WDM and T carrier system

Wavelength Division Multiplexing (WDM)

WDM is the multiplexing of several optical signals on one optical fibre by giving each a different wavelength (colour) of light. It is FDM in the optical domain.

 l1 -->|\                    /|--> l1
 l2 -->| MUX ==== fibre ==== DEMUX|--> l2
 l3 -->|/   (l1+l2+...+lN)   \|--> l3
 lN -->                         --> lN
 l = wavelength (lambda)

• Each channel has its own laser at wavelength $\lambda_1, \lambda_2, \ldots, \lambda_N$ (typically in the 1550 nm band).
• A passive multiplexer (prism, diffraction grating or thin-film filter) combines them; at the far end a demultiplexer separates them. Erbium-doped fibre amplifiers boost all wavelengths together.
• CWDM uses wide spacing (20 nm, up to 18 channels); DWDM uses close spacing (0.8 nm/100 GHz or less, 40–160 channels), giving terabits per second on one fibre.

T carrier system

The T carrier system is the North American digital TDM hierarchy (AT&T). The basic unit is a 64 kbps PCM voice channel (DS0): 8000 samples/s × 8 bits.

T1 frame structure: 24 channels × 8 bits + 1 framing bit = 193 bits per 125 µs frame.

 1 bit  8 bits  8 bits        8 bits
+-----+-------+-------+-----+--------+
|  F  |  Ch1  |  Ch2  | ... |  Ch24  |
+-----+-------+-------+-----+--------+
<--------- 193 bits in 125 us -------->
F = framing bit

$$\begin{aligned} \text{Bits per frame} &= 24 \times 8 + 1 = 193\ \text{bits} \\ \text{Frame rate} &= 8000\ \text{frames/s}\ (T = 125\ \mu\text{s}) \\ R_{T1} &= 193 \times 8000 = 1.544\ \text{Mbps} \end{aligned}$$

• Framing bits from 12 frames form a superframe pattern; signalling is carried by robbing the LSB of each channel in frames 6 and 12.

Multiplexing levels:

24 x DS0 --> [MUX] --> T1  1.544 Mbps (24 ch)
 4 x T1  --> [M12] --> T2  6.312 Mbps (96 ch)
 7 x T2  --> [M23] --> T3  44.736 Mbps (672 ch)
 6 x T3  --> [M34] --> T4  274.176 Mbps (4032 ch)

$$\begin{aligned} T2 &: 4 \times 1.544 = 6.176;\ \ 6.312 - 6.176 = 0.136\ \text{Mbps overhead} \\ T3 &: 7 \times 6.312 = 44.184;\ \ 44.736 - 44.184 = 0.552\ \text{Mbps overhead} \\ T4 &: 6 \times 44.736 = 268.416;\ \ 274.176 - 268.416 = 5.76\ \text{Mbps overhead} \end{aligned}$$

The overhead at each higher level carries framing, pulse-stuffing (justification) control and alarm bits.