Published online 13 July 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/wcm.1165
RESEARCH ARTICLE
Automatic event-triggered call-forwarding mechanism
for mobile phones
Yi-Bing Lin1*, Ren-Huang Liou1, Yuan-Kai Chen2and Zheng-Han Wu1 1Department of Computer Science, National Chiao Tung University, Hsinchu, Taiwan
2Chunghwa Telecom, Taipei, Taiwan
ABSTRACT
Call forwarding is a traditional telecom service that allows a user to forward incoming calls to another telephone number. This service requires the user to manually activate and deactivate the feature and therefore may not be very convenient. This paper proposes an automatic call-forwarding algorithm (CFA) for mobile phones. By installing a software in a smartphone, call forwarding is automatically triggered (e.g., when the phone is plugged in a charger or is turned off) or disabled (e.g., when the phone is unplugged from the charger or is turned on). We investigate the performance of the CFA through ana-lytic analysis, simulation, and measurement. Our study indicates that CFA is very feasible for commercial usage. Copyright © 2011 John Wiley & Sons, Ltd.
KEYWORDS
call forwarding; mobile telecom; UMTS *Correspondence
Yi-Bing Lin, Department of Computer Science, National Chiao Tung University, Hsinchu, Taiwan. E-mail: liny@cs.nctu.edu.tw
1. INTRODUCTION
When a person returns home, he or she may turn off his or her user equipment (UE; mobile phone) or plug the UE into its charger. The person may miss the calls to the UE when he or she is in the house (but away from the UE). In this case, it would be desirable that the calls are automati-cally forwarded to the line phone (and its extensions) in the house so that the person can still pick up his or her calls. Call-forwarding setup can be done manually, but it is con-sidered as a tedious process for many people. Furthermore, when call forwarding is not needed anymore, people may forget to manually disable this service and will not be able to receive calls from their original mobile phones.
Several solutions have been proposed to support auto-matic call forwarding. In [1], if the incoming call is not answered from the destination phone, the incoming call is automatically forwarded another pre-configured phone number. In [2], the location of the user is tracked by the sensor network so that the incoming call can be auto-matically forwarded to the phone nearest the user. These solutions require modifications to the telecom networks. In this paper, we propose an automatic event-triggered
call-forwarding algorithm (CFA) that does not incur any
modification to the telecom networks. Moreover, our solu-tion can be easily installed in a smartphone (i.e., a UE).
1.1. Concept of call-forwarding algorithm We first describe the concept of CFA, which is imple-mented by the functions in Microsoft Windows CE (WinCE) platform. For other smartphone platforms, the implementations are similar and will not be elaborated.
Our CFA solution consists of four parts:
Part 1. Detection of triggering events: The events ‘when the UE is turned off’ or ‘when the UE is plugged into its charger’ trigger the call-forwarding ser-vice. Such triggering events are automatically detected in CFA. This feature is implemented by using the WinCE function RegistryNotifyCall-back [3] to monitor the battery status. Another triggering event considered in our current imple-mentation is ‘when a special short message is received’ (this scenario will be elaborated in Section 4).
Part 2. Selection of the forwarded-to number (i.e., incoming calls to the UE are redirected to
this phone number): After a triggering event is detected, the UE selects the corresponding forwarded-to number. In particular, this selec-tion may be associated with locaselec-tion service. For example, the home phone number may be associated with the home’s Global Positioning
System (GPS) information. The UE then uses its
current location obtained from its assisted GPS (A-GPS) receiver to identify the forwarded-to number. Note that A-GPS can be used in indoor environment [4]. Location ambiguity may occur because of inaccuracy of location measurement, and in this case, the UE may query the user to select one from these ambiguous forwarded-to numbers. If ambiguity does not occur, the UE automatically selects the number without bothering the user.
Part 3. Activation of call forwarding: After the forwarded-to number is determined, the UE auforwarded-tomatically conducts the standard call-forwarding registra-tion procedure with the telecom network. This feature is implemented by invoking the WinCE function lineForward [3].
Part 4. Deactivation of call forwarding: When the cause of the triggering event disappears, call forward-ing is deactivated. For example, when the UE is unplugged from its charger, it automatically dis-ables the call-forwarding service by executing the standard call-forwarding erasure procedure with the telecom network. As in Part 3, the call-forwarding deactivation is also supported by the lineForward function.
