1

WAN Optical Network Design WAN Optical Network Design

Telcom 2110 Network DesignDavid Tipper

Graduate Telecommunications and Networking P

gProgram

University of PittsburghSlides 13

• WAN typically have a mesh or ring design • Many algorithms/optimization formulations/design

tools for WAN packet network designT d t b i b dd d i t k t h l /d t t l

WAN Network Design

– Tend to be imbedded in network technology /data rate layer– Different design techniques and metrics at different layers

• IP• MPLS – VPN design• WDM – circuit switched routing and wavelength assignment

– In general techniques are either • Graph theory based • Optimization based

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• Optimization based• Heuristics

– Considered Routing heuristics for IP layer design– Consider Optimization and heuristic techniques for VPN

design– Examine Optical Layer design

2

Optical Layer Equipment

WDM Terminal

Wavelength Division Multiplexing Terminal

Multiplexes/demultiplexes multiple optical analog signals at different wavelengths/frequencies into a single fiber.

OTS Optical Transport System Pair of WDM terminals in a point-to-point configuration.

OT Optical Transponder Converts wavelengths between switches or OXCs to specific wavelengths of OTS (regenerates specific digital, optical signal)

OADM Optical Add-drop Multiplexer A WDM terminal that can add/drop individual wavelengths from a WDM multiplexed signal

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wavelengths from a WDM multiplexed signal.

OXC Optical Cross-Connect Cross-connects wavelengths, either with electrical fabric (does O/E/O conversion) or optical fabric (no electrical conversion).

Equipment Across the WAN

custrouter

Accessrouter

BBrouter

packetpacket

OC-3ADM

ADM

ADMOC-48

UPSR OC-3

OC-192ADM

ADM

ADM

OC-12

OC-12OC-48

BLSR

packetpacketADMADM

OXC OT WDM WDM OT OT WDM WDM OT OXC1 2

multi OC-192 OC-192

multi

OTS OTS

OXC

OC-1922 1

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ADM SONET Add/Drop MultiplexerUPSR SONET Uni-directional Path Switched RingBLSR SONET Bi-directional Line Switched RingOXC Optical Cross-ConnectOTS Optical Transport SystemOT Optical Transponder

custrouter

Accessrouter

BBrouter OC-3

ADMADMOC-48

UPSROC-3OC-12

OC-12OC-192ADMADM

OC-48

BLSR

3

Optical Transport Network (OTN) (ITU-T G.709/Y.1331)

• Transparent support for different client signals

IP ATM ETHERNET STM -N

Optical Payload Unit ( OPU k )

IP ATM ETHERNET STM -N

Optical Payload Unit ( OPU k )

signals

• Includes leased lines/wavelengths

• Easy interconnection of different administrative

Optical Transport Unit ( OTUk )

Optical Channel ( OCh )

Optical Multiplex Section ( OMSn )OTM

Physical

Optical Data Unit ( ODUk )

Optical Transport Unit ( OTUk )

STM -N GbE

-

STM -N GbE

Optical Channel ( OCh )

Optical Multiplex Section ( OMSn )OTM

Physical

Optical Data Unit ( ODUk )

5

administrative domains Optical Transmission Section ( OTSn )

Optical Transport Module of order n

(OTM -n, n 1)

Section

(OPSn )

OTM - 0

OTM -nr, n>1

Optical Transmission Section ( OTSn )

Optical Transport Module of order n

(OTM -n, n 1)

Section

(OPSn )

OTM - 0

OTM -nr, n>1

Pure Glass Core

Glass Cladding8.3 micron*

Lightpack Cable Design

Fiber

g

Inner Polymer Coating

Outer Polymer Coating

125 micron

250 micron

Protection Layers

Protects “core”Serves as a “Light guide”

