Efficient Location Management for Hybrid Wireless ATM Networks

Efficient Location Management for Hybrid Wireless ATM Networks: Architecture and Performance Analysis Sihui Zhou♣ Aruna Seneviratne▲ and Terry Percival♣ ♣

CSIRO Telecommunications and Industrial Physics PO Box 76, Epping NSW 1710, Australia Phone: +61-2-93724340, Fax: +61-2-93724490 Email: [email protected] or [email protected]

▲

Abstract-Location management is one of the most important aspects in the realization of mobility in wireless networks. Following developments of ATM (Asynchronous Transfer Mode) technology, more core networks are becoming ATM-based. Therefore research into location management schemes for wireless networks with ATM cores is attracting more attention. In recent years several location management schemes have been proposed for classical wireless ATM networks, which have end-to-end native ATM connections. Most of these proposals assume that a wireless ATM network is built using End-user Mobile ATM Switches (EMAS). However, current ATM networks only consist of normal ATM switches that are not mobility enabled. In this paper, we present a location management scheme for hybrid wireless ATM networks, which have an ATM based core and IP based wireless tails. From a performance analysis, we show that the proposed scheme has better cost performance than the conventional GSM/IS-41 based location management scheme. I. INTRODUCTION Cellular communications and mobile computing have experienced rapid growth in the last decade. The wide acceptance of cellular communications and mobile computing has led to the development of a new generation of wireless communication system that provides users with free roaming, and relatively high speed communication links.

School of Electrical Engineering and Telecommunications University of New South Wales, Sydney 2052, Australia Phone:+61-2-93855389, Fax:+61-2-93855993 Email: [email protected]

prohibitively expensive. Secondly, the inability to provide QoS guarantees in wireless environment makes the provision of ATM QoS guarantees impossible. Finally, the lack of ATM based applications, makes the probability of user requiring this type of system low. On the other hand, a combination of the wide acceptance of ATM as backbone networks and the availability of IP applications makes an alternative solution, in which IP based wireless tails used in conjunction with backbone ATM networks much more attractive for the foreseeable future. We call this type of wireless network a hybrid wireless network. In recent years, a number of research projects have addressed issues associated with wireless IP networks The work on Mobile IP [5,6] has provided mechanisms for managing mobility in the Internet. A number of studies have investigated the behaviour of TCP/IP protocol suite over wireless links [7,8,9]. Also there has been some studies on handover latency [10,11]. Similarly, there has been considerable amount of work on wireless ATM networks [1,2,3]

To overcome this in the recent past, considerable research into wireless ATM networks, which extend ATM through wireless networks have been undertaken [1-4].

Although some of the above work has direct relevance and can be applied to hybrid wireless networks, little work has been explicitly done on hybrid wireless networks, especially on location management. In this paper, we present the architecture and performance analysis of a location management scheme for hybrid wireless networks. It is based on the GSM/IS-41 location management scheme. However, the proposed scheme has a distributed structure to overcome the inefficiencies of the generic GSM/IS-41 scheme. The rest of the paper is organized as follows. In section 2, we briefly review related work. Section 3 then presents the architecture of proposed location. In section 4, we describe basic operation including location registration/update and call delivery of our location management scheme. Then a performance analysis, which compares the proposed scheme with conventional GSM/IS-41 location management, is presented in section 5. The final section presents conclusions.

Although significant progress has been made in the mechanisms for providing wireless ATM connectivity, it is not a viable solution for three reasons. Firstly, due to wireless link level inefficiencies the transportation of ATM cells is

II. RELATED WORK ATM networks are connection-oriented. This allows ATM networks to have a more efficient location management scheme based on its signaling architecture. Following the

In parallel, broadband Asynchronous Transfer Mode (ATM) technologies are being deployed in the fixed networks. These ATM networks provide high data transfer speeds and the ability to provide users with Quality of Service (QoS) guarantees in terms of loss ratio, delay and throughput. Therefore despite the advances in mobile communications systems, the disparity with fixed networks has increased.

