To appear in WINET Special Issue on Mobility Management 28. [27] Steedman, R. The Common Air Interface MPT Cordless Telecommuinications

Transcription

1 To appear in WINET Special Issue on Mobility Management 8 [7] Steedman, R The Common Air Interface MPT 1375 Cordless Telecommuinications in Europe, 1990 Tuttlebee, WHW ed, (Springer Verlag) [8] Watson, EJ Laplace Transforms and Applications Birkhauserk, 1981 [9] Wong, WC Dynamic allocation of packet reservation multiple access carriers IEEE Trans Veh Technol, 4(4), 1993

2 To appear in WINET Special Issue on Mobility Management 7 [15] Lin, Y-B and Chlamtac, I Heterogeneous Personal Communications Services, 1996 To appear in Encyclopedia of Microcomputers [16] Lin, Y-B and DeVries, SK PCS Network Signaling Using SS7 IEEE Personal Commun Mag, pages 44{55, June 1995 [17] Lin, Y-B, Chang, LF, Noerpel, AR, and Park, K Performance Modeling of Multi-Tier PCS System To appear in Intl J of Wireless Information Networks [18] Lin, Y-B, Mohan, S, and Noerpel, A Queueing Priority Channel Assignment Strategies for Hando and Initial Access for a PCS Network IEEE Trans Veh Technol, 43(3):704{71, 1994 [19] Noerpel, AR Private Communications, 1995 [0] Noerpel, AR, Ziegler, RA, and Chang, LF PACS-UB, a protocol for the unlicensed spectrum ICC'95, 1994 [1] Padgett, JE, Gunther, CG, and Hattori, T Overview of Wireless Personal Communications IEEE Communications Magazine, pages 8{ 41, January 1995 [] Park, KI, and Lin, Y-B Registration Methods for Multi-Tier Personal Communications Services Submitted for Publications [3] Radio Advisory Board of Canada CTPlus class : Specication for the Canadian common air interface for digital cordless telephony, including public access services, annex 1 to radio standards specication 130, 199 [4] Rappaport, SS Blocking, hand-o and trac performance for cellular communication systems with mixed platforms IEE PROCEEDINGS-I, 140(5):389{401, 1993 [5] Rizzo, JF, and Sollenberger, N Multitier Wireless Access IEEE Personal Communications Magazine, (3):18{31, 1995 [6] Russell, JE Emergence of a Paradigm Shift in the Communications Industry Intl J Wireless Information Networks, July 1994

3 To appear in WINET Special Issue on Mobility Management 6 [3] Chang, LF, Noerpel, AR, and Park, KI Private Communications, 1995 [4] Cook, CI Developement of air interface standards for PCS IEEE Personal Communications Magazine, 1(4):30{34, 1994 [5] Cox, DC Wireless Personal Communications: What Is It? IEEE Personal Commun Mag, pages 0{35, April 1995 [6] EIA/TIA Cellular radio-telecommunications intersystem operations: Automatic roaming Technical Report IS-413-B, EIA/TIA, 1991 [7] ETSI Common air interface specication to be ued for the interworking between cordless telephone apparatus in the frequency band 8641 MHz, including public access services, I-ETS :1990, June 1991 [8] ETSI Digital European Cordless Telecommunications Services and Facilities Requirements Specication Technical Report ETSI DI/RES 300, European Telecommunications Standards Institute, 1991 [9] Hong, D and Rappaport, SS Trac model and performance analysis for cellular mobile radio telephone systems with prioritized and noprotection hando procedure IEEE Trans Veh Techol, VT-35(3):77{ 9, August 1986 [10] Hu, LR and Rappaport, SS Micro-Cellular Communication Systems with Hierarchical Macrocell Overlays: Trac Performance Models and Analysis WINLAB Workshop, pages 143{174, 1993 [11] Johnson, NL and Kotz, S Continuous Univariate Distributions-1 John Wiley& Sons, 1970 [1] JTC Personal Access Communications System Air Interface Standard Technical Report JTC(AIR)/ SP-3418, T1/TIA Joint Technical Committee, 1995 [13] Kelly, FP Reversibility and Stochastic Networks John Wiley & Sons, 1979 [14] King, PJB Computer and Communication Systems Performance Modeling Prentice Hall, 1990

