Intelligent roaming in AGV application

Details Intelligent roaming in AGV application Intelligent roaming in AGV application

1999-07-29

2001-12-18

Nguyen, Lee (Department: 2683)

Telecommunications

Radiotelephone system

Zoned or cellular telephone system

C455S434000, C455S435100, C370S331000, C370S338000

Reexamination Certificate

active

06332077

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a roaming algorithm in a wireless LAN, and more specifically to a method for associating a mobile station with an access point in a wireless LAN system used in an automatic guidance vehicle (AGV) application.
BACKGROUND OF THE INVENTION
Description of a Wireless Local Area Network (LAN) Systems:
According to the wireless LAN standard (IEEE 802.11), a wireless LAN station needs to connect to an AP (assess point) for accessing a file server or other workstations of a network system. The AP connects distributed systems and a set of basic service areas. It provides the communication between a local station in a wireless LAN and a remote station in another wireless LAN. It also provides the capability of accessing distributed systems for a station.
FIG. 1
shows the typical hardware architecture of a wireless LAN system. As illustrated in
FIG. 1
, an Ethernet LAN
100
may have two basic service sets
101
(BSS
1
) and
102
(BSS
2
), one workstation
103
and one file server
104
. Each basic service set comprises one access point and two wireless stations. Basic service set BSS
1
includes the access point AP
1
and two stations STA
1
and STA
2
. Basic service set BSS
2
includes the access point AP
2
and two stations STA
3
and STA
4
. Each wireless station should associate itself with one AP before it can communicate with the file server
104
or the workstation
103
in the Ethernet LAN
100
.
When a wireless LAN station is powered on, it first looks for an AP. After it finds an AP, the wireless LAN station registers itself with the AP. The station can then synchronize with the AP and, thereafter, transmit and receive information packet to and from the AP.
According to the wireless LAN standard FHSS (Frequency Hopping Spread Spectrum) (IEEE 802.11), the bandwidth used in the RF (radio frequency) is between 2.40 Ghz and 2.50 Ghz among 79 channels in the US and Canada areas. The frequency hopping radio signal shall hop in the frequency band between 2.40 Ghz and 2.50 Ghz as shown in FIG.
2
. The AP shall be the timing master and shall perform the TSF (timing synchronization function) to keep the timers for all wireless stations in the same BSS synchronized. For each channel, the AP shall periodically transmit special frames called beacons each containing a copy of its TSF timer and hopping sequence to synchronize other wireless stations in a BSS. The AP shall broadcast a generated beacon frame at a fixed time interval referred to as a beacon interval. A receiving wireless STA shall always accept the timing information in beacons sent from the AP servicing its BSS. If a wireless STA's TSF timer is different from the timestamp in the received beacon frame, the receiving wireless STA shall set the value of its local timer to the received timestamp value.
Traditionally, when an AP is powered on in a basic service set, it periodically broadcasts a beacon frame to all wireless STAs in the service set. The beacon frame contains the information for synchronizing with the AP and the information is referred to by the wireless STA looking for the AP. Because the beacon interval used in the traditional approach is long (100 mini seconds), it could increase the search time when wireless STA is looking for the AP.
A wireless LAN station needs to do a registration before it can communicate with an AP. According to 802.11 standard, the registration should include two phases. The first phase is authentication. This process is used to identify whether the station has the legal right or not to access the wireless network via a medium access control (MAC) layer. After the authentication is passed, the station needs to establish a connection link with the AP by sending an association request packet to the AP and waiting to receive a response frame from the AP which acknowledges the association.
The general steps taken by a traditional MAC sub-layer management architecture for a wireless STA from search to registration with an AP are shown in FIG.
3
. According to the design of
FIG. 3
, a wireless STA shall perform the three phases: scanning phase, authentication phase and association phase.
In the scanning phase, all IEEE 802.11 STAs initially start from “IDLE” state (step
301
). The radio hopping channel shall be set by the hardware (step
302
). To search for the AP, a wireless STA shall listen in each channel and scan for any beacon frame transmitted by the AP (step
303
). Upon finding the channel and receiving the AP beacon, the scanning phase is completed. Otherwise, the wireless STA shall listen in the next channel and return to step
302
. A new radio hopping channel shall be set.
After receiving the AP beacon, the receiving wireless STA will join a BSS by setting its local hopping time and channel sequence according to the information contained in the AP beacon (step
304
) in order to maintain synchronization with the AP. The remaining steps constitute the authentication phase and association phase.
In the authentication phase, IEEE 802.11 in general requires bi-directional authentication. At step
305
, the wireless STA exchanges some packets with the AP for authentication. It may further include three steps of assertion of identity, challenge of assertion and response to challenge.
In the association phase (step
306
), the wireless STA exchanges the association packets with the AP to accomplish the AP registration. The communication link between the wireless STA and the AP is thus setup. The wireless STA can start exchanging data frames with the AP (step
307
).
Because the radio frequency hops between 2.40 Ghz and 2.50 Ghz, a wireless LAN station may take a long time to find an AP among the 79 channels in Federal Communications commissions (FCC) regulation.
Each frame used in authentication and association should be sent in complying with the rule of to 802.11 standard (Carrier Sense Multiple Access with collision Avoidance, CSMA/CA). The time consumed for a wireless station to complete all the processes of authentication and association may vary greatly, and is unpredictable, especially when the traffic in the wireless medium is busy. Therefore, the total time from searching for an AP to accomplish the registration is unpredictable for the wireless LAN stations that comply with 802.11 standard.
From the foregoing discussions of prior arts, it can be seen that the disadvantages as pointed out earlier make the conventional access scheme inappropriate for a wireless LAN station to connect with an AP. There exists a strong need for having a more efficient access scheme for the wireless LAN station to quickly access a wireless LAN through an AP.
DESCRIPTION OF PROBLEM IN THE PRESENT INVENTION
In general, the wireless LAN is used in a working field. A vehicle travels along a predetermined track through a plurality of BSS. NDC's existing roaming algorithm can not support the application need for AP's hand over (switching the radio connection from the current AP to the next one which is similar to cellular phone application) without delay. Due to AGV's (Automatic Guidance Vehicle) mobility, this delay is critical to its application in the factory. For example, if the search time for each channel is 200 mini seconds, since there are 80 hopping channels, totally, an average time of 8 seconds is required for an AGV to establish a connection with the AP in the following BSS. This delay is a long period of time. Often, in this time interval, the vehicle has traveled through a long distance. The result is that some messages can not be communicated between the AGV and other control unit. If the work is strictly controlled from a remote controller, then the work can not be continued. Therefore, not only the work process is not successful but also dramatic fault may be generated.
Therefore, an intelligent roaming algorithm is desperately needed to resolve this issue. Thereby, by this roaming algorithm, we hope the search time during handing over from one BSS to another BSS may be reduced greatly.
SUMMARY OF THE INVENTIO

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