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    EPON vs. GPON

    A Comparative Study

    November 22, 2004

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    EPON vs. GPON

    Table Of Contents

    1  INTRODUCTION ...............................................................................................................................................3 

    2  EPON AND GPON OVERVIEW ......................................................................................................................4 

    2.1  PON CONCEPTS..............................................................................................................................................5 

    2.1.1   Layering and Multiplexing ...... ........... .......... ........... .......... ........... .......... ........... .......... ........... .......... .....5 

    2.1.2   Media Access .......... .......... ........... .......... ........... .......... ........... .......... ........... .......... ........... .......... ........... .7  

    2.1.3  ONT Discovery & Activation.................................................................................................................8  

    2.1.4   Encryption .......... ........... .......... ........... .......... ........... .......... ........... .......... ........... .......... ........... .......... .....9 

    2.1.5  Protection Switching..............................................................................................................................9 

    2.1.6   PHY Related Features ...........................................................................................................................9 

    2.2  DEPLOYMENT ASPECTS ................................................................................................................................10 

    2.2.1  Quality of Service ................................................................................................................................10 2.2.2  Services................................................................................................................................................10 

    2.2.3   Bandwidth and Efficiency .......... .......... ........... .......... ........... .......... ........... .......... ........... ........... .......... .11 

    2.2.4   Migration from BPON .......... .......... ........... .......... ........... .......... ........... .......... ........... .......... ........... ......12 

    2.2.5   Network Management ........... .......... ........... .......... ........... .......... ........... .......... ........... .......... ........... ......12 

    2.2.6   OSS Integration Options......................................................................................................................13 

    2.2.7    Network Uplink Options .......... ........... .......... ........... .......... ........... .......... ........... .......... ........... .......... ...13 

    2.3  PON DEVELOPMENTS...................................................................................................................................13 

    CONCLUSIONS................................................................................................................................................15 

    4  APPENDIX ........................................................................................................................................................17 

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    1 Introduction Background

    PON standardization activities have been going on for about ten years. With the continuing availability of moreadvanced technology, PON line rates have increased from 155Mbps up to 2.4Gbps. The timeline is shown in Fig. 1.

    1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

    FSAN starts first formal PONactivity . 155Mbps APON ITU G.983

    series

    Enhanced to BPON 622/155Mbpswith 3

    rd lambda, protection and DBA

    EFM starts work on 802.3ah inclEPON

    FSAN starts GPON work G.984series = extension of G.983 series,

    up to 2.4Gbps

    EPON Ratified by IEEE

    ITU approves GPON

     

    Figure 1: PON standardization timetable

    With the explosion of the Internet, it didn’t take too long before ATM-based BPON systems proved to be veryinefficient, as the vast majority of traffic through the access network consists of large, variable-sized IP frames.This created the opportunity for the development of the pure-Ethernet based EPON, taking advantage of emergingQOS-aware GigE switches and cost-effective integration with other Ethernet equipment. Ethernet has proven overtime to be the ideal transport for IP traffic.

     EPON and GPON

    As a result, the IEEE 802.3 tasked the 802.3ah “Ethernet in the First Mile” work group with the development ofstandards for point-to-point and point-to-multipoint access networks, the latter specifying Ethernet PONs. EPON iscurrently part of standard Ethernet.

    Development of the Gigabit-capable Passive Optical Network (GPON) standard (G.984 series) really started afterproposals by FSAN members (Quantum Bridge et. al) for a protocol-independent ATM/Ethernet Gbps PONsolution were not very popular within the IEEE 802.3ah work group. FSAN then decided to continue this as adifferent competing standard in the ITU.

    EPON and GPON both draw heavily from G.983, the BPON standard, when it comes to general concepts that workwell (PON operation, Optical Distribution Network (ODN), wavelength plan, and application). They both offer theirown version of enhancements in order to better accommodate variable sized IP/Ethernet frames at Gbps rates.

     Deployments

    Today, BPON has gained a decent level of maturity representing about a quarter of the over 1.5 million FTTH(data-only) lines deployed in Japan so far. Maturity and stability may have motivated SBC, Verizon, and Bellsouthto commit to BPON for their multi-$Billion FTTP deployments, in spite of its obvious shortcomings.

    In the mean time, however, as a clear testimony to the future of PON, NTT is already upgrading and furtherexpanding their FTTH network with EPON, not GPON. This is the common trend elsewhere in Asia. EPON isclearly taking off!

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    This begs the question: Do we really need GPON  next to EPON? In order to answer this question we will take acloser look at these two flavors, and compare their different approaches on technical and practical merits. We willshow that EPON represents a far more elegant solution that is well in line with the evolution of the rest of the

    network towards an all-IP/Ethernet strategy. 

    2 EPON and GPON OverviewAs pointed out earlier, EPON and GPON both draw heavily from the BPON specification. This is evident in thetable in Appendix A. In this section we take a closer look at the areas where they differ.