maturity of ATM technologies, especially the establishment of the wireless ATM working group in the ATM Forum, several such location management schemes have been proposed for classical wireless ATM networks [1,2,3,4,14]. However, few location management schemes [5,6] can be applied to hybrid wireless ATM networks, which have ATM connections terminated at base stations and IP based wireless tails. These schemes are detailed below. A. Location Management Schemes for Classical Wireless ATM Networks

• The Mobile PNNI Scheme The mobile PNNI scheme [1] enhances the ATM PNNI protocol to handle mobile users. It uses limited-scope reachability updates, forwarding pointers and a route optimization procedure to track and locate mobile terminals. There is no explicit mobile location prior to the connection set-up. Connections are set up to mobile terminals according to the reachability information at switches. The use the mobile PNNI scheme requires mobility enabled ATM switches. • The Location Register Scheme In the Location Register scheme [1], location registers are placed within the peer group structure of an ATM network. Location registers are used to track the location of a mobile terminal and to respond to location queries. It has an explicit mobile locating procedure, which occurs prior to the connection setup. If the Location Register scheme is used in a mixed classical wired and wireless ATM network, the ATM end system address space needs to be partitioned into separate spaces for mobile terminals and for fixed terminals. • The Integrated Scheme The Integrated scheme proposed by NEC Research Laboratories [2] is the basis of ATM Forum location management contributions [3,4], and the draft specification [14]. As in the mobile PNNI scheme, the Integrated scheme combines the location management process with the connection setup process. Thus, it is not necessary to acquire mobile terminal location information prior to connection setup. The fundamental requirement of the Integrated scheme is that the home switch of a mobile terminal must be mobility enabled. • The External Location Service Modular Scheme The External Location Service Modular scheme [2,3,4,14] is an alternative solution to the Integrated scheme for cases where the inter-switch signaling protocol is not mobile aware. It resolves a mobile terminal name to its current location prior to the connection setup process. The advantage of the External Location Service Modular scheme is that it is not necessary to change inter-switch connection setup process to accommodate mobile terminals. However, an additional mechanism may be needed to distinguish mobile terminal identifications from static terminal identifications to ensure

that the location management process is only invoked for mobile terminals. Among four proposed location management schemes for classical wireless ATM network, the Mobile PNNI and the Integrated schemes have similar merits. They provide simple location management solutions trading off with the complexity of ATM switches. The major disadvantage of the mobile PNNI and Location Register schemes is that they are not backward compatible. Therefore, they cannot be deployed on standard ATM networks. The Location Register and the External Location Service Modular schemes give a more unified solution. They can be used both in future wireless ATM networks and current wireless ATM networks, at the expense of structural complexity. B.

Location Management Schemes for Hybrid Wireless Networks

Although they are no location management schemes proposed explicitly for hybrid wireless ATM networks, the Mobile IP and GSM/IS-41 schemes can be used. The Mobile IP is relevant as hybrid wireless networks have IP based wireless tails and GSM/IS-41 because the backbones of hybrid wireless networks are connection-oriented. • The Mobile IP Scheme The Mobile IP (MIP) scheme is considered to be the only current means for offering seamless roaming in IP based networks (Internet) [5,6]. It has been discussed within the Internet Engineering Task Force (IETF) for a number of years. In MIP, a mobile terminal has a home address, which is defined as its permanent IP address on its home network. It also has a care-of-address, which is associated with the network to which the mobile station is currently attached. The home agent maintains the information location for each mobile terminal registered with it. This feature may be suitable for pure IP based networks, but it is very inefficient for hybrid wireless networks because of the connectionoriented backbone. • The Generic GSM/IS-41 Location Management Scheme The generic GSM/IS-41 [15] location management scheme uses a two-level location management structure. Two types of database, a home location register (HLR) and a visitor location register (VLR), are used. The HLR is similar to a home agent, and stores user profiles of its assigned mobile users. The VLR is analogous to a foreign agent, and each VLR stores replicas of user profiles of the users currently residing in its associated Registration Area (RA). A mobile terminal must do location register and update when it moves into a new RA. The call delivery includes location query and paging processes. The location query process gives the location of RA of the called mobile terminal. The paging process provides the cell allocation in a RA. Both MIP and the generic GSM/IS-41 schemes are not efficient in a hybrid wireless-networking environment. In