Note that in Part 2, if the UE’s A-GPS receiver is not activated under some power-saving strategy [5], then it is automatically turned on when the UE’s CFA detects the triggering event. After the UE’s CFA has retrieved its GPS position, it may turn off the A-GPS receiver again to avoid power consumption of mobile phone.
1.2. Mobile telecom network for call-forwarding algorithm execution
We use a simplified circuit-switched Universal Mobile
Telecommunications System (UMTS) network architecture
as an example to explain how call-forwarding service works [6,7]. The call-forwarding service also can be sup-ported in packet-switched telecom network (e.g., IP
Multi-media Subsystem) through the standard Parlay X interface
[8]. In the UMTS architecture (see Figure 1), a mobile user with a UE (UE1; Figure 1(1)) is connected to a serving mobile switching center/visitor location register (MSC/VLR; Figure 1(2)) to receive telecom services. The MSC and the VLR are responsible for call processing and mobility management, respectively. Each UE is assigned an E.164 mobile telephone number (e.g., 0911111111 for UE1), and every number is mapped to a Gateway Mobile
Switching Center (GMSC; Figure 1(3)). In other words, for
every incoming call to UE1, the call is first routed to its GMSC. The home location register (HLR; Figure 1(4)) is a database that indicates the MSC/VLR location of mobile users. The MSC/GMSC connects to the Public Switched
Telephone Network (PSTN; Figure 1(5)). In the PSTN, the service switching points (SSPs; Figure 1(6) and (7)) are
telephony switches that support call processing.
Based on this architecture, we will describe the mes-sage flows (Part 3 and Part 4) for CFA by using Chunghwa Telecom’s call-forwarding unconditional service [7].
2. MESSAGE FLOWS FOR
CALL-FORWARDING ALGORITHM
This section describes the message flows for CFA includ-ing activation, incominclud-ing call setup, and deactivation. In these procedures, we assume that user 1’s UE (UE1 with phone number 0911111111) is installed with CFA, and the number 031111111 of user 1’s line phone (Phone1; Figure 1(8)) is selected as the forwarded-to number (i.e., incoming calls to UE1 are forwarded to Phone1 when call-forwarding service is activated). Then, we investigate the performance of CFA by deriving the probability of incoming call arrival during CFA activation.2.1. Call-forwarding algorithm activation procedure
When user 1 plugs UE1 into the phone charger, CFA in UE1 detects the charging status, and automatically executes the CFA activation procedure through the stan-dard 3rd Generation Partnership Project (3GPP) call-forwarding registration procedure [7,9]. Figure 2 illustrates the CFA activation procedure with the following steps:
Step A.1. When the WinCE RegistryNotifyCallback function detects the charging status, CFA automatically dials the special number **21 *031111111# where 21 is the service code of Chunghwa Telecom’s call-forwarding uncon-ditional service and 031111111 is the forwarded-to number (call-forwarding dial-ing methods for other telecom operators are similar and will not be elaborated). The MSC/VLR (Figure 1(2)) sends the call-forwarding registration request to the HLR (Figure 1(4)) by the Signaling System
Num-ber 7 (SS7) MAP_REGISTER_SS request.
This message indicates that UE1 (with the number 0911111111) wants to enable the call-forwarding unconditional service with the forwarded-to number 031111111. Step A.2. The HLR checks if UE1 is allowed to enable
the call-forwarding service. If so, the HLR stores the forwarded-to number, and returns the SS7 MAP_REGISTER_SS response to the MSC/VLR indicating that the registration
(2) MSC/ VLR (3) GMSC (1) UE1 0911111111 (5) PSTN (8) Phone1031111111 (6) SSP1 (7) SSP2 (9) Phone2 (4) HLR
Figure 1. Network architecture for call forwarding. UE, user equipment; MSC, mobile switching center; VLR, visitor location register; HLR, home location register; GMSC, Gateway Mobile Switching Center; PSTN, Public Switched Telephone Network; SSP, service
switching point.
HLR A.1 SS7 MAP_REGISTER_SS request
A.2 SS7 MAP_REGISTER_SS response UE1
0911111111 MSC/
VLR
A.2 Call Forwarding Mapping 0911111111 031111111
Figure 2. Call-forwarding algorithm activation procedure. MSC, mobile switching center; VLR, visitor location register; HLR,
home location register; UE, user equipment.
procedure is successful. Otherwise, the HLR returns an error.
