http://www.internettelephony.com/archive/4.19.99/cover/covfig3.htm
As Internet connections worldwide are doubling every year, and more users are upgrading to faster access, it seems to be a great time for both internet users and service carriers. Users who subscribe to faster access will enjoy a speedier Internet experience while service carriers can cash in on the increased user base. Unfortunately, reality is far from ideal: users aren’t getting the bandwidth advertised by the provider, and internet carriers are having a hard time balancing between upgrading their infrastructure to meet the internet traffic growth and keeping their shareholders happy by making a profit.
The bandwidth crisis does not only apply to the Internet sector, it applies to all voice, fax, and data traffic. However, data traffic is gaining more importance. Internet traffic had been growing exponentially from 1990-1997, compared to traditional telecommunication traffic of 10-17% (Bray and Baney)
Among various mediums such as coaxial cables, twisted pair, optic fiber, radio frequency, satellites, and microwave wireless, optic fiber is the only medium with enough bandwidth to handle the rapidly growing high-capacity backbone in the world today. The most common optic fiber cable installed is OC-48, it has a transmission rate of 2.488Gbps, while the deployment of 10 Gbps OC-192 is getting more feasible. From a user standpoint, 2.5 or 10 Gbps may sound like enormously high bandwidth, but 2.5Gbps can support only approximately 1600 DSL users connecting at 1.544 Mbps. That may be fine for a local carrier, but the major network access points around the US will have trouble keeping up if DSL and other broadband services are as widespread as 56kbps modems are today. The largest access point – MAE-East (Washington D.C) and MAE-West (San Jose, California), “have combined traffic that has more than quadrupled in each of the two years since 1995. … combined traffic has reached about 1.5Gbit/s on average in mid 1997” (Bray and Baney) An equivalent of 12,000 phone calls, that figure indicates that the internet is “insufferably slow” because there had to be more internet traffic than the equivalent 12,000 phone call around the US in 1997.
There are various solutions to ease the congestion problem. First, carriers can install more optic fiber cable into their infrastructure. For a long distance carrier whose network easily exceeds 10,000+ miles worth of cable across the US, installing more optic fiber is not logistically feasible. And even for small distance carriers (300km or below), laying extra cables in a metropolitan area may take months to clear government paperwork before construction can begin. In both cases, carriers are turning to dense wave division multiplexing (DWDM) as a solution. By combining multiple wavelengths, each representing a separate data channel, the same optic fiber suddenly has the bandwidth capacity of multiple cables. An additional benefit of DWDM is that repeaters commonly used in time division multiplexing (TDM) network, such as synchronous optical network (SONET) are replaced by optical-amplifiers. Unlike repeaters, optical amplifiers can amplify the multiplexed signal without de-multiplexing it first, which reduces intermittent bottleneck. Optical Amplifiers can also be placed farther apart than repeaters.
With time division multiplexing, bandwidth is inversely proportion to the pulse width twice. In other words, a higher bandwidth in TDM system will create a shorter pulse width = higher frequency, making it more susceptible to fiber dispersion. As the bandwidth requirement increases in the future, researchers are faced with the dispersion issue if TDM is to be competitive. On the other hand, since DWDM increases its bandwidth by adding extra wavelengths, it has a theoretical bandwidth of 5000 Ghz in the 1550 nm region.
Notice that in applications where recoverability is essential, SONET (TDM)
will still be the first choice because of SONET’s ~50ms recoverability in case
of a path interruption, something DWDM cannot achieve today by itself.
DWDM is a technology that transmits multiple data signals using different wavelengths of light through a single fiber. Incoming optical signals are assigned to specific frequencies within a designated frequency band. The capacity of the fiber is increased when these signals are multiplexed out onto one fiber.
The best way to describe DWDM technology is by using the car analogy explained in the Dense Wavelength Division Multiplexing (DWDM) Tutorial, Section 3: Capacity Expansion and Flexibility: DWDM (http://www.webproforum.com/dwdm/topic03.html). This analogy compares one fiber to a multilane highway. It states that traditional TDM systems only use one lane of the "highway". Moving the “cars”, or signals, faster within the one lane increases the capacity of the highway. DWDM technology accesses the unused lanes "(increasing the number of wavelengths on the embedded fiber base)" in order to fully utilize the untapped capacity within the fiber.
