optical network control

Optical Network Control

Jorge M. Finochietto

Cordoba – 2014

LCD EFN UNCLaboratorio de Comunicaciones DigitalesFacultad de Ciencias Exactas, Físicas y NaturalesUniversidad Nacional de Córdoba, Argentina

Optical LayerOverview

Control 2 / 59

Outline

1 Network Management

2 Network Protocols

Control 3 / 59

Outline

1 Network ManagementConfiguration ManagementPerformance ManagementFault Management

2 Network Protocols

Control → Network Management 4 / 59

Optical LayerLightpath as a Service

Provides lightpaths to upper layers (SONET, IP, ATM, ETH)

Behaves as a server layer which provides services to client layers


Optical LayerRequirements

Lightpaths need tobe setup and taken down as required by the client layerprovide the amount of bandwidth required by the client layerimplement adaptation functions on the client layer to make itcompatible with optical layerguarantee a level of preformance (e.g., BER ≤ 10−12)offer some level of protectionmeet jitter requirements to offer transparency to the client layersupport fault management to address failures and reportroot-cause alarms

A management and control interface between the optical layerand the client layer is required

allows to setup or tear down lightpaths with configurationparametersprovides performance and fault management information to theclient layer


Optical LayerManagement and Control

Today lightpaths are set up fairly infrequently and remainednailed down for longs periods of time

In the future, lightpaths could be provisioned dynamically

To this end, there is no need of a signaling interface between theoptical layer and the client oneA network management system (NMS) is used to communicatewith the optical layer network elements (NE)

NEs (e.g., OLT/TLE, ROADM, OXC) are managed by elementmanagement system (EMS)EMS connect to the managed NEs using a data communicationnetwork (DCN)Besides, NEs can communicate each other through a fastsignaling channel (FSC) to exchange real-time control informationEMSs communicate with a NMS through a management network

Besides EMS, a local management system is usually provided toenable craftpeople to manage individual NEs


Optical LayerManagement System


Optical LayerData Communication Network (DCN)

EMS communicates with NEs through the DCN

DCN can be realized in several ways

Out-of-band separate network outside the optical layer (e.g.,existing TCP/IP network)In-band same network as the optical layer

In-band implies associating a control overhead to

a line (multiple wavelengths) → optical domaina path (one lightpath) → electronic domain


Optical LayerOptical Supervisory Channel (OSC)

Available for optical equipment that process multiple wavelengthsMakes use of a separate (dedicated) wavelength from where dataare being transportedOptical amplifiers (i.e., EDFAs) and ROADMs can be managedthis way

For WDM systems in the C-band, the popular choices for theOSC wavelength include 1310nm, 1480nm, 1510nm, or 1620nm.ITU-T has adopted the 1510nm as the preferred choice


Optical LayerRate-Preserving Overhead

Available for optical equipment that processes lightpathsUses the same wavelengths where data are being transportedOLT/LTE and OXC with regeneration can be managed this wayOverhead is already required by protocols for other purposeswhich include forward error correction (FEC) overheadIn this context, overhead can be processed at each regenerationstage (endpoints of a lightpath)


Optical LayerNetwork Management Functions

Fault Management: detects failures and isolates failedcomponents

Configuration Management: tracks equipment, establishesconnections (manually), and adapts signals to the optical layer

Accounting Management: gathers billing and usage data

Performance Management: monitors and measure performancemetrics

Security Management: authenticates users and assignspermissions


Outline


2 Network Protocols

Control → Network Management → Configuration Management 13 / 59

Configuration ManagementOverview

Equipment Management

Keep track of actual equipment on the network and its capabilities

Connection Management

Set up lightpaths. keep track of them and tear them down

Adaptation Management

Converts clients signals to a form that can be used inside the opticallayer


Configuration ManagementConnection Management

Lightpaths pass through multiple nodes

Each lightpath has an unique end-to-end identifier: section trace

Additional identifiers can be used to identify concatenatedlighpaths through 3R regeneration

tandem trace user-defined set of consecutive lighpathspath trace end-to-end connection where client signal travels

