Comparison between TDM and WDM

It is best to compare TDM and WDM on the basis of link design flexibility, speed and impact on BER.

Link Design Flexibility - TDM can be engineered to accommodate different link types. In other words, a TDM scheme can be designed to carve a given fiber optic cable into a multiplicity of links carrying different types of traffic and at different transmission rates. TDM can also be engineered to have different time slot assignment strategies. Slots may be permanently assigned. Slots may be assigned upon demand (Demand Assignment Multiple Access - DAMA). Slots may vary depending upon the type of link being configured. Slots may even be dispensed with altogether with data instead being encapsulated in a packet with Source and User addresses (statistical multiplexing). However, within the context of premises environment there is strong anecdotal evidence that TDM works best when it is used to configure a multiplicity of links all of the same traffic type, with time slots all of the same duration and permanently assigned. This simplest version of TDM is easiest to design and manage in premise data communications. The more complex versions are really meant for the WAN environment.

On the other hand, in the premises environment WDM, generally, has much greater flexibility. WDM is essentially an analog technique. As a result, with WDM it is much easier to carve a fiber optic cable into a multiplicity of links of quite different types. The character of the traffic and the data rates can be quite different and not pose any real difficulties for WDM. You can mix 10Base-T Ethernet LAN traffic with 100Base-T Ethernet LAN traffic with digital video and with out of band testing signals and so on. With WDM it is much easier to accommodate analog traffic. It is much easier to add new links on to an existing architecture. With TDM the addition of new links with different traffic requirements may require revisiting the design of all the time slots, a major effort.

With respect to flexibility the one drawback that WDM has relative to TDM in the premises environment is in the number of simultaneous links it can handle. This is usually much smaller with WDM than with TDM. Nonetheless, advances in DWDM for the WAN environment may filter down to the premise environment and reverse this drawback.

Speed - Design of TDM implicitly depends upon digital components. Digital circuitry is required to take data in from the various Sources. Digital components are needed to store the data. Digital components are needed to load the data into corresponding time slots, unload it and deliver it to the respective Users. How fast must these digital components operate? Roughly, they must operate at the speed of the composite link of the multiplexer. With a fiber optic cable transmission medium, depending upon cable length, a composite link of multiple GBPS could be accommodated. However, commercially available, electrically based, digital logic speeds today are of the order of 1 billion operations per second. This can and probably will change in the future as device technology continues to progress. But, let us talk in terms of today. TDM is really speed limited when it comes to fiber optic cable. It cannot provide a composite link speed to take full advantage of the tremendous bandwidth presented by fiber optic cable. This is not just particular to the premises environment it also applies to the WAN environment.

On the other hand, WDMdoes not have this speed constraint. It is an analog technique. Its operation does not depend upon the speed of digital circuitry. It can provide composite link speeds that are in line with the enormous bandwidth presented by fiber optic cable.

Impact on BER - Both TDM and WDM, carve a multiplicity of links from a given fiber optic cable. However, there may be cross talk between the links created. This cross talk is interference that can impact the BER and affect the performance of the application underlying the need for communication.

With TDM cross-talk arises when some of the data assigned to one time slot slides into an adjacent time slot. How does this happen? TDM depends upon accurate clocking. The multiplexer at the Source end depends upon time slot boundaries being where they are supposed to be so that the correct Source data is loaded into the correct time slot. The multiplexer at the User end depends upon time slot boundaries being where they are supposed to be so that the correct User gets data from the correct time slot. Accurate clocks are supposed to indicate to the multiplexer where the time slot boundaries are. However, clocks drift, chiefly in response to variations in environmental conditions like temperature. What is more, the entire transmitted data streams, the composite link, may shift small amounts back and forth in time, an effect called jitter. This may make it difficult for the multiplexer at the User end to place time slot boundaries accurately. Protection against TDM cross-talk is achieved by putting guard times in the slots. Data is not packed end-to-end in a time slot. Rather, there is either a dead space, or dummy bits or some other mechanism built into the TDM protocol so that if data slides from one slot to another its impact on BER is minimal.

With WDM cross-talk arises because the optical signal spectrum for a given link placed upon one particular (center) wavelength is not bounded in wavelength (equivalently frequency). This is a consequence of it being a physical signal that can actually be generated. The optical signal spectrum will spill over onto the optical signal spectrum for another link placed at another (center) wavelength. The amount of spillage depends upon how close the wavelengths are and how much optical filtering is built into the WDM to buffer it. The protection against cross-talk here is measured by a parameter called isolation. This is the attenuation (dB) of the optical signal placed at one (center) wavelength as measured at another (center) wavelength. The greater the attenuation,  the less effective spillage and the less impact on BER.

At the present time, clock stability for digital circuitry is such that TDM cross-talk presents no real impact on BER in the context of premises data communications and at the composite link speeds that can be accommodated. The TDM cross-talk situation may be different when considering WANs. However, this is the case in the premise environment. The situation is not as good for WDM. Here, depending upon the specific WDM design, the amount of isolation may vary from a low value of 16 dB all the way to 50 dB. A low value of isolation means that the impact upon BER could be significant. In such situations WDM is limited to communications applications that can tolerate a high BER. Digital voice and video would be in this group. However, LAN traffic would not be in this group. From the perspective of BER generated by cross-talk TDM is more favorable thanWDM.