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Heterogeneous Wireless Networks Using a Wireless ATM Platform - INTRODUCTION, TECHNICAL BACKGROUND, Wireless Mobile Network Overview, Heterogeneous Wireless Networks Overview, ATM Overview

registration radio users user

Spiros Louvros
COSMOTE S.A., Greece

Dimitrios Karaboulas
University of Patras, Greece

Athanassios C. Iossifides
COSMOTE S.A., Greece

Stavros A. Kotsopoulos
University of Patras, Greece

INTRODUCTION

Within the last two decades, the world of telecommunications has started to change at a rapid pace. Data traffic, where the information is transmitted in the form of packets and the flow of information is bursty rather than constant, now accounts for almost 40 to 60% of the traffic that is transmitted over the backbone telecommunication networks (Esmailzadeh, Nakagawa, & Jones, 2003). In addition to data traffic, video traffic (variable rate with real-time constraints) was made possible by low-cost video-digitizing equipment (Houssos et al., 2003).

Asynchronous transfer mode (ATM) technology is proposed by the telecommunications industry to accommodate multiple traffic types in a very high-speed wireline-backbone network. Briefly, ATM is based on very fast (on the order of 2.5 Gbits/sec or higher; Q.2931 ATM network signaling specification, ITU, n.d.) packet-switching technology with 53-byte-long packets called cells being transmitted through wireline networks running usually on fiber-optical equipment.

Wireless telecommunications networks have broken the tether in wireline networks and allow users to be mobile and still maintain connectivity to their offices, homes, and so forth (Cox, 1995). The wireless networks are growing at a very rapid pace; GSM-based (global system mobile) cellular phones have been successfully deployed in Europe, Asia, Australia, and North America (Siegmund, Redl, Weber, & Oliphant, 1995). For higher bit-rate wireless access, the Universal Mobile Telecommunications System (UMTS) has been already developed. Finally, for heterogeneous networks, including ex-military networks, ad hoc cellular and high altitude stratospheric platform (HASP) technologies are under development, and standardization for commercial data transmissions in heterogeneous environments has launched.

A wireless ATM transmission network provides a natural wireless counterpart to the development of ATM-based wireline transmission networks by providing full support for multiple traffic types including voice and data traffic in a wireless environment. In this article, an architecture for a wireless ATM transmission platform is presented as a candidate for the interconnection of heterogeneous, wireless cellular networks.

TECHNICAL BACKGROUND

Wireless Mobile Network Overview

In 1991 the European Telecommunication and Standardization Institute (ETSI) accepted the standards for an upcoming mobile, fully digital and cellular communication network: GSM. It was the first Pan-European mobile telephone-network standard that replaced all the existing analogue ones.

Broadband integrated-services data networks (B-ISDNs) are the state-of-the-art technology in today’s wired telecommunication links. The main feature of the B-ISDN concept is the support of a wide range of voice and nonvoice applications in the same network. Mobile networks have to follow the evolution of fixed networks in order to provide moving subscribers with all the services and applications of fixed subscribers. The result of this effort (although somewhat restrictive in terms of realizable bit rates) was another evolution in mobile networks: general packet radio services (GPRSs) and the enhanced data for GSM evolution (EDGE) network (usually referred to as 2.5G), with rates of up to 115 Kb/s and 384 Kb/s, respectively, when fully exploited.

UMTS is the realization of a new generation of telecommunications technology for a world in which personal services will be based on a combination of fixed and mobile services to form a seamless end-to-end service for the subscriber. Generally speaking, UMTS follows the demand posed by moving subscribers of upgrading the existing mobile cellular networks (GSM, GPRS) in nonhomogeneous environments.

3.5G and 4G systems (Esmailzadeh et al., 2003) are already under investigation. Aiming to offer “context-aware personalized ubiquitous multimedia services”(Houssos et al., 2003), 3.5G systems promise rates of up to 10 Mb/s (3GPP [3 rd Generation Partnership Project] Release 5), while the use of greater bandwidth may raise these rates even more in 4G (Esmailzadeh et al.). On the other hand, in the last five years a standardization effort has started for the evolution of WLANs (wireless local-area networks) in order to support higher bit rates in hot spots or business and factory environments with a cell radius in the order of 100 m. For example, IEEE 802.11 variants face rates of up to 11 Mb/s (802.11b) and 54 Mb/s (802.11a/g), while rates in excess of 100 Mb/s have already been referred (Simoens, Pellati, Gosteau, Gosse, & Ware, 2003). European HIPERLAN/2 supports somewhat lower rates but with greater cell coverage and enhanced MAC (medium access control) protocols. In any case, 4G and WLAN technology are going to be based on an IP (Internet protocol) backbone between APs and access controllers, or routers and the Internet. Mobile IPv4 and IPv6 are already under investigation (Lach, Janneteau, & Petrescu, 2003) to provide user mobility support for context-type services.