4 To appear in WINET Special Issue on Mobility Management 5 than the single low tier system in most cases and generates less than 0% registration trac in the worst cases It is clear that for the service availability, the two-tier system is better than the single one tier system We also studied the probability that a call is forced terminated in the single low tier system because the low tier becomes unavailable during the call (such a call can be continued in the two-tier system) Our study indicated that such a probability can be as large as 10% (5-10 times the engineered blocking probability of a typical PCS system) These forced terminated calls can be avoided in the two-tier system Our study can be extended in the following three directions To consider the impact of nite channel capacity in the availability model To consider the impact of the tier monitor frequency of a SR handset To relax the exponential assumptions in Situations 1 and by the simulation study Acknowlwdgement The assistance of Chris Rose and Ramesh Sitaraman and the valuable comments of the three reviewers have signicantly improved the quality of this paper References [1] ANSI/EIA/TIA Mobile station-land station compatibility specication Technical Report 553, EIA/TIA, 1989 [] Bellcore Network and operations plan for access services to personal communications services (PCS) providers Technical Report SR-TSV- 0040, Bellcore, 1993

5 To appear in WINET Special Issue on Mobility Management : V l = 100 l : k = : V l = 0:01 l : k = (%) (%) = l (a) The eect of V l (k = ) = l (b) The eect of V c (V l = 1= l )

6 To appear in WINET Special Issue on Mobility Management 3 For k = 41? f = L() l? d ds f L(s) s= =! l 4 l 1? + l + l 3 5 l + l!?13 5 (0) Figure 10 plots as a function of = l If the expected call holding time is 1= = 3 minutes, then = l = 10 and 100 correspond to the low tier available periods 30 minutes and 5 hours respectively Figure 10 indicates that 1-10% of the calls may be forced terminated in the single low tier system because the system becomes unavailable during the calls This percentage is large compared with the 1-% engineered blocking probabilities for a typical PCS system Figure 10 (a) indicates that is an increasing function of the variance V l of the low tier available period [t 0 ; t ] This phenomenon is explained as follows When V l! 0, the low tier available time period t L = t? t 0 = 1= l is a constant Since 1 l > 10 in our study, a small is expected As V l increases, more short and long periods of t L are observed More short t L periods imply that it is more likely to have t L periods shorter than the t c call holding time in Figure 9 Thus increases as V l increases Figure 10 (b) indicates that the variance V c of t c does not has signicant eect on when = l is large When = l is small, is aected by k, and smaller variance (a larger k) of the call holding times results in a larger value 5 Conclusions We compared the performance of the two-tier PCS system and the single low tier system in two aspects: the registration trac and the service availability The two-tier system generates more registration trac than the single low tier system Under the range of the input parameters in our study, we showed that the two-tier system generates less than 10% extra registration trac

7 To appear in WINET Special Issue on Mobility Management the period t l = t?t 1 From the residual life theorem, the Laplace transform f l (s) of f l is similar to (6): f l (s) = l s [1? f L(s)] (1) Two Laplace transform rules are used in this derivation Let f (s) and g (s) be the Laplace transforms of two density functions f(t) and g(t) respectively Then from [8] f(t) = Z t f(t) = tg(t), f (s) =? d ds g (s) (13) =0 g()d, f (s) = 1 s g (s) (14) R tl Let F l (t l ) = f =0 l()d be the distribution for t l, and let its Laplace transform be F (s) Then from (1) and (14), we have l The probability is derived as follows = Pr[t c > t l ] = = F l (s) = l[1? f L (s)] s (15) Z 1 Z tc tc=0 tl=0z 1 k (k? 1)! = (?1)k?1 k (k? 1)! f c;k (t c )f l (t l )dt l dt c tc=0 = (?1)k?1 l k (k? 1)! t k?1 F l (t c )e?tc dt c (16) d (k?1) ds F (s) k?1 l s= " 1? f d (k?1) ds k?1 L(s) s # s= (17) (18) Equation (17) is derived from (13) and (16), and equation (18) is the direct consequence of (17) and (15) Suppose that t L has a Gamma distribution with mean 1 and variance V l = l 1=( ), then for k = 1, (18) is re-written as l " = l l!# 1? (19) + l

