Optical Networking (part 2)

Mark E. Allen, Ph.D.mark.allen@ieee.org

Review of Transmission(Transport) Technologies,

Architectures and Evolution(Adapted from Shikuma (RIT) Notes

Asynchronous Data Rates

•Digital Signal Level 0 DS0 64 Kb/s– internal to equipment

•Digital Signal Level 1 DS1 1.544 Mb/s– intra office only (600 ft limit)

•Digital Signal Level 3 DS3 45 Mb/s – intra office only (600 ft limit)

•T1 Electrical (Copper) Version of DS1 1.544 Mb/s– repeatered version of DS1 sent out of Central Office

•T3 Electrical (Copper) Version of DS3 45 Mb/s– repeatered version of DS3 sent out of Central Office

Asynchronous Digital Hierarchy

DS1 DS3

Asynchronous Optical Line SignalN x DS3s

28 DS1s = 1 DS324 DS0s = 1 DS1

DS0 (a digitized analog POTS circuit @ 64 Kbits/s)

Asynchronous Lightwave Systems typically transport traffic in multiples of DS3s i.e.... 1, 3, 12, 24, 36, 72 DS3s

Asynchronous NetworkingManual DS1 Grooming/Add/Drop

LW M13

• Manually Hardwired Central Office• No Automation of Operations• Labor Intensive• High Operations Cost• Longer Time To Service

DS3 DS3

Some Review Questions

– What does the acronym SONET mean?– What differentiates SONET from

Asynchronous technology?– What does the acronym SDH mean?

The Original Goals of SONET/SDH Standardization

•Vendor Independence & Interoperability

•Elimination of All Manual Operations Activities

•Reduction of Cost of Operations

•Protection from Cable Cuts and Node Failures

•Faster, More Reliable, Less Expensive Service to the Customer

SONET RatesDS3s are STS-1 Mapped

DS3STS-1

Mbits/s

SONET Optical Line SignalOC-N = N x STS-1s

N is the number of STS-1s (or DS3s) transported

28 DS1s = 1 DS3 = 1 STS-124 DS0s = 1 DS1

(= 1 VT1.5)

DS0 (a digitized analog POTS circuit @ 64 Kbits/s)

OC level STM level Line rate (MB/s) OC-1 - 51.84 OC-3 STM-1 155.52 OC-12 STM-4 622.08 OC-48 STM-16 2488.32 OC-192 STM-64 9953.28

SONET and SDH

STESTELTE

PTEPTEPTEPTEPTEPTE

STESTELTELTE

PTEPTEPTEPTEPTEPTE

DS-3DS-3

DS-3DS-3DS-3DS-3DS-3DS-3DS-3DS-3

OC-3 TMOC-3 TMOC-3 TMOC-3 TM

SONET LineSONET Path

SONET Section

TM = Terminal MultiplexorDS = Digital Signal

PTE = Path Terminating ElementLTE = Line Terminating ElementSTE = Section Terminating Element

SONET Layering for Cost Effective Operations

SONET Point-to-Point NetworkRepeater Repeater

Section

STS-1FrameFormat Line

Overhead

SectionOverhead Path

Overhead

STS-1 Synchronous Payload Envelope

STS-1 SPE

Protection Schemes: 1 + 1

(Source) (Destination)

Working Facility

Protection Facility

1 + 1 Protection Switching(50% bandwidth utilization)

Network Protection

1 for N (1:N)

(Source) (Destination)

Working Facility

Protection Facility

1:n Protection Switching(Bandwidth Efficiencies)

Network Protection

Protection and Restoration

Path Protection Line Protection (Loopback)

Unidirectional/Path Switched Ring (UPSR)

WorkProtect

Bidirectional/Line Switched Ring (BLSR)2 fiber, 4 fiber

WorkProtect

4 fiber supports span switching2 fiber doesn’t

Typical Deployment of UPSR and BLSR in RBOC Network

Regional Ring (BLSR)

Intra-Regional Ring (BLSR) Intra-Regional Ring (BLSR)

Access Rings (UPSR)

WB DACs

BB DACs

WB DACS = Wideband DACS - DS1 GroomingBB DACS = Broadband DACS - DS3/STS-1 GroomingOptical Cross Connect = OXC = STS-48 Grooming

DACS=DCS=DXC

Emergence of DWDM

• Some Review Questions– What does the acronym DWDM mean?– What was the fundamental technology that

enabled the DWDM network deployments?

