Optical Network Unit for ATM-Based Passive Optical Networks

During the last year, the world witnessed the rising of a great diversity of new broadband services, including electronic commerce, video on demand, high-speed Internet, and videoconferencing, among others. More and more bandwidth is needed in a communications network in order to be able to provide these new services.

The main problem in achieving this objective appears in the final portion of a communications network, the access network, which offers the services to the final user. Here, in the access network, a bottleneck exists due to the fact that the bandwidth available to the user is limited by the copper wire-based technology these networks commonly use.

This lack of available bandwidth to deliver new broadband services handicaps service operators, and various alternative technologies have been proposed in order to solve this problem. For example, the traditional telephony companies, in an attempt to reuse their already installed copper lines, have proposed the x-Digital Subscriber Line (xDSL) technology which uses new modulation schemes to pack data onto the copper wires. However, this solution can only be viewed as an intermediate step, mainly due to the fact that the bandwidth with xDSL still isn't very high (between 128 Kbps and 1.5 Mbps). Hybrid Fiber-Coaxial (HFC) networks also deserve a special mention. These types of networks employ fiber-optic technology in the backbone network, where prices are shared between a lot of subscribers, and then use coax cable to reach the customer premises due to its more attainable cost. But again, the maximum bandwidth is somewhat limited, and this solution isn't considered as a final one.

In the open battle between alternative technologies for the access market, fiber optic cable appears as a strong candidate to triumph, mainly due to its unlimited bandwidth. In a Fiber to the Home (FTTH) environment, extending fiber all the way to the home or office isn't the most cost-effective solution. A more sensible approach is to share the fiber between a number of users—then the service operator can amortize the cost of equipment and infrastructure between those users. In this scenario, Passive Optical Networks (PONs) are probably the most attractive fiber-communications infrastructures. These networks show a point-to-multi-point topology and an important characteristic is that there isn't any active component that requires powering in the outside plant.

PONs were first envisioned by the Full Services Access Network (FSAN) Society in 1995. The objective of FSAN was to define a common optical-access system that was cost-effective and supported full service of voice, video, and data. These early PONs were based on the Asynchronous Transfer Mode (ATM) protocol, due to ATM's capacity to integrate different types of traffic with quality of service (QoS) guarantees. In 1998, ITU-T adopted FSAN's initiative in its G.983.1 recommendation. This article introduces the main guidelines for the design of the customer premise equipment, named the Optical Network Unit (ONU), required in a PON architecture. The design will provide full service of voice, video and data, and will take into consideration the G.983.1 recommendation of ITU-T regarding ATM-based PONs (APONs).

In the last year, the strong growth of Internet Protocol (IP) data traffic has led to some sectors claiming that ATM is not appropriate for the access network. Transporting that type of traffic in ATM requires the segmentation of variable-length (maximum of 1518-byte) data packets into fixed-length (53-byte) ATM cells, which causes a considerable delay in the communication process. For this reason, IEEE is currently developing a new standard relative to Ethernet-based PONs (EPONs). A main differentiator of these new PONs is the use of the well-known Ethernet protocol in its Gigabit Ethernet version. In addition to its higher available bandwidth (1 Gbps vs. 622 Mbps for ATM), Ethernet is one of the most widely used Local Area Network (LAN) protocols in the world. If Ethernet were used in the access network, it would eliminate the conversion between protocols required in ATM-based networks. This article introduces the main characteristics of the new standard, and makes a comparison to APONs to reach a conclusion about which of both standards is more appropriate for use in the access network.

Passive Optical Networks aim to solve the existing access-network bottleneck by bringing the fiber closer to users and by taking advantage of the unlimited bandwidth of the fiber in order to provide a complete range of present and future broadband services.