8 To appear in WINET Special Issue on Mobility Management 1 The low tier becomes available t L t l The low tier becomes unavailable t t t t 0 1 t 3 c Call arrival Call completion Figure 9: The timing diagram The remainder of this section derives the probability that the low tier becomes unavailable during a phone call (ie, is the probability that a call cannot be completed in the single low tier system) We make the following assumptions: The call originations/terminations are a Poisson process The call holding times have an Erlang density function (a special case of the Gamma density function where the parameter = k is an integer) f c;k with mean 1= and variance V c = 1 k That is f c;k (t c ) = (t c) k?1 (k? 1)! e?tc Most studies [9, 10, 18, 4, 9] assumed that the call holding times are exponentially distributed Our study generalizes the exponential call holding times by introducing dierent variance values (note that the exponential distribution is an Erlang with k = 1) The low tier available times have a general density function f L (t L ) with mean 1= l and the Laplace transform fl(s) Consider the timing diagram in Figure 9 In this diagram, t L = t? t 0 is the period when the low tier is available Suppose that a call arrives at time t 1 where t 0 < t 1 < t Since the call arrivals are a Poisson process, t 1 is a random observer point to the period [t 0 ; t ] Let f l be the density function for

9 To appear in WINET Special Issue on Mobility Management 0 (%) ? : V h = 100 h : V h = 1 h? : V h = 0:01 h?? (%) ? : V m = 100 () : V m = 1 ()? : V m = 0:01 ()?? (%) : Situation 1 : Situation (a) Situation 1 ( = 0:8, = 8 l ) (b) Situation ( = 0:8, = 8 l ) (c) = 0:8, = 8 l, V h = 0:01 h, V m = 0:01 () Figure 8: The eects of V h and V m on the two tier registration trac

10 To appear in WINET Special Issue on Mobility Management 19 also (8)) Since decreases as Pr[T 3 > T ] decreases (as we discussed before), we have the conclusion that is a decreasing function of Note that a large factor is expected in a two-tier system where the low tier system allows high user speed (such as PACS) Thus, a twotier system generates less registration trac if the low tier allows higher user speed The eects of V h and V m : Figure 8 illustrates the eect of the variances for periods [T 0 ; T 3 ] and [T 1 ; T ], respectively In Situation 1, we consider dierent V h values while V m = 1=() In Situation, we consider dierent V m values while V h = 1=h Figures 8 (a) and (b) indicate that increases as V h and V m increase We do not have any intuitive explanation for this phenomenon However, this result can be observed from (5) and (7) In these equations, the component increases as increases Thus Pr[T 3 > T ] +h increases as increases, which implies that is an increasing function of V h (V m ) Note that decreases as V h (V m ) increases We also note that the change of V h on is more signicant than that of V m (see Figure 8 (c)) 4 The Availability of the PCS Systems It is clear that the availability of the two-tier system is better than the single tier systems For the single high tier system, of the phone calls are delivered at a more expensive cost compared with the two-tier system where these calls are delivered through the inexpensive low tier For the single low tier system, (1? ) of the phone calls are lost compared with the two-tier system Also, when the low tier becomes unavailable during a phone call, the user cannot complete the call in the single low tier system On the other hand, the user may continue the call through the high tier in the two-tier system Although hando between tiers may be dicult under the current technologies, it is possible to continue the conversation by re-dialing a new call (either automatically done by the network or manually done by the user) through the high tier

11 To appear in WINET Special Issue on Mobility Management : : = 8 l : = 0:4 18 : 1 18 : = 4 l 18 : = 0: (%) 10 8 (%) 10 8 (%) (a) = 0:8; = 8 l (b) = 0: (c) = 8 l Figure 7: The eects of,, and on the -tier registration trac

