24.2 System Issues

The aim of this section is to discuss some technical issues which are at the basis of the system we have developed to support the distribution of mobile musical services to UMTS-enabled devices.

Some of the most prominent technical issues for the development of our system are those related to the problems of integrating UMTS wireless access technology with the Internet. It is well known that choosing UMTS wireless technology as a means to provide mobile access to the Internet poses a number of obstacles. In this context, the first problem is to decide if advanced TCP/IP (Transmission Control Protocol/Internet Protocol)-based applications will behave well over UMTS-type radio communications protocols. ^[11], ^[12] With regard to this fact, it is important to notice that the Internet TCP/IP protocol stack has not been especially designed for wireless communications. The standard (TCP) provides a sliding window-based ARQ (automatic repeat request) mechanism that incorporates an adaptive time out strategy for guaranteeing end-to-end reliable data transmissions between communicating peer nodes over wired connections. Because the ARQ mechanism of TCP essentially uses a stop-and-resend control mechanism for ensuring connection reliability, the question here is whether this mechanism may trigger a TCP retransmission at the same time the radio link level control mechanism is retransmitting the same data.

An even more significant problem of mobile wireless is that of temporary link outages. If a user enters an area of no signal coverage, there is no way that the standard TCP protocol may be informed of this link-level outage. ^[13]

After having considered all these challenges, an additional problem is strictly related to the internal architecture of those advanced Internet-based applications that should be accessed through radio interfaces. Those applications, in fact, must exhibit a high rate of robustness and availability, because mobile access to those applications should not be influenced by possible problems occurring on the Internet side.

To overcome these obstacles we adopted a number of important strategies:

In order to ensure both the availability and the responsiveness of our mobile musical service, we have structured our application according to the special technology of replicated Web servers. ^[14] Following this technology, a software redundancy has been introduced on the Internet side by replicating the multimedia resources across a certain number of Web servers distributed over the Internet. A typical approach to guarantee service responsiveness consists of dynamically binding the service client to the available server replica with the least-congested connection. An approach recently proposed to implement such an adaptive downloading strategy on the Internet side amounts to the use of a software mechanism, called the client-centered load distribution (C²LD) mechanism. ^[15] With this particular mechanism, each client's request of a given multimedia resource is fragmented into a number of subrequests for separate parts of the resource. Each of these subrequests is issued concurrently to a different available replica server, which possesses that resource. The mechanism periodically monitors the downloading performance of available replica servers and dynamically selects at run-time those replicas to which the client subrequests can be sent, based on both the network congestion status and the replica servers' workload.
Our wireless application has been structured following an all-IP approach, where a wireless session level has been developed additionally (on the top of the standard TCP protocol) to guarantee connection stability in case of possible temporary link outages.
As the download of musical resources over wireless links may experience long duration (and high costs), our wireless application has been designed to exploit the UMTS background service class. This is the UMTS service class with lower costs, because it has been designed for supporting non-interactive, best-effort traffic.

^[11]Staehle, D., Leibnitz, K., and Tsipotis, K., QoS of Internet Access with GPRS, in Proc. 4th ACM International Workshop on Modeling, Analysis and Simulation of Wireless and Mobile Systems, Rome, 2001, 57–64.

^[12]Kalden, R., Meirick, I., and Meyer, M., Wireless Internet access based on GPRS, IEEE Personal Commun., 7 (2), 8–18, 2000.

^[13]Huston, G., TCP in a wireless world, IEEE Internet Computing, 5(2), 82–84, March-April, 2001.

^[14]Ingham, D., Shrivastava, S.K., and Panzieri, F., Constructing dependable Web services, IEEE Internet Computing, 4 (1), 25–33, 2000.

^[15]Ghini, V., Panzieri, F., and Roccetti, M., Client-centered load distribution: a mechanism for constructing responsive Web services, in Proc. 34th IEEE Hawaii International Conference on System Sciences, Maui, 2001.