Active application oriented vertical handoff in next-generation wireless ...

Active Application Oriented Vertical Handoff in Next-Generation Wireless Networks Wen-Tsuen Chen and Yen-Yuan Shu Department of Computer Science, National Tsing-Hua University, Hsin-Chu, Taiwan 300, R.O.C. Tel: +886-3-5742896 Fax: +886-3-5711484 {wtchen, mr914368}@cs.nthu.edu.tw time to get maximum Quality of Service (QoS) and best balance of network resources usage. An efficient interface management for MT with multiple wireless interfaces is very important since it affects the discovery of other available networks. The power consumption of those interfaces is also critical because we are talking about MTs that use battery.

Abstract- Coexistence of heterogeneous wireless networks to provide service anywhere at anytime is an inevitable trend in the development of the next- generation wireless data networks. Vertical handoff is the switching of the Mobile Terminal (MT) among different types of wireless networks. How and when to carry out vertical handoff affects directly the performance and quality of network services. In this paper we propose a novel vertical handoff scheme in which the MT can request and initiate the handoff actively, in contrary to other schemes where the MTs participate passively during the handoff process. Our Active Application Oriented scheme provides an efficient interface management for multi-interface MTs to reduce the power consumption caused by unnecessary interface activation. By treating the application running at the MT as the main vertical handoff decision factor, the proposed scheme is able to switch the MT at right time to the most suitable network to minimize the waste of network resources. Finally, simulation results are presented to show the improved performance over passive schemes. I. INTRODUCTION Since there is no one single wireless network technology that can satisfy the requirements of all today’s and upcoming wireless services, the coexistence of heterogeneous wireless networks to provide service anywhere at anytime is an inevitable trend in the development of the next-generation wireless data networks. These different networks overlap each other hierarchically as shown in Fig. 1, and a multi-interface Mobile Terminal (MT) can select an appropriate network to use. Handoff is the process in which a MT switches from one cell to another cell. Handoff can be classified into two types: horizontal and vertical [6]. Horizontal handoff refers to the switching of the MT between different Base Stations (BS) of the same network, while vertical handoff (VHO) is the switching between different BSs belonging to different networks. Many research works have been done to improve the performance of horizontal handoff since it plays an important role in today’s 2G cellular system (e.g. GSM) [13]. As more and more different wireless technologies come out, and so do the MT with multiple wireless network interfaces, an efficient VHO scheme is necessary to guarantee that the users can experience satisfactory seamless wireless services. The MT needs to switch to an appropriate network at right

Fig. 1 An example of wireless overlay networks

A Location Service Server (LSS) is introduced to provide the information such as coverage area, bandwidth and latency of available wireless networks around a MT [7]. The design and implementation of LSS involve business agreements between different service providers, hence are beyond the scope of this paper and will not be discussed further in this paper. Information is stored in database and is searched primarily by geographical location. Then the BS, which requests information from the LSS, will periodically send information of nearby networks according to the location of each MT to the MTs in its coverage area. Subsequently based on this information the MT carries out some predefined evaluation procedures. After the evaluation the MT decides whether to stay in the current serving network or handoff to a better network. In here we say that the MT passively receives nearby networks’ information from the BS to distinguish from our proposed active scheme. Most of today’s VHO solutions have the following problems. First, the management of the MT’s interfaces is not efficient. In most cases all interfaces of a MT are assumed being turned on all the time to facilitate the contact with their corresponding networks. This introduces

This work was partially supported by the Ministry of Education of Republic of China under Grant No. 89-E-FA04-1-4 and National Science Council of Republic of China under Grant No. NSC 92-2213-E-007-017.

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In [1] a structure for the integration of 802.11 and 3G networks is proposed. The system is constructed in loosely coupled fashion [2]; i.e. 802.11 and 3G networks are treated as independent peer networks. An IOTA (Integration Of Two Access technologies) gateway is used to connect 802.11 and 3G networks and takes care of all traffics between these two networks. This gateway has several other functions including: RADIUS Server, Mobile IP Agent, Dynamic Firewall, QoS module, Accounting module and Web cache. There is a management of interfaces, but not very effective. The current signal strength and the priority of the interfaces are used for interface selection. Equations (1) and (2) are used for this selection. The variables wi, si, pi, Li and Hi stand for weight, normalized signal strength, priority, low threshold and high threshold of interface i, respectively. If i is the current interface,

