Multiplexing

Multiplexing

Multiplexing

Multiplexing

Multiplexing

Multiplexing

Synchronization

Multiplexing - Digital Data and the Standards

Multiplexing - E1 Frame

Multiplexing - Hierarchy

Multiplexing

Multiplexing

Multiplexing

Multiplexing

Multiplexing

Multiplexing

Multiplexing

- Data from different sources put together on an optical fiber, each signal

carried at the same time on its own separate light wavelength

- Using Wavelength Division Multiplexing (WDM) or Dense Wavelength

Division Multiplexing (DWDM)

- more than 16 ,

- realized up to 160

- theoretically 15,000 channels pronounced

separate wavelengths or channels of data can be multiplexed into a

transmitted lightstream on a single optical fiber

Multiplexing - WDM

- Each channel carries a (TDM) signal.

- In a system with each channel carrying 2.5 Gbps (30,720 telephone

channels)

- Up to 160 x 2.5 Gbit/s (total of 4,915,000 telephone channels)

realizable by the same optical fiber

- If 15,000 channels can be realized -> total of 460,800,000 telephone

channels

Multiplexing – Fiber Losses

- The International

Telecommunications Union (ITU)

specified a grid of standard

frequencies separated by

increments of 100 GHz

(approximately 0.8 nm),

referenced to a frequency of 193.1

THz (corresponding to a

wavelength of 1552.52 nm)

- These wavelengths are in the "conventional" or C band of the erbium-

doped fiber amplifier (EDFA) at 1530 to 1570 nm

- Other bands of interest are the "long" or L band (approximately 1570 to

1610 nm) and the "short" or S band (approximately 1490-1530 nm)

- The importance of DWDM is for exploiting the enormous capacity of

optical fiber to carry information

Multiplexing – Fiber Communication Windows

• Earliest fiber optic systems developed for a wavelength of 850 nm

• This wavelength corresponds to the so-called 'first window' in a silica-based optical fiber

• This window refers to a wavelength region that offers low loss

• It sits between several large absorption peaks caused primarily by moisture in the fiber and Rayleigh scattering.

Multiplexing – Communication Windows

• The 850 nm region was initially attractive because the technology for light emitters at this wavelength had already been perfected

• As technology progressed, the first window became less attractive because of its relatively high 3 dB/km loss limit

• Most companies jumped to the 'second window' at 1310 nm with lower attenuation of about 0.5 dB/km.

• In late 1977, Nippon Telegraph and Telephone (NTT) developed the 'third window' at 1550 nm.

• It offered the theoretical minimum optical loss for silica-based fibers, about 0.2 dB/km

Multiplexing - Communication Windows

• Today, 850 nm, 1310 nm, and 1550 nm systems are all manufactured and deployed along with very low-end, short distance, systems using visible wavelengths near 660 nm

• Each wavelength has its advantage:

• Longer wavelengths offer higher performance, but have higher cost

• Shortest link lengths can be handled with wavelengths of 660 nm or 850 nm

• The longest link lengths require 1550 nm wavelength systems

• A 'fourth window,' near 1625 nm, is being developed

• While it is not lower loss than the 1550 nm window, the loss is comparable, and it might simplify some of the complexities of long-length, multiple-wavelength communications systems.

Multiplexing - Communication Windows

Multiplexing – Source Wavelengths

• In 2002 the ITU standardized a channel spacing grid for use with CWDM (ITU-T G.694.2), using the wavelengths from 1270 nm through 1610 nm with a channel spacing of 20 nm.

• CWDM is also being used in cable television networks, where different wavelengths are used for the downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example the downstream signal might be at 1310 nm while the upstream signal is at 1550 nm.

• Dense wavelength division multiplexing (DWDM) refers originally to optical signals multiplexed within the 1550 nm band so as to leverage the capabilities (and cost) of erbium doped fiber amplifiers (EDFAs), which are effective for wavelengths between approximately 1525–1565 nm (C band), or 1570–1610 nm (L band).

