During the last decades, with the increasing demands of bandwidth and high speed, the technologies of optical communication have been growing rapidly and achieved significant performance. But due to the fiber attenuation, dispersion and nonlinearity, the achievable transmission capacity of conventional fiber-optic communication systems is still limited. Wavelength Division Multiplexing (WDM) and Optical Time Division Multiplexing (OTDM) are the technologies that can increase the transmission capacity of optical fiber at present. However, there are some defects of WDM, e.g. the appearance of fiber nonlinearities, or the unequal gain spectrum of the amplifiers. OTDM can overcome these defects of WDM based on its much more attractive features. It is considerd as a long-term network technology and develops constantly. Today, Fiberstore’s Blog will introduce the basic knowledge of OTDM, as well as the difference from WDM. In addition, TDM-PON and its difference from WDM-PON, as well as WDM/TDM-PON are also introduced in the paper.
OTDM, short for optical time division multiplexing, is a channel multiplexing technology which multiplexes signals in different bit slots in the time domain. In other words, it’s practical to combine a set of low-bit-rate streams, each with a fixed and pre-defined bit rate, into a single high-speed bit stream that can be transmitted over a single channel. In contrast to WDM, OTDM only uses one wavelength, intuitively speaking, only a “color” of light in a fiber. OTDM provides a user the full channel capacity but divides the channel usage into time slots. Maybe you are still confused with OTDM just via these boring description. Here is a simple example to help you understand OTDM intuitively. To suppose that a channel capable of transmitting 192 kbit/s from Los Angeles to New York. And there are three sources, all located in Los Angeles. So, each have 64 kbit/s of data that they want to transmit to individual users in New York. As shown in Figure 1, the high-bit-rate channel can be divided into a series of time slots, and the time slots can be alternately used by the three sources. The three sources are thus capable of transmitting all of their data across the single, shared channel. Clearly, at the other end of the channel (in this case, in New York), the process must be reversed. The system must divide the 192 kbit/sec multiplexed data stream back into the original three 64 kbit/sec data streams, which are then provided to three different users. This reverse process is called demultiplexing. OTDM makes the most of these advantages in the optical domain and is another important technique for the construction of photonic networks in addition to the development of highspeed signal processing.
Figure 1. Example of Time division multiplexing
Just as its definition, the basic working principle of OTDM is to multiplex a number of low bit rate optical channels in time domain. The overall OTDM system can be viewed as three big blocks. They are transmitter block, line system, and receiver block. The transmitter block consists of Laser sources, modulators, channel alignment systems, and multiplexer. The line system contains optical amplifiers and transmission fibers. And the receiver block is made of synchronization/timing extraction circuit and channel demultiplexing.
Channel allocation by time division multiplexing is dependent on the fundamental electrical data rate and the optical pulse width. With fixed electrical clock, one must shorten the optical pulse width in order to multiplex more channels within the clock period. In addition, the shortened pulse width can help reduce the crosstalk between channels because of more room left in bit slot. However, short optical pulses are subjected to heavy dispersion penalty as traveling distance increases. But take it easy, the use of transform-limited pulse and dispersion slope compensation technique can reduce the dispersion effect on OTDM.
In addition, accurate control on the channel alignment is also critical as transmission speed increases because more channels are multiplexed in a fix clock period. Any misalignment can affect the performance of the OTDM system because of crosstalk and dispersion. Electro-optic switching technique or all-optical switching technique can achieve demultiplexing at receiver end. The electro-optic technique is great for transmission speed at less than 40Gb/s. It is more difficult to achieve for speed over 40Gb/s due to restraint on electrical drive power. All-optical switching is based on third order nonlinear effect of the optical fiber. It is highly suitable for ultra-fast speed transmission because the non-linear response is in fs range. It is also allowing add/drop of an individual channel or a number of channels, which is great feature for network operation. However, the all-optical switch is very bulky and expensive to made. Successful demultiplexing can only be accomplished with accurate timing extraction. The timing jitter from the extraction circuit can directly affect the Bit Error Rate (BER) performance of the OTDM system.