First Driver for DWDMLong Distance Networks

• Limited Rights of Way• Multiple BLSR Rings Homing to a few Rights of Way• Fiber Exhaustion

BLSR Fiber PairsBLSR Fiber Pairs

Key Development for DWDM Optical Fiber Amplifier

120 km

OLSTERM

OLSRPTR

OLSTERM

120 km 120 km

Fiber Amplifier Based Optical Transport - 20 Gb/s

OC-48OC-48

Conventional Optical Transport - 20 Gb/s

1310RPTR

1310RPTRTERM TERM

40km 40km 40km 40km 40km 40km 40km 40km 40km

1310RPTR

1310RPTRTERM TERM1310

RPTR1310RPTR

1310RPTR

RPTR1310RPTR

1310RPTR

1310RPTRTERM

TERM1310RPTR

1310RPTR

RPTR1310RPTR

1310RPTR

RPTR1310RPTR

1310RPTR

TERMTERM1310

RPTR1310RPTR

1310RPTR

1310RPTRTERM

OC-48OC-48

Increased Fiber Network Capacity

Transporting BroadbandTransporting Broadbandacross Transmission across Transmission

NetworksNetworksdesigned for Narrowbanddesigned for Narrowband

T1/T3/OC3FRS and CRS

ATMAccess

ATMSwitch

Public/PrivateInternet Peering

ATMAccess

AccessRouter

T1/T3 IPLeased-LineConnections

CoreRouter

AccessRouter

ATM Access

AccessRouter

EtherSwitch

CoreRouter

BackboneSONET/WDM RAS Farms

T1/T3 FRand ATM IPLeased-LineConnections

ATM Switch

ATMSwitch

CoreRouter

Data SP

High Capacity Path Networking

•Existing SONET/SDH networks are a Existing SONET/SDH networks are a BOTTLENECKBOTTLENECK for Broadband Transport for Broadband Transport– Most Access Rings are OC-3 and OC-12 UPSRs while most Backbone Rings are OC-48. Transport of rates higher than OC-48 using the existing SONET/SDH network will require significant Most Access Rings are OC-3 and OC-12 UPSRs while most Backbone Rings are OC-48. Transport of rates higher than OC-48 using the existing SONET/SDH network will require significant

and costly changes. Clearly upgrading the SONET/SDH network everytime broadband data interfaces are upgraded based increased IP traffic is not an appropriate solution.and costly changes. Clearly upgrading the SONET/SDH network everytime broadband data interfaces are upgraded based increased IP traffic is not an appropriate solution.

Existing SDH-SONET Network

IP router

IP router IP router

STS-3c

STS-12c/48c/...

IP/SONET/WDM Network Architecture

Core IPNode

SONETADM/LT

OC-3/12[STS-3c/12c]

OC-12/48

OC-3/12[STS-3c/12c/48c]

SONET Transport Network

SONETNMS

Core IPNode

Access Routers/EnterpriseServers

SONETADM/LT

SONETXC

WDMLT1, 2, ...

OC-3/12/48[STS-3c/12c/48c]

Pt-to-Pt WDM Transport Network

OC-3/12/48[STS-3c/12c/48c]

OTNNMS

IP = Internet ProtocolIP = Internet ProtocolOTN = Optical Transport NetworkOTN = Optical Transport NetworkADM = Add Drop MultiplexorADM = Add Drop MultiplexorWDM = Wavelength Division MultiplexingWDM = Wavelength Division Multiplexing

LT = Line TerminalLT = Line TerminalEMS = Element Management SystemEMS = Element Management SystemNMS = Network Management SystemNMS = Network Management System

Optical Network Evolution mirrorsSONET Network Evolution

Multipoint NetworkWDM Add/Drop

Point-to-Point WDM Line System

Optical Cross-ConnectWDM Networking OXC

WDMADM

IP/OTN Architecture

Core DataNode

mc: multi-channel interface(e.g., multi-channel OC-12/OC-48)

mcmcOptical Transport Network

OTNNMS

Core Data Node

.Access RoutersEnterprise Servers

Core Data Node

IP = Internet ProtocolIP = Internet ProtocolOTN = Optical Transport NetworkOTN = Optical Transport NetworkOXC = Optical Cross ConnectOXC = Optical Cross ConnectWDM = Wavelength Division MultiplexingWDM = Wavelength Division Multiplexing

