NGN WDM Technology - NGN

What is WDM technology?

WDM is a technology that supports different optical signals to be transmitted by a single fiber. This is similar to frequency-division multiplexing (FDM). Different signals are transmitted with different carriers, occupying non-overlapping parts of a frequency spectrum. In case of WDM, the spectrum band used is in the region of 1300 or 1550 nm, which are two wavelength windows at which optical fibers have very low signal loss.
Here each and every window is used to transmit a single digital signal. With the advance of optical components such as distributed feedback (DFB) lasers, erbium-doped fiber amplifiers (EDFAs), and photo-detectors, each transmitting window was used by different several optical signals, each occupying a small traction of the total wavelength window available.
While many optical signals multiplexed within a window is restricted by the precision of these components. You can multiplex over 100 optical channels into a single fiber with latest technology which is known as dense WDM (DWDM).
The main advantage of DWDM's is to increase the optical fiber bandwidth many folds. The large network of fibers in existence around the world can suddenly have their capacity multiplied manifold, without the need to long new fibers, an expensive process. Let’s connect the new DWDM equipment with these fibers.
Let’s use the number and frequency of wavelengths to standardize by the ITU (T). The wavelength set used is important not only for interoperability, but also to avoid destructive interference between optical signals.
Let’s see the following Table which gives nominal, central frequencies based on the 50 GHz, minimum channel spacing anchored to 193.10 THz reference. Note that the value of C (velocity of light) is taken equal to 2.99792458 x 108 m/sec. for converting between frequency and wavelength.
The ITU-T Grid (within C-band), ITU (T) Rec. G.692
Nominal central frequencies (THz) for spacing of 50 GHz Nominal central frequencies (THz) for spacing of 100 GHz Nominal central wavelengths (Nm)
196.10 196.10 1528.77
196.05 1529.16
196.00 196.00 1529.55
195.95 1529.94
195.90 195.90 1530.33
195.85 1530.72
195.80 195.80 1531.12
195.75 1531.51
195.70 195.70 1531.90
195.65 1532.29
195.60 195.60 1532.68
195.55 1533.07
195.50 195.50 1533.47
195.45 1533.86
195.40 195.40 1534.25
195.35 1534.64
195.30 195.30 1535.04
195.25 1535.43
195.20 195.20 1535.82
195.15 1536.22
195.10 195.10 1536.61
195.05 1537.00
195.00 195.00 1537.40
194.95 1537.79
194.90 194.90 1538.19
194.85 1538.58
194.80 194.80 1538.98
194.75 1539.37
194.70 194.70 1539.77
194.65 1540.16
194.60 194.60 1540.56
194.55 1540.95
194.50 194.50 1541.35
194.45 1541.75
194.40 194.40 1542.14
194.35 1542.54
194.30 194.30 1542.94
194.25 1543.33
194.20 194.20 1543.73
194.15 1544.13
194.10 194.10 1544.53
194.05 1544.92
194.00 194.00 1545.32
193.95 1545.72
193.90 193.90 1546.12
193.85 1546.52
193.80 193.80 1546.92
193.75 1547.32
193.70 193.70 1547.72
193.65 1548.11
193.60 193.60 1548.51
193.55 1548.91
193.50 193.50 1549.32
193.45 1549.72
193.40 193.40 1550.12
193.35 1550.52
193.30 193.30 1550.92
193.25 1551.32
193.20 193.20 1551.72
193.15 1552.12
193.10 193.10 1552.52
193.05 1552.93
193.00 193.00 1533.33
192.95 1553.73
192.90 192.90 1554.13
192.85 1554.54
192.80 192.80 1554.94
192.75 1555.34
192.70 192.70 1555.75
192.65 1556.15
192.60 192.60 1556.55
192.55 1556.96
192.50 192.50 1557.36
192.45 1557.77
192.40 192.40 1558.17
192.35 1558.58
192.30 192.30 1558.98
192.25 1559.39
192.20 192.20 1559.79
192.15 1560.20
192.10 192.10 1560.61

