Roadmap to optical internetworking - dynamic synchronous transfer mode, or DTM, as a bridge between circuit-switched networks and packet-switched IP - Technology Information

Fiber optics is one of the primary technical advances driving the development and deployment of next-generation networks. The advent of advanced fiber-optic technologies, such as wavelength division multiplexing (WDM), has dramatically increased raw capacity in a cost-effective manner for carriers and service providers.

However, fiber itself will not solve the challenges faced by network operators today. The challenge is not merely capacity constraints, in that it is not enough just to upgrade the networks. It is just the beginning of the Internet revolution in terms of number of new users. More importantly, we are barely beginning to understand these new users' demands for added services.

We are approaching the "New Services Era" which will drive the need for a generation of networks. Due to rapid advancements in technology and increased competition among the carriers, it has become more profitable for them to invest in new infrastructure instead of upgrading the existing infrastructure, which was not the case five years ago.

REQUIREMENTS FOR NEXT-GEN NETWORKS

It is becoming increasingly clear that the next-generation network will be IP-centric. There is little doubt that IP will evolve as the prevailing service layer, and the network has to be optimized to realize current and future IP services. However, during the transition phase, it is also important that legacy services be supported in an efficient manner.

The reality is that whatever capacity is placed on the modern packet-switched hardware, the operator can never guarantee quality of service (QoS). Internet telephony has a long way to go before it meets the high expectations of users spoiled by the reliable circuit-switched network. Disappointing quality will severely hamper the growth of VoIP and, ultimately, force the operators to manage two networks in parallel--at a very high cost.

Another requirement of the next-generation network is to scale in terms of capacity, so that the full potential of the fiber-optic technology can be released. The demand for bandwidth is expected to explode, along with additional Internet users and more advanced services. It is clear that IP directly over the fiber media will not be sufficient. There needs to be some framing mechanism for the IP packets.

The network also must offer superior availability as customers move more mission-critical information to the Internet. Availability translates into resilience and restoration features on one or more network layers.

Furthermore, the optical network should be able to offer manageability at high speeds, since traffic engineering and efficient billing will be crucial as differentiated services are offered on the Internet.

What is needed is a technology that enables the true convergence of data communications and telecommunication services in a single network. DTM (dynamic synchronous transfer mode) is positioned as a thin, highly scalable transport and switching layer between IP and fiber. The protocol is media-independent and can run on all major fiber-transport infrastructures, including dark fiber, WDM, and SONET/SDH point-to-point or ring.

Hosts connected to a DTM network communicate with each other on channels. The DTM channels differ from ordinary circuits by several means. The channels are:

* Simplex. DTM channels are simplex, which makes it feasible to achieve high bandwidth utilization with asymmetric traffic.

* Multirate. ADTM channel is a dynamic resource that can be set up with a bandwidth ranging from 512 kbps in quantum steps of 512 kbps up to the full capacity of the link. To each channel a set of network time slots is initially associated. The slots may be changed dynamically during the lifetime of the channel, which enables efficient network management and billing capabilities.

* Multicast. DTM channels are multicast by nature. This means that any channel at any given time can occupy one sender and any number of receivers. Over the network, any number of multicasting groups can be active simultaneously. This is important when distributing video or other multicast services.

* Fast setup. Signaling delay associated with creation and deletion of communication channels determines much of the efficiency of fast circuit switching. Therefore, DTM is designed to create channels fast--in less than a millisecond.

DTM is the only protocol designed to be used in integrated services networks, combining the quality of circuit switching with the flexibility of packet switching. For the first time, strict QoS in IP networks can be realized.

The inherent simplicity and predictability of the DTM technology allows for scaling to OC-768 and beyond, using simple and standardized hardware. Today's implementations support 10 Gbps without the use of any application-specific integrated circuits (ASICs).

Since DTM is based on time-division multiplexing (TDM), control and data are separated. While scaling the capacity of the network, the signaling overhead remains constant. Running a network with 20 nodes connected to a 2.5-Gbps DTM ring translates into a signaling overhead of less than 0.5%.

With the distributed switching architecture, the node will process only traffic that is actually being routed by the node. This means that the routers and switches connected to the network will be offloaded, as opposed to having the full capacity of the link terminated in the router, as in packet-over-SONET (POS) networks. As routers and switches evolve in next-generation networks, the distributed switching architecture becomes an important feature in scaling to OC-192 and higher speeds.

For the future of optical internetworking, it is not only a question of high capacity but also a means of enabling new services to the end user. Linking the optical capabilities to the IP level by using a DTM layer will provide an unmatched platform for new services and revenue opportunities.

Schagerlund is the CEO and co-founder of Dynarc, Inc., Sunnyvale, Calif.

COPYRIGHT 1999 Nelson Publishing
COPYRIGHT 2004 Gale Group