These identifiers enables the NMS to identify, verify and managelightpaths


Configuration ManagementAdaptation Management

Client signals may need adaptation to enter the optical layerWavelength conversion signal conversion from one wavelengthto another oneEncapsulation add/remove overhead to manage the signal insidethe optical layerJustification add/remove stuffing to compensate bit ratemismatchesMultiplexing subrate multiplexing of lower-speed client streams

Adaptation is done at transponders / muxponders


Outline


2 Network Protocols

Control → Network Management → Performance Management 17 / 59

Performance vs. Fault ManagementOverview

Performance Management

Measures the performance of the network

Monitors quality-of-service (QoS) provided to client layer

Ensures client layer complies with requirements

Provides input to detect anomalous conditions

Fault Management

Detects failures when they happen

Reports root-cause alarm to supress other correlated alarms

Isolates faults by replacing faulty signals with special ones


Performance ManagementBER Measurement

Besides signal measurements (power level, chromatic dispersion,etc) bit error rate (BER) is the key performance attributeassociated to a lightpath

Difficult to measure BER accurately on the optical domain (i.e.,based on the SNR)

BER can be computed when signal is available in the electricaldomain

Lightpaths use framing protocols which include overhead byteswhich can be used for in-service BER estimation

Errored Blocks: verifies whether a data block contains errors ornot by means of parity checksCorrected Errors: counts the corrected errors by the ForwardError Correction (FEC) code


Performance ManagementBit-Error-Ratio (BER) Measurement

Besides signal measurements (power level, chromatic dispersion,etc) bit error rate (BER) is the key performance attributeassociated to a lightpath

Difficult to measure BER accurately on the optical domain (i.e.,based on the SNR)

BER can be computed when signal is available in the electricaldomain

Lightpaths use framing protocols which include overhead byteswhich can be used for in-service BER estimation

Corrected Errors: counts the corrected errors by the ForwardError Correction (FEC) codeErrored Blocks: verifies whether a data block contains errors ornot by means of parity checks


Performance ManagementErrored Block Measurements

Measurements are based on one-second intervals but typicallyregistered in

15-minute counters24-hour counters

A Severely Errored Second (SES) is thus a one-second periodthat contains at least

15 % of errored blocks, orone or more defects such as TIM, PLM, AIS, OCI (more later)

Otherwise, block errors are considered as Background BlockErrors (BBE)

Both SES and BBE can be expressed as the ratio (SESR andBBER) in a time interval (15-minute, 24-hour)


Outline


2 Network Protocols

Control → Network Management → Fault Management 22 / 59

Fault ManagementAlignment + Connectivity + Payload

Alignment

Lightpaths transport both data and overhead in a structure knownas frameAn alignment signal is inserted to delineates these frame andextract overhead bytes from the signal

Connectivity

Lighpaths are identified by traces at different levelsIt is possibe then to detect incorrect connections (i.e., tracemismatch)

Payload

Lighpaths carry client signals on their payloadIt is possibe then to detect incorrect payloads (i.e., payloadmismatch)


Fault ManagementAlarm Management

A signal failure may cause multiple alarms all over the network

When a link (fiber) fails, all lightpaths on that link fail

Besides the nodes at the end of the failed link, all nodes throughwhich these lightpaths traverse could detect the failure and issuealarms

Alarm supression is acomplished by using

forward defect indication (FDI)backward defect indication (BDI)


Fault ManagementAlarm Supression

When a link fails, the node downstream of the failed link detectsit and generates a defect condition.

A defect condition could be generated because of a high bit errorrate on the incoming signal or an outright loss of light on theincoming signal.

If the defect persists for a certain time period (typically a fewseconds), the node generates an alarm and inserts an FDI signaldownstream to the next node

The FDI is also referred to as the alarm indication signal (AIS)

A node detecting a defect also sends a BDI signal upstream tothe previous node, to notify that node of the failure

If this previous node did not send out an FDI, it then knows thatthe link to the next node downstream has failed.


