Over the past decade optical networks have provided the foundation for the growth of the telecommunications market through the supply of ever increasing bandwidth aimed at meeting growing requirements from both voice and data networks. Enormous investments have been made in metro and core networks, but growing bandwidth requirements in the Local Area Network (LAN) have outstripped supply, creating a chasm known as the access bottleneck.

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Figure 1: Typical PON architecture
The access network presents an even greater challenge when high bandwidth requirements are combined with high price sensitivity; legacy solutions based on either SONET/SDH or Ethernet are neither cost-effective nor scalable as an alternative for an optical access solution. This is where Passive Optical Networks (PON) in general--and the emerging GPON technology specifically--come into play by offering a cost-effective, flexible, scalable means of provisioning voice and data services reliably over the access network.
What is a PON?
A PON consists of an optical line terminator (OLT) located at the Central Office (CO) and a set of associated optical network terminals (ONTs) located at the customer premises. Between them lies the optical distribution network (ODN) comprised of fibers and passive splitters or couplers (see Figure 1). The fibers and passive optical splitters eliminate the need for and associated maintenance cost of active electronics in the distribution facility of the access network.
Downstream data is broadcast from the OLT to each ONT, and each ONT processes the data destined to it by matching the address at the protocol header. Upstream traffic is more complicated due to the shared media nature of the ODN; transmissions between each of the ONTs to the OLT must be coordinated to avoid collisions. Upstream data is transmitted according to control mechanisms in the OLT, using a Time Division Multiple Access (TDMA) protocol in which dedicated transmission time slots are granted to each individual ONT. The time slots are then synchronized so that transmission bursts from different ONTs do not collide.
The history of PON
In order to fully understand the various PON alternatives and the merits of GPON as the next leading technology, it is important to first cover the history of PON development over the past few years.
The first PON activity was initiated in the mid-1990s when a group of major network operators established the Full Service Access Networks (FSAN) consortium. The result of this first effort was the 155-/622-Mbit/sec PON system specified in the ITU-T G.983 series of standards. This system has become known as the Broadband PON (BPON) system or the APON (ATM PON) system, as it uses ATM as its bearer protocol.
Realizing the enormous prospect that lies ahead in the optical access market the IEEE established the Ethernet in the First Mile (EFM) group in early 2001. The EFM's (802.3ah) work is concentrated on standardizing a 1.25-Gbit/sec symmetrical system for Ethernet transport only.
Also in 2001, the FSAN group initiated a new effort for standardizing PON networks operating at bit rates above 1 Gbit/sec. Apart from the need to support higher bit rates, the overall protocol has been opened for reconsideration with the aim of efficiently supporting multiple services.
As a result of this latest FSAN effort, a new solution has emerged in the optical access market place, Gigabit PON (GPON), which offers unprecedented high bit rate support while enabling the transport of multiple services--specifically data and TDM--in native formats and with extremely high efficiency.
GPON--The native mode PON
GPON carries the two-fold promise of higher bit rates and higher efficiency when carrying multiple services over the PON. The main features of GPON, based upon requirements set forth by service providers within the FSAN group, can be summarized as follows:
• Full Service Support, including voice (TDM), Ethernet, ATM, leased lines, and others
• Physical reach of at least 20 km with a logical reach support within the protocol of 60 km
• Support for various bit rate options using the same protocol, including symmetrical 622 Mbits/sec, symmetrical 1.25 Gbits/sec, 2.5 Gbits/sec downstream, 1.25 Gbits/sec upstream, and others
• Strong Operations, Administration, Maintenance, and Provisioning (OAM&P) capabilities offering end-to-end service management
• Security at the protocol level for downstream traffic due to the multicast nature of PON
Efficiency and system performance
The most important factor in analyzing a solution's overall cost is the efficiency factor, providing the overall bandwidth that can be sold as services over the system.
When comparing various PON systems such as APON, EPON, or GPON, and assuming a similar bit rate of 1.25 Gbits/sec, it can be safely assumed that the system cost itself will be very similar. A substantial portion of the system cost originates from the optical interface, which is independent of the PON protocol. The rest of the system components should be similarly priced based on application-specific integrated circuits (ASICs) and other standard components.
Assuming similar cost figures for the system itself, efficiency is the single most dominant factor when determining the cost per bit or the amount of "revenue bits" that can be extracted from the network. A 100% efficient network will provide 1.25 Gbits/sec of available throughput, while a 50% efficient network would provide only 622 Mbits/sec of throughput. Thus two systems would be required for the same network configuration--doubling the network operator's cost.

Total bandwidth
Revenue throughput

APON 622 Mbits/sec 72% 448 Mbits/sec
EPON 1.25 Gbits/sec 49% 612 Mbits/sec
GPON 1.25 Gbits/sec, 2.5 Gbit/sec 94% 1.18 Gbits/sec, 2.36 Gbits/sec

Table 1 summarizes the overall PON efficiency for the different protocols, based upon a traffic model analysis performed as part of the FSAN work. Coupling the efficiency figures mentioned above with the different line rates provided by each protocol provides us with the overall available bandwidth for services or customer traffic, which comprises what we've defined as the "revenue bits" available from each system.

Scalability in a multi-service environment
GPON not only provides substantially higher efficiency as a transport network, but also delivers simplicity and superb scalability for future expansion in supporting additional services.
GPON, through the Generic Framing Procedure (GFP)-based adaptation method, offers a clear migration path for adding services onto the PON without disrupting existing equipment or altering the transport layer in any way. In contrast to both APON and EPON--which require a specific adaptation method for each service and the development of new methods for emerging services--the core foundation of GPON is a generic adaptation method, which already covers adaptation schemes for any possible service.
GPON is the most advanced PON protocol in the marketplace today, offering multiple-service support with the richest possible set of OAM&P features. It offers far higher efficiency when compared to ATM- and Ethernet-based PON technologies.
GPON also offers the lowest cost for all modes of operation. Not only is the system cost itself expected to be lower as no external adaptation is required, but exceptionally higher efficiency also leads to more "revenue bits" from the same system, i.e., a much shorter payback period.
Ensuring simplicity and scalability when dealing with new and emerging services, GPON offers a clear migration path for emerging services without any disruption to existing GPON equipment or alterations to the transport layer.

Oren Marmur is founder, chief technology officer, and board member of FlexLight Networks, headquartered in Kennesaw, GA. He may be reached via the company's Web site at www.flexlight-networks.com.

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Figure 2: PON/GPON network topology