Protocols

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The transmission and the reception of information by an optical fiber follow methods of codification or protocols similar to those of another means of transport nonoptician when it settles down between two certain points. The simplest system is to modulate the optical signal varying the electrical intensity that is applied to the generating device of the light. This allows to send a single signal. In the cases in that it is used to transmit different signals in the same channel, the simplest technique is the one of modulation of subcarriers. The signal is superposed in auxiliary subcarriers and soon combined using the resulting electrical signal to modulate optical carrier (SCM: Sub Carrier Multiplexing). These previous techniques are inherited of the systems of radio frequency communications. Own of the optical communication, for this type of connections, it is the technology of combination of a number of wavelengths in same fiber (WDM: Wavelength Multiplexing Division). The transmission of light by optical fibers presents/displays different properties based on the wavelength of the beam that crosses it. The main characteristic that takes advantage of for this type of information transmission is the attenuation that presents/displays the fiber for the different frequencies from the light source.

Attenuation in the optical fiber based on the 
wavelength of the light

In order to see the WDM potential, first we will examine the characteristics of a light source of high quality. Like example, the modulated exit of a laser of type DFB (Distributed Feedback Laser) has a phantom of 50 frequencies from 10 to MHz, which is equivalent to wide of 1E-3 nm (1 thousandth of nanometer or picómetro). When a source is used as this usually settles down a guard band - separation between 1.6 adjacent channels from 0.4 to nm anticipating possible alterations with the passage of time or elasticity effects. In the previous figure we can see two regions of little attenuation of a fiber monkey-way. By a side the rank of the 1270 to 1350 nm (call 1310 window nm) and on the other hand rank from 1480 to 1600 nm (the 1550 window nm). In order to find the wide bandwidth corresponding to a spectral one in individual, we used the relation c=λv that relates the wavelength λ, with carrier frequency v, where c is the speed of the light. Differentiating this we have for Δλ<<λ² |Δv| = (c/λ²) |Δλ|

Of the previous equation we have Δv = 14 THz (Tera Hertz) for a U.S.ABLE spectral band of Δλ = 80 nm in the 1310 window nm. And, also we obtain that Δv = 15 THz for a usable spectral band of Δλ = 120 nm in the 1550 window nm. This gives like result a total bandwidth of the fiber of about 30 THz in the two windows of low attenuation. Using different light sources, each one emitting with a wavelength that sufficiently is spaced of its neighbor, in such a way that they are not interfered with, the integrity of the independent messages of each source stays for a later conversion to electrical signals in the receiver. The definition of these communication channels based on the wavelength, settles down according to ITU (International Telecommunication Union) in frequencies. The reason fundamental to select fixed frequencies for the spaced one of channels, instead of wavelengths, is in which when fixing the way of operation of a laser is the frequency which is selected. Recommendation ITU-T G.692 specifies that the channels have to be selected of 193.100 referenciadas frequencies to THz (1552.524 nm) and to space them 100 GHz (0.8 nm to 1552 nm). Other 50 alternatives of spaced are GHz (0.4 nm) and 200 GHz (1.6 nm). The fundamental advantage of WDM is that the discreet wavelengths form an orthogonal set of carriers that can separated, enrutadas and be exchanged without interfering with one in the other.

With the arrival of the lines of communication of optical fiber, the following passage in the evolution of TDM (Time Division Multiplexing) was the creation of a standard format of called signal SONET (Synchronous Optical Network) for North America and SDH (Synchronous Digital Hierarchy) in other parts of the world. Protocols SONET and SDH specify formats for the optical signals that they can be shared between different networks (European and American). The most excellent characteristics of these standards cover the normalization with the structure of the data (it dates-frame), the specifications of the optical interface and the fundamental architectures of call. Although there are some differences in the implementation between SONET and SDH, all specifications SONET fulfill recommendations SDH.

Basic structure of a plot STS-1 SONET

In the previous figure we can see the basic structure of a plot SONET. She is one structures bidimensional consisting of 90 columns of 9 rows of octetos. The fundamental plot has 125 µs of duration. We have then that, the speed of transference of a basic signal SONET is: STS-1 = (90 octetos/fila) * (9 filas/trama) * (8 bitios/octeto)/(125 µs/trama) = 51.84 Mb/s This is what a signal STS-1 is called, where STS means Syinchronous Transport Signal. All other signals SONET are multiple whole numbers of this rate of transference, so a signal STS-N has a bit-rate (rate of transference) of N 51.84 times Mb/s. After the electrical conversion to optics, the optical signal of the physical layer that is is denominated OC-N, where OC means Optical Carrier. He is very current to talk about to connections SONET as I connect OC-N.

In SDH the basic speed of transference that is taken is equivalent to 155.22 STS-3 that correspond to Mb/s; to that STM-1 is called (Synchronous Transport Module - Level 1). At the highest speeds one denominates them of form STM-M. The values of M (in SDH) supported by recommendations ITU-T are M = 1, 4, 16, and 64. They are equivalent to SONET OC-N, where N = 3M. We see that, actually to maintain the compatibility between SONET and SDH, N has to be a multiple of 3.

Rates of transference used normally in SONET and 
SDH

As much SONET as SDH treats the first signal before their transmission to prevent long sequences with 1 or 0 that loss of synchronism would produce.

Transmission range

Window 1310 nm

Window 1550 nm

Attenuation to 1310 nm

Attenuation to 1550 nm

< 15 km
< 40 km
< 80 km

1260-1360 nm
1260-1360 nm
1280-1335 nm

1430-1580 nm
1430-1580 nm
1480-1580 nm

3,5 dB/km
0,8 dB/km
0,5 dB/km

Not specified
0,5 dB/km
0,3 dB/km

Fig: Ranks of wavelength and attenuation of the fiber according to the distance of the transmission

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24-1-2008

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