A PON is a point-to-multi-point optical-access network that enables the service provider to share one optical fiber between a number of users (typically 32). The operator can amortize the cost of equipment and infrastructure between those users and, also, can assign bandwidth in a dynamic manner between the subscribers, leading to higher system efficiency. The reference configuration for a PON is shown in Figure 1. The system consists of an Optical Line Termination (OLT) located in the operator's central office, the Optical Network Units (ONUs), located at the customer premises for FTTH connections, and the Optical Distribution Network (ODN) between the OLT and ONUs. The OLT is connected by a single fiber to an optical power splitter, which supplies the optical signal to several ONUs.

Figure 1:  Reference configuration for an ITU-T G.983.1-based PON

The main feature of a PON is that, in the ODN, between the OLT and the ONUs, there do not exist any active elements. Only passive optical elements such as optical connectors, single-mode optical fibers, attenuators, and splitters, are used at this point, reducing cost relative to maintenance and power. Another advantage is that a PON can be easily upgraded only by changing electronics at both extremes of the network, while the infrastructure remains the same. In this way, you have a highly flexible system in the sense that a broad range of future services would be able to be easily added.

In the ODN, simultaneous transmission on the same fiber is enabled by using different wavelengths for each direction: 1550 nm for the downstream (from the OLT to the ONUs), and 1310 nm for the upstream (from the ONUs to the OLT). In an ATM cell-based PON, the downstream signal is broadcast to all the ONUs, as shown in Figure 2a. Each ONU discards or accepts the incoming cells depending on the cell header addressing. Encryption is necessary to maintain privacy, since the downstream signal is broadcast and each ONU receives all the information. In the upstream direction, the system uses a Time Division Multiple Access (TDMA) protocol to accede the bandwidth (Figure 2b). The OLT controls the transmissions from each ONU by sending grants or permissions to them. In order to avoid collisions between transmissions from different ONUs a technique, ranging, is executed to measure the logical distance between the ONUs and the OLT. As a result, each ONU adjusts its transmission time properly and, thus, the effects of propagation delays are overcome.

Figure 2:  (a) In an ATM cell-based PON, the downstream signal is broadcast to all ONUs while (b) a TDMA protocol is used in the upstream

Nowadays, nearly all of the most important communications companies in the world offer well-equipped OLTs. In this work, the main objective has been the design of an ONU for an FTTH connection, whose features were specially adapted for the planned requirements (basically the desired services to be provided). Also, the work includes the possibility of reusing such an ONU, with only minor modifications, in a Fiber-to-the-Cabinet (FTTCab) connection. In this particular case, the fiber, using a PON architecture, only reaches the cabinet, and the modified ONU, ONU-Headend, would be located at this point. From the cabinet to the customer premises, Power Line Communications (PLC) technology would be used to take advantage of the high penetration of electrical networks. This last case is currently under development.

At this point, it's interesting to review what solution has been adopted in the ODN to provide the one-way analog video service. Such a service could be provided in an integrated manner with the rest of services, in other words, using the ATM protocol to carry it to the ONUs and sharing the available bandwidth in the downstream direction. However, a Wavelength Division Multiplexing (WDM) technique is used to multiplex the optical signal that transports information relative to voice and data with the one carrying information relative to video. Compared with the first method, this technique allows you to take advantage of the downstream bandwidth. This enables the possible evolution of a HFC network to an all-optical network in an easy fashion, due to the fact that, in a HFC network, the video signal is sent over the network in a non-integrated manner with respect the other services.

As shown in Figure 3, the design of the ONU has been organized as three main blocks with well-defined functions. The first one, the ODN Interface, basically performs the required optical-electrical conversion between the ODN and the customer's network. The APON Transport System has a special importance, since it's responsible for controlling all the management and operation functions required in the APON system, according to ITU-T G.983.1. Finally, the ATM Core will adapt into ATM cells the voice and data traffic coming from the user's network, in order to carry it to the OLT. Figure 3 shows the relation between the different blocks and also the interfaces provided to the user: one for POTS, two for high-speed data, and one for analog video.