12 To appear in WINET Special Issue on Mobility Management 17 SR1 vs SR: Figure 7 (a) plots 1 and as a function of assuming that 80% of the time the low tier is available ( = 0:8), and the user is expected to visit 8 RAs during the period when the low tier is available ( = 8 l ) The gure indicates that 1 = (11) Equation (11) is true for all cases From (1), (), and (3), we have n sr1? n l = Pr[T 3 > T ] and n sr? n l = + Pr[T 3 > T ] which lead to equation (11) In the remainder of this section, we only consider The results are also true for 1 The eect of : Figure 7 (b) plots assuming that = 0:8 (80% of the time the low tier is available), and = 4 l and 8 l (ie, the user is expected to visit 4 and 8 low tier RAs, respectively, during the period when the low tier is available) The gure indicates that decreases as increases This phenomenon is due to the fact that N increases as increases (because N is the expected number of RA crossings during (T 4 ; T 1 ); see (9)) For a large N the boundary eect of (n sr? n l ) becomes insignicant Thus, from (10) is a decreasing function of The eect of : Figure 7 (c) plots assuming = 8 l, and = 0:4 and 08 respectively The gure indicates that increases as increases (ie, as the low tier is more available) For a xed length of [T 4 ; T 1 ], a large implies a small length of [T 1 ; T ] A small [T 1 ; T ] period implies a large probability Pr[T 3 > T ] (ie, it is more likely that the user does not cross any RA boundary during the period [T 1 ; T ]) A large Pr[T 3 > T ] value implies a large value (note that n sr?n l = 4? 0 = 4 when T 3 > T, and n sr? n l = 6? 4 = when T 3 < T ) Thus, is an increasing function of The eect of : Figures 7 and 8 indicate that decreases as increases A large implies a small probability Pr[T 3 > T ] (if the user moves fast, it is likely that the user will cross RA boundaries during [T 1 ; T ]; see

13 To appear in WINET Special Issue on Mobility Management 16 = 1?Z 1 tm=0 e? h tm f m (t m )dt m = 1? h [1? f M ( h)] If f m (t m ) is a Gamma density function with mean 1 and the variance 1 V m =, then () Pr[T 3 > T ] = 1? h " 1? + h Situation 3 (Exponential high/low tier available times and exponential RA crossing times) This situation is a special case of Situations 1 and If f h in Situation 1 and f M in Situation are exponentially distributed, then (5) and (7) are the same: Pr[T 3 > T ] =!# (7) h + h (8) The number N of messages sent during [T 4 ; T 1 ] (including registrations, cancelations, and acknowledgments) is N = 4 E[T 1? T 4 ] = 4 (1? ) h (9) Let the percentages of the extra trac created by SR1 and SR over the L system be 1 and respectively Then 1 = n sr1? n l n l + N and = n sr? n l n l + N (10) Based on (10), we study the eects of dierent parameters on the percentage of the extra registration trac created by SR over the L system Figures 7 (based on Situation 3) and 8 (based on the three situations) indicate that the two-tier system generates less than 10% extra registration trac than the single low tier system in most cases and generates less than 0% registration trac in some worst cases The eects of the input parameters are discussed as follows

14 To appear in WINET Special Issue on Mobility Management 15 Then the probability Pr[T 3 > T ] is Pr[T 3 > T ] = Pr[t m > t h ] = = Z 1 Z 1 th=0 tm=th Z 1 th=0 = f h () e?tm f h (t h )dt m dt h e?t h f h (t h )dt h If f h (t h ) is a Gamma density function with mean 1 h and the variance V h = 1, then from [8] h Pr[T 3 > T ] = f h () = h h +! (5) Note that we are interested in the Gamma distribution because it can be used to approximate other distributions as well as experimental data collected from the real networks [11, 13] Situation (Exponential high/low tier available times and general RA crossing times) If the lengths of the intervals [T 4 ; T 1 ] and [T 1 ; T ] are exponentially distributed with the same mean, then T 1 is a random observer point (see pages in [14]) of [T 0 ; T 3 ] If t M = T 3? T 0 has a general density function f M (t M ) with mean 1 and the Laplace transform f (s), then from the residual life theorem (see page 71 in [14]), M t m = T 3? T 1 has the Laplace transform Thus, f m(s) = s [1? f M(s)] (6) Pr[T 3 > T ] = Pr[t m > t h ] = = Z 1 Z tm tm=0 th=0 Z 1 tm=0 h 1? e?h tmi h e? h t h f m (t m )dt h dt m fm (t m )dt m

