Patent Application: US-201113298868-A

Abstract:
the invention relates to a method for network discovery in a wireless communication network comprising communication devices sending announcement signals regularly with a period being equal to or exceeding a predefined minimum announcement interval , wherein a first communication device a ) communicates with a second communication device during a data exchange phase on a first channel ; b ) freezes the communication with the second device by signalling a freezing message terminating the data exchange phase ; c ) scans for the announcement signal of third communication devices on a second channel in a scan phase , wherein the scan phase duration is shorter than the minimum announcement interval ; d ) unfreezes the communication with the second communication device by signalling an unfreezing message ; and e ) repeats steps a ) through d ).

Description:
the preferred embodiment of the present invention will be best understood by reference to the drawings , wherein identical or comparable parts are designated by the same reference signs throughout . it will be readily understood that the present invention , as generally described and illustrated in the figures herein , could vary in a wide range . thus , the following more detailed description of the exemplary embodiments of the present invention , as represented in fig1 - 8 is not intended to limit the scope of the invention , as claimed , but is merely representative of presently preferred embodiments of the invention . an exemplary embodiment of the invention will now be explained in further detail , wherein a detailed performance analysis of opportunistic scanning using the 802 . 11 power save to pause any ongoing communication while scanning for other technologies , will be discussed . the following topics will be addressed : assessing the performance limits while considering implications of the 802 . 11 - architecture and protocol such as delayed beacons and clear channel assessment accounting for random backoff due to a busy media ; comparing results for an idle communication channel to analytically derived performance limits ; evaluating the influence of background traffic on the performance of opportunistic scanning ; and quantifying the cost of opportunistic scanning including a detailed discussion of the introduced protocol overhead . first , an exemplary embodiment of system model will be introduced followed by a short description of the opportunistic scanning approach and how it may be applied to a 802 . 11 system using power save as a signaling protocol . a set of 802 . 11 access points ( aps ) is considered , each having a high capacity connection to the internet . these aps may be located at user premises ( home networks ) or in highly populated urban areas ( public hot spots ). 802 . 11 devices have a standard compliant implementation of the mac but are not necessarily limited to operate only on the 2 . 4 and 5 ghz frequency band defined in the standard . hereby , our architecture implicitly allows 802 . 11 - based devices to be run in the “ white space ” recently opened for unlicensed operation . no constraints are imposed on the backhaul connection except that we assume that the backhaul &# 39 ; s media access is strictly separated from the last hop . this assumption allows a wide range of architectural choices ranging from wired links ( e . g ., ethernet ), over having heterogeneous ( wireless ) technologies for the last hop and backhaul ( e . g ., 802 . 11 and wi - max ), up to using homogeneous technologies on different frequencies ( e . g ., 802 . 11b / g vs . 802 . 11a ). this assumption is feasible as backhaul connectivity is usually set up by a service provider which tries to avoid any effects of end - usersystems on its backhaul technology . regarding the last hop , each 802 . 11 ap forms an infrastructure basic service set ( bss ). all considered bsss belong to the same extended service set ( ess ). if a bss does not belong to the same ess , clients may not use this bss for roaming . hence , detecting such bsss is conceptually identical to the detection of any other technology ( e . g . the presence of a primary user for white space operation ) which is not used for communication purposes . bsss may overlap and hence frequently operate on different channels to reduce interference . also , the coverage area of a bss may overlap with the one of a secondary technology ( e . g . wimax or a 3 / 4g network ). we assume that any present technology announces their existence at regular time intervals , e . g . by broadcasting a beacon or frame header . ( 802 . 11 or wimax ) or by a recurring energy pattern which can be identified but not necessarily decoded by the scanning sta ( footprint - based detection of primary users in white space ). in our system model , users are 802 . 11 - based stations ( sta ) located within a bss and are connected to the internet via their associated ap . under the “ best - connected network paradigm ”, stas may continuously choose among alternative links , i . e . another 802 . 11 ap or secondary technology . thus , they continuously scan in order to detect alternative technologies or evaluate the link properties on other frequencies . such continuous scanning is also used to detect primary users for white - space operation . the general concept of opportunistic scanning is driven by the paradigm that seamless connectivity ( as seen by the end user ) does not mean guaranteeing zero - delay , interruptionfree communication but limiting the duration and frequency of possible interruptions to an upper bound not affecting the qos constraints of the user application , hence not being noticeable to the user . as a result , our scanning approach is driven by two main constraints : the approach should enable real - time communication with small packet interarrival times and hard qos constraints requiring low packet loss and relative small extra delay at mac . such applications include , e . g ., voice over ip ( voip ) as well as telemetry applications . second , the scanning approach shall only passively scan the scanned channel . this assures that opportunistic scanning does scale with the number of stations employing this approach and does not ( unproductively ) affect any communication on the scanned channel . the following sections highlight the general idea of the opportunistic scanning approach and show how opportunistic scanning can be applied to an ieee 802 . 