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Single Fiber

* Single Mode Fiber; Multi-Mode Has A 50 Micron Core

Typical Loss: 0.2 – 0.25 dB/kmPlus Connector Loss

4

Wavelength Division Multiplexing

• The optical fiber has a bandwidth of ~ 30 THz

• Several transmissions can proceed on the fiber simultaneously using different optical frequencies (wavelengths) – termed WDM

D WDM > 18 l th fib h 0 240• Dense WDM > 18 wavelengths per fiber - can have 0-240 or more wavelengths

• Fiber bandwidth cannot be fully utilized by a single communication channel because of the electronic bottleneck

FiberTx Rx

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Tx

Tx

Rx

Rx

Wavelength channel bandwidth 2.5 to 10 Gb/sUnidirectional and Bidirectional options

1R

R

Frequency-registeredtransmitters

Receivers

All-Optical AmplificationOf Multi-Wavelength Signal!

Optical Amplifier/WDM

40 - 120 km(80 km typically)

Up to 10 000 km

OA OA

2

3

N

WDMMux

R

R

R

WDMDeMux

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Up to 10,000 km

WDM: Wavelength Division MultiplexingOA: Optical Amplifier

DWDM Links: Long Haul (0-600 km) Extended Long Haul (600-2000 km)Ultra Long Haul (3000+ km)

5

DWDM Transmission Example

Conventional Transmission - 20 Gb/s

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE

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

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE1310

RPTR1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE1310 13101310 1310 13101310 1310 1310

120 km

OAOA120 km 120 km

OC-48OC-48

OC-48OC-48

OC-48OC-48

OC-48OC-48OC-48

DS3OC3/12

DS3

RPTR RPTRRPTR RPTR RPTRRPTR RPTR RPTR LTE1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE1310

RPTR1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE1310

RPTR1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE1310

RPTR1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE1310

RPTR1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE

DS3

DS3

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE1310

RPTR1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE1310

RPTR1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE1310

RPTR1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

LTELTE

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In Each Direction:• 12 Fibers• 36 Regenerators

OC-48OC-48

OC-48OC-48

OC-48OC-48

OC-48DS3OC3/12

12 fibers 1 fiber; 36 regenerators 1 optical amplifier

OA: Optical Amplifier

ITU G.694.1 grid for wavelengths and timeslotsto support SDH transmission

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Optical Crossconnect: Typical Arrangement

1-Bay Capacity:640 Gb/s

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6

• Use same wavelength end-to-end• Easier use of multi-vendor equipment• Cheaper and simplifies Fault-localization• However, poorer utilization of capacity

Opaque Networking

FibersIn

FibersOut

-Mux

......

...

......

...

Intra-OfficeConnectivity

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Add ports Drop ports

....

...

....

...

Transparency= node-bypass

- TransponderSource: E. Goldstein & L. Lin

Optical Switch or Crossconnect (OXC)

Optical Crossconnect (OXC)(NO BUFFER) switches wavelengths

Switch Time < 1 ms

Si substrate

Microlens

Free-rotatingswitch-mirror array

Siliconsubstrate

8x8 is 1 cm x 1 cm

Measured Switching TimesUnder 1 ms (500s)

3-D MEMS (2 degrees ofFreedom) seems to be theCurrently preferred

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OXC reconfigured by actuating selected micromirrors

Input fiberssubstratearchitecture

7

An 8 x 8 OXC

Chip size: 1 cm x 1 cm

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Source: L-Y. Lin

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Effect Of Wavelength Continuity

Maybe unable to get a wavelength end-to end

X

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8

Wavelength Conversion

Conversion is possible but expensive

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Design Issues in All Optical Networks• Routing and Wavelength Assignment

– Pick Routes and assign wavelength so a signal can stay on the same wavelength from its source to its destination

– Continuity constraint – If wavelength conversion permitted - want to minimize number of

wavelength used minimize cost

• DWDM Bandwidth is very large– Concern: a fiber cut or link failure can result in a loss of

a large volume of data.– Solution: mesh or ring restorationg

– Concern: a large gap between the capacity of one wavelength channel and the bandwidth requirement for a connection request