DWDM Within The Network

Each SDH network contains two fibers on each side of every node, one to transmit to its neighbor on and one to receive from its neighbor on.
dwdm_within_network (1)
It’s not so bad that having two fibers between a site and there will be many systems running between sites, even though they don’t form part of the same network.
Let’s explain the two networks shown above, four fibers are now required between sites C & D, and laying between sites is extremely expensive. This is where DWDM networks come into play.
fibre_network_problem (1)
With a DWDM framework, the measure of strands required between sites C and D is lessened to a single fiber. Current DWDM gear would multiplex be able to up to 160 channels, speaking to a gigantic sparing in fiber speculation. Since DWDM gear works just with the physical flag, it doesn't influence the SDH layer of the system by any stretch of the imagination. The SDH flag is not ended or intruded, similarly as the SDH organize is concerned. There is as yet an immediate association between the destinations.
dwdm_network_solution (1)
DWDM networks are protocol independent. They transport wavelengths of light and do not operate at the protocol layer.
protocol_independence (1)
DWDM systems are helpful to save network operators’ large amounts of money when laying fiber, even more over the long distances. Using optical amplifiers, it is possible to transmit a DWDM signal to long distances.
An amplifier collects receives a multi-wavelength DWDM signal and simply amplifies it to reach the next site.
An op-amp will increases the red or blue lambdas when amplifying the red lambdas then it will drop out the received blue channels and vice versa. You need one amplifier to amplify the both directions.
dwdm_amplifier
For the DWDM system to operate in a satisfactory way, the incoming wavelengths to the optical amplifier should be equalized.
This process involves setting all the incoming optical sources to the DWDM system with same optical power levels. Wavelengths which will not be same may show errors when carrying traffic.
Some manufacturers DWDM equipment helps field technicians by calculating the optical powers of the incoming channels and recommending, which channels require power adjustment.
wavelength
You can equalize the wavelengths in many ways to be fitted between the fiber management frame and the DWDM coupler where an engineer can adjust the signal at the DWDM coupler side.
The source equipment contain variable output optical transmitters to support an engineer to make adjustment to the optical power through software at the source equipment.
An engineer is enabled to adjust every channel at the DWDM access point as some DWDM couplers have attenuators built in for every received channel.
When multiple frequencies of light travel through a fiber, a condition known as four wave mixing may occur. New wavelengths of light are generated within the fiber at wavelengths/frequencies determined by the frequency of the original wavelengths. The frequency of the new wavelengths is given by f123 = f1 + f2 - f3.
The scenario of the wavelengths can affect the optical signal to noise ratio within the fiber, and affect the BER of traffic within a wavelength.

WDM COMPONENTS

WDM components are built on the basis of various optics principles. Following Figure explains about a single WDM link. DFB lasers are used as transmitters, one for each wavelength. An optical multiplexer adds these signals into the transmission fiber. The main use of optical amplifiers are to pump the optical signal power up, to compensate for system losses.
dwdm_coupler
The other side of receiver side, optical de-multiplexers separate each wavelength, to be delivered to optical receivers at the end of the optical link. Optical signals are added to the system by the optical ADMs (OADMs).
These optical devices are similar to the digital ADMs, grooming and splitting optical signals along the transmission path. OADMs are built with arrayed-waveguide gratings (AWG), and other optical technologies like fiber bragg gratings also used.
Optical switch is known as the WDM component. This device is having the capacity to exchange optical signals from a given input port to a given output port which is similar to an electronic crossbar. Optical switches supports optical networks to enrooted to the optical signal towards its appropriate destination.
The other important optical component is the wavelength converter. A wavelength converter is a device which exchanges an optical signal from a given wavelength to another signal on a different wavelength with same digital content. This capability is important for WDM networks because it provides more flexibility in routing optical signals across the network.

OPTICAL TRANSPORT NETWORKS

WDM networks are built by connecting wavelength cross connect (WXC) nodes in a specific topology of choice. WXCs are considered by wavelength multiplexers and demultiplexers, switches, and wavelength converters.
The following Figure depicts a generic WXC node architecture.
dwdm_signal
Usually Optical signals are multiplexed in the same fiber which arrive at an optical de-multiplexer. The signal is decomposed into its several wavelength carriers, and sent to a bank of optical switches. The optical switches route the several wavelength signals into a bank of output.
Multiplexers are transmitted into the outgoing fibers for transmission. Wavelength converters are used to switch between the optical switch and the output multiplexers to provide more routing flexibility.

