The accumulation of advances in many key enabling technologies has made it possible to conceive the implementation of an advanced NII today. Among these are advances in materials that underlie electronic components and optical/opto-electronic components, including optical fibers; advances in electronic integrated circuits, including both the speed at which these circuits can perform their functions, as well as the achievable complexity that allows a single chip to perform complex tasks; advances in signal processing techniques that use electronic circuits and software to convert information and information-carrying signals into forms suitable for transport over distance and for storage; and advances in systems that process, store and route information through networks. Even advances in technologies that many engineers and scientists take for granted, like batteries, have played a key enabling role. While advances in these enabling technologies have made it possible to conceive the implementation of the NII, significant further progress is required before the vision of affordable multimedia communication and information services for all citizens can be realized. In the following Sections 2.1 through 2.3, we will identify areas where further research in foundational, transmission and switching technologies is most needed.
A goal of the research on internetworking for the NII should be to leverage the knowledge and experience from existing telephone, cable television and computer communications networks to identify a key set of interfaces and protocols that will allow seamless and transparent uses of a heterogeneous NII by a diversity of applications and users. For example, as one reviews the success of the Internet, one can identify certain features of the existing research Internet architecture that have contributed materially to its success to date. In particular, one of the noticeable characteristics of the Internet design is its internetworking approach, which is based on an hourglass-shaped communication protocol hierarchy.
A wide variety of application and transport layer protocols rest atop the common Internet Protocol (IP) that forms the "waist" of the hourglass. Similarly, below this IP layer a wide variety of physical layer transmission and switching services has been incorporated into the global Internet over its 20-year history. This successful Internet feature should be taken into account in defining the internetworking architecture of the emerging NII. On the other hand, the wide diversity of applications and services envisioned in the NII, which go well beyond those of the current Internet, reveal areas where today's internetworking approaches are deficient and where there is basic uncertainty as to how to proceed.
Accepting the hourglass model as the organizing principle of the NII internetworking architecture, we state the following as a grand challenge: to maintain the hourglass architectural model demonstrated in the Internet, and at the same time embrace the full range of applications as well as the full range of communication networks and services envisaged for the NII. In Section 2.4, we offer further observations about those aspects of the Internet (and other network architectures) that have lent it longevity, as well as speculation on areas of deficiency that need repair. It is in these areas that new research initiatives in internetworking may bear fruit.
However, some kinds of modern electronic telephones that plug into outlets in residences and businesses will not continue to function during a commercial power outage because they do not include battery back-up subsystems. Furthermore, various types of multiplexing systems that may be employed on customers' premises in emerging multimedia applications may not provide even basic telephone services during commercial power outages unless they are designed properly. Fibers cannot carry power into homes and businesses except perhaps in very small amounts.
Alternative solutions are possible, including co-installed power buses to carry power into homes and residences from telecommunications networks, local commercial powering with battery back-up and lower-priced interruptible services. With presently available technologies, these solutions do not meet the cost and performance objectives implicit within the vision of the emerging NII. In addition, there is a significant need to improve power storage and use parameters for wireless (mobile) terminal equipment. Powering considerations severely limit today's wireless appliances in communications range, available bandwidth and usage time between battery charges.
Research and corresponding advances are needed in battery technologies (to increase energy stored per unit weight and per unit volume, to increase recharging speeds and the number of recharging cycles before the end of useful battery life, etc.) and in alternative approaches to meeting powering challenges.
Further advances in compression technologies may result from such approaches as "wavelets" and "model-based coding." Transport and switching breakthroughs in the future may very well be based, or at least depend, on advances in these areas.
The future (more than five years away) requirement for the NII is to provide interactive broadband access to the home capable of providing two-way, multimedia service from any point to any point, on an affordable basis. The main barrier to deploying broadband to the home is cost. Therefore, research efforts must be focused on technologies leading to lower costs. Examples of specific enabling subgoals are a fiber trunking architecture allowing the cost of the fiber termination to be shared over many subscribers; low-cost laser technology enabling the use of the high bandwidth of the fiber (compared to LEDs) and the installation of longer fiber loops (due to higher coupled power); and wavelength division multiplexing providing high-bandwidth, transparent connectivity in a signal/transmission-format-independent way.
