Wireless Convergence Architecture: A Case Study Using GSM and Wireless LAN

1. Wireless Convergence Architecture: A Case Study Using GSM and Wireless LAN NIKOS A. NIKOLAOU KONSTANTINOS G. VAXEVANAKIS, SOTIRIOS I. MANIATIS and IAKOVOS S. VENIERIS NICHOLAS A. ZERVOS 通訊所 簡精政

2. Abstract • The evolution of wireless networks has motivated the expansion of the static business environment to a mobile and wireless one. • This paper presents the Wireless Convergence Architecture (WCA) that incorporates different wireless interfaces under the same mobile terminal. • A specific implementation is presented, based on two complementary wireless technologies – in terms of coverage area – an IEEE 802.11-compliant in the short local and a GSM in the wide area.

3. Outline • Introduction • Wireless Convergence Architecture overview • Supervisor • Resource Repository • IP Mobility Enhancement Client, Server • Abstract Socket Layer and Communication Manager • Reference network environment • Experimental results • Conclusions

4. 1. Introduction(1/4) • The trend in network evolution favours the presence of various network technologies that reveal different characteristics, in terms of coverage area, underlying physical medium, medium access control (MAC), available bandwidth and delay. • At the same time, various wireless technologies, with different pros and cons, are becoming more popular, as they eliminate the disadvantage of having the user restricted in a particular location. • in the near future, the expansion of the working environment to a global one will be based on different wireless access technologies, interworking with each other transparently.

5. 1. Introduction(2/4)

6. 1. Introduction(3/4) • Current wireless networks provide connectivity, targeting specific environments that pose restrictions in terms of available bandwidth, coverage area and cost. • A common difficulty faced by mobile users, which actually contradicts their “mobile” nature, is that they are constrained by the coverage area of the wireless technology used. • This paper introduces the Wireless Convergence Architecture (WCA) that caters for the aforementioned requirements, introducing new software components, both at the terminal and the local Intranet.

7. 1. Introduction(4/4) • Seeking for complementary technologies that could provide connectivity for both indoors and outdoors, two specific wireless technologies have been selected, namely the IEEE 802.11 Wireless LAN and the Global System for Mobile communications. • Taking into account this specific implementation, a reference network environment has been determined and experiments were performed using standard and proprietary networking applications.

8. 2. Wireless Convergence Architecture overview(1/4) • In order to free the mobile user from location restrictions the mobile terminal must be equipped with multiple wireless network adapters. • Following the distinction between indoors and outdoors communications, it seems reasonable that two adapters – one for each case – would be sufficient. • There are two major issues that must be sufficiently resolved, namely the location transparency and the resilience to temporal wireless link disconnection.

9. 2. Wireless Convergence Architecture overview(2/4) • To adequately address them, new software components have to be incorporated both at the terminal and the network side (local Intranet), as shown in figure 2. • As far as the network side is concerned, all the modifications are introduced in a dedicated machine called Gateway/Proxy. • Location transparency involves the automatic switching from one wireless interface to the other, when moving from indoors to outdoors and vice-versa. • IPMEC(IP Mobility Enhancement Client) • IPMES(IP Mobility Enhancement Server)

10. 2. Wireless Convergence Architecture overview(3/4) • A separate pair of modules has been introduced, consisting of the Abstract Socket Layer (ASL) and the Communication Manager (CM), placed at the mobile terminal and the Gateway/Proxy, respectively. • Two more components have been identified and introduced in the terminal side, namely the Supervisor and the Resource Repository (RR).

11. 2. Wireless Convergence Architecture overview(4/4)

12. 2.1 Supervisor • The Supervisor is responsible for monitoring the connectivity status of the underlying wireless adapters. • Supervisor continuously examines the signal-to-noise ratio (SNR) of the active interface. • Supervisor takes into account predefined priorities among the available wireless interfaces, to resolve conflicts where more than one wireless adapter is operational.

13. 2.2 Resource Repository • The Resource Repository (RR) is used for storing critical information for the proper operation of the mobile terminal. • RR provides an interface through which registered modules or user-applications are asynchronously notified when the value of a specific parameter changes. • All the information is organized in tree-like structure, thus, making future extensions easy.

14. 2.3 IP Mobility Enhancement Client, Server(1/2) • The IP Mobility Enhancement Client and Server team up with the IP protocol to achieve location transparency and hide mobility from higher layer protocols and, eventually, applications • IPMEC is mainly responsible for redirecting IP packets over the correct underlying wireless interface. • IPMEC does not determine by itself which wireless interface is active or inactive. • The Supervisor, through RR’s asynchronous notification mechanism, conveys this kind of information to IPMEC.

