White Paper

GPON - EPON Comparison CommScope Solutions MarketingOctober 2013

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Introduction 3

Protocol fundamentals 4

Framing/service adaption 4

Basic operation 5

Service hierarchy 7

Bandwidth allocation 7

Dynamic Bandwidth Allocation (DBA) 8

Control messages 9

ONT discovery & activation 9

Encryption 9

Bandwidth and efficiency 9

Ethernet service support 10

Bridging 10

Transparent LAN Services (TLS) 10

Conclusion 10

Contents

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Passive Optical Networking (PON) technology has been available since the mid-1990s. Significant development activities occurred during the early 2000s to develop Gigabit rate solutions designed to deliver Ethernet and IP services. Two very different solutions were developed by the IEEE and ITU-T—EPON and GPON, respectively. While the general concepts (PON operation, ODN framework, wavelength plan, and application) are the same for both EPON and GPON, their operation is very different, as are the features and services supported by each. EPON is a native Ethernet solution that leverages the features, compatibility and performance of the Ethernet protocol, while GPON leverages the techniques of SONET/SDH and Generic Framing Protocol (GFP) to transport Ethernet.

IntroductionPON standardization activities have been ongoing for the past fifteen years within the ITU-T and IEEE standards bodies. EPON and 10G-EPON are the latest ratified IEEE standards and GPON is the latest ratified ITU-T standard. Figure 1 shows some of the key historical milestones:

Initial PON offerings in the early and mid-1990s were based on ATM framing (APON, BPON). With the explosion of Internet- and intranet-based traffic in the years following, ATM-based BPON systems proved to be very inefficient, as the vast majority of traffic through the access network consisted of variable-length IP traffic. This became the catalyst for the development of a purely Ethernet-based PON (EPON), taking advantage of emerging QOS-aware Gigabit Ethernet (GbE) switch silicon, evolving Ethernet standards (VLANs, prioritization, OAM), and cost-effective integration with other Ethernet equipment.

Development of the Gigabit-capable Passive Optical Network (GPON) standard started after proposals by FSAN members (Quantum Bridge et al) for a dual Gigabit speed ATM/Ethernet PON solution were not able to gain support within the IEEE 802.3ah work groups and decided to continue this work within the ITU.

EPON and GPON both draw heavily from G.983, the BPON standard for their general concepts (PON operation, ODN framework, wavelength plan, and application). Also, both were designed to better accommodate variable-length IP frames at Gigabit line rates. There are, however, significant differences in the approaches used by each.

Figure 1: PON Standardization

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

Enhanced to BPON 622/155Mbps with 3rd

lambda, protection and DBA EPON ratified by IEEE

FASN starts first formal PON activity. 155MBps APON ITU

G.983 series

EFM starts work on802.3ah incl EPON

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

ITU approves GPON

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Protocol fundamentalsEPON is based upon IEEE 802.3 Ethernet that was modified to support point-to-multipoint (P2MP) connectivity. Ethernet traffic is transported natively and all Ethernet features are fully supported. GPON, on the other hand, is fundamentally a transport protocol, wherein Ethernet services are adapted at the OLT and ONT Ethernet interfaces and carried over an agnostic synchronous framing structure from end to end.

Framing/service adaptionThe GPON Transmission Convergence (GTC) layer is responsible for mapping service-specific interfaces (e.g. Ethernet) into a common service-agnostic framework.

Ethernet frames are encapsulated into GTC Encapsulation Method (GEM) frames, which have a GFP-like format (derived from Generic Frame Procedure ITU G.7401). GEM frames are, in turn, encapsulated into a SONET/SDH-like GTC frame (in both upstream and downstream directions) that is transported synchronously every 125 µsec over the PON.

Layer 5T1/E1TDM POTS Data Video

Layer 4

Layer 3

Ethernet frame

PON-PHY

EPON layering

TCP+UDP etc

Layer 1

Layer 2

IP

MAC layer

Layer 5T1/E1TDM

Variousservices POTS Data Video

Layer 4

Layer 3

GTC TC frame

PON-PHY

GPON layering

TCP+UDP etc

Layer 1

Layer 2

IP

Ethernet

GTC sub-layer

ATM cell GEM frame

Figure 2: GPON vs. EPON Protocol

Figure 3: Downstream GTC Frames

PCBd

PCBd = Physical Control Block Downstream

T = 125 msec

PCBdPayload

PURE ATM cells selection TDM and data fragments overGEM section

Payload

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In contrast, EPON carries Ethernet frames natively on the PON with no changes or modifications. There is no need for extra adaption and encapsulation.

Basic operationAs shown above, EPON is an IEEE Ethernet architecture. Its downstream media access control (MAC) works in the same manner as a standard GbE MAC, where all Ethernet traffic is broadcast. Optical splitters are used to passively divide the same downstream signal among each of the ONT end points.

