EPON vs. GPON - 1.0 Overview

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					                    EPON vs. GPON
                       A Comparative Study




November 22, 2004
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

3     CONCLUSIONS................................................................................................................................................15

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




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EPON vs. GPON




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

                     FSAN starts first formal PON           Enhanced to BPON 622/155Mbps
                 activity. 155Mbps APON ITU G.983          with 3rd lambda, protection and DBA                                EPON Ratified by IEEE
                                series


                                                                                                                    ITU approves GPON




                              1995      1996        1997       1998       1999      2000         2001    2002      2003         2004


                                                                                         FSAN starts GPON work G.984
                                                                                        series = extension of G.983 series,
                                                                                                  up to 2.4Gbps



                                                                                       EFM starts work on 802.3ah incl
                                                                                                   EPON




                                     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 very
inefficient, 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 emerging
QOS-aware GigE switches and cost-effective integration with other Ethernet equipment. Ethernet has proven over
time 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 of
standards for point-to-point and point-to-multipoint access networks, the latter specifying Ethernet PONs. EPON is
currently part of standard Ethernet.
Development of the Gigabit-capable Passive Optical Network (GPON) standard (G.984 series) really started after
proposals by FSAN members (Quantum Bridge et. al) for a protocol-independent ATM/Ethernet Gbps PON
solution were not very popular within the IEEE 802.3ah work group. FSAN then decided to continue this as a
different competing standard in the ITU.
EPON and GPON both draw heavily from G.983, the BPON standard, when it comes to general concepts that work
well (PON operation, Optical Distribution Network (ODN), wavelength plan, and application). They both offer their
own 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 Bellsouth
to 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 further
expanding their FTTH network with EPON, not GPON. This is the common trend elsewhere in Asia. EPON is
clearly taking off!




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EPON vs. GPON



This begs the question: Do we really need GPON next to EPON? In order to answer this question we will take a
closer look at these two flavors, and compare their different approaches on technical and practical merits. We will
show 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 Overview
As pointed out earlier, EPON and GPON both draw heavily from the BPON specification. This is evident in the
table 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

                                                                               ATM and Generic Frame
Layering and
                         Native Ethernet (includes TDM)                        (including Ethernet and   (see sec 2.1)
Multiplexing
                                                                               TDM )

Media Access             TDMA via granting -- derived from BPON                Equivalent

ONT Discovery and
                         Auto-discovery of new ONTs                            Equivalent
Activation

PHY related features:

                                                                               Max 64 at the PHY
•     Number of          Spec: > 16; 64 feasible
      branches                                                                 Max 128 at the TC layer
                         # logical splits not specified
                                                                               (logical splits)

                         Downstream:
                         1480-1500 nm            Capable of multiplexing
•     Wavelength
                                                 downstream for video          Same                      (BPON)
      arrangement        Upstream: 1260-         distribution (1550-1560 nm)
                         1360 nm

•     ODN class                                                                Class A; Class B; Class
                         Class A(5-20 dB), 10 km; Class B(10-25 dB), 20 km                               GPON adds Class C
      classification                                                           C (15-30 dB), 20 km

                                                                                                         FEC (forward error correction) reduces an
•     FEC (optional)     Reed Solomon                                          Same                      optical module cost, and aims to ease Tx power
                                                                                                         and Rx optical sensitivity

•     Encryption                                                               AES-128 on ATM and
                         AES-128 on Ethernet payload planned                                             Equivalent strategy, slightly different scope
      (optional)                                                               GEM frame payload

                                                                               Down: 1.2, 2.4 Gbps
                         Downstream: 1 Gbps
•     Bit rate                                                                 Up: 155 Mbps, 622
                         Upstream: 1 Gbps                                      Mbps, 1.2 Gbps, 2.4
                                                                               Gbps

                                                                                                         In GPON, the ONT optical output can be
                         OLT informs ONT of receiver stabilization time at     ONT optical output
•     Other (optional)                                                                                   adjusted in 2 steps to relieve automatic power
                         discovery                                             power leveling
                                                                                                         distribution (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 variable
sized IP/Ethernet frames. The justification for the continued support of ATM is often explained as serving the
need for backhauling of first-generation DSL traffic. This remains to be seen, considering that today’s IP DSLAMs
are all Ethernet based.