    The following table shows the key features of EPON and GPON.

    EPON and GPON Features

    EPON GPON Comments

    Service Full service, triple-play + RF video Same

    Layering andMultiplexing

    Native Ethernet (includes TDM)ATM and Generic Frame(including Ethernet andTDM )

    (see sec 2.1)

    Media Access TDMA via granting -- derived from BPON Equivalent

    ONT Discovery andActivation

    Auto-discovery of new ONTs Equivalent

    PHY related features:

    •  Number ofbranches

    Spec: > 16; 64 feasible

    # logical splits not specified

    Max 64 at the PHY

    Max 128 at the TC layer(logical splits)

    •  Wavelengtharrangement

    Downstream:1480-1500 nm

    Upstream: 1260-1360 nm

    Capable of multiplexingdownstream for videodistribution (1550-1560 nm)

    Same (BPON)

    •  ODN class

    classificationClass A(5-20 dB), 10 km; Class B(10-25 dB), 20 km

    Class A; Class B; Class

    C (15-30 dB), 20 kmGPON adds Class C

    •  FEC (optional) Reed Solomon SameFEC (forward error correction) reduces anoptical module cost, and aims to ease Tx powerand Rx optical sensitivity

    •  Encryption(optional)

    AES-128 on Ethernet payload plannedAES-128 on ATM andGEM frame payload

    Equivalent strategy, slightly different scope

    •  Bit rateDownstream: 1 Gbps

    Upstream: 1 Gbps

    Down: 1.2, 2.4 Gbps

    Up: 155 Mbps, 622Mbps, 1.2 Gbps, 2.4Gbps

    •  Other (optional)OLT informs ONT of receiver stabilization time atdiscovery

    ONT optical outputpower leveling

    In GPON, the ONT optical output can beadjusted in 2 steps to relieve automatic powerdistribution (APD) tolerance of OLT.

    The gray areas indicate similarities.

    The strategy of GPON is to continue to support “legacy ATM” as in BPON, but additionally support Gbps rates,better encryption, as well as a new frame-orientated mode that can better accommodate native TDM and variablesized IP/Ethernet frames. The justification for the continued support of ATM is often explained as serving theneed for backhauling of first-generation DSL traffic. This remains to be seen, considering that today’s IP DSLAMsare all Ethernet based.

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    2.1 PON ConceptsThis section presents several key concepts and how they have been addressed in EPON and GPON. Since these twoPON flavors really have a lot in common, we focus on conceptual differences in the following areas.

    1.  Layering and Multiplexing

    2.  Media Access

    3.  ONT Discovery and Activation

    4.  Encryption

    5.  Protection Switching

    2.1.1 Layering and Multiplexing

    In EPON, Ethernet frames are carried in their native format on the PON. This greatly simplifies the layering modeland the associated management. Services are all mapped over Ethernet (directly or via IP).

    In order to accomplish the same in GPON, two layers of encapsulation are required. First, TDM and Ethernetframes are wrapped into GTC Encapsulation Method (GEM) frames, which have a GFP-like format (derived fromGeneric Frame Procedure ITU G.7401). Secondly, ATM and GEM frames are both encapsulated into GTC frames

    that are finally transported over the PON. See the figure below.

    Ethernet

    IP

    TCP+UDP etc

    GEM frame

    GTC TC frame

    ATM cell

    PON-PHY

    AAL 1/2/5

    Variousservices

    T1/E1TDM

    POTS Data VideoLayer 5+

    Layer 4

    Layer 3

    Layer 2

    Layer 1

    GTC sub-layer

    GPON Layering

    IP

    TCP+UDP etc

    Ethernet frame

    PON-PHY

    T1/E1TDM

    POTS Data VideoLayer 5+

    Layer 4

    Layer 3

    Layer 2

    Layer 1

    MAC layer

    EPON Layering

    Figure 2: EPON vs. GPON Layering

    The main purpose of the GEM frame is to provide a frame-oriented service, as an alternative to ATM, in order toefficiently accommodate Ethernet and TDM frames. Both ATM and GEM modes are mandatory at the OLT, but anONT can be configured to support either one, or both.

    As an evolution step from the ATM-based BPON, this may sound like a big improvement. However, whencompared to the simple EPON model, it becomes clear that the GEM/GTC encapsulation and inclusion of ATM areadding unnecessary complexity to solve the same problem. The different transport schemes are illustrated in Figure

    3.

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    ATMXC

    ATMXC

    Service adaptation

    Service adaptation

    IP/ATM

    TDM

    IP/Ethernet

    TDM

    ODNOLT ONU

    ingress uplink    transport 

    ATM & GEM

    XC

    ATM & GEM

    XC

    Service adaptation

    Service adaptation

    ?

    TDM

    ?

    TDMATM

    Ethernet TDM

    GEM

    Ethernet IP/Ethernet

    Ethernet

    XC

    Ethernet

    XC

    Service adaptation

    Service adaptation

    TDM TDM

    TDM/ Ethernet

    Ethernet

    IP/Ethernet IP/Ethernet

    ATM

    BPON

    GPON

    EPON

    GFP/SONET?

