ATM/Sonet technologies converge voice, video, and data services
Combining Asynchronous Transfer Mode and Synchronous Optical Network technologies allows placement of diverse voice, video, and data services on a single fiber-optic network and enables a higher level of network integration and control
Steven J. CLENDENING
fujitsu network communications inc.
Wide segments of the North American telephone network have been deployed using Synchronous Optical Network (Sonet) technology and network elements that rely on fiber-optic transmission. These network elements can transport a variety of services; most deployed Sonet equipment uses Synchronous Transfer Mode (stm) and time-division multiplexing transmission technologies. However, industry analysts surmise that atm technology is expected to advance the integration of voice, video, and data communications.
To globalize their networks, network planners and providers are incorporating the Synchronous Digital Hierarchy (sdh) standard--the international counterpart of Sonet--as the basis of virtually all optical network installations outside North America. This worldwide deployment forces additional interworking issues between Sonet and sdh. Fortunately, due to the maturity of the sdh and Sonet standards plus recent technological advances, the two network technologies can be directly interworked. Moreover, by being able to transverse these global networks, atm technology enables the integration of diverse services all over the world.
Network desegregation
Although telephony and data networks sometimes share infrastructure, they are normally managed and operated separately, even within the same network provider`s organization (see Fig. 1). The video network is typically provided as a separate platform. Consequently, the three individual networks lead to a duplication of effort in operation, management, and maintenance. For example, a regional Bell operating company furnishes the telephony network; a cable-TV provider supplies the video network; and a separate service provider delivers the data network.
Increasingly, these communications networks are being built on fiber-optic network technology using the Sonet standard. Due to the diversity of applications, vendors, and deployment timing of the telephony, data, and video networks, the three network types are generally designed and installed separately. Some of the reasons for these deployments can be attributed to differing regulatory rules and the unavailability of atm technology, standards, and products. However, the size and technological maturity of the telecommunications market have now increased to the level where integrating these networks makes economic sense.
The atm payload can be mapped into network elements and transported by the Sonet core network. This payload is most efficiently transported by large-bandwidth, fiber-optic transport networks. Because Sonet transmissions can be upgraded from 51 Mbits/sec to greater than 10 Gbits/sec, they provide an economical and upgradable transport platform for networking. Moreover, the overall bandwidth and the low-loss characteristics of fiber-optic technology allow an economical, lower-speed interface to be deployed initially, which can then be upgraded over time without having to change the optical facilities.
Because atm technology can operate independently of Sonet and sdh technologies, the high-speed OC-3c/stm-1 interface is ideal for interworking. Thus, carriers can build flexible global networks operating at 155-Mbits/sec that span different countries and services. Furthermore, fiber-optic technology and standards provide a flexible network architecture that allows growth commensurate with the expected increase in global information exchange.
To simplify diverse voice, video, and data networks, communications services could be integrated into fewer networks, eventually resulting in a single unified network. atm technology can integrate these services and offers the following benefits:
It allows diverse services to be mapped into a single cell stream. Data, video, and voice signals can be mapped into cells traveling inside the same physical medium. This task is accomplished in a standardized method referred to as aal (atm adaptation layer). As standards evolve, adaptations for new services can be developed on atm, thereby avoiding the addition of another network for each new service.
atm technology is rate-adaptive--the cell speed can be varied depending on service requirements. In today`s stm networks, the connection speed typically cannot be varied without replacing network equipment. Increasing network speeds from "plain old telephone service" to Integrated Services Digital Network, DS-1 (operating at 1.544 Mbits/sec in the U.S. and 2.048 Mbits/sec in Europe), and DS-3 (operating at 44.736 Mbits), requires drastic changes to the access network. In the case of atm, the cell speed is set at the time of call setup and can vary widely between connections.
An example--and some problems
Consider a near-term application that integrates atm, Sonet, and sdh technologies to form a cohesive global network (see Fig. 2). In this setup, Sonet serves as the core network transport infrastructure, and sdh performs as the secondary transport technology. The reverse situation is equally viable in countries outside North America, where sdh is the dominant transport technology in the core network, and Sonet acts as a secondary feeder or access device at the edge of the network.
This network example is facilitated by the convergence of Sonet and sdh standards and by mapping algorithms for placing atm cells into a Sonet/sdh payload. With careful equipment design planning, the differences between Sonet and sdh technologies can be mitigated. The more significant differences that need to be interworked are the following:
Bits 5 and 6 of the H1 byte, known as the `ss` bits, are not defined in the same way for the Sonet and sdh standards. The differences have been maintained over the years to support compatibility with deployed equipment. To allow interworking of Sonet and sdh technologies, how ever, the newer standards recommend that the Sonet and sdh network elements ignore the `ss` bits.
Most sdh traffic runs at 2.048 Mbits/ sec (E1) mapped to the tug-2, tug-3, and AU-4 levels. At the high-speed rate, only one pointer value exists at the AU-4 level. In the case of Sonet, DS-1 lines are mapped to a VT1.5 line, which is then mapped to an sts-1 (52-Mbit/sec) line. Therefore, for an OC-3 interface, basically three high-speed pointers are used, one for each sts-1 line. This approach leads to a basic incompatibility in Sonet-to-sdh interworking. That is, the sdh equipment does not recognize the three separate point values. In the reverse transport direction, the Sonet network elements are capable of transporting an OC-3c (155-Mbit/ sec) signal. That is, three sts-1 signals are concatenated into a single payload with a resulting single pointer value. It is therefore possible for a Sonet network element to transport an stm-1 (155-Mbit/sec) signal as an OC-3c signal by using only the single pointer value. sdh-to-Sonet interworking uses an OC-3c signal, whereas the Sonet-to-Sonet interfaces usually employ an OC-3 signal (see Fig. 2).
Note that the internal payload of the respective sdh or Sonet signal is not being modified. Rather, the existing infrastructure is used to transport Sonet and sdh traffic seamlessly. Extracting an individual payload and converting it into the other type of signal--that is, converting a DS-1 signal to an E1 signal--is a complicated process.
Today`s networks rely primarily on an electrical interface at the core network switching elements, as shown in Fig. 1. This necessitates placing additional transport network elements by the switch to provide the optical-to-electrical conversion at the central office. To simplify this conversion, full Sonet/sdh optical interfaces are used directly on the switch. This method involves moving several functions that traditionally reside in the transport network element into the switch element. These functions include
survivability through 1+1, unidirectional path-switched ring/bidirectional line-switched ring, or meshed protection algorithms,
data communications channel termination and integrated management of optical transport functions and alarm reporting,
possible termination and control of user channels such as orderwire and user data channels.
This method could prove complicated, since there could be hundreds of optical interfaces on a large switch. A large load would be placed on the switch`s processing power--but it would simplify network management if the adjacent network element could be eliminated. The combined switching and optical transport element is referred to as the network node (see Fig. 3). Note that the architecture in this approach is much simpler than that of the network shown in Fig. 1. u
Steven J. Clendening is senior manager, transmission strategic planning, at Fujitsu Network Communications Inc., Richardson, TX.