Flex-WiFi: a mixed infrastructure and ad-hoc IEEE 802.11 network for datatraffic in a home environment

Carlo Parata, Vincenzo Scarpa, Gabriella ConvertinoSTMicroelectronics, Advanced System Technology, Lecce, Italy[carlo.parata, vincenzo.scarpa, gabriella. convertino] @st.com

Abstract two stations in the network, without traversing the AP.The DLS must be negotiated between the two peers

This paper presents a novel architecture, called through a specific protocol that directly involves theFlex-WiFi, specifically designedfor IEEE 802.11 home AP (Figure 1); steps la/lb/2b/2a involve four differentnetworks. Objective of the new approach is to improve frames.legacy network capacity in such home scenarios.Network capacity is increased by allowing setting upmultiple contemporary IEEE 802.11 connections usinga single WLAN card. All connections share the same la lbdata link and physical layers but use different bXXchannels; the MAC is responsible to handle the

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channel switching as well as to allocate/de-allocate the QSTAI QSTA2direct link sessions. In particular we aim implementinga solution where stations can be associated to an Figure 1. Four steps 802.lle direct link setup.access point and can have ad-hoc connections on a

diferet pysialchanelat he am tie. he deais In this paper we propose an enhancement of the802.1 le Direct Link Session that introduces three

to maximize the overall network capacity by features: the DLS negotiation protocol does not rely ontransferring some traffic load from the infrastructure AP; the DLS may exploit a dedicated and negotiatedto a direct link session. Results have been provided by PHY channel; stations in direct link may go in powerimplementing Flex-WiFi in the Madwifi open source

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driveroftheAheros chipsets.saving (in 802.11le DLS stations in DLS cannot go indriver of the Atheros chipsets.poesaig power saving)1. Introduction According to our proposal, the stations in a BSS

can dynamically switch between an AP-relayedconnection and a direct connection. Besides, using aThe most used network topology for IEEE 802.11 difrnphsclhaelorted-ccnetos

WLAN [1]stheinrastructre mode.different physical channel for the ad-hoc connectionsWLAN [1] is the infrastructure mode...In the infrastructure mode, wireless stations (STAs) would allow further efficiency improvements for the

are associated to an access point (AP). The set of infrastructure network.We define two different transmission modes:associated stations (including the AP) is referred to as a

Basic Service Set (BSS). A station can communicate 1) Infrastructure mode (IM): when two stations

only with the AP even if the traffic is addressed to a belonging to the same BSS communicate through theAP (obviously this mode is mandatory if one out of thestation in the same BSS. This last rule guarantees thattosain steA)

communication is possible between all stations, even if two stationsthey are out of range to each other. But the mechanism 2)oDirect lin me (dLM)l whentoatinsintroduces~ ~unane infiinywe.tetosain belonging to the same BSS directly communicate usingratroducesundnge.o lm this prob stionst any physical channel different from the one used by theare in the each other range. To limit thAs problem n the PIEEE 802.1 le extension a new mechanism ,called

direct linkession (DLS)[2], has bee itoue .Ti The two modes introduce two alternative paths tomechanism~~ ~~alosecagn.aadrcl ewe connect the stations within a BSS: a two-hops path

1-4244-0992-6/07/$25.OO ©2007 IEEE

STAI -* AP -* STA2, for IM, and a single-hop In [3] authors propose methods to integrate ad-hocpath STAI -* STA2, for the DLM. operations into the infrastructure mode by inserting two

The main elements that characterize our approach schemes: 23 Ad-Hoc Awareness Direct Connectionare:

manelmnt ha haatele u apoah (AHADC) and Direct Cut-Through Forwardinga > ~~~~~~~~~~~~(DCTF).The first scheme brings ad-hoc awareness to

1) AP legacy compatibility. No changes apply to the TFr.aTue .st and minis th ener o

AP sack Staiontha imlemetDM ar reerre tothe infrastructure setthng and minimizes the number of