When this procedure is finished, all calls to UE1 are re-directed to Phone1.
2.2. Incoming call setup procedure
After user 1 has enabled the call-forwarding service, if user 2 (Figure 1(9)) dials user 1’s mobile phone num-ber 0911111111, the call setup procedure is illustrated in Figure 3 with the following steps:
Step B.1. SSP2 (Figure 1(7)) issues the SS7
ini-tial address message (IAM) message to
the GMSC (Figure 1(3)) of 0911111111 (i.e., UE1).
Step B.2. To obtain the routing information for this call, the GMSC queries the HLR via the SS7 MAP_SEND_ROUTING_INFORMATION request.
Step B.3. The HLR replies with the SS7 MAP_SEND_ ROUTING_INFORMATION response that contains the routing number (the SS7 address) of SSP1 that serves the forwarded-to number 031111111 (i.e., Phone1).
Step B.4. The GMSC forwards the SS7 IAM message to SSP1.
Step B.5. SSP1 alerts Phone1 and returns the SS7
address complete message (ACM) message to
SSP2 through the GMSC.
Step B.6. When user 1 picks up Phone1, SSP1 issues the SS7 answer message (ANM) to SSP2 through the GMSC.
At the end of Step B.6, user 1 and user 2 start a conversa-tion.
2.3. Call-forwarding algorithm deactivation procedure
When user 1 unplugs UE1’s charger, UE1’s CFA detects this triggering event and automatically executes the CFA deactivation procedure to disable the call-forwarding ser-vice through the 3GPP call-forwarding erasure procedure [7,9]. Figure 4 illustrates the CFA deactivation procedure with the following steps:
Step C.1. Similar to Step A.1, when the WinCE Reg-istryNotifyCallback function detects the trig-gering event of unplugging UE1 from the charger, UE1’s CFA automatically dials the special number ##21#. Then the MSC/VLR sends the SS7 MAP_ERASE_SS request to the HLR to indicate that UE1 wants to disable the call-forwarding service.
Step C.2. The HLR removes the corresponding forwarded-to number of UE1 and replies with the SS7 MAP_ERASE_SS response to indicate that the erasure procedure is successful.
After CFA deactivation is finished, all calls to UE1 are routed to UE1 instead of Phone1.
2.4. Call-forwarding algorithm delay analysis
After user 1 has plugged UE1 into the phone charger, he or she expects that incoming calls to UE1 should be for-warded to Phone1. However, it is possible that an incoming call arrives at UE1 before CFA activation is complete. In
B.1 SS7 IAM B.4 SS7 IAM B.3 SS7 MAP_SEND_ROUTING _INFOMATION response B.5 SS7 ACM B.5 SS7 ACM B.6 SS7 ANM B.6 SS7 ANM Phone1 031111111 Conversation Phone2 B.2 SS7 MAP_SEND_ROUTING _INFOMATION request SSP2 GMSC SSP1 0911111111 031111111 Call Forwarding Mapping
HLR
Figure 3. Incoming call setup procedure. SSP, service switching point; GMSC, Gateway Mobile Switching Center; HLR, home location register; IAM, initial address message; ACM, address complete message; ANM, answer message; SS7, Signaling System Number 7.
C.1 SS7 MAP_ERASE_SS request C.2 SS7 MAP_ERASE_SS response UE1 0911111111 HLR MSC/ VLR
Figure 4. Call-forwarding algorithm deactivation procedure. MSC, mobile switching center; VLR, visitor location register; HLR, home location register; SS7, Signaling System Number 7. this case, if user 1 expects that the call will ring Phone1, then he or she is not notified of this call until he or she removes UE1 from the charger (and is informed by the missing call list in UE1). We will formally show that such missing calls occur with very low probability, and the issue can be either ignored or resolved by a notification mechanism described in Section 3.
Let pcbe the probability that an incoming call arrives at UE1 during CFA activation (before the 3GPP call-forwarding registration procedure is complete). It is clear that the smaller the pcvalue, the better the user experience about CFA.
Figure 5 illustrates a timing diagram for deriving pc. Let tcbe the inter-call arrival time and tabe the delay of CFA activation (Steps A.1–A.2 in Figure 2). The inter-val cbetween when CFA starts the activation procedure
tc
Call arrives Call arrives
CFA activation starts CFA activation ends ta
time
c
Figure 5. Timing diagram for derivingpc. CFA, call-forwarding algorithm.
and when the next call arrives is called the excess life of the inter-call arrival time. Then, pcis the probability that c< ta.
