oft group1 pli aware networks

8/12/2019 OFT Group1 PLI Aware Networks

1/36

1


2/36

2


3/36

Physical layer impairments (PLIs) are incurred by non- ideal optical

transmission medium, accumulate along an optical path, and determine the

feasibility or transmission quality of the light paths.If the received signal quality is not within the receiver sensitivity threshold, the

receiver may not be able to correctly detect the optical signal, causing the light

path (and the corresponding reserved resources) to be useless.

Hence, it is important for network designers and operators to know

1) various important PLIs;

2) their effects on lightpath feasibility;

3) PLI analytical modeling, and monitoring and mitigation techniques;

4) various techniques to communicate PLI information to network layer and

control plane protocols;

5) finally, how to use all these techniques in conjunction with control and

management plane protocols to dynamically set up and manage optically feasible

lightpaths.

The PLIs and their significance depends on:

network typeopaque, translucent, or transparent;

the reachaccess, metro, or core/long-haul;

the number and type of network elementsfiber, wavelengths, amplifiers,

switching elements, etc.;

3


4/36

the type of applicationsreal-time, non-real time,

3


5/36

Types Of PLIs:

Linear PLIs: intensity- independent, static in nature

Non-Linear PLIs: intensity-dependent, dynamic in nature

4


6/36

Linear PLIs:

Power Losses

Power loss can be defined as the optical loss that is accumulated from source to

destination along fiber-links and is normally made up of intrinsic fiber losses and

extrinsic bending losses.

Let P in be the power launched at the input of a fiber of length L; then the output

power Pout is given by Pout = Pin eL, where is the fiber attenuation

coefficient.

Chromatic Dispersion

The degradation of an optical signal caused by the various spectral components

traveling at their own different velocities is called dispersion.

CD causes an optical pulse to broaden such that it spreads into the time slots of

the other pulses.

Most commonly deployed compensation techniques are based on DCF

Polarization mode Dispersion

Polarization mode dispersion (PMD) is a form of modal dispersion where two

different polarizations of light in a waveguide, which normally travel at the same

5


7/36

speed, travel at different speeds due to random imperfections and asymmetries, causing

random spreading of optical pulses.

Unless it is compensated, which is difficult, this ultimately limits the rate at which data

can be transmitted over a fiber.

Figure shows the pulse broadening of a pulse due to the fact that different polarizationcomponents traveling at different speeds.

Polarization Dependent loss

The two polarization components along the two axes of a circular fiber suffer different

rates of loss due to irregularities in the fiber, thereby degrading signal quality in an

uncontrolled and unpredictable manner and introducing fluctuations in optical signal to

noise ratio (OSNR)

The combined effect of PMD and PDL can further degrade the optical signal quality. PDL

is a measure of the peak-to-peak difference in transmission of an opticalcomponent/system w.r.t. all possible states of polarization

Amplifier Spontaneous Emission

The primary source of additive noise in optically amplified systems is due to the ASE

produced by the optical amplifiers used as intermediate repeaters and as preamplifiers at

the receiver end. This noise is often quantified with noise figure (NF).

Excess ASE is an unwanted effect in lasers, since it dissipates some of the lasers power.

In optical amplifiers, ASE limits the achievable gain of the amplifier and increases its

noise level.The ASE noise is very broadband ( 40 nm) and needs to be carefully analyzed to

evaluate its degrading effect on system performance.

CrossTalk

Linear crosstalk arises due to incomplete isolation of WDM channels by optical

components such as OADMs, OXCs, multiplexers/demultiplexers, and optical switches,

i.e., the effect of signal power leakage from other WDM channels on the desired channel.

5


8/36

Non-Linear PLIs:

Fiber nonlinearity originates from the intensity dependence of the fiber refractive

index. The impact of nonlinearity increases as the optical power increases. Thus itenforces the upper limit to launch power.

The importance of non-linear effects is growing due to

1) increase in optical power levels to increase the optical reach,

2) recent developments in optical components such as EDFA and DWDM

systems to build more flexible networks,

3) increase in channel bit-rate to increase the traffic carrying capacity of

wavelengths,

4) decrease in channel spacing to increase the number of wavelengths and overall

network capacity.

6


9/36

Self-Phase Modulation (SPM)

SPM refers to the self-induced phase shift experienced by an optical pulse during

its propagation in optical fibers. An ultra-short optical pulse, when traveling in amedium, will induce a time varying refractive index of the medium, i.e., the

higher intensity portions of an optical pulse encounter a higher refractive index of

the fiber compared with the lower intensity portions. This results in a positive

refractive index gradient (dn/dt) at the leading edge of the pulse and a negative

refractive index gradient (dn/dt) at its trailing edge. This temporally varying

refractive index change results in a temporally varying phase change leading to

frequency chirping, i.e., the leading edge of the pulse finds frequency shift

towards the higher side whereas the trailing edge experiences shift towards the

lower side. Hence, the primary effect of SPM is to broaden the pulse in the

frequency domain, keeping the temporal shape unaltered.

