optical_nw.ppt

7/28/2019 optical_NW.ppt

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Optical Networks


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Overview

Network Concepts Network Topologies

SONET/SDH

High-Speed Light wave Links

Optical Add/Drop Multiplexing

WDM Network Examples

Passive Optical Networks

IP over DWDM Optical Ethernet

Generations of Optical NWs

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Network Terminology Stations are devices that network subscribers use to communicate.

A networkis a collection of interconnected stations.

A nodeis a point where one or more communication lines terminate.

A trunkis a transmission line that supports large traffic loads.

The topologyis the logical manner in which nodes are linked together by

information transmitting channels to form a network.

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Segments of a Public Network A local area networkinterconnects users in a large room or work area, a department,

a home, a building, an office or factory complex, or a group of buildings. A campus networkinterconnects a several LANs in a localized area.

A metro networkinterconnects facilities ranging from buildings located in several city

blocks to an entire city and the metropolitan area surrounding it.

An access networkencompasses connections that extend from a centralized

switching facility to individual businesses, organizations, and homes.

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Protocol Stack Model Thephysical layerrefers to a physical transmission medium

The data link layerestablishes, maintains, and releases links that directlyconnect two nodes

The function of the network layeris to deliver data packets from source

to destination across multiple network links.

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Network Layering Concept

Network architecture: The general physical arrangement andoperational characteristics of communicating equipment

together with a common set of communication protocols

Protocol: A set of rules and conventions that governs the

generation, formatting, control, exchange, andinterpretation of information sent through a

telecommunication network or that is stored in a database

Protocol stack:Subdivides a protocol into a number of

individual layers of manageable and comprehensible size The lower layers govern the communication facilities.

The upper layers support user applications by structuring and

organizing data for the needs of the user.

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Optical Layer

The optical layer is a wavelength-

based concept and lies just above

the physical layer

The physical layer provides a physical

connection between two nodes

The optical layer provides lightpathservices over that link

The optical layer processes

include wavelength

multiplexing, adding anddropping wavelengths, and

support of optical switching

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SONET/SDH The SONET/SDH standards enable the interconnection of

fiber optic transmission equipment from various vendors

through multiple-owner trunk networks.

The basic transmission bit rate of the basic SONET signal is

In SDH the basic rate is 155.52 Mb/s.

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Basic formats of (a) an STS-N SONET frame and (b) an STM-N SDH frame


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Common values of OC-N and STM-N

OC stands for optical carrier. It has become common to refer

to SONET links as OC-N links.

The basic SDH rate is 155.52 Mb/s and is called the

synchronous transport modulelevel 1 (STM-1).

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SONET/SDH Rings SONET and SDH can be configured as either a ring or mesh architecture

SONET/SDH rings are self-healing rings because the traffic flowing alonga certain path can be switched automatically to an alternate or standby

path following failure or degradation of the link segment

Two popular SONET and SDH networks:

2-fiber, unidirectional, path-switched ring (2-fiber UPSR)

2-fiber or 4-fiber, bidirectional, line-switched ring (2-fiber or 4-fiber BLSR)

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Generic 2-fiberUPSR with acounter-rotatingprotection path


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BLSR Recovery from Failure Modes

If a primary-ring device fails in either node 3 or 4, the affected nodes detect aloss-of-signal condition and switch both primary fibers connecting these nodes to

the secondary protection pair

If an entire node fails or both the primary and protection fibers in a given span

are severed, the adjacent nodes switch the primary-path connections to the

protection fibers, in order to loop traffic back to the previous node.

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High-Speed Multimode LinksMultimode fibers with different bandwidth grades exist for 10Gb/s use

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A link may contain a mixture of fibers, e.g. OM2 and OM3.

The fiber bandwidths determine the effective maximum link length Lmax. If all geometric parameters of the interconnected OM2 and OM3 fibers

are the same, then


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Optical Add/Drop Multiplexing An optical add/drop multiplexer(OADM) allows the insertion or

extraction of one or more wavelengths from a fiber at a network node. Most OADMs are constructed using WDM elements such as a series of

dielectric thin-film filters, a set of liquid crystal devices, or a series of

fiber Bragg gratings used in conjunction with optical circulators.

