EXHIBIT 7

Case 2:23-cv-00444-JRG Document 1-8 Filed 09/27/23 Page 1 of 13 PageID #: 128
(12) United States Patent 

Van Erven 

USOO8432892B2 

(10) Patent No.: US 8,432,892 B2 

(45) Date of Patent: *Apr. 30, 2013 

(54) 

(75) 

(73) 

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(21) 

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(65) 

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METHOD AND SYSTEM FOR WLAN 

SYNCHRONIZATION 

Inventor: Niels van Erven, Santa Clara, CA (US) 

Assignee: Hewlett-Packard Development 

Company, L.P., Houston, TX (US) 

Notice: Subject to any disclaimer, the term of this 

patent is extended or adjusted under 35 

U.S.C. 154(b) by 0 days. 

This patent is Subject to a terminal dis 

claimer. 

Appl. No.: 13/215.937 

Filed: Aug. 23, 2011 

Prior Publication Data 

US 2012/OO51247 A1 Mar. 1, 2012 

Related U.S. Application Data 

Division of application No. 1 1/943,297, filed on Nov. 

20, 2007, now Pat. No. 8,023,495, and a continuation 

of application No. 1 1/102,436, filed on Apr. 8, 2005, now abandoned. 

Int. C. 

H04.3/06 (2006.01) 

U.S. C. 

USPC ............................ 370/350; 370/345; 370/445 

Field of Classification Search .......... 370/229 240, 

370/445-448,210, 350, 345 

See application file for complete search history. 

s 

(56) References Cited 

U.S. PATENT DOCUMENTS 

5,923,702 A 7/1999 Brenner et al. 

6,693.915 B1 2/2004 Lappetelainen et al. 

6,816.510 B1 1 1/2004 Banerjee 6,917,804 B2 7/2005 Takayama et al. 

7,126,937 B2 10/2006 Crosbie et al. 

8,023,495 B1* 9/2011 van Erven ..................... 370/350 

2003. O145092 A1 7/2003 Funato et al. 

2005, OO58121 A1 3/2005 Santhoff et al. 

2005/0058153 A1 3/2005 Santhoff et al. 

2005/017.4953 A1 8, 2005 Ho 

2005/0243954 A1 11/2005 Li 

2006/0098606 A1 5/2006 Pandey et al. 

2006, O133394 A1 6/2006 Ware et al. 

* cited by examiner 

Primary Examiner — Christian Hannon 

(57) ABSTRACT 

A first access point for a multi-channel radio system includes 

a wireless transceiver to send and receive data; and a clock, 

with the first access point to: Scan the multi-channel radio 

system to receive beacon signals transmitted from other 

access points operating in the multi-channel radio system; 

determine a Received Signal Strength Indication (RSSI) level 

for each beacon signal received from another access point operating in the multi-channel radio system; identify a second 

access point from which a beacon signal was received having 

a lowest RSSI level of all beacon signals received from other 

access points in the multi-channel radio system; and synchro 

nize the clock with the second access point Such that a beacon 

signal transmitted by the first access point is transmitted at the 

same time as a beacon signal from the second access point. 

19 Claims, 5 Drawing Sheets 

 

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US 8,432,892 B2 

1. 

METHOD AND SYSTEM FOR WLAN 

SYNCHRONIZATION 

RELATED APPLICATIONS 

The present application is a divisional application and 

claims the priority under 35 U.S.C. S120 of U.S. application 

Ser. No. 11/943,297, filed Nov. 20, 2007, now U.S. Pat. No. 

8,023,495 which is allowed, and is a continuation and claims 

the priority under 35 U.S.C. S 120 of U.S. application Ser. No. 

1 1/102,436, filed Apr. 8, 2005, which is abandoned. These 

previous related applications are hereby incorporated by ref 

erence in their respective entireties. 

BACKGROUND 

1. Field of the Invention 

The present invention is directed to telecommunications. 

More particularly, the present invention is directed to meth 

ods and systems for synchronizing Access Points within a 

Wireless Local Area Network. However, aspects of the inven 

tion may be equally applicable in other scenarios as well. 

2. Description of Related Art 

Over the recent years, the market for wireless communica 

tions has enjoyed tremendous growth. Wireless technology 

now reaches or is capable of reaching virtually every location on the face of the earth. One reason for this tremendous 

growth has been the IEEE wireless Local Area Network 

(“LAN) standard 802.11. 

An 802.11 LAN is based on a cellular architecture where 

the system is subdivided into cells. Each cell or Basic Server 

Set (“BSS) is controlled by a Base Station. This Base Station 

is often referred to as an Access Point (AP). Even though a 

wireless LAN may be formed by a single cell, with a single 

Access Point, and in certain arrangements it can also work 

without an Access Point. Generally, however, most LAN 

installations will comprise several cells. The Access Points 

connected through some kind ofbackbone called Distribution 

System (“DS”), typically Ethernet, and in some cases wire 

less itself. 

The whole interconnected Wireless LAN including the 

different cells, their respective Access Points and the Distri 

bution System, is seen to the upper layers of the OSI model, 

as a single 802 network, and is called in the Standard an 

Extended Service Set (“ESS”). 

