Implementation and performance evaluation for mobility management of a wireless PBX network

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1138 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 19, NO. 6, JUNE 2001

Implementation and Performance Evaluation for

Mobility Management of a Wireless PBX Network

Phone Lin and Yi-Bing Lin, Senior Member, IEEE

Abstract—As wireless technology advances, wireless products are integrated with enterprise networking to offer cordless ter-minal mobility. Most corporations have deployed wireless PBXs at the departmental level. However, the mobility management mecha-nism that integrates these facilities at the corporation level may not be available. This paper describes a mobility management mecha-nism for an enterprise wireless telephone network. We show how to modify the call model of the private branch exchange (PBX) to accommodate mobility management for an enterprise network. Our design was implemented on a commercial PBX product called Jupiter. An analytical model is proposed to evaluate the perfor-mance of the implemented system. Our study shows that with a large number of WPBXs and long Internet message delays, the mis-routing probability can be limited to within 1%. This performance result is considered satisfactory.

Index Terms—Handset call routing, mobility management, reg-istration, wireless PBX.

I. INTRODUCTION

M

ANY corporate employees are away from their assigned wired phones but are still in their offices, either at other locations in the same building or in other buildings of the cor-poration. As wireless technology advances, wireless products are integrated with enterprise networking to provide employee mobility (the so-called cordless terminal mobility) in the com-pany. Unlike its public cellular counterpart, the enterprise wire-less telephone network [1] is free from the air time charges, which provides employees the access to the mobile telephone service within the corporation. An enterprise wireless telephone network can be divided into two levels: the departmental level and the corporation level.

Departmental Level: In a building, the wireless or wire-line voice/data traffic is exchanged within the same PBX. The PBX is deployed to connect the telephone links from the office building to the public switched telephone network (PSTN). If calls occur between the internal lines, then call routing is handled by the PBX without involving the PSTN. A wireless PBX (WPBX) connects radio base stations to provide mobility at the departmental level. (We assume that the reader is familiar with WPBX. Details of WPBX can be found in [3].)

Corporation Level: For a large corporation with multiple lo-cations, the private lines or virtual networks are used to pro-vide hard-wired connections among corporate locations. These leased lines are billed to the company on a flat, monthly basis. To

Manuscript received June 1, 1999; revised May 1, 1999 and January 16, 2001. The authors are with the Department of Computer Science and Information Engineering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C. (e-mail: liny@csie.nctu.edu.tw).

Publisher Item Identifier S 0733-8716(01)04206-8.

offer mobility at the corporation level, mobility management is required to coordinate the WPBXs connected by the enterprise network.

Many corporations have deployed WPBXs at the depart-mental level. However, integration of these facilities at the corporation level may not be available. In this paper, we describe a mobility management mechanism for an enterprise wireless telephone network. This mechanism utilizes the Internet to deliver mobility management messages among WPBXs, which can be implemented with minor modifications to the call model of the WPBXs. Compared to the mobility management in public cellular networks (e.g., IS-41 [4] and GSM MAP [5] that utilize complex SS7 protocols), our mech-anism is simpler and easier to deploy. Our design has been implemented on a commercial PBX product called Jupiter (a trademark of Wincomm Corp.).

Fig. 1 shows our mobility management architecture with three WPBXs. The WPBXs are connected to PSTN through digital trunks [see (1) in Fig. 1]. The control or signaling messages for mobility management are delivered through the Internet or Intranet [see (2) in Fig. 1]. A mobile handset accesses the telephone services through the base station (BS) connected to the WPBX [see (3) in Fig. 1]. The mobility management procedure works as follows.

When a handset moves from WPBX A to WPBX B, it makes a registration request to WPBX B. Then WPBX B an-nounces the location of to other WPBXs by broadcasting the location update messages. When a call termination for arrives at a WPBX, the WPBX routes the call to the correct WPBX (where resides) according to the routing information. Details for mobility management will be described in Sections III and IV. Our current implementation consists of two WPBXs, which can be easily extended to include more WPBXs.

In this paper, we first describe the basic call switching model (BCSM) for Jupiter. Then we show how to modify the call model to accommodate mobility management. An analytical model is proposed to evaluate the performance of the implemented system.