EMS = Element Management SystemEMS = Element Management SystemNMS = Network Management SystemNMS = Network Management System

Restoration on the backbone

• SONET rings– Simple and do the job today– Inefficient and inflexible– Diversely routed working and protect

• Next generation options– “Virtual rings”– Mesh with shared protect– Optical rings– Optical mesh

What are the restoration requirements?

• Recovery from failures– Equipment failures– Cable cuts

• Four 9’s? – Down 52 minutes per year.

• Five 9’s?– Down 5 minutes per year.

• Need to satisfy the users requirements: Service Level Agreement (SLA)– Service degradation varies by application– 911 calls, voice, video, ATM, Frame, IP

• Do customers want to pay for 50ms recovery from a cut?– Wide area rings vs. Local area

Protection & Restoration of Optical Networks

Terminology

• Protection– Uses pre-assigned capacity to ensure survivability

• Restoration– Reroutes the affected traffic after failure

occurrence by using available capacity• Survivability

– Property of a network to be resilient to failures

Classification of Schemes

Reactive / Proactive• Reactive

– When an existing lightpath fails, a search is initiated to find a new lightpath which does not use the failed components. (After the failure happens)

– It cannot guarantee successful recovery,– Longer restoration time

• Proactive– Backup lightpaths are identified and resources are

reserved along the backup lightpaths at the time of establishing the primary lightpath itself.

– 100% restoration guarantee– Faster recovery

Link Based vs. Path Based

• Link-based– Shorter restoration time– Less efficient.– Can only fix link failures

• Path-based– longer restoration time– More efficient.

Dedicated vs. Multiplexed Backup

• Dedicated backup– More robust– Less efficient.

• Backup multiplexing– Less robust– More efficient.

Primary Backup MUX

• Wavelength channel to be shared by a primary and one or more backup paths

Resilience in Optical Networks• Linear Systems

– 1+1 protection– 1:1 protection– 1:N protection

• Ring-based– UPSR: Uni-directional Path Switched Rings– BLSR: Bi-directional Line Switched Rings

• Mesh-based– Optical mesh networks connected by optical cross-connects

(OXCs) or optical add/drop multiplexers (OADMs)– Link-based/path-based protection/restoration

• Hybrid Mesh Rings– Physical: mesh– Logical: ring

Unidirectional WDM Path Protected Rings

• 1+1 wavelength path selection• Signal bridged on both protection and

working fiber.• Receiver chooses the better signal.• Failure:

– Destination switches to the operational link.– Revertive /Non revertive switching– No signaling required.

Bidirectional Line switched Ring

• Shares protection capacity among all the spans on the ring

• Link failure– Working traffic from 1 fiber looped back onto

opposite direction.– Signaling protocol required

• Node failure– Line switching performed at both sides of the

failed node.

2-Fiber WDM Ring

BLSR - 4 Fiber

• Fibers– 2 working– 2 protection

• Protection fiber: no traffic unless failure.• Link Failure.

– APS channel required to coordinate the switching at both ends of a failure.

4-Fiber WDM Ring.

4-Fiber WDM Ring After a Link Failure

4-Fiber WDM Ring After a Node Failure

Path Layer Mesh Protection• Protect Mesh as a single unit

• Pre-computed routes– 1+1 path protection– Protection route per light path– Protection route per failure.

• On the fly route computation.– Centralized route computation and coordination– Route computation and coordination at end nodes.– Distributed route computation at path ends.

• Decompose into protection domains.• Pure rings• P cycles

Mesh Topologies

• Fibers organized in protection cycles.– Computed offline

• 4 fibers of each link is terminated by 4 2X2 protection switches

• Before link failure, switches in normal position.

• After failure, switches moved to protection state and traffic looped back into the protection cycles.

