The metamorphosis of the network – Bell Canada plans ahead for a state-of-the-art broadband network
The metamorphosis of the network
Most field trials for the narrowband integrated services digital network are nearing completion, and in some cases services already are being offered to customers.
Bell Canada has conducted a two-year trial for basic rate ISDN involving more than 300 stations. A primary rate trial is currently under way involving over 1000 stations. Commercial service should be available soon.
Although narrowband ISDN serves a market need for integrated voice-data services up to 1.5 Mb/s, there also is a longer term need for user access to bandwidths of 150 Mb/s and eventually up to 600 Mb/s for video communications and intercomputer communications. With 600 Mb/s, local, metropolitan and wide area networks can communicate with each other.
Today’s computer networks can communicate over short distances at multimegabit speeds through such devices as specialized bus interconnections or LANs. For diverse systems to communicate over large distances requires a public, widely available, standardized network that can accommodate these broadband requirements.
These market trends herald the coming of a broadband era, fueled by the ever-increasing power of computers to process and exchange data, the continuing computerization of commercial systems and the prevailing tendency toware more natural face-to-face communications involving simultaneous voice and video. These factors dictate the need for multi-media network services that can convey large quantities of data on demand (Table 1).
The availability of broadband services will stimulate new applications that exploit these capabilities. In the business market, broadband will be used to interconnect high-speed LANs from different locations to form MANs or WANs. It will be possible to offset distributed computing by interconnecting computer components such as disk drives and high-speed printers via remote broadband connections. In the mass market segment, entertainment services such as pay-per-view television and video telephony will drive the deployment of broadband capabilities all the way to the home.
The development of broadband technologies is progressing in all the key areas: photonics, software and microprocessors. Fiber transmission systems are accepted and widely deployed in the core network. There are many ongoing trials for the use of fiber in the loop plant. The conversion from copper to fiber is a vital step on the road to broadband since copper cannot practically transmit information above the 2-Mb/s range. The capacity of fiber, however, is practically limitless. More importantly, fiber can be easily upgraded to higher speeds by simply changing the optoelectronic components, a relatively inexpensive and simple upgrade compared to the magnitude of the total implementation.
Suppliers increasingly are concentrating on modular structures for the software and hardware components of their switching products. This emphasis is vital to satisfy pressing requirements for open network architecture in the United States and to lay the foundation for intelligent network structures. Modularity also helps to satisfy carriers’ need for backward compatibility and longevity of their investments because modular systems allow changeouts as technology advances.
A planned evolution of the switch software architecture is the best approach and will not cause costly discontinuities. The trend is to separate the switch software into layers so the application software is independent of the underlying logical switching functions, which are common to any or all services.
In theory, applications software could then be made to run on switches from different suppliers, using a standardized interface between the various software layers. This move will facilitate the development of services, especially for carriers that use switches from several suppliers. It will significantly affects costs and be a major impetus for system enhancements, since software costs already are the dominant and growing component of overall system costs. The drive toward increasing software modularity is facilitated further by ongoing progress in the development of international open systems interconnection standards. These standards comprise logical structures that can be used to layer the software along well-defined lines for internal and external system operation.
Bell Canada’s strategy is to transform the narrowband network into a broadband-capable network through a process of progressive evolution. The objective is to develop a network infrastructure that can cost-effectively meet the telco’s needs for current and nearterm new services, and also has inherent broadband capabilities. This infrastructure will provide platforms that can be augmented with additional broadband features as technology develops and customer needs arise.
The infrastructure includes switching, access, transport, signaling and management elements. These elements consist of equipment and systems that can be enhanced through the application of additional broadband modules or other, similarly uniform upgrades, with minimal additional cost and minimal disruption to the network architecture.
Switch modernization is key in the march toward broadband services. The deployment of appropriate digital switches, which are cost-effective now for basic telephone service, will lead to a ubiquitous, intelligent network fabric with broadband capabilities. The switch must be properly structured, with modular components that can be individually upgraded and can evolve from the DS0 base to broadband operations in step with service needs.
For example, the core processing module should have, or be upgradeable to, the switching and throughput capacities required for broadband. There should be control structures to assemble variable bandwidth channels in multipes of 64 kb/s and to manipulate bandwidth at broadband levels.
It should be possible to interface subsequently developed high-speed peripherals with photonic interfaces and to evolve to multi-fabric switches by plugging in new types of network switching modules such as for broadband-level asynchronous or synchronous transfer mode. Figure 1 illustrates this type of switch architecture.
Bell Canada has chose Northern Telecom SuperNode technology to meet this objective. The switches are being used in all new network installations, and the telco has an aggressive retrofit program for replacing existing non-digital switches.
The replacement of electromechanical and non-digital, stored program control switches results in operational savings without any reliance on new broadband services. Although the SPC technology was available in the 1970s, the telco decided to limit its deployment in anticipation of fully digital switches. These latter systems now have been developed into proven and stable platforms that can form the basis for the service and operational requirements of the future, including broadband.
Out of a total base of 7.9 million working lines, almost 50% already are digital. Through a switch modernization program that will replace 660,000 working lines per year, the telco’s lines will be 90% digital by 1995. In addition, various components of existing DMS switches are being upgraded with SuperNode technology as capacity and service needs arise.
For example, the processor module is being upgraded to provide a twofold improvement in call-processing capacity with the potential for higher gains if necessary. The network switching matrix also can be upgraded to a non-blocking network with the potential to achieve a twofold to fourfold increase in switch channel capacity.