– Solution: traffic grooming to multiplex traffic

9

RWA Problem Formulation(no wavelength Conversion)*

Given a network {N,E} of nodes and edges and vector D of bi-directional connections. A connection or demand is characterized by a source/destination (unordered) pair of endpoint nodes, {(d), (d)}. The objective is to find a routing policy (i.e., select routing and wavelength assignment variables x and defined below) that minimizes the expected value of (D) over all realizations of the random vector D

)A3( , 1

)A2( } and 1:{max)(

to subject

)A1( ])([min

dp

dj

Ddx

DdjD

DE

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(A6) }1,0{and

(A5) and , 1

)A4( , 1

),(

)(

djdp

edPpdpdj

Dd

Jjdj

dPp

x

JjEex

Dd

where P(d) is the set of allowable paths over edges in E for routing connection dD (for example the set of k-shortest paths), P(d,e) is the subset of P(d) that routes over edge e, xdp is a 0/1 variable representing the assignment of connection d to path p, and dj is a 0/1 variable representing the assignment of connection d to wavelength j,

RWA Problem Formulation (Cont.)

where jJ and J = {1,...,W} for some sufficiently large value of W. Note here (the case without wavelength translation) for a given connection, a single wavelength j is assigned to all edges of the path. Constraint A2 defines the largest wavelength assigned. Constraint A3 says that every connection must be routed over exactly one path. Constraint A4 says that every connection must be assigned exactly one wavelength. Constraint A5 says that no wavelength may be assigned to more than one connection on any given edge and constraint A6 says that xdp and dj are binary variables.

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Many optimization based formulations in the literature – are NP –Hard – in practice use heuristics to provide good solution

*Doverspike, Robert, John Strand, Guangzhi Li, “Importance Of Wavelength Conversion In

An Optical Network,” Optical Networks Magazine, Vol 2 (3), May/June 2001, pp. 33-44.

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Multi-Route First Fit with K-Shortest Path

When wavelength conversion is not available, a wavelength is route-available if it is not in use (“unassigned”) on every link of R.

(R) = lowest ordered route-available wavelength of R

MRFF-KS Heuristic

(R) lowest ordered route available wavelength of R

(i,R) = lowest ordered, unassigned wavelength on link i of route R

max(R) = max {(i,R): i is a link of route R}

Pk(d) = {R1, R2, ... , Rk} = the set of k shortest routes to route demand d.

MRFF- KS without wavelength conversion – The route R* Pk(d) which minimizes (R) is chosen and wavelength (R*) is assigned.

MRFF- KS with wavelength conversion – The route R* Pk(d) which minimizes (R) is chosen On each link i of R* wavelength (i R*) is

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minimizes max(R) is chosen. On each link i of R , wavelength (i,R ) is assigned.

R

R) = 4max(R) = 2

321

4

wavelengthnumber

Optical Layer Protection

• Protection mechanisms exist in the higher layers, so why need protection in optical layer?

• ProsCapacity efficient due to protection capacity sharing across– Capacity efficient due to protection capacity sharing across multiple pairs of higher layer equipments (IP/MPLS/SONET)

– Significant savings in equipment cost– Handle fiber cuts more efficiently than the higher layers– Provide an additional degree of resilience (e.g., protect against

multiple failures)– Only way to protect layer 1 customers

• Cons

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– Can’t handle all failures: higher layer equipment failures must be dealt with by the higher layer

– Protect traffic in units of lightpaths: can’t protect only part of the traffic within a lightpath

– Need pay careful attention to interworking of protection schemes between different layers

11

OC-192(logical link)

Example Equipment Model & Failure Modes

Router failure

BR BR

IP layer

Router line card failure

OXC

OXCOXC OXC

WDM

WDM

WDM

WDM

OTsOTs OTsWDM

WDM

OTs OTs

OXC layer

WDM layer

OXC interface failure

WDM/Amplifier failure

OT failure

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fiber spans

Media layer

BR = Backbone Router OXC = Optical Cross-ConnectOT = Optical Transponder WDM = Wavelength Division Multiplexing

Fiber cut

Fiber and Node Failures

TxRx

TxRx

SRSR

LR LR

Amp

Node

Regen

TxRx

Add/Drop

fiber fails at 1 cut per 1000km per year, network is 50,000km of fiber (50 cuts in 1 year)Statistically….a cut every 7 days!