The OADM architecture depends on factors such as the number of

wavelengths to be dropped/added, the OADM modularity for upgradingflexibility, and what groupings of wavelengths should be processed.

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Reconfigurable OADM (ROADM) ROADMs can be reconfigured by a network operator within

minutes from a remote network-management console.

ROADM architectures include wavelength blockers, arrays of

small switches, and wavelength-selective switches.

ROADM features:

Wavelength dependence. When a ROADM is independent of

wavelength, it is colorless or has colorless ports.

ROADM degree is the number of bidirectional multiwavelength

interfaces the device supports. Example: A degree-2 ROADM has 2

bidirectional WDM interfaces and a degree-4 ROADM supports 4

bidirectional WDM interfaces.

Express channels allow a selected set of wavelengths to pass through

the node without the need for OEO conversion.

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Wavelength Blocker Configuration

The simplest ROADM configuration uses a

broadcast-and-select approach:

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Optical Burst Switching(OBS)

Optical burst switching provides an efficient solution for

sending high-speed bursty traffic over WDM networks.

Bursty traffichas long idle times between the busy periods

in which a large number of packets arrive from users.

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Passive Optical Networks (PONs) Apassive optical network (PON) uses CWDM over a single

bidirectional optical fiber. Only passive optical components guide traffic from the central

office to the customer premises and back to the central office.

In the central office, combined data and digitized voice are sent

downstream to customers by using a 1490-nm wavelength.

The upstream (customer to central office) uses a 1310-nm wavelength.

Video services are sent downstream using a 1550-nm wavelength.

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Active PON Modules The optical line termination (OLT) is located in a central office and

controls the bidirectional flow of information across the network. An optical network termination (ONT) is located directly at the customer

premises.

The ONT provides an optical connection to the PON on the upstream

side and to interface electrically to the local customer equipment.

An optical network unit (ONU) is similar to an ONT, but is located nearthe customer and is housed in an outdoor equipment shelter.

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PON Protection Methods

PON failure protectionmechanisms include a

fully redundant 1 + 1

protection and a

partially redundant1:N protection.

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IP over DWDM Early IP networks had redundant management functions in each layer, so

this layering method was not efficient for transporting IP traffic.

An IP-SONET-DWDM architecture using Multiprotocol Label Switching

(MPLS) provides for the efficient designation, routing, forwarding, and

switching of traffic flows through the network.

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Optical Ethernet The IEEE has approved the 802.3ah Ethernet in the First Mile (EFM) standard.

The first mile is the network infrastructure that connects business orresidential subscribers to the Central Office of a telecom carrier or a service

provider.

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Three EFM physical

transport schemes are:1. Individual point-to-point

(P2P) links

2. A single P2P link to

multiple users

3. A single bidirectional

PON


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Generations of Optical NWs

First Generation Optical Networks

Fiber Distributed Data Interface (FDDI)

Synchronous Optical Network/Synchronous Digital

Hierarchy (SONET/SDH)

Second Generation Optical Networks

Wavelength Division Multiplexing (WDM)

Optical Networking Components

Wavelength Routing Networks


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Fiber Distributed Data Interface (FDDI)

Dates back to the early 1980s

FDDI uses token-passing scheme

Uses two fiber pairs, each operating at 100 Mbits/s.

Data rates approaching 90% of its 100 MB/s operatingrate

FDDI was, and in some locations still is, commonly used

at the Internet Service Provider (ISP) peering points that

provide interconnections between ISPs.

Relatively expensive


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FDDI Position in the OSI Reference Model

FDDI is defined as the two bottom layers of the seven-layer OSI reference model

It provides a transport facility for higher-level protocols such as TCP/IP

Physical layer is subdivided into:

physical-medium-dependent (PMD)sublayer defines the details of the fiber-

optic cable used

the physical (PHY) layer specifies

encoding/decoding and clocking

operation


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FDDI 4B/5B Coding The selection of the 4B/5B coding was based on the need to reduce the

signaling level from 200 MHz to a 125-MHz rate (cost reduction)

Each bit is encoded using non-returnto-zero-inversion (NRZI) transmission Because 4 bits are encoded into 5 bits, this means there are 16, 4-bit

patterns.