15. 2.3 IP Mobility Enhancement Client, Server(2/2) • When switching occurs, the terminal’s IP address is not modified. • This is one of the prerequisites that enables the terminal to switch between interfaces, without having the active transport connections terminated. • IPMES component resides in the Gateway/Proxy and implements the complementary functionality of IPMEC.

16. 2.4 Abstract Socket Layer and Communication Manager(1/3) • The Abstract Socket Layer (ASL) and Communication Manager (CM) pair of modules provide application resiliency to wireless link disruptions. • The basic idea behind the operation of those two modules is to split an end-to-end TCP session into three consecutive parts – Application-ASL, ASL-CM and CM-Server – as shown in figure 3. • In this manner, the wireless part of the communication is isolated and the possible side effects of a temporal wireless link disconnection can be absorbed. • ASL resides at the mobile terminal side and provides a socket-like Application Programming Interface (API).

17. 2.4 Abstract Socket Layer and Communication Manager(2/3) • Whenever an interruption occurs, ASL re-establishes transport connections to CM, and, subsequently, re-associates sessions with physical connections. • 以下圖解說明ASL和CM間的運作。

18. 2.4 Abstract Socket Layer and Communication Manager(3/3)

19. 3. Reference network environment(1/4) • It accommodates two wireless networks, based on different technologies, namely the IEEE 802.11 Wireless LAN and the GSM. • Wireless terminal is equipped with a WLAN NIC, for indoors communication, and a combined GSM/PSTN adapter, for outdoors communication. • Gateway/Proxy maintains the control of a pool-of-modems also maintains the control of a poolof-modems, providing access through the public GSM/PSTN network.

20. 3. Reference network environment(2/4)

21. 3. Reference network environment(3/4) • When it moves towards the boundaries of WLAN and the Wireless LAN SNR drops below a predefined threshold, the system switches to GSM. • The selection of the aforementioned wireless interfaces has been triggered mainly by two factors. • First of all, the GSM technology is mature, well established and used by millions of people in a daily basis. • The second reason is correlated with the requirement to have two different wireless interfaces that can be used in a complementary manner. • WLAN and GSM are a perfect match, because they provide wireless communication for indoors and outdoors, respectively.

22. 3. Reference network environment(4/4) • The coverage area of WLAN is a sub-area within the GSM coverage, but, whenever it is feasible, the WLAN must be favoured over the GSM. • The transition from GSM to WLAN implies that more bandwidth becomes available for the running applications. • Through that API user-level applications are informed about the bandwidth capabilities of the active wireless interface and warned when a switching is going to be performed.

23. 4. Experimental results(1/6) • The implementation of the mobile terminal components, presented in section 2, has been realized in two different Operating Systems (OS), including Windows NT 4.0 and Linux (SuSE 6.1). • The former was selected because it has a largebase of end-users and, at the time of the decision, it was themost stable among the various clones of MS Window OSs. • The Linux OS was considered so as to prove the implementation feasibility and the generality of WCA.

24. 4. Experimental results(2/6) • The testing scenarios were based on standard networking applications, including Telnet, Web browsing (Internet Explorer for Windows NT and Netscape for Linux) and videoconference (NetMeeting for Windows NT). • Two testing scenarios were used, depicted in figure 5:

25. 4. Experimental results(3/6)

26. 4. Experimental results(4/6) This happens because as soon as Windows NT detects that the GSM (PSTN) link is down, it releases all resources referring to the Point-to-Point protocol, with the imminent result of terminating any previously active TCP connection.

27. 4. Experimental results(5/6) • Based on the results, we have concluded that the switching between different wireless interfaces, while maintaining TCP sessions intact, is accomplished successfully in almost all the experiments, apart from the cases restricted by the operating system’s limitations. • Moreover, measurements collected during the execution of the testing scenarios revealed that the average time to set up the GSM link is about 20 s. • ASL and CM gave a good solution to this problem, provided that the networking applications do not utilize the standard socket API, but the one offered by ASL. • Although ASL and CM provide resiliency to wireless link breaks, they also introduce extra delay.

28. 4. Experimental results(6/6)

29. 5. Conclusions(1/2) • Wireless communications seem to be more appealing than wired • Combining the capabilities of different wireless interfaces, a working environment can be built-up for both indoors and outdoors communications, allowing the user to move seamlessly in the resulting virtual world • Seamless and uninterrupted connectivity cannot be achieved by simply installing some network interfaces on a mobile host.

30. 5. Conclusions(2/2) • Another major concern regarding WCA is the support of a large number of mobile hosts connecting to the home Intranet through the Gateway/Proxy. • Scalability problems may arise, as a result of the simultaneous existence of multiple active TCP/IP flows from various mobile hosts. • In spite of the fact that there are many open points regarding the provision of seamless handoff between different wireless mediums, WCA offers a realized and tested paradigm of the feasibility of uninterrupted connectivity.