No. of nodes 20-200

No. of links 30-300

Path Unavailability(Assume work_path = protect_path)

100.0

1000.0

10000.0

100000.0

abili

ty, m

in Fiber only : unp

Fiber+Equip : 1+1

Fiber+Equip : unp

p

Note: “min” is minutes/yr = 24hrs/day * 60min/hr * 365days/yrNote: “min” is minutes/yr = 24hrs/day * 60min/hr * 365days/yr

Average node degree

2.2 – 2.5

Typical Link Distances

500-1500 km

Max Total Network Dist.

50,000 km

0.1

1.0

10.0

0 2000 4000 6000 8000 10000Path, km

Un

avai

la

[1-A] * min

[1-prod(A)] *min

Unp

1+1

Fiber only : 1+1

99.999%

12

Optical Layer Protection Schemes

• Optical channel (OCh) layer (or path layer) protection schemes (Path Restoration)– Restore one lightpath at a time

N d d lti l ll l th– Need demultiplex all wavelengths

• Optical multiplex section (OMS) layer (or line layer) protection schemes (Link Restoration)– Restore the entire group of lightpaths on a link– Require less equipment

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• OXCs and OADMs can provide both OCh and OMS layer protection in mesh or ring configurations

Optical Layer Protection Schemes

• OMS layer protection schemes– 1+1 (hot standby)

1:1– 1:1

– OMS-Dedicated Protection Ring (DPR)

– OMS-SPRing

• OCh layer protection schemes– 1+1

OCh SPRing

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– OCh-SPRing

– OCh-Mesh

– Och- P-Cycle

13

1+1 OMS Protection

• Dedicated protection in point-to-point links

• At one end, the composite WDM signal is bridged onto both the working fiber and a protection fiber using an optical splitter

• At the other end, an optical switch selects the better among the two signals

HOT t db h

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• HOT standby approach

1:1 OMS Protection

• Shared protection in point-to-point links• The composite WDM signal is transmitted over

only the working fiber y g– Use a switch at the transmitter, instead of a splitter

• If the working fiber is cut, both ends switch over to the protection fiber– Need an APS protocol

• Support low priority traffic on the protection fiber

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• Allow N working systems to share a single protection system

14

OMS-DPRing

• Dedicated protection ring• Two fibers operate in opposite directions• Each node transmits on both directions of the ringg

– Different nodes must transmit at different wavelengths

• Normal operation: the ring functions as a bus, with one pair of amplifiers turned off and all the others turned on

• When a link fails: an amplifier pair next to the failed link are turned off and the ones that were originally inactive are turned on

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inactive are turned on• Equivalent to a Sonet USHR

OMS-SPRing

• Shared protection ring• Four fibers, analogous to a SONET BLSR/4

(BSHR)(BSHR)• The two protection fibers do not have attached

WDM equipment• Use either span switch or ring switch• Two-fiber version

– Dedicate half the wavelengths on each fiber for

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gprotection purposes

– Make the protection wavelengths on one fiber correspond to the working wavelengths on the other fibersignals can be rerouted w/o requiring wavelength conversion

15

1+1 OCh Dedicated Protection

• Works in point-to-point, ring (OCh-DPRing), and mesh configurations

• Two lightpaths on disjoint routes are setup for each client connection

• The client signal is split at the input, the destination selects the better of the two lightpaths