    GTC frame

     

    Figure 3: Frame Transport in BPON, GPON, and EPON

    EPON clearly offers a much simpler and more straightforward solution than GPON. The support of ATM

    and the double encapsulation of GPON serve no real benefit over a pure Ethernet transport scheme. 

    2.1.1.1 TDM Support

    In both EPON and GPON systems, TDM is supported by assigning sufficient network resources to avoidunnecessary blocking and latency, by periodic scheduling of upstream bandwidth, and by distributing the CO clockdownstream throughout the PON in order to avoid jitter and drifting.

    The Service Adaptation block in Figure 3 is responsible for extracting and transferring native TDM from/to theassociated frames. In EPON these are standard Ethernet frames, while in GPON this is done via GEM frames.

    In EPON systems the clock is commonly embedded in the downstream signal, which allows an 8kHz clock to berecovered at the ONT for synchronization. In a similar way, this is achieved in GPON by transmitting a marker

    with downstream GTC frames at 125 µsec boundaries.

    2.1.1.2 Control messages

    EPON is managed like any other Ethernet switch via SNMP through IETF MIBs. Additional control messages are

    Multi-Point Control Protocol (MPCP) GATEs/REPORTs for BW granting, as well as EFM OAM messages.MPCP and EFM OAM frames are multiplexed with regular Ethernet traffic

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    In GPON there are three different types of control messages: OMCI, OAM, and PLOAM. Their roles are shown inthe table below. In either case, REPORTs are transported upstream as payload traffic.

    Control Messages in EPON and GPON

    Control function EPON GPON

    Provisioning of ONT service defining layers above L2 IETF MIB / SNMP OMCI (Ethernet or ATM)

    BW granting, Encryption key switching, and DBA MPCP (higher layer for encryption key) Embedded OAM (Header overhead)

    Auto discovery, and all other PMD and GTC mgt info. MPCP and EFM OAM PLOAM (ATM)

    2.1.1.3 Multiplexing architecture

    The multiplexing architecture in EPON is based on the point-to-point emulation concept, where the OLT containsmultiple MACs, each having a 1:1 relationship with an ONT off the PON. This is represented by the Logical LinkID (LLID), which is used for addressing. The figure below illustrates the roles of LLIDs in point-to-pointemulation. Notice the significance of the bridge at the OLT in this model, which is not present in GPON.

    Standard 802.1 Bridge

    MAC

    MACMACMACMACMACMACMAC

    MAC MAC MAC MAC MAC MAC

    LLID

    LLID

    ONT

    OLT

     

    Figure 4: Point-to-Point Emulation in EPON

    An ONT is identified by the LLID. In addition, the VLAN_ID can be used for further addressing. A given VLANis identified as LLIDVLAN_ID. In the downstream direction, the OLT attaches the LLID to the preamble offrames, which is used to identify the right port on the bridge.

    Similar to the LLID, GPON uses a parameter called T-CONT to address ONT’s. In the ATM mode, a given VC isaddressed via ONT_IDT-CONTVPI/VCI. In the GEM mode, a ‘port’ can be identified via ONT_IDT-CONTPort_ID. 

    Both the LLID and T-CONT provide a form of point-to-point emulation, except that GPON has no relationship to802.1 bridge, and hence bridging has to be achieved upstream of the OLT.

    2.1.2 Media Access

    2.1.2.1 Granting / Resource Allocation

    In EPON, grants are sent per-LLID, as separate MAC-Control client frames (GATEs), in-between regular Ethernetframes. Each grant specifies an upstream transmission opportunity for a given ONU via {LLID+Start+Length}

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    Similarly, GPON grants per T-CONT. Grants are carried in the downstream frame header overhead, via a map thatholds multiple grants specifying {Alloc-ID+Start+End} for each granted upstream Window (timeslot). The twodifferent schemes are shown in the figure below.

    Downstream

    Upstream

    LLID Start Length LLID Start Length LLID Start Length

    1 100 200 2 400 100 3 520 80

    LLID 1(ONU1)

    LLID 2(ONU2)

    LLID 3(ONU3)

     

    Alloc-ID Start End Alloc-ID Start End Alloc-ID Start End

    1 100 300 2 400 500 3 520 600

    US BW Map

    Frame header (PBCd)Payload

    T-CONT 1(ONU1)

    T-CONT 2(ONU2)

    T-CONT 3(ONU3)

    Downstream

     

    EPON GPON

    Figure 5: EPON And GPON Media Access Control

    2.1.2.2 Dynamic Bandwidth AllocationDynamic Bandwidth Allocation (DBA) refers to an optional flexible upstream timeslot assignment mechanism usedin a PON. It allows a system to assign upstream timeslots in real-time, based on the instantaneous demand of agiven ONT, and hence use the upstream bandwidth more efficiently. In a typical FTTH deployment today wherethere is ample upstream bandwidth, DBA is not very effective since traffic patterns are still asymmetric andupstream bandwidth demands tend to be relatively low. However, in situations where the upstream demand isrelatively high (e.g., FTTB, or emerging gaming services), DBA could be useful.

