INTRODUCTION TO

OPTICAL NETWORKS

Presentation Overview• Why Optical Networks..?!• Generations of Optical Networks• The Classical Layered Hierarchy• The Optical Layer

• Functions of Optical Layer• Advantages of Layering

• Architectures Of Networks• Access Networks: Introduction

• Why Passive Optical Networks..?!• Passive Optical Access Network • Ethernet Passive Optical Network (EPON)

• Downstream and Upstream Operation• WDM-Passive Optical Network (WDM-PON)• Ring-Based WDM-PON Architecture

• Downstream and Upstream Operation

Why Optical Networks ...??Dramatic changes in the telecommunicationindustry. • Need for more capacity in the network.• Tremendous growth of the Internet and the World

Wide Web in terms of number of users & the amount of time bandwidth taken by each user – internet traffic growing

rapidly.• Businesses rely on high speed networks.• Need for more bandwidth.• Deregulation of the telephone industry.• Need of providing quality of service(QoS) to carry

performance sensitive applications ( real-time voice, video etc.)

Optical Networks Definition: An Optical Network is a telecommunication network • with transmission links that are optical fibers and• with an architecture that use designed to exploit the unique features if fibers.

High performance lightwave network –involve complex combination both optical and electronic devices.

Low-cost broadband services – Internet based applications continues to increase.

The “glue” that holds the purely optical network together consists of :• optical network nodes (ONN) connecting the fibers within the network• network access stations (NAS) interfacing user terminals and other non-

optical end systems to the network

Critical role :• Reducing communications costs• Promoting competition among carriers & service providers• Increasing the demand for new services

Generations of Optical Networks

First Generation: • Optics used for transmission & provide

capacity• Switching & other intelligent network

functions were handled by electronics• ex. SONET (synchronous optical network)• SDH ( synchronous digital hierarchy)

Second Generation:• have routing ,switching and intelligence in

the optical layer• use multiplexing techniques – provide the

capacity needed

The Classical Layered Hierarchy

The OSI Model Physical layer

• Provides a “pipe” with a certain amount of bandwidth to the data link layer.

Data link layer• Framing• Multiplexing• Reliable transmission –acknowledgment frames• Error detection and correction• Flow control• Demultiplexing data send over the physical

layer.

Network Layer• Performs the end-to-end routing function

of taking a message at its source• And delivering it to its destination• Controls congestion

Transport Layer• Ensuring the end-to-end • In-sequence• Ensuring error-free delivery of the

transmitted messages

The Classical Layered Hierarchy

Session Layer• Sessions restoration• Token management• Synchronization

Presentation Layer • Encoding data

Application Layer• Compatibility betweenapplications

The Classical Layered Hierarchy

The architecture is composed of anunderlying optical infrastructure Physical layer

• Contains optical components executing linear(transparent)operations on optical signal.

• provides basic communication services to a number of independent logical networks (LNs).

LNs are residing in the Logical layer. • Contains electronic components executing

nonlinear operations on electrical signal

The Optical LayerLayered View of the Optical Network

The Optical LayerLayered View of the Optical Network

Multiplexes lightpaths into a single fiber.

Allows individual lightpaths to be extracted efficiently from the composite multiplex signal at the network nodes.

Incorporates sophisticated service restoration techniques.

Incorporates management techniques.

Provides lightpaths – used by SONET and IP network elements.

Functions Of The Optical Layer

Advantages of Layering1. Independently control and manage each logical

network simplifying these functions. 2. Share the total resources of the physical layer among

several logical network exploiting them more efficiently.

3. Customize each logical network to provide specialized user services improving the QoS.

4. Dynamically reconfigure each logical network equipment failures and changing traffic patterns.

5. Use both optical and electronic degrees of freedom provide flexibility, survivability, manageability and capacity for growth and change.

Architectures Of NetworksBackbone Networks

• networks in the same building, in different buildings in a campus environment, or over wide areas.

• exchange of information between different LANs

Metro Area Networks (MAN) • network that interconnects users with computer

resources in a geographic area or region larger than that covered by even a large local area network (LAN) but smaller than the area covered by a wide area network (WAN).

Access Networks• Distributed EPON architectures• Distributed ring-based WDM-PON architectures • Converged Optical/Wireless Access Networks

Architecture Of Networks

• Access Networks• Tremendous growth in both backbone and

Metro Access Network (MAN) capacity.

• End users are becoming more sophisticated• Rich multimedia• Real-time services

• The “Last Mile” remains a bottleneck.• Current “Last Mile” capacity has increased from

56Kb/s (dialup modem) to a few Mb/s (cable modem or digital subscriber line (DSL) connection).

• Still far short of the Gigabit line speed necessary to support rich multimedia and real-time services.