The standard also defines the concept of a Portal where a 

Portal is a device that interconnects between an 802.11 and 

another 802 LAN. This concept is an abstract description of 

the functionality of a “translation bridge.” As any 802.x pro 

tocol, the 802.11x protocol covers that MAC and Physical 

Layer, the Standard currently defines a single MAC which 

interacts with three PHY's (all of them running at 1 and 2 

Mbit/s up to 54 Mbit/s). Frequency hopping spread spectrum 

in the 2.4 Ghz Band, Direct Frequency Spread Spectrum in 

the 2.4 GHz Band, and Infrared. Beyond the standard func 

tionality usually performed by MAC Layers, the 802.11 

MAC performs other functions that are typically related to 

upper layer protocols, such as Fragmentation, Packet 

Retransmissions, and Acknowledges. The MAC Layer 

defines two access methods, the Distributed Coordination 

Function and the Point Coordination Function. 

The basic access mechanism, called Distributed Coordina 

tion Function, is a Carrier Sense Multiple Access with Colli 

sion Avoidance mechanism (“CSMA/CA). CSMA proto 

cols are known, where the most popularis the Ethernet, which 

is a CSMA/CD protocol (CD standing for Collision Detec 

tion). 

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With a CSMA protocol, a station desiring to transmit 

senses the medium. If the medium 20 is busy (i.e., some other 

station is transmitting), then the station will defer its trans 

mission to a later time. If the medium is sensed free or avail 

able for transmission, then the station is allowed to transmit. 

These kinds of protocols are effective when the medium is not 

heavily loaded, since it allows stations to transmit with mini 

mum delay. However, there is a chance of stations transmit 

ting at the same time (collision), caused by the fact that the 

stations sensed the medium free and decided to transmit at 

OCC. 

These collision situations should be identified, so that the 

MAC layer can retransmit the packet by itself and not by 

upper layer, which would cause significant delay. In the Eth 

ernet case, this collision is recognized by the transmitting 

stations which go to a retransmission phase base on an expo 

nential random backoff algorithm. While these Collision 

Detection mechanisms are a good idea on a wired LAN. Such 

Collision Detection mechanisms are typically difficult to 

implement in a Wireless LAN environment. For example, 

implementing a Wireless LAN based Collision Detection 

Mechanism would require the implementation of a Full 

Duplex radio, capable of transmitting and receiving at once. 

Such a full duplex approach would also tend to increase the 

complexity and price of the system significantly. 

In addition, on a wireless environment one cannot assume 

that a JI stations hear each other. This is generally a basic 

assumption of the Collision Detection scheme. The fact that a 

station willing to transmit and senses the medium free, does 

not necessarily mean that the medium is free around the 

receiver area. In order to overcome these two concerns, the 

802.11 uses a Collision Avoidance mechanism together with 

a Positive Acknowledge scheme. In this Positive Acknowl 

edge scheme, a station willing to transmit senses the medium 

is busy then it defers transmission. If the medium is free for a 

specified time (called DIFS, Distributed Inter Frame Space), 

then the station is allowed to transmit. 

A receiving station will check the CRC of the received 

packet and send an acknowledgment packet (ACK). 

Receipt of the ACK will indicate to the transmitter that no 

collision occurred. If the sender does not receive the acknowl 

edgment then it will retransmit the fragment until it gets 

acknowledged or thrown away after a given number of 

retransmissions. 

In order to reduce the probability of two stations colliding 

because they cannot hear each other, 802.11 defines a Virtual 

Carrier Sense mechanism. A station willing to transmit a 

packet will first transmit a short control packet called RTS 

(Request to Send). The RTS includes the source, destination, 

and the duration of the following transaction (i.e., the packet 

and the respective ACK). The destination station responds (if 

the medium is free) with a response control Packet called CTS 

(“Clear to Send'). The CTS will include the same duration 

information. 

Stations receiving either the RTS and/or the CTS, will set 

their Virtual Carrier Sense indicator (called NAV. for Network 

Allocation Vector), for the given duration, and will use this 

information together with the Physical Carrier Sense when 

sensing the medium. This mechanism reduces the probability 

of a collision on the receiver area by a station that is “hidden' 

from the transmitter, to the short duration of the RTS trans 

mission, because the stations will hear the CTS and “reserve” 

the medium as busy until the end of the transaction. The 

duration information on the RTS also protects the transmitter 

area from collisions during the ACK (by stations that are out 

of range from the acknowledging stations). 

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Because RTS and CTS are short frames, it also reduces the 

overhead of collisions, since these are recognized faster than 

it would be recognized if the whole packet was to be trans 

mitted, (this is true if the packet is significantly bigger than the 

RTS, so the standard allows for short packets to be transmitted 

without the RTS/CTS transaction, and this is controlled per 

station by a parameter called RTS Threshold. 

