Tdm Passive Optical Networks Computer Science Essay

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This wiki is about Time Division Multiplexing Passive Optical Network. There is only a limited number of books in this area that give a coherent and comprehensive review of PON technologies. We start with providing some history and introduction about PON. We cover different flavors and nomenclatures of PON technologies (EPON, BPON, G-PON, APON, TDM-PON). We first present the APON/BPON system and then the more recent GPON and EPON systems. We explain different bandwidth allocation schemes that are utilized in TDM PONs. We also presents recent work and current research on next generation systems.


1. History

PON has a long history which can be retraced back to the ATM era. At that time, ATM technique was considered to be the only solution to unifying various types of communication networks ranging from back to local area networks. However, after several years ATM technique fail to achieve its objective, several key issues arises in ATM that made this technique infeasible and unreliable. These factors include high cell header tax, which causes wastes of network bandwidth, (2) if there will be loss only a single cell an entire IP packet will also be retransmitted, which may result in low efficiency of network capacity as well, (3) highly complicated sublayer architecture makes the cost of ATM facilities incomparable to widely deployed Ethernet facilities, etc <1>. This factors eventually make people lose interest in the ATM techniques, but refer to recent MPLS techniques. At that time when the ATM techniques dominate, the first generation of PON, was developed and standardized as well.

Full Service Access Network (FSAN) working group had done early work on fiber to the home architectures in 1990s. After that ITU (international Telecommunication Union) did further work and has standardized on two generations of PON. The first standard ITU-T G.983 is referred to as APON (ATM PON) and is based on ATM <2>. APON was not able to compete the DSL techniques because of its high cost and falling out of favor of ATM as a protocol, which provide the base for creation of next version of ITU-T G.983 being referred to as broadband PON, or BPON. Some years later, the ITU-T standardized Gigabit - capable PON (GPON) as a newer <3> implementation of a TDM PON with improved capabilities.

The IEEE 802.3 Ethernet PON standard was completed in 2004 as part of the Ethernet First Mile project.


2. Introduction

A passive optical network (PON) is a type of access network that uses optical fiber and it is a point to multipoint network architecture. The term passive is used which mean that there is no need of the electrical power and the fiber plant connecting the terminal, located at the Central Office (CO) of a service provider, to the customer's terminal is completely passive. Using only passive components in the path between the central office and the customer reduces the cost for the service provider and makes PONs an attractive and affordable solution for deploying broadband access networks in a large scale <3> . PONs belong to the Fiber to the X (where X = Home/ Premise/ Curb/ Office/ Neighborhood) broadband access network technologies, along with Active Optical Networks and Point to Point (P2P) links.

A PON consist of an Optical line terminal (OLT) and a number of Optical network units (ONUs). The Optical Line Terminal (OLT) is located at the side of the service provider's central office and Optical network units (ONUs) also know as Optical Network Terminals (ONTs) are located near the end users. OLT broadcasts a signal going downstream towards a power splitter, which is a passive component that, as its name implies, splits the (same) signal to N different fibers. Each of these fibers leads to a ONU at the customer side. As a result, the ONUs receive the same signal from the OLT, which contains TDM frames destined for all the customers. Each ONU recognizes its own frames by some kind of destination address contained in the frame header.

PONs are divided into Time Division Multiplexing (TDM) PONs, Wavelength Division Multiplexing (WDM) PONs and Hybrid TDM-WDM PONs. The First PON technology to be standardized and deployed was the TDM PON.

At the optical level all PON systems have the same theoretical capacity. The electrical overlay is used to set the limits on downstream bandwidth and upstream bandwidth, the protocol used to manage the connection and allocate the capacity.

2.1 TDM-PON Infrastructure

The architecture of a standard TDM-PON structure is shown in the figure 2.1. APON, EPON, and G-PON are covered in this architecture.

A 1:32 splitter is used in this architecture to connect OLT with the ONUs. Usually 10-20 km is the maximum transmission distance that is covered. 1.3-mm wavelength is used to carry upstream traffic from ONUs and 1.49-mm wavelength is used to carry the downstream traffic from OLTs &lt;7>.

Figure 2.1 Standard TMD-PON architecture &lt;7>.

Backbone switch or cross-connect is used to connect multiple OLTs in the CO to the backbone network. OLTs line cards are inserted into a chassis. The chassis that host the backbone switch provide the interconnect to the OLTs through a back plane.