DWDM is able to reach transmission capabilities that are four to eight times those of the traditional TDM systems. This high-speed, high-volume transmission is made possible by the technology within the optical amplifier. An optical amplifier is a section of fiber optic cable that has been doped with erbium to amplify the optical signal. Erbium-doped fiber amplifiers (EDFAs) have two advantages: they increase the optical signal and at the same time do not have to regenerate the signal to boost its strength. A typical optical signal must be regenerated every 100 km. This is accomplished by converting the optical signal to an electrical signal, and then back to its optical form for re-transmission. EDFAs can lengthen the distance of transmission to more than 300 km before regeneration is needed.
The amplifiers should be automatically adjusted when a channel is added or
removed. This will help achieve an optimal system performance, which is
important because "degradation in performance through self-phase modulation" may
occur if there is only one channel on the system with high power. The reverse is
true if there is too little power: there will not be enough gain from the
amplifier. Some questions have been raised as to which type of optical
amplifiers will enhance the system performance the most, fluoride- or
silica-based fiber amplifiers. Silica-based optical amplifiers with filters and
fluoride-based optical amplifiers show comparable performance within the 1530 to
1565 nm range, but the fluoride-based amplifiers are more expensive and the long
term reliability of fluoride-based amplifiers has yet to be proven.
DWDM is protocol and bit rate independent so data signals such as ATM, SONET, and IP can be transmitted through the same stream regardless of their speed differences. Each individual protocol remains intact during the transmission process because there is not optic-electric-optic conversion with DWDM. The fact that the signals are never terminated within the optical layer allows the independence of the bit-rate and protocols, thusly allowing DWDM technology to be easily integrated with the existing equipment in the network. This gives service providers the flexibility to expand capacity within any portion of their networks. No other technology allows this. Service providers are also able to partition dedicated wavelengths for customers who would like to lease just one wavelength instead of an entire fiber.
Service providers may also begin to increase the capacity of the TDM system
currently connected to their networks. This is due to the fact that OC-48
terminal technology and related operations support systems (OSSs) match with
DWDM systems. OC-192 systems may be added later to expand the capacity of the
current DWDM infrastructure to at least 40Gbps.
"Analysts estimate that while annual growth in voice traffic proceeds at a steady rate of between 5 and 10 percent, data traffic is forging ahead at a rate of 35 percent per year in North America." (Pulse Online) Numbers like this show that any network technology currently used must already be able to handle such an increase in traffic, or be expandable to handle the increase when necessary.
"'DWDM technology gives us the ability to expand out fiber network rapidly to meet the growing demands of our customers,' said Mike Flynn, group president for ALLTEL's communications operations. 'DWDM, coupled with the recent deployment of Asynchronous Transfer Mode (ATM) switches, will allow us to simplify the network, reduce network costs and continue to offer new services.'" (Computer Protocols)
In addition to the expansion methods mentioned in the flexibility section, DWDM allows service providers to establish a "grow-as-you-go" infrastructure; they can add current and new TDM systems to their existing technology to create a system with virtually endless capacity expansion. DWDM is also considered to be a "perfect fit" for networks trying to meet bandwidth demands. To accomplish this, DWDM systems must be scalable. As an example: a system of OC-48 interfacing with 16 channels per fiber line may be viewed as overkill with the present network usage, but this will allow the system to run efficiently as early as two years from now. It is projected that future DWDM terminals will carry up to 80 wavelengths of OC-48 or up to 40 wavelengths of OC-192, for a total of 200 Gbps or 400 Gbps, respectively. This is enough capacity to transmit 90,000 volumes of an encyclopedia in one second.
Returning to the car analogy used earlier, this diagram outlines how a service provider can integrate DWDM with their current technology and increase their system capacity.
Ultrawideband optical-fiber amplifiers may also be used to boost lightwave
signals carrying over 100 channels of light. By using these amplifiers, a
network could handle information in the terabit range. All of the world’s
television channels or about half a million movies could be transmitted at once
at this rate.
Every new technology is pushed by three main needs/wants: more, cheaper, and faster. Knowledge of DWDM is in itself fascinating. Academic arenas have studied the technology for years, but the real push for advances in this technology are the promises for more data sent cheaper and faster. Earlier, DWDM has been explained to have several “advantages”: less amplification, easy addition to existing systems. These are advantages in cost. Similar performance could be achieved using existing technologies, but the cost would be very high in comparison. DWDM cost can be broken down into a comparison with SONET (TDM) and WDM and long-haul vs. short haul data links.