additional power consumption. Second, the MT passively receives its nearby networks’ information even when it’s not necessary. Clearly the bandwidth and power is consumed by receiving unnecessary packets. This information is obtained from the BS by searching the LSS according to the geographical location of MT, in case the LSS is deployed, or detected by MT’s always on interfaces. Third, the time to handoff can not be decided precisely. In most occasions the application that is running at the MT is not considered. Therefore the MT may not be able to switch to an appropriate network for the current application at right time. To overcome these shortcomings we present an Active Application Oriented (AAO) scheme. The goal is to achieve efficient interface management that gives a better power balance and to perform the handoff to the most suitable network at right time. The basic idea of our solution is simple: allow the MT actively decides when to handoff and which network to attach on according to the running application at the MT [9], i.e. allow the application be served by the most suitable network. No more passively turning on all interfaces to receive nearby networks’ information then evaluate if handoff is necessary when it is not necessary. Or worse, execute handoff when the MT does not need to perform handoff. The remaining part of this paper is organized as follows. Several related works are discussed in Section II. Our proposed scheme is explained in Section III. Simulation results are presented in Section IV. Finally conclusions and future work are given in Section V.

1000 * pi + 2 si , s i ≥ Li wi =  2 si , si < Li 

If i is not the current interface, 1000 * pi + s i , s i ≥ H i wi =  si , si < H i 

(2)

The weight is calculated periodically. The equations introduce a hysteresis effect and its purpose is to let the MT stay with the current interface as long as possible to prevent oscillation. The application running at the MT is still not considered, only the above variables are considered for the handoff decision. As a result, some inappropriate decisions to the application may occur. On the other side, the MT still passively evaluates the handoff conditions even when the application runs just fine under the current network. This introduces unnecessary network resource and power consumption.

II. RELATED WORKS In this section we describe some of the current researches on VHO and discuss their pros and cons. In [3] a VHO scheme for integration between GSM/GPRS and WLAN networks is proposed. To minimize the handoff delay and packet loss the multi-tunnel technique is used: the Home Agent (HA) [14] copies the same IP packet destined to the MT and sends various copies to multiple destinations through multi-channel. Although the delay and packet loss can be decreased to minimum, the impact of this multi-casting action to the network performance is not mentioned. Two thresholds for the Received Signal Strength (RSS), TOFF and TON, are used to control the handoff from and to the WLAN network and two dwell timers, TDWELL1 and TDWELL2, are used to avoid the ping-pong effect. When the MT is using WLAN and detects that RSSWLAN < TOFF and this status persists for TDWELL1, the MT handoffs to GSM/GPRS. When the MT is using GSM/GPRS and detects that RSSWLAN > TON and this status persists for TDWELL2, the MT handoffs to WLAN. Although these thresholds and dwell timers facilitate the handoff decision, the energy consumption issue has not been taken into consideration here. Both GSM/GPRS and WLAN interfaces are assumed activated all the time. Only by doing this the MT can monitor continuously the signal strength of both GPRS and WLAN networks and decide whether to handoff or not. Two interfaces activating all the time will consume a large part of the MT’s energy even they are merely in stand-by mode. The situation will get worse for the upcoming MTs with more than two network interfaces. IEEE Communications Society / WCNC 2005

(1)

III.

THE PROPOSED SCHEME

In this section we describe our proposed scheme. In section A, an evaluation method to measure and quantify the QoS requirement of an application is presented. In section B and C, we present the system discovery method and two handoff decision algorithms to help the MT to switch to an appropriate network at right time. And finally in section D, the entire AAO vertical handoff process is presented. A. Application QoS requirements evaluation Every application has its own QoS requirements and different networks provide different QoS level [8]. To obtain the best balance between performance and network resource usage, the application at the MT must be served by the most suitable network. For example for a voice conversation service which uses PCM compression a bandwidth of 64kbps is just fine and the QoS is satisfactory. Allocating bandwidth over 64kbps such as WLAN’s 11Mbps for this service is viable but will not result in a better QoS level. To find the most suitable network we need an application to define its requirement first. The requirement consists of the network factors that each application concerns, such as bandwidth and network latency. For each network factor fn, an application defines its requirement boundaries: an upper bound Ufn and a lower bound Lfn to specify the maximum and minimum requirement, respectively. With these boundaries we can check if the serving network satisfies the 1384