    DBA is optional in EPON, and out of scope. The right ‘hooks’ are supported, however, allowing ONT’s to sendREPORT messages including multiple 802.1p queue states, but it’s up to the scheduler at the OLT whether / how tointerpret this information.

    GPON uses a very similar scheme, but there the DBA is part of the standard. Elements of the two schemes areoutlined in the table below.

    DBA Elements in GPON and EPON

    GPON DBA EPON DBA

    Granting unit GTC Overhead MPCP GATE frame

    Control unit T-CONT LLID

    Identification of control unit Alloc_ID LLID

    Reporting unit ATM: ATM cell / GEM: fixed length block MPCP REPORT frame

    Reporting mechanism Embedded OAM Separate REPORT frame

    Negotiation procedure GPON OMCI N/a

    2.1.3 ONT Discovery & ActivationEPON supports a discovery mechanism that allows the OLT to automatically detect a newly added ONT, learn itsMAC Address, assign an LLID, and activate the ONT. At this point it is up to the provisioning application toconfigure it with appropriate user bandwidth and other subscriber parameters.

    Standard EPON does not require the ONT Serial Number for authentication; however, higher-level authenticationschemes can do so.

    GPON uses the Serial Number for ONT authentication. This can be either pre-provisioned, or discovered by theOLT. Once the Serial Number is detected, an ONT-ID is assigned, and the ONT is activated.

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    2.1.4 Encryption

    Both EPON and GPON have endorsed 128-bit Advanced Encryption Standard (AES) encryption. 128-bit keysmeans that there are 3.4 x 1038 possible keys, i.e., very strong encryption.

    The GPON standard already includes this scheme and encrypts the GEM payload, which means that Ethernet framesand TDM data are completely encrypted. Key management messages are exchanged via PLOAM cells.

    EPON is expected to include this in the standard in 2005, encrypting the Ethernet payload. This includes completeIP payloads as well as TDM data. A group key protocol is additionally required for multicasting (e.g., IPTV).Details, including key management are still under discussion.

    Notice that some may consider the GPON approach stronger than that of EPON, citing concerns about exposingMAC addresses over the PON link. However the true severity of this threat has always been a controversial topic.

    2.1.5 Protection Switching

    GPON survivability features are derived from G.983, so there is the benefit of a standardized scheme. Upondetecting downstream signal loss, an ONT and sends a Loss of Window (LOW) alarm to the OLT. As a result,either the OLT switches all ONT’s to the protection fiber, or the OLT switches itself.

    Protection switching is out of scope for EPON. Vendors offer various proprietary solutions, and interoperability

    can potentially be an obstacle in deployments where multi-vendor solutions are required by the service providers.

    2.1.6 PHY Related Features

    2.1.6.1 Physical Medium Dependent layer (PMD)

    EPON and GPON both support the same wavelength plan as BPON, i.e., Upstream {1260-1360 nm}, Downstream{1480-1500 nm} and Video distribution {1550-1560 nm}.

    Three different PMD classes, defined in G.982, are specified for transceivers in GPON. Key parameters are shownin the table below, compared with EPON .

    ODN Classes

    GPON EPON Note

    Rates 155, 622Mbps; 1.25Gbps 1.25Gbps EPON 1000PX-10

    Reach 10km 10km

    Splits 16 16

    OLT Rx sensitivity ? ?

    Class A

    ONT Tx on/off time 16ns 512ns

    Rates 155, 622Mbps; 1.25Gbps 1.25Gbps EPON 1000PX-20

    Reach 20km 20km

    Splits 32 32

    OLT Rx sensitivity ? ?

    Class B

    ONT Tx on/off time 16ns 512ns

    Rates 155, 622Mbps; 1.25Gbps 2.4Gbps N/S

    Reach > 20km N/S

    Splits 64 N/S

    OLT Rx sensitivity ? N/S

    Class C

    ONT Tx on/off time 16ns N/S

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    2.1.6.2 Line Coding

    As part of standard Ethernet, EPON uses 8B10B line coding for DC balancing to allow reasonable clock recovery.This implies an overhead of 20% of the 1.25Gbps line rate, i.e., a maximum capacity of 1Gbps

    GPON uses NRZ line coding, and frames are scrambled using a frame-synchronous scrambling polynomial. As aresult, there is no line-coding overhead, and the maximum available capacity is 1.25Gbps.

    2.1.6.3 Power Leveling

    GPON supports an optional power leveling mechanism, where the OLT can instruct the ONT to change its powerlevel to one of three states (Normal, Normal-3dB, or Normal-6dB), based on the measured levels.