15 To appear in WINET Special Issue on Mobility Management 14 The probability Pr[T 3 > T ] is derived with the following notation > 1 is a speed factor When the low tier is not available, the average speed of the user is times of the average speed when the low tier is available = E[T 1? T 4 ] E[T? T 4 ] available is the proportion of the time when the low tier is is the low tier RA crossing rate when the low tier is available Thus, when the low tier is not available, the low tier RA crossing rate is E[T? T 1 ] = 1= h E[T 1? T 4 ] = 1= l Note that l = 1? h We consider three situations Situation 1 is used to study the impact of the variance of the high tier available times Situation is used to study the impact of the variance of the low tier RA residence times Situation 3 is used to study the impact of other parameters such as,, and Situation 1 (General high tier available times and exponential RA crossing times) If the low tier RA residence times when the user is in both the low and the high tiers are exponentially distributed (with rates and respectively), then from the memoryless property of the exponential distributions, t m = T 3? T 1 has a exponential density function f m (t m ) with rate : f m (t m ) = e?tm Suppose that t h mean 1 h = T? T 1 has a general density function f h (t h ) with and the Laplace transform f h (s) = Z 1 th=0 e?t h s f h (t h )dt h (4)

16 To appear in WINET Special Issue on Mobility Management 13 the handset detects that the RA visited at T is dierent from the RA visited at T 1, and based on the IS-41 protocol (see Figure 3 (b)) four mesage exchanges are required in the registration/cancellation operations at T Thus, the number n l of the registration/cancellation messages (including the acknowledgement messages) sent during [T 1 ; T ] is n l = 0 Pr[T 3 > T ] + 4(1? Pr[T 3 > T ]) = 4? 4 Pr[T 3 > T ] (1) The SR1 system Eight signaling messages are required for tier switching whether the handset crosses the low tier RA boundaries during [T 1 ; T ] or not At time T 1, the handset detects that the low tier is not available, and four messages are exchanged as in Figure 4 (c) Similarly, another four messages are required at time T as in Figure 4 (d) The number n sr1 of the registration/cancelation messages (including the acknowledgment messages) sent during [T 1 ; T ] is n sr1 = 8 () The SR system If the user does not cross any low tier RA boundary during [T 1 ; T ] (see Figure 6 (a)), then four signaling messages are exchanged during [T 1 ; T ] At time T 1, the handset detects that the low tier is not available, and a message is sent to MHLR to modify the tier eld (see Figure 5 (c)) and an acknowledgment is sent back to the handset Since we assume that all low tier RAs are covered by one high tier RA, the previously visited high tier RA is always the same as the currently visited high tier RA, and no registration/cancelation operations are required At T, the handset detects that the low tier is available again, and another two messages are exchanged for tier switching On the other hand, if the user crosses the low tier RA boundaries during [T 1 ; T ] (ie, T > T 3 as shown in Figure 6 (b)), then 6 messages are exchanged as shown in Figures 5 (c) and (d) The number n sr of the registration/cancelation messages (including the acknowledgment messages) sent during [T 1 ; T ] is n sr = 4 Pr[T 3 > T ] + 6(1? Pr[T 3 > T ]) = 6? Pr[T 3 > T ] (3)

17 To appear in WINET Special Issue on Mobility Management 1 T0 T3 T0 T3 t M t M t m t m T4 t h t h T1 T T4 T1 T (a) The handset does not cross any low tier RA boundaries during [T1, T] (b) The handset crosses at least one RA boundaries during [T1,T] T0: The last low tier RA boundary crossing before the low tier is not available T1: The low tier becomes unavailable T4,T: The low tier becomes available T3: The first low tier RA boundary crossing after the low tier is not available Figure 6: The timing diagram 1