Those patterns were selected to ensure that a transition is present at leasttwice for each 5-bit code. DC balance: important for thresholding at receiver

For some input data sequences the worst case DC unbalance is 10%

Because 5-bit codes are used, the remaining symbols provide specialmeanings or represent invalid symbols.

Special symbols

I symbol is used to exchange handshaking between neighboring

stations, J and K symbols are used to form the Start Delimiter for a

packet, which functions as an alert to a receiver that a packet is arriving.


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FDDI Fiber Specifications

OPTICAL FIBER SUPPORT FDDI can support 62.5/125-, 50/125-, and 100/140-m multimode fiber sizes.

Maximum distance 2 Km. FDDI also supports the use of single-mode fiber,

Long-distance transmission (up to 40 Km)

FDDI single-mode fiber is commonly specified as 8/125, 9/125, and 10/125.

OPTICAL TRANSMITTER 850, 1300, and 1550 nm

850 and 1300 nm for multimode fiber 1300 and 1500 nm for single-mode fiber

For single-mode fiber laser diodes must be used

ATTENUATION For multimode fiber

PMD standard specifies a power budget of 11.0 dB

Maximum cable attenuation is 1.5 dB/km at 1300 nm.

single-mode fiber power budget extends from 10 to 32 dB

Core/Cladding

this means that up to

11 dB of the optical

signal can be lost.


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FDDI Ring Structure

FDDI backbone consists of two separate fiber-optic rings,

primary ring: active

secondary ring: on hold, Station Types

Class A:dual-attachment stations, Class B: single-attachment station.


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SONET/SDH(1)

Current transmission and multiplexing standard for high speed signals

North America: Synchronous Optical Network (SONET)

Europe, Japan and rest of the world: Synchronous Digital Hierarchy (SDH)

Prior to SONET and SDH: Plesiochronous Digital Hierarchy (PDH)

4KHz sampled at 8KHz quantized at 8 bits per sample 64kb/s

Level North America [Mb/s] Europe [Mb/s] Japan [Mb/s]

0 DS0 0.064 0.064 0.064

1 DS1/T1 1.544 E1 2.048 1.544

2 DS2/T2 6.312 E2 8.448 6.312

3 DS3/T3 44.736 E3 34.368 32.064

4 139.264 E4 139.264 97.728

Transmission rates for PDH


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SONET/SDH(2) PDH versus SONET/SDH

Multiplexing

PDH: Difficult to pick low bit rate stream from high bit rate stream In PDH, clocks of lower bit streams are not perfectly synchronous

Higher rates are not integral multiples of 64Kb/s

Bit stuffing needed

Mulltiplexers and Demultiplexers complicated

In SONET/SDH a master clock is usedMUX and DEMUX much easier

Management Unlike PDH, SONET/SDH standards are rich of management and traffic

performance monitoring information

Interoperability SONET/SDH define standard optical interfaces

PDH: different vendors define different line coding, optical interfaces,...

Networking SONET/SDH: Service restoration time is less than 60 ms

PDH: restoration time is several seconds to minutes


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SONET/SDH(3) SONET/SDH

Lower speed PDH is mapped into synchronous payload envelope (SPE),or synchronous containerin SDH

Path overhead bytes are added to the SPE

Path overhead unchanged during transmission

Allows PDH monitoring end-to-end

SPE+path overhead = virtual tributaryVT (containerin SDH) VT may be placed at different points within a frame (125s)

Many small VTs can be multiplexed into a larger VT (see next slide)

The overhead of each VT includes a pointer to smaller VTs multiplexed

into the payload of the larger VT

This hierarchical structure simplifies extraction of low speed stream

from high speed stream


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SONET/SDH(4)

