Brief Introduction:

The standards of GPON and EPON came out almost at the same time.However,the WDM-PON has no standard to come out, so here we only introduce the GPON and EPON and the WDM-PON will be introduced in the next sections(Key Points For PON and Solutions).

From the view of today , they are all the good implementation of FTTx(Fiber-to-the-x, i.e FTTB, FTTH, etc). Fiber-to-the-home (FTTH) is considered an ultimate solution to meet higher bandwidth requirements in access networks. Additionally, the next-generation access (NGA) network based on optical fiber should be passive to reduce the overall cost. Recently, network service providers have begun to deploy time-division-multiplexing passive optical networks (TDM-PONs) such as ethernet PON (E-PON), or gigabit PON (G-PON).They are all the TDM-PON,and these standards have been sufficiently matured to be employed in several countries, such as GPON in North American and EPON in Japan and Korea. Our country has constructure the EPON architecture in some cities. Also, our country pay a lot of attention to the business status of EPON and GPON.

GPON:

The gigabit-capable PON (G-PON) is specified by International elecommunication Union — Telecommunication Standardization Sector (ITU-T) G.984 series.G-PON definition began in the Full Service Access Network (FSAN) consortium in 2001.

The G-PON network architecture supports a two-wavelength WDM scheme for ownstream and upstream digital services (Fig. 1). Additionally, another downstream wavelength is allocated for distribution of analog video service. The network supports up to 60 km reach, with 20 km differential reach between optical network units (ONUs). The split ratio supported by the standard is up to 128. Practical deployments typically would have lower reach and split ratio, limited by the optical budget.

Figure3.1 GPON Physical Network Architecture

Figure3.2 GPON Functional Relationships and Layering

Active transmission equipment in GPON network consists only of Optical Line Termination (OLT) and Optical Network Unit (ONU). Starting at the central office, only one singlemode optical fiber strand runs to a passive optical power splitter near users’ locations. At this point the splitting device simply divides the optical power into N eparate paths to the subscribers. The number of splitting paths can vary from 2 to 64. From the optical splitter, individual single-mode fiber strand run to each user (home, businesses, etc.). The optical fiber transmission span from the central office to the each user can be up to 20 km.

GPON standard defines a lot of different line transmission rates for downstream and upstream direction.

Although all combination are possible (except downstream 1.2 Gbit/s and upstream 2.4 Gbit/s), the most often vendors offers only 1.2 Gbit/s in upstream and 2.4 in downstream direction.

Operating wavelength: The operating wavelength range is 1480-1500 nm for the downstream direction and 1260-1360 for upstream direction. In addition, the wavelength range 1550-1560 nm can be used for downstream RF video distribution.

Forward error correction: Forward Error Correction (FEC) is a mathematical signal-processing technique that encodes data so that errors can be detected and corrected. With FEC, redundant information is transmitted along with the original information. The amount of redundant information is small so FEC doesn’t introduce a lot of overhead. FEC results in an increased link budget by approximately 3-4 dB. Therefore, higher bit rate and longer distance from the OLT to the ONU can be supported, as well as higher number of splits per a single PON tree.

Dynamic bandwidth allocation: Dynamic bandwidth allocation (DBA) is a methodology that allows quick adoption of user’s bandwidth allocation based on current traffic requirements. DBA is controlled by OLT, which allocates bandwidth volume to ONUs. This technique works only in upstream direction, in downstream direction traffic is broadcasted.To determine how much traffic to assign to an ONU, the OLT needs to know the traffic status of the T-CONT associated with the ONU. In status reporting method, as part of its traffic status a T-CONT indicates how many packets are waiting in its buffer. Once the OLT receive this information, it can reapportion the grants to various ONUs accordingly. When an ONU has no information waiting to be transported, upon receiving a grant it sends an idle cell upstream to indicate that its buffer is empty. This informs the OLT that the grants for that T-CONT can be assigned to other T-CONTs. If an ONU has a long queue waiting in its buffer, the OLT can assign multiple T-CONTs to that ONU.