AP stack. Stations that implement DLM are referred to packets forwarded by the AP: the idea is that, if theas enhanced station (E-STA). DLM cannot be used to destination is directly reachable, the packet is receivedcommunicate to legacy STAs (including the AP). directly in ad-hoc mode, otherwise it is forwarded byWhile operating in DLM E-STAs cannot receive data the AP as normal. The DCTF scheme is introduced atfrom the AP since they operate on a different channel. the AP to minimize the packet forwarding delay ofIEEE 802.11 Power Saving Mode (PSM) [1] is intra-cell packets that cannot be directly delivered, byexploited in order to avoid packet losses: just before eliminating access contention and immediatelystarting a direct link connection an E-STA forwarding them after a short inter-frame space (SIFS).communicates its transition to PSM to the AP. This The two proposed methods improve the performancecauses the AP to store all the traffic directed to the (i.e. throughput) of WLAN systems with intra-cellPSM station until it returns in IM (this is an indication packets but do not include any support to Quality ofthat all buffered data can now be delivered). Service (QoS).

2) Efficiency improvement. Bandwidth savings occur In [4] the idea of ad-hoc connections that use differentsince some intra-cell traffic can now be managed channels is proposed to increase the networkthrough a single hop connection. performance since it allows multiplying the system

3) User transparency. Switching from IM to DLM bandwidth. The paper presents a framework (M2-(and vice versa) is implemented at the MAC layer and WLAN) where a node can dynamically switch between

the infrastructure mode and the ad-hoc mode accordingis completely transparent to the protocol higher layers the instruction of the aP. Theswitching(the IP address for the STA is always one), this means

orsentrto the And deensontrficthat aSTA ma have oncurrnt IM nd DL transparent to the users and depends on traffic

that a STA may have concurrent IM and DLM condition in the cell: if the traffic load is high, the APsessions. requests some local groups to switch to the ad-hoc

4) The exploitation of IMand DLM is negotiated on mode, otherwise the nodes remain in infrastructurea per flow basis. A traffic stream (TS) is the traffic mode. This is a very centralized approach since the APgenerated by a specific application that can be not only controls the modes of the nodes, but alsorepresented as a set of QoS requirements (average data manages the channel usage and monitors the trafficrate, maximum delay, maximum packet error rate, etc). condition in the network.Our system includes a decision module that selects the Authors of [5] propose a software based approach,mode that better fits such QoS requirements. Operation called Multinet that facilitates simultaneousmodes can dynamically be changed based on channel connections to multiple networks by virtualizing aconditions and network topology changes. single wireless card. Such virtualization is obtained by

The rest of this paper is organized as follows: introducing an intermediate layer below IP, whichSection 2 presents the state of the art in the field of continuously switches the card across multipleschemes that integrate ad-hoc mode with Wireless networks: the switching algorithm is completelyLAN infrastructure; Section 3 presents the basic idea transparent to the user. Multinet requires changes tothat characterizes the Flex-WiFi implementation; either the data link or the device driver layer of theSection 4 and 5 include a description of the system networking stack and requires that each stationarchitecture and the negotiation protocol; finally, in (including the AP) maintains associated stateSection 6 a scenario is analyzed in order to explain the information for each network the card is connected to.potential applications of the Flex-WiFi. The paper also describes different switching algorithms

and buffering protocols that ensure reliable packet2. State of art delivery to all the nodes in the involved networks.

This approach has several similarities with the one we

The approach to combine infrastructure and ad-hoc propose in this paper, including the possibility to trickconnection modes in a single network is widely the AP by using the Power Saving Mode (PSM) toadrse*n ieaue achieve data buffering while the requesting station is

switching to a different network. Unlike our scheme,

Multinet is still centralized since it requires the AP to number of stations increases, while the OFFSET ismaintain the state of all the cards that are associated to negotiated between the peer stations based on the timeit. The network switching is handled at the user level availability within the MIXED_SI interval (TXOPs(IP level) while our idea is to let the MAC to perform belonging to different flows should not overlap).such operation. Our approach also introduces support In Figure 3 a scenario with three E-STAs associatedto QoS since we claim to use the virtualization of to an AP is represented. E-STA2 and E-STA3 aremultiple networks in home environments that include connected to the external network to perform an e-mailvery delay sensitive traffic. download. E-STA1 sends a video to both E-STA2 and