A Wavelength Cross-Connect Node

Optical transport networks (OTNs) are WDM networks which provides support to transport services via light paths. A light path is a high-bandwidth pipe carrying data at up to several gigabits per second. You can decide the speed of the light path with the technology of the optical components (lasers, optical amplifiers, etc.). Speeds on the order of STM-16 (2488.32 Mbps) and STM-64 (9953.28 Mbps) are currently achievable.
While an OTN is a combination of WXC nodes, plus a management system, which controls the set-up and teardown of light paths with supervisory functions like monitoring of optical devices (amplifier, receivers), fault recovery, and so on. The set up and teardown of light paths are to be executed over a large time scale such as hours or even days, given that each of them provides backbone.
There is a great deal of adaptability in how OTNs are sent, contingent upon the vehicle administrations to be given. One reason for this adaptability is that most optical segments are straightforward to flag encoding. Just at the limit of the optical layer, where the optical flag should be changed over back to the electronic space, does the encoding make a difference bandwidth capacity.
Thus, transparent optical services to support various legacy electronic network technologies, such as SDH, ATM, IP and frame relay, running on top of the optical layer, is a likely scenario in the future.
  • The optical layer is further divided into three sublayers −
  • The optical channel layer network, which interfaces with OTN clients, providing optical channels (OChs).
  • The optical multiplex layer network, which multiplexes various channels into a single optical signal.
  • The optical transmission section layer network, which provides the transmission of the optical signal across the fiber.

OTN FRAME FORMAT

It is same as the use of a SDH frame, which can be accessed with the OCh is expected to be through an OC frame. The main criteria of frame size corresponds to STM-16 speed or 2488.32 Mbps, which includes the basic OCh signal. The following Figure depicts a possible OCh frame format.
otn_frame_format

An Optical Channel Frame

If you observe the given figure the leftmost region of the frame is allocated for overhead bytes. These bytes are to be used for OAM&P functions same as the overhead bytes of the SDH frame, discussed earlier.
It can also supports the additional functions like the provision of dark fibers (reservation of a wavelength between two end points for a single user) and wavelength-based APS. The rightmost region of the frame is allocated for a forward error correction (FEC) scheme to be exercised on all payload data. An FEC in an optical transmission layer is used to increase the maximum span length, and it also reduces the number of repeaters. A Reed-Solomon code can be used.
Various OChs are to be combined in the optical domain to create the optical multiplexer signal (OMS). This parallels to the multiplexing of several STM-1 frames into an STM-N SDH frame format. Multiple OChs can be multiplexed to form OMS.
Usually the optical client signal is fixed within the OCh payload signal. Client signal is not restricted by the OCh frame format. Other than this the client signal is used to be only a constant bit rate digital signal. This format is also not relevant to the optical layer.

WDM RINGS

Technically a WDM ring is similar to a SDH ring. WXCs are interconnected in a ring topology, similar to SDH ADMs in a SDH-ring. The key difference between a SDH ring and a WDM ring is rooted in the WXC capabilities of wavelength switching and conversion.
The various features are used to provide various levels of protection with no parallel in SDH technology. While, wavelength or light path protection can be included along with the path and line protection.
Optical APS protocols are bit critical as SDH APSs. Get protected at the OCh level or the optical multiplex section/optical transmission section level. The extra protection capabilities can be implemented with no parallel in SDH rings. Unlike, a failed light path (e.g. a laser failure) can be fixed by converting an optical signal from a given wavelength into a different one, avoiding the rerouting of the signal.
This is equivalent to span switching in SDH, with the difference that even two fiber WDM rings can provide such capability for OCh protection. In the OMS layer, however, span protection will require four fiber rings, as in SDH. These extra features will undoubtedly introduce extra complexity in the optical-layer APS protocols.
Once the WDM ring is up, light paths need to be established in accordance with the traffic pattern to be supported.

MESH WDM NETWORKS

Mesh WDM networks are built with the similar optical components like WDM rings. The protocols used in mesh networks are quietly different from those used in rings. Like the protection in mesh networks is a more critical proposition as is the problem of routing and wavelength assignment in WDM mesh networks.
Mesh networks are considered as the major support infrastructures connecting WDM rings. Some of these connections may include be optical, avoiding optical/electronic bottlenecks and providing transparency. Others will require the conversion of the optical signal into the electronic domain for monitoring management, and perhaps billing purposes. Let’ see the following Figure depicts a WDM network.
backbone_network
Infrastructure − In this figure, three following topology layers are shown −
  • Access Network
  • Regional Network
  • Backbone Network

WDM Network Infrastructure

Both SDH rings and passive optical networks (PONs) as access networks are included. They are generally based on a bus, or star topology and medium access control (MAC) protocol is used to coordinate transmissions among the users. No routing functionality is provided in such networks.
These architectures are practical for networks supporting at most a few hundred users over short distances. Although PONs are less expensive networks than WDM rings, due to the lack of active components and features such as wavelength routing, the lasers necessary at the PON sources make the first generation of such equipment still more expensive than SDH rings. This favors the SDH solution at the access network level, at least in the near future.
Backbone networks contain active optical components, hence providing functions such as wavelength conversion and routing. The backbone networks will have to somehow interface with legacy transport technologies, such as ATM, IP, PSTN, and SDH.
The overall scenario is depicted in the following Figure. Several types of interface involved in the figure.
wdm_network_infrastructure (1)
Overlaying a WDM Transport Network carrying ATM/IP Traffic.