The signals transported between OLT and ONU can be multiplexed and encoded in different formats which depend on the PON standard. Beyond the connection Between OLT and ONU, standard signals are used for switching and cross-connect.

In the uplink direction, all the ONUs share the same fiber through a Time Division Multiple Access (TDMA) protocol. Generally, the OLT coordinates the ONUs in order to transmit their frames in strictly specified time intervals. The problem with this strategy is that the ONUs are placed in different distances from the power splitter and therefore experience different delays when sending upstream signals towards the OLT. In order to solve this problem, a TDM PON implements a procedure called ranging, in which the OLT determines the distances from the various ONUs by sending special control frames to them. It uses this information to calculate the exact time that each ONU should start transmitting, so that signals from the ONUs arrive at the ONT without overlapping.


3. Current Technologies

3.1 APON/BPON

ITU-T study group 15 had standardized the APON. APON (ATM-PON) and BPON (broadband PON) are based on the ITU-T G.983 standards . APON was proposed by the Full Service Access Network and established in 1998. After establishment many new standards were added in the G.983 series to further improve APON. Basically ATM PON and Broadband PON are the same specification which is commonly referred to as BPON. The term BPON is also used to describe the same architecture. To refer this class of PON design we will use the term APON, while BPON was introduced mainly for marketing reasons.

Full service access network (FSAN) was stared initial work on APON and after that ITU-T SG15 start working on the APON as the G.983 standards. APON uses Asynchronous Transfer Mode (ATM) protocol for transport purposes. At that time ATM was the most appropriate protocol for integrated networking services as the ITU believed at that time. Another reason was that at that time many telecommunication companies had a large deployed ATM switching infrastructure and it was easier for them to extend their ATM network using APON technologies in the access network <7>.

APON follows the general architecture of a TDM PON, which was described in the Introduction section. The maximum distance between the OLT and the ONUs is 20km. The initial downstream rates specified in the G.983.1 standard were 155.52 or 622.08 Mbps, but a newer version of the same standard, issued in 2005<>, specified a maximum of 1244.16 Mbps downstream rate. The upstream rate can be up to 622.08 Mbps. Upstream traffic is transmitted in a wavelength of 1.31 µm and downstream traffic in a wavelength of 1.49 µm. A separate wavelength of 1.55 µm can also be used for broadcasting video signal in the downstream direction. In all implementations of the standard, both the upstream and the downstream signals are carried on the same fiber, although the specification allows for the use of separate fibers and the same 1.31µm wavelength for upstream and downstream traffic.

APON uses ATM cells to transmit data between the OLT and the ONUs. An ATM cell is a fixed length packet that has 48 bytes of payload and 5 bytes of header, which includes fields for determining the virtual circuit in which the cell belongs, the type of the payload and a header error checksum. ATM is a connection-oriented network technology that supports virtual circuits between sources and destinations and differentiates between QoS requirements for each virtual circuit by defining service classes.

3.2 EPON

IEEE proposed Ethernet passive optical network (EPON) which provide a solution for the IEEE 802.3ah standard also known as Ethernet-in-the-First-Mile (EFM). By applying Ethernet encapsulation methods in the physical layer and data link layer, it is easier to have interoperability with Metropolitan and WAN networks that use the same technology.

EPON (Ethernet passive optical network) is a new addition to the Ethernet family. The work of EPON was started by the IEEE 802.3ah study group in March 2001 and finished in June 2004 . EPON provide low cost method of deploying access lines between a CO and a customer site.

3.2.1 Ethernet Layering Architecture and EPON

Ethernet operate on the data link layer and physical layer of the open system interconnect (OSI) reference model. Figure 3.2.1 shows a comparison of the layering model of the point-to-point (P2P) and the point-to- multipoint (P2MP) EPON architecture. The EPON architecture is based on the standard 802.3 Ethernet architecture. EPON characterized as a combination of a shared medium and a point-to-point network .

Figure3.2.1 EPON layering architecture, Point-to-point (P2P) Ethernet and point-to-multipoint (P2MP) <7>.

As show in the figure there is clear similarity between EPON layering and P2P Ethernet. OSI data link layer and physical layer further divided into sublayer by the ethernet standard and media-independent interface (MII) or gigabit media-independent interface (GMII) are used for connecting the physical layer to the data link layer <7>.

Packets are broadcasted to all terminals in the downlink (from OLT to ONUs) and then packets are processed by the destination terminal, approacing the P2MP model. While in the uplink, packets are only sent from the ONU to the adjacent OLT, approaching the P2P model.

