Letters

Development of a Gigabit Ethernet Passive
Optical Network (GE-PON) SystemKoji Ochiai, Tsutomu Tatsuta†, Takashi Tanaka,Osamu Yoshihara, Noriyuki Oota, and Noriki MikiAbstract
The gigabit Ethernet passive optical network (GE-PON) is a high-speed optical access method that has
been standardized according to IEEE 802.3ah and is attracting attention as some operators have started
to provide optical access service with it. NTT Access Network Service Systems Laboratories has developed a GE-PON system that should capture a major share of the FTTH (fiber to the home) market, whichis expected to become very competitive in the near future.

1. FTTH market overview
Since 2001, point-to-point and passive optical network (PON) fiber to the home (FTTH) access systems have been widely deployed in Japan, in responseto rapid growth of high-speed Internet access services
and competition in both monthly access charges andtransmission speed, driven by the popularity ofADSL (asymmetric digital subscriber line) access.
Currently, besides conventional telecom operators,
major power utility companies and ADSL service
operators are also providing FTTH. The monthly risein the number of FTTH users has reached 100,000
and the total number of FTTH subscribers exceeded
2 million in September 2004 [1].
2. Trend of PON systems
Network costs need to be reduced further in orderto persuade more home users to use FTTH. The PON
architecture lets operators make significant savings infiber cost in the access network. It has been favoredby telecom operators and some power utility companies for FTTH access to individual home users.
In Japan, a considerable number of PON systems
deployed in the access network have been GE-PON† NTT Access Network Service Systems Laboratories
Chiba-shi, 261-0023 Japan
E-mail: tatsuta@ansl.ntt.co.jp

Vol. 3 No. 5 May 2005

since major operators announced the installation ofGE-PON for residential FTTH access last year.
Japanese operators prefer GE-PON because they arefocusing on data transmission services [2].
3. Outline of the GE-PON standard
GE-PON was specified by the IEEE 802.3ah Ethernet in the First Mile (EFM) task force as one of theEthernet interface series in June 2004 [3]. To provide
a wide variety of broadband services and to allow forfuture capabilities, the 1000BASE-PX series wasspecified in the IEEE 802.3ah standard. 1000BASEPX10 (up to 10 km) and 1000BASE-PX20 (up to 20km) physical media dependents (PMDs) have been
prepared for GE-PON. The wavelengths of opticalsignal transmission are based on ITU-T* G.983.3, sothat additional services like video broadcasting usinga wavelength of 1550 nm are possible. The maximum
splitting is not specified by IEEE 802.3ah because it
depends on PMDs and actual optical link losses. GEPON provides 2 octets (16 bits) for the logical linkidentifier (LLID) and 15 bits can be assigned to eachmedium access control (MAC). Therefore, more than
30,000 optical network units (ONUs) can be logically accommodated in a PON.
A key feature of GE-PON is that data is transmitted* ITU-T: International Telecommunication Union Telecommunication Standardization Sector

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in variable-length MAC frames of up to 1518 octets(1522 octets for a tagged frame). The downstream
transmission is based on the original Ethernet mechanism keeping a minimum inter-frame gap of 12octets. An optical line terminal (OLT) broadcasts
MAC frames to every optical network unit (ONU)
accommodated in a PON, so each ONU receives allthe MAC frames. However, a two-octet frame-header
called an LLID contains address information. As aresult, only the addressed ONU reads its MACframes; ONUs discard frames that are not addressedto them. On the other hand, upstream transmission isbased on TDMA (time division multiple access) arbitration to avoid data collisions between ONUs. The
OLT allocates a transmission window called a gate toeach ONU. When an ONU receives the gate frame, ittransmits MAC frames at a rate of 1 Gbit/s during thetime slots assigned by the gate. Multipoint control
protocol (MPCP) contains the gate allocation foreach ONU, gate request from the ONU, and discovery and registration of ONUs with the PON.
4. Main specifications of NTT’s GE-PON system
IEEE 802.3, which includes IEEE 802.3ah, specifies the data link and physical layers, but regards
other specifications as out of its scope (Fig. 1). For
instance, because there is no specification for how thesystem activates the MPCP, the GE-PON system can-not work only by referring to the IEEE 802.3 standard. During the design of a GE-PON as an accesssystem, these specifications will be defined by referring to other standards, proprietary proposals, andoperator’s requirements.
We have developed our own set of specifications for
optimizing the deployment of a GE-PON access system that includes interoperability between different
vendors [4]. The construction of our GE-PON system
is shown in Fig. 2, its function stack is shown in Fig.
3, and the external appearances of the ONU and OLT
are shown in Fig. 4. The main specifications outside
the IEEE 802.3ah standards are explained below.
4.1 Dynamic bandwidth allocation (DBA)
DBA is a function that allocates the upstream bandwidth to each ONU. As the features of the provided
service depend on this function, we developed a DBA
function that achieves a balance between high bandwidth efficiency and low delay [5].
In the GE-PON system, an upstream bandwidth of
1 Gbit/s is shared among up to 32 ONUs. One methodof allocating the upstream bandwidth is to set theassigned bandwidth for each ONU to a fixed value(static allocation) regardless of the upstream traffic.
However, since bandwidth is allocated to an ONUeven when it has no upstream traffic, a lot of bandwidth is wasted. To eliminate this wastage, a DBA
algorithm is installed in the OLT. The DBA algorithm

