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Passive Optical Networking

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PCQ Bureau
New Update

The installed base of copper-twisted pair and coaxial cables is insufficient

to handle the ever-increasing demand for network bandwidth which



is driven by Internet usage, video conferencing, storage networks and large file
transfers. The most widely deployed broadband solutions today are DSL and CM

(Cable Modem) networks. Although they are an improvement as compared to 56 Kbps

dial-up lines, they are unable



to provide enough bandwidth for emerging services such as VOD (Video-on-
Demand), interactive gaming or two-way video conferencing.



A new technology is required to meet such high bandwidth needs. It should be
inexpensive, simple, scalable and capable of delivering bundled voice, data and

video services to the end user over a single network.


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What is PON?



The Passive Optical Networking technology represents the convergence of low-cost
Ethernet equipment and low cost fiber infrastructure. The PON (Passive Optical

Networking) is a point-to-multipoint optical access architecture designed to

bridge the last mile. The fundamental benefits of PON technology are

flexibility, reliability and simplicity. This is an emerging access network

technology that could allow dual-wired/wireless transmission of the same content

such as converged data, video and voice over a single optical access system.

Direct

Hit!
Applies

to:
ISPs, network managers
USP:

Learn about the flexibility, reliability and simplicity of this optical network
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Passive Optical Access Network

'Passive' describes the fact that optical transmission has no power

requirements or active electronic parts when the signal is going through the

network. Depending on where the PON terminates, the system can be described as

FTTC (Fiber-To-The-Curb), FTTB (Fiber-To-The-Building) or FTTH

(Fiber-To-The-Home).

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PON architecture



A PON consists of a central office node called an OLT (Optical Line Terminal),
one or more user nodes called ONTs (Optical Network Terminals), and the fibers

and splitters between them referred to as the ODN (Optical Distribution

Network).

The OLT provides the interface between the PON and the backbone network while

the ONTs provide service interface to the end user. Optical signals traveling

across the PON are either split onto multiple fibers or combined onto a single

fiber by optical splitters depending on whether the light is traveling up or

down the PON. PON is deployed in a single-fiber, point-to-multipoint,

tree-and-branch configuration for residential applications. It may also be

deployed in a protected ring architecture for business applications or in a bus

architecture for campus environments.



PON combines, routes and separates optical signals through the use of passive
optical filters. It distributes and routes signals without the need to convert

them to electrical signals for routing through switches. It also takes advantage

of WDM (Wavelength Division Multiplexing), using one wavelength for downstream

traffic and another for upstream traffic. This allows for two-way traffic on a

single fiber-optic cable.

EPON is

a single, layer 2 network that uses only one protocol-IP-for

everything from data, voice and video

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The latest specification calls for downstream traffic to be transmitted at a

wavelength of 1490 nm and the upstream traffic to be transmitted at 1310 nm. The

1550 nm band is purposely left open in case the servce provider wishes to share

the PON fiber with an HFC (Hybrid Fiber-Coax) network, which is the traditional

Cable TV architecture. 

A PON is a shared network in that the OLT sends a single stream of downstream

traffic that is seen by all ONTs. Each ONT reads the content of only those

packets that are addressed to it. Encryption is used to prevent unauthorized

snooping of downstream traffic. The OLT also communicates with each ONT in order

to allocate upstream bandwidth to each node. When an ONT has some traffic to

send, the OLT assigns a timeslot in which the ONT can send its packets.

As bandwidth is not explicitly reserved for each ONT but allocated

dynamically, a PON allows statistical multiplexing and over-subscription of both

upstream and downstream bandwidth. This gives PON yet another advantage over

point-to-point networks, in that not only the fiber but also the bandwidth can

be shared across a large group of users, without sacrificing security.

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Being a passive network technology, the network elements don't amplify the

signal in PON. So, trunk lengths and signal splits are limited. FSAN trunk

lengths can be upto 12 miles, and as many as 32 users and 64 endpoints can share

access to a single PON at the current speeds and with the current splitter

technologies using the features of DWDM (Dense Wave Division Multiplexing).

Some newer PONs use HDWDM (High-Density Wave Division Multiplexing). The use

of HDWDM increases the number of Optical Network Users per PON from 32 to 64.