Assume that tc is exponentially distributed with the mean 1= (i.e., the call arrivals are a Poisson process) and ta has an arbitrary distribution with the density function fa./ and the Laplace transform fa.s/. From the memo-ryless property of the exponential distribution, chas the same exponential distribution as tc, and pcis derived as
pcD PrŒc< ta D Z 1 taD0 fa.ta/ Z ta cD0 ecd cdta D 1 fa./ (1)
If tais a Gamma random variable with the Laplace trans-form fa.s/ D
s C k
, where k is the shape parameter and is the rate parameter, then Equation (1) is re-written as pcD 1 C k (2)
We consider the Gamma distribution because this distri-bution is widely used in telecom modeling; see [5,10] and the references therein. Equation (2) is validated against the Monte Carlo simulation, which generates the delays cand ta, and then compares the lengths of these delays to pro-duce the pcvalue. The simulation experiments show that the discrepancies between the analytic (i.e., Equation (2)) and simulation results are within 0.2%.
We have also measured the tavalues in the commercial UMTS system of Chunghwa Telecom. We installed CFA in a smartphone CHT 9110 (Chunghwa Telecom, Taiwan) with Microsoft Windows Mobile 6.0 operating system and collected the delays tafrom more than 3000 CFA activa-tion execuactiva-tions. We obtained statistics EŒta D 7:88266 s and the variance VaD EŒta2 EŒta2D 0:0139717EŒta2.
With the average measured EŒta value (i.e., 7.88266 s), we assume that 100EŒta EŒtc 1000EŒta (i.e., the inter-call arrival time ranges from about 13 min to 2.18 h). Figure 6 plots pc (i.e., the probability that an incoming call arrives at UE1 before CFA activation is complete) against EŒtc=EŒta and Va. The figure shows the trivial result that pcdecreases as EŒtc=EŒta increases. The non-trivial result is that pcdecreases as Vaincreases. This phe-nomenon is explained as follows. For a fixed EŒta value, when Vais large (Va > 10EŒta2), if Vaincreases, there are more short taperiods than long taperiods. For short ta, it is unlikely that c < ta. Therefore, pcdecreases as Va increases.
Measurements indicate that the Vaof Chunghwa Tele-com’s network is very small, and 0.1% to 1% of the incom-ing calls may still arrive at UE1. However, such calls will occur in less than 8 s after CFA activation is executed, while user 1 is still around UE1. Therefore, user 1 will hear the ringing tone, and these calls are answered through UE1. In some telecom networks, large Vavalues may be observed, which result in long ta. For a very long ta, it is possible that incoming calls arrive at UE1 after user 1 has moved away from UE1 (before CFA activation is com-plete). In this case, he or she may not hear the ringing and miss the calls (with probability pc < 0:2% in Figure 6). To resolve this issue, we propose the CFA notification procedure described in Section 3.
3. CALL-FORWARDING ALGORITHM
NOTIFICATION AND FAILURE
DETECTION
After user 1 plugs UE1 into the charger, he or she may move to another room in the house (e.g., from bedroom to kitchen). Immediately after CFA activation, it is desirable
0 0.002 0.004 0.006 0.008 0.01 pc 10−4 10−3 10−2 10−1 100 101 102 103 104 Va (unit: E[ta]2) : E[tc] = 100E[ta] E[tc] = 400E[ta] E[tc] = 700E[ta] E[tc] = 1000E[ta] : : × :
Figure 6. Effects ofEŒtc=EŒta and Vaonpc(tcis exponentially distributed).
to notify user 1 that call forwarding is correctly activated to the target line phone. This section proposes a CFA notifica-tion procedure to serve for this purpose. Besides successful CFA notification, this procedure also notifies user 1 of unsuccessful call-forwarding activation through a thresh-old mechanism with a timer T . The CFA notification is executed to inform user 1 that the call-forwarding activa-tion is either successful or failed (i.e., T expires; due to possibly lost message).
3.1. Call-forwarding algorithm notification procedure
Figure 7 illustrates the CFA notification procedure with the following steps:
Step D.1. UE1’s CFA initiates a call to Phone1 by auto-matically dialing the forwarded-to number 031111111. The MSC/VLR sends the SS7 IAM message to SSP1.