Typical LAN protocols use packets of several hundred of 

bytes (e.g., Ethernet longest packet could be up to 1518 bytes 

long), ona Wireless LAN environment there are some reasons 

why it would be preferable to use smaller packets. For 

example, because of the higher Bit Error Rate of a radio link, 

the probability of a packet to get corrupted increases with the 

packet size. In addition, in the case of packet corruption 

(either because of collision or noise), the smaller the packet 

the less overhead it causes to retransmit it. In addition, on 

Frequency Hopping system, the medium is interrupted peri 

odically for hopping (in our case every 20 milliseconds), so 

the smaller the packet, the smaller the chance that the trans 

mission will be proposed to after the dwell time. 

The mechanism is a simple Send-and-Wait algorithm, 

where the transmitting station is not allowed to transmit a new 

fragment until one of the following happens: receives an ACK 

for the said fragment or decides that the fragment was retrans 

mitted too many times 15 and drops the whole frame. 

Exponential Backoff is a method that attempts to resolve 

contention between different stations willing to access a 

medium. The method requires each station to choose a Ran 

dom. Number n between 0 and a given number. Then, wait for 

this number of Slots before accessing the medium, and then 

checking whether a different station has accessed the medium 

before. The Slot Time is defined in such a way that a station 

will be capable of determining if another station has accessed 

the medium at the beginning of the previous slot. This reduces 

the collision probability by half. 

Exponential Backoff means that each time the station 

chooses a slot and happens to collide, it will increase the 

maximum number of the random selection exponentially. 802.11 defines Exponential Backoff Algorithm that will be 

executed in the following case. Where, if when the station 

senses the medium before the first transmission of a 5 packet, 

and the medium is busy. 

When a station wants to access an existing BSS (either after 

power-up, sleep mode, or just entering the BSS area), the 

station needs to receive synchronization information from an 

Access Point (or from the other stations when in an ad-hoc 

mode). The station can get this information by one of two 

means: passive scanning or active scanning. First, it may 10 

get this information by passive Scanning. In the passive scan 

ning case, the station waits to receive a beacon frame from the 

AP. The beacon frame is a periodic frame sent by the AP with 

synchronization information. 

Alternatively, active scanning may be used where a station 

tries to find an AP by transmitting Probe Request Frames, and 

waiting for Probe Response from the AP. Either of these 

methods may be chosen according to the power consumption/ 

performance itself 

Once the station has found an AP, and the station has 

decided to join the AP's BSS, the station will go through the 

Authentication Process. This Process is an interchange of 

information between the AP and the station, where each side 

proves the knowledge of a given password. 

When a station is authenticated, then the station will start 

the Association Process. This process is the exchange of 

information about the stations and BSS capabilities, and 

which allows the DSS (the set of APs to know about the 

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current position of the station). Only after the association 

process is completed, a station is capable of transmitting and 

receiving data frames. 

Stations need to keep synchronization since this is needed 

for keeping hopping synchronized, and other functions like 

Power Saving. On an infrastructure BSS this is performed by 

all the stations updating their clocks according to the AP’s 

clock. This synchronization occurs as the AP transmits peri 

odic frames called Beacon Frames. These Beacon Frames 

contain the value of the AP's clock on the moment of trans 

mission. 

Note that this is the moment when the transmission really 

occurs, and not when it is put in the queue for transmission, 

since the Beacon Frame is transmitted using the rules of 

CSMA, the transmission may be delayed significantly. 

The receiving stations check the value of their clock at the 

receiving moment and correct it to keep synchronizing with 

the AP's clock. This prevents clock drifting which could 

cause loss of synchronization after a couple hours of opera 

tion. 

In a typical wireless network, such as a Wireless Local Area 

Network (“WLAN) system, 15 each Access Point (AP) 

will have its own internal clock. Each independent clock will 

also define the Time Division Multiplexing (“TDMA) tim 

ing of the SSID area that this AP covers. Consequently, this 

means that all AP's, provided in a certain WLAN SSID area, will be not be synchronized. Consequently, these unsynchro 

nized BSS cells may interfere with one another. These unsyn 

chronized BSS cells will interfere with each other even if they 

are operating on the same channels or alternatively adjacent 

yet different channels. 

In an effort to reduce such interference concerns, it is 

generally known that the overall system throughput may be 

improved when there is at least beacon synchronization and 

scheduling between all stations in the ESS environment. 

Existing mechanisms like fragmentation, data rate back-off. 

adaptive contention window threshold, as other mechanisms, will help improve the system throughput to a certain extent. 

However, there is, therefore, a general need for a method and 

system for synchronizing WLAN APs. There is also a general 

need for a method and system for synchronizing WLAN APs 

while also improving system capacity and QoS by utilizing 

AP synchronization and beacon scheduling. 

SUMMARY 

According to an exemplary embodiment, a method of syn 

chronizing a first Access Point and a second Access Point is 

provided to increase an overall system capacity due to a 

reduction of inherent packet collisions in an un-synchronized 

WLAN deployment. Preferably, the method comprises the 

steps of deploying the first and the second Access Point within 

a wireless network and utilizing the first Access Point to 

detect a presence of the second Access Point. A first periodic 

Beacon Frame of the first Access Point is synchronized with 

a first periodic Beacon Frame of the second Access Point. 

According to another exemplary embodiment, a system for 

synchronizing a wireless cellular architecture is provided. 