Figure3.3 DBA Process

FUTURE G-PON EXTENSIONS
A few G-PON enhancements are currently in the works. They include the following:
? Definition of wavelength blocking filters. The filters would be supported at G-PON ONUs to ensure that next-generation ONUs using additional wavelengths could in the future be installed on currently deployed G-PON optical data networks (ODNs) side by side with G-PON ONUs.
? Extension of a G-PON’s optical budget to allow deployment of longer reach and higher split ratio. This may require an active extender box to be deployed at the ODN.
? Inclusion of higher data rates. The downstream rate would likely be 10 Gb/s, but the upstream rate is still an open question of 2.5, 5, or 10 Gb/s.

EPON:

In November 2000 IEEE 802.3 announced a call for interest for a new study group called Ethernet in the First Mile (EFM). The group was to extend Ethernet into the subscriber access area. Ethernet over point-to-multipoint (P2MP) fiber (also known as EPON) became one of the focus areas of this group, along with Ethernet over copper, Ethernet over point-to-point (P2P) fiber, and OAM tracks. In September 2001 the IEEE Standards Board approved the EFM Project Authorization Request, resulting in the formation of the P802.3ah task force.

EPON technology provides bidirectional 1 Gb/s links using 1490 nm wavelength for downstream and 1310 nm for upstream, with 1550 nm reserved for future extensions or additional services, such as analog video broadcast. EPON’s rapid adoption was driven by the early decision to define the physical layer specification using relatively minor modifications to inexpensive high-volume 1 Gb/s optical components. This has greatly reduced optics cost to levels comparable to those of continuous mode optics. Using the same philosophy of “define the specification for rapid high-volume deployment,” the EPON upstream burst lock timing was relaxed to use available continuous mode mixed signal components. The downside is somewhat lower upstream utilization, but since other access technologies are far more asymmetric, this slight difference was deemed minor.

EPON’s Ethernet roots are unmistakable. EPON traffic uses the same Ethernet packet format, with standard IPG, as found in any enterprise switch. For that matter, EPON uses the same MAC found in any IEEE 802.3-compliant device. The new P2MP connectivity is supported by a protocol called Multipoint Control Protocol (MPCP), which uses standard Ethernet packets generated in the MAC control sublayer.

EPON does not use encapsulating framing in either the upstream or downstream direction; instead, the content of the Ethernet preamble is modified. An upstream burst is simply a sequence of Ethernet packets with regular IPG between them, preceded by a longer sequence of IDLE codes used for receiver synchronization. Any management or control information is delivered in normal Ethernet frames.

Figure3.4 EPON Downstream Operation

Figure3.5 EPON Upstream Operation

All time-driven events are synchronized to the PON clock, a 16 ns resolution counter that is carried in all MPCP messages. The ONU uses the received timestamp to lock to the OLT time base. The OLT uses returned timestamps to measure ONU round-trip delay and schedule collision-free upstream transmissions.

EPON’s packet preamble contains additional fields not found in packets sent over P2P Ethernet links. In downstream transmission the logical link ID (LLID) field defines the destination ONU. An ONU filters the received frames based on the LLID in the frame’s preamble and its own unique LLID value assigned by the OLT (Fig. 5a). A special value is reserved for broadcast messages sent to all ONUs. In upstream transmissions the LLID field marks the source ONU (Fig. 5b). A cyclic redundancy check (CRC) field validates preamble integrity. Most ONU equipment registers as a single ONU and uses a single LLID for data transport. However, some equipment registers as multiple virtual ONUs, thereby establishing multiple LLIDs. This allows EPON to access the same traffic granularity on the PON as G-PON.

When a physical ONU registers as multiple virtual ONUs, the OLT treats each virtual ONU as a separate ONU. Correspondingly, the OLT grants each virtual ONU separately, including repeated allocation of the optical overhead. The OLT also maintains a separate management channel to each virtual ONU, and has to identify the SLA allocated to each virtual ONU.

