Managing the light

Despite some carrier concerns about forgoing quality in metro area networks, IP over DWDM is proving to be a viable alternative to Sonet

The debate rages on over the makeup of the next generation network. Will it be IP over ATM, IP over Sonet or IP over DWDM? Increasingly, the last scenario looks the most promising. Dense wave division multiplexing already benefits long-haul networks and is making significant inroads into the metro market. Furthermore, because the majority of originating and terminating traffic is packet based, it is the most cost- and technology-effective way to create the all-optical metro network.

So given the obvious benefits of DWDM for networks, why is rapid deployment not occurring as quickly as one would think? The answer is guaranteed quality of service (QOS). Quite simply, metro service providers are not yet convinced that they can incorporate DWDM or replace tried-and-true Sonet upgrades with DWDM, while keeping Sonet-like service commitments (Telephony, April 19, page 26).

However, those fears are losing their basis rapidly. Today, DWDM offers a viable alternative to Sonet - even for service providers concerned about guaranteeing QOS. Metro DWDM now is capable of monitoring performance, providing protection and provisioning optical channels - highly valued and expected Sonet-like capabilities.

The state of the market

Local exchange carriers (LECs) are facing an increasing amount of competition. Every day a new competitive LEC (CLEC) or utility company enters the telecom m arket, eager to grab business away from an incumbent with new services at lower cost. The clear winners will be the carriers that are able to guarantee better services at a reduced cost.

The profound shift of power from service provider to customer means that carriers no longer can expect users to front the cost of running data over their networks. Specifically, carriers no longer can demand that customers purchase an ATM router or a Sonet interface to connect to their networks. With thousands of new bandwidth providers entering the market - many that are able to carry traffic in its native Ethernet- or IP-based format - a carrier no longer has the luxury of requiring customers to pay for unnecessary protocol conversion or aggregation equipment.

Subscribers want to connect at the current speed of their backbone, whether that is fast Ethernet, switched-fast Ethernet or gigabit Ethernet. They want to make a direct connection, through metro and even long-haul networks, with the associated protocols. And increasingly, subscribers do not need - nor do they want to pay for - Sonet equipment.

Power to the subscriber

Today's customer demands service guarantees. In a typical network, a customer would buy a T-1, T-3 or OC-3 pipe and a dedicated amount of bandwidth on the carrier network. Because of its reliability, service providers typically have used Sonet to guarantee QOS.

Today, however, carriers can provide those same QOS guarantees to subscribers by using an IP-over-DWDM solution. The new generation of IP-based switches provide the capability to meet QOS commitments. Built-in Layer 3/Layer 4 switching services allow the switch to prioritize and guarantee packets based on pre-determined criteria within a switch, and higher-layer protocols, such as RSVP, can reserve bandwidth across an entire network (Figure 1).

Furthermore, the carrier now can sell unlimited amounts of bandwidth, while incurring far less costs in constructing the underlying network. This creates a value proposition for the service provider: It can deliver higher bandwidth - in a format the user wants for less cost - while approximating the QOS guarantees the end user expects for high-priority traffic. With IP-based systems, a carrier can use statistical multiplexing to over-subscribe its network bandwidth, which can be four, five or even 10 times the network's aggregate capability. Therefore, the carrier can sell 10 Gb/s of bandwidth even though the underlying network may support only a total of 1 Gb/s of bandwidth. Bandwidth reservation and intelligent IP switches can prioritize voice and video traffic to ensure that high-priority traffic gets the first shot at the underlying bandwidth.

Using IP over DWDM for voice, video or data also enables carriers to provide better service because less equipment is in the network and, therefore, fewer conversions are necessary. This decreases the time it takes to transport voice messages or real-time packets from Point A to Point B because there is less aggregation and no conversion. If Ethernet goes in through the multiplexer, then Ethernet comes back out on the other side. This makes it easier to provide better QOS. When service providers need to aggregate traffic, next generation switches enable true QOS to be implemented at the IP layer. Reservation protocols and switches that didn't exist a year ago now are sophisticated enough to guarantee this service.

Criteria to consider

The same protocols meet QOS requirements for both Sonet and DWDM. If a Sonet network fails, it has built-in protocol recovery steps to detect service outage and begin recovery. IP-based protocols, such as gigabit Ethernet, traditionally have not provided the same level of recovery protocols. When the customer picks up the phone, regardless of whether the network is IP over DWDM or Sonet, the subscriber should not detect a network break or service outage. Even if construction work damages or cuts a fiber, calls should not be lost. The key is to provide Sonet-like protection and restoration services with a combination of IP and optical layer recovery services.

When addressing network manageability, service providers must emphasize reliability. Providing better value to subscribers by reducing cost also improves reliability. IP over DWDM offers greater reliability because it requires less equipment in the network, and, therefore, less hardware to manage and repair.