Multiplexing – CWDM & DWDM

• The main characteristic of the recent ITU CWDM standard is that the signals are not spaced appropriately for amplification by EDFAs

• This limits the total CWDM optical span to somewhere near 60 km for a 2.5 Gbit/s signal

• This is suitable for use in metropolitan applications

• The relaxed optical frequency stabilization requirements allow the associated costs of CWDM to approach those of non-WDM optical components.

Multiplexing – CWDM & DWDM

Multiplexing – Band Names

• Fiber types and WDM regions

Multiplexing – CWDM & DWDM

- Most DWDM systems support standard SONET/SDH short-reach

optical interfaces to which any SONET/SDH compliant "client" device

can attach

- Clients may be SONET/SDH terminals or add/drop multiplexers

(ADMs), ATM switches, or IP routers

- Within the DWDM system a device called a transponder converts the

SONET/SDH compliant optical signal from the client back to an

electrical signal

- This electrical signal is then used to drive a WDM laser

- WDM laser is a very precise laser operating around the 1550-nm

wavelength range

Multiplexing - WDM

• Wavelength-converting transponders served originally to translate the transmit wavelength of a client-layer signal into one of the DWDM system's internal wavelengths in the 1,550 nm band

• In the mid-1990s, however, wavelength converting transponders rapidly took on the additional function of signal regeneration

• Signal regeneration in transponders quickly evolved through 1R to 2R to 3R and into overhead-monitoring multi-bitrate 3R regenerators

Multiplexing - Wavelength-converting transponders

Multiplexing - WDM

• A transponder converts the incoming signal from the end or client device to a WDM wavelength or lambda.

• Transponders are available with single or multiple lanes per module.

• A quadruple transponder, for example, has four client and four WDM network ports per module

• Typically client and network ports have an equal number.

• The building blocks of a datacenter optimized transponder design are shown in the figure below.

Multiplexing

• 1R – Retransmission: Early transponders were "garbage in garbage out" - output was nearly an analogue "copy" of the received optical signal, with little signal cleanup occurring. Signal monitoring was basically confined to optical domain parameters such as received power

• 2R - Re-time and re-transmit. Transponders of this type were not very common and utilized a quasi-digital Schmitt-triggering method for signal clean-up. Some rudimentary signal-quality monitoring was done by such transmitters that basically looked at analogue parameters

• 3R - Re-time, re-transmit, re-shape. 3R Transponders were fully digital and normally able to view SONET/SDH section layer overhead bytes. They are able to perform regeneration on signals, and reporting on signal health by monitoring SONET/SDH section layer overhead bytes.

Multiplexing - Wavelength-converting transponders

- Each transponder within the system converts its client's signal to a

slightly different wavelength

- The wavelengths from all of the transponders in the system are

then optically multiplexed onto a single fiber

- In the receive direction of the DWDM system, the reverse process

takes place

- Individual wavelengths are filtered from the multiplexed fiber and

fed to individual transponders, which convert the signal to electrical

and drive a standard SONET/SDH interface to the client

Multiplexing - WDM

Multiplexing - WDM

Multiplexing

Drawbacks of the TDM approach:

- TDM requires a service-affecting, “all-at-once” upgrade to the new

higher rate

- Therefore, network interfaces must be replaced by units with four times

their capacity, whether or not all the capacity is immediately required

- Whereas DWDM is non-service affecting, capacity upgrades in 2.5Gbps

increments, 2.5Gbps to 10 Gbps as demand increases

- TDM constrains the capacity of the fiber to the speed of the available

electronics.

- The highest transmission rate in commercially available electronics is

10Gbps while the capacity of the fiber is orders of magnitude higher

- Electronic components capable of operating at this speed are costly to

construct, operate and maintain.

- With DWDM, electrical components operate at 2.5Gbps while the

multiplexing is done in the optical domain