Optical cdma offers all-optical network alternative
Optical cdma offers all-optical network alternative
Birendra (Raj) Dutt and Bill Johnson
Commercial Technologies Corp.
Don`t think of optical code-division multiple access (cdma) as just another remedy for fiber exhaust in the local loop. It`s a new architecture that can potentially reduce the cost of every aspect of an optical network: equipment, outside plant, facilities, and last but not least, operational support systems. The concept of an all-optical cdma network is quite simple: Place hardware where traffic originates and terminates on the network and perform all provisioning and restoration functions within the network using photons instead of electrons--photons in, photons out.
The market for data services is booming, and data, rather than voice transmission, promises to be the limiting factor in many networks. Data is estimated to be 50% of network traffic today, growing at a rate of 40% to 100% per year in most enterprises. Voice traffic, including fax and pagers, is growing at 8% to 10% per year and Internet traffic is skyrocketing at 6% per month--an amazing 72% a year. Extrapolate this growth to the year 2005 and it is easy to see why forecasters say there is no immediate limit to the need for increased fiber capacity. Just two years ago in 1996, telecommunications carriers spent $574 million to expand fiber-optic bandwidth. By 1997, that amount had nearly tripled to $1.453 billion, yet the demand for more fiber capacity continues to grow at a staggering rate.
To facilitate this growth, most would agree that an all-fiber network clearly outperforms its copper counterpart on every front. But cost continues to be a problem. When we consider the overall lifecycle cost of an optical network using current technology, only a small segment of the business community can be served.
The equipment manufacturers currently are focused on a solution for one high-priority network problem: fiber exhaust. Their solution--high-density wavelength-division multiplexing (wdm)--has one major drawback: It incrementally adds cost to a network architecture that is already too expensive to serve the majority of the business community. The alternative offered by optical cdma is to develop a network approach that not only increases the efficiency of the existing fiber plant, but also significantly reduces the fiber count for future builds, while eliminating many of the intermediate time-division multiplexing steps required in conventional Synchronous Optical Network (sonet) infrastructures. The elimination of equipment within the network not only reduces equipment cost, but also reduces the need for buildings, batteries, and infrastructure, while simplifying network management by eliminating layers of network elements.
How optical cdma works
It is very important from the onset to point out that optical cdma is not a replacement for sonet, Asynchronous Transfer Mode (atm), or Ethernet, but rather a more cost-effective method to transport, provision, and protect these standard protocols. Optical cdma can best be described by comparing it to wdm. Consider that wdm divides the fiber spectrum into narrow optical wavelengths, each with an individual laser. Optical cdma divides the fiber spectrum into individual codes, all derived from a single broadband optical source.
The real power of optical cdma can be found in its coding scheme, which can be explained with a simple analogy. Think of a flashlight with your fingers spread in front of the lens. Your fingers pass and block light. Now push the button on top of the flashlight off and on; this represents modulation. Using this analogy, your fingers represent a complex spatial filter placed in front of a 28-nm optical source (the flashlight). The coded light then passes through an optical modulator (the on/off switch) which is modulated with a digital signal, for example, an OC-12 (622 Mbits/sec). It is a simple three-step process: source, filter, and modulator. The spatial filter can be thought of as an optical bar code and can be either fixed or programmable.
cdma is not new. It has been successfully used for years as a multiple addressing solution for applications that span from satellites to cellular telephones. All of these cdma applications have one thing in common: Their signals are electromagnetic in nature and thus contain a positive and negative phase characteristic. The property of phase is essential when decoding and separating a single cdma channel from hundreds of other channels that can potentially interfere with the targeted user.
Implementing cost-effective optical cdma has posed the difficult technical challenge of adding the property of phase to an optical signal that constantly turns from light to dark, especially given that the state of darkness has no phase or amplitude characteristics at all. This challenge has been met by a simple technical solution that effectively carries an artificial phase component along with the optical signal without actually operating lasers in phase--something that can be accomplished in the laboratory but certainly not mass-produced for optical networks.
Optical cdma and all-optical networks
From a network perspective, an all-optical network has four basic requirements: a technology that permits access to a large portion of the fiber spectrum, a means for dropping and inserting traffic, the ability to crossconnect traffic, and a restoration methodology, all at the photonic layer.
Figure 1 illustrates three users on an optical cdma network. Traffic flow is shown in only one direction for simplicity. Starting at the central office, a broadband optical source is first split into three paths, each providing optical power to a transmitter. Individual transmitters contain spatial filters, referred to as masks, that encode each leg of the spread spectrum source with a unique signature. This code, similar to a bar code, is just one from a selected code set. The encoded spectrum is then on/off modulated by the user`s data stream. These time-modulated and spectrally encoded optical signals, together with other transmit channels, are coupled to the optical network. This coding method satisfies the first requirement of the all-optical network; optical cdma is a technology that permits access to a large portion of the fiber spectrum through its spatial coding scheme.