This system includes a first Access Point and a second Access 

Point deployed in the wireless cellular architecture. A pres 

ence of the second Access Point is detected by the first Access 

Point. A first periodic Beacon Frame of the first Access Point 

is synchronized with a first periodic Beacon Frame of the 

second Access Point. 

These as well as other advantages of various aspects of the 

present invention will become apparent to those of ordinary 

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skill in the art by reading the following detailed description, 

with appropriate reference to the accompanying drawings. 

BRIEF DESCRIPTION OF THE DRAWINGS 

An exemplary embodiment of the present invention is 

described herein with reference to the drawings, in which: 

FIG. 1 illustrates one arrangement for an ESS comprising 

a plurality of AP's operating on various channels; 

FIG.2 provides one illustration of various transmitted Bea 

con Frames from a plurality of unsynchronized Access 

Points; 

FIG. 3 illustrates beacon timing graphs from two AP hav 

ing inconsistent clock frequencies; 

FIG. 4 illustrates a plurality APs (and/or ACs) transmitting 

over different channels; and 

FIG. 5 illustrates an arrangement of sequential beacons. 

DETAILED DESCRIPTION 

There are several interference scenarios which can arise in 

a BSS/ESS and related interference characteristics that will 

need to be discussed. Each interference scenario depends how 

the pathloss and selectivity filter works out. 

Pathloss is calculated as follows: 

Pathloss (PL)=(32.4+20 log F (MHz)+20 log D (km)), 

where F is the Frequency in MHZ and D is the Distance in 

Kilometers. 

In one arrangement, where F-2.4 GHz, this results in a 

path-loss: PL=32.4+67.6+20 log D 

R-radius of the coverage circle (axMbps, Xe 1.2.5.5.11, 6.9,12,18,24.36.48.54} SIR, SIR required to get |O PER for xMbps at a R' 

radius with adjacent/alternate channel int reference 

SIR, SIR required to get |O PER for xMbps at a R' 

radius with co-channel int reference 

SIR, SIR 4.6 dB (with ideal sphere packing) In the above equations, SIR represents a signal to inter 

ference ratio that arises between two cells that are operating 

on different or alternate channels. And SIR represents a 

signal to noise ratio that may arise between two cells that are 

operating on the same or similar channels. Regarding the 

SIR case (i.e., adjacent/alternate channel interference), a 

selectivity filter attenuates the signal in general by about 35 

dB. Each multi channel radio system, such as 802.11, has 

minimal requirements on how much the adjacent and alter 

nate channels has to be suppressed by the selectivity filter. Its 

up to the radio vendor how much they want to improve on this. 

Such is generally not the case for the SIR case (co-channel 

interference). So at the boundary of the coverage circles for 

e.g. Rss (to allow some coverage overlap for R1 and R2, 

required for roaming), the co channel interference cell results 

in about 30 dB more interference than the adjacent channel 

cells. This is an important phenomenon when considering 

Access Point deployment within a BSS or ESS. 

The change of Access Points interfering with one other. In 

a normal Access Point deployment scenario, the various AP’s 

will be located generally e.g. about 20 to 25 meters apart from 

each other. With Such a general placement location, this 

would mean that the signals received from a first AP by a 

second AP (operating on a different channel) will be about 

-80 dBm. This value may be calculated using the pathloss PL 

formula +35 dB for selectivity filter provided above. The 

sensitivity of 1 Mbps can be around-93 dBm. This indicates 

that the change that every received package will be corrupted 

due to an AP in the vicinity operating on a different channel 

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can not be ignored. The data rate Backoff Mechanism dis 

cussed above generally may help in reducing this IS corrup 

tion issue to a certain degree. 

There are a variety of scenarios that may arise for which 

CSMA/CA initiates the Backoff scenario. These scenarios 

will result in reduced capacity or where interference can 

occur regardless of CCA. Consequently, because of Such 

interference, there is a general likelihood that data packages will be destroyed and that further packet retransmission will 

be required. 

FIG. 1 illustrates one arrangement for an ESS 10 compris ing a plurality of APs operating on various channels. ESS 10 

comprises a plurality of APS Some of which are operating on 

the same channel and some of which are not operating on the 

same channel. More specifically, ESS 10 comprises APs oper 

ating on channels 1, 6, and 11. The first dotted line of each AP 

is indicative of 11 Mbps coverage. Note that two coverage 

areas of 1 Mbps coverage areas are also shown. The first 1 

Mbps coverage 38 is shown for AP16 and the second 1 Mbps 

coverage area 28 is illustrated for AP 30. 

ESS 10 comprises a first set of APs operating on channel 1 

(AP 22, 26, and 36), a second set of APs operating on channel 

6 (AP 16, 18, and 30), and a third set of APs operating on 

channel 11 (AP12, 20, 24). Note the spatial orientation of the 

APs operating of the different channels. The locations of the 

various APs in ESS 10 present a number of interference 

possibilities. 

