WAN-WLAN Roaming for Voice Application

WAN-WLAN roaming for voice application

Qionghai Dai , Liu Xin and Qiufeng Wu

Broadband Networks & Digital Media Lab of Automation Dept, Tsinghua University, 100084, China

This work is supported by the National Natural Science Foundation of China (No. 60172026) and by the Basic Research Foundation of Tsinghua University (No. JC2001028).

Abstract

Current developments mean that the separation between voice and data transmission is no longer necessary. Voice and data service have converged with both using the same network, opening up a basis for “Integrated Business Solutions”. However, most of prior work focuses on data integration between WAN and WLAN. In contrast little work considers voice convergence. This paper gives an overview of the emerging WAN-WLAN seamless roaming, especially for voice application, then proposes a WAN-WLAN roaming architecture using voice over WLAN with VoIP technology, describes the procedure of handoff.

Keywords: WAN-WLAN, roaming, voice

1. Introduction

Third generation (3G) wireless wide-area network (WWAN) technology promises to bring high-speed packet services to mobile subscribers and pave the way for true converged voice and data wireless services. However, the 3G-network rollout has been plagued with delays caused by huge infrastructure costs, lack of 3G-compatible mobile phones, and uncertainty about subscriber’s willingness to pay for the services.

Wireless LAN (WLAN) technology, an evolution of local-area networking (LAN), offers an attractive alternative to 3G WWAN for delivering mobile high-speed packet applications. Improved standards are yielding faster data rates——now in excess of 10Mbps. The infrastructure cost is a tiny fraction that of 3G. Network installation is relatively simple and painless. No cables are required, and since it operators in the 2.4-GHz radio band, no licenses are necessary. Some analysts predict that the number of users of public hot spot networks will rise from just over half a million this year to 21 million by 2006.

One of the growing areas of focus is how WLAN and WAN will be interconnected seamlessly for the user. The initial focus for these deployments is high-speed data access, but riding voice applications over the same airwaves would create truly converged services for mobile subscribers and complement future 3G data offerings. [1] The following is an example of this solution in use. For instance, when an employee is inside the enterprise (or another "equipped" building/area), their voice and data communications will be carried over the IP Telephony network. While inside, phone access to the IP network will be through the WLAN Access Points throughout the Enterprise. When that same employee moves outside the WLAN, voice traffic will be carried over the WAN (public cellular network), ensuring contiguous communications. The transition between networks is handled by the solution's mobility manager and provides a seamless, uninterrupted "hand-off."

This paper provides an overview of the emerging WAN-WLAN seamless roaming, especially for voice application. The paper is organized as follows. Section 2 presents the related works. Section 3 states the proposed WAN-WLAN seamless roaming for voice application architecture. Section 4 describes the WAN-WLAN roaming procedures. Finally, summary and conclusion is given in Section 5.

2. Related works

There has been some prior work on WAN-WLAN roaming for voice application. GSM on the Net [2] utilizes a corporate intranet to integrate an enterprise communications network with the public GSM network. This system has two important characters. Firstly, the system supports user mobility where, by using various types of terminals, a user can move around the service area without losing contact with the system. Secondly, the system supports terminal mobility which means that a terminal can be moved around geographically without losing contact with the system. The GSM on the Net architecture is illustrated in Fig 1, and consists of GSM and corporate networks. All elements in the corporate network are connected to an intranet. They communicate with each other using an intranet protocol. These elements are described below. The service node enables user mobility, controls calls among different types of terminals, and translates addresses between the PSTN and GSM on the Net. The access node resembles the MSC, VLR, and BSC. GSM/BTS provides the GSM MS with wireless access to the IP network. The gateway provides interfaces between GSM on the Net and other networks (particularly the GSM network). But the GSM on the Net integrates not a general WLAN with the public GSM network but a corporate network with the public GSM network, and it introduces wireless access equipment (i.e., the GSM BTS) into the IP network.