Optical cdma is a broadcast technology, with all information going to all parts of the network. When a receiver is placed anywhere on the network with a bar code that matches a transmitter, that signal alone is decoded and extracted from the network. The second requirement for an all-optical network, the ability to economically add users, is made possible by this broadcast functionality. A simple tap and insert coupler is installed in the lateral fiber run to multiple users, and a receiver is installed at each terminating location. Even though all information is tapped from the network to each drop location, only that information with a matching receiver bar code will be received. All other information will be rejected. It should be emphasized that optical cdma, like its cellular counterpart, is a spread-spectrum technology and therefore highly secure.
The third requirement for an all-optical network, the ability to crossconnect traffic, is satisfied by a small liquid crystal mask that is programmable under the network-management architecture. For example, a 128-channel optical cdma network can be viewed as a 128 x 128 crossconnect matrix.
One significant impact is the effect of optical cdma on the central office. Voice and data transceivers can now be grouped with their respective services and connected to their users by simply programming matching receiver and transmitter bar codes. This automated feature provided by the programmable mask can substantially reduce the size of both optical and electrical digital signal crossconnect (dsx) frames and eliminate the manual processes associated with crossconnecting traffic.
The programmable mask also provides the fourth and final function of an all-optical network, ring protection. When protection is desired, two mask codes are transmitted around a ring, one clockwise, the other counter-clockwise. This technique is similar to the 1+1 protection provided by sonet in that information is constantly present at the receiver from two sources and all switching is accomplished at the receive end of the network. This arrangement eliminates the need for a complex network-management application that must communicate with every node on the ring and maintain tables for restoration and provisioning.
Impact on the network
A sonet network might start with OC-48 (2.5-Gbit/sec) equipment at the central office, branching off to OC-12 at the curb and dropping to OC-3 (155 Mbits/sec) to the customer (see Fig. 2). With optical cdma, the network provider installs optical cdma equipment at the customer premises, with the other end of the termination resident at the central office. However, all of the other sonet steps in the middle of the network are eliminated.
The impact on operational support systems is tremendous. There is no need to go through and assign time slots to all the sonet elements every time the service provider turns up or moves a circuit. Provisioning is accomplished between the two end points. As an example, suppose a customer requires a T1 protocol (1.554 Mbits/sec). This voice-grade service would be put on the network with an optical cdma bar code. At the central-office end, the terminating T1s would sit next to the voice switch. If a customer required additional service, say Ethernet, that termination would sit next to a router or Ethernet switch. There would be no need to pass circuits through dsx frames. At the central office, all that is required is to set up two cdma codes and make the connection. The result is the elimination of the manual dsx part of the station, including the building cost that goes with them.
Optical cdma can be interfaced to any standard protocol by changing the personality card in front of the common engine. From the customer premises perspective, interfaces include OC-3 and below as well as 10 Mbits/sec, 100 Mbits/sec, Gigabit Ethernet, Fibre Channel, and atm. Video can be transmitted over optical cdma by means of a simple analog-to-digital conversion process, allowing uncompressed transmission without compression delay. As with other applications, video provisioning is accomplished using bar-code management in either a point-to-point or point-to-multipoint configuration. In addition, high-speed applications can be accomplished with OC-12 and OC-48 interfaces.
How much bandwidth, how many channels?
Perhaps the greatest advantage of optical cdma as a network solution is high granularity. This technology, unlike dense wdm, is not dependent on thinner and thinner lasers, but rather on more and more codes. Current technology using 28-nm optical sources can generate 256 codes with an aggregate data rate of 120 Gbits/sec. A system could therefore transport 50 OC-48s, 190 OC-12s, or 256 OC-3s (or smaller links) on a single fiber-ring infrastructure. Data rates can be mixed in any manner as long as the 120-Gbit/sec total throughput is not exceeded. In addition, multiple groupings of optical cdma signals can be placed alongside one another in the optical spectrum, i.gif., 1310 and 1550 nm.
Optical cdma also can be used in conjunction with other technologies, including wdm and optical crossconnects (oxcs). The oxc, in its simplest form, is a switch that provides the ability to transfer information from one fiber to another based on traffic demand.
Optical networks have been around for years, have proved to be beneficial, and have come down in price, although experts in the industry tell us that relying on photons to expand bandwidth will not be practical until five to 10 years in the future. Optical cdma, however, could conceivably provide this all-optical networking solution as early as 18 months from now. u
Birendra (Raj) Dutt is chairman and chief executive and Bill Johnson is chief operating officer of Commercial Technologies Corp., Richardson, TX.