Physical Layer and Overview of PL Switching

Unit-II

Physical Layer and Overview of PL Switching

Multiplexing: Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.

1.      In a multiplexed system, n lines share the bandwidth of one link. Multiplexing is done using a device called Multiplexer (MUX) that combine n input lines to generate one output line i.e. (many to one).

2.      At the receiving end a device called Demultiplexer (DEMUX) is used that separate signal into its component signals i.e. one input and several outputs (one to many).

The word link refers to the physical path. The word channel refers to the portion of a link that carries a transmission between a given pair of lines.

Advantages of Multiplexing:

·         More than one signals can be sent over single medium or link

·         Effective use of the bandwidth of medium

Categories of multiplexing:

Frequency-Division Multiplexing:

Ø  Frequency-division multiplexing (FDM) is an analog technique that can be applied when the bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be transmitted.

Ø  In FDM, signals generated by each sending device modulate different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the link.

Ø  Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal.

Ø  Channels can be separated by strips of unused bandwidth called guard bands used to prevent signals from overlapping.

Ø  In addition, carrier frequencies must not interfere with the original data frequencies.

Multiplexing Process:

Each source generates a signal of a similar frequency range. Inside the multiplexer, these similar signals modulates different carrier frequencies (f1,f2, and f3). The resulting modulated signals are then combined into a single composite signal that is sent out over a media link.

Demultiplexing Process:

The demultiplexer uses a series of filters to decompose the multiplexed signal into its constituent component signals. The individual signals are then passed to a demodulator that separates them from their carriers and passes them to the output lines.

Wavelength-Division Multiplexing:

Ø  Wavelength-division multiplexing (WDM) is designed to use the high-data-rate capability of fiber-optic cable. The optical fiber data rate is higher than the data rate of metallic transmission cable.

Ø  Using a fiber-optic cable for one single line wastes the available bandwidth. Multiplexing allows us to combine several lines into one.

Ø  WDM is an analog multiplexing technique. WDM is conceptually the same as FDM; In WDM different signals are optical or light signals that are transmitted through optical fiber.

Ø  Very narrow bands of light from different sources are combined to make a wider band

of light. At the receiver, the signals are separated by the demultiplexer.

Wavelength-division multiplexing (WDM)

Ø  We want to combine multiple light sources into one single light at the multiplexer and do the reverse at the demultiplexer.

Ø  The combining and splitting of light sources are easily handled by a prism. Recall from basic physics that a prism bends a beam of light based on the angle of incidence and the frequency.

Prisms in wavelength-division multiplexing and demultiplexing

One application of WDM is the SONET network in which multiple optical fiber lines are multiplexed and demultiplexed. A new method, called dense WDM (DWDM), can multiplex a very large number of channels by spacing channels very close to one another. It achieves even greater efficiency.