18 To appear in WINET Special Issue on Mobility Management 11 PACS-UA/PHS [19], and less than MPH in DECT [8]) On the other hand, the user speed can be more than 70 MPH in a typcially high tier system We assume that the PCS service area is covered by both the high tier and the low tier In other words, the high tier is always available, and the low tier is not available when the user moves too fast The case that the low tier is not available due to the lacking of the coverage for the low tier radio system can be found in [17] A performance model is proposed to compare the registration trac generated by the single low tier system (the L system) and the SR system (both SR1 and SR are considered) In this model, when the user moves at a low speed, the low tier (in both the L and the SR systems) is available to serve the user When the user moves at a high speed, the lower tier is not available to serve the user In this case, the behaviors of the L and the SR systems are dierent: In the L system, the user will not receive any service, and no registration messages are sent when the user crosses the low tier RA boundaries In the SR system, the handset switches to the high tier by sending a high tier registration message to the network The user continues to receive the service through the high tier When the lower tier is available again, the user receives the service from the low tier in both the L and the SR systems Figure 6 illustrates two consecutive periods [T 4 ; T 1 ] (where the low tier is available) and [T 1 ; T ] where the low tier is not available because the user is moving too fast Let T 0 be the time of the last low tier RA boundary crossing before T 1 and T 3 be the time of the rst low tier RA boundary crossing after T 1 The L system If the user does not cross any low tier RA boundary during [T 1 ; T ] (ie, T < T 3 ; see Figure 6 (a)), the low tier RA visited at T is the same as the one visited at T 1, and the handset takes no action at time T On the other hand, if the user crosses the low tier RA boundaries during [T 1 ; T ] (ie, T > T 3 as shown in Figure 6 (b)), then

19 To appear in WINET Special Issue on Mobility Management 10 the handset to MHLR for registration and tier switching Since the low tier RA (ie, L ) visited is dierent from the previously visited low tier RA (ie, L 1 ), a cancellation message is sent from MHLR to L 1 as in the IS-41 protocol The advantage of SR1 is that the network can utilize the existing IS-41 protocol (thus MHLR database record structure needs not be modied) for tier switching, and there is no need to distinguish the low tier RAs from the high tier RAs The advantage of SR is that the cancellation operations may not be necessary during tier switching (see Figure 5 (c)) and the protocol generates less signaling trac than SR1 The call delivery procedure for SR (both SR1 and SR) is exactly the same as the procedure for the single tier system That is, the network delivers the call based on the tier and RA indicated in MHLR 3 Registration Trac of the Two-Tier System This section evaluates the registration trac created by the two-tier system For the comparison purpose, we made the the following assumptions As mentioned before, every VLR covers exactly one RA, and we will use the terms VLR and RA interchangeably Our results generalize to the case where a VLR covers several RAs All low tier RAs under study are overlaid with one high tier RA This assumption is reasonable because a high tier RA may cover a US state (such as New Jersey) while a low tier RA typically covers a town (such as Morristown) within a state The low tier system is not available to a user if the area is not covered by the low tier radio system or if the user moves too fast Note that the user speed is typically limited to less than 40 MPH in a low tier system (less than 38 MPH in PACS [1], less than 19 MPH in PACS-UB [0], less than 1 MPH in CT-/CT-Plus [7, 3, 7], less than 5 MPH in

20 To appear in WINET Special Issue on Mobility Management 9 MHLR p1 H H1 L1 MHLR p1 L H1 L1 tier switching H1 L1 L H1 L1 L (a) Step 1 (b) Step MHLR p1 H H1 L1 tier switching MHLR p1 L cancellation H1 L 1 registration and tier switching H1 L1 L H1 L1 L (c) Step 3 (d) Step 4 Figure 5: The SR protocol

21 To appear in WINET Special Issue on Mobility Management 8 MHLR p1 H1 MHLR p1 L1 cancellation 1 registration H1 L1 L H1 L1 L (a) Step 1 (b) Step MHLR MHLR p1 H1 p1 L cancellation 1 registration cancellation 1 registration H1 L1 L H1 L1 L (c) Step 3 (d) Step 4 Figure 4: The SR1 protocol

22 To appear in WINET Special Issue on Mobility Management 7 HLR p1 L1 HLR p1 L cancellation 1 registration L1 L L1 L VLR1 VLR VLR1 VLR (a) Before the movement (b) After the movement Figure 3: The IS-41 protocol for the single tier PCS systems RA L 1 (see Figure 4 (b)) Tier switching is achieved by a registration operation from L 1 (handset) to MHLR and a cancellation operation from MHLR to H 1 In Figure 4 (c) and (d), the handset moves back to H 1 and then another low tier RA L The tier switching operation is the same as the IS-41 protocol shown in Figure 3 Alternative (SR) In this alternative every MHLR record consists of two location elds (one for the high tier and one for the low tier) and one bit to indicate which tier is available as shown in Figure 5 In Figure 5 (a), the handset is in H 1, and the tier eld of the MHLR record is set \H" (high tier) The record also indicates that the previously visited low tier RA is L 1 The handset sends a message to MHLR when it moves to L 1, and tier switching is performed by setting the tier eld as \L" (low tier) No RA registration/cancellation operations are required because the RA currently visited (ie, L 1 ) is the same as the previously visited RA (The location information of L 1 is still in MHLR record) When the handset moves to H 1 again (see Figure 5 (c)), a message is sent from the handset to MHLR to switch the tier Again, no registration/cancellation operations are performed When the handset moves into L (see Figure 5 (d)), a message is sent from