For example, one Such interference scenario arises where 

AP's are operating on the same channel and these APs will 

interfere with one another. For example, referring to FIG. 1, 

AP 16 and AP 30 are both operating on channel 6 and will 

consequently interfere with one another as illustrated by the 

11 Mbps coverage area for AP16 and 30. The 1 Mbps power 

level 38 of AP 16 is illustrated as overlapping with the 11 

Mbps power level of 15 client 32 residing in AP 30 coverage aca. 

Another interference scenario will arise where AP's are 

operating on different channels and will interfere with one 

another. For example, the cluster of three AP's 36, 30, 24 

operating on channels 1, 6, and 11, respectively, will interfere 

with one another. Therefore, client 34 residing between these 

three APs will experience interference from two out of these 

three APs (one AP is the AP it communicates with). 

Another interference scenario will arise where the APs 

operating on different channels interfere with one another as 

adjacent and alternate channel interference. Such interfer 

ence may be seen by the various Small dashed circles defining 

the operating area of each AP. Yet another interference situa 

tion arises where a client detects an AP on the same channel 

but is not necessarily associated with that particular detected 

AP. For example, referring back to FIG. 1, such an interfer 

ence situation could arise for client 32 by the 1 Mbps operat ing coverage of AP 16, resulting in co-channel interference. A 

similar interference situation could arise where a client 

detects AP's on different channels. For example, a client 

could detect AP 16 operating on channel 6 while also detect 

ing AP 36 operating on channel 1. 

Another concern that may arise occurs where a client inter 

feres with a client(s) on same channels of different BSS. And 

yet another concern is client interference with clients on 

different channels of a different BSS. 

Typically, in an ordinary BSS or ESS layout, AP's operat 

ing on a similar channel will be located further away as that is 

the only “selectivity” mechanism available to reduce interfer 

ence. Typically, adjacent and alternate channels can be 

located closer to each other as the selectivity filter will enable 

that. In general a selectivity filter can Suppress approximately 

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35 dB for an alternate channel, and can Suppress somewhat 

less for an adjacent channel. Consequently, an AP on an 

alternate channel may be located on the order of approxi 

mately 5 to 6 times closer to the AP under consideration, then 

cochannel AP. Therefore, in a typical BSS/ESS cellular 

deployment, such as the cellular deployment depicted in FIG. 

1, co-channel AP's have a higher chance to interfere with each 

other. 

As previously discussed, WLAN uses DCF as the control 

method to allow clients to have access on the BSS. Within 

these BSS, clients are time synchronized to a specific AP with 

whom they are associated with. All other clients and AP's in 

a ESS deployment are not synchronized with each other. For 

Example, AP's illustrated in FIG. 1 are not synchronized with one another. 

Stations within a BSS need to keep synchronization, this is 

needed for keeping hopping synchronized, and other func 

tions like Power Saving. On an infrastructure BSS this is 

performed by all the stations updating their clocks according 

to an AP clock. This synchronization occurs as the AP trans 

mits periodic frames called Beacon Frames. These Beacon 

Frames contain the value of the AP's clock on the moment of 

transmission. Not that this is the moment when the transmis 

sion really occurs, and not when it is put in the queue for 

transmission, since the Beacon Frame is transmitted using the 

rules of CSMA, the transmission may be delay significantly. 

The receiving stations check the value of their clock at the 

receiving moment and correct it to keep synchronizing with 

the AP's clock. This prevents clock drifting which could 

cause loss of synchronization after a couple hours of opera 

tion. Within a ESS, the stations from one BSS with a other 

BSS will however not be synchronized. 

FIG. 2 provides one illustration 50 of how various trans 

mitted Beacon Frames from a plurality of unsynchronized 

Access Points may interrupt or corrupt a data packet in an 

ESS, such as the ESS illustrated in FIG.1. As shown in FIG. 

2, four beacon timing graphs 52, 54,56, and 58 are illustrated. 

Beacontiming graphs 52,54,56, and 58 are generated by four 

different APs operating in a ESS. For example, in one 

arrangement, beacon timing graph 52 could be generated by 

an AP operating on channel 1, beacon timing graph 54 could 

be generated by an AP operating on channel 6, beacon timing 

graph 56 could be generated by an AP operating on channel 

11, and beacon timing graph 58 could be generated by an AP 

operating on yet another different channel. 

Since each of the four APs are operating on a different 

channels, each AP will generate a different beacon timing graph. The top most timing graph 52 a first beacon timing 

graph from a first AP, a second beacon timing graph from a 

second AP, and a third beacon timing graph from a third AP. 

As shown intiming graph 52, the beacons from three different 

AP are not synchronized and arise at different times. For 

example, in this arrangement, beacon 52 from a first AP occurs at time t1 while the second beacon 54 from a second 

AP occurs at time t2, a third beacon 56 from a third beacon 3 

occurs at time t3, and yet the fourth beacon occurs at time ta. 

The fifth timing graph 60 in FIG. 2 illustrates a timing of a 

data package 62 received from a fifth AP Comparing the five 

timing graphs 52, 54, 56, 58, 60, it can be seen that the 

beacons generated by the first AP, the second AP and the 

fourth AP's interfere with the data 62 received by the fifth AP 

in this unsynchronized ESS. 