    In EPON systems only one state is supported. Notice that during EPON auto discovery, the OLT informs the ONTsof its synchronization time, allowing ONTs to adjust their upstream transmission timing accordingly.

    2.2 Deployment AspectsIn order to further understand the differences between GPON and EPON, we next take a look at how they fare whenit comes to actual deployment. The following aspects are briefly addressed:

    •  Quality of Service (QoS)

    •  Services

    •  Bandwidth and efficiency

    •  Migration from BPON

    •  Network management

    •  OSS integration options

    •  Network uplink options

    2.2.1 Quality of Service

    Traditionally, BPON solutions were often referred to as having better QoS features than EPON, based on the factthat historically ATM-based solutions enjoyed a set of QoS features that were not supported in Ethernet-based ones.

    Not anymore, though, as switches available today use common QoS engines and offer similar features. A goodexample is the fact that GPON, which was designed as a superset of BPON, uses GEM cross-connects for TDM

    services, not ATM. In a very similar way, EPON systems utilize state-of-the-art QoS-aware Ethernet switches.

    At the end, effective QoS is really a product of systems architecture fundamentals, not the PON flavor.

    2.2.2 Services

    It is sometimes claimed that EPON is only appropriate for data-only services and GPON for triple-play. The realityis however, that today EPON-based systems are being deployed worldwide, successfully delivering carrier-gradetriple-play services.

    The EPON protocol was deliberately designed to allow the simultaneous support of loss- and delay-sensitive traffic.Combining this with versatile QoS-aware switches and proper system architecture techniques (including VLANs,queue design, priority-based scheduling, etc) results in powerful solutions, capable of transporting any type ofservice (IP Data, TDM, POTS, VOIP, IPTV, RF Video). In fact, when it comes to certain IP/Ethernet services, itturns out that GPON is the one that fall short, as is shown below.

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    2.2.2.1 IPTV support

    IPTV consists of Video on Demand (VOD) and Switched Digital Video (SDV). With VOD, each IP stream isviewed by one viewer, while with SDV multiple users can view a single IP stream.

    VOD relies on IP Unicast and SDV on IP Multicast. Both require high bandwidth to minimize frame loss, and fastsystem response to minimize channel change times.

    IPTV SDV solutions that utilize IGMP work very well over the broadcast topology of a PON. A key benefit is itsinherent single-copy broadcast capability, resulting in bandwidth consumption as a function of channels, rather thanthe number of viewers. However, the overall performance scalability of the service relies heavily on themulticasting and broadcast capabilities of the underlying transport network.

    This is where Ethernet-based networks have a strong advantage, as these features are inherent to Ethernet

    switches. In an EPON system, additional efficiency can be achieved by implementing filtering and proxy

    functions at strategic points within the PON subnetwork.

    Even though GPON systems are designed to transport  Ethernet frames efficiently, they cannot benefit from themulticast advantages of Ethernet, since they do not switch Ethernet traffic (but rather GEM and/or ATM frames). 

    2.2.2.2 Bridging

    Since in the GPON architecture the cross-connect at the OLT is not an Ethernet switch, GPON cannot supportstandard Bridging, which can be of interest in FTTB deployments. Some form of ‘GEM-bridging’ could probablybe implemented that allows port-port bridging (TDM or Ethernet), but this would not be very inefficient.

    In order to support standard bridging, there would be the need for an Ethernet switch upstream of the OLT cross-connect, either in an aggregation point in the same chassis, or externally.

    Bridging is a standard feature of EPON systems, supported via point-to-point emulation (see figure 4).

    2.2.2.3 Transparent LAN Services (TLS)

    TLS is another popular business application that is not directly supported by a standard GPON system, as it isachieved via VLAN tunneling (Q-in-Q) in Ethernet switches. Without these in the OLT chassis, one would need touse external Ethernet switches in order to achieve the same result.

    TLS is commonly supported in EPON systems implemented in the Ethernet switches at the OLT and ONTs.

    2.2.3 Bandwidth and Efficiency

    2.2.3.1 Bandwidth

    Probably the most heralded benefit of GPON is the fact that it is specified to scale up to 2.448 Gbps in both theupstream and downstream directions. Rates are shown in the table below.

    GPON Rates

    Downstream (Mbps) Upstream (Mbps)

    1,244 155.52; 622.08; 1,24416

    2,488 155.52; 622.08; 1,24416; 2.48832

    One apparent advantage of the multi-tiered bandwidth scheme is that it can be configured for 1.2 or 2.4 Gbpsdownstream and 622 Mbps upstream, and take advantage of lower cost lasers at the ONT.

    Today, however, rates of 1.2Gbps/622Mbs for downstream vs. upstream are a more realistic target (similar to‘extended BPON’), sharing similar technology with EPON.

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    Notice also that 2.4 Gbps is not a common rate, and lacks volumes to draw from in order to drive down ONTtransceiver costs. Upstream rates higher than 622Mbps are also not economical due to mode partition noise, untilnarrow spectral width FP lasers become economical.

