Technological networks category, one comparison of interest is among infrastructure networks such as the power generation and distribution networks, the public switched telephone networks (PSTN), the Internet, and various transportation networks that we have come to rely heavily upon. In this paper we will use network analysis to study the PSTN.
.In, wavelength-division multiplexing ( WDM) is a technology which a number of signals onto a single by using different (i.e., colors) of. This technique enables communications over one strand of fiber, as well as multiplication of capacity.The term WDM is commonly applied to an optical carrier, which is typically described by its wavelength, whereas typically applies to a radio carrier which is more often described.
This is purely conventional because wavelength and frequency communicate the same information. Specifically, frequency (in Hertz, which is cycles per second) multiplied by wavelength (the physical length of one cycle) equals the velocity of the carrier wave.
In vacuum, this is the velocity of light, usually denoted by the lower case letter, c. In glass fiber, it is substantially slower, usually about 0.7 times c. The data rate, which ideally might be at the carrier frequency, in practical systems is always a fraction of the carrier frequency. 's WDM SystemA WDM system uses a at the to join the several signals together and a at the to split them apart. With the right type of fiber, it is possible to have a device that does both simultaneously and can function as an.
The optical filtering devices used have conventionally been (stable solid-state single-frequency in the form of thin-film-coated optical glass). As there are three different WDM types, whereof one is called 'WDM', the notation 'xWDM' is normally used when discussing the technology as such.The concept was first published in 1978, and by 1980 WDM systems were being realized in the laboratory. The first WDM systems combined only two signals. Modern systems can handle 160 signals and can thus expand a basic 100 system over a single fiber pair to over 16. A system of 320 channels is also present (12.5 GHz channel spacing, see below.)WDM systems are popular with because they allow them to expand the capacity of the network without laying more fiber. By using WDM and, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network.
Capacity of a given link can be expanded simply by upgrading the multiplexers and demultiplexers at each end.This is often done by use of optical-to-electrical-to-optical (O/E/O) translation at the very edge of the transport network, thus permitting interoperation with existing equipment with optical interfaces.Most WDM systems operate on which have a core diameter of 9 µm. Certain forms of WDM can also be used in (also known as premises cables) which have core diameters of 50 or 62.5 µm.Early WDM systems were expensive and complicated to run. However, recent standardization and better understanding of the dynamics of WDM systems have made WDM less expensive to deploy.Optical receivers, in contrast to laser sources, tend to be devices. Therefore, the demultiplexer must provide the wavelength selectivity of the receiver in the WDM system.WDM systems are divided into three different wavelength patterns: normal (WDM), coarse (CWDM) and dense (DWDM). Normal WDM (sometimes called BWDM) uses the two normal wavelengths 1310 and 1550 on one fiber.
Coarse WDM provides up to 16 channels across multiple of silica fibers. Dense WDM (DWDM) uses the C-Band (1530 nm-1565 nm) transmission window but with denser channel spacing. Channel plans vary, but a typical DWDM system would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing. Some technologies are capable of 12.5 GHz spacing (sometimes called ultra dense WDM).
New amplification options enable the extension of the usable wavelengths to the L-band (1565 nm-1625 nm), more or less doubling these numbers.Coarse wavelength division multiplexing (CWDM), in contrast to DWDM, uses increased channel spacing to allow less-sophisticated and thus cheaper transceiver designs. To provide 16 channels on a single fiber, CWDM uses the entire frequency band spanning the second and third (1310/1550 nm respectively) including the critical frequencies where OH scattering may occur.
OH-free silica fibers are recommended if the wavelengths between second and third transmission windows is to be used. Avoiding this region, the channels 47, 49, 51, 53, 55, 57, 59, 61 remain and these are the most commonly used. With OS2 fibers the water peak problem is overcome, and all possible 18 channels can be used.WDM, CWDM and DWDM are based on the same concept of using multiple wavelengths of light on a single fiber but differ in the spacing of the wavelengths, number of channels, and the ability to amplify the multiplexed signals in the optical space.