Access Networks : Introduction

Access Networks …

CentralOffice

End Users

Last/First Mile

Copper-based access networks will soon no longer be able to meet the ever-growing consumer demand for bandwidth.

PON-based fiber-to-the-curb/home (FTTC/FTTH) systems are considered as possible successors to current copper-based access solutions.

Two most viable architectures:• Single channel Time-Division Multiplexed PON

(TDM-PON) • Multi-channel Wavelength-Division Multiplexed PON

(WDM-PON)

Access Networks …

18

Why Passive Optical Networks? A natural step in access evolution

Concentration Switch in the neighborhood

PON Minimum fiber usage/ N+1 transceivers Path transparency Passive network elements Much longer distance (~20km) than DSL (~5.5 km). Higher bandwidth due to deeper fiber penetration. Downstream video broadcasting.

Point-to-Point links

CO

CO SC

Passive Star Coupler

CO

~20 km ~1 km

PON

Passive Optical Access Network

Time Division Multiplexing (TDM) :A type of multiplexing that combines data streams by assigning each stream a different time slot in a set. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel.Wavelength Division Multiplexing (WDM):A technique of sending signals of several different wavelengths of Light into the Fiber simultaneously. In fiber optic communications, wavelength-division Multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single Optical Fiber by using different wavelengths (colors) of Laser light to carry different signals.WDM is similar to frequency-division multiplexing (FDM).

Multiplexing Techniques

Optical Network Terminal and Optical Network UnitONT (Optical Network Terminal):An ONT is a media converter that is installed either outside or inside your premises, during fiber installations.

The ONT converts fiber-optic light signals to copper/electric signals.

Three wavelengths of light are used between the ONT and the Optical Line Terminal :•1310 nm voice/data transmit•1490 nm voice/data receive•1550 nm video receive

Each ONT is capable of delivering: Multiple POTS (plain old telephone service) lines Internet dataVideo

ONU (Optical Network Unit):

•An Optical Network Unit (ONU) converts optical signals transmitted via fiber to electrical signals.

•These electrical signals are then sent to individual subscribers. ONUs are commonly used in fiber-to-the-home (FTTH) or fiber-to-the-curb (FTTC) applications.

•Using different wavelengths for each service makes it possible to transmit high-speed Internet and video services at the same time. • The 1310nm and 1490nm bands are used for Internet transmissions on the uplink and downlink, respectively,•The 1550nm band is used for multi-channel video broadcasts. •Wavelength multiplexing is performed at the central office and a wavelength demultiplexing mechanism is provided at the customer's house.

Transmission between ONT and ONU Example …

OLT – Optical Line Terminal OLTs are located in provider’s central switching office.

This equipment serves as the point of origination for FTTP (Fiber-to-the-Premises) transmissions coming into and out of the national provider’s network.

An OLT, is where the PON cards reside. The OLT's also contain the CPU and the GWR and VGW uplink cards. Each OLT can have a few or many dozens of PON cards.

PON = Passive Optical NetworkGWR = Gateway RouterVGW = Voice Gateway

Each PON card transmits 1490nm laser data signal to the ONT, and receives the ONT transmission of the 1310nm laser data signal.

The one-way 1550nm laser video signal to the ONT is injected into the fiber at the CO.

Optical Splitter and Combiner

Fiber optic splitter is used to split the fiber optic light into several parts at a certain ratio. For example, a 1X2 50:50 fiber optic splitter will split a fiber optic light beam into two parts, each get 50 percent of the original beam.An optical combiner is a passive device that combines the optical power carried by two input fibers into a single output fiber.

Ethernet PON (EPON) Architecture

Passive Optical Splitter/Coupler

Dow

nstr

eam

ope

rati

onU

pstr

eam

ope

rati

on

Downstream: Operates as Broadcast & Select

Network Each ONU extracts those packets that

contain the ONU’s unique MAC address

Upstream:• ONUs employ arbitration mechanism

to avoid collisions.

• OLT arbitrates transmissions via a Dynamic Bandwidth Allocation (DBA) module.

• A Multi-point Control Protocol (MPCP) was developed.OLT and ONUs exchange control messages, namely, REPORT and GATE messages.

• REPORT message contains the ONU’s bandwidth requirements. GATE message has the start time and the duration of the granted time slot.

• The average dedicated bandwidth per user is limited to a few percent of the channel capacity, i.e., a few tens of Mb/s.

10-20 km

EPON employs a point-to-point emulation mechanism, which makes the EPON medium behave as a collection of point-to-point links.

Emulation mechanisms rely on tagging Ethernet frames with a unique value called the Logical Link ID (LLID).

To allow point-to-point emulation, the OLT must have N MAC ports (interfaces), one for each logical link .

When sending a frame downstream (from the OLT to an ONU), the emulation function in the OLT will insert the LLID associated with a particular MAC port on which the frame arrived. Even though the frame will be delivered to each ONU, only one ONU will match that frame’s LLID with its own assigned value, and thus accept the frame and pass it to its MAC layer for further verification.