Step D.2. SSP1 alerts Phone1 and returns the SS7 ACM message to the MSC/VLR. Then, the MSC/VLR notifies UE1 that Phone1 starts ringing.
Step D.3. After user 1 picks up Phone1, SSP1 sends the SS7 ANM message to the MSC/VLR. Through, for example, voice announcement, UE1’s CFA informs user 1 about the status of the call-forwarding setup and indicates if there are incoming calls during CFA activa-tion. We note that 0.1%–1% calls that still arrived at UE1 (described in Subsection 2.4) will be notified to user 1 in this step. The voice announcement requests user 1 to dial
Phone1 031111111 UE1 0911111111 D.1 SS7 IAM D.2 SS7 ACM D.5 SS7 RLC D.4 SS7 REL D.3 SS7 ANM Voice Announcement MSC/ VLR SSP1
Figure 7. Call-forwarding algorithm notification procedure. MSC, mobile switching center; VLR, visitor location register; SSP, service switching point; UE, user equipment; SS7, Signaling System Number 7; IAM, initial address message; ACM, address complete message; ANM, answer message; REL, release; RLC,
a secret digit (e.g., ‘1’) to confirm receipt of call-forwarding setup status. If a wrong forwarded-to number is accidentally selected, another person who picks up this call does not know the secret digit, and the call-forwarding service is canceled.
Step D.4. After user 1 has hung up Phone1, SSP1 issues SS7 release (REL) message to the MSC/VLR to terminate the call.
Step D.5. The MSC/VLR replies to SSP1 with the SS7
release complete (RLC) message. The
proce-dure exits.
After the call-forwarding service is successfully activated, UE1 enters the charging mode. For the ‘turning off UE’ scenario, UE1 is actually turned off after the aforemen-tioned procedure is complete. Note that Step D.3 ensures that call-forwarding service is correctly enabled through confirmation of user 1.
3.2. Call-forwarding algorithm activation failure detection
The proposed CFA activation failure detection scheme uti-lizes a threshold T computed as follows. Every time the CFA activation is executed, the elapsed time tais measured and stored. UE1’s CFA accumulates the m most recent tasamples. Let ta;i be the i th previous tasample. When UE1’s CFA executes CFA activation, T is computed as
T D ˛ m X i D1 ta;i ! m (3)
where ˛ > 1 is a weighting factor used to ensure that T is not shorter than the actual tavalue. If CFA activation is not finished in the T period (i.e., CFA does not receive the response from the HLR within T ), then CFA activation is considered failed. In this case, CFA notification will inform user 1 of unsuccessful setup.
From Equation (3), if ˛ is set too small, CFA may cancel a successful call-forwarding setup. On the other hand, if ˛ is set too large, CFA activation failure can not be detected early. Therefore, it is important to select an appropriate ˛ value. We will show that ˛ D 1:5 is sufficient for CFA acti-vation failure detection in Chunghwa Telecom’s network. We will also investigate the performance of the CFA acti-vation failure detection scheme under different Va(i.e., ta’s variance).
Let psD PrŒta< T be the probability that CFA activa-tion is complete within T . It is clear that the larger the ps value, the better the performance of failure detection.
Let ta be a random variable with the density func-tion fa./ and the Laplace transform fa.s/. Let T be a random variable with the density function fT./ and the Laplace transform fT.s/. If we re-write Equation (3) as
T D m X i D1 ˛t a;i m
, then the Laplace transform of the T distribution is
fT.s/ Dhfa ˛s m
im
(4)
If tais an Erlang random variable with the shape param-eter k and the rate paramparam-eter , then its density function and the Laplace transform are
fa.ta/ D ktak1eta .k 1/Š and f a.s/ D s C k (5)
Substitute Equation (5) into Equation (4) to yield
fT.s/ D m ˛s C m km (6)
We selected the Erlang distribution because this tion can be easily extended into a hyper-Erlang distribu-tion, which has been proven to be a good approximation to many other distributions as well as measured data [11,12].