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g p• No signaling requiredfast restoration• No protection bandwidth

sharingbandwidth inefficient

OCh-SPRing

• Shared protection ring • Similar to SONET BLSR/4, but operate at the

ti l h l loptical channel layer• Working lightpaths are set up on the shortest

path along the ring• When a working lightpath fails, it’s restored

using span switch or ring switch• Non-overlapping lightpaths in the ring can

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• Non-overlapping lightpaths in the ring can share a single wavelength around the ring for protectionmore efficient than OCh-DPRing

16

OCh-Mesh Protection

• For mesh networks, OCh-mesh protection schemes are more bandwidth-efficient than rings– Efficiency improvements range from 20% to 60%

• Path-based: the connection is rerouted end to end on an alternate path– Need notify the source node upon a failure

– Can share backup path bandwidth if working paths disjoint

– Can implement P-cycle scheme

• Link-based: the connection is rerouted on an alternate

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path around the failed link– Need not notify the source node upon a failure

– Enable faster restoration than path-based schemes

Offline v.s Online Protection

• Offline protection – Protection path and wavelengths are reserved at

the time of connection setupthe time of connection setup• In path-based scheme, a link-disjoint protection path is

reserved• In link-based scheme, protection paths are reserved

around each link of the working lightpath

– Fast and guaranteed restoration

• Online protection

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– Search for protection paths using the spare capacity in the network upon a failure

– Capacity efficient– Slow and no guarantee of restoration

17

Dedicated v.s Shared Protection

• An offline scheme can use either dedicated protection or shared protection

Dedicated protection each orking lightpath is• Dedicated protection: each working lightpath is assigned its own dedicated protection bandwidth

• Shared protection: if two working lightpaths are link-disjoint, they can share protection bandwidth– More capacity efficient than dedicated protection

Protection lightpaths are set up after a failure occurs

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– Protection lightpaths are set up after a failure occurs

Classification of OCh-Mesh Protection Schemes

OCh-mesh protection schemes

Path-based

offline online

Link-basedSharedDedicated

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Link-based Link-based Path-based

P-cycle

Path-based

Algorithms for finding backup similar to MPLSMay need to include distance and Amp/Termination cost in algorithms

18

2

6

3No Protection

2 2

64

3Dedicated

Example Demands: (1,4) of 1 , (1,3) of 2 , (2,5) of 4

2 24

44 22

4

46

15

4

TE = 34 A = 45 Cost = 31.48

23P-Cycle

6

15

4

TE = 70 A = 47 Cost = 66.48

23Shared

22 4

24

4 622

244

4

6

15

4

TE = 70 A = 88 Cost = 62.30

6

15

4

TE = 72 A = 88 Cost = 63.12

222

4

4

4

6

6

2

2 2

22

28

Mesh Restoration Method*

service path

restoration path

= OXC

• restoration paths are– pre-planned and stored in

endpoints

– node & span disjoint from service path

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• Upon failure, network restores connection by sending messages to nodes along restoration path

*ROLEX = Robust Optical Layer End-to-end X-connectionDoverspike, Robert D., G. Sahin, J. L. Strand, R. W. Tkach, "Fast Restoration in a Mesh Network of Optical Cross-connects," OFC '99, San Diego, CA, February 1999.

19

Problem:

• given set of edges, E, traffic demands, D, and

Mesh Restoration Network Design*

associated service edge-paths, Pd for each d D

• calculate restoration paths such that restoration capacity required to restore all failed traffic for any single failure event is minimized

• failure event set typically consists of all fiber

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spans (cables between a pair of buildings)

*Doverspike, Robert D., Steven Phillips, Jeffery R. Westbrook, "Transport Network Architectures

in an IP World," INFOCOM-2000, Tel-Aviv Israel. March 2000.