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application’s requirements. Factors to be considered depend on each application. The following lists some factors that may be considered when calculating the QoS level provided by a network: z z z

z z

z z

MT can request the BS to get nearby networks’ information and evaluate those networks’ QoS level. The MT then compares the results from the quantification of current serving network and nearby networks. For instance, if the network b grants better QoS level than the current network, i.e. QoSb > QoScurrent, for current running application, the MT can handoff to network b, which is more suitable. Otherwise the MT will stay in current network. B. System discovery To switch to an appropriate network we need to know what networks are currently available. The most intuitive and effective way to discover the existence of other wireless networks is to turn on all interfaces of the MT. These always activated interfaces will detect the existence of their corresponding networks as soon as the MT enters their coverage area. This method will speed up the power consumption of the MT. Another means is to open the interfaces at certain predefined time interval, e.g. open the WLAN interface every 3 seconds to detect the WLAN network signals. This solution seems better than the first one, but still not efficient enough. The MT may be located 10 kilometers away from its nearest WLAN network; activation of WLAN interface is useless unless the MT enters the coverage of the nearest WLAN hotspot.

Bandwidth: data-intensive programs like FTP or video streaming will perform better when higher bandwidth is available. Latency: real-time programs will need low latency network while non-real-time programs will not be so sensitive to latency. Packet error rate: dropping some packets in video streaming or voice conversation service won’t degrade much the performance but this is not the case in Email or FTP applications. Usage charge: user may want to use the cheapest network available. Power consumption: different network interface consumes different amount of energy, the user may want to stay in the network with lowest power consumption to get longer battery life. MT moving speed: most networks have speed limit, the MT will be out of service if its speed exceeds the limit. Network coverage: if the coverage area is too small and the MT’s moving speed is too fast, then it may not be necessary to handoff.

For example a voice conversation application that takes three factors into consideration. To check whether the conditions of the serving network satisfy the requirements, we compare each factor to its corresponding real value from serving network, as shown in Table I. We can clearly see that the serving network satisfies the requirements of the application. This is the case when all requirements of an application are satisfied. But there will be cases where not all or none of the requirements are satisfied and then it is time to find a more suitable network. Equation (3) is a function to quantify the QoS level provided by a network m. The variable wn is the weight of factor fn and it’s assigned by the application according to each factor’s importance in the application. The variable fm,n is the normalized value of factor fn in network m, the calculation of the normalized value comes from (4) and (5). (3) QoS m = f m,1 * w1 + f m , 2 * w2 + ⋅ ⋅ ⋅ + f m ,n * wn

Factors Bandwidth Latency Packet error rate

The above methods keep the MT to open its interfaces even when there is no need of handoff action or no corresponding network exists. In our scheme, with the help of LSS, the MT will know it’s under the coverage of what type of networks. The MT opens its corresponding interface to handoff from the network of current interface only when handoff is necessary. This can avoid unnecessary interface activation and thus can reduce unnecessary power consumption. The LSS stores information of different wireless networks in certain areas, and provides this information according to the MT’s geographical location. The position of the MT can be obtained with the help of mobile positioning system such as GPS. In passive schemes this information is sent by the BS periodically to the MT even it’s unnecessary, while in the AAO scheme the MT requests this information only when it is needed. C. Handoff decision algorithm Traditionally the RSS has been used as the main decision factor in horizontal handoff [12]. The following are some schemes based on RSS:

We further classify the factors into high factors and low factors to facilitate the normalization. High factors are factors with value that should be as high as possible, such as bandwidth and throughput. Low factors are factors with value that should be as low as possible, e.g. latency and cost. Two boundary variables URm,n and LRm,n represent the range of the real value of factor fn in the network m. High factor normalization: UR m , n + LR m , n  ≤ Lf n  0, 2    UR m , n + LR m , n  f m , n = 1, ≥ Uf n 2   UR m , n + LR m , n  − Lf n UR m , n + LR m , n  2 Lf n < < Uf n ,  Uf n − Lf n 2 