E-STA3; furthermore E-STA3 downloads a file from3. Basic idea of Flex-WiFi E-STA2. Figure 4 shows a possible allocation of the

TXOPs related to each flow using our proposedIn the proposed architecture a wireless E-STA can neotiatinp tool.

operate in one out of two possible transmission modes, It can be noted that the three negotiated TXOPs doIM and DLM, and uses a different PHY channel for not overlap and this allows using the direct link channeleach mode, referred to as C (IM channel) and C1 in a more effective way. Even if our goal is to minimize(DLM channel) in the following. Such operations are TXOP overlapping, it is not guaranteed by this solutionallowed by dividing the beacon interval into sub- as it is of improved solution

intervals as shown in Figure 2. The beacon interval TXO overlapin is an imp oratian terestinrepresents the time interval between two successive rsac areapand i ar or ftr work.Target Beacon Transmission Time. The beacon is a

rsac raadwl epr forftr ok

TaragetmBeaont Transmissicontainime. T eacon isoua An analysis of the delay in the direct link connectionmanagement frame [1] containing information about and in the infrastructure connection has been done insynchronization, association and power management to function of the duration of the beacon interval. If webe used by all the stations associated in the BSS. consider that there are only two flows, one in theTwo sub-intervals have been identified: IM_SI and infrastructure connection and the other one in the direct

MIXED_SI. link connection, there is only a TXOP allocated and theIM_SI is the time interval used by a station to contention time for the two flows can be neglected

transmit/receive while in IM on channel C. This time because they are sent on two disjoint channels. Theinterval is also used to receive the beacon frame from duration of the TXOP is a fraction of the beaconthe AP and setup the DLM connections on channel C1. interval and this value depends on the amount of trafficMIXED_SI is the time interval that an E-STA can that has to be transmitted. The TXOP duration can be

use to exchange data both during DLM and IM. calculated as:While the channel access scheme in IM follows the TX _ RATE

rules defined by the standard IEEE 802.11, the TXOP = BIl MACtransmission opportunities (TXOPs) during the DLM MAXTHmust be negotiated between the sender and the receiver Where BI is the duration of the beacon interval,using a proprietary protocol. This protocol is MAX is the maximum useful throughput that, if wecompletely transparent to the AP. The negotiated TH

TXOPs are bounded time intervals during which the E- suppose to send data over a 11 Mbps radio channel,STA is allowed to transmit a burst of frames in direct can be fixed to 7.74 Mbps taking into account thelink mode, with limited channel contention. Each overhead needed at the MAC and physical layers forTXOP is defined by a starting time and a maximum the transmission of the packets (see [6]) andduration. An allocated TXOP starts at a time t given TX RATEMAC is the transmission rate of the flowby: over the MAC layer.

ft = TBTT + OFFSET The maximum delay in the direct link connectionOFFSET . IM _SI can be computed as:

The TXOP duration is computed on a per-flow basis MAX - DELAYDL = BI - TXOP(i.e. based on the flow specific QoS requirements) and The maximum delay in the infrastructure connectiontakes into account possible channel variations (i.e. instead can be computed as:fluctuations of the channel capacity) and a certain time MAX _ DELAYINF TXOPof contention that cannot be neglected when theINR

In Figure 5 it is possible to see the plots of the

maximum delay in the infrastructure connection and inthe direct link connection when the TXOP is allocated Dlay AnalySiSrespectively for a 500 kbps flow and a 2 Mbps flow 1100over the MAC layer. 900 -

As expected, the delay on both IM and DLM 800 - Delay DL (600lincreases when the beacon interval increases since in 600- -Delaynrfa (01k: l00 - -. Delay DL (2M)this case the TXOP duration increases too. We can : Del0ay nfra (2M)3006 ~ ea hr Nnotice as with the decrease of the amount of the 2 -

transmitted data, the duration of the TXOP decreasescetoo. Consequently, the maximum delay in the o 200 400 G00 800 1000infrastructure network decreases too, while the BeAcon ifiteivall (tis)maximum delay in the direct link network increases. Figure 5. Analysis of the maximum delay in

IBeacon Interval the direct link connection and in theinfrastructure connection.