The beacon timing interference concerns as illustrated in 

FIG. 2, will not occur at fixed time instances. This relates to 

the accuracy of the AP clock frequencies. For example, FIG. 

3 illustrates clock timing graphs from two AP's having incon 

sistent clock frequencies. More specifically, FIG.3 illustrates 

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the effect if two clocks having different frequencies. Such 

inconsistent clock frequencies may be due to the 25 or 20 ppm clock accuracy requirement. 

As shown in FIG. 3, the frame 82 of a first AP (such as AP 

16 in FIG. 1) may be slightly shorter than frame 84 for a 

second AP (such as AP 36 in FIG. 1), having the exact same 

amount of bits/symbols in the frame. Such a frame difference 

is due to the clock differences between the two APs. Conse 

quently, there is a first difference delta 86 that quantifies the 

difference in frames between the first and second frames 82 

and 84, respectively. Therefore, over a certain period of time, 

the frames from the first AP may slide over the frames from 

the second AP2. For example, in one arrangement comprising 

a system having a 25 ppm clock accuracy, this may result in a 

time slip of about 5 minutes that two frames will side over 

each other from beginning to end. For example, after a certain 

period of time, there will be a second difference delta 88 

between the two frames 82 and 84. The effect mentioned 

above is very much present for all the beacons sent out by the 

APS 

As there are a number of potential interference scenarios 

present in a WLAN system where APs are not synchronized, 

and as a WLAN system has a highly random behavior, only 

one case will be highlighted to give some qualitative insight. 

Eventually extensive discrete event simulation can be used to 

determine the detailed improvements for different scenarios 

and applications. 

For example, when client 32, in FIG. 1, is associated with 

AP30, the beacons of the other APs, such as AP36 and AP 20, 

can interfere with the data frames transmitted by 15 AP30 and 

intended for client 32. This provides that e.g. the beacon 

power of AP36, received at client32, may be above a Gaus 

sian noise floor when client 32 receives that power. This 

would results in client 32 having a lower data rate or even 

loosing data package send out by AP 30. 

There are several ways to eliminate this un-synchronized 

packet interference phenomena between different BSS cells. 

A first ideal solution is a complete synchronous TDMA 

approach over an ESS. This should be done for each AP and 

each Access Controller (“AC). An access controller, also 

referred to as a wireless Switch, can be considered as a Sub 

ESS. Within an AC, the timing control of packet is much 

better than for fit (standalone AP) AP's as each fit AP (fit AP: 

AP's controlled by the AC), inherently has a tight control with 

the AC. But between different AC's the same problem exist as 

between fat AP's. So the synchronization and scheduling 

mechanism should be applicable for both fat AP's as AC’s 

over different BSS and even ESS deployments. 

As each fit AP and AC can be considered independent stand 

alone units, they must synchronize amongst one another. 

Synchronization may be accomplished among the plurality of 

APs and ACs comprising the ESS by having these entities 

communicate amongst themselves. There are a number of 

methods for Such communication. For example, in one 

arrangement, the APs and ACS synchronize with each other 

by methods used in Bluetooth, DECT, GSM, etc. 

These methods use the following approach: during acqui 

sition/bootstrapping each AP and/or AC scans the operating 

environment for other APs or ACs. When there is no other AP 

or AC present, the AP or AC it simply begin a normal opera 

tion. Alternatively, if the AP or AC determines during the 

acquisition or bootstrapping process that there is another 

AP/AC present, the AP it will synchronize to it such that its own beacons will not collide with the beacons of the other 

APs and ACs. 

Over the air synchronization will inherently protect syn 

chronization for its associated client with respect to neigh 

Case 2:23-cv-00444-JRG Document 1-8 Filed 09/27/23 Page 11 of 13 PageID #: 138
US 8,432,892 B2 

9 

borhood BSSS/EES's cells. An AC, which does not use over 

the air synchronization, but only uses hardware synchroniza 

tion, can’t guarantee this as it will not adjust itself to other 

AC’s or fit AP's. 

One method of synchronizing AP's that preserves overall 

system bandwidth comprises the steps of combining beacons 

of various APS and ACs in a sequential manner. Such a 

sequential arrangement may depend on certain factors. For 

example, such a sequential arrangement may depend on the 

size and/or the number of AP's deployed within a specific 

BSS or ESS 

For example, FIG. 4 illustrates a ESS comprising a plural 

ity APs operating on three different channels. In the ESS 

arrangement illustrated in FIG. 4, these three channels 

include 1, 6, and 11. For example AP 104 is operating on 

channel 11, AP 106 is operating on channel 1, and AP 108 is 

operating on channel 6. Preferably, the plurality of APs illus 

trated in FIG. 4 are so configured in an attempt to reduce 

adjacent/alternate channel interference as well as co-channel 

interference. As those of ordinary skill in the art will recog nize, however, alternative arrangements and/or deployments 

may be used as well. 

