Each wireless technology in the integrated networks has ... WLANs with 3G systems has been studied actively by research communities and standardization ...
Efficient WLAN Discovery Schemes Based on IEEE 802.21 MIH Services in Heterogeneous Wireless Networks Wan-Seon Lim, Dong-Wook Kim, Young-Joo Suh
Jeong-Jae Won
Department of Computer Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-Dong, Pohang, 790-784, Korea Email:{kiki,shkim80,yjsuh}@postech.ac.kr
Telecommunication and Network R&D Center Samsung Electronics Co., LTD. 416, Maetan-3dong, Suwon, 443-742, Korea Email:{jjwon.won}@samsung.com
Abstract— Discovering currently available radio access networks (RANs) is one of the most challenging issues in the heterogeneous wireless network environment. Especially, discovering WLANs, which support high data rates but have limited service coverage, has a significant effect on the energy consumption of multi-mode terminals (MMTs). In this paper, we propose three WLAN discovery schemes which are based on the upcoming IEEE 802.21 standards. In the proposed schemes, a MMT exploits the information on neighboring WLANs from a MIH information server to discover available WLANs as soon as possible while minimizing energy consumption. Our simulation results show that the proposed schemes can enhance the performance of MMTs in terms of energy consumption and the detection time of WLANs compared to conventional WLAN discovery schemes.
I. I NTRODUCTION The integration of different wireless communication technologies is one of key trends in the next generation wireless communications. The integrated network environments offer the Internet access to end users anytime, anywhere, and with better Quality of Service (QoS). The integration of heterogeneous wireless networks raises interesting issues such as vertical handoff (i.e., handoff across heterogeneous access networks), admission control, security guarantee, and power management for a multi-mode terminal (MMT). With the growing importance of these issues, standard group activities including the IEEE 802.21 WG have occurred. The upcoming IEEE 802.21 standard defines Media Independent Handover (MIH) mechanisms that enable the optimization of handovers in heterogeneous networks [1]. Each wireless technology in the integrated networks has its own characteristics that complement others. For example, Wireless Local Area Networks (WLANs) which are based on the IEEE 802.11 standards support higher data rates than other wireless access technologies. In contrast, the IEEE 802.16 (WiMAX) or 3G networks cover relatively large areas, but they provide smaller data rates than WLANs. Due to such distinctive characteristics, the integration of WLANs with other wide area access networks is one of the most famous examples in network integration scenarios. From late 1990s, interworking WLANs with 3G systems has been studied actively by research communities and standardization bodies such as 3GPP/3GPP2 consortium, IEEE, and ETSI [2-4]. Most recently, interworking
between WLANs and the IEEE 802.16 based networks also has gained much attention. In such integration scenarios, it is typically assumed that a MMT always turns on its WLAN interface and attempts to connect to WLAN access points (APs). However, this assumption can cause an inefficient use of energy since WLANs are not always accessible due to their small service coverage. For efficient power managements of MMTs, which are usually battery-powered, keeping the WLAN interface turned on should be avoided. The most intuitive way to reduce energy consumption is to turn on the WLAN interface periodically. In particular, a MMT turns on the WLAN interface for a certain predefined time interval and tries to associate with the WLAN AP. This is quite simple but more efficient discovery schemes are possible if there is additional information on WLANs. Information on WLANs can be provided to a MMT through protocol-specific broadcast messages or an external server. Choi et al. proposed that 802.16e base stations (BSs) periodically broadcast the information on the density of WLAN APs within their cell coverage [6]. By using the density information, a MMT decides the scan interval of the WLAN interface by considering the energy consumption. Cao et al. proposed a WLAN discovery scheme of utilizing 3GPP networks to broadcast the channel information on WLANs [7]. These protocol-specific approaches need to modify the message format, and thus may cause compatibility problems with existing devices. In [5], the authors utilized the location service server (LSS) which stores the geographical information of WLAN AP locations. With the help of the LSS, a MMT can turn on its WLAN interface only if the WLAN is available. In this paper, we propose efficient WLAN discovery schemes for heterogeneous wireless networks to reduce the energy consumption and detection time of WLANs. The proposed schemes utilize the IEEE 802.21 MIH services to provide information on WLANs to a MMT. Since the IEEE 802.21 standard supports media independent services, a MMT can obtain the information on WLANs regardless of the currently connected network type. A main strong point of our schemes compared to existing work is that we consider a case where some APs are not managed by an information server. Note that most of WLAN APs are independently owned and managed unlike commercial networks. If a MMT tries to
978-1-4244-2324-8/08/$25.00 © 2008 IEEE. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.