Provide an efficient wideband amplification for the, Raman amplification adds a mechanism for amplification in the L-band. For CWDM, wideband optical amplification is not available, limiting the optical spans to several tens of kilometres.Coarse WDM. Series of SFP+ transceivers for 10 Gbit/s WDM communicationsOriginally, the term coarse wavelength division multiplexing (CWDM) was fairly generic and described a number of different channel configurations.
In general, the choice of channel spacings and frequency in these configurations precluded the use of (EDFAs). Prior to the relatively recent ITU standardization of the term, one common definition for CWDM was two or more signals multiplexed onto a single fiber, with one signal in the 1550 nm band and the other in the 1310 nm band.In 2002, the ITU standardized a channel spacing grid for CWDM (ITU-T G.694.2) using the wavelengths from 1270 nm through 1610 nm with a channel spacing of 20 nm.
ITU G.694.2 was revised in 2003 to shift the channel centers by 1 nm so, strictly speaking, the center wavelengths are 1271 to 1611 nm. Many CWDM wavelengths below 1470 nm are considered unusable on older G.652 specification fibers, due to the increased attenuation in the 1270–1470 nm bands. Newer fibers which conform to the G.652.C and G.652.D standards, such as Corning SMF-28e and Samsung Widepass, nearly eliminate the 'water peak' attenuation peak and allow for full operation of all 18 ITU CWDM channels in metropolitan networks.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, which 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.CWDM Applications CWDM is being used in 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. Some and small form factor pluggable transceivers utilize standardized CWDM wavelengths. GBIC and SFP CWDM optics allow a legacy switch system to be 'converted' to enable wavelength multiplexed transport over a fiber by selecting compatible transceiver wavelengths for use with an inexpensive passive optical multiplexing device. The 10 Gbit/s standard is an example of a CWDM system in which four wavelengths near 1310 nm, each carrying a 3.125 gigabit-per-second (Gbit/s) data stream, are used to carry 10 Gbit/s of aggregate data.Passive CWDM is an implementation of CWDM that uses no electrical power. It separates the wavelengths using passive optical components such as bandpass filters and prisms. Many manufacturers are promoting passive CWDM to deploy fiber to the home.
Dense WDM 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 (EDFAs), which are effective for wavelengths between approximately 1525–1565 nm (C band), or 1570–1610 nm (L band). EDFAs were originally developed to replace optical-electrical-optical (OEO), which they have made practically obsolete. EDFAs can amplify any optical signal in their operating range, regardless of the modulated bit rate. In terms of multi-wavelength signals, so long as the EDFA has enough pump energy available to it, it can amplify as many optical signals as can be multiplexed into its amplification band (though signal densities are limited by choice of modulation format). EDFAs therefore allow a single-channel optical link to be upgraded in bit rate by replacing only equipment at the ends of the link, while retaining the existing EDFA or series of EDFAs through a long haul route.
Furthermore, single-wavelength links using EDFAs can similarly be upgraded to WDM links at reasonable cost. The EDFA's cost is thus leveraged across as many channels as can be multiplexed into the 1550 nm band.DWDM systems At this stage, a basic DWDM system contains several main components. WDM multiplexer for DWDM communications. A DWDM terminal multiplexer. The terminal multiplexer contains a wavelength-converting transponder for each data signal, an optical multiplexer and where necessary an optical amplifier (EDFA). Each wavelength-converting transponder receives an optical data signal from the client-layer, such as Synchronous optical networking SONET /SDH or another type of data signal, converts this signal into the electrical domain and re-transmits the signal at a specific wavelength using a 1,550 nm band laser.
These data signals are then combined together into a multi-wavelength optical signal using an optical multiplexer, for transmission over a single fiber (e.g., SMF-28 fiber). The terminal multiplexer may or may not also include a local transmit EDFA for power amplification of the multi-wavelength optical signal. In the mid-1990s DWDM systems contained 4 or 8 wavelength-converting transponders; by 2000 or so, commercial systems capable of carrying 128 signals were available. An intermediate line repeater is placed approximately every 80–100 km to compensate for the loss of optical power as the signal travels along the fiber. The 'multi-wavelength optical signal' is amplified by an EDFA, which usually consists of several amplifier stages.