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EPON vs. GPON



2.1 PON Concepts
This section presents several key concepts and how they have been addressed in EPON and GPON. Since these two
PON 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 model
and 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 Ethernet
frames are wrapped into GTC Encapsulation Method (GEM) frames, which have a GFP-like format (derived from
Generic 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.

                         T1/E1                                                         Various       T1/E1
                                     POTS           Data      Video                                              POTS         Data         Video
         Layer 5+        TDM                                                Layer 5+   services      TDM



         Layer 4                              TCP+UDP etc                   Layer 4                                       TCP+UDP etc

         Layer 3                                     IP                     Layer 3                                            IP

                                                                                       AAL 1/2/5                            Ethernet


         Layer 2                                                            Layer 2    ATM cell               GEM frame


                                 Ethernet frame                                                      GTC TC frame
                                                  MAC layer                                                               GTC sub-layer
         Layer 1                   PON-PHY                                  Layer 1                    PON-PHY


                    EPON Layering                                                                  GPON 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 to
efficiently accommodate Ethernet and TDM frames. Both ATM and GEM modes are mandatory at the OLT, but an
ONT 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, when
compared to the simple EPON model, it becomes clear that the GEM/GTC encapsulation and inclusion of ATM are
adding unnecessary complexity to solve the same problem. The different transport schemes are illustrated in Figure
3.




                                                                                                                                          Page 5 of 17
EPON vs. GPON



               uplink                                                            transport                                    ingress


                                         OLT                                           ODN                 ONU
                             Service                                                                              Service
                            adaptation                                                                           adaptation


                IP/ATM                                                                                                         IP/Ethernet


                                                ATM                                              ATM
                                                 XC                                               XC
                 TDM                                        ATM                                                                  TDM




                                                                                        BPON



                             Service                                                                              Service
                            adaptation                                                                           adaptation


                   ?                                                                                                                ?


                                          ATM & GEM                                            ATM & GEM
                                             XC                                                   XC
                 TDM                                                                                                             TDM
                                                           ATM                GEM
               Ethernet                                                                                                       IP/Ethernet
                                                          GTC frame
                                                                      Ethernet   TDM
            GFP/SONET?


                                                                                       GPON




                             Service                                                                              Service
                            adaptation                                                                           adaptation

                 TDM                                                                                                             TDM
                                               Ethernet                                         Ethernet
                                                                                                                              IP/Ethernet
              IP/Ethernet                        XC                                               XC
                                                              Ethernet


                                                                    TDM/
                                                                   Ethernet



                                                                                       EPON




                             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 avoid
unnecessary blocking and latency, by periodic scheduling of upstream bandwidth, and by distributing the CO clock
downstream 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 the
associated 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 be
recovered 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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EPON vs. GPON



In GPON there are three different types of control messages: OMCI, OAM, and PLOAM. Their roles are shown in
the 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 contains
multiple MACs, each having a 1:1 relationship with an ONT off the PON. This is represented by the Logical Link
ID (LLID), which is used for addressing. The figure below illustrates the roles of LLIDs in point-to-point
emulation. Notice the significance of the bridge at the OLT in this model, which is not present in GPON.