Figure 1 The GSM on the Net architecture

The iGSM[3] service provides user mobility to subscribers, which allows them to use either GSM handsets or H.323 terminals (IP phones or PCs) to access telecommunications services. The iGSM is a voice-over-IP value-added service for mobile network. The iGSM consists of GSM and H.323 network. Figure 2 illustrates the iGSM architecture. A GSM subscriber ordering the iGSM service enjoys the standard GSM services when he/she is in the GSM network. When the person moves to the IP network he/she can utilize an H.323 terminal to receive call delivery to his/her mobile station ISDN (MSISDN) number. This system only solves the calling delivery after its roaming from GSM to IP network, without mentioning the calling happening during roaming. The terminal is either GSM handset or H.323 terminal, not an integrated one handset.

Figure 2 The iGSM architecture

Some companies are also applying themselves to the integration of WAN and WLAN for voice application. In January 2003, for example, mobile communications company Motorola joined forces with networking vendor Avaya and wireless LAN giant Proxim to collaboratively push a new converged solution for voice over IP that would allow for roaming between cellular and WLAN networks, as well as data services. The jointly developed, standards-based solutions will support contiguous voice and data service to users across enterprise networks, public cellular networks, and public hotspot WLANs based on 802.11 technology (commonly known as Wi-Fi). The convergence of these technologies will enable businesses to gain new cost savings, user efficiencies, and enhanced communications capabilities. Motorola will develop Wi-Fi/cellular dual-band/ dual-technology handsets. Avaya will contribute its Session Initiation Protocol (SIP) IP Telephony Software. And Proxim will supply its voice-enabled WLAN infrastructure. The IP Telephony application is enabled by Avaya MultiVantage™ Software, which provides reliable and scalable communications. Motorola will create network mobility management components that control the hand-off between local WLANs and cellular networks. And Proxim will provide voice-enabled Wi-Fi WLAN infrastructure, quality-of-service software and centralized management systems to facilitate hand-offs between access points. Figure 3 is a pattern interpretation about its solution overview. Alan Reiter, an analyst at Wireless Internet & Mobile Computing in Chevy Chase, Md., said this is the first venture he knows of that has attempted to combine voice and data services for both cellular and Wi-Fi networks. Unfortunately, there are no specific descriptions on the solution could be found by now.

Figure 3. Converged WLAN and WAN solution overview

3. The WAN-WLAN architecture

When the mobile user roams between WAN and WLAN, the voice should be contiguous without interrupting. Since the voice is sent by WLAN (an IP network) when WLAN is available, we consider transport the voice stream by utilizing VoIP technology. Although we use H.323[4] as the VoIP protocol, our results can be generalized to accommodate other protocols such as SIP (Session Initial Protocol)[5]. In order to reduce cellular network management cost, we realizing voice over WLAN by communication between mobile and access point directly. For the purpose of contiguous voice during handoff, firstly we set up a new VoIP connection from mobile to the other terminal user over WLAN and other IP network without changing the former public cellular network connection. After the new connection is completely finished, the public cellular network connection is then cut off.

The WAN-WLAN architecture that we supposed is as follows. One of the terminals is a mobile user, while the other terminal can be divided into two cases: a PSTN fixed phone or a mobile user.

Case 1:One of the terminations is PSTN fixed phone:

Figure 4a WAN-WLAN roaming for voice application architecture, one of the terminals is PSTN fixed phone

Case 2: Both of the terminations are mobile phone:

Figure 4b WAN-WLAN roaming for voice application architecture, both of the terminals are mobile phones

The upper line is former public cellular network connection and the under line is later voice over WLAN connection by using VoIP technology. GW is the abbreviation of Gateway. The GW next to the access point performs the similar function as the gateway in H.323 protocol. The AP and GW are separated in function and can be integrated in device. The IP phone termination proxy serves as a gatekeeper in H.323 whose function including telephone number analysis and authentication.