3.2.2 Physical Layer

In EPON, Point-to-Multipoint (P2MP) optical fiber links are used for distances up to 20 km and for 1-Gbps maximum speeds. The standards for the physical interfaces of the transceivers are defined as 1000BASE-PX10 for links reaching up to 10 km and 1000BASE-PX20 for reaches up to 20 km (clause 60 of <26>). As in the case of the other PON standards, the wavelengths used for voice and data are 1490 nm for downstream transmission to the ONUs and 1310 nm for the upstream return link to the OLT. The standard specifies a minimum splitting ratio of 1:16, but other ratios (like a 1:32 ratio similar to GPON) are implemented by commercial applications, with the maximum number of logical ONU IDs supported being 32,768.

The line coding specified by the Physical Coding Sublayer (PCS) is the 8B /10B , which adds a 25% overhead to the data, meaning that the symbolrate of the line increases to 1250 Mbaud /s. FEC is an optional functionality of EPON. ONUs that don't support FEC are still compatible with FEC encoded frames, since the coding doesn't affect the information bits.

As ONUs located in varying distances from the OLT transmit data in bursts, the power level of the optical signal received from the OLT is bound to vary greatly in successive time slots. It is therefore necessary to use burst-mode receivers that can adjust to the different power levels. This is not necessary in the side of ONUs, which always receive signals from the same OLT.

3.2.3 Ethernet Framing

Almost all the fields of an Ethernet frame remain unchanged in the case of EPON frames. In order to add extra information for the identification of the ONUs , the Preamble and SFD fields have changed, now containing the following fields:

  • The Start of Packet Delimiter (SPD), which contains clocking information

  • Three bytes reserved for future use

  • The Logical Link Identifier (LLID), used the identification tag for ONUs

  • The Frame Check Sequence (FCS), responsible for error detection in the previous fields.

The LLID field's last 15 bits are used to identify each of the ONU's to a different number, thus leading to a possible 32,768 logical ONUs . When data is transmitted in the downstream direction, all ONUs get copies of the broadcasted data, but unless frames carry its own LLID, an ONU will reject the packet. It is also possible, as in the case of GPON with Port IDs, to assign multiple LLIDs to a single ONU (for example to provide different QoS flows).

The first bit is the Mode Indicator. When set to 0, the RS layer works in the P2P emulation mode, where each ONU (identified by the LLID of the frames sent) is mapped to a different logical MAC in the OLT. Then, the frames are forwarded to a Level 2 switch or a Level 3 router. When the Mode Indicator bit is sent to 1, all frames are mapped to the Single Copy Broadcast (SCB) MAC, and packet forwarding must be performed by a router. In this case, downstream packets carry a default LLID that is recognised and accepted by all ONUs

3.3 GPON

As the demand from different applications for bandwidth increased, ATM did not succeed to carry these applications and fail to becoming the universal network protocol, so ITU-T designed a new standard for TDM PONs with Gigabit capabilities, to better cope the changes and meet fast-growing demand in communication technologies. This new ITU-T standard was given the name GPON (Gigabit-capable PON). GPON is specified in the G.984 series of ITU-T recommendations <11,12> and it improved APON/BPON capabilities in order to support transmission of variable length packets in Gbps rates, that was its main characteristic. To support ATM switching functionality, GPON does not require the OLT or the ONUs (which was a relatively expensive characteristic of APON components), and also supports encapsulation of various types of service data.

General architecture and high level characteristics of GPON, the physical and link layer, MAC and control functionality are given below in the following subsections.

3.3.1. General architecture

The network architecture and a high level overview of GPON are specified in the G.984.1 standard which was issued by ITU-T in 2003 <11>. The network topology remains the same as in APON. However, the downstream and upstream rates have been increased as compared to APON and defined in G.984. GPON supports 1244.15 or 2488.32 Mbps for downstream and 155.52, 622.08, 1244.15 or 2488.32 Mbps for upstream traffic. GPON is capable of carrying various types of traffic, including Ethernet frames, plain analog telephone signal, T1/E1 traffic and ATM cells. The wavelengths used for upstream and downstream traffic remain the same as the ones for APON, namely 1.34 and 1.49 µm respectively and are carried on the same fiber. The maximum splitting ratio is 64 and maximum transmission distance can be 10 or 20 km.

