless system. The mobile device then transmits a PSM awake message to the then connected wireless access point as a result of which the wireless access point releases the packets stored in the buffer to the mobile node at block 184. Also the mobile node releases its buffer and restarts the transmission. At block 198, the available wireless systems list 161 is reordered based on the results from the background monitoring. The background monitoring may occur periodically or upon the available wireless systems list 161 becoming depleted as shown at block 200. At block 210, the available wireless systems list 161 is updated.
The updating of the backup wireless systems lists is now discussed. The channels which are to be probed are identified at block 212. At block 214, a delay between the periodic probing occurs. The mobile device instructs the then-connected wireless system to begin buffering by sending a PSM message to the wireless system at block 216. The mobile device switches the channel and sends a PROB_REQ at blocks 218 and 220. A determination of whether a wireless system with an acceptable RSSI has responded is performed at block 222. If a wireless system with an acceptable RSSI has responded, then an IP address is reserved and the available wireless systems list 161 is updated based on the subnet. Otherwise, the mobile device switches back to the original channel at block 226. The mobile device then requests the then-connected wireless system to flush its buffer at block 228. The wireless system will respond by transmitting any buffered data packets to the mobile device. The probing continues until the number of wireless systems discovered satisfies the backup wireless systems lists. The mobile device is switched back into a normal processing mode at block 232.
Referring to
The present invention thus includes a computer program which may be hosted on a storage medium and includes instructions which perform the processes set forth in the present specification. The storage medium can include, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, flash memory, magnetic or optical cards, or any type of media suitable for storing electronic instructions.
Obviously, many other modifications and variations of the present invention are possible in light of the above teachings. The specific embodiments discussed herein are merely illustrative, and are not meant to limit the scope of the present invention in any manner. It is therefore to be understood that within the scope of the disclosed concept, the invention may be practiced otherwise then as specifically described.
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of IIS-0326505 awarded by the National Science Foundation (NSF).