EPON uses a frame-based FEC mechanism based on the RS(255,239) algorithm. Each frame is encoded separately, and all per-frame parity bytes are added at the end of the frame. This approach allows ONUs without FEC capabilities to receive FEC-encoded frames, ignoring the appended parity data. FEC can be selectively activated per ONU.

Although not defined in the IEEE 802.3ah specification, all EPON implementations incorporate encryption. Encryption is AES-based with the exception of a special algorithm defined by the major carrier in China for its network.

EPON MANAGEMENT LAYER: OAM functionality is another important EPON breakthrough. Ethernet now includes link layer management that enables OLTs to remotely manage attached ONUs. OAM is established after the discovery process and is maintained by periodic message transmission. Information about remote failures is conveyed using flags in OAM messages to indicate failure status. The remote ONU can be instructed to return incoming packets as part of the remote loopback functionality. Link monitoring, where any Ethernet variable of the remote port can be retrieved by the OLT, is arguably the most useful EPON OAM function. OAM link information can be extended beyond the OLT by placing a Simple Network Management Protocol (SNMP) agent at the OLT. A soon to be finalized RFC, “Managed Objects of EPON,” details the EPON MIBs. An EPON CPE device contains much more than a MAC. OAM includes vendor extension mechanisms to provide a convenient and lightweight method to manage the additional functionality. This can lead to differing OAM variants as carriers customize their products.

FUTURE EPON EXTENSIONS
A significant EPON enhancement to run at higher speed has begun. The IEEE has formed the P802.3av task force to consider the definition of an EPON PHY that operates at 10 Gb/s downstream and 1 or 10 Gb/s upstream. This enhancement would provide a significant capacity increase for TDM PON systems.

 

SIMILARITIES AND DIFFERENCES

Similarities
PON systems in general allow for longer distances between central offices and customer premises, since it can operate at distances of up to 20 km, which considerably exceeds the maximum coverage afforded by DSL. This way, the fiber deployment in both local exchange and local loop is minimized, since only one strand of fiber is needed in the trunk and only one port per PON is required in the central office, which allows for a very dense CO equipment (smaller footprint) and low power consumption.

PON systems provide higher bandwidth due to deeper fiber penetration. While the fiber-to-the-building (FTTB), fiber-to-the-home (FTTH), or even fiber-to-the-PC (FTTPC) solutions have the ultimate goal of fiber reaching all the way to customer premises, fiber-to-the-curb (FTTC) seems to be most economical today, at least in the cases where the customer population is dispersed and there is an existing in-field copper-based (more likely DSL) distribution network installed.

Additionally, PON systems allow for inherent downstream video broadcasting (currently, in the form of an analog video overlay service, in the future - most likely delivered as IP Video), due to their underlying Point- To-Multipoint structure. Multiple wavelength overlay channels can be added to PON without any modifications in the terminating ONU modules, thus adding flexibility and extensibility for once deployed fibre plant.

PON eliminates the necessity of installing multiplexers / demultiplexers in the splitting locations, thus relieving network operators from the task of maintaining and providing power to them. Additionally, PON allows simplified upgrades to higher bit rates, additional wavelengths and/or advanced modulation formats. Passive splitters and combiners provide complete path transparency both in terms of data rate and modulation formats.

Both EPON and GPON systems operate in the upstream channel in the so-called burst mode, where the transmissions (slots) from the two different ONUs are TDM multiplexed and have a dead zone (guard band) between them to allow for turning the laser off and on between subsequent transmissions, adjust power levels (EPON), align to the incoming data stream, and retrieve clock (both EPON and GPON).

In terms of security, all xPON systems suffer from the same set of problems related with the P2M system architecture due to the underlying physical infrastructure (see Section 4.2 for details). Dynamic Bandwidth Allocation mechanisms are standardized (along with security mechanisms) by respective responsible international bodies (IEEE for EPONs and ITU for GPONs).