II. BASICCALLSWITCHINGMODEL

The Jupiter basic call switching model (BCSM) is based on ITU-T Q.931 specification [7], and the PBX software was implemented in C . Consider a call from subscriber A (the calling party) to subscriber B (the called party). Fig. 2 illustrates the BCSM state diagram for the calling party (A party). Several states are defined in the state diagram.

means that no call exists.

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Fig. 1. Mobility management architecture with three WPBXs.

Fig. 2. A simplified BCSM state diagram for calling party (A denotes calling party; B denotes called party).

means that the PBX has acknowledged the call establishment request and has prepared to receive additional call information (if any) from the calling party.

means that the PBX has sent an ac-knowledgment to the calling party to indicate that all in-formation necessary to effect call establishment has been received.

means that the PBX has awarded the call to the called party, and the call path between the calling and called parties has been established.

means that the PBX has requested the corre-sponding user (either the calling party or the called party) to release the call and is waiting for a response.

Based on the state diagram in Fig. 2, call setup for the calling party is described in the following steps.

Step I—Calling Party Off-Hook: Initially the calling party BCSM is at state ; i.e., no call presents. Suppose that the calling party picks up the phone. The PBX detects the off-hook

situation, creates a call record for the calling party, and sends a dial tone to the calling party. In the call record, the calling party’s BCSM state Astate is set to to reflect the transition in BCSM.

Step II—Telephone Number Dialing: After receiving the dial tone, the calling party starts dialing the telephone number of the called party. Since the calling party is at state

, the function in Fig. 3

is invoked when a dialed digit is received. At Line 2 of this function, the variable is the message passed from the calling party to the PBX. For example, the command _ _ means that the calling party dials the digit “0.” An incomplete list of the Jupiter commands are listed in Table I. For each PBX task, a timeout timer is set. If the task is not complete before the timer expires, some actions are taken to ensure that the PBX can return to an operational state. The command _ indicates a timeout timer expiration. The variable _ represents the telephone number of the calling party, and the variable _ represents the telephone number of the called party. The function works as follows.

Line 1: The function first stops the _ _

timeout timer for the calling party. Then one of the three parts in this function is executed.

Part 1: Lines 3–7: This part contains the instructions for the registration procedure. We will describe this part in Sec-tion III.

Part 2: Lines 8–24: This part handles the dialed digits sent from the calling party to the PBX. At Line 10, every dialed digit is collected in the buffer _ . Line 11 sets the next timeout timer for the calling party. The calling party is expected to take the next action before the timer expires. Depending on the number of dialed digits received so far, Lines 12–24 of the function process the dialed digits in two cases.

Case 1: Lines 12–14: The received digits do not pro-vide enough routing information to identify the called party. The

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Fig. 3. TheQOverlapSending function in C++-like description. TABLE I

ANINCOMPLETELIST OF THEJUPITERCOMMANDS

control exits the function and the PBX con-tinues to wait for more dialed digits. If only one digit is received, the dial tone for the calling party is removed at Line 12.

Case 2: Lines 15–24: The received digits provide enough routing information. In this case, the number of received digits stored in _ specifies the address of the called party. If the called party is a handset, the call setup procedure for the handset is executed (Lines 16–23). We will describe this part in Section IV. If the called party is a fixed telephone line

user, then the _ function is invoked

to set up the call path from the calling party to the called party. In the function, if the called party is free, the calling party’s state Astate is set to

to reflect the transition in BCSM. If the called party is busy

or does not exist, Astate is set to to reflect a call setup failure.

Part 3: Lines 25–28: If the calling party hangs up

the phone at the state, an _

command is sent from the calling party to the PBX. If the _ _ timeout timer for the calling party expires, a _ command is issued to the PBX. In either case, the PBX resources (e.g., DTMF receiver and pulse code modulation stream) for the calling party are released, and Astate is set to . Call setup is terminated at this moment. Step III—Called Party Off-Hook: If the called party picks up the phone, then is set to . The calling and called parties start conversation.

