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
Page 2 of 17
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 activity . 155Mbps APON ITU G.983 series
Enhanced to BPON 622/155Mbps rd with 3 lambda, protection and DBA
EPON Ratified by IEEE
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 EPO N, 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, N TT 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!
Page 3 of 17
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
Service
Full service, triple-play + RF video
Same
Layering and Multiplexing
Native Ethernet (includes TDM)
ATM and Generic Frame (including Ethernet and TDM )
Media Access
TDMA via granting -- derived from BPON
Equivalent
ONT Discovery and Activation
Auto-discovery of new ONTs
Equivalent
Comments
(see sec 2.1)
PHY related features: •
•
Number of branches
Wavelength arrangement
Spec: > 16; 64 feasible # logical splits not specified Downstream: 1480-1500 nm Upstream: 12601360 nm
Capable of multiplexing downstream for video distribution (1550-1560 nm)
Max 64 at the PHY Max 128 at the TC layer (logical splits)
Same
(BPON)
•
ODN class classification
Class A(5-20 dB), 10 km; Class B(10-25 dB), 20 km
Class A; Class B; Class C (15-30 dB), 20 km
GPON adds Class C
•
FEC (optional)
Reed Solomon
Same
FEC (forward error correction) reduces an optical module cost, and aims to ease Tx power and Rx optical sensitivity
•
Encryption (optional)
AES-128 on Ethernet payload planned
AES-128 on ATM and GEM frame payload
Equivalent strategy, slightly different scope
Down: 1.2, 2.4 Gbps Downstream: 1 Gbps •
Bit rate
•
Other (optional)
Upstream: 1 Gbps
Up: 155 Mbps, 622 Mbps, 1.2 Gbps, 2.4 Gbps
OLT informs ONT of receiver stabilization time at discovery
ONT optical output power leveling
In GPON, the ONT optical output can be adjusted in 2 steps to relieve automatic power 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.
Layer 5+
T1/E1 TDM
POTS
Data
Video
Layer 5+
Various services
T1/E1 TDM
POTS
Data
Layer 4
TCP+UDP etc
Layer 4
TCP+UDP etc
Layer 3
IP
Layer 3
IP AAL 1/2/5
Layer 2
Layer 2
Ethernet
ATM cell
GEM frame
Ethernet frame
GTC TC frame MAC layer
Layer 1
Video
PON-PHY
GTC sub-layer Layer 1
PON-PHY
GPON Layering
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 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
ingress
transport
OLT
ODN
ONU
Service adaptation
Service adaptation IP/Ethernet
IP/ATM
ATM XC
ATM XC
TDM
TDM
ATM
BPON
Service adaptation
Service adaptation ?
?
ATM & GEM XC
TDM
ATM & GEM XC
TDM
GEM
ATM Ethernet
IP/Ethernet GTC frame
GFP/SONET?
E th er ne t
T DM
GPON
Service adaptation TDM
Service adaptation
Ethernet XC
IP/Ethernet
Ethernet XC
TDM IP/Ethernet
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 o n 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_IDT-CONTVPI/VCI. In the GEM mode, a ‘port’ can be identified via ONT_IDTCONTPort_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}
Page 7 of 17
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) Downstream
LLID 1
Start 100
Length
LLID
200
2
Start 400
Length 100
LLID 3
Start 520
Length 80
Payload
US BW Map
Alloc-ID
Start
End
Alloc-ID
Start
End
Alloc-ID
Start
End
1
100
300
2
400
500
3
520
600
Upstream LLID 1 (ONU1)
LLID 2 (ONU2)
LLID 3 (ONU3)
T-CONT 1 (ONU1)
EPON
Figure 5:
T-CONT 2 (ONU2)
T-CONT 3 (ONU3)
GPON
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.
Page 8 of 17
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 10 38 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 .
ODN Classes GPON Class A
Class B
Class C
EPON
Rates
155, 622Mbps; 1.25Gbps
1.25Gbps
Reach
10km
10km
Splits
16
16
OLT Rx sensitivity
?
?
ONT Tx on/off time
16ns
512ns
Rates
155, 622Mbps; 1.25Gbps
1.25Gbps
Reach
20km
20km
Splits
32
32
OLT Rx sensitivity
?
?
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
ONT Tx on/off time
16ns
N/S
Note EPON 1000PX-10
EPON 1000PX-20
Page 9 of 17
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 N ormal-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, w hich was designed as a superset of BPON, uses GEM cross-connects for TD M 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 O LT 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 crossconnect, 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.
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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 BPO N OLT blades and their associated ONT’s with GPON or EPON ones off the same chassis. A clever BPONGPON 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, GPO N 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 futureproof 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 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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EPON vs. GPON
• 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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EPON vs. GPON
Product-wise, the common trend is still: Triple play in the North America; data-only in Asia. PON Vendors Loc.
BPON
GPON
EPON
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
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?
-
Nayna
USA
S
-
S
NEC
Japan
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
?
-
Sumitomo
Japan
-
-
S*/L
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
Zhone
USA
S
?
-
AFC
*) lower-rate pre-standard version
Comments
200k data-only ONTs deployed by NTT . Partners with Wave7 for triple-play ONT.
Co-author of GPON spec (with Flexlight)
S*
Broadlight chipset Working with Passave
Apparently upgrading to EFM compliant system. (old NEC eLumnant BPON)
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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 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 lower ONT laser costs, e.g., 1.2G/622M or 2.4G/622M -Encrypts the full payload, ie., full Ethernet frames -- no exposed MAC addresses
Disadvantages -Complex layering model Ethernet/GEM/GTC encapsulation 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
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] [2] [3] [4]
GPON spec ITU G.984.3 EPON spec IEEE 8023ah “How efficient is PON?” By Glen Kramer/Teknovus 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 framework
Based 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 by wavelength allocation
Based on this framework
Based on this framework
G.983.4
A broadband optical access system with increased service capability using dynamic bandwidth assignment (DBA)
Based on this framework
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 with protection 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-connections and other select functions
Included
Equivalent MIBs via IETF
G.983.9
B-PON ONT management and control interface (OMCI) support for wireless Local Area Network interfaces
Included
Equivalent MIBs via IETF
G.983.10
B-PON ONT management and control interface (OMCI) support for Digital Subscriber 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, relaxed parameters
G.984.3
Gigabit-capable Passive Optical Networks (G-PON): Transmission convergence layer specification
(=GPON)
Pure-Ethernet based
G.984.4
Gigabit-capable Passive Optical Networks (G-PON): ONT management and control interface specification
(=GPON)
Equivalent MIBs via IETF
The gray areas indicate where GPON and EPON are more or less equivalent.
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