Smaller VT

Pointer

Small VT Small VTSmall VT

Pointer

Pointer

Pointer

Big VT

Hierarchical multiplexing structure employed in SONET and SDH

In SONET: VTs with four sizes

VT1.5, VT2, VT3, VT6 that carry 1.5, 2, 3, 6 Mb/s PDH streams VT group = 4 VT1.5s or 3 VT2s or 2 VT3s or a single VT6

Basic SONET SPE (STS-1) = 7 VT groups = 51.84 Mb/s

STS-N = N STS-1 (byte interleaved) STS = Synchronous Transport Signal STM-1 = synchronous Transport Module = 155 MB/s


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SONET/SDH(5)

VT1.5 SPE VT1.5

(SPE + path overhead)DS1

1.544 Mb/s

VT2 SPE VT2

E1

2.048 Mb/s

VT3 SPE VT3

DS1C

3.152 Mb/s

VT6 SPE VT6

DS2

6.312 Mb/s

VT group

STS-1 SPE

DS3

44.736 Mb/s

ATM

48.384 Mb/s

STS-3c SPE

E4

139.264 Mb/s

ATM

149.760 Mb/s

STS-1

STS-3c

STS-N

4

3

2

1 7

byteinterleaved

N

N/3

The mapping of lower-speed

PDH streams into VTs in

SONET

Lockedpayload: not possible to demultiplex into

lower-speed streams

SONET Signal SDH

signal

Bit rate

[Mb/s]STS-1 51.84

STS-3 (OC-3) STM-1 155.52

STS-12 (OC-12) STM-4 622.08

STS-24 1244.16

STS-48 (OC-48) STM-16 2488.32

STS-192 (OC-

192)

STM-64 9953.28

Optical Carrier


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Optical Layers(1)

Physical Layer

First generation networks:

Point-to-point, full bandwidth over single wavelength to layers above Second generation networks:

Variable amounts of bandwidth

Optical layer: Provide lightpaths to varaity of first-generation opticallayers

ATM layer

SONET/SDH layer ATM layer

Optical layer

ESCON layer

Virtual circuitsVirtual circuitsVirtual circuits

User applications

SONET/SDH connections

Lightpaths

Enterprise

Serial

Connection


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Optical Layers(2)

Optical layer OC (lightpath layer): end-to-end connections Each lightpath traverses a number of links, each link carries multiple wavelengths

(WDM).

Optical multiplex section OMS: point-to-point Consists of several segments

Multiplex

Section

connection

WDM

node

Amplifier

WDM

node

WDM

node

Optical

Channel

Amplifier

Section

Multiplex

Section

Optical

Channel

Amplifier

Section

Amplifier

Section

Multiplex

Section

Amplifier

Section

Path

Line

Section

Physical

Multiplex

Section

Channel

AmplifierSection

SONET/SDH

layer

Opt

icallayer

Example:

SONET over optical layer


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WDM OPTICAL NETWORKS

Considerable increase in traffic became a driving force for WDM and itsevolution into dense WDM (DWDM).

WDM refers to the technology of combining multiple wavelengths ontothe same optical fiber.

Each wavelength is a different channel.

At the transmitting end, there are Windependent transmitters. Eachtransmitter Txis a light source, such as a laser, and is independentlymodulated with a data stream. The output of each transmitter is anoptical signal on a unique wavelength i, i= 1, 2, . . . , W.

WDM: ~200 GHz spacing

DWDM: ~50 GHz spacing


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Wavelength Routing Optical Networks

Lightpaths

Circuit-switched network. This connection is a circuit-switching connection and is established by using a

wavelength on each hop along the connections path.

Exmaple Lightpaths from router A to C over OXCs

1 and 2; from B to D over OXCs 1 and 3;

and from C to D over OXCs 2

and 3.

OXC 3 contains wavelength

converter

Assumed single fiber carrying

Wwavelengths,

Unidirectional transmission.


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References

REFERENCES:

Rajiv Ramaswami and Kumar N. Sivarjan, Optical Networks, A practical

Perspective, Morgan Kaufmann.

Keiser , Optical Communications, PHI

Gilbert Held, Deploying Optical Networking Components, McGraw-Hill.

GOVIND P. AGRAWAL, Fiber-Optic Communications Systems, Wiley & Sons.

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