Outline


2 Network ProtocolsAlignment OverheadFEC OverheadOTU OverheadODU OverheadOPU Overhead

Control → Network Protocols 26 / 59

Optical Transport NetworkOverview

A set of NEs connected by optical fiber links, able to providefunctionality of transport, multiplexing, switching, management,supervision and survivability of optical channels carrying clientsignals

Standarized mainly in ITU-T G.709 + G.798

OTN was designed to provide support for WDM unlike itspredecessor SONET/SDH as well as

to carry much more FEC overheadto provide monitoring of user-defined segments (tandemconnections)to support protocol transparency (CBR service)for asynchronous timing (i.e., frames can be generated usingfree-running oscillators)


Optical Transport NetworkOptical Layers + Electronic Layers


Optical Transport NetworkOptical Transmission Section (OTS)

Manages fiber segments between optical components such asbetween optical amplifiers, or optical amplifiers and WDM muxes.

Made up of i) a payload signal (OTS-P), consisting of allwavelengths with data traffic, and ii) an overhead signal(OTS-O) conveying optical supervisory channel (OSC)


Optical Transport NetworkOTS Continuity Supervision

Loss of Signal Payload (LOS-P) indicates that the incomingpayload signal is absent or that the incoming power level hasdropped below some critical threshold

Loss of Signal Overhead (LOS-O) indicates that the incomingoverhead signal is absent or that the incoming power level hasdropped below some critical threshold

These defects indicate either

a transmitter / receiver failurea path break (e.g., fiber cut, amplifier shutdown)


Optical Transport NetworkOptical Multiplex Section (OMS)

Each link between OLTs or (R)OADMs represents an OMS

Consists of several OTS segements carrying multiple wavelengths

Has access to the optical supervisory channel (OSC)


Optical Transport NetworkOptical Channel (Och)

Takes care of end-to-end routing of the lightpaths

A lightpath traverses many fiber links, wherein it is multiplexedwith many other wavelengths carrying other lightpaths

Manages optical connections between 3R regeneration (i.e.,transponders)


Optical Transport NetworkOch Transport Unit (OTU)

Manages lightpaths provided by the optical layer (OCh) in theelectronic domainDelineates frames, provides identification of the lightpath,monitors its BER performance, carries alarm indicators, andprovides a communication channel between the end pointsAdds the FEC to frames and scrambles them before transmission


Optical Transport NetworkFrame Structure


Optical Transport NetworkControl Overhead


Outline



Control → Network Protocols → Alignment Overhead 36 / 59

Alignment OverheadOverhead Fields

Frame Alignment Signal (FAS) A fixed patter consisting of 6bytes are used to delineate each frame

Multiframe Alignment Signal (MFAS) Some of the overheadfields carry information that is dispersed over multiple frames,referred to as multiframes. The MFAS byte is incremented everyframe providing 256 values indicating the number of the framewithin a multiframe.


Alignment OverheadLoss Conditions (Defects)

Loss of Frame (LOF) indicates that it has not been possible todetect framing for a period of time (3 ms)

Loss of Multiframe (LOM) indicates that it has not beenpossible to detect multiframing for a period of time (3 ms)

Alignment memory: even if in out-of-(multi)frame condition“last” alignment is mantained; thus, the system could enter andrecover from these loss states without changing the actualalignment assumption.


Outline



Control → Network Protocols → FEC Overhead 39 / 59

FEC OverheadCoding Gain

An forward error-correcting code is a technique for reducing thebit error rate on a communicaton channel

It involves transmitting additional bits, called redundancy, alongwith the data bitsThese redundant bits are used by the receiver to correct errors inthe data bits

Increasing tx power may not decrease BER due to non-lineareffects, FEC techniques are typically used for this purposeFEC codes can be characterized by the

overhead, which represents the rate expansion to appendredundancycoding gain, which is the difference (dB) in the OSNR of thecoded and uncoded systems

Since the overhead introduces an OSNR penalty, the coding gainis typicaly defined as the Net Equivalent Coding Gain (NECG)which is the FEC coding gain minus the overhead penalty