MIP, the location management scheme is decoupled from the handoff procedures. The MIP handoff procedure is inefficient in hybrid wireless networks, as the backbone of a hybrid wireless network is connection-oriented. Using MIP also results in the loss of the QoS guarantees, which is the main advantage of ATM networks. The generic GSM/IS-41 location management scheme has takes advantage of the separation of location management and handoff management. However, its efficiency is reduced because of its two level location management structure.

The main functions of the proposed scheme are registration, location update, location query, and call delivery. Registration is required when a mobile terminal powers on/off or enters a new registration area. Location updates need to be performed every time a mobile terminal makes a new registration. A location query is needed when a mobile is not at its home registration area. It provides an updated location to the called or the home base station. Call delivery is a procedure to deliver calls to a called mobile terminal. IV.

III.

ARCHITECTURE

Figure-1 shows the system model for a location management scheme for hybrid wireless networks with ATM cores. The network consists of standard (non-mobility enabled) ATM switches, base stations which act as IP/ATM gateways, mobile terminals and fixed IP terminals. In addition it has distributed home location registers and visitor location registers. In this system model external location registers are used to track and locate mobile terminals. Each mobile terminal has a pre-assigned permanent address that is associated with a base station. The base station is its home base station. Each base station has a home location register and a visitor location register. The home location register is used to store users’ permanent profiles. It also includes a set of pointers to point to the current location of a moving mobile terminal. The visitor location register is used to store temporary user profiles. Such profiles are updated when the mobile terminal moves from one cell to another. HLR VLR Gateway

Fixed IP network

Fixed ATM network BS

VLR

HLR BS

VLR

BS

cell

MT

Fixed IP terminal cell

HLR MT

VLR MT

HLR

cell

Figure-1: The system logical architecture model In this environment, if a fixed IP terminal wishes to communicate with a mobile terminal, it needs to acquire the location information of the mobile terminal prior to the connection setup. The location information can be found in the distributed location registers (HLRs and VLRs). In addition, the IP/ATM gateway has to perform address mapping to enable the setting up of the connections across the ATM backbone network. If a mobile terminal wishes to communicate with another mobile terminal, it also needs to determine the current location of that mobile terminal before setting up connections by querying the location registers (HLRs and VLRs).

BASIC OPERATION

The most important operations of our location management scheme are that registering, updating mobile locations, and delivering a call to a callee. Each mobile is assigned a permanent address and a permanent home location when first turned on. The two basic operations in our location management scheme are location registration/update and call delivery. • Location Registration/Update When a mobile powers on, it registers its permanent address with its HLR. Once it moves to a different cell, it gets a temporary address and registers both its temporary and permanent addresses with VLRs. When a registration request is received from a mobile, the VLR forwards the request to the mobile’s HLR to update the location. In turn, the HLR sets its forwarding pointer to current VLR servicing the RA in which the mobile is currently located. Figure-2 shows an example of the location registration/update operation. In Figure-2 (a) a mobile powers on at its home location, it then registers its mobile address with its HLR. In Figure-2 (b) the mobile moves from its home cell to a foreign cell and sends a location update request and the corresponding VLR forwards its request to the HLR. The HLR then sets up its forwarding pointer to the foreign location. In Figure-2 (c) the mobile moves from a foreign location to another foreign location and updates both VLRs and HLR forwarding pointers. • Call Delivery The main aim of call delivery is to find out where the callee is located and setup the connection. Therefore, two basic operations have to be accomplished during call delivery, namely location query and the connection initiation. Figure-3 shows an example of call delivery operation when the callee is at a foreign location. In Figure-3, the callee is located at a foreign location. The caller tries to query the callee’s HLR to find out where it is. After receiving the location query, the HLR forwards the query to the VLR. VLR in turn returns an acknowledgement with the information of the current location of the callee. Once the caller receives the acknowledgement, it sets up a connection to the callee.