Differences
EPON systems relay the Ethernet encapsulated data arriving at the LAN subscriber port of the ONU or the MAN/WAN Ethernet port connected to the OLT. Thus, EPON is a natural extension of the LAN systems and bridges the gap between the LAN and Ethernet based MAN/WAN structures, which have witnessed increasing proliferation in recent years. GPON on the other hand uses legacy ATM or novel GEM encapsulation mechanisms to relay any data streams which are delivered at the subscriber / OLT port. GPON strips the incoming Ethernet frames (99.500 of subscriber ports are Ethernet based, nowadays) from preambles and encapsulates them in the GEM frames, in which format they are delivered to the OLT module for further
processing. The incoming data frames are non fragmentable (rules of Ethernet) in EPON systems, while GPON has the ability to fragment and assemble frame fragments, providing that such a need arises, adding some computational complexity.

In terms of the data rate, EPON operates at 1.25 Gbit/s data rate (effective data rate 1.0 Gbps) in accordance with the IEEE 802.3ah standard, but non-compliant systems are available (so-called Turbo EPON) with the downstream data rate of 2.5 Gbit/s. GPON operates at 1.25 Gbit/s and 2.5 Gbit/s in the downstream direction (from Central Office to customer) and 155 Mbit/s, 622 Mbit/s, 1.5 Gbit/s and 2.5 Gbit/s in the upstream direction (from customer to Central Office). GPON is backward compatible with the APON and BPON systems, and can transport their frames natively. Comparable system set-ups (16 ONUs, 1 logical entity per ONU, similar data rate - 1.25 Gbit/s for EPON and GPON) produce 9500 efficiency for GPON and 89% efficiency for EPON, when measured relatively to the pure subscriber payload.

EPON hardware parameters are very relaxed, and thus low grade optical and electronic components can be applied, minimizing the CAPEX for SPs when deploying the active equipment modules. GPON requires shorter guard bands and faster electronics (laser drivers, etc.), which significantly increase the technical challenge level and boost the equipment cost. The hardware layer parameters determine the resulting system price. EPON systems, due to their lowered hardware requirements, are more cost effective (commonly quoted costs for SPs include 80 USD per ONU, 450 USD per OLT port). GPON systems, due to their more strict hardware requirements, are more expensive (ONU costs are 2 - 3 times higher and OLT port cost is roughly 5 - 10 times higher than for EPON).

GPON deployments are mainly in the trial phase - no real commercial deployments have been reported up to this moment (04.2007), while EPON had approximately 8 million subscriber ports and 16 million CO port capacity deployed, by the end of March 2007. The growth rate is of the order of 3 to 4 million subscriber ports per 6 months, occurring mainly in the Asian market (Japan, Korea, China, etc.). Small deployments are present in the USA (mainly cable operators) and South America.

 

NEXT-GENERATION PON SOLUTIONS

Historically, data rates associated with broadband consumer service offerings have increased at a rate of approximately 1.3 times/year. This growth has been driven by services such as convergent subscription television and the Internet, high-definition television, digital photography and video, new models for content production, distribution, and marketing, possible re-emergence of thin client computing, and so on. Projecting this trend into the future, in the long term we will face bandwidth demands beyond current G-PON capabilities, requiring R&D in this field already.

Different groups around the world have recently started to address this topic. Both FSAN and IEEE are now discussing ways how to extend their standards to 10 Gb/s line rates. Several research projects around next-generation PON (NG-PON) are investigating the topic on a wider scope, for example, the European PIEMAN and MUSE II projects in which different hybrid network solutions are evaluated that combine the classical TDM/time-division multiple access (TDMA) PON with WDM channel allocations as well as with optical amplification and transparent long-haul feeder transport.

Expecting large-scale deployments of GPON systems to start soon, network operators and system vendors are seeking NG-PON solutions that can coexist with G-PON on the same fiber plant and enable gradual network capacity upgrades. At the same time, it is highly required to keep the fiber plant as transparent as possible while moving to NG-PON in order not to block further evolution paths. The time consuming and costly deployment of optical fibers, especially in the distribution plant and drop sections, must remain in place for decades without needing modifications or replacements.

 

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