PASSIVE OPTICAL NETWORKS (PON)
Access network plays a pivotal role in any telecommunication setup. The backbone has skilled noticeable advancements in recent past, however, a little progress has been made with regards to the access network Recent advancements in the communication field has accelerated the increasing hindrance capacity of access network; however, this issue still remains the holdup area. The PON technology is designed to overcome this bottleneck of the local loop. PON provides a low cost solution to Fiber to the Home (FTTH by serving a single optical subscriber line between several subscribers. This feature has brought about a tremendous revolution in the area of optical communication deployment at the doorstep of end user.
Passive Optical Networks (PONs) are the primary and the most vital group of optical fiber access systems throughout the world in the present era. PONs have now become the choice of people across the globe and have gathered much attention as a prospective minimal operational expenditure OPEX. It also provides large capacity broadband service to every home and business user with FTTH technology. Apart from that, its crystal clear optical connection and low maintenance cost adds further glory to its attractive profile as the ultimate user choice.
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PONs are point-to-multipoint as well as fiber to the premises networks design in which various devices such as splitters and utilizing newer optical principles combine to enable a single OFC to dish up numerous localities. On an average, it can typically serve approximately 32 to 128 subscribers simultaneously. Thanks to the low cost optical splitters that are employed to split the single fiber into individual strands providing access to each customer. The biggest advantage is that these novel optical networks are passive, since apart from CO and subscriber endpoints, there are simply no active element that consume power.
An economical Fiber to the Home (FTTH) has been found feasible by sharing an optical subscriber line amongst multiple subscribers. The proposed configuration of optical access networks is depicted in 1 below. Typically, such optical subscriber line is in a point-to-point (P2P) configuration, to convey aggregate Ethernet Traffic, and ideal for use at places like residential flats/apartment. Large number of end-users subscribe to the Ethernet Switch which is equipped with LAN cable or Very High Bit-rate DSL. As the line termination, a Media Converter (MC) is adopted and a PON system (with no split) is also used so that the equipment at the central office can be shared with individual house application. For the individual houses too, PON (Passive Optical Network) may be adopted where a single optical subscriber line is split by the splitter. PON system requires less space in and consumes lesser electrical power, and its maintenance cost is small. Hence it is the mainstream of the FTTH these days.
Technology of the local loop, the "last mile" of copper wiring connecting businesses to the Public Switched Telephone Network (PSTN), has put significant limitation upon traditional telco data services. ‘Copper wiring' implies that the rates of data delivered to customers are far below than speeds in the core of telco networks. Passive optical networks provide a way of working around this bottleneck and help effectively achieve higher data rates.
As shown above, a PON consists of an optical line terminal (OLT) at the service provider's central office and multiple optical network units (ONUs) near end users. Another positive aspect of a passive optical network is that it minimizes the cost affect incurred upon OFC and other connected equipment in comparison with point to point installations. We can achieve duplex transmission hierarchy by employing Wavelength Division Multiplexing on the same single fiber. (Mukai, Yokotani and Kida n.d.)
Time Division Multiplexing (TDM) is employed for multiplexing the downstream signals. These downstream signals are encrypted between the OLT and the individual ONU. Afterwards, decryption of the specified downstream signals can only be achieved by using the specific ONU only. (Mukai, Yokotani and Kida n.d.).
Contrary to this, the upstream signals are then multiplexed by employing the concept of Time Division Multiple Access (TDMA). In order to avoid collisions, each ONU is scheduled to send its upstream data during specified time interval. After this, OLT performs data scheduling of transmission from the entire ONUs. OLT is acting as master over here, and it allots the free time slot present in the upstream to every ONU by the grant information multiplexed in the downstream signal. This control mechanism is the primary technology for Passive Optical Netwroks.. (Lee, Sorin and Kim 2006)
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B-PON, E-PON and G-PON are a few variants of the PON system. With the deployment of B-PON (Broadband-PON) in 2002, there began a rapid growth of FTTH users. The EPON (Ethernet PON) started in 2005 (especially in Asia) and G-PON (Gigabit PON) deployment in 2006 in North America. (Mukai, Yokotani and Kida n.d.)