    2.2.3.2 Efficiency

    Due to its use of NRZ scrambling as opposed to 8B10B encoding, GPON does not pay the 20% overhead penalty asin Ethernet. This makes it appear even more attractive, with efficiency potentially in the upper 90% (of 1.244Gbps). This is often contrasted to EPON, which is frequently incorrectly claimed to be “only 50% efficient”.

    Efficiency has to be considered in both directions of a PON. Each PON protocol introduces its own overhead ineither direction. The downstream efficiency is significantly more important because of the asymmetric nature ofPON bandwidth usage. Notice e.g., that for data services in a typical FTTH deployment at least 40% of theupstream BW consists of a low load of small packets (internet TCP ACKs). In addition, one has to take inconsideration the actual upstream demand.

    PON efficiency is a function of protocol encapsulation and scheduling efficiencies. In the downstream direction,the impact of either one is relatively low.

    EPON efficiency can be shown [3] to reach theoretically up to about 72% (downstream) and 68% (upstream) of

    1.25Gps (i.e., about 900Mbps/850Mbps) while GPON in GEM mode can achieve about 95% of 1.25Gbps in either

    direction [4].

    In practice, upstream efficiency values are often much lower due to vendor’s design choices and componentselection. Often, however, a few 100’s Mbps upstream is sufficient for standard FTTH applications, especiallywhen DBA is used. Of course, what actually matters is the remaining usable bandwidth, and whether or not it issufficient for the intended PON application (e.g., FTTH, 32 splits, triple play, HDTV or regular IPTV, etc.).

    2.2.4 Migration from BPON

    Since BPON, GPON and EPON are based on different technologies, their OLTs and ONTs are not interoperable.Migration from BPON therefore requires replacing the OLT and all associated ONTs, i.e., significant truck rolls.The good news is that the common ODN in-between remains untouched, which is one of the main attractivefeatures of FTTP.

    The least painful migration scenario is probably a same-vendor one, i.e., replacing BPON OLT blades and theirassociated ONT’s with GPON or EPON ones off the same chassis. A clever BPONGPON strategy is what manyBPON vendors are selling today; it validates the investment in outdated BPON gear, creates a future for GPON, andat the same time locks in the customer.

    For service providers with large investments in their ATM-based core networks, GPON may initially have someappeal because of its support of ATM. However, notice that existing ATM core networks are quickly running outof capacity and are simply too expensive, and too complicated to manage, and are being replaced by GigE systems.On the access side, most operators are installing IP DSLAMs today that support GigE uplinks, eliminating the needfor ATM altogether.

    Migration to EPON, not GPON, as a true IP/Ethernet solution, is therefore a far more realistic and future-

    proof scenario, consistent with the evolution of the rest of the network.

    2.2.5 Network Management

    Conventional Ethernet, traditionally used exclusively in enterprise environments, has historically been relativelyweak in areas of network management that are important to large-scale subscriber access networks. Key areas ofconcern have been: 

    • Limited performance monitoring and servicelevel agreement (SLA) assurance metrics• Limited or non-existent diagnostics, fault management and isolation capabilities• Lack of a clear service demarcation point

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    • Non-standard provisioning and control architectures• Incompatible network management interfaces

    These areas have been addressed with the introduction of Ethernet in the First Mile (802.3ah), and continue to bethe focus in different standards organizations. EPON system vendors have been successful in filling any remaininggaps with innovative solutions.

    These areas should also be reasonably well covered in GPON, as it inherits some of the ATM functionality and wasdesigned with the input of a few service providers.

    For an EPON system, management of PON operation and OLT/ONT interoperability are defined through EFMOAM and IETF MIBs. Equivalently, GPON has outlined its management capabilities through a series of OMCIspecs.

    Because of its inherent Ethernet simplicity and collapsed layering model, management should be much

    simpler for EPON than GPON.  Regardless of the flavor of PON, the rest of the system is commonly managedthrough traditional telecom system models using MIBs that include management functions for equipment, servicecards, services, alarms, PM data, etc. (ITU, Telcordia, ANSI, ETSI, etc)

    2.2.6 OSS Integration Options

    OSS options vary widely. However, a common trend is that many operators are updating their OSS software infavor of next generation 3rd party software. Common choices are Micromuse Netcool for assurance and SyndesisNetprovision for inventory and provisioning. These systems are very flexible to customize, and typically wellpositioned for supporting IP/Ethernet based systems and services, as they have been for the many routers andswitches in today’s carrier networks.

    EPON management is added as an extension to the already widely supported Ethernet MIB. Integrating an

    EPON system is therefore a very reasonable effort, significantly easier than GPON.

    2.2.7 Network Uplink Options

    Network uplink options are similar for EPON and GPON. Once switched at the OLT, services are adapted tothe familiar service interfaces (DS3, T1E1, ATM, SONET/SDH, GigE, 100BT, etc).