discover APs based on the obtained information only, (e.g., location information from the LSS [5]), then it may fail to use APs that are not registered in the information server. The proposed schemes are designed to discover APs efficiently even if not all APs are managed by the server. The remainder of this paper is organized as follows. In Section II, we briefly describe the WLAN scanning procedure and IEEE 802.21 standard. Section III presents the detailed operation of the proposed schemes. Then, the performance of the proposed schemes is evaluated in Section IV. Finally, this paper is concluded in Section V. II. BACKGROUND A. Scanning procedure in IEEE 802.11 WLANs The discovery of potential WLAN APs is accomplished by means of a medium access control (MAC) layer function called scanning according to the IEEE 802.11 standard. There are two types of scanning mode: passive mode and active mode. In the passive scanning mode, a terminal listens to each channel of the physical medium one by one for beacon frames. During the passive scanning mode, the terminal waits for at least a period of time longer than the beacon interval, before it switches to the next channel. On the other hand, a terminal in the active scanning mode transmits a probe request frame containing the broadcast address as its destination and waits for probe responses from APs. If no response has been received by MinChannelTime, then the next channel is scanned. If one or more responses are received by MinChannelTime, the terminal waits for more responses at most MaxChannelTime. Since the channel switching delay is negligible, the average probe delay in the passive scanning mode can be represented as a function of the beacon interval and the number of available channels. On the other hand, the probe delay in the active scanning mode can be determined by the device-dependent MinChannelTime and MaxChannelTime values. If N is the number of channels available, then the probe delay bound is between N*MinChannelTime and N*MaxChannelTime. B. IEEE 802.21 Standard The IEEE 802.21 is a recent effort that aims at enabling seamless service continuity among heterogeneous networks including 3GPP, 3GPP2, and the IEEE 802 standards family. The standard defines a logical entity, Media Independent Handover Function (MIHF), which is located between the lower layer (L2 and below) and the upper layer. At the lower layer, a MMT has multiple interfaces for different wireless access technologies. The role of MIHF is providing media independent services to upper layer entities to facilitate mobility management and handover process. There are three primary services in the IEEE 802.21 standard: Media Independent Event Service (MIES), Media Independent Command Service (MICS), and Media Independent Information Service (MIIS). MIES may indicate or predict changes in a state and transmission behaviors of physical and link layers. Common MIES entities provided through MIHF are: “Link Up”, “Link Down”, “Link Parameters Change”,
and “Link Going Down”. MICS enables higher layers to configure, control, and obtain information from lower layers. MIIS provides a unified framework for obtaining neighboring network information that exists within a geographical area. For MIIS, the IEEE 802.21 standard defines information structures called Information Elements (IEs) which are classified into two groups: access network specific information and Point of Attachment (PoA) specific information. Fig. 1 shows an example network model. A MIH-capable MMT has multiple wireless interfaces based on different access technologies. It can connect concurrently to multiple PoAs which are network side endpoints of the layer 2 link. Each access network provides one or more MIH Points of Service (PoS) node(s). The MIIS server located in the network side maintains the information on neighboring access networks. III. P ROPOSED WLAN D ISCOVERY S CHEMES Before proposing new discovery schemes, we introduce a conventional WLAN discovery scheme based on periodic switching. Fig. 2 shows the timing model of the periodic switching scheme. When a MMT turns on its WLAN interface, it scans all channels via active or passive scanning modes. If one or more APs are detected after scanning, the MMT associates with a new AP and starts to use the WLAN interface for data communications. Otherwise the WLAN interface is turned off during the switching interval T. Although this simple periodic switching can effectively reduce energy consumption compared to the continuous scanning, our aim is to design an advanced WLAN discovery scheme based on MIIS of the IEEE 802.21 standard. We assume that a MIH-capable MMT obtains the information on WLANs as follows: When the MMT cannot access a WLAN and is connected to an access network which covers a wide area, it sends a MIH Get Information request including the address of the currently connected BS to a MIIS server. Then the MIIS server replies by sending a MIH Get Information response,
Access network 1 WLAN PoA
Core network 1 with PoS MIIS server Core network 2 with PoS
Access network 2 WiMAX PoA MIH-capable MMT
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Fig. 1.