Table 1 - Comparison between PON Technologies
ITU-T G.983 Series
ITU-T G.983 Series
ITU-T G.984 Series
The consideration about the service architecture is important in the PON systems. A large variety of optical sources are employed in PONs architectures on the basis of the chosen wavelength plans. Every class of the passive networks posses a peculiar system property by the passive remote node (RN) in order to provide various downstream services. The Ethernet-PON (E-PON) at the RN side employs power splitter that are autonomous of wavelengths in use. Keeping in view the wavelength schedule, arrayed waveguide grating routers (AWG or WGR) are used in WDMPON; it uses the depending on the operating plan. (Hann, et al. 2008)
The main idea behind development of PON architecture is to provide a low cost solution that not only ensures abundance with regards to bandwidth, but it should at the same time provide a seamless connectivity using all the advantages of optical communication systems. The basic theme of PONs employs a single mode fiber that provides a connection between the he optical network units and the line terminal with an optical distribution network (ODN). The TDM-PON hierarchy is also based on this concept and it has achieved maturity over the past few years. Other PON architectures like B-PON, G-PON and E-PON are very popular and they are being deployed on large scale.
A-PON. (Lam 2007)
Full Service Access Network (FSAN) Consortium commenced the work on A-PON and afterwards transferred to ITU-T SG15 as the G.983 standards. APON systems were mostly deployed in North America by RBOCs for their FTTP projects. Many ideas covered in the G.983 standards were carried over to the G.984 G-PON standards.
The original G.983.1 standard published in 1998 defined 155.52-Mbps and 622.08-Mbps data rates. A newer version of the standard published in 2005 added 1244.16-Mbps downstream transmission rate. APON vendors can choose to implement symmetric or asymmetric downstream and upstream transmission rates.
In addition to the one-fiber wavelength diplex solution explained earlier, both the G.983.1 and G.984.1 standards specify a two-fiber solution with dedicated upstream and downstream transmission fibers. The 1:3-mm wavelength is used in both directions in the two-fiber solution. However, to the knowledge of the author, no system has been deployed with the two-fiber solution.
Connections between ONU and OLT are established as ATM virtual circuits in an APON system. ATM services are connection-oriented. Each virtual circuit is identified by a virtual path identifier (VPI) and a virtual channel identifier (VCI) which are embedded in the cells comprising its data flow. VPI and VCI are indices providing different levels of ATM signal multiplexing and switching granularity. Multiple virtual circuits (VCs) can exist within a single virtual path (VP). An ATM connection is identified by its VPI/VCI pair. 2.15 illustrates the idea of ATM signal switching.
Services in APON are mapped to ATM virtual circuits through the ATM adaptation layer (AAL). ATM cells include information cells, signaling cells, OAM cells, unassigned cells, and cells used for cell-rate decoupling . AAL implements different levels of quality of service (QoS). ITU-T G.983.2 includes three different ATM adaptations for APON : AAL-1, AAL-2, and AAL-5. AAL-1 provides adaptation functions for time-sensitive, constant bit-rate, and connection-oriented services such as T1 and E1 circuits. AAL-2 is used for variable-bit-rate connection-oriented services such as streaming audio and video signals and AAL-5 is used for connectionless data services such as TCP/ IP applications.
The OLT and ONU4 as a whole can function as a VP or VC switch. Depending on the implementation, an ONU can cross-connect traffic at the VP level or VC level.
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Gigabit Passive Optical Networks (GPON) provides a capacity boosts in both the total bandwidth and bandwidth efficiency through the use of larger, variable-length packets in PON technology. GPON is standardized in ITU-T G.984 (GPON) that permits several choices of bit rate, but the industry has converged on 2.488 Gbps of downstream bandwidth, and 1.244 Gbps of upstream bandwidth. (htt) GPON Encapsulation Method (GEM) allows very efficient packaging of user traffic, with frame segmentation to allow for higher QoS for delay-sensitive traffic such as voice and video communications. (Networks n.d.)
The beneficial features of GPON based upon requirements set forth by service providers are 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 Mbps, symmetrical 1.25 Gbps, 2.5 Gbps downstream, 1.25 Gbps 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
When comparing various PON systems such as APON, EPON, or GPON, and assuming a similar bit rate of 1.25 Gbps, 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.