Heterogeneous Wireless Networks Overview

In the near future, the offered communication services to mobile users will be supported by combined heterogeneous wireless networks. This situation demands actions in the following engineering issues.

  • Integration with existing technologies in the radio network and in the switching levels of the involved combined wireless communication networks.
  • Reengineering of the appropriate interface units at the link layers of the involved networks in order to support optimum access procedures to the corresponding media.
  • Implementation of systemic handover procedures in order to combine the independent handover and roaming procedures of the involved wireless networks.
  • Introduction of new methods and techniques to provide a number of effective security measures.
  • Introduction of advanced ATM procedures in order to support optimum information routing between the main nodes of the combined wireless network.
  • New protocol versions of the existing technologies in order to support interoperability demands.

    It is worthwhile to mention that the possible involved wireless networks that are going to set the futuristic heterogeneous environment belong to the following categories.

  • WLANs covering small geographical areas. In this case the WLANs with the adopted protocols IEEE 802.11a and IEEE 802.11g, and supporting user services on the orthogonal frequency-division multiple-access (OFDM) technique seem to appear as the great scientific interest (Simoens et al., 2003).
  • Ad hoc networks, operating in specific geographical areas using the IEEE 802.11b protocol, will be involved on nested schemes under the technology of the existing cellular communication systems.
  • Cellular mobile networks of 2.5G (i.e., GPRS) and 3G (wideband CDMA [code division multiple access]) will cover geographical areas with mixed cell sizes (i.e., pico-, micro-, and macrocell). In this case, cellular-aided mobile ad hoc networking becomes a very interesting and “hot” research area for reaching the heterogeneous combination of the involved two different types of wireless networks.
  • High-altitude stratosphere platforms will soon cover the non-line-of-sight communication applications and are going to support satellite-like communications with the advantage of small energy demands on the used portable and mobile phones. The SkyStation, SkyNet, SkyTower, and EuroSkyWay projects declare new promises to the applications for a large-scale geographical coverage (Varquez-Castro, Perez-Fontan, & Arbesser-Rastburg, 2002).
  • Satellite communications networks using low earth-orbiting (LEO) and medium earth-orbiting (MEO) satellites will continue to offer their communication services and to expand the communication activities of the terrestrial wireless communication networks.

The futuristic technology convergence on the heterogeneous wireless networking environment is depicted in Figure 1. The lower layers consist of the land mobile networks (GSM, UMTS, WLAN, general ad hoc networks). Above these layers exist the HASP platform either as an overlay umbrella cell or as an overlay switching and interconnecting platform among the different switching protocols of the lower layers. Finally, on the top of all is the high-altitude satellite network.


ATM Overview

ATM technology is proposed by the telecommunications industry to accommodate multiple traffic types in a very high-speed wireline network. The basic idea behind ATM is to transmit all information in small, fixed-size packets called ATM cells over all transmission channels (wired or wireless). Having fixed-size packets of information for transmission can emulate the circuit-switching technique of traditional telephony networks and at the same time take advantage of the best utilization of the transmission-line bandwidth. Hence, it operates asynchronously and it can continuously switch information from and to different networks (voice, video, data) with variable bit rates. The responsible nodes for asynchronous operation are called ATM switches . They consist of interfaces in order to communicate with various heterogeneous networks such as LANs (local-area networks), WANs (wide-area networks), and so forth. All these networks transmit information in different bit rates, and the ATM switches (through the ATM layer of B-ISDN or IP hierarchy) divide this heterogeneous information (using special ATM adaptation layers in terms of OSI [open systems interconnection] layer structure) into fixed-size packets of 48 bytes to accommodate them into the ATM cells.

ATM supports a QoS (quality of service) concept, which is a mechanism for allocating resources based upon the needs of the specific application. The ATM Forum (1996; Rec. TM 4.0) has defined the corresponding service categories (constant bit rate [CBR] for real-time applications, such as videoconferences with strict QoS demands; real-time variable bit rate [rt-VBR] for bursty applications such as compressed video or packetised voice; and so forth).

CHALLENGES IN WIRELESS ATM NETWORKS

The ATM-network architecture has to be redesigned to support wireless users. The use of wireless ATM networks as an interconnection medium among several wireless platforms in a heterogeneous environment is important. So far, WLANs using wireless Ethernet and wireless ATMs have been considered during the evolution toward 4G and beyond-4G wireless mobile heterogeneous networks (Figure 3). Supporting wireless users presents two sets of challenges to the ATM network. The first set includes problems that arise due to the mobility of the wireless users. The second set is related to the provisioning of access to the wireless ATM network.