Example of network model with MIH Services
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Fig. 2.
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Timing model of periodic switching scheme
978-1-4244-2324-8/08/$25.00 © 2008 IEEE. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.
which includes the information on APs within the coverage of the BS, according to the PoA specific IEs. The IEEE 802.21 standard defines two useful IEs for a WLAN discovery: TYPE IE POA CHANNEL RANGE and TYPE IE POA LOCATION, which represent the channel range and geographical location of the PoA, respectively. As a result, the MIH-capable MMT can utilize the channel and location information for a WLAN discovery. In a design phase, there are two considerable points. First, a MIIS server cannot manage all running APs within its service area. In contrast to the commercial networks such as 3G and WiMAX, numerous WLAN APs are individually deployed and managed without centralized management system. Unlike other schemes, the proposed schemes include the operation for unregistered APs at the MIIS server. Second, it is not always guaranteed that a MMT can utilize both the channel and the location information. For example, to exploit location information, a MMT should learn the current location of itself via the Global Positioning System (GPS) or other location services. Therefore, we propose three different WLAN discovery schemes according to the available information.
Disconnected from an AP
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Turn off WLAN interface Scan_count = 0 Wait until switching timer expires Perform full scanning Restart switching timer Find APs ?
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Scan_count++ Yes Turn on WLAN interface
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Fig. 3.
Operations of channel information based discovery scheme
A. Channel information based discovery From Fig. 2, we can observe that if we reduce the scanning time in a given switching interval T, then the WLAN interface of a MMT will remain in the turn off state for a longer time and thus the energy consumption will be reduced. Moreover, a shorter scanning time leads to faster AP detection when the MMT moves into the service coverage of APs. Based on the above observation, we propose a channel information based discovery scheme (CID) that reduces the scanning time by limiting the number of scanned channels. In CID, on receiving the MIH Get Information response, a MMT constructs a list of channels used by nearby APs to perform scanning for selected channels (selective scanning) rather than for all channels (full scanning). There already have been several approaches that adopt selective scanning for the fast AP discovery. CID, however, is designed to reduce the energy consumption of MMT as well as the discovery time. Fig. 3 shows the overall operations of CID. After disconnecting from an AP, the MMT starts ‘switching timer’ with the switching interval T and turns off its WLAN interface. When the switching timer expires, the MMT restarts the timer and increases the value of Scan count which is set to zero initially. Then it turns on the WLAN interface and decides whether to perform selective scanning or full scanning. In case Scan count does not exceed the predefined threshold Max scan, the MMT tries to discover APs by selective scanning. If the MMT fails to discover APs until Scan count exceeds Max scan, then it resets Scan count to zero and performs full scanning. (How to determine the optimal value of T and Max scan is outside the scope of this paper). After the scanning operation, if the MMT fails to detect any APs, it turns off its WLAN interface again and waits until the next switching timer expires. If one or more APs are found, then it uses the WLAN interface for data transmission and
stops the switching timer. Note that full scanning allows the MMT to find APs on channels which are not announced by the MIIS server. In case the MMT finds APs on such channels, it updates the channel list for later selective scanning. B. Location information based discovery The second scheme is a location information based discovery scheme (LID) which exploits the location map of APs. The location map, which consists of the address, geographical location and transmission range of APs, is constructed when a MMT receives the MIH Get Information response. (If the transmission ranges of some APs are unknown, then they can be set to the pre-defined maximum transmission range Rmax , e.g., 250m for common 802.11b APs.) If the MMT constructs the location map successfully and has the capability to know its current location, then it can use LID to discover WLANs. Although the concept of exploiting a location server was proposed in [5], we define detailed operations for MMTs including the update process of the location map. Fig. 4 describes how a MMT discovers APs in LID. Initially, the switching interval is decided according to the location map and current location of the MMT. If the MMT determines that it is located within the service coverage of an AP, then the switching interval is set to Tshort . Otherwise, it is set to Tlong which is relatively longer than Tshort . When the MMT enters into the service coverage of an AP, the expiration time is adjusted to a smaller value between the remaining time of the current switching timer and Tshort . When the switching timer expires, the MMT restarts the timer according to the current location and performs scanning to find APs. If the MMT finds an unmanaged AP via scanning, then it needs to register the AP to the location map. Unfortunately, the update process in LID is more complicated than CID since
978-1-4244-2324-8/08/$25.00 © 2008 IEEE. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.