DWDM is a sub-category of WDM. Both DWDM and WDM (as explained earlier) send data over multiple wavelengths of light. The data is sent in different “channels” of light. This is in contrast to a TDM system that breaks the different “channels” into time slots sent over one wavelength of light (when talking about fiber optic transmissions). Traditionally, long haul links between high speed carriers used TDM over optical fiber. To instantiate this, an electrical regenerator must be installed every 40 km to 100km to boost the signal. The optical signal attenuates and requires a boost. Boosting this signal can be complicated. The TDM signal has strict timing requirements (it is a time division signal) so the signal must be re-shaped, re-timed, and then re-transmitted. The equipment used to perform this function is expensive. It must be housed, powered, and maintained for proper operation. The propagation delay for the signal is also increased, because the optical signal is converted into an electrical signal, and then re-converted into an optical signal. This means less data can be transferred in the same amount of time. This equals more cost for the data provider. Over very long distances, these factors can dramatically increase the cost of using TDM.
WDM can be less costly in such long-haul links. This is due to several reasons. First of all, the optical signal still attenuates and requires a boost. This can be performed differently. WDM signals can use an optical amplifier that does not require the costly electrical regeneration. As explained earlier, this system uses EDFAs and can be spaced at a distance of up to 1000km. This is the second main advantage. These amplifiers do not need to de-multiplex and process each signal. The optical amplifier simply amplifies the signals. It has no need to reshape, retime, or retransmit the signals, and the process does not need to convert the optical signals to electrical signals and back again. Combine the simple amplification with the increased distance between the amplifiers, and dramatic savings can be seen. One optical amplifier on a 40-channel WDM system or a 150-channel DWDM system can replace 40 or 150 separate regenerators. This factor can be increased even more, since the distance between amplifiers is greater than between regenerators. Also, because the signal is always transmitted as light, the amplifiers do not slow the link.
Another cost saving applies to long-haul carriers that already implement WDM over their links. The upgrade from the existing system to a system employing DWDM is relatively simple and cost effective (another advantage of DWDM that was mentioned earlier). One reason this upgrade is simple has to do once again with the amplifiers used to boost the signal. The EDFAs already in place for WDM systems can amplify the DWDM signal as well. Systems that employ TDM have an even greater cost associated with them when upgrades are necessary. Even more complicated electrical regenerators replace the old, and this equals an even greater cost (especially over greater distances).
One final cost advantage of DWDM over other systems is the ability to easily transmit several different protocols and speeds over the same link. The traditional TDM system, SONET, required a total separate link or complicated (and expensive) conversion software to link routers and hubs with different protocols together. DWDM can send different protocols and different speed links over separate optical channels. One fiber can carry multiple protocols easily and cost effectively.
Below is a graphical representation comparing the DWDM system with a conventional TDM system. Notice that the OC-48 speeds on the transmit and receive sides can be replaced with higher rates or even different rates.
All of the “advantages” of DWDM mentioned before apply mostly to long-haul links between major carriers. Most of the cost savings come from the decreased need to amplify or the increased distances between amplifiers. However, in short link applications, these savings cannot be realized. Both DWDM and TDM systems have no need to amplify under 40 km, and most LANs do not span any distance wider. In these cases, it is usually cheaper to lay more line or upgrade the existing TDM systems to an increased capacity. Here, the DWDM routers necessary to implement the system are more expensive than the older SONET TDM systems. These TDM systems have been around for longer, and are more readily available.
DWDM will be necessary in areas where extremely high traffic will occur and
over relatively large areas. An example of this is the metropolitan area of New
York and New Jersey. Here, the need for extremely high-speed networks may
actually exceed the capacity of current TDM systems. Also, these areas put speed
and QoS, not cost, as the largest priority.
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is Wave Division Multiplexing (WDM)? Cisco Systems, Inc.
3. Computer
Protocols. CIENA's Multiwave Sentry Enables Protocol Flexibility”. Worldwide
Videotex. August 1, 1999
4. Hanley, Michael. DWDM
Rising. Ed. Curt Harler.
5. Lucent Technologies. Dense Wavelength
Division Multiplexing (DWDM) Tutorial.
6. Pulse
Online. “Carriers Turn to
DWDM for Enhancing Fiber Optic Networks. May 1999.