    An area where GPON’s GEM format is sometimes said to offer some benefit is that, because of its similarity toGFP, GPON vendors could design the OLT to pass native GFP frames to the uplink via SONET/SDH payloads (seefigure 3). This could help accomplish transparent trunking of Ethernet or TDM payloads. Of course, TLS is a moreelegant way of doing this, as supported in EPON systems.

    2.3 PON DevelopmentsToday, there are about twice as many BPON vendors than there are EPON ones (see table below). About half of theBPON vendors have indicated they are developing GPON products (although they don’t seem to be in a rush); onlytwo companies have released GPON products today. The table below shows the state of PON developments ofover 30 vendors today.

    Notice how this growing pool of PON vendors can roughly be divided into broad camps:-  Early generation, sub-rate APON and EPON systems by Japanese vendors (e.g., Fujitsu and Fujikura)

    -  BPON an GPON systems by FSAN members (Alcatel, Quantum Bridge (now Motorola), Hitachi)

    -  EPON systems by Asian or Asian-focused vendors – (Hitachi, Sumitomo, UTStarcom)

    -  Traditional DSLAM, DLC etc. vendors integration BPON/GPON blades – Lucent, Calix, AFC etc.

    -  Pioneering private companies with mostly US-focus (Alloptic, OSI, Wave7, Flexlight)

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    Product-wise, the common trend is still: Triple play in the North America; data-only in Asia.

    PON Vendors

    Loc. BPON GPON EPON Comments

    AFC USA S L - (incl. Marconi product) Embarking on large BPON deployment in 2005 (Verizon)

    Alcatel France S L - Embarking on large BPON deployment in 2005 (SBC)

    Alloptic USA - - S C-COR reselling

    Calix USA L ? - Blade in C7

    Carrier Access USA S - - ONT vendor

    Ciena USA S L - (Old Catena Networks BPON product) Working on a GPON blade

    ECI Telecom Israel S L - Nortel reselling

    Entrisphere USA L - - ONT vendor; Partnering with Fujitsu

    Fiberhome Networks China - - L Teknovus chipset

    Flexlight Israel - S - Key author of GPON spec

    Fujikura / Alcoa Japan S* - - Half-Gig EPON

    Fujitsu Japan S - S 155Mbps APON product did fairly well for NTT

    GW Technologies China - - S

    Hitachi Japan S L S 200k data-only ONTs deployed by NTT . Partners with Wave7 for triple-play ONT.

    iamba Israel S - -

    LG Korea S - -

    Lucent USA - L (S) GPON blade for Stinger planned; Reselling Salira EPON

    Mitsubishi Japan S ? S*/L Working with Passave – awarded NTT contract

    Motorola (QB) USA S L? - Co-author of GPON spec (with Flexlight)

    Nayna USA S - S

    NEC Japan S* S*

    Oki Japan - - S Oki+Fujitsu venture OFN working with Centillium – awarded NTT contract

    Optical Solutions USA ? S - CWDM PON – shares GPON market with Flexlight today

    Salira USA - - S Teknovus chipset; Lucent reselling

    Samsung Korea - - S? Doing something with ETRI

    Siemens Germany L ? - Broadlight chipset

    Sumitomo Japan - - S*/L Working with Passave

    Terawave USA S L? L? Active participant in 802.3ah WG

    TTS Communications USA ? - - Hybrid PON?

    UTStarcom USA - - L Working With Passave / huge presence in China

    Vinci Systems USA S L L ONT only “AnyPON”

    Wave7 USA - - S*/L Apparently upgrading to EFM compliant system.

    Zhone USA S ? - (old NEC eLumnant BPON)

    *) lower-rate pre-standard version

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    Notice that AFC, ECI, Entrisphere, Calix, Siemens, Zhone and Ciena are currently all developing BPON systemsusing Broadlight’s chipset, which includes 1.25Gbps/622Mbps ‘extended BPON’ rates as well as pin-compatibleGPON options. This provides them with a built-in migration path to GPON.

    3 Conclusions

    Key advantages vs. disadvantages of GPON and EPON are compared in the table below.

    GPON vs. EPON

    Advantages Disadvantages

    GPON -Can be operated at different rates w/different transceivers

    -Higher theoretical BW, up to 2x that of EPON

    -Can be configured in asymmetric fashion and take advantage of lowerONT laser costs, e.g., 1.2G/622M or 2.4G/622M

    -Encrypts the full payload, ie., full Ethernet frames -- no exposed MACaddresses

    Better chances for interoperability

    -Standard supports TDM

    -Standard ONT service-level management

    -Complex layering model Ethernet/GEM/GTCencapsulation means complex management

    -More expensive at comparable rates as EPON

    -Transceivers at 2.4Gbps rates are expensive today,no volumes

    -upstream BW limited to 622Mbps today

    EPON -Native transport of Ethernet frames

    -Simple and familiar, lower cost management

    -Benefits from true Ethernet switching

    -Fully compatible with IP

    -Supports TLS

    -Broadcast, Multicast --IGMP

    -IGMP support means better support for IPTV with high scalability

    -lower costs optics due to relaxed timing parameters

    Mostly interoperability obstacles:

    -Non-standard service-level interoperability

    - non-standard TDM

    -non-standard encryption

    -non-standard protection switching

    In conclusion, GPON can be seen as a ‘me-too’ specification that duplicates EPON functionality, but than in arather complex way. The actual practical need for the GPON standard in addition to EPON remains questionable.