OSI reference model

LAN layer OLT

LAN layer ONU

Application

Higher layer

Higher layer

Presentation

SessionTransport

Network

Datalink

Physical

OAM: operation, administrationand maintenance
MPMC: multipoint MAC control

MAC
clientMAC
client

OAM

OAM

•••••

MAC
client

MAC client

OAM

OAM

MPMC
MAC

MAC

MPMC
•••••MAC

MACRSRSGMII

GMIIPCS

PCS

PMA

PMA

PMD

PMD

MDI

MDI
Medium

RS: reconciliation sublayer
GMII: gigabit media independent interface
PCS: physical coding sublayer

PMA: physical medium attachment
PMD: physical medium dependent
MDI: medium-dependent interface

Fig. 1. GE-PON protocol stack.

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Customer’s premises

NTT central officeDownstream
wavelength:1490 nmBroadbandrouterService network A

ONU

Layer-2switch

OLT

Service network B

Upstream
wavelength:1310 nm
Optical splitter
OSS: operation support system

Datacommunication
network

OSS

Fig. 2. System construction.

System OAM function

System OAM function

ONU authentication function

ONU authentication function

Bridge/priority control function

Bridge function

Encryptionfunction

UNI
port
function

DBA
function

Encryptionfunction

IEEE 802.3ah functions

IEEE 802.3ah functions

OAM

OAM

MPCP

MPCP

RS/PCS/PMA

RS/PCS/PMA

PMD

PMD

SNI
port
function

OLT
UNI: user network interface
SNI: service node interface

ONU

Fig. 3. Function stack.

flexibly allocates bandwidth according to the amountof upstream traffic. Therefore, unused bandwidth canbe allocated to other ONUs, allowing the availablebandwidth to be utilized more efficiently. As shownin Fig. 5, it is possible to share bandwidth evenly
among the ONUs using the upstream bandwidth.
We developed a new DBA algorithm specificallyfor the GE-PON system. It is designed to pack variVol. 3 No. 5 May 2005

able-length Ethernet frames efficiently and leads tolower delay and higher bandwidth efficiency. Its features are described below.
(A) Bandwidth control
It is possible to configure the minimum guaranteed bandwidth and the maximum bandwidth ofeach ONU. The bandwidth is distributed to theONUs in proportion to each ONU’s minimum guar53

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Less than 1000 cc

Less than 5 U
1 U = 1.75 inches (high) × 19 inches (wide)

19 inches

ONUOLTBandwidthFig. 4. External appearances of the ONU and OLT.