Speed is another issue. IP over DWDM does not require the conversion steps of Sonet or ATM, so it offers faster detection and restoration of service interruptions. Instead of taking 30 to 60 seconds to re-converge routes, newer protocols can detect and re-route packet streams in less than a second. Combined with optical layer protection that can detect and restore fiber failures within 25 seconds, IP-over-DWDM finally becomes a viable alternative to Sonet at a small fraction of the cost and at the complexity of an equivalent Sonet network (Figure 2).

Because the support of carriers' management interfaces is critical to widespread acceptance of DWDM, vendors already have begun to address this issue. Many larger carriers have trillions of dollars of installed equipment, so any new equipment must interoperate with legacy equipment. In contrast, an entirely new network can be maximized using IP because the reduced complexity of the network leads to a simpler management structure. In fact, many of the protocols that are commonly used in LANs, such as SNMP, can provide end-to-end management over a large regional DWDM network, effectively creating an extended enterprise management system.

DWDM components

Other optical layer components also add to the increased level of network reliability and the ability of IP-over-DWDM networks to approximate the service guarantees provided by traditional Sonet networks. These include optical add/drop multiplex-ers (ADMs), optical probes and protocol-sensitive interfaces.

Optical add/drop multiplexers. Incorporating optical ADMs into a DWDM system offers more reliability with fewer network interruptions and greater cost savings (Figure 3). With Sonet, data traffic must be decoded at each node, and the information must be converted into electrical form and passed through a router or cross-connect. At the other side of the local node, the information then is converted back into an optical signal for transmission. When an OC-3 node drops part of a message, the entire OC-48 packet must be decoded. And when that node goes down, the entire OC-48 connection is lost.

On the other hand, when DWDM drops a wavelength, the whole process occurs at the optical level. With a passive optical network design, only the information being dropped is affected in an electrical outage; the wavelengths passing through the node are not affected. Therefore, the optical layer adds reliability to the core network because traffic is not being demultiplexed and routed at every service point.

Optical service channels. An optical service channel creates a management interface for each piece of equipment on an optical network. This management interface operates over the same fiber as the data portion of the network that is being managed.

The value of an optical service channel is primarily in its use for initial provisioning and managing throughout the network. For instance, if a ring network has 13 nodes, how can a carrier detect a problem on the sixth node? The easiest way is for the network technician to directly monitor the fiber going through that node. An optical service channel is useful in the initial installation because the working service channel can be active, and a communications link between the node and the central office can be established before the channel cards even are installed.

The optical service channel also can help technicians through repair or provisioning procedures. The optical service channel provides the flexibility to service and communicate with each node on the ring for systemwide network management. Although DWDM has the ability to manage on a node-by-node basis, optical service channel management covers the entire network. It talks to all the boxes on the network through IP. Because most networks now run IP-based services, an IP-based service channel fits extremely well.

Optical probes. Optical probes compensate for the only difficulty in a protocol-independent architecture: monitoring. If a service provider is unsure of the protocol in a particular environment, the ability to carry any and all types of protocols over the fiber seamlessly is vital. Optical probes can diagnose the optical signal integrity and simultaneously look at other wavelength-based operational parameters (Figure 4). The probe provides rudimentary optical spectrum analysis on the entire system. Technicians might want to know how each optical layer is running if there is a sufficient signal-to-noise ratio and if the lasers are hot enough and still on target.

Optical probes should be able to monitor all the channels on a given DWDM system and then communicate that information back to other systems so that automatic adjustments and re-tuning can be completed. This type of technology gives a systemwide view of optical performance.

Personality modules. While metro DWDM solutions typically provide protocol-transparent services (for example, the ability to run any protocol over any wavelength), additional monitoring and management of individual interfaces sometimes is necessary. A personality module allows network administrators to evaluate how well protocols are running. The module sits at the edge of a DWDM network and analyzes a protocol-specific channel - such as ESCON or fast Ethernet - on each wavelength entering or leaving the DWDM-based optical network (Figure 5). In environments where the protocol is known - for example, gigabit Ethernet - a personality module can monitor bit-error rate, framing errors, management information and performance without affecting the data.

Doesn't this additional equipment just add complexity to the DWDM's supposedly simpler-than-Sonet architecture? No. Instead, it adds considerable management functionality - without adding complexity - because the interfaces are integrated into a single DWDM device. Service providers can build some of these technologies and elements into the DWDM unit, and others can be added during the installation. By combining the optical probe, personality module and optical service channel from a central site, administrators have the ability to look into the network at both the optical and protocol levels. Then the information can be relayed back to a central location for repair and restoration.

The introduction and improvement of IP-layer QOS protocols, optical layer protection switching and other optical layer improvements, such as service channels, probes and personality modules, provide an environment where eliminating Sonet from next generation optical networks not only is possible but desirable. IP over DWDM provides a cost-effective alternative to Sonet, reduces network complexity and management issues and can provide equivalent QOS guarantees.