Fiber Optics for Telecommunication–A Complete and Quick Tutorial (Part 3)

—————————————————————————————————————

Fiber Optics for Telecommunication–A Complete and Quick Tutorial (Part 1)

Fiber Optics for Telecommunication–A Complete and Quick Tutorial (Part 2)

Fiber Optics for Telecommunication–A Complete and Quick Tutorial (Part 3)

Fiber Optics for Telecommunication–A Complete and Quick Tutorial (Part 4)

Fiber Optics for Telecommunication–A Complete and Quick Tutorial (Part 5)

Fiber Optics for Telecommunication–A Complete and Quick Tutorial (Part 6)

Fiber Optics for Telecommunication–A Complete and Quick Tutorial (Part 7)

—————————————————————————————————————

>> What is Multiplexing?

The purpose of multiplexing is to share the bandwidth of a single transmission channel among several users. Two multiplexing methods are commonly used in fiber optics:

1. Time-division multiplexing (TDM)
2. Wavelength-division multiplexing (WDM)

 

1. Time-Division Multiplexing (TDM)

In time-division multiplexing, time on the information channel, or fiber, is shared among the many data sources. The multiplexer MUX can be described as a type of “rotary switch,” which rotates at a very high speed, individually connecting each input to the communication channel for a fixed period of time. The process is reversed on the output with a device known as a demultiplexer, or DEMUX.

After each channel has been sequentially connected, the process repeats itself. One complete cycle is known as a frame. To ensure that each channel on the input is connected to its corresponding channel on the output, start and stop frames are added to synchronize the input with the output.

TDM systems may send information using any of the digital modulation schemes described (analog multiplexing systems also exist). This is illustrated in the following figure.

The amount of data that can be transmitted using TDM is given by the MUX output rate and is defined by the following equation

where N is the number of input channels and the maximum input rate is the highest data rate in bits/second of the various inputs.

The bandwidth of the communication channel must be at least equal to the MUX output rate. Another parameter commonly used in describing the information capacity of a TDM system is the channel-switching rate. This is equal to the number of inputs visited per second by the MUX and is defined as

 

2. The Digital Telephone Hierarchy

The North American digital telephone hierarchy defines how the low-data-rate telephone signals are multiplexed together onto higher-speed lines. The system uses pulse code modulation (PCM) in conjunction with time-division multiplexing (TDM) to achieve this.

The basic digital multiplexing standard established in the United States is called the Bell System Level 1 PCM Standard or the Bell T1 Standard. This is the standard used for multiplexing 24 separate 64-kbps (8 bits/sample × 8000 samples/s) voice channels together. Each 64-kbps voice channel is
designated as digital signaling level 0 or DS-0. Each frame in the 24-channel multiplexer consists of

8 bits/channel × 24 channels + 1 framing bit = 193 bits

The total data rate when transmitting 24 channels is determined by:

193 bits/frame × 8000 frames/s = 1.544 Mbps = T1 designation

If four T1 lines are multiplexed together, we get

4 × 24 channels = 96 channels = T2 designation

Multiplexing seven T2 lines together we get

7 × 96 = 672 channels = T3 designation

The following figure shows how the multiplexing takes place.

 

3. SONET

Fiber optics use Synchronous Optical Network (SONET) standards. The initial SONET designation is OC-1 (optical carrier-1). This level is known as synchronous transport level l (STS-1). It has a synchronous frame structure at a speed of 51.840 Mbps. The synchronous frame structure makes it easy to extract individual DS1 signals without disassembling the entire frame.

OC-1 picks up where the DS3 signal (28 DSI signals or 672 channels) leaves off. With SONET standards any of these 28 T1 systems can be stripped out of the OC-1 signal.

The North American SONET rate is OC-48, which is 48 times the 51.840-Mbps OC-1 rate, or approximately 2.5 billion bits per second (2.5 Gbps). OC-48 systems can transmit 48 × 672 channels or 32,256 channels, as seen in the following table.

One fiber optic strand can carry all 32,256 separate 64-kbps channels. The maximum data rate specified for the SONET standard is OC-192 or approximately 9.9538 Gbps. At this data rate, 129,024 separate voice channels can be transmitted through a single fiber. Even though OC-192 is the maximum data rate specified by SONET, recent developments in technology allow for transmission as high as 40 Gbps. This, coupled with the availability of 32-channel wavelength-division multiplexers, has led to the development of systems capable of 1.2-terabit/s transmission. As can been seen, the data rates achievable through the use of fiber optics are dramatically greater than those achievable with copper. In addition, the distance between repeaters in a fiber optic system is considerably greater than that for copper, making fiber more reliable and, in most cases, more cost-effective.

 

4. Wavelength-Division Multiplexing (WDM)

In wavelength-division multiplexing, each data channel is transmitted using a slightly different wavelength (different color). With use of a different wavelength for each channel, many channels can be transmitted through the same fiber without interference. This method is used to increase the capacity of existing fiber optic systems many times.

Each WDM data channel may consist of a single data source or may be a combination of a single data source and a TDM (time-division multiplexing) and/or FDM (frequency-division multiplexing) signal.

Dense wavelength-division multiplexing (DWDM) refers to the transmission of multiple closely spaced wavelengths through the same fiber. For any given wavelength λ and corresponding frequency f, the International Telecommunications Union (ITU) defines standard frequency spacing ∆f as 100 GHz, which translates into a ∆λ of 0.8-nm wavelength spacing. This follows from the relationship ∆λ = (λ*∆f)/f. You can check out the ITU GRID wavelengths in the following table.

DWDM systems operate in the 1550-nm window because of the low attenuation characteristics of glass at 1550 nm and the fact that erbium-doped fiber amplifiers (EDFA) operate in the 1530-nm–1570-nm range. Commercially available systems today can multiplex up to 128 individual wavelengths at 2.5 Gb/s or 32 individual wavelengths at 10 Gb/s. You can check out this configuration in the following figure.

Although the ITU grid specifies that each transmitted wavelength in a DWDM system is separated by 100 GHz, systems have been demonstrated that reduce the channel spacing to 50 GHz and below (< 0.4 nm). As the channel spacing decreases, the number of channels that can be transmitted increases, thus further increasing the transmission capacity of the system.