From Equations (5) and (6), psis derived as ps D PrŒta< T D Z 1 T D0 fT.T / Z T taD0 fa.ta/dtadT D Z 1 T D0 fT.T / 2 41 k1X i D0 eT.T /i i Š 3 5 dT D 1 k1X i D0 " i.1/i i Š # 2 4difT.s/ dsi ˇ ˇ ˇ ˇ ˇsD 3 5 D 1 k1X i D0 ˛ ˛ C m i.km C i 1/Š i Š.km 1/Š m ˛ C m km (7)
Equation (7) is used to validate the simulation model (following the same Monte Carlo methodology described in Section 2.4). Experiments show that the discrepancies between the analytic (i.e., Equation (7)) and simulation results are within 0.1%. In the remainder of this paper, we used the validated simulation experiments to investigate the performance of the CFA activation failure detection scheme. Specifically, we extend the validated simulation model from the Erlang ta distribution to the Gamma ta distribution. Then, we used the Gamma simulation model with EŒta D 7:88266 s and Va D 0:0139717EŒta2 to approximate the measured data from Chunghwa Telecom’s network (mentioned in Section 2.4). Figure 8 plots ps (the probability that CFA activation is complete within T ) against ˛ and m for simulation and measurement. It is clear that ps increases as ˛ increases. Figure 8(a) shows that
0.5 0.6 0.7 0.8 0.9 1 1 1.2 1.4 1.6 1.8 2.0 Dashed: Measurement Solid: Simulation
•:
:
:
= 1 = 10 = 20 0.95 0.96 0.97 0.98 0.99 1 1 5 10 15 20 25 Dashed: Measurement Solid: Simulation•:
:
= 1.5 = 2(a) Effect of
(b) Effect of
Figure 8. Effects of ˛ andm on ps(EŒta D 7:88266 seconds and VaD 0:0139717EŒta2).
when ˛ is small (i.e., ˛ 1:3), the simulation results are lower bounds of the measurement. When ˛ is large (i.e., ˛ 1:5), the result reverses. The trends of ps are simi-lar for both the Gamma tadistribution and the measured histogram, and these values are close when ˛ is large (i.e., ˛ 1:5).
Figure 8(b) indicates that when m 20, psis not sen-sitive to the change of m. In other words, it is sufficient to store 20 most recent tasamples in CFA for computing the T value in Equation (3). When m D 20, the discrepancies between the simulation and the measurement are within 0.5% for ˛ 1:5. Figure 8 also shows that selection of a small ˛ (i.e., ˛ D 1:5) suffices to yield good ps perfor-mance (e.g., ps>0.99) in Chunghwa Telecom’s network. For a telecom network where Va is large, Figure 9 shows that psdecreases and then increases as Vaincreases (when ta has a Gamma distribution). This phenomenon is explained as follows. When Va is small (i.e., Va < EŒta2), if Va increases, more short ta and more long ta are observed. These long ta result in smaller ps. When Va is large (i.e., Va > 10EŒta2), if Va increases, much more short ta are observed. Much longer ta are also observed. However, the number of these very long ta is much fewer than the number of short ta. There-fore, a larger ps is observed. We note that if ˛ D 4:5 (i.e., EŒT D 35:47197 s) is selected, ps > 0:9 for all Va values under our study.
0.75 0.8 0.85 0.9 0.95 1 10−4 10−3 10−2 10−1 100 101 102 103 • : = 1.5 : : : Va (unit: E[ta]2) = 2.5 = 3.5 = 4.5
Figure 9. Effects of ˛ andVaonps(m D 20).
4. CALL-FORWARDING ALGORITHM
FOR TELEMATICS
In telematics, a car is typically equipped with a personal
mobile communications capabilities (e.g., GSM, GPRS, or UMTS). In the hands-free phone service, when a person turns on the PND, all incoming calls are forwarded to the PND, and the person can receive hands-free calls (i.e. he or she can listen and talk through the car speaker and the PND’s microphone). Existing hands-free car phone service is typically provisioned in two ways: the wire-line and the Bluetooth solutions. Both solutions require manual con-nection between the mobile phone and the communication device installed in the car.
With CFA, we can provide automatic call forwarding for telematics, assuming that user 1’s PND is installed with a software that can detect the triggering event ‘when the PND is turned on/off’. Many PND products manu-factured in Taiwan allow such modifications to accommo-date telecom operators’ needs. When the triggering event is detected, the PND sends a short message to UE1 to enable/disable the call-forwarding service. The CFA works as follows. After user 1 gets on her car and turns on the PND, the following steps are executed:
Step E.1. The PND retrieves its position from the GPS receiver and sends a short message to UE1. This short message contains the PND’s GPS position and the request for enabling call forwarding to the PND’s phone number. Step E.2. After UE1 has received the short message,
UE1’s CFA obtains its position from the A-GPS mechanism and compares the PND’s position with UE1’s position. If their posi-tions are close enough (e.g., within 10 m), the CFA considers that UE1 is in the car and rings user 1 to ask if he or she wants to acti-vate the call-forwarding feature. User 1 sim-ply presses one key to accept (or reject) the call-forwarding activation request. Then, the CFA activation is executed as described in Section 2.1.