ONU #4
ONU #3
ONU #2
ONU #1Time
3 ONUs2 ONUs4 ONUsFig. 5. Concept of DBA.

anteed bandwidth though the bandwidth distributed
to a certain ONU must be less than its maximum
bandwidth. When this limitation generates surplus
undistributed bandwidth, it is distributed recursively.
(B) Delay control
It is possible to choose one of two classes, lowdelay or normal delay, as required. The low-delayclass can be used for delay-sensitive services such
as VoIP (voice over Internet protocol) or videocommunication. The low-delay class can achieve
high TCP (transmission control protocol) throughput because it can send TCP ACK (responseacknowledgment) messages back with low delay.
The mechanism of the DBA function is explained
below (see Fig. 6).
(1) Each ONU sends a bandwidth-request message, which indicates the amount of upstream
data stored in the ONU’s transmission buffer.
54(2) The DBA algorithm calculates the allowedtransmission slot consisting of the transmission
start time and transmission duration for eachONU. This calculation is done based on theamount of upstream data stored in ONU transmission buffers.
(3) Each ONU is notified of two allowed transmission slot calculated in step (2) by a transmission-allowed message. One is for sending thenext bandwidth-request message and the otheris for sending upstream data.
(4) In accordance with the allowed transmission
slot notification, the ONU sends a bandwidthrequest message and upstream data that had
been stored in its buffer.
Steps (1), (3), and (4) conform to MPCP, which
was standardized according to IEEE 802.3ah. The
DBA algorithm that we developed operates using
the MPCP framework; however, any DBA algorithms themselves, i.e., step (2), are outside thescope of standardization, so the DBA algorithm is aservice-differentiating element.
4.2 ONU authentication
When the GE-PON system is used for a personal
access service like B-FLET’S, there is a concern thatthe automatic discovery and registration function
could allow access by unauthorized users. Therefore,
an ONU authentication function has been included
to make it impossible for unauthorized ONUs tocommunicate with the service network via the OLT(Fig. 7).
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Letters

ONU

OLT

Allowed transmission notification
Upstream data
(1) Bandwidth request(2)
Bandwidth calculationby the DBA algorithm(3) Allowed transmission
notification(4)

Upstream data

Time

Bandwidth request

Fig. 6. Mechanism that enables DBA.

If authentication succeeds, the user canconnect with the service network.
User networkONUAuthorized user

User network

OLTServicenetwork

ONU

Unauthorized user

If authentication fails, the user is notallowed to connect to the service network.

Fig. 7. Connection to the service network through authentication.

4.3 Encryption
PON downstream transmission is a broadcast, so aframe destined for a certain ONU also reaches otherONUs. To prevent unintended ONUs from eavesdropping on and analyzing the frames destined foranother ONU, the system uses encryption with a different encryption key for each LLID.

4.5 System OAM
IEEE 802.3ah specifies operation, administration,and maintenance (OAM) only for GE-PON datalinks. OAM for the whole commercial system is outof its scope. Therefore, we defined system OAM
functions and managed objects that satisfy our carrier-grade operating demands.

4.4 Priority control
In the ONU, multiple queuing, classified according
to IEEE 802.1D/Q priority, is performed for both theupstream and downstream directions, and high-priority frames are transferred first. This makes it possible
to support delay-sensitive applications such as VoIP.

5. Assuring interoperability

Vol. 3 No. 5 May 2005

Although the various specifications discussed
above give the GE-PON system flexibility and enablesystem differentiation in term of features, they may
lead to interoperability problems among multiple sys55

Letterstems. In particular, functions implemented in hardware like the encryption function will require a common specification, otherwise GE-PON will turn intoa custom-made service operator’s system like ones in
the past, and further cost reductions and service
extensions will be limited. To ensure that the GEPON system can be widely procured, multi-vendor
interoperability between the OLT and ONU is essential. Our system achieves interoperability among theproducts of different system vendors.
6. Future plans
We plan to work on adopting standardized technologies such as IEEE 802.1AE/af, which specifies
encryption and authentication mechanisms, to promote interoperability and the incorporation of ONUs

References
[1]
[2]

[3]
[4]

[5]

http://www.soumu.go.jp/s-news/2005/050126_1.html (in Japanese).
M. Abrams, P. C. Becker, Y. Fujimoto, V. O’Byrne, and D. Piehler,“FTTP Deployments in the United States and Japan—Equipment
Choices and Service Provider Imperatives,” J. Lightwave Tech, Vol.
23, No. 1, pp. 236-246, 2005.
IEEE Std 802.3ah-2004, http://www.ieee802.org/3/ah/index.html
K. Ochiai, T. Tatsuta, Y. Fujimoto, T. Tanaka, O. Yoshihara, N. Oota,
and N. Miki, “Development of a Gigabit Ethernet Passive Optical
Network (GE-PON) System,” NTT Technical Journal, Vol. 17, No. 3,
pp. 75-80, 2005 (in Japanese).
O. Yoshihara, N. Oota, and N. Miki, “Dynamic Bandwidth Allocation
Algorithm for GE-PON,” COIN.MoA1 p. 22, COIN2002, July 2002.