GPON also supports protection schemes and defines a fully redundant 1+1 configuration and a partially redundant 1:N configuration. In 1+1 configuration, the signal from the ONU to the ONT and vice versa is fully duplicated on two fibers and there are two transceivers in both the OLT and the ONUs. In a 1:N configuration, there is an additional backup fiber every N working fibers and a switching mechanism that enables the OLT and the ONUs to switch over to the backup fiber in case of a failure. This configuration is less costly as compared to the 1+1 configuration and can cope with most practical cases of failures but demands a switching mechanism in the OLT and the ONUs.

3.3.2 Physical layer

The physical layer is defined in the G.984.2 standard <12> and is called the Physical Media Dependent (PMD) layer.

Power budget for the optical link is an important parameter in the physical layer. The power budget is the maximum attenuation that in its path from the ONU to the ONT (or vice versa) an optical signal can suffer and still be recognizable at the receiver. There are three classes of optics defined in the G.984.2: First one is a Class A with a power budget of 5 to 20db, Second one isClass B with a power budget of 10 to 25db and the last one is Class C with a power budget of 15 to 30db. The class of optics used to influences the maximum distance between the ONUs and the OLT consequently and also influences the attenuation that the signal experiences.

The ranging procedure which specified in G.984.2 is another feature. The ranging procedure is similar to the one specified for APON and aims at determining the distances of the various ONUs from the OLT. After performing the ranging procedure, the OLT has a rough idea of the distances to the ONUs and can instruct them to transmit at appropriate time instances so that their signals do not collide. Because of the fact that ranging is not so accurate, guard times are inserted between the upstream bursts generated from the ONUs in order for the separation of upstream bursts to be guaranteed. The value for the guard time is specified at 25.6 ns and is independent of the transmission rate.

An additional feature that is completely new in GPON is the support for power level control of the optical transmitters at the ONUs. Because of the different distances of the ONUs from the OLT, the upstream signals from ONUs that are close to the OLT arrive at a high power level at the OLT. This can result into overloading and damaging the optical receiver of the OLT, which has to be very sensitive in order to receive signals from far-away placed ONUs. The power level mechanism in GPON provides a means to the OLT to instruct the ONUs to transmit at different power levels depending on their distances from the OLT. Three distinct power levels are introduced, namely a full-power level, a 3db-reduced power level (half power) and a 6db-reduced power level (¼ of the maximum power). In order to cope with higher transmission rates, GPON uses higher power transmitters to comply with the power budget requirements

3.3.3. Link Layer and MAC

The G.984.3 recommendation from ITU-T <9> defines the Transmission Convergence (TC) layer which corresponds to the link and MAC layer. This standard defines the structure of the downstream and upstream frames and the medium access control protocol that coordinates the transmissions from the ONUs in the upstream direction. Additional features specified in the same standard include ranging, registration of ONUs, Forward Error Correction (FEC) techniques (optional capability), encryption of downstream data (optional capability), Dynamic Bandwidth Allocation (DBA) and Physical Layer Operation, Administration and Management (PLOAM) functionality.

3.4 GPON and EPON Comparison

In the following table we can see the comparison between EPON and GPON.

Table1: Comparison of EPON and GPON <7>

Data rates are an obvious advantage of GPON. GPON support higher data rates and it is also more flexible. GPON capability to supporting more than one standard data rate which make GPON systems to adjust to different needs. Since access markets is the target group for the two systems, it is expected that traffic is going to be asymmetric, meaning that the 1 Gbps upstream rate imposed by EPON might be a cost demanding feature, that might not be needed by the end user.

Another cost advantage of GPON as compared to EPON is the support of Class C optical distribution network (ODNs). Class C equipment allows distances beyond 20 kms and higher split ratios (up to 1:64), which translates into cost savings for the service provider.

In terms of technical features, GPON supports TDM natively. Signals are divided constantly into 125 µs frames. This allows for TDM cicuits with guaranteed speed and delay characteristics, which is highly important if voice or other delay sensitive applications are to be supported. In the case of EPON, Ethernet frames are of variable length and delay sensitive services are only supported through various complex QoS mechanisms, which are not yet well proven.

Finally, Ethernet is the only available encapsulation method for EPONs, which is efficient if we consider application throughput, particularly when compared with APON. But as more LLIDs are allocated, the Grant and Report MPCPDUs exchanged for each ONU, increase the overhead in the MPCP, and the system doesn't scale very well. GPON on the other hand inserts report and grant functions into the PCB overhead of the TC layer frame. The result is higher application throughput compared to EPON (93% overall efficiency versus 49% in EPON in similar data rate systems)<7><8>.