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.
Instead of deploying a full "fiber-to-the-curb" build out with its high cost and complexity, a PON connects an optical access switch (OAS) or optical line terminal (OLT) located at the telco central office (CO) using a single strand of fiber-optic cabling to a passive optical splitter or coupler located in the neighborhood of a group of customers. The fiber connecting the CO to the splitter is passive-that is, it has no active components such as repeaters or optical amplifiers. Instead, a high-power laser is used to ensure that signals maintain strength over the trunk length, which is typically limited to 12 miles (19 kilometers). Multiple splitters can be deployed on a single fiber, up to a maximum of 32 splitters, and these may be cond in various ways to create star or ring networks as needed and support both permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). (Tulloch and Tulloch 2002 )
Customers can then be connected to splitters in their neighborhood either by deploying intelligent optical terminals (IOTs) or optical network units (ONUs) located at the customer premises and connecting them to the splitters using fiber-optic cabling (if it has been deployed to the customer premises) or by using existing copper local loop cabling running high-speed Digital Subscriber Line (DSL) technologies. The result is that high-speed data services can be more easily and efficiently provisioned to customers without the need to lay a lot of fiber. (Tulloch and Tulloch 2002 )
PONs multiplex data at the splitters using either time-division multiplexing (TDM) for downstream traffic or time division multiple access (TDMA) for upstream. Two speed configurations are common: 155 megabits per second (Mbps) in both directions or asymmetric 622 Mbps downstream and 155 Mbps upstream. Some faster speeds have been achieved in test bed environments, such as OC-48 PONS running at 2.48 Gbps. (Tulloch and Tulloch 2002 )
Advantages and Disadvantages.
PONs help telcos offer high-speed services to more customers without the cost of building out excessive amounts of neighborhood fiber structure. The downside is that they are shared, rather than dedicated, services, but by overlaying dense wavelength division multiplexing (DWDM) on PONs, telcos can provide users with individual lambdas simulating dedicated links. Such services, however, are likely to be several years away. (Tulloch and Tulloch 2002 )
Several startups have reached market with PON switches, including Quantum Bridge and Terawave Communications. This market is likely to explode in the next few years as real-life PON rollouts accelerate. (Tulloch and Tulloch 2002 )
Downstream signals are broadcast to each premises sharing a single fiber. Encryption is used to prevent eavesdropping. Upstream signals are combined using a multiple access protocol, usually TDMA. The OLTs "range" the ONUs in order to provide time slot assignments for upstream communication. (Tulloch and Tulloch 2002 )
Evolution Paths for PON (Lee, Sorin and Kim 2006)
Currently, the most popular PON systems are based on TDM-PON solutions. These solutions have been commercially available since the early 1990s. One of the strengths of these approaches is that only a single shared transceiver is required at the CO location. Recently, relatively large deployments of EPON have occurred in Japan, and it appears that the BPON/GPON may start seeing significant deployment in North America.
A BPON system shared among 32 users provides a dedicated bandwidth of less than 20 Mb/s in the downstream direction and about 4 Mb/s for the upstream. The later generation EPON solution provides up to about 30 Mb/s per user assuming the same 32-split ratio. The next-generation TDM-PON solution will be GPON that will be capable of about 75 Mb/s [35 Mb/s] for the downstream [upstream] data rate. GPON systems are also capable of servicing 64 users per PON, but this lowers the dedicated data rate to about 37 Mb/s [17 Mb/s] per user. It may be noted that DBA, which can provide a statistical gain, may be used to enhance effective user bandwidths by allocating unused time slots to the busier ONTs , . However, the actual amount of statistical gain may be reduced for continuous data-rate applications such as video and voice streaming.
With the expected bandwidth growth for the impending HD video-centric world, these solutions will eventually need to be upgraded. The following will discuss some possible evolution paths assuming a network initially designed for providing a TDM-PON solution.
