Ethernet Passive Optical Network(EPON) : Building a Next- Generation Optical Access Network

COURSE – CEG 790Instructor – Dr. Bin Wang

Presenter – Ram Iyer

Authors : Glen Kramer and Gerry Pesavento(AllOptic Inc.).

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Overview

Introduction What are Passive Optical Networks ? Deployment Scenario of Next-Generation Access

Networks Types of PON technologies Different types of PON topologies What are EPONs ? How does an EPON work ? Issues related to EPONs Benefits of using EPONs IEEE P803.3ah status The market for EPONs Conclusion

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Introduction

Internet has spawned genuine demand for broadband services, leading to unprecedented growth in Internet Protocol (IP) data traffic. This humongous data traffic is putting pressure on carriers to upgrade their networks.

An improvement over 56 kb/s is unable to provide enough bandwidth for emerging services such as the IP telephony, Video on Demand (VoD), interactive gaming, or two-way video conferencing.

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Per-user bandwidth requirements for new services kept increasing as

shown

A new technology is required which would be able to handle the bandwidth hungry services.

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What is a Passive Optical Network (PON) ?

Passive Optical Network (PON) is a high bandwidth Point-to-Multipoint (P2MP) optical fiber network based on the Asynchronous Transfer Mode protocol (ATM), Ethernet or TDM.

Components used in Passive Optical Network

PONs generally consist of an OLT (Optical Line Termination), which is connected to ONUs (Optical Network Units). OLT and ONUs are explained in the later slides of the presentation.

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Properties of PONs

PONs rely on light waves for data transfer. Only passive optical components are used

such as optical fiber, splices and splitters. PONs minimizes the fiber deployment in

both the local exchange office and the local loop.

PONs provides higher bandwidth due to deeper fiber penetration, offering gigabit per second solutions.

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Range of operation of PONs

PONs aim to break the First Mile (once called as Last Mile) bandwidth bottleneck by targeting the sweet spot between T1s and OC-3s that other access network technologies do not adequately address. PONs are capable of delivering high volumes of upstream and downstream bandwidth (up to 622 Mbps downstream and 155 Mbps upstream).

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Deployment scenario of Next-Generation Access Network

A logical way to deploy optical fiber in the local access network is using a point-to-point (P2P) topology, with dedicated fiber which runs from the local access network to each end-user subscriber (Figure a)

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Deployment of Next-Generation Access Network contd..

Second method is to deploy a remote switch (concentrator) close to neighborhood since it reduces the fiber deployment as shown in (Figure b). The main downside of this curb switch architecture is it requires electrical power as well as the backup power at the curb unit and currently, one of the highest cost for local exchange carriers is providing and maintaining electrical power in the local loop.

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Deployment of Next-Generation Access Network contd..

In the third we can see that a PON actually minimizes the amount of optical transceivers, central office terminations, and the fiber deployment. As stated earlier a PON is a point-to-multipoint (P2MP) optical network with no active elements in the signals path from the source to destination. PONs basically use passive optical components, such as optical fiber, splices, and splitters. This is show in the Figure c.

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Decrease in the number of Fibers and Transceivers used

Point to point network - Number of Fiber - 32 Number of Transceivers - 64

Curb-switched network - Number of Fibers - 1 Number of Transceivers - 66

Passive Optical Network - Number of Fiber - 1 Number of Transceivers - 33

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Types of PON technologies

PON

Asynchronous transfer mode PONs (APONs)

Ethernet PONs (EPONs)

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Passive Optical Networks APONs Data is transmitted in fixed

length 53-byte cells as specified by ATM protocol.

APONs don’t deliver data, video and voice over a single platform.

APONs offer insufficient bandwidth

APONs are expensive APONs do not provide

broader service capabilities

EPONs Data is transmitted in variable-

length packets of up to 1,518 bytes according to IEEE 802.3 protocol for Ethernet.

EPONs deliver data, video and voice over a single platform

EPONs offer higher bandwidth EPONS are less expensive than

APONs EPONs provide broader service

capabilities

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Components used in PON topologies

All transmission in a PON are performed between an optical line terminal (OLT) and optical network units (ONUs).

What is Optical Line Terminal (OLT) ?An OLT resides in the local exchange (central office), connecting the optical access network to the metro back-bone.What are Optical Network Units (ONUs) ?The ONU provides the interface between the customer’s data, video,and telephony networks and the PON. Its function is to receive trafficin a optical format and convert it into customer’s desired format(Ethernet, IP multicast, T1, etc.)

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Typical PON architecture

EPONs

APONs

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Different types of PON topologies

Tree topology Bus topology Ring topology Tree with redundant trunk

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PON topologies

Figure 3

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Why do we require EPONs?

We require EPON technology since it has the following qualities:

it is inexpensive, simple, scalable and capable of delivering bundled voice, it provides data and video services to

an end-user subscriber over a single network.

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What are EPONs?

Ethernet passive optical networks (EPON) are an emerging access network technology that provides a low-cost method of deploying optical access lines between a carrier office (CO) and customer site.

We can say that, Ethernet Passive Optical Networks (EPONs) represents the convergence of low-cost Ethernet equipment and low-cost fiber infrastructure, to be the best candidate for the Next-Generation access network.

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How does an EPON work ? In a EPON the process of transmitting

data downstream from the OLT to multiple ONUs is fundamentally different from transmitting data upstream multiple ONUs to the OLT.

The different techniques used to accomplish the downstream and upstream transmission in a EPON are shown in Figure 4 and Figure 5.