GPON consists of multiple layer 2 networks over the same physical layer. Each network has a different protocol

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APON



This was the first PON standard. It was developed in the mid 1990s through the
work of FSAN (Full Service Access Network) initiative. APON (ATM Passive Optical

Networking) was used primarily for business applications, and was based on ATM.

When the ATM protocol is combined with PON system, it is called APON.

The APON format used by FSAN was accepted as an ITU (International

Telecommunications Union) standard ITU—T G.938. APONs support 16 wavelengths

with 200 GHz spacing and 32 wavelengths with 100 GHz spacing between channels.

BPON (Broadband PON) is also a standard which is based on APON. BPON adds

support for WWDM (Wide Wavelength Division Multiplexing). WWDM specifies regular

transmissions in the band from 1480—1500 nm, and an enhanced band for video

and other future applications between 1539—1565 nm.

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EPON



In 1995 when the FSAN initiative was started, ATM had high hopes of becoming the
prevalent technology in LAN, MAN and backbone. But Ethernet technology has

leapfrogged ATM. Ethernet has become universally accepted standard with over 320

million port deployments all over the world. It's easier to scale and manage

WAN and LAN on an Ethernet network.

One of ATM's shortcomings is that in this case a dropped or corrupted ATM

cell will invalidate the entire IP datagram. Thus, APON may not be the best

choice to interconnect two Ethernet networks. On the other hand, EPON (Ethernet

PON) looks like a logical choice for an IP data-



optimized access network.

EPON is an IEEE/EFM 802.3ah standard that carries data traffic encapsulated

in the Ethernet frames. It is based on MPCP (Multi-Point Control Protocol) and

uses a standard 8b/10b line coding (8 user bits encode as 10 line bits)

operating at standard Ethernet speed.

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The key difference between EPONs and APONs is that in EPONs, data is

transmitted in variable length packets of upto 1,518 bytes according to the IEEE

802.3 protocol for Ethernet, whereas in APONs, data is transmitted in fixed

length of 53 byte cells (48 byte payload and 5 byte overhead) as specified by

the ATM protocol. This format means it is difficult and inefficient for APONs to

carry traffic formatted according to the IP.

The main advantage of EPON is that it allows carriers to eliminate complex

and expensive ATM and SONET elements and to simplify their networks

dramatically. Thus, EPONs finally make it cost effective for service providers

to extend fiber into the last mile and reap all the rewards of the very

efficient, highly scalable, low maintenance, end-to-end fiber optic network.

GPON



GPON (Gigabit PON) technology was accepted as ITU-T G.984 standard in January
2003. It's an evolution of the BPON standard. There are four basic

recommendations under G.984 standard-G.984.1 which describes the service

provider requirements for the system; G.984.2



specifies the physical layer for all data rate combinations in G-PON; G.984.3
that defines the transmission convergence layer; and G.984.4 that defines the

OMCI on the system.

GPON supports higher rates, enhanced security, and a choice of three Level 2

networks: ATM for voice, Ethernet for data and proprietary encapsulation for

voice. This type of network promises 1.25 Gbit or 2.5 Gbit downstream, and

upstream bandwidth scalable from 155 Mbit to 2.5 Gbit, and supports upto 128

ONUs (Optical Network Users). It provides security at the protocol level for the

downstream traffic.

Why PON?



There are numerous advantages of using a Passive Optical Network. Some of them
are as follows.




-
Minimizes fiber deployment in the local loop and lowers the

initial deployment cost.




-
Provide higher bandwidth due to deeper fiber penetration,

offering gigabit-per-second solutions.




-
Operating in the downstream as a broadcast network, they allow

for video broadcasting either as IP video, or analog video.




-
Allow for long reach between the CO and customer premises,

operating at distance over 20 kms.




-
Eliminate the necessity of installing active multiplexers at

the splitting locations.




-
Being optically transparent end-to-end, these networks allow

upgrades to higher bit rates or additional wavelengths.




-
Uses bi-directional communications over a single fiber strand.




-
Low maintenance, faster and cheaper.



Now that you know the underlying basics behind the passive optical networking
technology, you must go ahead to cash all these advantages



offered by it.

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