Mesh Restoration Network Design Problem

)( ),( , ,

tosubject

)()(min

)(

41

FffDdp)(fx

eneB

dfPpd

Ee

(6) andinteger )(,},{,

,

(5) ,)(,)(),,()(

),(

010

enp)(fx

FfEe

Kenp)(fxdC

d

edfPpd

fDd

dfPp

{N,E} = nodes and edges D = demands S(d) embedded path of edges (a service path) for each demand F = set of failure events (e.g., all single fiber spans) E(f) E f d th t f il f h f il t f

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E(f) E of edges that fail for each failure event, f. B(e) =unit cost of edge e n(e) = the number of units of restoration capacity placed for edge eE C(d) =the number of units of capacity required by d K = the multiplexing capacity of a one unit of restoration capacity D(f) = the set of demands interrupted by failure f P(f,d) =the set of possible restoration paths for failure event fF. demand dD(f) P(f,d,e) = the subset of P(f,d) that routes over edge e xd(f,p) = 0/1 variable representing the assignment of d to p.

20

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Heuristic algorithm - example

C

B

Aservice *

• maintain array Rek = number of restoration channels needed on edge ewhen edge k fails

• define Se = max {Rek : k E } (total required restoration capacity)

D

CApath

restorationpath

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• for given demand, d D, calculate Se(d) = max {Rek : k Pd } and define edge weight (incremental amount of restoration capacity)

we = 0 if Se – Se(d) > 0; else we = 1• route demands (traffic) in orders and choose restoration path that

minimizes we over e Pd (shortest path problem)

• Do various heuristics to order demands and re-route to lower costs

B

CA

*

Heuristic algorithm - example

C

B

Aserviceth

*

1

D1 1

B*

C

B

A

11

D

Cpathrestorationpath

1

1 1

TELCOM 2110 40

D

CA

1 1 D

CA

1 1 *

21

Internetworking between Layers

• Need coordination between protection mechanisms in different layers

• Bottom-up sequential approach: start at the layer where the failure occurs, let the layer try to restore service first, then let the higher layer try– Option 1: have the restoration in the lower layer

happen so quickly that the upper layer doesn’t detect the failure

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the failure

– Option 2: impose an additional hold-off time in the higher layer before it attempts restoration

The Traffic Grooming Problem

• Number of wavelengths per fiber = 4 -200+

• Per wavelength capacity = 2.5 Gbps to10 Gbps

• Bandwidth requirements of most applications << 2.5Bandwidth requirements of most applications 2.5 Gbps

Group several sessions on the same wavelength channel in order to better utilize the available bandwidth

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Traffic Grooming: The intelligent allocation of traffic demands onto an available set of wavelengths in a way that reduces the overall cost of the network.

22

Entering The Transport Network

POTS 1 MU

OOO

OO

64 kb/s 1.5 Mb/s 45 - 622 Mb/s

2.5 - 10Gb/s

&MUX M

UX

X24

OO

WDM

BackboneFiber

Network*

1.5 Mb/s PL

45 - 2500 Mb/s PL[1Gb Ethernet]

2500 - 10,000 Mb/s PL, 10 Gb WAN Ethernet

&VG PL

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POTS: "Plain Old Telephone Service"VG: Voice GradePL: Private Line

Network

10 Gb WAN Ethernet• SONET Framed - 9.953 Gb/s• Asynchronous

*10 Gb WAN Ethernet

The Traffic Grooming Problem: CapEx

• Dominant cost factor: Electronic layer multiplexing; number of electronic layer Line Terminating Equipment (LTs):Line Terminating Equipment (LTs):– SONET/SDH ADMs

– IP/MPLS router ports

• Solution: Assign the traffic such that minimum number of LTs is used

3-4 times as expensive as OXC ports

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NP-Complete Problem

• Solution types:– Exact solutions (based on ILP or MILP)

– Heuristic and approximate solutions

– Bounds

23

Summary

• Optical Network Design

– Basic network components • (OXC, OADM, Fiber, OA, etc.)

– Network Design Problems• RWA with/without wavelength conversion

• SurvivabilityRings or Mesh

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– Rings or Mesh

• Traffic Grooming