FEC OverheadHard Decision FEC

Hard Decision (HD) FEC operate on bits, which means that adecision has been made a priori related to whether an inputsignal is a 1 or 0 valueDefault HD FEC for OTN signal is Reed-Solomon RS(255,239),which adds 16 redundant bits every 239 ones; thus, resulting inan overhead of 6.7 % and a NECG of 5.6 dB @ 1e-12Others HD FEC known as super FEC have been proposed inITU-T G.795.1


FEC OverheadSuper HDFEC


FEC OverheadSoft Decision FEC

Soft Decision (HD) FEC operate on probabilities, which meansthat a decision has not been made a priori related to whether aninput signal is a 1 or 0 value but its probability is used forenhancing teh error correction capacity of the code

An SD FEC is also proposed in ITU-T G.795.1 for the 6.67 %overhead; however, most SD-FEC make use of larger overhead(20 %) to maximize the NECG (11-12dB)


FEC OverheadSuper SDFEC


Outline



Control → Network Protocols → OTU Overhead 45 / 59

OTU OverheadOverhead Fields

Section Monitoring (SM): 3 byte for monitoring at the section(lightpath) level

General Communications Channel (GCC0) 2-byte field thatprovides a clear channel connection between OTU terminationpoints for use as DCN

RESERVED (RES): for future standarization (all zero)


Section MonitoringOverhead Fields


Section MonitoringOverhead Fields

Trail Trace Identifier (TTI) 64 bytes for lighpath identificationdistributed over 64 frames (1 byte on each frame), where 16bytes are used for endpoint ids and the remaining 32 bytes areoperator specific

BIP-8 Parity checksum to detect errored frames (i.e., blocks)

Alarm signals

BEI/BIAE (backward error indicator and backward incomingalignment error) indicates to upstream node the number of erroredblocks (BIP-8 fails) or if there is an incoming alignment errorBDI (backward defect indication (BDI) indicates to upstreamnode whether there is a signal defectIAE (incoming alignment error) indicates to downstream nodethat it has detected an alignment error


Section MonitoringTrail-trace Identifier Mismatch (TIM)

The TTI mismatch process reports the trail trace identifiermismatch defect (TIM)

The process is based on the comparison of expected xAPIs (i.e.,SAPI and DAPI) with the xAPIs in the incoming signal.

An acceptance process is required to define the xAPI values onthe incoming signal, which typically is based on receiving 3consecutive times the same value


Section MonitoringBit-Interleaved-Parity 8 (BIP8)


Section Monitoring(Backward) Incoming Alignment Error)


Outline



Control → Network Protocols → ODU Overhead 52 / 59

Optical Transport NetworkOch Data Unit (ODU)

Manages connections resulting from one or more consecutivelightpaths (i.e., includes 3R regeneration)

End-to-end paths (electropaths) or just ODU path (PM)Intermediate connections (tandem connections) or just ODU TC

Supports path and up to 6 TC monitoring (i.e, connectionsidentification, BER performance, alarm indicators, etc.)


Path & TC MonitoringOvehead Fields


Och Data Unit (ODU)Maintenance Signals

A status field (STAT) is used to indicate the presence of amaintenance signal

ODU-AIS: an alarm indication signal (AIS) is a signal sentdownstream as an indication that an upstream defect has beendetected. Encoded as all 1s signal.ODU-OCI: an open connection indication (OCI) is a signal sentdownstream as an indication that upstream the signal is notconnected (i.e., provisioned) Encoded as 0110 signal.ODU-LCK: a locked signal is sent downstream as an indicationthat upstream the connection is ”locked”, and no signal haspassed through. Encoded as 0101 signal.


Outline



Control → Network Protocols → OPU Overhead 56 / 59

Optical Transport NetworkOch Paylod Unit (OPU)

Adapts client signals to the OTN frames


Optical Transport NetworkOPU Fields

One-byte Payload Type (PT) is used to indicate the content ofthe OPU signal


Optical Network Control

Jorge M. Finochietto

Cordoba – 2014

LCD EFN UNCLaboratorio de Comunicaciones DigitalesFacultad de Ciencias Exactas, Físicas y NaturalesUniversidad Nacional de Córdoba, Argentina

optical network control

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