Perform scanning
of the MMT just like LID. When the switching timer expires, the MMT performs selective scanning if it is located within the coverage of APs and Scan count does not exceed Max scan.
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IV. P ERFORMANCE E VALUATION
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Fig. 4.
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Operations of location information based discovery scheme
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the MMT cannot know the actual location of the AP nor even the exact distance from the AP. A location estimation based on the received signal strength is not so accurate, especially in indoor environments. One fact is that if the MMT draws a circle whose radius is 2Rmax from the current location, then the circle includes service coverage of the AP. Thus, the MMT can register with the new AP by not Rmax but 2Rmax in the location map. For such a MMT-driven update case, it is allowed to register multiple locations per one AP. In particular, the MMT registers the location for an AP when it detects the AP by scanning or it is disconnected from the AP. Fig. 5 shows examples of location updates when a MMT passes through the service coverage of an AP. The MMT detects the AP at location A and then is disconnected from the AP at location B. After that, A and B are registered in the location map and then the intersection of two large circles becomes the estimated service coverage of the AP. If the MMT passes through the AP several times, it can estimate the service coverage of the AP more accurately. C. Channel and location information based discovery Lastly, we propose a channel and location information based discovery scheme (CLID). In CLID, a MMT constructs a location map which includes the channel number as well as the location and transmission range of APs. CLID basically inherits LID and adopts the scanning operation of CID. The expiration time of the switching timer depends on the location
Before evaluating the proposed schemes, we measured the scanning time and energy consumption of the 802.11b interface for the cases of full scanning (11 channels) and selective scanning (3 channels) with the active scanning mode. To support the selective scanning, we have extended the MadWifi driver which is an open device driver for 802.11 chipsets from Atheros. Table I shows our experiment results, where we can see that the selective scanning reduces both scanning time and energy consumption significantly. (Note that the results include wasted time and energy by turning on/off the interface.) Based on the above results, we compared the performance of the proposed schemes with that of the periodic switching scheme (PS) through the ns-2 simulator. We assumed that there is one BS of CDMA2000 which covers the entire area of 3km*3km square. The number of APs varies from 5 to 25 and 80% of the APs are managed by a MIIS server. The transmission range of the APs is 250m and the used channel numbers are 1, 6, and 11. A MMT has two network interfaces, each for WLAN and CDMA2000. The total simulation time is 4 hours and the MMT moves as the random way point model with pause time of 10 sec. The switching intervals T (for PS and CID), Tshort and Tlong (for LID and CLID) are set to 10, 3 and 50 seconds, respectively. Max scan (for CID and CLID) is set to 10. We used three performance metrics: 1) the average discovery time, defined as the time from entering the service coverage of an AP to discovering it, 2) the average energy consumption before connected to the AP, and 3) the normalized discovery overhead, computed by dividing the average energy consumption by the average AP usage time. Fig. 6 shows the simulation results as a function of the number of APs when the MMT’s average speed is 15m/s. First, let us discuss the performance of the average discovery time (Fig. 6 (a)). With the help of the selective scanning, CID reduces the average discovery time compared to PS. LID shows better performance than CID, and CLID is the best among the schemes. The average energy consumption decreases with the increases in the number of APs as shown in Fig. 6 (b). As shown in the figure, the proposed schemes show better performance than PS, and LID and CLID are more efficient than CID in terms of the energy consumption. The performance difference among the schemes becomes more significant as the number of APs decreases. Since the average usage time of one AP is independent of the density of APs, TABLE I E XPERIMENTAL R ESULTS
Full scanning Selective scanning
Scanning time (sec) 1.771 0.342
Energy consumption (mWh) 7.255 2.689
978-1-4244-2324-8/08/$25.00 © 2008 IEEE. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.