Mobility of Wireless Users

The ATM standards proposed by the International Telecommunications Union (ITU) are designed to support wireline users at fixed locations (Lach et al., 2003); on the other hand, wireless users are mobile. Current ATM standards do not provide any provisions for the support of location lookup and registration transactions that are required by mobile users (Lach et al.). They also do not support handoff and rerouting functions that are required to remain connected to the backbone ATM network during a move.

The user identification (UID) numbers in wireline networks may be used for the routing of connections to the user; in contrast, the identification number for a wireless user may only be used as a key to retrieve the current location information for that user. The location information for wireless users is usually stored in a database structure that is distributed across the network (Jain, Rajagopalan, & Chang, 1999; Rajagopalan, 1995; Simoens et al., 2003). This database is updated by registration transactions that occur as wireless users move within the wireless network. During a connection setup , the network database is used to locate and route connections to the user.

If a wireless user moves while he or she is communicating with another user or a server in the network, the network may need to transfer the radio link of the user between radio access points in order to provide seamless connectivity to the user. The transfer of a user’s radio link is referred to as handoff. In this article, mobility signaling protocols , designed to implement mobility-related functionality in an ATM network, are described.

Providing Access to the Wireless ATM Network

A key benefit of a wireless network is providing tetherless access to the subscribers. The most common method for providing tetherless access to a network is through the use of radio frequencies. There are two problems that need to be addressed while providing access to an ATM network by means of radio frequencies.

  • Error Performance of the Radio Link: ATM networks are designed to utilize highly reliable fiber-optical or very reliable copper-based physical media. ATM does not include error correction or checking for the user-information portion of an ATM packet. In order to support ATM traffic in a wireless ATM network, the quality of the radio links needs to be improved through the use of equalization, diversity, and error correction and detection to a level that is closer to wireline networks. There are a number of solutions that combine these techniques to improve the error performance of wireless networks. Some of these solutions may be found in Acampora (1994), ATM Forum (1996), Chan, Chan, Ko, Yeung, and Wong (2000), and Cox (1991, 1995).
  • Medium Access for Wireless ATM Networks: A wireless ATM network needs to support multiple traffic types with different priorities and quality-of-service guarantees. In contrast to the fiber-optical media in wireline networks, radio bandwidth is a very precious resource for the wireless ATM network. A medium-access control protocol that supports multiple users, multiple connections per user, and service priorities with quality-of-service requirements must be developed in order to maintain full compatibility with the existing ATM protocols. This medium-access protocol needs to make maximum use of the shared radio resources and needs to achieve full utilization of the radio frequencies in a variety of environments.

WIRELESS ATM-PLATFORM DESCRIPTION

This section introduces our wireless ATM-network architecture. It describes the components of the wireless ATM network and the functions of these components. It also describes the registration-(location) area concept.

Components of the Wireless ATM Network

A wireless ATM-network architecture is based on the registration-area concept. A registration area consists of radio ports, radio-port controllers (medium and small ATM switches), possibly a database, and the physical links that interconnect the parts of the registration area (Figure 2).

The wireless ATM network is designed as a microcellular network for the reasons described in Cox (1991), and Wang and Lee (2001). The typical coverage of a radio port in a microcellular network varies between 0.5 km to 1 km (Cox); therefore, a fairly large number of radio ports are required in order to maintain full coverage of a given geographical area. Consequently, the radio ports in a microcellular network must be economical radio modems that are small enough to be placed on rooftops and utility poles (Cox, 1991, 1995). In a wireless ATM network, where users are globally mobile, the tracking of users is one of the major functions of the wireless network. Each registration area may have a database that is used to support the tracking process (Jain et al., 1999; Marsan, Chiasserini, & Fumagalli, 2001; Rajagopalan, 1995; Siegmund et al., 1995; Simoens et al., 2003).

The ATM-network gateway (large ATM switch) manages the flow of information to and from the wireless ATM network to the wireline ATM networks. The ATM-network gateway is necessary to support connections between the wireline ATM-network users and wireless users, and is responsible for performing location-resolution functionality for wireline network users as described in Jain et al. (1999).

Registration-Area Concept

The wireless ATM network consists of registration areas, the wireless ATM-network backbone, and gateways to the wireline ATM network(s) as depicted in Figure 2. The registration areas of the wireless ATM network are responsible for supporting wireless users.