MAC layers in all other ONUs will never see that frame. (discard it)

EPON - Frame Transmission

•The LLID replace two bytes in the preamble. The OLT could distinguish frames of different ONUs by the LLIDs and thus the LLID equals the logical identification of the ONU.EPON frame

EPON – Logical Link ID (LLID)

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WDM-PONs Separate pair of dedicated upstream/downstream

wavelength channels to each subscriber (≥1 Gb/s of dedicated bandwidth per subscriber).

Provide dedicated optical connectivity to each subscriber with bit rate and protocol transparencies, guaranteed QoS, and increased security.

WDM-PON systems’ capacity is still too high compared to the access capacity needed. However, as bandwidth demand increases, the economics change. In terms of cost per bit rate, WDM-PON is more efficient and economical.

ONTλ1 λ2 λ3... λN

λ1

λ2

λ3

λN

Central Office

ONT

ONT

ONT

AW

G

L-Band

C-BandRemote Node

λ’, λ2’, λ3’... λN

OLT

WDM-PONsSimple Architecture

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WDM-PON Limitations

Traditional tree-based WDM-PON architectures suffer from several limitations including:

• Inability to efficiently utilize network resources. The unused dedicated channel capacities of lightly-loaded/idle subscribers cannot be shared by any of the other heavily-loaded users attached to the PON.

• Inability to provide private networking capability within a single PON

• Lack of simple and cost-effective protection and/or restoration capabilities.

Ring-Based WDM-PON Architecture

ONU-1

ONU-N

λ1,λ2,..,λN

λ1

λN

λLan

TX-1

TX-N

RX-1

RX-NλLanRX-Lan

12

3 1

23

λ1

λN

λ1,λ2,..,λN, λLan 90:10

Filter

MUX

λ2 λ2 λLanλLan

RXLAN TXLAN

PROCESS-APPEND-FORWARD

TXUpRXDown

OADM

λ1 λ1λLan λLan

ONU-2

OADM

λ2 λ2λLan λLan

OADMλN λNλLan λLan

WD

M M

UX

WD

M D

EM

UX

10-20km

Olt Trunk Ring Onu

Ring-Based WDM-PON Architecture

• Efficiently utilizes network resources. Provides dynamic allocation of unused capacities of lightly loaded/idle wavelengths to heavily loaded channels

• Provides truly shared LAN capability among PON end-users.

• Utilizes a fully distributed control plane among the ONUs that enables distributed provisioning and fault restoration by the ONUs

• Eliminates the OLT's centralized task of bandwidth provisioning and failure recovery • Reduction of processing complexities and delays at

the OLT.

ONU-1

ONU-N

TX-1

TX-N

RX-1

RX-N

RX-Lan

90:10

Filter

MUX

OADM

ONU-2

OADM

OADMW

DM

MU

XW

DM

DE

MU

X

TX-2

RX-2

Sche

dule

r

A downstream flow to ONU-

1

λ1

λ2A downstream flow to ONU-2

An upstream flow from ONU-1 to OLT

An upstream flow from ONU-2 to OLTλ1

λ2

Downstream & Upstream Operation(Without Sharing)

ONU-1

ONU-N

TX-1

TX-N

RX-1

RX-N

RX-Lan

90:10

Filter

MUX

OADM

ONU-2

OADM

OADM

WD

M M

UX

WD

M D

EM

UX

TX-2

RX-2

λLAN

LAN Operation (ONU-ONU Communication)

Downstream & Upstream Operation(With Sharing)

ONU-1

ONU-N

Q1

QN

RX-1

RX-N

RX-Lan

90:10

Filter

MUX

OADM

ONU-2

OADM

OADM

WD

M M

UX

WD

M D

EM

UX

Q2

Sche

dule

r

Arrival of a downstrea

mflow

destined to ONU1

λ1λ2 λLAN

More downstream

flows to ONU1 (i.e, Rin>Rout)

Scheduler runs SWS algorithm searching for a lightly loaded

downstream buffer to send ONU1’s newly

arriving excess flows (Assume Q2 is lightly

loaded )

ONU2 determines new flow as

ONU1’s downstream flow and forwards it to

ONU1 over λLAN

OLT discards all excess

downstream traffic

Congestion at downstream buffer, Q1

ONU-1

Q1

Q3

RX-1

RX-3

90:10

Filter

MUX

ONU-2

WD

M M

UX

WD

M D

EM

UX

Q2

ONU-3

RX-2

RX-Lan

Upstream Scheduling Algorithm (USA)

Arrival of an upstream flow

Arrival of more upstream flows (i.e, Ri,up>λi,up)

λ1 λLAN

OLT processes all TUS flows and

forwards them to their destinations