Figure 4: Framing in GPON and EPON

Ethernet

OLT

Service adaption

Ethernet

Ethernet

EthernetSwitch

Ethernet

EPON

EthernetSwitch

ONT

Ethernet

Ethernet

Service adaption

OLT

Service adaption

Ethernet

Ethernet

GEMXC

GTC FrameEthernet

GEM

GPON

GEMXC

ONT

Ethernet

Downstream broadcastAll data goes to all ONUs and the ONU address controls the downstream data.

Figure 5: EPON Downstream

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Upstream, the Ethernet MAC has been modified by the IEEE to support time division multiple access (TDMA) scheduling. The upstream Ethernet bandwidth (before 8B/10B coding) is scheduled for use by each of the ONT end points using a TDMA algorithm controlled by the OLT. The full bandwidth is available to each endpoint for the duration of its scheduled time. Dynamic Bandwidth Allocation (DBA) algorithms are implemented by vendors to dynamically change the allocation for each ONU based on the amount of traffic it has queued to send.

As shown in Figure 7, GPON is a synchronous transport network architecture. Its downstream MAC is derived from GFP-framed SONET, whereby it operates on a fixed time base of 125 µsecs and time division multiplexing (TDM) is used to divide the bandwidth to the 32 ONTs. Because it is synchronous in nature, idle characters are inserted whenever no asynchronous Ethernet traffic is available. Optical splitters passively split the same downstream signal to each of the 32 ONU end points.

Upstream, TDMA scheduling is used and incoming Ethernet traffic is again encapsulated and mapped into a 1.2 Gbps synchronous framed signal in a similar manner to the downstream.

Upstream TDMA operationONUs send information to the OLT in a specific time window.

Figure 6: EPON Upstream

Voice

Data

Data

Video

PABX

TDM

TDMA

1:32 (64) Optical Splitter

0-20 km physical reach(60 km logical reach supported by protocol)

Maximum bit rate 2488 Gbit/s both downstream and upstream

OLT

NB

CC

E1/T1

GbESTMn/OCn

Access Node

BB

ONU

PABX ........ Private Automatic Branch ExchangeONU ....... Optical Network UnitTDM ........ Time Division MultiplexTDMA ...... Time Division Multiple AccessOLT ......... Optical Line TerminationCC .......... Cross-ConnectNB .......... Narrow BandBB ........... BroadbandE1 ........... 2048 Mbit/sT1 ........... 1554 Mbit/sGbE ........ Gigabit EthernetSTMn ....... Synchronous Transport Module “n”OCn ........ Optical Carrier “n”

Downstream (two-fiber systems)................ 1310 nm + 50 nmDownstream (single-fiber systems) ............ 1490 nm + 50 nmUpstream ............................................... 1310 nm + 50 nm

ONU

ONU

Figure 7: GPON Upstream & Downstream

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Service hierarchySince PON is P2MP in nature, the OLT must be able to uniquely identify and communicate with each ONT. EPON uses a Logical Link ID (LLID) to uniquely address an ONT. In addition, VLAN_IDs are used for further addressing in order to deliver VLAN-based services. In the downstream direction, the OLT attaches the LLID to the preamble of the frame to identify the destination ONT.

In GPON, one or more Traffic Containers (T-CONT) are created between the OLT and an ONT. This T-CONT allows for the emulation of a point-to-point virtual connection between the OLT and ONT and the subsequent TDM multiplexing of the downstream bandwidth between T-CONTs. Within each T-CONT there can exist multiple Port IDs to identify individual ONT ports within a single ONT.

Bandwidth allocationThe use of TDMA in the upstream direction requires the OLT to schedule each ONT’s transmission to avoid collisions. Fundamentally, each ONT receives a grant telling it when to begin and end transmission. In GPON, grants are scheduled per T-CONT; in EPON, per LLID. In the case of GPON, grants are carried in the downstream frame header. A map field within the header specifies the specific T-CONT, start and end {Alloc-ID+Start+End} for each granted upstream window (timeslot).

In EPON, grant messages are sent per LLID, as separate MAC-Control client frames (GATEs), between regular Ethernet frames. Each grant specifies the {LLID+Start+Length}.

Figure 8: Service Hierarchy

PON

Identifiedvia

ONU_ID Identifiedvia VPI Identified

via LLID

Identified via Alloc_ID

Identifiedvia VCI

ONU ONU

ONU

ONU

T-CONT

T-CONT

T-CONT

PORT

PORT

PORT LLID

LLID

LLID

LLID

VP

VP

VP

VP

VP VC VLAN

VLAN

VLAN

VC

VC

VC

VC

Identified via Port_ID

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Dynamic Bandwidth Allocation (DBA) Optionally, both GPON and EPON support DBA. This is used for real-time variation of timeslot allocation to ONTs, which increases throughput as a function of upstream demand.