Koji Ochiai

Osamu Yoshihara

Chief, FLET’S Service Division, Broadband
Service Department, Consumer Business Headquarters, NTT East Corporation.
He received the B.E. and M.E. degrees inapplied physics from the University of Tsukuba,Tsukuba, Ibaraki in 1995 and 1997, respectively.
In 1997, he joined NTT Access Network Service
Systems Laboratories, Chiba, Japan and engaged
in R&D of access systems, providing both narrowband TDM services and broadband accessservices, such as the STM-PON system series andthe basic rate interface over synchronized symmetric digital subscriber line (BRI over SSDSL)
system and also engaged in R&D of the GE-PON
system. Since April 2005, he has been engaged inupgrading the operations of the B-FLET’S service
series in his present post.

Chief, Ubiquitous Services Department, NTT
Communications Corporation.
He received the B.E. and M.E. degrees inapplied physics from the University of Tokyo,
Tokyo in 1994 and 1996, respectively. In 1996,
he joined NTT Technology and Development
Support Center in Chiba, Japan, where he wasengaged in the development of the STM-PON
system. In 1997, he moved to NTT Access Network Service Systems Laboratories, Chiba. From
2001, he was engaged in R&D of the GE-PON
system. Since April 2005, he has been engaged indevelopment of access network services in his
present post. He is a member of the Institute ofElectronics, Information and Communication
Engineers (IEICE) of Japan.

Tsutomu Tatsuta
Senior Research Engineer, First Promotion
Project, NTT Access Network Service Systems
Laboratories.
He received the B.E. degree in electronic engineering from the University of Electro-Communications, Chofu, Tokyo in 1991. He joined NTTin 1991. In 1996, he moved to the Network Systems Development Department and worked on
the development of the STM-PON system. Since
2001, he has been engaged in R&D of the GEPON system. He contributed to the standardization of G.984.1 in ITU-T as an editor.

Takashi TanakaSenior Research Engineer, Head, Supervisor,
First Promotion Project, NTT Access Network
Service Systems Laboratories.
He received the B.E. degree in electrical engineering from Shibaura Institute of Technology,
Tokyo in 1986. He joined NTT in 1986. From1987, he was engaged in the development oftransport system equipment. From 1999, he wasengaged in the development of the FLET’S service series providing broadband Internet access.
Since 2002, he has been engaged in joint development of optical access equipment in his present post.

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into consumer products. As a result, we hope thatusers will be able to buy ONUs in ordinary electricalappliance stores and install them themselves. We alsoplan to improve maintainability by considering feedback about current services.

Noriyuki Oota
Optical Access Innovation Project, NTT
Access Network Service Systems Laboratories.
He received the B.E. and M.E. degrees in electrical and communication engineering fromTohoku University, Sendai, Miyagi in 1998 and2000, respectively. In 2000, he joined NTT
Access Network Service Systems Laboratories,Chiba, Japan. Since then, he has been engaged inresearch on a high-speed passive optical networkand systems. He is a member of IEICE and IEEE.

Noriki Miki
Senior Research Engineer, Supervisor, OpticalAccess Innovation Project, NTT Access Network Service Systems Laboratories.
He received the B.E. and M.E. degrees in electronics engineering from Shinshu University,
Nagano in 1982 and 1984, respectively. In 1984,
he joined the Yokosuka Electrical Communication Laboratory, Nippon Telegraph and Telephone Public Corporation (now NTT), Kanagawa, Japan, where he was engaged in R&D ofdigital subscriber loop transmission systems.
Since 1991, he has worked on R&D of fiberoptic access systems. In 1999, he received theIEICE Achievement Award. He is a member ofIEICE and IEEE.

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