4. Bandwidth Allocation Techniques for TDM PONs

4.1 Static Bandwidth Allocation

In the static Bandwidth alocation Fixed Bandwidth Allocation (FBA) algorithm is used.For the transmission in the upstream direction fixed length time slots are assigned to the ONUs.One of the main advantages of FBA is that it provides a fixed bandwidth to each user in the upstream link.The implementation of FBA is simple, it does not require the negotiation between the OLT and the ONUs.The efficiency is the main problem in the FBA algorithm if an ONU does not have data to transmit but it will reserve time slot that is the wastage of bandwidth.For this reason it is not considered to be used the TDM PON.

4.2 Dynamic Bandwidth Allocation (DBA)

4.2.1 DBA overview

Incontrast to FBA, DBA is used to allocate bandwidth according to the upstream traffic demand and requirements. It is efficient than that of FBA.The figure below illustrates how DBA works.

Figure 4.3.1 Dynamic bandwidth allocation <7>.

It follows the following steps

  1. The ONU stores upstream traffic in the buffer received from the user.

  2. The OLT is notified about data volume stored in the buffer.

  3. The start time and awailable duration (1/4 transmission window) to ONU are specified by the OLT.

  4. The ONU waits for the granted time and then transmit the data volume specified by OLT.

4.2.2 Target Service

The system must satisfy the functions and performance required for providing the target service and must also be designed at a minimum cost. This is because if the system is for general users and the number of users exceeds 10,000,000, the difference of several dollars per user will show up as a difference of tens of millions of dollars of investments. DBA depends largely on service specifications and if the services change, the DBA has no choice but to change drastically. Therefore, the DBA must be designed with clear definitions of the target services.


5. Industry Actors

PONs started being massively deployed in the Asia Pacific region (mainly Japan and South Korea) since 2002, initially with BPON, and then exponentially growing with EPONs beginning in late 2004.

Despite the advantages of GPON over EPON, it is EPON that currently leads in market penetration. By the end of 2006, EPON was the 64% of the total PON ports installed, while the same number for BPON was 36% and 3% for GPON. It is estimated that nearly 20 million EPONs ports are currently installed, partly due to China's EPON deploymenys in 2008. Still, it is expected that these percentages are about to get lower for EPON, since new GPONs are deployed in North America (from Verizon since early 2007) and in Europe (France Telecom and Neuf in France) Followings are the most popular industry partners of this technology, for more information you can visit their official websites.

Alphion

Alphion is the pioneer in developing all optical PON network extension solutions to extend the reach of broadband service today. Alphion Corporation is a full member of the ITU FSAN group and a leader in developing key technologies for the next generation of passive optical networks.

http://www.alphion.com/default.asp?contentID=10

Cambridge Industries(CIG)

Cambridge Industries (CIG) is one of the world-leading outsource center specialized for FTTH (fiber-to-the-home). CIG now is the leading independent telecom OEM supplier focused solely on GPON (Gigabit-Capable Passive Optical Network) CPE equipment (i.e. GPON ONT, ONU, MDU, etc.), which is an essential integral part of GPON OLT (Optical Line Terminal) vendors' and service providers' end-to-end GPON solutions.

http://www.ci-g.com/products.htm

Comtrend

Comtrend is member of FSAN and provides high speed optical network devices. Comtrend is bringing people closer to closer by providing superior communication services.

http://www.comtrend.com/links/i$residential-gpon$product.htm

Corning

Corning is one of the world leading company providing,cable, hardware eqiupment for telephone, optical fiber and telecommunication network solutions.Its optical fiber products are recognised as an excellent and innovatory in the industry markete.

http://www.corning.com/opticalfiber/

Ericsson

Ericsson is also one of the partners of PON.

http://www.ericsson.com/us/solutions/government/stimulus/stimulus_solutions.shtml


6. Next Generation Technologies

With the EPON and GPON technologies already being deployed in large scales, the task forces behind these standards have started developing the next generation TDM EPONs.

6.1 10G-EPON

In the up coming time GPON and EPON are going to be merged by providing next- generation standard supporting 10Gb/s downstram bandwidth allocation and higher upstream bandwidth.This standard is provided by IEEE 802.3 av task force. It will also support 1 Gb/s in order to have compatibility with existing EPON deployments.Both existing system will use differrent wavelenth in the down stream direction, a link of 10 G will be allocated in the 1570-1600 nm wavelength window.In the upstream direction due to some technical limitations of OLT 1310 nm wavelength will be used.