(4)

z z

Low factor normalization: UR m , n + LR m , n  ≥ Lf n 0 , 2    UR m , n + LR m , n  f m , n = 1, ≤ Uf n 2   UR m , n + LR m , n  − Lf n UR m , n + LR m , n  2 , Lf n > > Uf n  Uf n − Lf n 2 

z z

(5)

z

RSS: handoff when RSSnew >RSSold RSS + Threshold: handoff when RSSnew > RSSold and RSSold < T RSS + Hysteresis: handoff when RSSnew>RSSold+ H RSS + Threshold + Hysteresis: handoff when RSSnew > RSSold + H and RSSold < T Dwell timer: used with above algorithms, handoff when the dwell timer expires

These algorithms work well under horizontal handoff, but in VHO there are more factors needed to be considered. The main reason is that the RSS of different networks can

When not all or none of the requirements are satisfied, the

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TABLE I REQUIREMENT CHECKING App’s requirements The serving network provides U fn L fn 200kbps 9.6kbps 30 ~ 40kbps 1ms 500ms 10 ~ 20ms 0.1% 5% 1 ~ 2%

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application. If they do, then there is no more second step: system discovery, until the application changes and the current network conditions can not satisfy the requirements of the application.

not be compared directly and the characteristics of each network are also different. We proposed two decision algorithms that are based on the above mentioned application QoS measurement to reflect the real need of the MT and permit the MT to make precise decision. The first algorithm shown in Fig. 2 is executed each time the MT boots up or changes application, e.g. from stand-by mode to watching an online video clip. The requirements of the application are then checked, if they are satisfied, the algorithms ends here. Otherwise, the MT will request the BS for nearby networks’ information. If after the above information’s evaluation, the MT finds out a more suitable network, then it will handoff to that network; otherwise it will stay with current network. The dwell timer is used to ensure that the target network, which is the network that provides higher QoS level, is stable enough. If the target network can not maintain the measured QoS level for a certain time period then it is unstable. There is no necessity to switch to an unstable network. Some causes of instability includes: weather, number of users and network traffic. When there is more than one network available, the QoS level provided by each network must be measured. The MT chooses the least one from those networks that give just enough better QoS level than current serving network. The MT chooses the least one because it looks for the most suitable and not the best network to obtain the best balance between performance and network resource usage. This can avoid inefficient usage of network resources in situations where the MT attaches to a network that provides much more resources than the MT needs. When the signal strength of current BS is getting weak, the MT may need to perform horizontal handoff to another BS with stronger signal strength. The horizontal handoff scheme implemented in the MT will be in charge in this situation. But if no other BSs of current network exist, then the MT may need to switch to another type of network before the connection with current network becomes broken. Fig. 3 shows the decision algorithm when VHO may be carried out instead of horizontal handoff. When horizontal handoff should be executed but no BS of the same network exists, the algorithm tries to find a second most suitable network for the MT. D. AAO vertical handoff Commonly the VHO includes three steps: system discovery, handoff decision and handoff execution. Usually the first two steps are executed with repetition in certain time period, depending on the scheme applied, before the real handoff happens. The MT first finds out its nearby networks and then using some evaluation methods, depending on the scheme applied, to check the network conditions. No matter the evaluation result, the MT keeps executing these two steps routinely trying to find a better network, even when the application running on the MT runs just fine and does not need a better network. These unnecessary steps before the real handoff action will waste the energy of the MT and network resources. Our AAO scheme reverses the mentioned order to: handoff decision, system discovery and handoff execution. The MT will first check if the current network conditions can satisfy the current running IEEE Communications Society / WCNC 2005

Check whether the requirements are satisfied when MT boots up or changes application if satisfied then stay in current network else request BS for nearby networks’ information if nearby networks exist then calculate QoScurrent and QoSnearby if exists QoSnearby > QoScurrent then repeat until all candidate networks are tested select the network with least value of QoSnearby > QoScurrent as target network start dwell timer if condition persists until timer expires then handoff to target network exit else QoStarget unstableÆremove this network from candidate list end stay in current network else stay in current network Fig. 2 Pseudocode of decision algorithm 1 While signal strength of current network is getting weak do If other BS of current network exists then start horizontal handoff scheme else start dwell timer to check the instability if condition persists until timer expires then request BS for nearby networks’ information if nearby networks exist then calculate QoScurrent and QoSnearby if exists QoSnearby > QoScurrent then select the network with least QoSnearby as target network handoff to target network else select the network with highest QoSnearby as target network handoff to target network request BStarget for better network else stay in current network else stay in current network Fig. 3 Pseudocode of decision algorithm 2