An intermediate optical terminal, or optical add-drop multiplexer. This is a remote amplification site that amplifies the multi-wavelength signal that may have traversed up to 140 km or more before reaching the remote site. Optical diagnostics and telemetry are often extracted or inserted at such a site, to allow for localization of any fiber breaks or signal impairments.
In more sophisticated systems (which are no longer point-to-point), several signals out of the multi-wavelength optical signal may be removed and dropped locally. A DWDM terminal demultiplexer.
At the remote site, the terminal de-multiplexer consisting of an optical de-multiplexer and one or more wavelength-converting transponders separates the multi-wavelength optical signal back into individual data signals and outputs them on separate fibers for client-layer systems (such as ). Originally, this de-multiplexing was performed entirely passively, except for some telemetry, as most SONET systems can receive 1,550 nm signals. However, in order to allow for transmission to remote client-layer systems (and to allow for digital domain signal integrity determination) such de-multiplexed signals are usually sent to O/E/O output transponders prior to being relayed to their client-layer systems. Often, the functionality of output transponder has been integrated into that of input transponder, so that most commercial systems have transponders that support bi-directional interfaces on both their 1,550 nm (i.e., internal) side, and external (i.e., client-facing) side. Transponders in some systems supporting 40 GHz nominal operation may also perform (FEC) via technology, as described in the standard. Optical Supervisory Channel (OSC).
This is data channel which uses an additional wavelength usually outside the EDFA amplification band (at 1,510 nm, 1,620 nm, 1,310 nm or another proprietary wavelength). The OSC carries information about the multi-wavelength optical signal as well as remote conditions at the optical terminal or EDFA site. It is also normally used for remote software upgrades and user (i.e., network operator) Network Management information.
It is the multi-wavelength analogue to SONET's DCC (or supervisory channel). ITU standards suggest that the OSC should utilize an OC-3 signal structure, though some vendors have opted to use 100 megabit Ethernet or another signal format.
Unlike the 1550 nm multi-wavelength signal containing client data, the OSC is always terminated at intermediate amplifier sites, where it receives local information before re-transmission.The introduction of the ITU-T G.694.1 in 2002 has made it easier to integrate WDM with older but more standard systems. WDM wavelengths are positioned in a grid having exactly 100 GHz (about 0.8 nm) spacing in optical frequency, with a reference frequency fixed at 193.10 THz (1,552.52 nm). The main grid is placed inside the optical fiber amplifier bandwidth, but can be extended to wider bandwidths. The first commercial deployment of DWDM was made by Ciena Corporation on the Sprint network in June 1996.
Today's DWDM systems use 50 GHz or even 25 GHz channel spacing for up to 160 channel operation.DWDM systems have to maintain more stable wavelength or frequency than those needed for CWDM because of the closer spacing of the wavelengths. Precision temperature control of laser transmitter is required in DWDM systems to prevent 'drift' off a very narrow frequency window of the order of a few GHz. In addition, since DWDM provides greater maximum capacity it tends to be used at a higher level in the communications hierarchy than CWDM, for example on the and is therefore associated with higher modulation rates, thus creating a smaller market for DWDM devices with very high performance. These factors of smaller volume and higher performance result in DWDM systems typically being more expensive than CWDM.Recent innovations in DWDM transport systems include pluggable and software-tunable transceiver modules capable of operating on 40 or 80 channels. This dramatically reduces the need for discrete spare pluggable modules, when a handful of pluggable devices can handle the full range of wavelengths.Wavelength-converting transponders. This section's tone or style may not reflect the used on Wikipedia. See Wikipedia's for suggestions.
( December 2018) At this stage, some details concerning wavelength-converting transponders should be discussed, as this will clarify the role played by current DWDM technology as an additional optical transport layer. Main article:As mentioned above, intermediate optical amplification sites in DWDM systems may allow for the dropping and adding of certain wavelength channels.