One of the most obvious methods for upgrading a TDMPON system is to keep the PON architecture fixed and increase the transmission data rate shared by all the users. The IEEE standards body is already considering this concept by looking into the feasibility of increasing the EPON transmission data rate from 1.25 to about 10 Gb/s . Due to the technical challenges associated with such a high data rate over a PON, it is currently unclear if a cost-effective solution is possible.
Another difficulty with a bandwidth upgrade is that all the users sharing the PON must upgrade their ONTs at the same time. This can be inconvenient for many of the users if only a few of the subscribers on the PON actually need the higher bandwidth. It also results in a loss of service, while the hardware upgrade is being done.
Another upgrade solution would be to add additional wavelength channels in both the upstream and downstream directions. This would require inserting WDM filters at both the CO and ONT locations. These wavelength filters would have to be of high quality to avoid any data-modulation crosstalk from the multiple-broadcast wavelengths. To avoid these crosstalk issues, everyone on the PON would be required to simultaneously upgrade their ONT equipment causing inconvenience to many of the lower data-rate users. It may be possible to overcome this problem by initially including blocking filters into the design of the ONTs in advance of them being needed for the upgrade. This would allow for a wavelength upgrade without affecting the legacy PON users. The new wavelengths need to be chosen so as not to interfere with the wavelengths used in the legacy PON. This approach is currently under discussion in FSAN . With the addition of blocking filters in the ONTs, it may also allow for an easier upgrade to WDM-PON at a later time by allowing the existing feeder fiber to transmit both the TDM and the WDM signals. It should be noted that these preemptive approaches would reduce the current link budget and increase PON costs many years in advance of the need for the wavelength upgrade.
Another possible upgrade path is to reduce the number of users that are sharing a PON. Assuming that 32 subscribers initially share a PON, it might be feasible to reduce the number to 16 or possibly eight users as bandwidth demand increases.
This could provide a data-rate increase of either two or four times. This solution requires installing more feeder fibers and adding more power splitters at the RN locations. It also requires more OLTs at the CO that reduces the benefits of sharing the more costly and higher performance OLT controller and transceiver. This upgrade solution can also become very costly if no additional duct space is available either entering the CO or anywhere along the feed-fiber path. Since reducing the number of users on a PON requires upgrading the OSP, this may not be a desirable solution.
A likely scenario for upgrading a TDM-PON after it can no longer support the bandwidth demand is to convert it over to a WDM-PON system. This can be accomplished by replacing the power splitter at the RN with a wavelength mux/demux such as a 32-port athermal AWG. The OLTs and ONTs would also need to be upgraded, similar to two of the previous upgrade scenarios. Although there would be some disruption during this conversion, the resulting upgrade would result in an essentially futureproof PON that would provide dedicated point-to-point optical connectivity to each subscriber. Since the users sharing the PON would now be uncoupled, future bandwidth upgrades could be done on a case-by-case basis without affecting any of the other users. For service providers who have not yet committed to a TDM-PON solution, there may be advantages to initially install a WDM-PON solution since they could then avoid the complexities and difficulties of the above various upgrade options.
Another advantage of WDM-PONs is that they can allow for increased PON transmission lengths, which may offer significant cost savings in future access networks. These cost savings can be realized by reducing the number of COs between the metro network and the end customer , . The consolidation would also enhance the QoS by reducing the number of hops experienced by the data signals . Although extended ranges are also technically possible using TDMPONs, they may require relatively expensive optical amplification and dispersion compensation
The most widely deployed “broadband” solutions today are Digital Subscriber Line (DSL) and cable modem (CM) networks. Although they are an improvement compared to 56 Kbps dial-up lines, they are unable to provide enough bandwidth for emerging services such as Video-On-Demand (VoD), interactive gaming or two-way video conferencing. A new technology is required; one that is inexpensive, simple, scalable, and capable of delivering bundled voice, data and video services to an end-user over a single network. Passive Optical Networks (PONs), which represent the convergence of low-cost equipment and fiber infrastructure, appear to be the best candidate for the next-generation access network.
It is prudent to architect a PON that is scalable to meet future network expanding needs, connecting a large number of end-users at lower cost per user and delivering elastic bandwidth on-demand. Furthermore, it should be up-gradable without modification to the outside.
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