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Feature Article
April 2001


IP Over Optics: Where Are We Going With It?


[ Go Right To Sidebar: The Great VoIP Migration ]

Today's networks are struggling to meet the demand created by new users, new technologies, and new high-bandwidth applications. To meet this expanding market demand, carriers are re-evaluating how they architect their networks.

Traditional networks -- developed using a multilayer model where layers are built and managed independently -- make it difficult for carriers to operate, optimize network resources, and provide networks with cost-effective reliability. An IP network, for example, may run over a frame relay network, which runs over an ATM network, which runs over a SONET/SDH network, which runs over an optical (wavelength) network, and eventually over fibers. This multilayer model allows each layer to evolve independently, while continuing to support legacy services.

However, the large number of different devices in the network complicates the development, deployment, and management. In addition, each layer of the network typically has an independent management structure and associated processes that only have visibility of the topology and state information of that one layer. The number of management systems increases network cost while adding complexity to network-wide operation tasks such as provisioning, performance monitoring, and fault isolation. These challenges are the driving forces behind the need for a much simpler IP over smart optics architecture, where the vast majority of services -- voice, data, video, and private line -- are all carried through IP.

To make the transition from today's complicated multilayer network architecture to the much simpler IP Over Optics architecture, three key enablers are needed:

A standards-based intelligent optical layer -- Today's optical networks are mostly static. Services take a long time to provision, networks are susceptible to manual errors, and service providers must overprovision their networks to avoid congestion and provide carrier-grade availability. When bandwidth grows fast and unpredictably, there is an urgent need to switch optical bandwidth so that a lightpath should be set up and torn down when and where needed. The International Telecommunication Union (ITU) standard for Automatically Switched Transport Networks (G.ASTN) provides dynamic optical connectivity though automatic routing and switching of transport bandwidth. With G.ASTN, it is possible to sustain a simpler and less resource-intensive network with automation. Dynamic connectivity leads to significant reduction in capital, operational, and maintenance costs for both service providers and end-users.

The packet layer needs to be unified, from today's multilayer or separate networks to one network that runs the same protocol -- The Multi-Protocol Label Switching (MPLS) standard developed by Internet Engineering Task Force (IETF) allows service providers to carry potentially all traffic types over one universal packet core while continuing to provide ATM-like Quality of Service. The MPLS standard is based on the blending of connectionless protocols, like IP, with the virtual-circuit networking concept. The term "IP Over Optics" is indeed for new generation switching routers running IP or MPLS over intelligent optical switches.

Interworking of the packet and the optical layers -- With interworking, it is possible to monitor both packet and optical layers to recommend or automatically make changes to either the packet layer (create or tear down MPLS paths) or the optical layer (create or tear down new lightpaths). The optical layer can then dynamically adapt to the change of traffic pattern in the IP layer. The key issue is how to exchange routing information and control signaling between IP networks and optical networks.

New IP Over Optics architecture proposals can be mainly classified into overlay model and peer model. The overlay model has separate routing and signaling protocol sets for each layer, while the peer model has a single monolithic routing and signaling protocol set spanning both the IP and the optical layers. In the case of the overlay model with separate protocol sets for each layer, models can be further distinguished by the level and mode of interaction between the two layers.

Typically, accessing the optical layer is done through user-network-interface (UNI) while the interconnection between the optical components is done through network-network-interface (NNI). In the peer model, IP routers and optical switches act as peers. There is no distinguishing between the UNI and NNI, or between UNI and other router-to-router protocols. A single routing protocol runs across both layers to discover the topology of IP routers and optical switches. The IP and optical layer can be truly integrated. Over the short term, however, this seems unrealistic to implement, because it requires a single overall control plane, and consequently requires the optical switches to become as intelligent as the IP routers. The optical switches would have to be able to discover topology, and signal lightpath setup as easy as routers.

The overlay model is similar to traditional IP over ATM or ATM over SONET models. The main benefit of this model is that the optical layer can independently define its own control plane to support a variety of services such as wavelength private lines, optical virtual private networks (OVPNs), and transport bandwidth brokering. The optical layer control plane can also adapt to the optical layer infrastructure, be it electro-optical like today, or migrating to all-optical in the future.

Another advantage of the overlay model is isolation of IP and optical layer information. This allows for different life cycles and evolution of the two layers, and makes the implementation and deployment of UNI and NNI more straightforward. Finally, this model can be applied to incumbent or new multiservice carriers who either own or lease their transport facilities.

The overlay model does have overlapping functions in the IP layer and the optical layer, however. For example, both layers have the function of topology and reach-ability discovery, which leads to low efficiency. In addition, the intelligence in both layers might cause a coordination problem in areas such as protection and restoration.

To improve the coordination, information from one layer must be passed into the other. An enhanced overlay model (Overlay Plus) could be based on a trusted intelligent layer in between the IP and optical layers that uses topology and status information from both networks to prepare policy-based (e.g., SLA, revenue...) end-to-end connection and release requests. These requests can be rapidly invoked to avoid network abnormalities such as congestion and failures, increase infrastructure utilization, and automate engineering by rebalancing the network and forecasting needed resource upgrades (such as node and link capacity) for both the IP and optical layers.

Many vendors are working on products and solutions that usually classify themselves as "IP Over Optics." The meaning of the term "IP Over Optics" has moved away from the simple physical connection of router and optical equipment toward the full interworking and integration of the control and signaling as well as the management of routers and optical switches. The intelligence introduced to the picture with technology innovation is changing the way many carriers operate. Therefore:

  • Carriers should consider building or migrating to a simple IP Over Optics network architecture, with the three key enabling technologies: MPLS, G.ASTN, and Packet Optical Interworking.