Figure 3:  Diagram of the ONU functional blocks

Figure 4a shows the main modules that make up the ODN-interface block. First, we propose a Triplexer module. The Triplexer integrates a WDM filter, optical transceiver, and optical receiver to capture the broadcast analog video (Figure 4b). In the downstream direction, two multiplexed optical signals are received carrying the information relative to voice-data (at l₁), and analog video (at l_VIDEO), as explained at the beginning of this section. The WDM filter will extract and separate both signals in order to guide them to the proper receivers. As a result, the analog video signal is recovered and directly offered to the user; meanwhile, the ATM cells transporting voice, data, and management traffic are passed to the APON Transport System in order to continue with their processing.

In the upstream direction, this filter will only insert the optical signal carrying information relative to voice and data (at l₂) in the optical fiber. In addition to the Triplexer, the ODN Interface comprises the typical modules of an optical/electrical interface. Typically, this would consist of a laser driver to control the average power sent to the optical fiber by the optical transceiver's laser, an amplifier to recover the downstream signal with sufficient amplitude, and a clock and data-recovery unit.

In order to get this ODN Interface, all the selected components must be fully compliant with the physical medium-dependent requirements of an ITU-T based APON.

Figure 4:  (a) The ODN Interface comprises the typical modules of an O/E interface. (b) The optical transmitter, receivers, and the WDM filter are integrated in the Triplexer module.

Figure 5 shows the main functional modules that comprise the APON Transport System block. In receiving, you need to define the limits of each ATM cell to process it correctly. This is the function of the Cell Delineation module. Once ATM cell delineation is complete, and before extracting PLOAM cells from the rest of received cells, the ONU acquires the synchronization on the downstream frame defined in ITU-T G.983.1. After that step, and taking into account the header of each cell, PLOAM cells can be identified and guided to the module responsible of their processing.

The rest of ATM cells are passed to the module responsible for verification of the cells' Header Error Control (HEC) code in order to detect incorrect or idle cells. In all other cases, the cell is considered to be correct and, after executing Dechurning (the opposite of the encryption done in the OLT), the cell is passed through a Universal Test and Operations PHY Interface for the ATM (UTOPIA) interface to the ONU block ATM Core. Here, information carried in the cell (voice or data) will be extracted and sent to the proper interface.

In general, the main objective of the intermediate modules (Figure 5) relative to the treatment of PLOAM cells is the processing of the information carried by these types of cells. The information consists mainly of grants to control the transmission from the ONU and messages relative to ranging, as previously discussed. As a result, the proper actions are executed; for instance, the generation of messages to be included in the upstream PLOAM cells, and the control of the transmission time, taking into account the delay time obtained with the ranging function. For transmission, the main function is correct inclusion of the generated PLOAM cells into the flow of ATM cells to be transmitted to the OLT, which have been received through the UTOPIA interface from the ATM Core.

We have proposed this APON Transport System, fully compliant with ITU-T G.983.1, for implementation via an ASIC. Nevertheless, due to the high cost associated with ASIC fabrication, we will use an FPGA to get and probe the first prototypes of the ONU.

The ATM Core is the last block that supports the proposed ONU. Briefly, its primary purpose is the adaptation into ATM cells of voice and data traffic coming from the user's network in order to transmit a continuous flow of ATM cells to the OLT. To perform this function, the ATM Core includes, in addition to interfaces to the user's telephone line and data network, several operations. These operations are defined for the ATM Adaptation Layers (AALs) Type one (for voice), Type five (for data), along with the ATM Layer, according to the ATM protocol stack, as shown in Figure 6.

Figure 6:  Organization of the ATM core. The purpose of this block is to obtain a continuous flow of ATM cells carrying the user's voice and data traffic.

