The Principles of WDM and TDM System
Wave-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light, as is shown in Figure 1.This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. As an analog process, WDM is based on a well-known concept called frequency division multiplexing (FDM). With this technology, the bandwidth of a channel is divided into multiple channels, and each channel occupies a part of the large frequency spectrum. In WDM networks, each channel is referred to as a wavelength. This name is used because each channel operates at a different frequency and at a different optical wavelength. The wavelengths on the fiber are separated by unused spectrum. This practice makes the wavelengths separate from each other and helps prevent their interfering with each other. This idea is called channel spacing, or simply spacing.
Figure 1: WDM operating principle
Time-division multiplexing (TDM) is a technology of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern, as is shown in Figure 2. TDM is a type of digital (or rarely analog) multiplexing in which two or more bit streams or signals are transferred simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent time slots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during time slot 1, sub-channel 2 during time slot 2, etc. One TDM frame consists of one time slot per sub-channel plus a synchronization channel and sometimes error correction channel before the synchronization. After the last sub-channel, error correction, and synchronization, the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc.
Figure 2: TDM operating principle
The Features of WDM and TDM
WDM, possessing high transmission capacity, can save optic fiber resources. As to the single-wavelength fiber system needs to use a pair of optic fibers to receive and dispatch a signal, while the WDM system, no matter how many signals waited to be transmitted, only needs a pair of optic fibers. Being transparent to various service signals, WDM is able to transmit different kinds of signals, then compounding and decomposing them. As an optimal capacity-expanding method, WDM can introduce various services or expand capacity only by means of changing switch and adding an optical wavelength instead of using lots of fibers or high-speed networking devices. What’s more, using the optical add-drop multiplexer (OADM) and the optical cross connection (OXC), WDM can constitute the all-optic network of high flexibility, high reliability and high survivability.
TDM is designed to accomplish the high-capacity and high-speed transmission. Being able to adopt nonlinear soliton transmission and other useful technologies, TDM can eliminate the effect of chromatic dispersion in the high-speed transmission. At the same time, TDM is able to eliminate the rate effect of electronic devices to accomplish the high-speed transmission on a single wavelength. As an effectively optical multiplexing way, TDM can make full use of spectral resources and greatly improve the utilization of spectral bandwidth. Unlike WDM, TDM is free of the limitations resulted from the nonlinear effect of fibers, thus effectively utilizing optical wavelength and operating in various network of different distances and capacities. Though still immature, TDM is a more long-term technology than WDM.
The Differences between WDM and TDM
TDM and WDM are two methods of multiplexing multiple signals into a single carrier. Multiplexing is the process of combining multiple signals into one, in such a manner that each individual signal can be retrieved at the destination. Since multiple signals are occupying the channel, they need to share the resource in some manner.
The primary difference between WDM and TDM is how they divide the channel. WDM divides the channel into two or more wavelength ranges that do not overlap, while TDM divides and allocates certain time periods to each channel in an alternating manner. Due to this fact, we can say that for TDM, each signal uses all of the bandwidth and some of the time, while for WDM, each signal uses a small portion of the bandwidth and all of the time.
TDM provides greater flexibility and efficiency, by dynamically allocating more time periods to the signals that need more of the bandwidth, while reducing the time periods to those signals that do not need it. WDM lacks this type of flexibility, as it cannot dynamically change the width of the allocated wavelength.
WDM proves much better latency compared to TDM. Latency is the time it takes for the data to reach its destination. As TDM allocates time periods, only one channel can transmit at a given time, and some data would often be delayed, though it’s often only in milliseconds. Since channels in WDM can transmit at any time, their latencies would be much lower compared to TDM. WDM is often used in applications where latency is of utmost priority, such as those that require real-time information.
The Relationship between WDM and TDM
WDM and TDM are all ultrafast transmission technologies. TDM has dispelled the restriction of the speed of the electronic device and is free of the limitation of the nonlinear effect of fibers, realizing high-speed transmission on the single wavelength, but it is still at research and development stage for the present. As to WDM, it is a very mature technology and extensively used in communication networks, but its multiplexing wavelength and transmission diatance are restricted by the nonlinear effect of fibers. In the long term, WDM and TDM can be used in tandem and co-exist in the transmission network. As is shown in Figure 3, we can build a bigger optical transmission network by using TDM high-speed channels to connect the subnets composed of WDM. In the subnets, WDM can significantly improve flexibility and reliability of network. At the same time, TDM is effective to accomplish the high-speed and high-capacity transmission.
Figure 3: The optical transmission network of WDM and TDM
WDM, as a mature and high-capacity optical transmission technology, has already been extensively adopted in the network now. It is equipped with the advantages of transparency, reconfigurability and excellent network survivability. The future WDM optical network will develop towards the flexible networking direction based on the optical wavelength routing and exchanging, which possesses the ability of fast network recovery and reconfiguration and will play a main role in the future optical transmission network. As a very effective multiplexing technology, TDM can make full use of spectral resources and dispel some restrictions of WDM system caused by nonlinear effect. In recent years TDM has made great progress in the research field, but not ripe enough. In a nutshell, WDM and TDM have their own advantages and disadvantages as the optical multiplexing technology. With the relevant study lucubrating constantly, WDM and TDM can be combined together to be extensively applied to the ultra fast transmission network.
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