Step IV—Termination of the Conversation: In Jupiter, the ei-ther-control model is adopted to terminate the conversation. In this model, termination of a call conversation is controlled by either the calling party or the called party. If the calling party hangs up the phone first, Astate is set to and the con-versation is terminated. If the called party hangs up the phone first, Astate is set to , and the PBX alarms the calling party to hang up the phone by sending a warning tone. A timeout timer is set, and the calling party is expected to hang up the phone. After the timeout timer expires or the calling party hangs up the phone, Astate is set to to free the telephone circuit.

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Fig. 4. Mobility management mechanism for WPBX.

Fig. 5. Mobile handset record in the roaming table.

III. REGISTRATIONMECHANISM

To implement the registration mechanism for a network of WPBXs, every WPBX is associated with a roaming table as shown in Fig. 4 (1). The WPBX software is modified to include the registration functions [see Fig. 4 (2)] that access a remote roaming table through a TCP/IP server [see Fig. 4 (3)]. In our implementation, a handset has a unique telephone number, which allows the user to attach to different WPBXs without identification ambiguity. Unique telephone numbers can be easily arranged through the PBX numbering plan.

The roaming table is a database that records the cur-rent location of a mobile handset. The record consists of a field to indicate the mobile telephone number, and a field to indicate the routing address of the WPBX where the handset resides. Other ser-vice-related fields specify the services related to the mobile handset. Fig. 5 shows a mobile handset record in the roaming table, which indicates that handset 900 resides at WPBX 02. In the above example, a two-digit WPBX address is used just for demonstration purposes. In reality, the length for WPBX address can be extended to provide complete addressing for the WPBXs.

Three registration functions are defined in our implementa-tion:

• The function is invoked by

a WPBX A to modify A’s roaming table when an incoming handset (that moves into WPBX A) makes a registration request to A. The input argument represents the telephone number of the handset, which is used to search the roaming table. When the record is found, the current location field is set to A’s routing address.

• The function is invoked by

a WPBX to retrieve the current location of a handset when a call termination to the handset arrives at that WPBX. The

argument is used to search the roaming table, and the corresponding current-location value is returned.

• The

func-tion broadcasts a locafunc-tion update message to TCP/IP servers of all remote WPBXs through the Internet or Intranet. The location update message is of the format

.

The TCP/IP server continuously monitors the incoming loca-tion update messages sent from remote WPBXs. As menloca-tioned before, a remote WPBX invokes the function to issue the location update message. When a TCP/IP server B

receives the message , it invokes

the function. This function uses

to search the roaming table B. When the record is found, the current location field is set to . Note that the location update messages are delivered by using socket through the TCP/IP protocol that provides Reliable Stream Transporta-tion (RST) [2]. With RST, the locaTransporta-tion update messages are guaranteed to arrive at the destinations.

Although mobility management is functionally distinguished from call processing, we note that the registration procedure can be integrated with the call processing mechanism by reusing the call setup procedure.

When a handset enters a new WPBX area, it sends a registra-tion message to the base staregistra-tion (BS) through the air interface. The BS sends the WPBX a special sequence “#03 .” We con-sider the standard DECT [12] approach where the BS-WPBX connection implies the handset number. For other PCS systems such as GSM [6], a signaling message sent from the BS should also indicate the handset number. To accommodate handset registration, we modify the BCSM in Fig. 2 by creating two

new states: and . The modified

call switching model is illustrated in Fig. 6. • When the digit “#” is received at the

state, Lines 3–7 of the function

(see Fig. 3) are invoked to process the command _ _ . At Line 4, if the received digit “#” is the first dialed digit, it means that the BS attempts to set up services (e.g., handset registration), and Lines 5–7 are executed. The dial tone is removed, Astate is set to

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Fig. 6. The modified calling party call model state diagram for registration.

Fig. 7. TheQFeatures function in C++-like description.

, and the _ _ timeout

timer is set for the BS. The BS is expected to send the next digit before the timer expires.

• When the WPBX receives the next dialed digit at the state, the function in Fig. 7 is in-voked. The function first stops the _ _ timeout timer. Then one of the following two parts is executed.