SDH Frame Encapsulation

The OCh frame must be defined so that SDH frame encapsulation can be easily done. The entire STM-16xc, for instance has to be carried as an OCh payload. If a basic STM-16 optical channel is used, it might not be possible to encapsulate SDH-16xc into STM-16 optical channel, due to the OCh overhead bytes.
The OCh frame format is currently being defined. The following Figure exemplifies SDH frame encapsulation into OCh frame.
sdh_frame_encapsulation (1)

SDH Interfaces to WDM

WDM equipment with physical SDH interfaces will deliver optical signals to SDH devices. These interfaces must be for backward compatibility with SDH technology. Therefore, the SDH device need not be aware of the WDM technology used to transport its signal (e.g. the device can belong to a BLSR/4 ring).

In this case, the WXC will drop and add into the optical medium the wavelength originally used in the SDH ring. This way, WDM and SDH layers are completely decoupled, which is necessary for WDM interoperability with SDH legacy equipment.

This puts extra constraints on the selection of wavelengths in the optical layer, since the last-hop wavelength, the one interfacing with the SDH device, must be the same one used by SDH device to terminate the optical path, if wavelength conversion is not provided within SDH device.

SDH Interfaces to WDM

WDM equipment with physical SDH interfaces will deliver optical signals to SDH devices. These interfaces must be for backward compatibility with SDH technology. Therefore, the SDH device need not be aware of the WDM technology used to transport its signal (e.g. the device can belong to a BLSR/4 ring).

In this case, the WXC will drop and add into the optical medium the wavelength originally used in the SDH ring. This way, WDM and SDH layers are completely decoupled, which is necessary for the WDM interoperability with SDH legacy equipment. This puts extra constraints on the selection of wavelengths in the optical layer, since the last-hop wavelength, the one interfacing with the SDH device, must be the same one used by SDH device to terminate the optical path, if wavelength conversion is not provided within SDH device.

A WDM Link


Technology
Detection Restoration Details
WDM WDM-OMS/OCH 1-10ms 10-30ms Ring/P-P
SDH SDH 0.1ms 50ms Ring
APS 1+1 0.1ms 50ms P-P
ATM FDDI 0.1ms 10ms Ring
STM 0.1ms 100ms
ATM PV-C/P 1+1 0.1ms 10msxN Standby N=#hops
ATM PNNI SPV-C/P, SV-C/P 40s 1-10s
IP Border Gateway Protocol 180ms 10-100s
Interior Gateway Routing Protocol and E-OSPF 40s 1-10s
Intermediate System 40s 1-10s
Routing Internet Protocol 180s 100s

As per the Table shown above, although restoration is faster in WDM than the SDH technology, failure detection in WDM is slower. Safer overlay of WDM/SDH protection mechanisms calls for a faster WDM protection scheme. Alternatively, SDH APSs could be artificially slowed down if SDH clients can afford the performance degradation incurred by such procedures.

Unnecessary failure recovery at higher layers may cause route instability and traffic congestion; hence, it should be avoided at all costs. Fault persistence checks can be used at higher layers to avoid early reaction to faults at lower layers.

A failure recovery at the OMS sublayer can replace recovery procedures of several instances of the SDH signals being served by optical layer. Thus, a potentially large number of SDH clients are spared from starting failure recovery procedures at their layers. Therefore, a single failure recovery at the optical OMS sublayer can spare hundreds.

Evolution towards an All-Optical Transport Network

Evolution towards an all-optical WDM network is likely to occur gradually. First, WXC devices will be connected to existing fibers. Some extra components might be necessary in the optical link, such as EDFAs, in order to make legacy fiber links suitable to WDM technology. WXCs will interface with legacy equipment, such as SDH and fiber distributed data interface (FDDI).

A plus of an all-optical transparent transport network is that the transferring of SDH functions into either the layer above (IP/ATM) or below (WDM) SDH is likely to happen, bringing savings in terms of network upgradability and maintenance. Such layer re-organization could affect transport networks, assume that real-time traffic, including voice, is packetized (IP/ATM). This could lead to the extinction of VCs' SDH signals.

A key issue then would be how to most efficiently pack packets into SDH, or even directly into OCh frames. Whatever new encapsulation method emerges, back compatibility with IP/PPP/HDLC and ATM encapsulation is a must

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