The MT will attach to a predefined network as its home network when boots up. For example a 3G device will attach to 3G network when boots up and a laptop will attach to WLAN when boots up. After the boot process the MT executes algorithm 1 for the first time. In most cases the MT will be satisfied since after the boot process it will enter stand-by mode and this mode does not need much resources. After the boot process, the MT executes algorithm 1 when the running application is changed or algorithm 2 when the RSS of current interface deteriorates. After receiving the request from the MT, BS will request the LSS to search adjacent networks’ information according to the geographical location of the MT. The result is replied to BS and then passed to the MT. If there are available networks, the MT will use the information received from BS to quantify the QoS level of each network, determining 1386

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number of WLAN hotspots is arbitrarily selected from 50 to 70. To simplify the simulation the WLAN hotspots can be connected freely without authentication. Although in real life the connection to WLAN hotspots must be authenticated and authorized for security concerns. The factors considered by the applications at the MT are bandwidth, latency and packet error rate. Table II shows the value of these factors used in the simulation. The requirement of each factor of each application is shown in Table III. B. Precise handoff time Handoff at right time to a suitable network is vital to guarantee maximum performance and best balance of network resources. Unnecessary handoff will waste network resources and even degrade the QoS level. Fig. 4 shows the average number of unnecessary VHO in a time interval of 10 minutes when basic and AAO vertical handoff schemes are applied. The activity of the MT is 50% in stand-by mode, 30% in voice conversation and 20% in real-time data-intensive applications such as video streaming that requires continuous massive data transmission. The MT moves randomly in the area at different speed. Unnecessary VHO means handoff from GSM to WLAN network when the MT is in stand-by or voice conversation mode. Basic VHO scheme only compares the factor’s values, no more criteria or decision algorithm is involved. We can clearly see that the average number of unnecessary VHOs almost decreases to zero when AAO scheme is applied. But in basic passive VHO scheme the number of unnecessary VHOs increases with the increase of MT’s moving speed. Moreover, in the basic VHO scheme both WLAN and GSM/GPRS interfaces are activated all the time to facilitate the contact with both networks. With the proposed AAO scheme, the waste of network resources and

whether a more suitable network exists. If there is no other network available, the BS will keep requesting the LSS for the MT asking for available network until the MT tells BS that it does not need better network. If there is network available and has higher QoS, then the MT will set up a dwell timer to probe the stability of the new network. If the conditions of the new network persist until the dwell timer expires, then the MT can start the handoff action. During the handoff process the mobility management is done by using Mobile IP [10]. To avoid the loss of data packet destined to the MT, bicasting of packet is used, instead of multi-casting to possible next BS candidates. When the MT decides to handoff, it will notify its HA about the new BS in the target network. After that the HA will send two copies of data to both old and new BS until MT replies handoff success. Because the time that the bicasting lasts is very short, the cost is very low and the impact to the performance is minimum. The AAO scheme also avoids the ping-pong effect by making the handoff decision based on the application’s need and not on the RSS of the networks. IV.

SIMULATION RESULTS

In this section we present some simulation results to show the performance of our AAO scheme. The results are compared with those of conventional passive schemes, i.e. always on and periodically on schemes. TABLE II FACTOR VALUE

Bandwidth Latency Packet error rate

WLAN URm,n LRm,n 700 Kbps 400 Kbps 5ms 20ms 1%

GSM/GPRS URm,n LRm,n 35 Kbps 25 Kbps 3ms 30ms

1.5%

2%

3.5%

Average number of unnecessary VHOs

Factors

TABLE III APPLICATION REQUIREMENT Stand-by

Bandwidth

Packet rate

error

Data-intensive (Video streaming) 500 Kbps

Ufn

25 Kbps

Lfn

3 Kbps 10ms

9.6 Kbps

50 Kbps

5ms

5ms

Lfn

500ms

150ms

150ms

Ufn

2%

2%

2%

Lfn

10%

8%

8%

Ufn

Latency

Voice conversation 64 Kbps

16 AAO VHO Basic VHO

12

8

4

0 1

5 7 9 11 MT moving speed (m/s)