GPON DBA GPON DBA

Granting Unit GTC overhead MPCP GATE frame

Control Unit T-CONT LLID

Identification of control unit Alloc-ID LLID

Reporting mechanism Embedded OAM Separate REPORT Frame

Negotiate Procedure GPON OMCI N/A

Figure 9: GPON Media Access Control

Frame header (PBCd)Payload

Downstream

US BW Map

Alloc-ID

T-CONT 1(ONU 1)

T-CONT 2(ONU 2)

T-CONT 3(ONU 3)

1 2 3100 400 520300 500 600

Alloc-IDAlloc-IDStart StartStartEnd EndEnd

Figure 10: EPON Media Access Control

Downstream

Upstream

LLID

LLID1(ONU 1)

LLID 2(ONU 2)

LLID 3(ONU 3)

1 2 3100 400 520200 100 80

LLIDLLIDStart StartStartLength LengthLength

Figure 11: DBA Elements in GPON and EPON

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Control messagesIn GPON there are three different types of control messages: OMCI, OAM, and PLOAM. Their roles are shown in the table below.

Control channel Format Used for

OMCI Ethernet or ATM Provisioning of ONT service defining layers above the GTC (e.g., via EMS)

Embedded OAM Header overhead BW granting, encryption key switching, and DBA

PLOAM ATM Auto discovery and all other PMD and GTC management info. PLOAM messages are directed to ONTs or FF for broadcasts

In contrast, EPON utilizes IEEE 802.3ah OAM messages for provisioning, fault isolation and performance monitoring in conjunction with SNMP sets and gets through IETF MIBs. Additional control messages are MPCP GATEs/REPORTs for BW granting.

ONT discovery & activation Both EPON and GPON support automatic ONT discovery and activation mechanisms. GPON uses the Serial Number (SN) for ONT authentication. There are two methods for activating ONTs.

•Method A -- The SN of the ONT is registered in advance at the OLT by the operator •Method B -- The SN of the ONT is not registered at the OLT by the operator/EMS. Method B requires an automatic detection mechanism of the SN. If a new ONT is detected, an ONT-ID is assigned and the ONT is activated.

Traditionally, EPON does not authenticate via the SN, but, instead, uses the ONT MAC Address in order to assign an LLID. However, some vendor-specific EPON implementations optionally utilize the ONT’s SN.

Encryption Both EPON and GPON support AES-128bit encryption. For GPON, key management messages are exchanged via PLOAM cells. Upon request by the OLT, the ONT sends a new key three times. Once received, the OLT toggles a bit in the GTC header to initiate a key switch. For EPON, key management messages are either via a management VLAN or via IEEE 802.3ah OAM messagesm depending on vendor implementation.

Bandwidth and efficiency Probably the most heralded claim of GPON vendors is that it is 2.448 Gbps in the downstream direction and 1.24416 Gbps in the upstream direction, whereas EPON is symmetrically 1.25 Gbps (1.0 Gbps prior to 8B/10B coding). Efficiency has to be considered in both directions of a PON. Each PON protocol introduces its own overhead in either direction. Overall, PON efficiency is a function of protocol encapsulation and scheduling efficiencies. In the downstream direction, protocol overhead should be negligible. In the upstream direction, the total scheduling overhead within EPON is between 2.92 percent

Figure 11: Control Messages

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and 9.67 percent. In other words, EPON efficiency is from 90.33 percent to 97.08 percent compared to a GbE point-to-point link. In the downstream direction, EPON efficiency reaches from 97.13 percent to 98.92 percent of the efficiency of a point-to-point 1GbE link, while GPON in GEM mode can achieve ~ 95 percent efficiency of its usable bandwidth. It should be noted that, optionally, EPON can be operated in what is typically termed ‘turbo mode’. With turbo mode, the downstream EPON data rate is doubled to 2.5Gbps, thus enabling bandwidth throughput comparable to GPON.

Ethernet service support Ethernet features Since EPON is an IEEE Ethernet standard and utilizes Ethernet switches within its silicon, it can natively support all of the 802.1 and 802.3 features of Ethernet, including VLAN tags, prioritization, OAM, etc. All Ethernet services can be natively delivered in a manner identical to what is done with switched Ethernet today.

Since GPON only defines the transport of Ethernet frames, there is no native Ethernet functionality. Ethernet switches must be placed either in front of or within GPON OLTs and ONTs to provide any additional Ethernet capabilities. Capabilities are, thus, unique to each manufacturer’s implementation.

Bridging Since the cross-connect at the GPON OLT is not an Ethernet switch, GPON cannot support standard Ethernet bridging. Thus, in order to support standard bridging, there would be the need for an Ethernet switch upstream of the OLT cross-connect, either externally or in an aggregation point in the same chassis.

Transparent LAN Services (TLS) TLS is achieved via VLAN tunneling on Ethernet switches. Without these in the GPON OLT chassis, one would need to use external Ethernet switches in order to achieve the same result. The GEM cross-connect cannot inspect VLANs in order to make the appropriate forwarding decisions.

ConclusionThe developers of both EPON and GPON recognized the need to evolve PON to being a Gigabit-capable solution for transporting Ethernet and IP traffic. Their implementations, while utilizing a common optical infrastructure, are very different in execution. EPON extended native Ethernet to support the PON P2MP architecture, while GPON used techniques similar to GFP-framed SONET/SDH to create an efficient Ethernet transport mechanism.

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