                                                                    Standard 802.1 Bridge


                                                       MAC   MAC      MAC      MAC     MAC     MAC     MAC

                                                                                                            LLID
                                             OLT




                                                                                                            LLID

                                                       MAC   MAC      MAC      MAC     MAC     MAC     MAC

                                             ONT


                                      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 VLAN
is identified as LLID VLAN_ID. In the downstream direction, the OLT attaches the LLID to the preamble of
frames, 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 is
addressed via ONT_ID T-CONT VPI/VCI. In the GEM mode, a ‘port’ can be identified via ONT_ID T-
CONT Port_ID.
Both the LLID and T-CONT provide a form of point-to-point emulation, except that GPON has no relationship to
802.1 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 Ethernet
frames. Each grant specifies an upstream transmission opportunity for a given ONU via {LLID+Start+Length}



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EPON vs. GPON



Similarly, GPON grants per T-CONT. Grants are carried in the downstream frame header overhead, via a map that
holds multiple grants specifying {Alloc-ID+Start+End} for each granted upstream Window (timeslot). The two
different schemes are shown in the figure below.
                                                                                                                                                                                Downstream
                                                                                                 Frame header (PBCd)
                                                                                                                                        Payload
                                                                                 Downstream               US BW Map



                                                                                              Alloc-ID   Start   End        Alloc-ID   Start       End    Alloc-ID      Start      End
  LLID    Start   Length       LLID   Start    Length          LLID      Start      Length
                                                                                                 1       100     300           2       400         500       3           520       600
   1       100     200           2    400        100            3        520          80




   Upstream

                      LLID 1                   LLID 2           LLID 3                                           T-CONT 1                      T-CONT 2              T-CONT 3
                     (ONU1)                   (ONU2)           (ONU3)                                             (ONU1)                        (ONU2)                (ONU3)




                                        EPON                                                                                           GPON

                                      Figure 5:                  EPON And GPON Media Access Control

2.1.2.2 Dynamic Bandwidth Allocation
Dynamic Bandwidth Allocation (DBA) refers to an optional flexible upstream timeslot assignment mechanism used
in a PON. It allows a system to assign upstream timeslots in real-time, based on the instantaneous demand of a
given ONT, and hence use the upstream bandwidth more efficiently. In a typical FTTH deployment today where
there is ample upstream bandwidth, DBA is not very effective since traffic patterns are still asymmetric and
upstream bandwidth demands tend to be relatively low. However, in situations where the upstream demand is
relatively 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 send
REPORT messages including multiple 802.1p queue states, but it’s up to the scheduler at the OLT whether / how to
interpret this information.
GPON uses a very similar scheme, but there the DBA is part of the standard. Elements of the two schemes are
outlined 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 & Activation
EPON supports a discovery mechanism that allows the OLT to automatically detect a newly added ONT, learn its
MAC Address, assign an LLID, and activate the ONT. At this point it is up to the provisioning application to
configure it with appropriate user bandwidth and other subscriber parameters.
Standard EPON does not require the ONT Serial Number for authentication; however, higher-level authentication
schemes can do so.
GPON uses the Serial Number for ONT authentication. This can be either pre-provisioned, or discovered by the
OLT. Once the Serial Number is detected, an ONT-ID is assigned, and the ONT is activated.


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EPON vs. GPON



2.1.4 Encryption
Both EPON and GPON have endorsed 128-bit Advanced Encryption Standard (AES) encryption. 128-bit keys
means 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 frames
and 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 complete
IP 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 exposing
MAC 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. Upon
detecting 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 shown
in the table below, compared with EPON . <Review this table w/Ketan>

                                                      ODN Classes
                                                           GPON                    EPON         Note

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

                     Reach                   10km                             10km

                     Splits                  16                               16

                     OLT Rx sensitivity      ?                                ?

                     ONT Tx on/off time      16ns                             512ns

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

                     Reach                   20km                             20km

                     Splits                  32                               32

                     OLT Rx sensitivity      ?                                ?

                     ONT Tx on/off time      16ns                             512ns

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

                     Reach                   > 20km                           N/S

                     Splits                  64                               N/S

                     OLT Rx sensitivity      ?                                N/S

                     ONT Tx on/off time      16ns                             N/S




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EPON vs. GPON



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 a
result, 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 power
level 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 ONTs
of its synchronization time, allowing ONTs to adjust their upstream transmission timing accordingly.