11 based wlan . in general , opportunistic scanning aims at periodically leav ing the current communication channel only for a very short time to conduct a scanning procedure as indicated by the scan - intervals ( si ) in fig1 . the approach hereby assumes that the system / technology to be discovered announces its existence at regular time intervals on the scanned channel δt beacon . please note that the term beacon includes any kind of footprint identifying a technology — ranging from a decodable signaling packet ( as known from homogeneous technologies , i . e . 802 . 11 ) up to a unique energy pattern ( whose contents cannot be interpreted but only recognized as known from the primary user detection concept in the white space ). thus , two unique phases comprise the scan interval : a data exchange and a scan phase . in order to avoid packet loss , the former also involves signaling to any interlocutor to pause ongoing transmission ( sleep - procedure ) before it ends , as well as to continue sending data ( wake - up procedure ) in its beginning . depending on the implementation , opportunistic scanning allows to prioritize either the scanning or the data exchange process . the former guarantees a minimum scan duration and hence stops the data exchange even if user data packets are pending for transmission . such priority could be given in a white space environment where the upmost goal is detecting the primary user . the latter would always exchange any pending user data packets even at the cost of reducing the scan duration nearly to zero . thus , this second mode is suitable if qos - constrained data exchange is valued higher than network discovery and was hence our choice for this embodiment of the invention . the power save feature of ieee 802 . 11 [ 7 ] allows a station to signal its interlocutors to hold ( and buffer ) any pending traffic . though , it does not deem the signaling station to actually go into power save mode . hence we herein use this time for passively scanning another channel . it should be noted that in most common implementations , the “ sleeping ” station only returns from power save after the reception of a 802 . 11 beacon which would result in unacceptably long sleeping times . nevertheless , a rarely used sequence of power save signaling messages allows the station to resume communication at any time . this enables us to use the existing , standard compliant power save feature to apply opportunistic scanning to a 802 . 11 network . hereby , a sta signals to the ap that it will go into “ sleep mode ” for at most n beacon intervals . nevertheless , the sta may return from its “ sleep mode ” any time before this period expires . fig2 and 3 illustrate the resulting protocol details for the wake - up and sleep process . actually , all the signaling information can be piggy - backed in the transmission of pending up - link data packets . the only overhead for this approach comes if no uplink data is pending — at which a null - data packet has to be transmitted . also the standard requires power save stations to explicitly request all buffered packets in the downlink using a ps - poll frame [ 7 ]. for the sake of briefness , the reader is referred to [ 8 ] for a detailed discussion of the signaling procedure . therein , the theoretical performance limit of the approach is derived and prime numbers as optimal choices for the scan interval are recommended . in the following we aim at classifying the theoretical performance limits of the opportunistic scanning approach . in particular , we intend to answer the following questions : how large is the minimum duration just for the whole power save signaling ? how long does it take to find an existing station at a given probability ? answering the former quantifies the smallest possible turnaround time from data exchange to scanning and back to data exchange if 802 . 11 power save is used as the underlying signaling protocol . hence it is a measure for the smallest supportable service interval for user data . the latter in turn assess the time required in the overlap of adjacent cells to successfully complete the topology discovery under the optimistic assumption that the station scans only one channel on which an alternative mesh ap is known to be found . also , these results may be used to quantify an upper limit after which the opportunistic scanning process should start its topology discovery on a new channel if no station has been found . in the following , we employ analysis to assess these theoretical limits . we consider two adjacent mesh nodes having an overlapping coverage area . both mesh nodes un - synchronously transmit beacons to announce their existence at regular time intervals as defined by the 802 . 