10 G EPONs is going to be implemented in three phases

Phase 1- Asymmetric 10G/1G Optical Network Units
The availability of 10Gb/s link will only be to the downlink direction.The deployment of new transmitters in the OLT side is simple by using seprate wavelenths for different speeds.

Phase 2 - Symmetric 10G/10G Optical Network Units
Before the deployment of symmetric 10G system, for the 10Gb/s upstream the following factors should be addressed

  • Complicated TDM mechanisn

  • More powerfull Client side Transmitters

Phase 3 - Gradual Removal of 1G Equipment from the Network
The last phase includes the removal of symmetric 1 G 802.3ah ONU equipments. It might also require the removal of asymmetric 10G/1G equipmen. It will simplyfy the OLT.


7. Questions

  • (Q#1) What are the main goals of the bandwidth allocation algorithm? (Answer)

  • (Q#2) What are the differences between an EPON frame and a standard Ethernet frame? (Answer)

  • (Q#3) What is the difference between GPON and EPON ? (Answer)

  • (Q#4) What is the purpose of the ranging process and power level control functionality in a GPON? (Answer)

  • (Q#5) What are the mechanisms used in a TDM PON for sharing the medium in the downstream and upstream directions? (Answer)

8. Abbreviations

10G-EPON ----10 Gigabit-Ethernet Passive Optical Network

AAL ----ATM Adaptation Layer

APON ----ATM(asynchronous transfer mode) passive optical network

BPON ----Broadband passive optical network

CRC ----Cyclic Redundancy Check

DBA ----Dynamic bandwidth allocation

EFM ----Ethernet-in-the-first-mile

EPON ----Ethernet Passive optical network

FBA ----Fixed bandwidth allocations

FCS ----Frame check sequence

FEC ----Forward Error Correction

FSAN ---- Full Service Access Network

FTTX ---- Fiber To The X

GEM ---- GPON Encapsulation Mode

GPON ---- Gigabit Passive Optical Network

ONT ---- Optical Network Terminal

ONU ---- Optical Network Unit

PON ---- Pasive Optical Network


9. References

<1> "Ethernet passive optical network", Found at: http://www.optical-network.com/terminology.php?letter=all&id=2

<2> "Passive optical network", Found at: http://en.wikipedia.org/wiki/Passive_optical_network#History

<3> ITU-T. Recommendation G.984.1 (2003 and 2008 versions). "Gigabit-capable passive optical networks (GPON): General characteristics". 2003/2008. Found at:http://www.itu.int/rec/T-REC-G.984.1/en.

<4> C. Lin. "Broadband Optical Access Networks and Fiber-to-the-Home: Systems Technologies and Deployment Strategies". Wiley. 2006.

<5> ITU-T Recommendation G.983.1, Broadband Optical Access Systems Based on Passive Optical Networks (PON), 2005.

<6>ITU-T G.983.3, A broadband optical access system with increased service capability by wavelength allocation, 2001.

<7> CEDRIC F. LAM, "Passive Optical Network: principle and practice", Elsevier Press, 2007.

<8> FlexLight Networks and BroadLight, "Comparing Gigabit PON Technologies ITU-T G.984 GPON vs. IEEE 802.3ah EPON", Found at: http://www.broadlight.com/docs/pdfs/wp-gpon-vs-epon.pdf.

<9> ITU-T. Recommendation G.984.3 (2004 and 2008 versions), "Gigabit-capable Passive Optical Networks (G-PON): Transmission convergence layer specification", 2004/2008. Found at: http://www.itu.int/rec/T-REC-G.984.3/en.

<10> IEEE 802.3ah Task Force, Found at: http://www.ieee802.org/3/efm

<11> ITU-T. Recommendation G.984.1 (2003 and 2008 versions), "Gigabit-capable passive optical networks (GPON): General characteristics". 2003/2008. Found at: http://www.itu.int/rec/T-REC-G.984.1/en.

<12> ITU-T. Recommendation G.984.2. "Gigabit-capable Passive Optical Networks (GPON): Physical Media Dependent (PMD) layer specification". 2003. Found at: http://www.itu.int/rec/T-REC-G.984.1/en.

<13> G. Kramer and G. Pesavento.,"Ethernet Passive Optical Network (EPON): Building a Next-Generation Optical Access Network", Communications Magazine, IEEE Volume 40, Issue 2, Page(s):66 - 73, Found at: http://ieeexplore.ieee.org/Xplore/login.jsp?url=/ielx5/35/21205/00983910.pdf?arnumber=983910 .

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