BEACON AND "INTR DL TXOPS AND

NEGCiOTIATION PROTOCOL INTRADATA l4. Flex-Wifi architectureTBTT TBTT

The Flex-WiFi architecture is based on three mainFigure 2. Sub-intervals that constitute the modules that are present in each E-STA:beacon interval. The scheduler. This module is responsible to admit

or reject a traffic stream originated by an application.The duration of the TXOP is chosen according theTSPEC of the data flow provided by the application.

The buffering module. It is responsible for queuingdata packets received by the application. Our approachis based on an architecture where multiple connections

PortablePC(E-STA2)PDown Portabie PC (E-STA3) are managed with only one MAC/PHY instance.The_______File_Do_ __load MAC decides when it needs to switch from a

connection to the other one. The application is unawareVidec, Stream / Vidleo Straming about the current connection served by the MAC. For

iin this reason buffering is necessary in order to avoid theSet-top4ox (E-STAI) loss of application packets. When a packet is sent from

the application layer to the MAC layer, the last decidesthe buffer where the frame has to be queued. When a

Fssocigured3. Scenario withAthree.staticonnection becomes active, the respective buffer willassociated to the AP. be emptied.The best path selector. This module is for the

IffSI MIXED SI decision of the best path between the infrastructureL ___OFFSET1 connection and the direct link connection. The choice

OFFS0T2--------- ---- ----OFFSET3 depends on the channel condition on both direct link

STAI TX<0P1 Txb 3 and infrastructure link. The module should be able to

STP,.2 TXOP1 TXOP2 1 gather the statistics of the several links and provide analgorithm to perform the best path selection. A

STA3 1 0 T>,0i72 TXO93 l description of this algorithm is not the objective of this

article; any implementation can be chosen for thismodule.

Figure 4. Example of allocation of TXOPs.

5. The negotiation protocol maximum delay in the infrastructure connection is lessthan 100 ms; it means that the VoIP flow respects the

The negotiation protocol is performed only in delay constraint of Table 1. The direct link flow,infrastructure mode. Stations exchange messages instead, has a maximum delay of about 1 second; itwithout involving the AP. As we can see in Figure 6, means that also the video streaming respects the delaythe station QSTA- 1, that intends to setup a direct link constraint of Table 1. A pre-buffering technique is usedcommunication with another station, QSTA-2, sends a to the receiver in order to contain the effects of therequest frame to QSTA-2. The request contains delay in the direct link connection. If we could use ainformation about the PHY channel to use for smaller beacon interval, as it is possible to see incommunication, the TXOP offset and length. The Figure 5, the performance of the system in terms ofQSTA-2 may respond refusing the request, accepting delay would improve for both direct link andthe request as it is or proposing new values for TXOP infrastructure connections.duration and offset. Figures 8, 9 and 10 represent the offered load (i.e.

the traffic injected in the network), the goodput (i.e. theamount of traffic correctly received, as measured at the

QAP MAC level) and the PER (i.e. the packet error rate as

I-a measured by the transmitter at the MAC level) versuslb the time, related to the video streaming application

oSTA-i QStA2 while in DLM respectively.Table 2 represents the same performance results

Figure 6. Flex-WiFi setup protocol handshake. related to the VoIP connection, which is always in IM.Results demonstrate that, in the proposed scenario,

6. Scenario and results DLM allows to deliver the video streaming traffic atthe required rate and PER.

To evaluate the performances of the Flex-WiFiarchitecture we have used a scenario that can be seen inFigure 7. Mean Maximum Maximum Maximum

It represents a home environment where a user, Data PER (F.PER) elay Packet Size

already involved in a VoIP connection, decides to start VoIP 64 kbps 10% 100ms 120Ban off-line video streaming application. Objective is to Off-line 500 kbps 20% 2000ms 1350Bshow a typical case where a mixed IM/DLM approach Videoallows two connections to be active in the same time Streamingand an improvement in the bandwidth occupation. Table 2. Average rates and standard

