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8/9/2019 oc6thsem,PON

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Passive Optical Network (PON)

ITU-T recommendation Starting point of a PON is named asOptical Line Termination (OLT) and end

point as Optical Network Termination

(ONT) or Optical Network Unit (ONU).

In Industry Terminology, the starting pointis also referred to as Central Office (CO)

and the end point as ONU.

PON is a network, which carries data in the optical domain between the

OLT and the ONU or ONT and the transport path of the optical signal is

passive. This implies that the optical network devices (between thetransmitter and receiver) are non-powered, i.e., no electrical devices are

used.


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Standard Development

First PON activity 1980

Internal become common in the 1990s, which brought out the need forefficient broadband access.

A group of seven major network operators established the Full ServiceAccess Networks (FSAN) consortium in 1995 to device a common set ofrequirements for optical access systems.

In 1998, this resulted ITU-T recommendation G.902 and, in 1999,ITU-Tadopted the new specifications as the 155 Mb/s PON system [ITU-Trecommendation. G.983 series]. This was named as Broadband PON

[BPON] or more commonly as Asynchronous Transfer Mode [ATM] basedPON [A-PON].

While FSAN and ITU-T were actively improving B-PON, Ethernet wasgaining more and more popularity. IEEE established the Ethernet in theFirst Mile (EFM) study group, which later developed to IEEE802.3ah taskforce. This task force had an E-PON sub task force, which focused onstandardizing the Ethernet based 1Gb/s symmetrical PON system. Thework has been finalized and the first version of the standard was approved

in 2004. FSAN consortium initiated new effort to standardize PON networks

operating at bit rates above 1 Gb/s. The work was based on the earlier B-PON. In 2003, the first draft documents of ITU-T recomm. G.984 or Gigabit-Capable PON [G-PON] standard were published.


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PON Characteristics:-

1. Low-Cost Optical Components- The main source of cost in running an existingnetwork are the maintenance and powering of activenetwork equipment.

- Idea of PON use passive components.- no requirement for power or maintenance.

- two types of components- Optical splitter/combiner is used with TDM PONnetworks. It divides the optical power, originatingfrom OLT, to all ONUs and combines the upstreamsignals coming from the ONUs into a single fiber.

- In WDM networks, arrayed waveguide Gratings(AWG) devices are used for the traffic distribution.An AWG device can separate wavelengths androute them to different fibers. In the upstreamdirection, the wavelengths are combined into a

single fiber towards the OLT.


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- Although transport links of a PON

networks can do without electronicscomponents, powered transceivers areneeded at the terminating ends of thefiber, i.e. at the OLT and ONUs. Cost-

efficiency is still maintained, becausethe OLT side needs only one transceiverto communicate with the ONUs. Powerfeeding does not usually cause

additional expenses, since the OLT andONUs are located in places whereelectric power is available.


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2. Simple Network Architecture

General PON Architecture and Terminology

Optical Splitter or

AWG/Combiner


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In PON network, data signals are carried fromone to many in the downstream direction and frommany to one in upstream direction. Thus the powerof the downstream signal is divided in a splitter anddelivered to all ONUs, connected via fiber links to thesplitter.

The no. of ONUs that can be connected to asplitter is limited by the power loss, introduced inthe splitter and on the OLT-to-ONU fiber links. Whenthe power is divided uniformly between the ONU-links, the longest link sets the limit, because powerloss is a function of the transport distance.

One could use Linear Divider Combiner (LDC)components to adjust the signal power to be equal at

each ONU input interface. However, LCDs are in thedevelopment phase and therefore differentmultiplexing techniques, commonly referred to asWDM-PONs are used instead.


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PON concept allows an efficient way to offer one-way services.However when two-way services are concerned the PON solutionrequires some extra investigation. In the two-way transport, the most

critical point is the optical splitter/combiner, which does only passiveoperations. In the upstream direction this means that data streamfrom different ONUs are combined for transmission towards OLT. Ifnot synchronized, signals from different ONUs may overlap at thecombiner, which causes signal deterioration and data loss in thereceiver. The standardization bodies, working on the issue, haveovercome this problem by defining specific request and grantprocedures to be used between the OLT and ONUs for delaycalculations. As a result, all the three standards use TDM techniquefor the downstream and Time Division Multiple Access(TDMA) for upstream traffic.


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TDM

PON transport Principles

TDMA


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Upstream and Downstream signals can not be

carried on the same wavelength in the same fiber withoutdifficulties, e.g. due to cross talk. So if we use same

wavelength, then we have to use different fiber for

downstream and upstream, and if we use same fiber then

different wavelength.

The PON standards commonly reserve the 1490nm

wavelength for the downstream and 1310nm for the

upstream. Today, PONs operate at bit rates upto 2.5 Gb/s.