Step E.3. UE1’s CFA sends a short message to the PND to indicate the result of the activation. Then, the PND shows the result to user 1 through, for example, voice announcement.
Note that in Step E.2, another strategy is that whenever UE1 receives the short message, it always alerts user 1 that the PND is turned on (without considering the GPS information that may not be available in UE1).
When user 1 turns off the PND (e.g., turns off the car), call forwarding is disabled with the following steps:
Step F.1. Before the PND actually shuts down, it sends a short message to UE1 to disable the call-forwarding service.
Step F.2. Upon receipt of the short message, UE1’s CFA executes the deactivation procedure described in Subsection 2.3.
If the user leaves the car while the PND is still on, the aforementioned procedure does not work. To resolve this
issue, the PND may periodically check UE1 through steps similar to Steps E.1–E.3, and if the GPS locations indicate that they are too far away, then UE1 will ask the user if CFA deactivation should be performed.
5. CONCLUSIONS
This paper proposed an automatic CFA for mobile phone. Call forwarding for fixed line phone is typically triggered manually, which is a tedious process for a user. Unlike a fixed line phone, many triggering events may occur to a mobile phone, for example, battery charging, turn-off, and location change. By detecting these events, the CFA automatically triggers call-forwarding features.
With CFA, the user avoids tedious activation and deacti-vation actions of call forwarding; however, he or she must be notified if a CFA action is successful. We derived the value of a time-out period T such that the user is appro-priately informed of the CFA result before T expires. We conducted analytic analysis, simulation, and measurement in Chunghwa Telecom’s network to show that the CFA yields good performance and can be practically commer-cialized. As a final remark, CFA can be easily installed in a smartphone and does not make any modification to the telecom network.
ACKNOWLEDGEMENTS
Y.-B. Lin’s work was supported in part by NSC (grant nos. 98-2219-E-009-016, 98-2221-E-009-059-MY2, and 97-2221-E-009-143-MY3), Chunghwa Telecom, IBM, ITRI and the NCTU Joint Research Center, and the MoE ATU plan.
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AUTHORS’ BIOGRAPHIES
Yi-Bing Lin is the vice president and life chair professor of the College of Computer Science, National Chiao Tung University (NCTU), and a visiting professor of King Saud Uni-versity. He is also with the Insti-tute of Information Science and the Research Center for Information Technology Innovation, Academia Sinica, Nankang, Taipei, Taiwan. Lin is one of the authors of the books Wireless and Mobile Network Architecture (John Wiley & Sons, 2001), Wireless and Mobile All-IP
Networks (John Wiley & Sons, 2005), and Charging for Mobile All-IP Telecommunications (John Wiley & Sons,
2008). Lin received numerous research awards including 2005 NSC Distinguished Researcher and 2006 Academic Award of Ministry of Education. Lin is an ACM Fellow, an AAAS Fellow, an IEEE Fellow, and an IET Fellow.
Ren-Huang Liou received the BS and the MS degrees in Computer Sci-ence from National Chiao Tung Uni-versity (NCTU), Hsinchu, Taiwan, in 2007 and 2009, respectively. He is currently working toward the PhD degree at NCTU. His current research interests include design and analysis of personal communications services networks, mobile computing, and performance modeling.
Yuan-Kai Chen received the BS, MS, and PhD degrees in Computer Science and Information Engineer-ing from National Chiao Tung Uni-versity, Hsinchu, Taiwan, in 1989, 1991, and 2002, respectively. In 1991, he joined Chunghwa Telecom Co., Ltd., Taiwan. He has been involved in design of 2G/3G/WBA network, mobile value-added services, handset software develop-ment, and strategy of mobile network evolution. His current research interests include design and analysis of personal communications services network, B3G/4G, and cloud computing.
Zheng-Han Wu received the BS degree in Computer Science from National Chiao Tung University (NCTU), Hsinchu, Taiwan, in 2009. He is currently working toward the MS degree at NCTU. His cur-rent research interests are applica-tion developments for SmartPhone and IBM WebSphere software for Telecom.