Cross-Phase Modulation (XPM)

The non-linear refractive index seen by an optical pulse depends not only on the

intensity of the pulse but also on the intensity of the other co-propagating optical

pulses, i.e., the non-linear phase modulation of an optical pulse caused by

fluctuations in intensity of other optical pulses is called XPM

Four Wave Mixing (FWM)

FWM originates from third order non-linear susceptibility in optical links. If three

7


10/36

optical signals with carrier frequencies 1, 2 and 3, co-propagate inside a fiber

simultaneously, it generates a fourth signal with frequency 4, which is related to the

other frequencies by

4 = 1 2 3

In general for W wavelengths launched into a fiber, the number of FWM channelsproduced is

M = W 2(W 1)/2)

The FWM effect is independent of the bit-rate and is critically dependent on the channel

spacing and fiber dispersion. Decreasing the channel spacing increases the four-wave

mixing effect.

Stimulated Brillouin Scattering (SBS)

SBS occurs when an optical signal in fiber interacts with the density variations such as

acoustic phonons and changes its path. In SBS, the scattering process is stimulated byphotons with a wavelength higher than the wavelength of the incident signal.

Stimulated Raman Scattering (SRS)

In WDM systems, if two or more optical signals at different wavelengths are injected into

a fiber, the SRS effect causes optical signal power from lower wavelength optical

channels to be trans- ferred to the higher wavelength optical channels. This can skew the

power distribution among the WDM channels reducing the signal-to-noise ratio of the

lower wavelength channels and introducing crosstalk on the higher wavelength channels

7


11/36

PLI-Aware Service Level Agreements (SLA)

RWA algorithms need to consider SLAs that are specific to the optical layer in

order to realize dynamically reconfigurable generalized multi-protocol labelswitching (GMPLS)-based WDM optical networks

Optical Power:

The optical power at the end of a light path has to be within the dynamic range of

the receiver.

An optical receiver needs a minimum power, called receiver sensitivity, to

distinguish between 1s and 0s.

In addition to the receiver sensitivity, the minimum optical power required also

depends on the type of forward error correction (FEC) used.

Minimum Optical Signal to Noise Ratio (OSNR)

To correctly decode and interpret the received signal, it is important for the

received signal to be above the minimum OSNR level.

OSNR depends on several impairments such as ASE, CD, PMD, etc.

8


12/36

Bit-Error Rate (BER)

BER is a measure of service degradation in optical networks and should be below some

threshold level; otherwise false alarms may be sent to higher layers indicating a failure

which eventually may lead to setup of alternate lightpaths or rerouting of traffic.

Q-factor

This method can determine error ratios faster than the traditional BER test. Q-factor

measures the quality of an analog transmission signal in terms of its signal-to-noise ratio

(SNR).

It takes into account physical impairments to the signalfor example, noise, chromatic

dispersion and any polarization or non-linear effectswhich can degrade the signal and

ultimately cause bit errors.

In other words, the higher the value of Q-factor, the better the OSNR and therefore the

lower the probability of bit errors.

Q-factor is the difference between the mean values of the signal levels for a 1 and a 0

(1 and 0), divided by the sum of the noise standard deviation values (1 and 0) at

those two signal levels assuming Gaussian noise and the probability of a 1 and 0

transmission being equal, i.e.,

Q = (10)/(1+0)

8


13/36

RWA algorithms can be defined as a procedure to establish a route and assign a

wavelength for each connection request.

This algorithms try to satisfy the agreed parameters on the SLA when they are

calculating the lightpath.

They can be classified in five approaches according to:

1st The constrains used to verify the feasibility of a lightpath such

as OSNR, BER or Q-factor.

2

nd

The physical impairments that are considered in the feasibilityevaluation.

3rd The type of RWA algorithm that it is used. It can be:

Integrated: Route and wavelength are computed at

the same step.

2-step: Route and wavelength are computed

separately, one after the other.

4th The network scope that is used, such as centralized or

distributed.

5th

The PLI scope -> If PLIs are estimated using analytical models

9


14/36

either in a centralized server or in a distributed manner; or measure in real

time using monitors.

9


15/36

General flow chart for PLI-aware RWA algorithms is shown on the figure.

PLI-aware RWA algorithms establish the lightpaths following these procedure:

1. A new lightpath request arrives.

2. For each connection request a route and a wavelength are found using

a RWA algorithm. It can be 2-step or integrated procedure.