ATM Switch 3

ATM Switch 3 (5).ACK to foreign BS

ATM Switch1

ATM Switch1 ATM Switch 2

ATM Switch 2

(2). Record to HLR

V L R

H L R

BS1

V L R

H L R

BS2

V L R

H L R

ATM/IP Gateway

H L R

BS1(home)

(3). ACK to Home BS

V L R

H L R

BS2(foreign)

(4).Setup forward pointer to VLR

(4). ACK to mobile

(2).Record in VLR

V L R

(6).ack to Mobile

(1). Registration request

H L R

(3).Forward to homeBS

move Fixed terminal

ATM/IP Gateway

(1).Registration request

Fixed terminal

Mobile

Figure-2(a): mobile powers on at home

Mobile powers on/off

V L R

Figure-2(b): mobile moves from home to a foreign location

Mobile

ATM Switch 1

(5)

ATM Switch 4

ATM Switch2 (7). ATM Switch 3

(9)

(3)

H L R

V L R

H L R

BS2(foreign 1)

V L R

H L R

BS3(foreign2) (2)

(4)

(6)

H L R

BS1(home) (8)

V L R

ATM/IP Gateway

(1) (1). (2). (3). (4). (5). (6).

move Mobile

V L R

Mobile

Registration request Record in VLR2 Forward to Foreign1 Delete record in VLR1 ACK toForeign2 ACK to Mobile

(7). Forward update pointer to home BS (8). Update HLRforward pointer (9). ACK to current BS of Updatein HLR Fixed terminal

Figure-2(c): mobile moves from a foreign location to antoher foreign location

Figure-2: An Example of location registration/update operation registers. The search cost is incurred before the call set-up, and includes the cost for call delivery. Five situations are considered in the tracking cost and three situations are considered in the search cost as shown in Table-1.

ATM Switch 3 (5) ATM Switch 2

ATM Switch1

(4)

(1).

(3) H V L L R R BS1(home)

H V L L R R BS2(foreign)

H V L L R R ATM/IP Gateway

(2).

By defining probabilities and cost elements in Table-1, we can get the mean tracking cost Ctrack , the mean search cost Csearch and the average total cost per move

Cˆ total of the

proposed location management scheme as shown in equation (3.1), (3.2), and (3.3). 5

Ctrack = ∑ Ptrack( k )Ctrack ( k )

Called Mobile (1).Location query request to home BS. (2).Forwards toHLR. (3). HLR forwards to VLR (4).VLR acknowledges to home BS. (5).Forwards the ack to the calling fixed terminal.

Fixed terminal

Figure-3: An Example of call delivery operation V.

PERFORMANCE ANALYSIS

The performance of a location management scheme can be measured in terms of its cost [13,15]. The cost of a location management scheme normally is the performance measurement associated call-to-mobility ratio (CMR). Two cost components were defined by [1,12,13], namely the cost of tracking and the cost of searching. The tracking cost is the cost for reporting the new location, and updating the location

(3.1)

k =1 3

Csearch = ∑ Psearch ( k )Csearch ( k )

(3 . 2 )

k =1

Cˆ total = C track + ρ C search

(3.3)

ρ=CMR=λc/λm , λc is the call arrival rate and λm is the mobile terminal moves rate. Here, according to operations, we also have equation (3.4) and equations listed in Table-2: 5

∑P

track ( k )

k =1

=1

and

3

∑P

search( k )

=1

(3.4)

k =1

The performance of the proposed scheme was compared to a generic GSM/IS-41 scheme. Table-3 shows selective values of cost elements and numerical results.