The access network must be upgraded from its copper wire base to deliver broadband from the switch to the end user. The only practical means to do this is through the use of fiber technology. In addition, the access network architecture must be designed to distribute broadband services in a way that efficiently uses overall network bandwidth, that is integrated with narrowband services and that allows cost-effective delivery of basic telephony and other narrowband services. It also must support multiservice interfaces to the user while maintaining acceptable levels of service continuity.
The Access Network
The present access network is based upon the serving area concept. New growth is accommodated on remote distribution terminals connected to the local switching office by fiber. This arrangement is a “double star” configuration, using the RDT to create a copper-based carrier serving area.
Fiber eventually will be used to connect users to the RDT. The RDT will be placed either within buildings for large concentrations of users or externally as a community distribution point. Service continuity can be enhanced by appropriately interconnecting the RDTs such as in a ring configuration.
Fiber to residences and small businesses, known as residential fiber access, will likely occur during the early 1990s and will be based on the shared fiber concept, using either passive optical splitters or active pedestals. Large business buildings and high-density applications such as apartment complexes will use RDTs located on the promises. Service continuity in this case also could be enhanced by appropriately interconnecting the RDTs in a ring configuration.
For major business applications, the telco’s objective is to place fiber in more than 300 strategic feeder routes by 1991. Right now this program is 50% complete. By exploiting service triggers based on growth and churn, the fiber is being taken to all the major buildings along these routes and will be terminated by RDTs. This program will lay the foundation for a broadband distribution network. The fiber feeder can be subsequently upgraded to higher speeds, the customer interface loop can be changed to fiber and the RDT can be upgraded with a broadband application.
Bigger and Better
In a broadband world the transport network between switches must have significantly more capacity than today’s network. It also must be more dynamic and flexible to provide on-demand bandwidth and to react to rapidly varying bandwidth requirements. Finally, it must be self-healing to counteract failures.
Bell Canada is planning a high-capacity transport network using synchronous optical network-based systems. It will consist mostly of fiber routes, with some Sonet-compatible, high-performance digital radio for areas with difficult terrain. Sonet provides the capability for enhanced operations, administration and maintenance features and extensive flexibility for assembling and manipulating bandwidth.
For instance, channels of varying capacity can be formed through the concatenation of virtual tributaries and STS-1s. VT and STS-1 are signal rates equivalent to DS1 and DS3, respectively. The synchronous nature of Sonet systems allows for drop-and-insert capability at intermediate points that can be used to “groom” the component channels and engineer high levels of network survivability.
Currently, Bell Canada has 500 kilometers of fiber systems in its toll transport network, representing 13 million effective voice frequency channel-kilometers. By 1995, 70% of the toll network will be fiber. Most of the FO transmission systems transmit at 565 Mb/s on a single-mode fiber pair, which paves the way for the introduction of Sonet-based systems, within the next few years, with speeds up to 2400 Mb/s. This transition will be smoother and more economical if it is possible to interface with existing non-Sonet transmission systems such as asynchronous DS3 through Sonet-compatible digital cross-connects. The components of the transport network will be controlled and operated through a standard, networkwide operations support system that can perform automatic traffic rerouting and dynamic load balancing.
The final component of the broadband network infrastructure is signaling. A sophisticated signaling system is essential for multimedia broadband services through the network. The signaling requirements are between the network and the user, between the network and the network and between end users themselves.
Broadband services will require the allocation of resources on a dynamic basis throughout the network. Signaling information must be transmitted efficiently and reliably using a standard format.
Bell Canada also is deploying signaling system 7 throughout its network. The rollout is phased in based on the introduction of corresponding services such as call management services, which are similar to custom local area signaling services in the U.S., automatic calling card systems and 800 + services.
By the early 1990s, SS7 will be progressively rolled out to most of the telco’s digital and SPC switches, for database access and trunk signaling, in both the toll and the local portions of the network. The Bell Canada SS7 network will consist of two toll signal transfer point pairs and four to five local STP pairs.
What About Services?
This article has examined the evolution of the technological infrastructure leading to broadband. Such a progressive transformation would allow a related progression of new network service capabilities. For instance, the Sonet-based transport network allows service opportunities for broadband private lines at DS3 rates or higher. With respect to switched broadband service, opportunities exist for the introduction of a network-based LAN interconnection service, employing the IEEE 802.6 MAN standard. Such a service can interconnect high-speed LANs at 150 Mb/s or higher to form MAN or WAN user networks.
Similarly, today’s circuit switching services are limited to 56 kb/s. By upgrading to an enhanced switching matrix, it will be possible to switch channels with bit rates of 1.5 Mb/s, in multiples of 64 kb/s.
The eventual deployment of an ATM switching module and Sonet peripherals, superimposed on a Sonet transport layer, will provide a powerful structure for the realization of various types of end user services–circuit, packet, frame or burst mode–at speeds up to 600 Mb/s (Figure 2).
Strategic network modernization is necessary to meet the future broadband market requirements and to satisfy the ongoing market needs for timely and cost-effective services. Modernization will align the evolution of the major network elements to achieve near-term economies and market objectives, while at the same time embedding inherent broadband capabilities into the network.
The goal is to meld and coordinate the characteristics of switching, access, transport, signaling and OA&M to form a broadband-capable network infrastructure. End-user service can be developed and augmented progressively, in conjunction with the advancing capabilities of the network, leading eventually to a range of broadband services of various switching modes.
Bell Canada has an active program of network modernization to generate new revenues and reduce costs. A modern digital network will provide the basic platforms for incremental and efficient addition of technology that can provide sophisticated broadband-level services.
Anthony Lavia is Assistant Vice President-Business Development for Bell Canada, Ottawa.
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