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ATM based PON [ATM-PON]

The ATM-PON was first used to describe the ATM basedPON system, developed by FSAN, but it was later named asBroadband-PON [B-PON] by ITU-T.

Broadband Passive Optical Network [B-PON]

The B-PON transmission convergence layer exploits TDMtechnique in the downstream direction and TDMA technique inthe upstream direction. The physical layer specificationsdefine the maximum distance between an ONU and OLT to be20 Km. The logical reach of the B-PON medium access control

(MAC) protocol is limited to 20 Km between the nearest andthe farthest ONU and it can identify up to 64 separate ONUs.


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Currently, the following nominal line rates have beendefined for the downstream/upstream traffic:-

- 155.52 Mb/s / 155.52 Mb/s

- 622.08 Mb/s / 155.53 Mb/s

- 622.08 Mb/s / 622.08 Mb/s

- 1244.16 Mb/s / 155.52 Mb/s

- 1244.16 Mb/s / 622.08 Mb/s

In the case of a dual fiber system, the older B-PON standard reserves the 1260-1360 nmwavelength window for both the downstream and

upstream traffic. In the case of a single fiber system,the 1480-1580 nm window is used for thedownstream.


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In the new wavelength allocation scheme-

Downstream wavelength window is divided intothe normal downstream band (1480-1500nm) and

enhancement band. The enhancement band has two

options, 1539-1565 nm window for additional digital

services and 1550-1560 nm window for video

distribution and service alike. Separate guard bandsare left between the reserved wavelength windows

and some bands are reserved for the future use.


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Upstream Downstream

Future Use

1200nm 1300nm 1400nm 1500nm 1600nm

Basic Wavelength

Allocation withG.983.1

1200nm 1300nm 1400nm 1500nm 1600nm

Upstream FutureUseEnhancementWavelength

Allocation

Complaint

with G.983.3

EnhancementBand

Wavelength Allocation for a single fiber B-PON


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Ethernet Passive Optical Network [E-PON]

E-PON concept was developed by IEEE 802.3 oh and

standard was published in Sept. 2004.

TDM technique Downstream traffic.

TDMA technique Upstream traffic.

Transmission speed 1 Gb/s (Symmetrical)

Protocol sets no limits to the maximum logical distance

between the ONUs & OLT.

E-PON defines single fiber network.

1490nm downstream

1310nm upstream

1550nm open for additional services such as CATV, WDM

network

E-PON physical media dependent layer defines two alternative

optical range : 10 Km for short reach and 20 Km for long reach.


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Gigabit-capable PON [G-PON]

Aim of G-PON is to improve the B-PON system.

TDM downstream

TDMA upstream

The G-PON standard defines the following nominal system line rates

(downstream/upstream)

-1244.16 Mb/s / 155.52 Mb/s

-1244.16 Mb/s / 622.08 Mb/s

-1244.16 Mb/s / 1244.16 Mb/s

-2488.32 Mb/s / 155.52 Mb/s

-2488.32 Mb/s / 622.08 Mb/s

-2488.32 Mb/s / 1244.16 Mb/s

-2488.32 Mb/s / 2488.32 Mb/s


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In the G-PON Transmission Convergence layer (TC), the

max. logical reach between an OLT and an ONU is 60 Km, while

the max. differential fiber distance between the farthest & the

nearest the ONU is 20 Km.

The ONU addressing scheme allows upto 253 ONUs to beidentified in the same network.


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Wavelength Division Multiplexed PONs

WDM techniques have most commonly been applied in the

downstream direction and can be divided roughly into broadcast and

select PONs (WPONs) and wavelength routed PONs [WRPONs].

OLT AWG

ONU

ONU

ONU

ONU

WRPON1 - n

1

2

n-1

n


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ONU

ONU

ONU

SplitOLTWPON

The WDM-PON technique to be used on the upstream direction can be determined

separately from the downstream direction. The most common solution is to deploy asingle wavelength together with TDMA.

However, this combination is not capable of supporting symmetrical data

rates. If symmetry is required, the wavelength routed alternative might be a more

desirable solution also for the upstream direction.

1 - n

1-n

1-n

1-n


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BPON EPON GPON

Bit rates

Upstream

Downstream

622 Mb/s

1244Mb/s

1000 Mb/s

1000 Mb/s

2488 Mb/s

2488Mb/s

Logical Split

Ratio 1:64 1:32768 1:253

Max. Physical Distance

OLT-ONU 10 Km / 20 Km 10 Km / 20 Km 10 Km / 20 Km

Max. Logical Distance

OLT-ONU

ONU-ONU

N/A

20 Km

Not Limited

N/A

60 Km

20 Km

Max. link utilization

Upstream

Downstream

85.71%

87.33 %

98.00 %

98.27 %

97.44 %

98.53%

Protection APS N/A APS

N/A Not Announced

Summary of PON Concept

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