Part 1: Lines 3–15 handle the transition from state to state . The dialed digit is col-lected at Line 7. The _ _ timeout timer is set at Line 8, and the BS is expected to send the next dialed digit before the timer expires. At Line 9, if two dialed digits have been collected, then the WPBX decodes the digits. Lines 10-12 process the service code “03,” where a registration request from the handset is identified, and Astate is set to . Lines 13-15 process services other than handset registration. The details are omitted.

Part 2: Lines 16–19 handle the case when the BS ter-minates the communication before the registration procedure is complete; i.e., it sends an on-hook signal to the WPBX at the

state, or the _ _ timeout timer

expires. The actions taken are exactly the same as Lines 25–28

in the function.

Fig. 8. TheQRegistration function in C++-like description.

• If the WPBX receives the next dialed digit at state

, then the function in

Fig. 8 is invoked. The function first stops the _ _ timeout timer. Then the following actions are taken.

Lines 3–6: If the WPBX receives the dialed digit

“ ,” then the function is invoked to

modify the handset record in the local roaming table, and the function is invoked to modify the handset records in the remote roaming tables. At this point, the regis-tration procedure is completed. The PBX resource for the BS is released, and Astate is set to .

Lines 7–8: At the state, if a dialed digit

other than “ ” is received, the _ _ timeout timer expires, or the BS terminates the communication, the PBX resource for the BS is released, Astate is set to , and the registration request is considered as a failure.

Note that the registration messages delivered among the WPBXs are transmitted through the Internet or Intranet without involving the PSTN.

IV. HANDSETCALLROUTINGMECHANISM

Suppose that a call termination to handset arrives at WPBX A. WPBX A queries its roaming table to identify the WPBX B where resides, and sets up the trunk to WPBX B (if B is different from A). This call termination mechanism for handset is implemented in Lines 16–23 of the func-tion (see Fig. 3). At Line 17, the function is invoked to retrieve the routing address of WPBX B (where

resides). The returned value of the

function is stored in the variable . De-pending on the value of , Lines 18–22 of the function set up the call for handset in two cases.

Case 1: Intra-PBX Call Routing (Lines 18 and

19): The value indicates that A and B are

the same WPBX. The function is invoked to set up the call path from the calling party to (i.e., _ ). The call setup procedure follows BCSM described in Section II. Case 2: Inter-PBX Call Routing (Line 21): The value indicates that WPBXs A and B

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Fig. 9. The message flow for inter-PBX call routing.

function is invoked to set up the trunk between WPBXs A and B. Fig. 9 illustrates the message flow for this function. This function sends the SS7 Initial Address Message (IAM) and Subsequent Address Message (SAM) [11] to provide routing information of handset to WPBX B [see (1) in Fig. 9]. WPBX B sets up the circuit to based on the received routing information. Then it sends an SS7 Address Complete Message (ACM) to WPBX A [see (2) in Fig. 9], and starts to ring handset [see (3) in Fig. 9]. When WPBX A receives the ACM message, Astate is set to . If handset in WPBX B answers the call [see (4) in Fig. 9], WPBX B sends an SS7 Answer Message (ANM) to WPBX A [see (5) in Fig. 9]. Astate is set to , and the calling party and the called party (handset ) starts the conversation.

We note that the SS7 messages described in Case 2 are deliv-ered through PSTN.

V. PERFORMANCEISSUE

The delay of Internet signaling affects the registration pro-cedure. The roaming table may provide obsolete information during the transition of location update. Consider a handset moving from the service area of WPBX A to the service area of WPBX B. Handset makes a registration request to WPBX B. When WPBX B receives the registration request, it sends the location update messages to other WPBXs in the system as de-scribed in Section III. Let be the period between when the lo-cation update message is sent from WPBX B and when the mes-sage arrives at a WPBX C. During , any s call terminations (i.e., any call terminations to ) at WPBX C will be routed to a wrong WPBX since the roaming table in WPBX C has not been modified. These call terminations are misrouted. It is important to see how the delay of the Internet signaling affects misrouting of call terminations to .

In the Appendix, we derive the probability that call terminations to in a WPBX during period as

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where

Assume that the call-waiting service is available, which allows a user to answer other calls while already engaged in a call.

terminations at WPBX B (where moves to) are always routed to correctly. Thus, if we exclude the traffic to WPBX B, then (2) Similarly, (3) (4) and (5)

Since the call termination traffic of at a WPBX is a Poisson process with rate , the net call termination traffic to is

.