23 To appear in WINET Special Issue on Mobility Management 6 Step ; if the high tier is available, then Step 6 is exercised as to be described in Case C) If neither tiers is available, the handset keeps monitoring the two tiers Case B If the low tier is available, the handset sends a registration message to MHLR through the visited low tier VLR (see Step 3) The handset then monitors and receives service at the low tier The high tier is ignored as long as the low tier is available (see Steps 4 and 5) If the low tier becomes unavailable, the handset looks for the high tier (see Step 6 described in Case C) Case C If the high tier is available (and the low tier is not available), then the handset sends a registration message to MHLR through the visited high tier VLR (see Step 6), and receives service through the high tier (see Step 7) The handset keeps monitoring the low tier (see Step 8) If the low tier becomes available, the handset switches to the low tier by exercising Case B If the high tier becomes unavailable (see Step 9), then Case A is exercised Now we describe the two-tier PCS registration protocols For the comparison purpose, we rst describe the IS-41 registration protocol for the single tier PCS systems [6] (see Figure 3) We assume that every VLR covers one registration area (RA) and the terms VLR and RA will be used interchangably In Figure 3 (a), the handset p 1 resides at registration area L 1, and the HLR record for p 1 indicates that the current RA is L 1 When p 1 moves to L (see Figure 3 (b)), it sends a registration message to the HLR (through VLR) The HLR updates the location information, and acknowledges this action by sending an acknowledgement message to VLR (and then to p 1 ) The HLR also sends a cancellation message to VLR1 to delete the temporary record of p 1 at VLR1 An acknowledgement message is sent from VLR1 to HLR The two SR methods are described below Alternative 1 (SR1) does not distinguish the low tier RAs from the high tier RAs, and the switching between tiers is exactly the same as the IS-41 registration process Figure 4 illustrates the SR1 method by an example In Figure 4 (a), the handset is in the high tier RA H 1, and the location is recorded in MHLR Then the handset moves to the low tier

24 To appear in WINET Special Issue on Mobility Management 5 Turn on no 1 Is the low tier available? no Is the high tier available? yes 6 Register to the high tier yes 3 Register to the low tier 7 Receive Services at the high tier no 4 Receive Services at the low tier 5 Is the low tier available? 8 Is the low tier available? no yes yes yes 9 Is the high tier available? no Figure : The two-tier SR handset algorithm both registration protocols give the priority to the low tier That is, the PCS service is delivered through the low tier whenever it is available The details for the MR method can be found in [17, ] This paper will focus on two alternatives of the SR method Under the SR method, the multi-tier handset works as follows (see Figure ): Case A When the handset is rst turned on, it sequentially checks the low tier (see Step 1 in Figure ; if the low tier is available, then Step 3 is exercised as to be described in Case B), and then the high tier (see

25 To appear in WINET Special Issue on Mobility Management 4 MSC VLR BS IS41/SS7 Multi-Tier HLR (MHLR) AMPS IS41/SS7 High tier system Low tier system VLR/AM Multi-mode Mobile Station Swithch RPCU Licensed PACS RP Figure 1: The Multi-tier PCS System