(a) Average discovery time Fig. 6.
(b) Average energy consumption
(c) Normalized discovery overhead
Effect of the number of APs when MMT’s average speed is 15m/s
(a) Average discovery time Fig. 7.
(b) Average energy consumption
(c) Normalized discovery overhead
Effect of the MMT’s average speed when there are 20 APs
the normalized discovery overhead in Fig. 6 (c) shows very similar results to Fig. 6 (b). Fig. 7 shows the simulation results as a function of the MMT’s speed when there are 20 APs. An interesting fact is that CID shows the best performance in terms of the average discovery time when the MMT’s speed is 5m/s (Fig. 7 (a)). If the MMT moves slowly, then it cannot register unmanaged APs to the location map early and thus the average discovery time will be long. However, in Fig. 7 (b), we can see that LID and CLID show smaller energy consumption performance than CID regardless of the MMT’s speed. As a result, LID and CLID guarantee smaller normalized discovery overhead as shown in Fig. 7 (c). From Fig. 6 and Fig. 7, we can conclude as follows: 1) CLID shows the best performance. 2) The difference between CLID and LID is not so significant. 3) CID is the worst among the proposed schemes but much better than PS. V. C ONCLUSION In this paper, we proposed three WLAN discovery schemes for heterogeneous wireless networks. The proposed schemes have been designed to discover available WLANs efficiently by utilizing the MIIS of the IEEE 802.21 standard. In the proposed schemes, a MMT obtains the information on neighboring WLANs from a MIIS server and exploits them to discover WLANs as soon as possible, while reducing the energy consumption. Our simulation results show that the
proposed schemes outperformed the conventional discovery scheme in terms of the energy consumption and the discovery time of WLANs. ACKNOWLEDGMENT This research was supported by the MIC(Ministry of Information and Communication), Korea, under the ITRC(Information Technology Research Center) support program supervised by the IITA(Institute for Information Technology Advancement) (IITA-2008-C1090-0801-0045) and was supported by the Korea Science and Engineering Foundation(KOSEF) grant funded by the Korea government(MOST) (No. R01-2007-000-20154-0). R EFERENCES [1] V. Gupta, “IEEE802.21 Standard and Metropolitan Area Networks: Media Independent Handover Services”, Draft P802.21/D05.00, Apr. 2007 [2] 3GPP TS 23.060 v6.5.0, “3rd Generation Partnership Project: Technical Specification Group Service and System Aspects; General Packet Radio Service (GPRS); Service description; State 2 (Release 6),” Jun. 2004. [3] 3GPP2 S.R0087-0 v1.0, “3GPP2-WLAN Interworking, State 1 Requirements,” Jul. 2004. [4] ETSI, “Requirements and architecture for interworking between HYPERLAN/3 and 3rd Generation Cellular Systems,” Tech. Rep., Aug. 2001. [5] W.-T. Chen and Y.-Y. Shu, “Active Application Oriented Vertical Handoff in Next-Generation Wireless Networks,” IEEE WCNC 2005, Mar. 2005. [6] Youngkyu Choi and Sunghyun Choi, “Service Charge and Energy-Aware Vertical Handoff in Integrated IEEE 802.16e/802.11 Networks,” IEEE INFOCOM 2007, May 2007. [7] Z. Cao, J. Jiang, and P. Fan, “WLAN Discovery Scheme Delay Analysis and Its Enhancement for 3GPP WLAN Interworking Networks,” IEICE Transactions on Communication, Jun. 2007.
978-1-4244-2324-8/08/$25.00 © 2008 IEEE. This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2008 proceedings.