In order to meet the increasing demand of information transmission, the all-optical networks will be the trend of future networks. Some attractive features of OTDM makes it as a future all-optical network technology:
- Easy accesses to the line with high rates (up to hundreds of Gbit/s)
- Though the the total rate of the network is very high, electronic devices in the nodes of the network just work with a low data rate as the local
- Greatly simplifies the amplifiers cascade management and dispersion management as it is a single wavelength transmission
- The combination of WDM and OTDM can support the future ultra high-speed optical network implementation
- Data of each channel can be an arbitrary rate level
- Be compatible with the current technologies (such as SDH)
Look at the picture (Figure 2.) below. You may find OTDM is similar to WDM if you just scan this picture quickly. Because there are many channels both in OTDM and WDM. In fact, they are not the same. For OTDM, in a single fiber, there is only one wavelength, and also called one bandwidth. Channels are called time slots as they are divided according to the time domain. Signals are multiplexed in different bit slots. While, in WDM, channels are called wavelengths and there are multiple wavelengths in a singal fiber. Turn to Figure 3., you will obviously find these difference between OTDM transmission and WDM transmission. In OTDM, the signal wavelength (color red) transmits throughout the whole process, while in WDM, there are several wavelengths (several colors) and each wavelength is divided into a separate channels.
Figure 2. OTDM & WDM Axis
Figure 3. OTDM vs WDM Working Principle
Both OTDM technology and WDM technology are used in passive optical network (PON), which are respectively called TDM-PON and WDM-PON. The TDM-PON (Figure 4) splits the optical power vis the 1xN splitter, for which N is relatively small and so is the number of subscribers and also the deliverable data rate to each end user. In contrast, the WDM-PON with relatively few channels delivers traffice to as many Optical Network Units (ONUs) as the channels but at high data rate to each one. Because the medium is shared by all end users, the available bandwidth and the network resources are better used in TDM-PON than in WDM-PON, hence the TDM-PON is more efficient. In other aspect, the TDM-PON is based on a fixed number of well synchronized time slots. Thus, the TDM-PON is not easily scalable. Conversely, the WDM-PON is only limited by the number of wavelengths available in the grid.
Because of its broadcast nature, TDM-PON allows bad actors to “listen” to time slots that belong to other ONUs. Thus, the TDM-PON is less secure. The WDM-PON does not broadcast data and thus in that respect it is better than the TDM-PON. However, an eavesdropper may also extract data from an individual ONU by unauthorized access of ONU or by tapping the input or output of ONU. In all, security is an issue which needs to be examined seriously by encrypting data and by securing the fiber link.
In conclusion, the TDM-PON and the WDM-PON have advantages and disadvantage in a complimentary way. The advantages of the former are disadvantages of the latter and vice versa. As a result, to combine the TDM and WDM technologies into PON, i.e. WDM/TDM-PON is quite valuable because it allows multiple users to share one WDM wavelength’s capacity. With some exceptions, the capacity of one wavelength exceeds an individual user’s traffic capacity needs. Several features of WDM/TDM-PON as following:
Figure 4. TDM-PON
- Compromise between WDM-PON and TDM-PON
- Combine the advantages of Both technologies
- First WDM, then TDM
- One wavelength per ONU
- Several NTs (Network Terminal) connected to one OUN
- Each NT serves one or more users
- Traffic from/to NT are time multiplexed
WDM technology has been widely used in today’s network as it is the mature and practical optical transmission technology for large capacity transmission at present. With the advantages of transparency, reconfigurable, network survivability, WDM will be developed in the direction of flexible optical networks which are based on optical wavelength switching and wavelength routing. With the features of faster network restoration and reconstruction of capacities, WDM will be the main direction of future optical transport network. However, there are some unavoidable limitations of WDM. Thus, we need TDM technology in optical transmission, called OTDM due to its much more attractive features. OTDM is a very effective method of optical multiplexing. It can make full use of the spectrum resource, and remove some restrictions of nonlinear effects in WDM system. However, although we have made great progress in recent years on research of OTDM, it is not mature enough, because some of the key technologies have still to be resolved. Actually, we believe that, with the deepening of research, WDM and OTDM technologies will be combined to complement for each other and widely used in future ultra high-speed transmission networks.
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