3. The lightpath is tested for its feasibility. Then the OSNR, BER and Q-

factor is estimated on the lightpath. If the estimated OSNR, BER and

Q-factor satisfy the threshold requirement, then the lightpath is

feasible and is admitted into the network. Otherwise, the RWA

algorithm may select other route and/or wavelength and repeat the

procedure to check their feasibility.

10


16/36

Translucent networks: are optical networks where at some intermediate nodes

along the paths OEO conversion is done.

The main RWA challenges on translucent networks are:

Handling the physical layer impairments and the wavelength

contention for all possible lightpaths.

The optical-layer constraints and available resources may change

dynamically.

When there are a large number of nodes, it is necessary to use

regeneration resources inside of a domain.

Due to availability of network resources will change according to the change of

traffic. In order to cope with this variation, the resource should be managed in a

dynamic manner. In consequence, their availability status should be known by the

whole network and been dynamically updated.

11


17/36

Intradomain Dynamic Balancing Routing (IDDBR) is a RWA algorithm use on

translucent networks.

In order to search different routes between source (S) and destination (D) the

IDDBR uses the OLC-BFS procedure.

In addition, in order to achieve a possible path, this algorithm calculates the

Impairment Parameter Triplet (IPT) and the wavelength continuity constraint to

achieve the possible paths.

The IPT is composed by:

Chromatic Dispersion (CD).

Amplified Spontaneous Emission (ASE).

Crosstalk.

12


18/36

OLC-BFC in order to calculate the different routes follows this steps:

1st Discards the possible routes that traverse more optical links.

2nd It looks if there are possible routes that traverse the same

number of links. If there are it discards the routes that have higher

values of IPT. Then, it selects the route which traverses less

number of optical links. If there are not any possible routes that

traverse the same number of optical links, it selects directly the

route which traverses less number of optical links.

13


19/36

The IDDBR RWA algorithm follows 5 steps to get the path between the source

(S) and the destination (D).

Step 1: Trying optical bypass at all nodes

1.It checks if the wavelength of the source and the destination are the same, if it

is it tries to find a route between them using the OLC-BFS procedure. This route

has the same wavelength Ws along all the path.

2.If not, it passes to the step 2.

Step 2: Trying regeneration either at S or at intermediate interior node.

It tries to find a route which has an intermediate regenerator using OLC-BFS. If it

found it, the route will use wavelength Ws from the source (S) to the regeneration

resource (R) and wavelength WT from the R to the destination (D). If it fails, it

goes to step 3.

Step 3: Trying regeneration both at S and at an intermediate node R.

1.It identifies all possible intermediate regenerators that can be used between S

14


20/36

and D.

2. It uses OLC-BFS to select which route and which wavelength will be used between S

and R. It will use WT between R and D.

If it fails, Step 4 will be executed.

Step 4: Trying regeneration at D.

In this step the algorithm checks if there are any R available for this signal at node D. If it

is the regeneration of the signal is carried out at node D. Then, OLC-BFS is used to find

the different wavelengths that will be used along the path.

If it fails, it goes to step 5.

Step 5: Choice among candidate routes.

If Step 5 is executed means that all previous steps had failed and there are a group of

candidate routes.

The preference to select a route from the group of the possible candidates the following

step order has to be followed: 1)>2)>3)>4). If after executing again all the steps it still

results more than one route, the one with the lowest values of IPT will be chosen.

14


21/36

Transparent networks are optical networks where the lightpaths are switched

completely in the optical domain.

The lack of optical regeneration increases the impact of the Physical Layer

Impairments.

PLI-aware RWA algorithms on transparent networks have 2 different approaches:

Centralized: The RWA is done in a centralized manner. But, the

wavelength assignment can be done either centralized or distributed.

Distributed: The RWA is done using extensions of the GMPLS

protocol.

15


22/36

The example of PLI-aware RWA algorithms that uses the centralized approach

which is going to be explained uses the Network Management System.

16


23/36

The main elements that will be used to compute the route in this example of the

centralized approach are:

Network Element (NE): A single NE which is connected to all NEplay the role of a server.

Traffic Engineering Database (TED): It stores information about

the network topology, resource availability and physical parameters.

Network Management System (NMS): It is responsible for the

administration, computation and provisioning functionalities of the

whole network.

17


24/36

This algorithm in order to compute the path follows these steps:

1st A connection request is received by the NMS.

2nd The routing computation process is launched considering both the

current TED information and the requirements of the connection request.

3rd The NMS configures all the NEs optical switching equipment

involved in the path in a parallel way to set up the lightpath.

18


25/36

The distributed approach works in the same way than the centralized approach,

but it has 2 differences in respect of the other approach:

Each NE is responsible to setup the lightpaths.