2.2 Deployment Aspects
In order to further understand the differences between GPON and EPON, we next take a look at how they fare when
it 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 fact
that 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 good
example 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 reality
is however, that today EPON-based systems are being deployed worldwide, successfully delivering carrier-grade
triple-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 of
service (IP Data, TDM, POTS, VOIP, IPTV, RF Video). In fact, when it comes to certain IP/Ethernet services, it
turns out that GPON is the one that fall short, as is shown below.




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EPON vs. GPON



2.2.2.1 IPTV support
IPTV consists of Video on Demand (VOD) and Switched Digital Video (SDV). With VOD, each IP stream is
viewed 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 fast
system 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 its
inherent single-copy broadcast capability, resulting in bandwidth consumption as a function of channels, rather than
the number of viewers. However, the overall performance scalability of the service relies heavily on the
multicasting 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 the
multicast 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 support
standard Bridging, which can be of interest in FTTB deployments. Some form of ‘GEM-bridging’ could probably
be 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 is
achieved via VLAN tunneling (Q-in-Q) in Ethernet switches. Without these in the OLT chassis, one would need to
use 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 the
upstream 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 Gbps
downstream 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. <any comments on the cost of 1.2G/622M vs. 1G/1G?>



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EPON vs. GPON



Notice also that 2.4 Gbps is not a common rate, and lacks volumes to draw from in order to drive down ONT
transceiver costs. Upstream rates higher than 622Mbps are also not economical due to mode partition noise, until
narrow 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 as
in Ethernet. This makes it appear even more attractive, with efficiency potentially in the upper 90% (of 1.244
Gbps). 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 in
either direction. The downstream efficiency is significantly more important because of the asymmetric nature of
PON bandwidth usage. Notice e.g., that for data services in a typical FTTH deployment at least 40% of the
upstream BW consists of a low load of small packets (internet TCP ACKs). In addition, one has to take in
consideration 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 component
selection. Often, however, a few 100’s Mbps upstream is sufficient for standard FTTH applications, especially
when DBA is used. Of course, what actually matters is the remaining usable bandwidth, and whether or not it is
sufficient 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 attractive
features of FTTP.
The least painful migration scenario is probably a same-vendor one, i.e., replacing BPON OLT blades and their
associated ONT’s with GPON or EPON ones off the same chassis. A clever BPON GPON strategy is what many
BPON vendors are selling today; it validates the investment in outdated BPON gear, creates a future for GPON, and
at the same time locks in the customer.
For service providers with large investments in their ATM-based core networks, GPON may initially have some
appeal because of its support of ATM. However, notice that existing ATM core networks are quickly running out
of 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 need
for 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 relatively
weak in areas of network management that are important to large-scale subscriber access networks. Key areas of
concern have been:
• Limited performance monitoring and service level 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 be
the focus in different standards organizations. EPON system vendors have been successful in filling any remaining
gaps with innovative solutions.
These areas should also be reasonably well covered in GPON, as it inherits some of the ATM functionality and was
designed with the input of a few service providers.
For an EPON system, management of PON operation and OLT/ONT interoperability are defined through EFM
OAM and IETF MIBs. Equivalently, GPON has outlined its management capabilities through a series of OMCI
specs.
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 managed
through traditional telecom system models using MIBs that include management functions for equipment, service
cards, 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 in
favor of next generation 3rd party software. Common choices are Micromuse Netcool for assurance and Syndesis
Netprovision for inventory and provisioning. These systems are very flexible to customize, and typically well
positioned for supporting IP/Ethernet based systems and services, as they have been for the many routers and
switches 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 to
the 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 to
GFP, GPON vendors could design the OLT to pass native GFP frames to the uplink via SONET/SDH payloads (see
figure 3). This could help accomplish transparent trunking of Ethernet or TDM payloads. Of course, TLS is a more
elegant way of doing this, as supported in EPON systems.