In Table 1 the QoS requirements (included in the deviations for VolP traffic.TSPEC) of the two applications are shown. Ourframework has been obtained by modifying the Average Std Average StdMadwifi open source driver of two 802.11b wireless Transmission 88709 2875 87775 1039cards having Atheros chipset. Rate (bps)The IM_SI has been fixed to 50 ms (each E-STA in Received Rate 87121 1566 85701 3237

the network uses the same values). The beacon interval (bps)Transmission 0.14 0.02 0.02 0.01has been set to 1024 ms to satisfy some hardware PERa(%) 0_14 0_02 0_02 ____

constraints of the Atheros wireless cards: a smallervalue would have been preferable in terms of delay

performance.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~i Access Point (AP)performance.The video streaming data rate have been set to 500 Wesi

kbps while the voice has been coded at 64 kbps whichmeans a bandwidth of about 80kbps considering the IP, E-STAI E-STA2UDP and RTP overheads. The VoIP transmission rate mDLfoW (50 Kbps !has a standard deviation which is always less than 3.3Kbps. Comparing the parameters of the flows to themaximum delays seen in Figure 5, we can see as the Figure 7. Home scenario used in the test.

increase the network capacity as well as to guaranteethe required QoS to certain delay sensitive traffic by

iomow allocating them scheduled transmit opportunities on alow-contended direct link channel. Obviously the highpriority traffic streams can be transmitted in direct link

6eccco mode only when the channel conditions are satisfactory(i.e. low PER). While the transmission opportunities in'7-'1 'I.,the direct link channel are scheduled, the contentionbased access scheme is the only allowed policy in the

o infrastructure mode. The MAC level of the wirelesso 1X' 150 m~ 250 XI ^O eI . .J

Time2 card has been modified in order to allow a dynamicswitching from IM to DLM (and vice versa). The

Figure 8. Offered load of the direct link changes required by the Flex-WiFi only impact theflow in transmission. MAC level of the mobile stations and are completely

transparent to the AP. The compatibility with the1000c0o legacy APs is preserved using the standard Power

_cXJ Saving Management mode in order to avoid datain losses.

In this paper we presented a preliminary proposal for0 IJCIXIJ0 m. F -the Flex-WiFi architecture, further enhancements are

mboxb under development. Enhancements will include thel l l llanalysisof the allocation of non-overlapping TXOPs in

o so 2mIm 3o3m 0m very congested scenarios; finally the policy to allocateTi me s) the PHY channels to the DLM connections will be

investigated.

Figure 9. Goodput of the direct link flow inreception. 8. References

loo [1] IEEE Wireless LAN Edition. IEEE Std 802.1 1TM 1999(R2003).

80pS69- [2] IEEE 802.1 le-2005. IEEE Standard for Information

technology-Telecommunications and information exchangeff 40 between systems-Local and metropolitan area networks. 11-

Nov-2005. ISBN: 0738147885.20

oLt ;l;t=t-;l X ;t: .,:+ ,+. ii,. r.+ 1: [3] Z. Yin, V.C.M. Leung, "Performance improvements ofO 50 100 60 200 250 300 350 400 460 Integrating Ad Hoc Operations into Infrastructure IEEE

Time ls) 802.11 Wireless Local Area Networks", ComputerCommunications, Vol 28.10, 2005.

Figure 10. PER (at MAC level) of the direct[4] J. Chen, S.-H.G. Chan, J. He, S.-C. Liew "Mixed-Modelink in transmission. WLAN: The Integration of Ad Hoc Mode with WirelessLAN Infrastructure" in Proceedings of IEEE GLOBECOM,2003.

7. Conclusion and future work [5] R. Chandra, P. Bahl, P. Bahl, "Multinet: Connecting toMultiple IEEE 802.11 Networks Using a Single Wireless

Flex-WiFi is a new model for a mixed 802.11 Card" in Proceedings of IEEE INFOCOM, 2004.network which allows multiple contemporary

[6] M. Heusse, F. Rousseau, G. Berger-Sabbatel, A. Duda,,connections using a single wireless card. The idea is to "Performance anomaly of 802.1 lb" in Proceedings of IEEEallow a user to establish ad-hoc connections (on a INFOCOM, 2003dedicated PHY channel) while staying in itsinfrastructure network: this is in the direction to