We investigate the effects of the net call termination rate , the standard deviation of message sending delay, and the number of WPBXs on the performance (i.e., ), where and are normalized by . For example, if the expected value of is seconds (i.e., the average TCP/IP signaling message delay is 2.4 seconds), then

indicates that there is one call termination per 40 minutes to in the system. We consider two cases. In the first case, the WPBXs in various cities are connected through an uncongested Internet network (or an Intranet network). In this case, the average TCP/IP signaling message delay is less than 2.4 seconds. The call frequency to is one call termination per 40 minutes (i.e., ). In the second case, the WPBXs are connected through a congested Internet environment, where the average TCP/IP signaling message delay is 24 seconds. Like the first case, the call frequency to is one call termination per 40 minutes (i.e., ). Based on (2)–(5), we observe the following phenomena.

Effect of : Figs. 10 and 11 plot ( )

for various values, where and , respec-tively. In all cases considered in our study, %

for [Fig. 10(a)] and % for

[Fig. 11(a)]. In other words, in an uncongested Internet transmission environment (i.e., ), the mis-routing probability can be ignored (which is less than 0.1%). In a congested Internet transmission environment (i.e., ), the misrouting probability is reasonably small (which is less

than 1%). Furthermore, and are in the

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Fig. 10. Effects ofN and on Pr[M] ( = 0:01).

Fig. 11. Effects ofN and on Pr[M] ( = 0:001).

respectively. For , and are

in the order of [Fig. 11(c)] and [Fig. 11(d)], respec-tively. Thus, the misrouted probabilities for multiple calls can be ignored in all cases.

Effect of : Figs. 10(a) and 11(a) indicate that

decreases as increases. This phenomenon is due to the fact that for a fixed , the misrouted traffic to the network [which is ] increases as increases. Even if the net mis-routing traffic is fixed, the stochastic phenomenon [10] implies that with a fixed net arrival rate, if there are more arrival sources (i.e., more number of WPBXs), it is more likely to observe ar-rivals during a fixed period. Thus, adding more WPBXs in the network will degrade the performance of the network. However, this effect is not significant. When , by adding one more

WPBX, is degraded by 0.011% (for )

and 0.0012% (for ). On the other hand, when , by adding one more WPBX, is degraded

by 0.166% (for ) and 0.0166% (for ).

To conclude, the performance degradation due to adding extra WPBXs can be ignored.

Effect of : The standard deviation of message sending delay determines the probability of observing long/short Internet delay times. As increases, more long and short Internet delay times are observed. More short and long Internet

delay periods imply larger and . This

analysis is consistent with our observation in Figs. 10 and 11

which indicate that as increases, , ,

and increase, and decreases. With a

small , and increase as increases.

On the other hand, this misrouting probability decreases as increases for large . This nonintuitive phenomenon is due to complicate interaction among input parameters that we cannot

explain. Since and are very small

for large , the effect of can be ignored under the normal operation conditions.

VI. CONCLUSION

We designed an enterprise mobile phone network based on WPBX. We showed how to modify the PBX call model to ac-commodate mobility management where the location update messages are transmitted through Internet or Intranet. We have implemented an enterprise mobile phone network using a com-mercial PBX product called Jupiter. Although the configuration in the current implementation only consists of two WPBXs, the implementation can be easily extended to accommodate more WPBXs.

In our design, it is important to investigate if Internet mes-sage delay will cause misrouting of phone calls. In the current implementation with 2 WPBXs, message delay does not cause any misrouting problem. We used an analytic model to study the performance of the system with more than 2 WPBXs in the con-gested Internet environment. Our study indicated the following. • In an uncongested Internet transmission environment, the misrouting probability can be ignored (less than 0.1% in our study). For a congested Internet, the misrouting prob-ability is reasonably small (less than 1% in our study). • The misrouting probabilities for multiple calls can be

ig-nored in all cases (less than 10 in our study).

• The performance degradation due to adding extra WPBXs can be ignored (the degradation is less than 0.012% in our study).