2X2 Switch

Protection Cycles (cont’d)

• Criterion for protection cycles.– Recovery from a single link failure in any

optical network with arbitrary topology and bi-directional fiber links

• All protection fibers are used exactly once.• In any directed cycle both protection fibers in a

pair are not used unless they are in a bridge

Protection Cycles

Protection Cycles (cont’d)

Network With Default Protection Switching

Network After a Link Failure

P –cycles

• Ring like restoration needed for some client signals.

• Mesh topologies: bandwidth efficient.• P –cycles:Ring like speeds, Mesh like

capacity.• Addresses the speed limitation of mesh

restoration.

P –cycles (cont’d)

• Cycle oriented pre configuration of spare capacity.

• Can offer up to 2 restoration paths for a failure scenario.

• Span Failure– On cycle: similar to BLSR– Off the cycle: 2 paths.

• Time needed for calculating and connecting restoration path is needed in non-real time.

P - cycles

WDM Recovery

• Fiber based restoration– Entire traffic carried by a fiber is backed by

another fiber.– Bi-directional connection - 4 fibers.

• WDM based recovery– Protection for each wavelength. – Bi-directional connection - 2 fibers– Allows flexibility in planning the configuration of

the network.– Recovery procedure similar to BLSR.

Resilience in Multilayer Networks

• Why resilience in multilayer networks?– Avoid contention between different single-

layer recovery schemes.– Promote cooperation and sharing of spare

capacity

PANEL: Protection Across Network Layers

PANEL Guidelines• Recovery in the highest layer is recommended when:

– Multiple reliability grades need to be provided with fine granularity

– Recovery inter-working cannot be implemented– Survivability schemes in the highest layer are more mature

than in the lowest layer• Recovery in the lowest layer is recommended when:

– The number of entities to recover has to be limited/reduced– The lowest layer supports multiple client layers and it is

appropriate to provide survivability to all services in a homogeneous way

– Survivability schemes in the lowest layer are more mature than in the highest layer

– It is difficult to ensure the physical diversity of working and backup paths in the higher layer

Network Architecture

Classes of WDM Networks

• Broadcast-and-select• Wavelength routed• Linear lightwave

Broadcast-and-Select

Passive

Couplerw1

Wavelength Routed

• An OXC is placed at each node• End users communicate with one

another through lightpaths, which may contain several fiber links and wavelengths

• Two lightpaths are not allowed to have the same wavelength on the same link.

WRN (cont’d)

• Wavelength converter can be used to convert a wavelength to another at OXC

• Wavelength-convertible network.– Wavelength converters configured in the network– A lightpath can occupy different wavelengths

• Wavelength-continuous network– A lightpath must occupy the same wavelength

A WR Network

IP SONET

Linear Lightwave Networks

• Granularity of switching in wave bands• Complexity reduction in switches• Inseparability

– Channels belonging to the same waveband when combined on a single fiber cannot be separated within the network

Routing and Wavelength Assignment (RWA)

• To establish a lightpath, need to determine:– A route– Corresponding wavelengths on the route

• RWA problem can be divided into two sub-problems:– Routing– Wavelength assignment

• Static vs. dynamic lightpath establishment

Static Lightpath Establishment (SLE)

• Suitable for static traffic• Traffic matrix and network topology are

known in advance• Objective is to minimize the network capacity

needed for the traffic when setting up the network

• Compute a route and assign wavelengths for each connection in an off-line manner

Dynamic Lightpath Establishment (DLE)

• Suitable for dynamic traffic• Traffic matrix is not known in advance

while network topology is known• Objective is to maximize the network

capacity at any time when a connection request arrives at the network

Routing

• Fixed routing: predefine a route for each lightpath connection

• Alternative routing: predefine several routes for each lightpath connection and choose one of them

• Exhaust routing: use all the possible paths

Wavelength Assignment

• For the network with wavelength conversion capability, wavelength assignment is trivial

• For the network with wavelength continuity constraint, use heuristics

Wavelength Assignment under Wavelength Continuity Constraint

• First-Fit (FF)• Least-Used (LU)• Most-Used (MU)• Max_Sum (MS)• Relative Capacity Loss (RCL)

First-Fit

• All the wavelength are indexed with consecutive integer numbers

• The available wavelength with the lowest index is assigned

Least-Used and Most-Used

• Least-Used– Record the usage of

each wavelength– Pick up a

wavelength, which is least used before, from the available wavelength pool

• Most-Used– Record the usage of

each wavelength– Pick up a

wavelength, which is most used before, from the available wavelength pool

Max-Sum and RCL• Fixed routing• MAX_SUM Chooses the wavelength,

such that the decision will minimize the capacity loss or maximize the possibility of future connections.