Each registration area incorporates the signaling functionality required to support mobile users. Via the use of registration areas, the wireless ATM-network architecture is a completely distributed network. By dividing the wireless ATM network into registration areas, the need for addressing the granularity of the wireless ATM network is also reduced. The radio ports and radio-port controllers have only local significance within the registration area. In terms of locating and routing connections to wireless users, the wireless ATM network only considers the registration area of the user and not the particular radio port. In the other direction, the location of the user needs to be updated only when the user moves between the registration areas, which significantly cuts on the signaling traffic.

MOBILITY MANAGEMENT IN WIRELESS ATM NETWORKS

In a wireless ATM network, several procedures are required due to subscriber mobility. Registration is required to locate a user during information delivery. A connection setup is used to establish connections to other users or servers in the wireless network. Handoff provides true mobility to wireless users and allows them to move beyond the coverage of a single wireless access point. Existing ATM-signaling protocols do not support the registration, connection-setup, and handoff transactions that are required to support wireless users (Lach et al., 2003). In order to support wireless users in the ATM architecture, we need to adapt the registration, connection—setup, and handoff procedures used in existing wireless communication networks (Marsan et al., 2001; Siegmund et al., 1995).

During a study of wireless ATM mobility management, several ideas have been proposed. What is important is to explain the overlay-signaling technique (Chiasserini & Cigno, 2002). Overlay-signaling ATM connections are used to transport mobility-related signaling messages between the registration areas in the wireless ATM network and does not require any changes to the existing ATM protocols. The resulting signaling network is then overlaid on top of the existing ATM network. The motivation for implementing an overlay-signaling network is to remain compatible with the existing ATM protocols. Since there are no modifications to the ATM protocols, the overlay-signaling approach does not require any modifications to the existing ATM infrastructure.

Registration Using Overlay Signaling

Registration is performed to maintain information about the wireless users’ locations. It consists of several phases. First, the registration process starts with the transmission of the user identification number and the user’s previous registration-area identification from the portable device that enters a new registration area (Cox, 1991; Wang & Lee, 2001). Upon receiving the UID and the authentication information, an ATM connection is established and the user’s profile is updated with the new location information. The updated profile is transferred to the current registration area. The user’s profile in the previous registration area is deleted by establishing an ATM connection to the previous registration-area switch (PRAS). After the registration transaction is complete, the connection is released.

Session Setup Using Overlay Signaling

The session-setup procedure is used to establish a connection between two wireless network users. The originating registration area refers to the calling user’s registration area, and the destination registration area refers to the called user’s registration area. The called-user identification number (CUID) is transmitted from the portable device to the originating registration-area switch together with the session-setup parameters such as the required bandwidth, traffic type, and so forth. The originating registration-area switch forms a setup message using the incoming session parameters.

Handoff Using Overlay Signaling

Handoff is the transfer of a user’s radio link between radio ports in the network. The portable devices monitor the link quality in terms of received signal power to candidate radio ports, and when the link to another port becomes better, that port is selected and handoff is initiated (Cox, 1991; Wang & Lee, 2001). The link quality is determined by the portable devices because only these devices can determine the quality of the links to multiple radio ports and decide on the best link. In contrast, a radio port can only monitor the link between itself and the portable device. Starting the handoff, the device realizes that a link of better quality exists to a candidate radio port and sends a message to the previous registration-area switch, desiring a handoff to the candidate radio port. The PRAS transfers a copy of the user profile to the candidate registration-area switch (CRAS). the PRAS contacts the end point for the user connection and requests rerouting to the candidate registration area (Cox). Once the rerouting is complete, the PRAS contacts the portable device and relays the channel-assignment information, while the CRAS and the device verify the connection (Cox).

CONCLUSION AND FUTURE TRENDS

In this article, a wireless ATM network is described that can be used in combining future heterogeneous cellular systems (Figure 1). It will expand the range of offered services and the amount of resources available to wireless users.

The future convergence of several wireless networks in an interoperability environment is critical for the existence and reliability of services worldwide. The interconnection should take special care for mobility procedures, especially for handover, which in our case is considered to be intersystem handover. A common transmission-interconnection network should be implemented, capable of managing all mobility procedures that might take place during the movement and the required services of heterogeneous subscribers. Wireless ATM is a promising candidate since it consists of a robust architecture based on wired ATM, supports multiple services from different sources, and can interconnect different networks as a transport mechanism. The wireless environment poses main problems such as cell losses due to the radio environment, cells out of order in the case of handovers, and general congestion in the case of simultaneous resource demands. Future research on wireless ATM should concentrate on forward error connection (FEC) techniques to guarantee cell integrity, handover algorithms to preserve the cell sequence, call-admission control algorithms to take care of congestion, and priority services and special signaling over existing ATM networks to maintain mobility cases.

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