4. The WAN-WLAN procedures

The procedures of WAN-WLAN roaming for voice application can be described as follows.

Case 1:

· Mobile can detect the existence of WLAN. Mobile measures the signal from WLAN in every a few time slots.

· Mobile can exchange between the two operator modes. So when it measures the signal of available WLAN is strong enough (some indexes conform with the communication threshold values), it will send request to AP. AP forward this request to GW. GW answers the mobile with acknowledgement or rejection. (Maybe there is no idle wireless channel to use or the signal is not strong enough. The GW decision is more complex and comprehensive.)

· If the answer is acknowledgement, GW will also allocate a temp IP for mobile phone. The temp IP is to discriminate with other mobile phone. All the voice data sent to this temp IP should be intermediated from this GW.

· Mobile send the dial number to GW, GW forwards it to IP phone termination proxy. After its authentication (such as charge authentication), IP phone termination proxy is in charge of exchanging the address between dial number and available IP of destination GW.

· The protocol between GWs is based on H.323. But the communication between mobile and IP phone termination proxy is under our control. So GW near IP phone termination proxy should has some function of GW in H.323, and IP phone termination proxy should has some function of GK in H.323. For example, GW is in charge of providing H.323 signaling operation and handling message of H.225 and H.245. GK is in charge of authentication, routing, and bandwidth manage.

· Destination phone has applied the calling waiting service. So a PSTN phone user can hear the prompt voice and choose to handoff to another call when it is communicating with one call.

· AP can receive 13kbit/s numerical signal and trans-code it into voice data stream packets and vice versa. This can also be said that AP has the function of media gateway. In fact, AP is just a bridge in WLAN. Ideally, trans-code function should be realized by mobile phone, but this must consume too much power in mobile. So the function should be realized in AP.

· Mobile send request to GW to leave the WLAN when it detects the signal fading of WLAN, GW answers the mobile with acknowledgement or rejection.

· Mobile auto dial destination number through GSM

· The GK of calling party terminates connection with the GK of called party. The GW of calling party calls back the temporary IP from mobile.

· Handoff function: Mobile can handoff between different APs in different subnets. This can be realized with Mobile IP.

· Roaming function: There should be a VLR in WLAN that HLR in GSM can visit. Thus PSTN phone knows how to route to the destination mobile when the mobile is in WLAN as the called party.

Case 2:

· Mobile can detect the existence of WLAN. Mobile measures the signal from WLAN in every a few time slots.

· Mobile send the dial number to GW, GW forwards it to IP phone termination proxy. After its authentication (such as charge authentication), IP phone termination proxy is in charge of communicating with HLR to get the current location of called party.

· IP phone termination proxy can analyze the gateway IP of called party by current location.

· AP visit IP phone termination proxy to get the gateway IP of called party

· The protocol between GWs is based on H.323. But the communication between mobile and IP phone termination proxy is under our control.

· The called mobile phone has applied the calling waiting service. It can choose to switch to the new connection.

· Mobile phone cut off the GSM link

· AP can receive 13kbit/s numerical signal and trans-code it into voice data stream and vice versa

· Mobile send request to GW to leave the WLAN when it detects the signal fading of WLAN, GW answers the mobile with acknowledgement or rejection.

· Mobile auto dial destination number through GSM.

· The GK of calling party terminates connection with the GK of called party. The GW of calling party calls back the temporary IP from mobile.

· Handoff function: Mobile can handoff between different APs in different subnets. This can be realized with Mobile IP.

· Roaming function: There should be a VLR in WLAN that HLR in GSM can visit. Thus PSTN phone knows how to route to the destination mobile when the mobile is in WLAN as the called party

The deficiency of this architecture is that when the other terminal user processes the transfer waiting, he/she does not know the dialing is from the original user or other users. This can be solved via sending a message from mobile handoff user. The message includes the handoff user mobile number that can be displayed on the other terminal telephone.