Time-division multiplexing (TDM) was invented as a way of maximizing the amount of voice traffic that could be carried over a medium. In the telephone network before multiplexing was invented, each telephone call required its own physical link. This proved to be an expensive and unscalable solution. Using multiplexing, more than one telephone call could be put on a single link.
TDM can be explained by an analogy to highway traffic. To transport all the traffic from four tributaries to another city, you can send all the traffic on one lane, providing the feeding tributaries are fairly serviced and the traffic is synchronized. So, if each of the four feeds puts a car onto the trunk highway every four seconds, then the trunk highway would get a car at the rate of one each second. As long as the speed of all the cars is synchronized, there would be no collision. At the destination the cars can be taken off the highway and fed to the local tributaries by the same synchronous mechanism, in reverse.
This is the principle used in synchronous TDM when sending bits over a link. TDM increases the capacity of the transmission link by slicing time into smaller intervals so that the bits from multiple input sources can be carried on the link, effectively increasing the number of bits transmitted per second (see Figure 4-1).
With TDM, input sources are serviced in round-robin fashion. Though fair, this method results in inefficiency, because each time slot is reserved even when there is no data to send. This problem is mitigated by the statistical multiplexing used in Asynchronous Transfer Mode (ATM). Although ATM offers better bandwidth utilization, there are practical limits to the speed that can be achieved due to the electronics required for segmentation and reassembly (SAR) of ATM cells that carry packet data.
SONET and TDM
The telecommunications industry adopted the Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) standard for optical transport of TDM data. SONET, used in North America, and SDH, used elsewhere, are two closely related standards that specify interface parameters, rates, framing formats, multiplexing methods, and management for synchronous TDM over fiber.
SONET/SDH takes n bit streams, multiplexes them, and optically modulates the signal, sending it out using a light emitting device over fiber with a bit rate equal to (incoming bit rate) x n. Thus traffic arriving at the SONET multiplexer from four places at 2.5 Gbps will go out as a single stream at 4 x 2.5 Gbps, or 10 Gbps. This principle is illustrated in Figure 4-2, which shows an increase in the bit rate by a factor of four in time slot T.
The original unit used in multiplexing telephone calls is 64 kbps, which represents one phone call. Twenty-four (in North America) or thirty-two (outside North America) of these units are multiplexed using TDM into a higher bit-rate signal with an aggregate speed of 1.544 Mbps or 2.048 Mbps for transmission over T1 or E1 lines, respectively. Figure 4-3 depicts this multiplexing and aggregation hierarchy. Using a standard called virtual tributaries for mapping lower-speed channels into the STS-1 payload, the 28 DS1 signals can be mapped into the STS-1 payload, or they can be multiplexed to DS3 with an M13 multiplexer and fit directly into the STS-1. Note also that ATM and Layer 3 traffic, using packet over SONET (POS), can feed into the SONET terminal from switches equipped with SONET interfaces.
To summarize, the demand placed on the transport infrastructure by bandwidth-hungry applications and the explosive growth of the Internet has exceeded the limits of traditional TDM. Fiber, which once promised seemingly unlimited bandwidth, is being exhausted, and the expense, complexity, and scalability limitations of the SONET infrastructure are becoming increasingly problematic.
Wavelength Division Multiplexing
WDM increases the carrying capacity of the physical medium (fiber) using a completely different method from TDM. WDM assigns incoming optical signals to specific frequencies of light (wavelengths, or lambdas) within a certain frequency band. This multiplexing closely resembles the way radio stations broadcast on different wavelengths without interfering with each other (see Figure 4-4). Because each channel is transmitted at a different frequency, we can select from them using a tuner. Another way to think about WDM is that each channel is a different color of light; several channels then make up a "rainbow."
In a WDM system, each of the wavelengths is launched into the fiber, and the signals are demultiplexed at the receiving end. Like TDM, the resulting capacity is an aggregate of the input signals, but WDM carries each input signal independently of the others. This means that each channel has its own dedicated bandwidth; all signals arrive at the same time, rather than being broken up and carried in time slots.
The difference between WDM and dense wavelength division multiplexing (DWDM) is fundamentally one of only degree. DWDM spaces the wavelengths more closely than does WDM, and therefore has a greater overall capacity. The limits of this spacing are not precisely known, and have probably not been reached, though systems are available in mid-year 2000 with a capacity of 128 lambdas on one fiber. DWDM has a number of other notable features, which are discussed in greater detail in the following chapters. These include the ability to amplify all the wavelengths at once without first converting them to electrical signals, and the ability to carry signals of different speeds and types simultaneously and transparently over the fiber (protocol and bit rate independence).
WDM and DWDM use single-mode fiber to carry multiple lightwaves of differing frequencies. This should not be confused with transmission over multimode fiber, in which light is launched into the fiber at different angles, resulting in different "modes" of light. A single wavelength is used in multimode transmission.
TDM and WDM Compared
SONET TDM takes synchronous and asynchronous signals and multiplexes them to a single higher bit rate for transmission at a single wavelength over fiber. Source signals may have to be converted from electrical to optical or from optical to electrical and back to optical before being multiplexed. WDM takes multiple optical signals, maps them to individual wavelengths, and multiplexes the wavelengths over a single fiber. Another fundamental difference between the two technologies is that WDM can carry multiple protocols without a common signal format, while SONET cannot. Some of the key differences between TDM and WDM are graphically illustrated in
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