26 To appear in WINET Special Issue on Mobility Management 3 will focus on the second issue and study the impact of the multi-tier system on the network signaling trac The Two-Tier Architecture and Protocols Our previous studies [17, ] considered a three-tier (high-tier, licensed low tier, and unlicensed low-tier) multi-tier PCS system To simplify our discussion, this paper only considers a two-tier (high-tier and licensed low tier) system The extension to the three-tier system is trivial We rst illustrate the two-tier PCS system Then we describe two registraton protocols for the system 1 The Two-Tier PCS Architecture Consider the two-tier PCS architecture illustrated in Figure 1 In this architecture, the Advanced Mobile Phone Service (AMPS) [1] serves as the hightier, and the licensed PACS (Personal Access Communications Systems [1]) serves as the low-tier The two individual tiers have their respective Visitor Location Registers (VLRs) For PACS, the Access Manger (AM) [] and the VLR may be co-located The two tiers share a single multi-tier Home Location Register (HLR) or MHLR The AMPS VLR and the licensed PACS VLR/AM communicate with MHLR using the IS-41 Mobile Application Part (MAP) protocol [6] The underlying transport protocol for the VLR-to-MHLR connection is the Transaction Capabilities Application Part (TCAP) protocol of Signaling System No 7 (SS7) [16] The Registration Protocols Two registration protocols for the multi-tier system have been proposed [17, ] Under the Single Registration (SR) method, the handset is allowed to register to MHLR on only one tier at any given time Under the Multiple Registration (MR) method, the handset is allowed to register to MHLR on multiple tiers concurrently at any given time In the two-tier PCS system,

27 To appear in WINET Special Issue on Mobility Management 1 Introduction Several personal communications service (PCS) technologies have been developed to provide wireless service in dierent environments [5, 1, 5, 6] For example, cellular mobile radio (which is becoming known in the United States as high-tier PCS, particularly when implemented in the new 19 GHz PCS bands [4, 5]) was designed to provide two-way voice communications with high mobility (vehicular speeds) and wide-ranging (national-wide coverage) The high-tier technologies require a high radio transmitting power with antennas mounted on tall towers, and the equipment costs are high A gure often quoted is US $1 million for a cell site [5] On the other hand, low-tier PCS technologies (such as PACS [1], CT- [7, 3, 7], and DECT [8]) use a low radio transmitting power covering a small area (ie, a micro-cell) and are intended to operate at low speeds such as the pedestrian speeds The equipment costs for the low tier technologies are much cheapter than that for the high tier technologies (eg, US $5,000 for a CT- base station) A low tier system would be cost-eective for high density environments such as metropolitan areas and urban residential areas whereas a high tier system would be cost-eective for covering large areas The motivation for the multi-tier system [5, 15] is to integrate the high tier and low tier systems into a single system to provide the advantages of both tiers The multi-tier system is designed to be compatible with the existing low-tier and high-tier systems That is, the low-tier PCS subscribers will use the low-tier handsets to access service through the inexpensive low-tier system without noticing the existence of the high-tier system The situation is the same for the hightier PCS subscribers For a multi-tier PCS subscriber, a multi-tier handset is required to (automatically or manually) switch between the dierent tiers The developement of multi-tier handset has recently been conducted in the industry [3] To integrate dierent PCS systems, two basic issues must be considered Firstly, the signaling message formats are quite dierent for the high and the low tier radio systems Modications to the radio signaling protocols are required so that the wireline backbone network has a consistent view of the dierent radio systems This issue is being studied [3, 5], and is out of the scope of this paper Secondly, the location tracking protocol at the network level must be modied to accommodate the tier switching operations We

28 1 A Comparison Study of the Two-Tier and the Single-Tier Personal Communications Services Systems Yi-Bing Lin Dept Comp Sci & Info Engr National Chiao Tung University 1001 Ta Hsueh Road Hsinchu, Taiwan, ROC Tel: Fax: liny@csienctuedutw Abstract A two-tier PCS system integrates the high tier PCS system and the low tier PCS systems into a single system to provide the advantages of both tiers Such a system is expected to provide better service (more available and more cost eective to the users) at the expense of the extra tier switching management We compare the performance of the two-tier PCS system and the single low tier system in two aspects: the registration trac and the service availability Because of the tier management, the two-tier system generates more registration trac than the single low tier system Under the range of the input parameters in our study, we show that the two-tier system generates less than 10% extra registration trac than the single low tier system in most cases and generates less than 0% registration trac in the worst cases It is clear that for the service availability, the two-tier system is better than the single one tier system We also study the probability that a call is forced terminated in the single low tier system because the low tier becomes unavailable during the call (such a call can be continued in the two-tier system)