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conclude, our design provides mobility management for an enter-prise mobile phone network, which utilizes the Internet/Intranet for signaling. Our design does not modify the existing signaling capability of public switched telephone network, and thus re-sults in low costs for implementation and maintenance.

APPENDIX

DERIVATION FOR

Suppose that the call terminations at a WPBX are a Possion process with rate , and is a random variable with a Gamma density function with mean , standard deviation , and the Laplace transform where

(6) Note that our derivation applies to any message delay distribu-tions whose Laplace transforms have closed forms. The Gamma distribution is selected to present the message delay distribution because this distribution can be used to approximate many dis-tributions as well as the measured data obtained from the PCS fields [8]. Let random variable be the number of ’s call terminations at a WPBX during period . The probability that

during period is

(7) The derivation for is described as follows.

We validate (1) by showing that the expected value of is (8)

and (9) is rewritten as

(10) Since , it is apparent that (10) and (8) are the same. Equation (10) states that the expected number is indepen-dent of the distribution. However, is sensitive to the distribution.

REFERENCES

[1] I. Chlamtac, B. Khasnabish, and Y.-B. Lin, “The wireless segment for enterprise networking,” IEEE Network, vol. 12, pp. 50–55, July/Aug. 1998.

[2] D. E. Comer, Interworking with TCP/IP Vol. I: Principles, Protocol, and

Architecture, 3rd ed. Englewood Cliffs, NJ: Prentice-Hall, 1995. [3] R. A. Dayem, PCS and Digital Cellular Technologies: Prentice-Hall,

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[7] ITU-T, “Digital subscriber signaling system no. 1: Network layer,” Tech-nical Report Recommendation Q.931, ITU-T, Mar. 1993.

[8] N. L. Johnson, Continuous Univariate Distributions-1. New York: Wiley, 1969.

[9] L. B. W. Jolley, Summation of Series, 2nd ed: Dover Publications, 1961. [10] L. Kleinrock, Queuing Systems: Volume I—Theory. New York: Wiley,

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[11] Y.-B. Lin and S. K. DeVries, “PCS network signaling using SS7,” IEEE

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Phone Lin received the B.S.C.S.I.E. and Ph.D. de-grees from National Chiao Tung University, Taiwan, R.O.C., in 1996 and 2001, respectively.

He is currently an Assistant Professor with the Department of Computer Science and Information Engineering, National Taiwan University, Taiwan, R.O.C. His current research interests include design and analysis of personal communications services networks, mobile computing, and performance modeling.

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Yi-Bing Lin (SM’96) received the B.S.E.E. degree from National Cheng Kung University in 1983, and the Ph.D. degree in computer science from the Uni-versity of Washington in 1990.

From 1990 to 1995, he was with the Applied Re-search Area, Bell Communications ReRe-search (Bell-core), Morristown, NJ. In 1995, he was appointed as a Professor of the Department of Computer Science and Information Engineering (CSIE), National Chiao Tung University (NCTU). In 1996, he was appointed as Deputy Director of Microelectronics and Informa-tion Systems Research Center, NCTU. During 1997–1999, he was elected as Chairman of CSIE, NCTU. His current research interests include design and analysis of personal communications services network, mobile computing, dis-tributed simulation, and performance modeling. He is coauthor with I. Chlamtac of Wireless and Mobile Network Architecture (New York: Wiley).

Dr. Lin is an Associate Editor of IEEE NETWORK, an Editor of IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS Wireless Series, an editor of IEEE PERSONALCOMMUNICATIONS MAGAZINE, and an Editor of numerous other journals; as well as Guest Editor for IEEE TRANSACTIONS ONCOMPUTERSSpecial Issue on Mobile Computing, and a Guest Editor for IEEE COMMUNICATIONSMAGAZINESpecial Issue on Active, Programmable, and Mobile Code Networking. He served as Program Chair for the 8th Workshop on Distributed and Parallel Simulation, General Chair for the 9th Workshop on Distributed and Parallel Simulation, and Program Chair for the 2nd International Mobile Computing Conference. He received the 1998 and 2000 Outstanding Research Awards from National Science Council, R.O.C., and 1998 Outstanding Youth Electrical Engineer Award from CIEE, R.O.C. He is an Adjunct Research Fellow of Academia Sinica.