In most systems deployed as of August 2006 this is done infrequently, because adding or dropping wavelengths requires manually inserting or replacing wavelength-selective cards. This is costly, and in some systems requires that all active traffic be removed from the DWDM system, because inserting or removing the wavelength-specific cards interrupts the multi-wavelength optical signal.With a ROADM, network operators can remotely reconfigure the multiplexer by sending soft commands. The architecture of the ROADM is such that dropping or adding wavelengths does not interrupt the 'pass-through' channels. Numerous technological approaches are utilized for various commercial ROADMs, the tradeoff being between cost, optical power, and flexibility.Optical cross connects (OXCs). This section needs expansion. You can help.
( June 2008)When the network topology is a mesh, where nodes are interconnected by fibers to form an arbitrary graph, an additional fiber interconnection device is needed to route the signals from an input port to the desired output port. These devices are called optical crossconnectors (OXCs). Various categories of OXCs include electronic ('opaque'), optical ('transparent'), and wavelength selective devices.Enhanced WDM 's Enhanced WDM system combines 1 Gb Coarse Wave Division Multiplexing (CWDM) connections using SFPs and GBICs with 10 Gb Dense Wave Division Multiplexing (DWDM) connections using, or DWDM modules. These DWDM connections can either be passive or boosted to allow a longer range for the connection. ITU-T G.694.2, 'WDM applications: CWDM wavelength grid' 2012-11-10 at the. ITU-T G.652, 'Transmission media and optical systems characteristics –Optical fibre cables' 2012-11-10 at the.
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First discussion: O. Delange, 'Wideband optical communication systems, Part 11-Frequency division multiplexing'. 58, p. 1683, October 1970.
Multiplexing is a technique by which different analog and digital streams of transmission can be simultaneously processed over a shared link. Multiplexing divides the high capacity medium into low capacity logical medium which is then shared by different streams.Communication is possible over the air (radio frequency), using a physical media (cable), and light (optical fiber). All mediums are capable of multiplexing.When multiple senders try to send over a single medium, a device called Multiplexer divides the physical channel and allocates one to each. On the other end of communication, a De-multiplexer receives data from a single medium, identifies each, and sends to different receivers. Frequency Division MultiplexingWhen the carrier is frequency, FDM is used. FDM is an analog technology. FDM divides the spectrum or carrier bandwidth in logical channels and allocates one user to each channel.
Each user can use the channel frequency independently and has exclusive access of it. All channels are divided in such a way that they do not overlap with each other. Channels are separated by guard bands. Guard band is a frequency which is not used by either channel. Time Division MultiplexingTDM is applied primarily on digital signals but can be applied on analog signals as well. In TDM the shared channel is divided among its user by means of time slot. Each user can transmit data within the provided time slot only.
Digital signals are divided in frames, equivalent to time slot i.e. Frame of an optimal size which can be transmitted in given time slot.TDM works in synchronized mode. Both ends, i.e.
Multiplexer and De-multiplexer are timely synchronized and both switch to next channel simultaneously.When channel A transmits its frame at one end,the De-multiplexer provides media to channel A on the other end.As soon as the channel A’s time slot expires, this side switches to channel B. On the other end, the De-multiplexer works in a synchronized manner and provides media to channel B. Signals from different channels travel the path in interleaved manner. Wavelength Division MultiplexingLight has different wavelength (colors). In fiber optic mode, multiple optical carrier signals are multiplexed into an optical fiber by using different wavelengths. This is an analog multiplexing technique and is done conceptually in the same manner as FDM but uses light as signals.Further, on each wavelength time division multiplexing can be incorporated to accommodate more data signals. Code Division MultiplexingMultiple data signals can be transmitted over a single frequency by using Code Division Multiplexing.
FDM divides the frequency in smaller channels but CDM allows its users to full bandwidth and transmit signals all the time using a unique code. CDM uses orthogonal codes to spread signals.Each station is assigned with a unique code, called chip. Signals travel with these codes independently, inside the whole bandwidth.The receiver knows in advance the chip code signal it has to receive.