In an arrangement comprising a large deployment of APs, 

there may be many channel 1, 6 and 11 clusters defined. In 

such an arrangement as illustrated in FIG.4, AP132,134,130 operating on channels 1, 6 and 11, respectively are configured 

with the same cross hatching and therefore may define a first 

beacon cluster 111 (cluster 111 is not referred to anything. It 

should refer to FIG. 5 cluster 152). Although these three APs 

132, 134, and 130 are operating on different channels (i.e., 

channel 6 for AP132, channel 11 for AP134 and channel 1 for 

AP 130), these APs will be synchronized so that they will 

transmit their respective beacons at the same time. 

Similarly, AP 120, 124, and 122 operating on channels 1, 6 

and 11, respectively are configured with the same cross hatch 

ing and therefore may define a second beacon cluster 154 

(FIG. 5). Although these three APs 120, 124, and 122 are 

operating on different channels (i.e., channel 1 for AP 120, 

channel 6 for AP 124 and channel 11 for AP 122), these APs 

will be synchronized so that they will transmit their respective 

beacons at the same time. 

In an arrangement where only a small number of APS may 

be used, each AP may define a separate cluster. For example, 

in a system illustrated in FIG. 4, AP 104 may define a first 

cluster, AP106 may define a second cluster, and AP 108 may define a third cluster. Then, in Such an arrangement, a first 

beacon signal will be transmitted by AP 104, while the second 

beacon signal will be transmitted by the second AP 108 while 

a third beacon signal will be transmitted by AP 106. 

FIG. 5 illustrates an arrangement 150 of a plurality of 

sequential beacons. As illustrated in FIG. 5, three beacon 

signals 152, 154, and 156 are provided. Each beacon signal 

152, 154, and 156 is related to one of the AP clusters defined 

in FIG. 4. For example, for beacon cluster 102, a first beacon 

signal 152 may be representative of AP 104, while the second 

beacon signal 154 is representative of the second AP 108 

while the third beacon signal may be representative of AP 

106. 

There are a number of assignment algorithms that may be 

used in an arrangement comprising a randomly deployed 

environment of APs, such as the AP deployment illustrated in 

FIG. 4. For example, in one arrangement, a first AP listens to 

an ambient environment and detects the other deployed AP’s 

since these are transmitting beacons. The first AP performs 

this scanning on all channels (1, 6, and 11 for the 2.4 GHz 

ISM band, or all the 5 GHZ channels for 802.11a). When the 

beacon of a second AP is received at a RSSI level which is the 

10 

15 

25 

30 

35 

40 

45 

50 

55 

60 

65 

10 

lowest detected RSSI, the first AP will synchronize to that 

second AP. In this manner, when the first AP commences 

transmission of its beacons, the first and the second AP will 

transmit at the same time. However, the second AP will alter 

its channel before it starts transmitting its beacons. 

Other solutions schedule the beacons transmission one 

after the other for each AP in succession. One disadvantage of 

Such an approach is that this approach requires more band 

width. One advantage of the present Solution versus Such 

other approaches is that it results in beacon times being on 

average of about 300 us. Therefore, to utilize such beacon 

transmission schemes for a 48 port AC, it will take approxi mately 14 ms to synchronize these beacons in the proper 

sequence. Consequently, this will consume 14% of the avail 

able capacity/bandwidth. If the beacons are scheduled based 

upon channel clustering as suggested above, it will only con 

sume approximately 1% of the available capacity for 802.11b 

band g. 

To optimize this scheduling a second mechanism is pro 

posed and that involves accurate synchronization. Such a 

mechanism may be used to optimize the timing of CCA 

approach in CSMA/CA. For example, regarding compensat 

ing round trip delay for VoIP clients. When a packet is sent 

from an AP to a client, the air propagation delays reception. 

After the VoIP client transmits back it’s voice data, the same 

propagation delay will exist. This delay will be different for 

each client in a BSS (some are close to the AP, others are 

further away). To optimize expectation of reception and to 

reduce jitter (and indirectly system capacity), accurate timing is required. 

To reduce the number of times that fat AP's and AC's need 

to update their synchronization with its environment. The 

lower the ppm, the less scan efforts needs to be performed. 

This will result in a reduced demand for bandwidth. 

In an alternative arrangement where the WLAN system is 

operating in a PCF or HCF mode, for certain critical real time 

applications like VoIP, the AP sends out a voice packet to a 

VoIP client. For ideal Quality of Service and system perfor 

mance, it is required that the AP knows when to receive a 

packet back. As the time of the voice packet and the time of 

the handset processing time are known, the only variables are 

air propagation and Xtal offset between the stations. As the 

handset listens to the beacons, it can then self synchronize its 

own internal clock (VCXO). Consequently, this results in 

getting the clock of the AP and the clock of the handset 

synchronized to less than approximately 0.1 ppm. This accu 

rate synchronization approach is used in wireless TDMA 

systems throughout the industry and is considered common 

knowledge. 

Exemplary embodiments of the present invention have 

been described. Those skilled in the art will understand, how 

ever, that changes and modifications may be made to these 

embodiments without departing from the true scope and spirit 

of the present invention, which is defined by the claims. 