Research in the area of PCS/cellular will ensure full personal communication capability in the NII with mobility, and would be targeted at extension beyond current PCS/cellular applications to 1.5-megabit-per-second transmission rates.
Research in the broadband wireless area could lead to elimination of the need for cable infrastructure for the last mile to the subscribers. Specifically, high-quality entertainment video and data communications with very large file transfer capabilities to residential and business subscribers are needed, especially in certain environments (e.g., urban). The transmission capacity would likely be asymmetric: hundreds of megabits per second downstream, hundreds of kilobits per second upstream per subscriber.
Interactive bidirectional satellite communications to residential and business subscribers (clients) for data communications (and possibly voice and video telephony) are also deserving of further research. This would supplement DBS technology, which already has been developed for entertainment video. Transmission rates would be 64 kilobits per second through 1.5 megabits per second per subscriber. Perhaps the most significant usage of this technology would be in rural and remote regions. Research is needed in the areas of antenna design, signal processing, and coding, modulation and multiplexing methods for very large numbers of subscribers per satellite transponder.
In the time domain the limits are determined by the speed of the electro-optic transducers, the speed of required buffers and memory, and the speed of the switching and control logic required to manage the system. In addition, high-speed regeneration technologies play a pivotal role in delivering the benefits of the time division techniques to the system.
In long-distance systems, fiber properties such as loss and dispersion in the fiber limit the capabilities of the fiber span. Devices such as optical amplification and dispersion compensators redress performance impairments induced by the fiber properties themselves. Wavelength division multiplexers, wavelength converters, and wavelength filters and routers enable use of more of the available capacity of the fiber. Optical regeneration techniques permit clock recovery and lead to full regenerative capabilities in the optical domain, avoiding unnecessary optical-to-electrical conversion.
NII networking in the five- to 10-year time frame will likely depend on hybrid electronic/optical switching systems with electronics and optics each utilized in areas where they have advantages. For example, optical switching fabrics could be combined with electrical control. Research areas would include low-cost electrical-to-optical conversion; techniques for combining lasers, optical receivers and digital electronics on the same substrate; and studies of architectures that make optimal trade-offs of electronics and optics.
Despite the fact that electro-optic switching is here today, and is likely to be the initial switching technology path for the NII, end-to-end optical transport and switching will eliminate the inefficiencies introduced by electrical-to-optical conversion. Preliminary research is under way, but much more is needed.
The high bit rates demanded of switching systems supporting the NII will require high-speed interconnections between circuit boards. These demands will quickly exceed the ability of electrical backplanes to carry this traffic cost-effectively. A solution may be the use of optical backplanes with electrical-to-optical conversion performed on the individual circuit boards. This would follow the evolutionary trend of fiber optics first being used to interconnect switching systems, then used to interconnect modules within switching systems, and next used to interconnect individual circuit boards within modules. Research areas in optical backplanes would include low-cost electrical-to-optical conversion, guided wave techniques and hologram techniques.
A crucial issue in enabling the NII is designing switch control systems that can be used in conjunction with a variety of underlying switch fabrics. Research will be required in areas such as identification of appropriate switch fabric/control boundaries and design of protocol sets for multi-switch synchronization. This decoupling of the switch fabric from switch control will enable these components to be upgraded independently, to take advantage of successive generations of technology. It will also allow the capacity of control systems to be engineered separately from switch fabrics. This is essential, given the unpredictable nature of NII traffic and the need to maintain maximum flexibility in matching control capabilities to user demand.
"Seamlessness" and "transparency" as desired properties of the NII raise a large set of issues, ranging from interoperability across subnetworks, to infrastructure mechanisms that support "plug-and-play" service portability. From the perspective of an application or consumer, achieving seamlessness requires defining a set of critical interfaces along each of a set of dimensions of internetwork service management. Those dimensions could include naming, addressing, accounting, billing, authentication and access control, resource discovery and management, and performance/QOS management, in addition to the recognized need for common internetwork transport protocols/interfaces. We postulate that, for each of these service management dimensions, key interfaces must be defined that achieve adequate economy of scope. Economy of scope results when an interface supports or hides a usefully large set of technology options below it (as IP operates over a large set of underlying network technologies) and when the interface supports a usefully large set of applications and application users above it.