Name Ch Cv Cg Catm Ptrack(1) Ptrack(2)

Definition Cost for a query or an update of the HLR Cost for a query or an update of the VLR Cost to route a message at a gateway or a base station Cost to transfer a message through an ATM network Probability a mobile terminal powers on/off at home Probability a mobile terminal powers on/off at a foreign location Probability a mobile terminal moves from home cell to a foreign cell Probability a mobile terminal moves from one foreign cell to another foreign cell Probability of a mobile terminal moves from one foreign cell to the home cell Probability a mobile terminal calls another mobile terminal Probability a mobile terminal calls a fixed terminal Probability a fixed terminal calls a mobile terminal

Ptrack(3) Ptrack(4) Ptrack(5) Psearch(1) Psearch(2) Psearch(3)

Table-1: Probabilities and cost elements definition Name Ctrack(1)

Definition Cost of a mobile terminal powers on/off at home Cost of a mobile terminal powers on/off at a foreign location Cost of a mobile terminal moves from home to a foreign cell Cost of a mobile terminal moves from a foreign cell to another foreign cell Cost of a mobile terminal moves from a foreign cell to the home cell Cost of a mobile terminal calls another mobile terminal Cost of a mobile terminal calls a fixed terminal Cost of a fixed terminal calls a mobile terminal

Ctrack(2) Ctrack(3) Ctrack(4) Ctrack(5) Csearch(1) Csearch(1) Csearch(1)

Equation Ch Ch+Cv+2Cg+2Catm

The reason for the better performance is that the VLRs have a more distributed structure in our proposal than in the generic GSM/IS-41 based scheme. Instead of a two level update and search procedures, there is only a single update and search. However, this improvement can only be seen with high-speed networks. When the speed of the wireless and IP networks is very slow, the proposed scheme is only slightly better than the generic GSM/IS-41 scheme (Set 2). VI.

Ch+Cv+4Cg+4Catm Ch+Cv+2Cg+2Catm Ch+2Cg+2Catm Ch+2Cg+2Catm Ch+2Cg+2Catm

ACKNOWLEDGEMENT

The authors would like to acknowledge J. Chan, Dr. R. Malaney and D. Ostry for their helpful discussions and the support of a number of other colleagues at CSIRO. The permission of the Chief, CSIRO Telecommunications and Industrial Physics to publish this work is also acknowledged. REFERENCES [1] [2]

Relative Cost (our scheme/GSM)

Set

Ch

Cv

Cg

Catm

CMR0.1

1 2 3

13.5 13.5 13.5

13.5 13.5 13.5

2 3 1

1 1 1

78% 80% 77%

89% 91% 83%

[3] [4] [5] [6] [7]

Table-3: Performance numerical results* In Table-3, cost elements of Set 1 are values measured on the CSIRO Telecommunications and Industrial Physics experimental network. Two other sets of cost elements are considered. Set 2 represents an implementation of a slower network and Set 3 represents an implementation of a faster network. From the results, it can be seen that the proposed location management scheme has superior cost performance compared to a generic GSM/IS-41 based scheme. For the experimental network results (Set 1 cost elements), the cost of the proposed scheme is between 78% and 89% of the cost of the generic GSM/IS-41 scheme. For Set 3 of cost elements, the cost of

*

CONCLUSION

The paper describes the architecture and the performance analysis for a location management scheme for hybrid wireless networks. The proposed scheme has the advantage of backward compatibility with ATM switches. In addition it can be used in future networks with mobility enabled switches. Using location registers at both wireless and fixed network gateways, it provides better transparency for users. A cost performance analysis shows that the proposed scheme is more efficient than a generic GSM/IS-41 based scheme.

Ch+Cv+2Cg+2Catm

Table-2: Cost elements equations Cost Elements

proposed scheme is even lower, between 77% and 83% of the generic GSM/IS-41 scheme.

More detailed results are included in [16], and subject to publishing

[8] [9] [10] [11] [12] [13] [14] [15] [16]

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