What is claimed is: 

1. A first access point for a multi-channel radio system, said 

first access comprising: 

a wireless transceiver to send and receive data; and 

a clock, 

said first access point to: 

Scan the multi-channel radio system to receive beacon 

signals transmitted from other access points operating 

in the multi-channel radio system; determine a Received Signal Strength Indication (RSSI) 

level for each beacon signal received from another 

access point operating in the multi-channel radio sys 

tem; 

Case 2:23-cv-00444-JRG Document 1-8 Filed 09/27/23 Page 12 of 13 PageID #: 139
US 8,432,892 B2 

11 

identify a second access point from which a beacon 

signal was received having a lowest RSSI level of all 

beacon signals received from other access points in 

the multi-channel radio system; and 

Synchronize said clock with the second access point 

Such that a beacon signal transmitted by the first 

access point is transmitted at the same time as a bea 

consignal from the second access point. 

2. The access point of claim 1, wherein the first access point 

scans all channels of the multi-channel radio system to 

receive beacon signals from other access points in the system. 

3. The access point of claim 1, wherein said first access 

point, after synchronizing with the second access point, oper 

ates on a different channel than the second access point. 

4. The access point of claim 1, wherein said first access 

point, before synchronizing with the second access point, 

operates on a different channel than the second access point. 

5. The access point of claim 1, wherein the first access point 

scans 5 GHZ channels defined by the 802.11a standard. 

6. The access point of claim 1, wherein the first access point 

scans channels of 2.4 GHz Industrial, Scientific and Medical 

(ISM) radio bands. 

7. The access point of claim 1, wherein the first access point is a carrier sense multiple access with collision avoidance 

(CSMA/CA) access point. 8. The access point of claim 1, wherein the first access point transmits beacon signals comprising a value of the clock of 

the first access point at a time when the first access point transmits that beacon signal. 9. The access point of claim 1, wherein synchronizing with 

the second access point comprises adjusting the clock of the 

first access point using clock data in the beacon signal received from the second access point. 10. A first access point for a multi-channel radio system, said first access comprising: 

a wireless transceiver to send and receive data; and 

a clock, 

said first access point to: 

transmit a beacon signal, said beacon signal comprising 

a value of the clock of the first access point at a time 

when the first access point transmits that beacon sig 

nal, 

Such that a second access point operating in the multi 

channel radio system synchronizes a clock of that 

second access point by extracting the time data from 

the beacon signal of the first access point and adjust ing the clock of the second access point in accordance 

with that time data from the beacon signal of the first 

access point. 

11. The access point of claim 10, wherein said first access 

point transmits on a first channel prior to synchronizing with 

said second access point and then changes to transmit on a 

Second channel after synchronizing with said second access 

point. 

12. The access point of claim 10, the first access point to 

Scan the multi-channel radio system to receive beacon sig nals transmitted from other access points operating in 

the multi-channel radio system; 

10 

15 

25 

30 

35 

40 

45 

50 

55 

12 

determine a Received Signal Strength Indication (RSSI) level for each beacon signal received from another 

access point operating in the multi-channel radio sys 

tem; 

identify an access point from which a beacon signal was 

received having a lowest RSSI levelofall beacon signals received from other access points in the multi-channel 

radio system; and 

synchronize with that access point having the lowest RSSI 

level such that a beacon signal transmitted by the first 

access point is transmitted at the same time as a beacon 

signal from the access point having the lowest RSSI 

level. 

13. The access point of claim 12, wherein the first access 

point scans all channels of the multi-channel radio system to 

receive beacon signals from other access points in the system. 

14. The access point of claim 10, wherein said first access 

point, before synchronizing with the second access point, 

operates on a different channel than the second access point. 

15. The access point of claim 10, wherein the first access 

point transmits on a 5 GHz channel defined by the 802.11a 

standard. 

16. The access point of claim 10, wherein the first access 

point transmits on a channel of 2.4 GHz Industrial, Scientific 

and Medical (ISM) radio band. 

17. The access point of claim 10, wherein the first access 

point is a carrier sense multiple access with collision avoid 

ance (CSMA/CA) access point. 18. Access points for a multi-channel radio system, each 

access point comprising a wireless transceiver to send and 

receive data; and a clock, comprising: 

a first access point to: 

scan the multi-channel radio system to receive beacon 

signals transmitted from other access points operating in the multi-channel radio system; determine a ReceivedSignal Strength Indication (RSSI) 

level for each beacon signal received from another 

access point operating in the multi-channel radio sys 

tem; 

identify a second access point from which a beacon 

signal was received having a lowest RSSI level of all 

beacon signals received from other access points in 

the multi-channel radio system; and 

Synchronize with the second access point such that a 

beacon signal transmitted by the first access point is 

transmitted at the same time as a beacon signal from 

the second access point; and 

said second access point to: 

transmit said beacon signal having a lowest RSSI level at 

said first access point, that beacon signal comprising a 

value of the clock of the second access point at a time 

when the second access point transmits that beacon 

signal. 

19. The access points of claim 18, wherein said second 

access point transmits on a first channel prior to synchroniz ing with said first access point and then changes to transmit on 

a second channel after synchronizing with said first access 

point. ck ck ck k ck 

Case 2:23-cv-00444-JRG Document 1-8 Filed 09/27/23 Page 13 of 13 PageID #: 140