11 standard . the analysis considers the opportunistic scanning station being stationary located within the overlap . it is associated with one of the mesh nodes . apart from the beacon transmissions and communication between the opportunistic scan station with its associated mesh node , the channel is assumed idle . our analysis makes use of the following two metrics : power save mode duration and beacon reception probability . the power save mode duration defines the time from the beginning of the signaling involved to transition from the “ awake ” into the “ doze ” and back into the “ awake state ”. it quantifies the service interruption imposed on the application due to the opportunistic scanning approach . the beacon reception probability quantifies the number of scanning attempts / time required to successfully receive a beacon at a given probability . fig4 illustrates the signaling sequence involved in going from “ awake ” into “ doze ” and immediately back into “ awake state ”. as we do not spend any time in the “ doze state ”, we are actually not conducting any opportunistic scanning at all . this quantifies the smallest possible duration to switch back and forth between channels . in order to hold a specific qos constraint , the minimum power save duration represents the lower bound for the inter - arrival time of application data at mac level . the minimum time spent in power save mode ( t minpsm ) is given by assuming an idle channel , equation ( 1 ) can be directly simplified into t minpsm = t difs + 2 · t sifs + 2 · t data - ul ++ 2 · t ack + t probed ( 2 ) apart from phy specific parameters ( t difs , t sifs , and t probed ), t minpsm depends on the employed modulation and coding scheme ( mcs ) for the data and acknowledge frame [ 9 ]. fig5 shows the minimal achievable psm duration for parameterization and defined mcs for two situations : first assuming that the signaling is transmitted in a null data frame , and second , if it is piggy backed in a voip data stream packet assuming an underlying g . 711 codec and 10 ms packetization without silent suppression . obviously , the smallest achievable interruption of roughly 1 . 3 ms occurs for the low - est packet size ( null data frame ) at the highest data rate . but also a 2 . 6 ms - long interruption at the most robust mcs schemes is acceptable even for hard real time services [ 10 ]. also , the theoretical limits show that opportunistic scanning should be a feasible approach not noticeably affecting voip applications as service interruption for piggy backed signaling may be reduced to less than 6 ms for the most robust mcs . in order to detect a neighboring mesh ap during the nth + 1 opportunistic scanning attempt , the beginning of the scanning t ss has to be before the beginning of the beacon reception / start t bs and the end of the scan t se has to lie after the beacon &# 39 ; s end t be ( c . f . fig6 ): t ss = t offset + n scan · δt scan t bs = n beacon · δt beacon t be = t bs + t bencon t se = t ss + t scan where t offset is a random variable uniformly distributed over [ 0 , δt beacon ), δt beacon the target beacon transmission time , δt scan the scan interval , and t scan the ( effective ) scan duration remaining after the involved signaling is deducted from the time span given by δ tscan . equation 3 can accordingly be rewritten into which gives the condition if beacon number m beacon is successfully received within scan attempt n scan . solving the latter equation numerically and due to the stochastic nature of t off - set , we obtain the probability functions of detecting a beacon at a given scan attempt / after a given time ( c . f . fig7 ). obviously , t offset and δt beacon may not have a common divider to guarantee beacon detection . as we assume that a provider will employ common values with multiples of 10 ms for the target beacon transmission time ( e . g ., 100 ms ) we choose prime numbers for δt scan . as expected , longer scan intervals yield to better results but interestingly , the effect is less noticeable if one considers the time required to find a beacon as compared to the number of scanning attempt . a topology discovery in two target beacon transmission times ( tbtt ) is possible . this is only twice the time needed as compared to traditional passive scanning resulting in long service interruptions . but even unsuitable scan intervals resulting in a high duration can accomplish a successful discovery within five tbtts . fig8 shows an exemplary embodiment of a communication device 10 capable of network discovery in a wireless communication network comprising devices which send announcement signals regularly with a period being equal to or exceeding a predefined minimum announcement interval . communication device 10 comprises a transmitting unit 20 and a receiving unit 30 adapted to communicate with a second communication device during a data exchange phase on a first channel . communication device 10 further comprises a control unit 40 adapted to freeze and unfreeze the communication with the second device by signalling a freezing message terminating the data exchange phase . receiving unit 30 is configured to scan for the announcement signal of third communication devices on a second channel in scan phases . the scan phase duration is preferably shorter than the minimum announcement interval . e . perahia , “ vht 60 ghz par plus 5c &# 39 ; s ,” ieee 802 . 11 vht sg very high throughput study group , denver , colo ., usa , doc . 11 - 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whitespace - 09 / 0039r3 . “ fcc part 15 . 711 : interference avoidance mechanisms for telefision band devices , fcc rules for radio frequency devices ,” http :// www . hallikainen . com / fccrules / 2009 / 15 / 711 /, april 2009 . ieee 802 . 11 - 2007 — wireless lan medium access control ( mac ) and physical layer ( phy ) specifications , ieee std . 802 . 11 - 2007 , 2007 . m . emmelmann , s . wiethölter , and h .- t . lim , “ opportunistic scanning : interruption - free network topology discovery for wireless mesh networks ,” in international symposium on a world of wireless , mobile and multimedia networks ( ieee wowmom ), kos , greece , jun . 15 - 19 2009 . ieee 802 . 11 - 2007 — wireless lan medium access control ( mac ) and physical layer ( phy ) specifications , 2007 virtual automation networks consortium . real time for embedded automation systems including status and analysis and closed loop real time control . deliverable d04 . 1 - 1 , ec information society technology , july 2006 .