• RCL will choose the wavelength which minimize the relative capacity loss.

Applications for Free Space Optics (FSO)

Mark E. AllenSignalWise LLC

mallen@signalwise.com

Outline

• Where does FSO fit in the network?• FSO design issues• What is the performance of FSO?• Applications for FSO• Future directions

Intro to FSO

• The last-mile problem continues to be an issue.– Fiber doesn’t exist everywhere. – Trenching new fiber can cost upwards of $250K /mile

• Often impossible in congested metro areas• Not cost effective in sparse areas• Nobody has any money left

– DSL / Cable / Copper ?• DSL/T1/DS3 (when available) are limited in speed and

distance (~1.5M for DSL/T1), (45M for DS3)• Provisioning times/errors often a problem• Monthly recurring charges can be substantial

Lasers through the air

• Laser sources normally in the 850nm, 1310 or 1550 ranges. – Some debate on what’s best, 1550

generally more eye-safe• Receiver optics capture the light and

converts back to electrical signal (OEO)• Several factors can impair the signal as

it propagates through the air.

Two major markets for FSO

• Enterprises looking for:– Increased bandwidth and connectivity throughout

the campus– Reduced monthly recurring costs from Telco– Unconstrained expansion of their GigE LANs

• Service providers want:– Access to more customers– Reduced capital infrastructure costs

• Military has also been very interested in “LaserCom”

FSO and Wireless

• FSO– Range ~3km– More than 1Gbps– No rain fade– Fog interferes – No license required– Indoor (through window)

or outdoor installation– No licensing required– 3-4 nines typical– Line of sight

• Wireless– Range ~ 5-25km– 10 – 100 Mbps– Rain fade– Fog OK– Outdoor installation – Licensing may be

required– 3-4 nines typical– Line of sight required?

• No (MHz carrier)• Yes (GHz carrier)

FSO Impairments

• Atmospheric Impairments– Scattering of light from particles

• Fog,smoke have diameter in the micron range– Turns out visibility and FSO path loss are

directly correlated• On a clear day, FSO path will incur low

loss, but must be engineered for worst case.

Visibility and corresponding loss

60200 m

128100 m

21500 m

dB loss / km

Visibility

60200 m

128100 m

21500 m

dB loss / km

Visibility

lossdB(L) 10 * L/Visibility

Scintillation (heat waves)

• These are caused by localized changes in the density of the air.

• Can be mitigated– Multiple beams– Aperture averaging (large beam)– Adaptive Optics (time-varying corrective lens)

• Other than fog, this is the biggest challenge for FSO.

Other impairments

• Mispointing losses– Inaccuracy or building shake/vibration can

cause signal dropouts– Active control systems can correct this. $$

$• Divergence losses

– As the beam travels, it spreads out.– Can be tightened, but this complicates the

mispointing problem.

Sample budget

Description FSO

Transmit power +20dBm

Internal losses (total for both ends) 8dB

Window losses 6dB

Path attenuation (clear air) 0dB

Scintillation loss 4dB

Mispointing loss 1dB

Geometric spreading loss 4dB

Required receiver sensitivity -30dBm

Available weather margin 27dB

The statistics of visibility

Visibility vs. Cumulative Time

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Visibility (km)

Tulsa, OK

Ex: Computing expected uptime

• Assume link with 27dB “weather” margin• 1km in length• 400m visibility >> 27dB/km of loss• So: The 1km link goes down when visibility

drops below 400m.• Statistics of different cities vary widely.

– 2-3 “nines” are usually attainable for shorter links.

FSO Applications

• Metro Fiber Extension– Services providers extending their reach

into areas where they don’t have (or can’t lease) fiber

– OC-N mux can be terminated at the end of the FSO system

– 1+1 Redundancy with fiber can also used.

Optical Networking (part 2)

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