A digital voice channel operates at 64 kbps. Fiber optic communication allows transmission of data at rates more than 100 Gpbs in some of the deployed systems, while data speeds of 10 Gbps and 40 Gbps are common. Earlier systems were operating at 2.5 Gbps or lower. In order to make use of the capacity of an optical fiber and therefore e the fiber optic system, it is necessary to transmit many channels simultaneously through multiplexing.
Multiplexing or muxing combines many signals and allows to utilize the resources effectively. It is a method by which multiple analog signals or digital data signals are combined into one signal over a shared medium. In fiber optic telecommunications, the shared medium is optical fiber. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing was originated in telegraphy in 1870s. In telephony, George Owen Squier introduced telephone carrier multiplexing in 1910.
Multiplexing or simply combining of multiple signals can be accomplished through Time-Division Multiplexing (TDM) or Frequency-Division Multiplexing (FDM). Bits associated with different channels are interleaved in the time domain to form a composite bit stream in Time Domain Multiplexing. For a single voice channel operating at 64 kbps, the bit slot is around 15 microseconds. If the bit streams of successive channels are delayed by 3 microseconds, five channels can be multiplexed through Time Division Multiplexing.
Channels will be spaced apart in the frequency domain in the case of Frequency Domain Multiplexing. Each channel is carried by its own carrier wave. In order to avoid overlap of channel spectra, the carrier frequencies are spaced more than the channel bandwidth. Frequency Domain Multiplexing can be employed in both digital and analog signal transmission. Frequency Domain Multiplexing is used in radio and television broadcasting. On the other hand Time Division Multiplexing has been employed in digital systems, especially in telecommunication networks.
Both Time Division Multiplexing and Frequency Domain Multiplexing techniques can be used in optical and electrical domains. When frequency domain multiplexing is implemented in optical systems, it is often referred as Wavelength division multiplexing or WDM. Time division multiplexing implemented in electrical domain is used world over to multiplex large number of voice channel. Digital hierarchy systems were formed by using Time Division Multiplexing concept. In developed markets such as United States and Japan, 24 voice channels were multiplexed with a composite bit rate of 1.544 Mbits per second. Europe went a little farther by multiplexing 30 voice channels that gave a total bit rate of 2.048 Mbits per second. Since extra bits for control needed to be added, the bit rate of the multiplexed signal is slightly higher than that of the 64 kbits per second with the number of channels. Control bits are necessary to demultiplex the channels at the receiver end.
The next level of hierarchy in digital telecommunication was obtained by multiplexing 4 DS-1 Time division multiplexed channels. The composite bit rate was 6.312 Mbits per second known as DS-2, which is the second level of hierarchy in North America and Japan. 8.448 Mbits per second was the corresponding bit rate in Europe. As the developments continued by improving hierarchical levels, Japan’s NTT reached 396 Mbits per second bit rate while it was 565 Mbits per second in Europe at the fifth level of hierarchy.
International standardization committees were not in control of the development or introduction of new technologies during the 1980s. This led to the introduction of SONET (Synchronous Optical Network) standard, which later establish itself as SDH (Synchronous Digital Hierarchy). As the name indicates, SDH defines a synchronous frame structure for transmitting TDM digital signals.
The basic building block of SONET, named as OC-1 has a bit rate of 51.84 Mbits per second, while the basic building block of SDH starts from 155.52 Mbits per second, whose corresponding level in SONET is OC-3. It is termed as STM-1. STM stands for Synchronous Transport Module. Advantage of SDH and SONET is that the higher bit rates are exact multiples of the basic building bloks. SDH data rates are internationally followed now and fiber optic networks operating at 40 Gbits per second are common now, while operators are busy in updating their networks to 100 Gbits per second.
The below table gives a glance to the SONET and corresponding SDH data rates in megabits per second.