    Notice the following observations:

    •  Ethernet can be transported in it’s native format and support all services very well, as demonstrated withcarrier-grade TDM suport in EPON

    •  ATM traffic is insignificant or not-existent in today’s access networks, adding unnecessary complication toGPON

    •  GPON and EPON are equally capable of providing the QoS capabilities required for triple play servicedifferentiation

    •  I.e., EPON is not limited to data-only services, but can support triple-play services as well as GPON

    •  Even though GPON is capable of transporting Ethernet traffic, it lacks several key capabilities inherent to pureEthernet switches. EPON is more appropriate for IP/Ethernet services:

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    •  Large-scale IPTV deployment, which is often seen as a key driver for Gbps PON deployments

    •  Business applications that include TLS, Bridging

    •  EPON solutions are more popular with service providers where IPTV and VOIP strategies areimportant. Today this is mostly the case in Asia.

    •  Management and OSS integration of EPON is much easier than GPON, due to the following

    •  EPON is part of standard Ethernet, which is very simple to manage

    •  The collapsed layering model of EPON results in less complex management than GPON

    •  OSS integration is much easier with EPON due to the wide support of IP/Ethernet in most NextGenOSS systems.

    •  EPON is receiving considerable endorsements in Asia today, while in the US leading RBOCs are embarkingon large-scale, ATM-limited BPON deployments

    •  The lack of any significant GPON RFP clearly illustrates its relevance today

    •  Most key BPON vendors are working on GPON follow-on products, often sold as a ‘future-proof’ strategy

    •  Most key Asian vendors are focused on EPON

    •  GPON’s flagship Class-C ODN and 2.4Gbps transceivers are in reality very expensive and do not have theadvantage of volumes that can help drive down costs. Partition Noise currently limits the GPON upstreamspeed to 622 Mbps, until narrow spectral width FP lasers become economical

    •  I.e., in reality, GPON and EPON solutions offer about the same bandwidth today with GPONslightly better in the downstream direction, and EPON slightly better upstream 

    •  GPON, in reality, represents an evolutionary step from BPON to EPON 

    References

    [1] GPON spec ITU G.984.3

    [2] EPON spec IEEE 8023ah

    [3] “How efficient is PON?” By Glen Kramer/Teknovus

    [4] GPON efficiency paper by Alcatel 

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    4 Appendix

    Recommendations for BPON and GPON

    Rec. Title GPON comments EPON comments

    BPONG.983.1 Broadband optical access systems based on Passive Optical Networks (PON) Based on this

    frameworkBased on this framework

    G.983.2 ONT management and control interface specification for B-PON Included Equivalent MIBs via IETF

    G.983.3 A broadband optical access system with increased service capability bywavelength allocation

    Based on thisframework

    Based on this framework

    G.983.4 A broadband optical access system with increased service capability usingdynamic bandwidth assignment (DBA)

    Based on thisframework

    Out of IEEE scope

    G.983.5 A broadband optical access system with enhanced survivability Included Out of IEEE scope

    G.983.6 ONT management and control interface specifications for B-PON system withprotection features

    Included Out of IEEE scope

    G.983.7 ONT management and control interface specification for dynamic bandwidth

    assignment (DBA) B-PON system

    Based on this

    framework

    Out of IEEE scope, controls

    supported

    G.983.8 B-PON OMCI support for IP, ISDN, video, VLAN tagging, VC cross-connectionsand other select functions

    Included Equivalent MIBs via IETF

    G.983.9 B-PON ONT management and control interface (OMCI) support for wireless LocalArea Network interfaces

    Included Equivalent MIBs via IETF

    G.983.10 B-PON ONT management and control interface (OMCI) support for DigitalSubscriber Line interfaces

    Included Equivalent MIBs via IETF

    GPON

    G.984.1 Gigabit-capable Passive Optical Networks (GPON): General characteristics (=GPON) See table 1

    G.984.2 Gigabit-capable Passive Optical Networks (GPON): Physical Media Dependent(PMD) layer specification

    (=GPON) Class A and B, relaxedparameters

    G.984.3 Gigabit-capable Passive Optical Networks (G-PON): Transmission convergencelayer specification

    (=GPON) Pure-Ethernet based

    G.984.4 Gigabit-capable Passive Optical Networks (G-PON): ONT management andcontrol interface specification

    (=GPON) Equivalent MIBs via IETF

    The gray areas indicate where GPON and EPON are more or less equivalent.

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