This same hourglass notion applies in part to the Public Switched Telephone Network (PSTN). The PSTN has a global numbering plan and a simple service abstraction, the 3-KHz analog channel. The original application of this channel was limited to human-to-human communication in the beginning but has been extended to support applications like fax and modem transmissions in modern times. At lower layers, the 3-KHz service abstraction has been implemented out of a continually changing set of technologies, from direct analog transmission and switching through today's modern fully digital switched and transmitted ISDN networks. Unfortunately, the service abstraction of the PSTN is an analog channel, which does not map well into the sorts of applications that are realizable over end-to-end digital channels. It is clear that the service abstraction of the NII will have to be some sort of digital service.
Why do "hourglass" network architectures appear in such seemingly different networks as the Internet and PSTN, and is this type of architecture a good abstraction for the NII? There is some belief that an hourglass is an important network simplifying tool. Consider the problem of trying to make N different applications work atop N different network technologies. One can develop a common intermediate abstraction, the N mappings from applications to intermediate abstraction, and the N mappings from intermediate abstraction to technologies. For the price of 2N mappings, full N2 communication is possible. Furthermore, the addition of new technologies and applications requires only constant (unit) work.
Assuming there is a central "waist" in the NII protocol hourglass, one essential challenge is to define what functionality is already understood to be needed, what functionality might be added at this level to provide full NII capability, and then what techniques are needed for implementing these functions. The fixed and uniform functionality (i.e., the service abstraction) of the Internet layer, which has been projected over all supporting networks, might be extended in three ways: new basic features and functionality required of all underlying networks; new optional features and functionality supported by some but not all networks; and evolution over time of the service abstraction (i.e., a time-varying service abstraction).
Indeed, some networks may support some features and other networks other features. This makes for a vexing interoperability problem. If we break the uniformity of function at the Internet layer, we will have to introduce mechanisms to negotiate the achievable commonality between communicating end points (and intervening networks). This has been a very difficult area to address in the past, and there is no reason to think this is going to be any easier with today's technology.
A contemporary phenomenon is the rapid growth of commercial networks providing data communication services. The planning for future networks providing integrated services would benefit from knowledge, based on analyses of network models, on whether there are fundamentally important reasons to justify coexistence of diverse networks. Is it possible that the benefits of resource sharing and economy of scale are outweighed by the overhead of managing a network that must satisfy a very broad spectrum of quality-of-service requirements? Topics that need to be examined are the consequences on network design and operations of diversity in quality of service as measured by guarantees, response times and flexible allocation of bandwidth, reliability and error rates.
Another research question is whether or not a single NII protocol hourglass is necessary and sufficient. If there are multiple NII protocol stacks, can interworking between the stacks be accomplished at another level, and does this interworking lead to cost efficiencies, increased flexibility or better backwards compatibility and evolution?
The basic datagram service on the Internet is a best-effort service to deliver a variable-length packet from source to destination--without guarantee of delivery. Internet's longevity is due, in part, to the ability to derive this simple service over virtually any transmission and switching architecture. This notion is largely based on the notion of "encapsulation" of datagrams in lower-level network frames. We believe that encapsulation will continue to be fundamental to NII implementation.
While this service has served the current population of the Internet well, it is clearly insufficient for an NII that must carry all forms of communication. The NII will need to carry not only the computer communications traffic prevalent on today's Internet, but also real-time traffic such as voice and video. Many services we would like to offer within the NII framework will require some form of resource assurance or allocation. Internet's simple best-effort's datagram service can handle only limited amounts of packet voice and video, and only in an uncongested network.
Commercial networks have devised innovative algorithms for providing bandwidth on demand, i.e., for adaptively allocating uncommitted bandwidth to active users on the basis of feedback on the congestion levels at distant nodes. These algorithms are optimized for local traffic and network conditions. Interworking has been given little thought, partly because of the lack of means and partly because of a lack of information. In fact, heterogeneous conditions, and also history, have inevitably led to a variety of protocols and algorithms that sometimes interact destructively. A subject of research is to understand better the interactions of these dynamic processes and to develop tools for their analyses and specifications, in which the end goal is to make these processes cooperative.
A final research topic relates to the operation of IP-style protocols, with full addressing and routing, over networks that themselves provide these functions.