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Downstream traffic in EPON

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Downstream Traffic flow in an EPON

Consider the downstream traffic in EPON

In the Figure 4, the data broadcasted downstream from OLT to multiple ONUs in variable-length packets of up to 1,518 bytes, according to IEEE 802.3 protocol. Each packet carries a header that uniquely identifies it as data intended for ONU-1, ONU-2 or ONU-3.At the splitter the traffic is divided into three separate signals, each carrying all of the ONU specific packets. When the data reaches the ONU, it accepts the packets that are intended for it and discards the packets that are intended for other ONUs. For example, in figure 4, ONU-1 receives packets 1, 2 and 3; however only two packets are delivered to end user 1.

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Downstream Frame Format in an EPON

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Upstream traffic in EPON

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Upstream Traffic flow in a EPON

Figure 5 shows the upstream traffic is managed utilizing TDM technology, in which transmission time slots are dedicated to ONUs. The time slots are synchronized so that upstream packets from the ONUs do not interfere with each other one the data is couple onto the common fiber. For example, ONU-1 transmits packet 1 in the first time slot, ONU-2 transmits packet 2 in the second non-overlapping time slot, and ONU-3 transmits packet 3 in a third non-overlapping time slot.

Consider the downstream traffic in EPON

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Upstream Frame Format in an EPON

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Transceiver Issues

There are number of issues which have surfaced by the use of transceivers (A transceiver is a device which is capable of transmitting and receiving signals)

Due to the unequal distances between the central office and ONUs, optical signal attenuation in the PON is not same for each ONU i.e. the power level received at the OLT will be different for each ONU (this is also called as near-far problem)

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Transceiver Issues Contd. As shown in the Figure below, one ONUs signal strength

is lower at the OLT, which is most likely due to the longer distance.

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Approaches suggested to solve the attenuation

problem

There are couple of approaches which are suggested in this paper but they have not been implemented since they have their own drawbacks

One of the approaches suggested is:

To allow ONUs to adjust their transmitter power such that power levels received by the OLT from all the ONUs becomes the same.

Drawback of this approach: This method is not favored by the transceiver designers

because it makes the ONU hardware more complicated, requires special signaling protocol for feedback from the OLT and ONU and most importantly degrades the performance of the all the ONUs to that of the of the most distant unit.

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Security

Encryption mechanism is necessary since a malicious ONU if placed in promiscuous mode would be able to read all the downstream packets.

Is Encryption mechanism necessary in Passive Optical Network ?

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On which layer of the OSI model the encryption must

be placed? If the encryption is placed in the MAC layer then it

will encrypt the MAC frame payload only, and leave the headers in plain text. This method prevents malicious ONUs from reading the payload, but they may still learn other ONUs MAC address.

Implementing the encryption scheme on the physical layer would encode the entire bit stream, including the frame headers and CRC. In this scheme no information is learned by a malicious ONU. But the difficulty is the physical layer is a connectionless layer. Requiring the Physical layer in a OLT to apply different keys for different ONUs will make it connection-aware.

So encryption in EPON still remains an open question.

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Benefits of Ethernet PONs Higher bandwidth: up to 1.25 Gbps symmetric

Ethernet bandwidth Lower Costs: lower up-front capital equipment

and ongoing operational costs More revenue: broad range of flexible service

offerings means higher revenues

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Higher bandwidth More subscribers per PON More bandwidth per subscriber Higher split counts Video capabilities Better QoS

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Lower Costs

Cost reduction in the case of EPONs are achieved by simpler architecture, more efficient operations, and lower maintenance needs of an optical IP Ethernet network.

Eliminate complex and expensive ATM and SONET elements and dramatically simplify the network architecture

Long-lived passive optical components reduce outside plant maintenance

Standard Ethernet interfaces eliminate the need for additional DSL or cable modems

No electronics in outside plant reduces need for costly powering and right-of-way space

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More Revenue

EPONs support for legacy TDM, ATM and SONET services.

Delivery of new Gigabit Ethernet, fast Ethernet, IP multicast and dedicated wavelength services.

Provisioning of bandwidth in scalable 64 Kbps increments up to 1 Gbps.

Tailoring of services to customer needs with guaranteed SLAs (Service License Agreement).

Quick response to customer needs with flexible provisioning and rapid service reconfiguration.

Revenue opportunities from EPONs include:

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IEEE P802.3ah status The standards work for Ethernet in the local

subscriber access network is being done in the IEEE P802.3ah Ethernet in the First Mile (EFM) Task Force.

In order to evolve Ethernet for local subscriber networks, P802.3ah is focused on four primary standards definitions:

Ethernet over copper Ethernet over P2P fiber Ethernet over P2MP fiber Operation, administration, and maintenance

(OAM)

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IEEE 802.3ah has been approved. Materials concerning the P802.3ah standards effort and the presentation materials can be found at:

•http://www.ieee802.org/3/efm/index.html

•http://www.ieee802.org/3/efm/public/index.html

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The Market for EPONs Analysts expect the optical access

market to grow rapidly. CIBC (Canadian Imperial Bank of

Commerce) forecasts the market for PON access system to reach $1 billion by 2004 from $23 million in 2000.

P2P optical Ethernet offer the best possibility of a turnaround in the telecom sector.

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Conclusion

The future of broadband access network is likely to be a combination of point-to-point and point-to-multipoint Ethernet, optimized for transporting IP data, as well as time critical voice and video.

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References Topics in Lightwave: Ethernet Passive

Optical Network (EPON): Building the Next-Generation Optical Access Network

Glen Kramer and Gerry Pesavento, Alloptic, Inc.

http://www.iec.org/online/tutorials/epon/