13

15

Fig. 4 Unnecessary VHOs

energy can be reduced to minimum and the handoff occurs only when the MT needs a better network. C. Accumulated activating time of interfaces Here we compare the accumulated activating time of WLAN interface when using the following methods: always on, periodically on and AAO. In always on scheme the WLAN interface is activated all the time. In periodically on scheme the WLAN interface is activated for 500ms in every 3 seconds. In AAO scheme the WLAN interface is activated when the MT needs higher bandwidth for data-intensive applications. The activity of the MT is 50% in stand-by mode, 30% in voice conversation and 20% in real-time data-intensive applications. The MT moves randomly in the area at speed of one meter per second. Higher accumulated

A.

Simulation model The MT in the simulation is a device with two interfaces: one for WLAN and one for GSM/GPRS. IEEE 802.11b standard is used for WLAN and the GPRS standard is class 10 (4 + 2, 53.6Kbps for downlink, and 26.8Kbps for uplink). The MT is configured to use GSM/GPRS as main access radio, i.e. a cellular phone [11]. A battery with capacity of 1000 mAh is used. The simulation environment is a one kilometer square plain area under GSM/GPRS coverage. To obviate the radio signal interference issue, the area is supposed to be free of obstacle and buildings. Within this area there are randomly allocated WLAN hotspots of diameter of 40 meters. The IEEE Communications Society / WCNC 2005

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devices capable of running several applications concurrently would come out shortly. The support for multi-tasking devices should be the next step for our research. Another issue is that even the AAO scheme is able to find the most suitable network for the current running application, there will be moments where no network with enough QoS level is available. How to adapt the application’s QoS requirement to a network with much lower QoS level is another step in our research.

24

Battery remaing capacity (mAh)

WLAN interface activated time (hours)

activating time means higher power consumption. It’s desirable to decrease the accumulated activating time as less as possible. The simulation result is shown in Fig. 5. After 24 hours of MT activating time, the AAO scheme gives a substantial decrease in WLAN interface activating time. In always on scheme the activating time of the WLAN interface is the same as the activating time of the MT, which is 24 hours. In periodically on scheme the activated time is decreased to less than 10 hours. In AAO scheme the activated time is decreased substantially to 4 hours. Always on Periodically AAO

20 16 12 8 4 0 1

3

5

7

500 400 300 200 100 0 3

5

7

9 11 13 15 17 19 21 23 25

MT activated time (hours)

MT activated time (hours)

Fig. 6 Power consumption

D.

Energy consumption Battery is the main power source for portable devices and should be used as efficient as possible to obtain maximum battery life. The activity of the MT in this simulation is 75% in stand-by mode, 20% in voice conversation and 5% in data-intensive applications. The MT moves randomly in the area at speed of one meter per second. As we can see from the previous result, the AAO scheme does reduce effectively the activated time of WLAN interface. With this reduction the AAO scheme does save some extra energy. The result is shown in Fig. 6. With a battery of capacity of 1000 mAh, the MT merely operates for 10 hours with always on scheme. The periodically on scheme gives a battery life of 20 hours, while the AAO scheme grants a battery life of 24 hours.

[1] [2] [3] [4] [5] [6] [7]

CONCLUSIONS AND FUTURE WORK

In this paper, we have presented an active application oriented VHO scheme for next generation wireless data networks. The active style permits less resource usage. The handoff decision based primarily on running applications makes the handoff process more accurate and appropriate. It is possible to execute handoff only when the MT needs, therefore the waste of network resources and battery produced by unnecessary handoffs is decreased to minimum. A method to quantify the requirements of the applications is used to help the handoff decision. The simulation showed the effectiveness of the AAO scheme. Unnecessary VHOs, excessive interface activating time and redundant resource usage are decreased to minimum. In battery life test we obtained an improvement of 240% over always on scheme and 20% over periodically on scheme. Consequently, the proposed scheme meets the goal of power efficient and provides an efficient interface management. Currently we are dealing with devices that run one application at same time. As the technologies advance, IEEE Communications Society / WCNC 2005

Always on Periodically AAO

1

9 11 13 15 17 19 21 23 25

Fig. 5 Accumulated WLAN interface activating time

V.

1000 900 800 700 600

[8] [9] [10]

[11] [12] [13] [14]

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