The new network architecture with agile and auto provisioning will reduce operating costs as fewer personnel are needed to operate the network. The simple architecture should also help in reducing capital cost. New technology like MPLS overcomes the scalability problem of ATM or TDM networks. It is also easier to grow the network, thanks to the automation of topology discovery and end-to-end path establishment/tear down. Coordinated and unified cross-layer protection can lead to differentiated SLA options. Such scalability and reliability eventually means more revenue to the carriers.

  • Carriers should choose a model based on their existing network architecture, network evolution plan, and service offering.

With the variation in today's service provider environment, which model applies will likely depend on a number of factors such as: Metro versus regional versus long haul networks; incumbent versus new providers; multiservice versus IP centric providers; and facility ownership.

More importantly, the maturity of technology and practicality of deployment in service providers' network should be carefully examined. The overlay model and its variations offer the smooth evolution from today's network, to a fully integrated IP Over Optical network. While the peer model represents the future of network architecture, it is not realistic to implement it today for the reason mentioned earlier.

Some other approaches call for a smart IP layer to attempt to use rudimentary impairment and fiber-type primitives of a relatively dumb optical layer, to assess what lightpath connections can be established. These approaches ignore the fact that optical network is quickly developing its intelligence and is emerging to provide rich optical services directly, and may consequently bypass the IP layer.

  • Carriers should fully leverage new network applications enabled by the new architecture and network to differentiate their service offerings from competitors.

The new IP Over Optics architecture opens a whole range of new network applications such as protection/restoration. A number of protection mechanisms exist across packet and optical layers including guaranteed 1+1, protected 1:N, non-pre-emptible unprotected, and pre-emptible unprotected. Carriers can choose to protect their networks at the packet layer only, the optical layer only, or a combination coordinated through cross-layer interworking intelligence. Such flexibility allows carriers to provide rich SLA options to their customers.

The other application is resource allocation that provides optical resources for the packet layer in an automatic manner. Capacity is added when needed, then relinquished as an available resource later when the need subsides.

All of these can be performed dynamically and automatically to enable service providers to provide faster, better services to their customers. With a new network architecture and maturing of the technology, more network applications will be discovered and consequently benefit both the carriers and their customers.

Dr. Billy Wu is senior manager of technology marketing for Nortel Networks' Optical Internet business. He can be reached at [email protected]. Nortel Networks is a global Internet and communications leader with capabilities spanning optical, wireless, local Internet, and e-business. 

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The Great VoIP Migration


History is marked with many migrations that have forever changed the face of the earth. Today, we are on the verge of another great migration -- one that will drastically change the way we communicate.

Communications have gone through immense changes in the last one hundred years -- the development of Morse code telegraphy, telephony, radio, television, cable TV, pagers, e-mail, and wireless communications -- each advancement has brought new devices and procedures, along with a particular set of limitations. By now, each of us has myriad communications devices: Several telephones in many locations, multiple mobile phones, pagers, PDAs, PCs, answering machines, and fax machines. So what's next? More appliances and disparate modes?

Fortunately, the next revolution in communications -- VoIP -- will bring diverse methods of communications together, enabling us to communicate more effectively and efficiently. More importantly, this "communications revolution" will deliver control, allowing individuals to easily select when, how, and where they wish to communicate.

In The Beginning
VoIP began in the center of the network, providing the ability to make high-quality telephone calls between phones using the public or private Internet Protocol (IP) networks. This development shattered a monopolistic model that enabled carriers to levy steep charges on calls made across countries or between carriers.

As IP networks were connected, the clearinghouse function was conceived to determine which network should be used to complete a call. As a result, the early stages of VoIP allowed for cheap, high-quality calls that could originate and terminate anywhere with just a plain telephone.

The next steps of VoIP include migration of VoIP to the network edge, and adding features and applications to span various existing networks, making communication easier, better, and more complete.

To The Edge
VoIP technology is primed for migration from the network to individuals in the form of cable and DSL modems, wireless local loops, IP phones, PCs, and iPBXs. Most importantly, softswitch technology exists that can power this move and still provide the useful call processing features that individuals are comfortable using today, such as call waiting, call forwarding, calling number identification, and three-way calling.

Essential to the softswitch (and therefore migration) is scalability, meaning a carrier's customers can grow from hundreds to millions, and built-in provisioning, billing, maintenance, and management capabilities make it possible to administer the larger networks. In turn, softswitch technology is based on a distributed, layered architecture that allows features, processing power, and memory to be added where needed as business demands evolve.

What does this flexible architecture mean for the communications migration? The softswitch architecture is open, allowing innovative applications to be added and interface with existing capabilities of VoIP. Existing PSTN or wireless networks are proprietary, with no easy way to add applications. VoIP network elements can easily communicate with the intelligence in other networks such as mobile or corporate networks. Additionally, the software in the VoIP network is simpler, more structured and easier to modify, making it easier to produce APIs (application programming interfaces) and add service execution programs into the network.

Viva La Revolucion!
VoIP will revolutionize the way individuals communicate, migrating from the depths of the network to the edge and giving form to innovative applications, while streamlining our currently disparate modes of communications. And, applications will exist throughout the VoIP network to provide capabilities and control across all networks and end devices. 

Dr. Peter Bohacek is senior vice president, business development and product marketing for Clarent Corporation. Clarent is a leading IP telephony technology supplier to mainstream carriers and service providers. Clarent's strength and advantage comes from its customer experience and market position; its products and overall technical architecture; the application of its technology in the marketplace; the quality, skills, and experience of its employees and executive team; and its solid financial backing.

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