A GMPLS control plane is needed to establish the end-to-end path.

19


26/36

Traditional transport networks primarily consist of a transport plane and a

management plane. Two sides of the boxes shown in the above slide. Here, the

transport plane is responsible for carrying the user data across the network andcomprises of network equipment such as, line interface cards, switch fabrics,

backplanes and fiber plants. In this scenario all the (operations, administration,

maintenance and provisioning)OAM&P are handled by the management plane.

Now, we have an addition of optical control planes that essentially lie between

management plane and the transport planes. The function of the control plane is

to synchronize the intelligence between NEs. This paves way for NEs to have the

information about the complete network topology and resource information

which can be used to plan, establish and maintain user services. So, the

introduction of control plane is required for fast and flexible resourceprovisioning, high reliability and scalability.

In general, ITU-T has defined the architecture and requirements for an optical

control plane. However, IETF is more focused on developing the protocols for an

OCP.

A third, standard body OIF (Optical Internetworking Forum) is deploying

GMPLS protocols into ASON architecture.

.

20


27/36

The main application of GMPLS in the context of optical networks is the

dynamic establishment of lightpaths. Moreover, its drawback is lack of physical

layer information details such as PLI, transponder characteristics and availability,regenerator/WC avail. Information etc.

GMPLS serves to support switching techniques in multi-layered network. IP at

L3 to OTN at L1

21


28/36


29/36

The extension of routing protocols are used to carry the wavelength availability

and PLI information. The route computation is based on constrained shortest path

first (CSPF) algorithm at the source node. Moreover, a wavelength is selected aswell. During the CSPF calculation, PLIs can be considered and hence no extra

modifications are required for signaling protocol like, resource reservation

protocol(RSVP-TE) in order to guarantee the optical feasibility of the lightpath.

Modeling of PLIs should take into consideration the type of network elements

and vendors.

Several issues and challenges must be explored: most important parameters to

carry, representation of these parameters, delay..etc..

23


30/36

This extension calculates the route at the source node using CSPF (of course

using TED). Then, RSVP-TE is used to select an optically/physically wavelength

before setting up the lightpath. Then, the source node launches the SETUPrequest on the network. This SETUP message accumulates the estimated PLI

performance between any traversed link from source to the destination. The

admission control interface at the destination checks whether the required QoS is

met by analyzing the accumulated PLI performance. If the accumulated PLI

performance is compliant with the receiver sensitivity, the a positive response is

sent back and the lightpath is established. Otherwise, a negative message is sent

back and the process can be repeated using another route. In this method, no

modifications are required in the routing protocol.

In the above process, each node keeps and updated local PLID and models toevaluate PLI performance.

Issues: bi-directional path feasibility checking because non-linear impairments

are asymmetric. Best wavelength to minimize future connection requests.

24


31/36

Routing protocol to carry wavelength availability information and linear

impairments as these are relatively static in nature.

25


32/36

PCE is a new standard from IETF. It is capable of computing a network path or

route taking in account the network graph and PLI constraints. Using this method

the path computation can be either centralized or distributed. In the centralizedversion, PCE is aware of the whole network topology, resource availability, and

physical parameters in a central repository called Traffic Engineering

Database(TED). PCE also has another database, i.e. PLID, obtained from either

NMS or PMS(performance monitoring system).

When a request arrives at NE, it sends query to the PCE, which computes the

required path using TED and PLID information. Next, PCE sends back the

explicit path to the source node. Then, the source node establishes the path using

signaling protocol (PATH/RESV/RSVP-TE).

More traditional approach would be NMS based. Where NMS receives a

connection request and it calculates the route using TED and received set of

requirements. Then, NMS configures all the optical switching elements involved

in the calculated path in parallel [6]. But, in [5] NMS works exclusively with

PCE, and uses signaling protocol to establish path.

SDN/OpenFlow based approach is more appreciated in closed network platforms

like Campus networks and data center where IP/MPLS protocols are not used

heavily.

26


33/36

PCE calculates the path in a centralized or distributed manner taking into account the

global network topology, resources, and physical parameters etc. Which makes it

preferable to guarantee the optimum resource utilization.

26


34/36

PLI has to be considered in designing optical networks and managing off-line

design network.

Different approaches for RWA algorithms has to be taken in considerationdepending on the type of the network. Because, PLI have more influence on

transparent networks.

1. PLIs are incurred due to the non ideal optical transmission media and

when ever there is an optical switching in optical domain PLIs cause severe

damage to the received signal.

2. PLI has to be considered while designing and maintaining the optical

network.

3. Monitoring and modeling of both Linear and Non linear PLIs in the

optical layer are significant.

27


35/36

28


36/36

oft group1 pli aware networks

Documents