Figure 7 shows the main functional modules of this ATM Core block. The main function of the Telephonic Interface is to obtain a flow of 64 Kbps of Pulse Code Modulation (PCM)-coded voice. This PCM flow is passed to the AAL1 Unit responsible for the adaptation of this type of traffic into ATM cells. Voice traffic has some special properties, and so requires some special requirements to be satisfied in order to be transmitted in a proper manner. For instance, the traffic has a constant bit rate and requires the exchange of some kind of information that enables synchronization between both extremes of the network, in other words, the ONU and the OLT.

Data traffic is received from two Ethernet interfaces running at 10 or 100 Mbps. The module Ethernet Interface and Switch receives the data packets and decides if these packets are local to the user's network; if they aren't, the variable-length data packets are sent to the AAL5 Unit, which adapts this variable bit-rate traffic into ATM cells.

Figure 7:  Functional blocks of ATM core

At this point, we have two types of ATM cells with different requirements, depending on the traffic they transport. In order to get a continuous flow of cells, you must do a multiplexing of both cells. This function is performed in the selected AAL1 Unit, in which a higher priority is given to cells carrying information relative to voice due to the fact that jitter and delay requirements are stronger for this type of traffic. Finally, the flow of ATM cells is passed through the UTOPIA Interface to the APON Transport System block, in which PLOAM cells, locally generated in the ONU, are inserted as previously discussed.

In this section, we introduce the new standard on Ethernet-based PONs (EPONs) which have been under development since 2000 by the IEEE 802.3 "Ethernet in the First Mile (EFM)" study group. The main difference between these new PONs and ATM-based PONs is the use of the well-known Ethernet protocol, in its Gigabit Ethernet version, to carry the information between OLT and ONUs. The topology and principles of the EPON system's performance (ranging, upstream TDMA protocol controlled by the OLT, and so on) remains the same.

Advocates of EPON claim that ATM is not appropriate for use in the access network because of the following arguments. In recent years, IP data traffic has become very important to the service providers due to its unprecedented growth. Transporting this type of traffic in ATM is quite inefficient, since it requires the segmentation of variable-length (up to a maximum of 1518-byte) data packets into fixed-length (53-byte) ATM cells. This excessive segmentation causes a considerable delay in the communication process. By contrast, Ethernet is tailor-made for carrying IP data traffic. In addition, the available bandwidth in APON (a maximum of 622 Mbps) is smaller than the bandwidth available in EPON (1.25 Gbps). Finally, Ethernet is a widely used LAN protocol all over the world. If Ethernet were used in the access network, it would be unnecessary to convert between protocols as required in ATM.

The final version of the new standard has been forecast to be approved in 2003. It would be different than the rest of already existing 802.3 standards, since the new functionality relative to PON performance must be included (inter-exchange of messages and data between OLT and ONUs, control of the laser's transmission from the ONU, and so on). Also, using Ethernet in an access network requires the definition of new physical parameters in order to support the desire distance between both extremes of the network (typically 20 kilometers).

Once the new standard has been approved, EPONs will become an important rival to APON systems because of EPON's numerous advantages. Obviously, this type of network will be particularly useful in communications environments in which nearly all the traffic between OLT and ONUs comprises data. The successful adoption of EPONs in place of APONs will depend on the development of technologies such as video over IP or voice over IP (VoIP). Meanwhile, APONs are more appropriate for use in environments offering an integrated service of voice, video and data to subscribers, mainly due to ATM's capability of providing a high degree of QoS.

Conclusions
Passive Optical Networks, either APONs or EPONs, are among the most attractive options to solve the existing bottleneck in the access network. Compared to other solutions, PONs offer more available bandwidth to provide a broad range of services, and more reliability due to the use of optical fiber. We have introduced the main guidelines for the design of the customer premise equipment, the ONU. We have also estimated the costs required to get 100 units of the first prototype of the ONU—that estimation has shown great promise. Finally, the present work contributes to the standardization process of the new EPONs by outlining their main attributes as an alternative to APONs.