2.3 PON Developments
Today, there are about twice as many BPON vendors than there are EPON ones (see table below). About half of the
BPON vendors have indicated they are developing GPON products (although they don’t seem to be in a rush); only
two companies have released GPON products today. The table below shows the state of PON developments of
over 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     <Q: what about China’s ZTE and Huawei?>




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EPON vs. GPON



Notice that AFC, ECI, Entrisphere, Calix, Siemens, Zhone and Ciena are currently all developing BPON systems
using Broadlight’s chipset, which includes 1.25Gbps/622Mbps ‘extended BPON’ rates as well as pin-compatible
GPON 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              -Complex layering model Ethernet/GEM/GTC
                                                                                   encapsulation means complex management
         -Higher theoretical BW, up to 2x that of EPON
                                                                                   -More expensive at comparable rates as EPON
         -Can be configured in asymmetric fashion and take advantage of lower
         ONT laser costs, e.g., 1.2G/622M or 2.4G/622M                             -Transceivers at 2.4Gbps rates are expensive today,
                                                                                   no volumes
         -Encrypts the full payload, ie., full Ethernet frames -- no exposed MAC
         addresses                                                                 -upstream BW limited to 622Mbps today
         Better chances for interoperability
         -Standard supports TDM
         -Standard ONT service-level management
EPON     -Native transport of Ethernet frames                                      Mostly interoperability obstacles:
         -Simple and familiar, lower cost management                               -Non-standard service-level interoperability
         -Benefits from true Ethernet switching                                    - non-standard TDM
                    -Fully compatible with IP                                      -non-standard encryption
                    -Supports TLS                                                  -non-standard protection switching
                    -Broadcast, Multicast --IGMP
         -IGMP support means better support for IPTV with high scalability
         -lower costs optics due to relaxed timing parameters



In conclusion, GPON can be seen as a ‘me-too’ specification that duplicates EPON functionality, but than in a
rather 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 with
    carrier-grade TDM suport in EPON
•   ATM traffic is insignificant or not-existent in today’s access networks, adding unnecessary complication to
    GPON
•   GPON and EPON are equally capable of providing the QoS capabilities required for triple play service
    differentiation
        •    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 pure
    Ethernet switches. EPON is more appropriate for IP/Ethernet services:



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EPON vs. GPON



          •   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 are
              important. 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 NextGen
              OSS systems.
•     EPON is receiving considerable endorsements in Asia today, while in the US leading RBOCs are embarking
      on 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 the
      advantage of volumes that can help drive down costs. Partition Noise currently limits the GPON upstream
      speed 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 GPON
              slightly 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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EPON vs. GPON




4 Appendix
                                          Recommendations for BPON and GPON
     Rec.                                          Title                                    GPON comments        EPON comments

                                                                    BPON

G.983.1      Broadband optical access systems based on Passive Optical Networks (PON)       Based on this   Based on this framework
                                                                                            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 by         Based on this   Based on this framework
             wavelength allocation                                                          framework

G.983.4      A broadband optical access system with increased service capability using      Based on this   Out of IEEE scope
             dynamic bandwidth assignment (DBA)                                             framework

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 with      Included        Out of IEEE scope
             protection features

G.983.7      ONT management and control interface specification for dynamic bandwidth       Based on this   Out of IEEE scope, controls
             assignment (DBA) B-PON system                                                  framework       supported

G.983.8      B-PON OMCI support for IP, ISDN, video, VLAN tagging, VC cross-connections     Included        Equivalent MIBs via IETF
             and other select functions

G.983.9      B-PON ONT management and control interface (OMCI) support for wireless Local   Included        Equivalent MIBs via IETF
             Area Network interfaces

G.983.10     B-PON ONT management and control interface (OMCI) support for Digital          Included        Equivalent MIBs via IETF
             Subscriber Line interfaces

                                                                    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      (=GPON)         Class A and B, relaxed
             (PMD) layer specification                                                                      parameters

G.984.3      Gigabit-capable Passive Optical Networks (G-PON): Transmission convergence     (=GPON)         Pure-Ethernet based
             layer specification

G.984.4      Gigabit-capable Passive Optical Networks (G-PON): ONT management and           (=GPON)         Equivalent MIBs via IETF
             control interface specification



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




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