The traditional copper telephone wire network still in use today was designed to operate at a bandwidth of up to 4 kHz--the top-end of the voice range. With analog modems, ISDN, and finally xDSL technologies, access carriers have managed to increase the bandwidth over this same network. However, this bandwidth is significantly reduced over distance, and as such, access carriers find themselves unable to deliver the high-speed broadband services to most of their customers using only the existing copper loops. Carriers looking to use the existing coax infrastructure also face the same problem--bandwidth over coax cable degrades over distance.

Due to this distance limitation, many operators are converting to fiber technology. Upgrading to fiber-to-the-premises (FTTP) and Passive Optical Network (PON) devices help access networks meet the demand for more bandwidth and enable such additional revenue generating services as HDTV, Voice-over-IP (VoIP), and high-speed data service to a larger number of customers.

Point-to-Point (P2P) networks, a competing PON methodology where each subscriber has a direct line (or fiber) to the central office (CO), have proven too costly on a large scale due to the complexity of managing all of the connections and the sheer number of transceivers required. PON systems reduce costs for service providers since the number of transceivers decrease due to consolidated aggregation on the trunk side.

PON subscribers are expected to grow dramatically in North America and Asia Pacific, with GPON gaining traction in China and North America, and EPON dominating Japan. European broadband subscribers will largely continue to use DSL, although each region has its own variations.

What is PON?
A PON is a point-to-multipoint FTTP network in which passive optical splitters enable a single optical fiber to serve multiple customers. A PON configuration reduces the fiber and central office equipment required compared to point-to-point architectures.

PON systems broadcast downstream signals on a shared fiber, using encryption to keep data secure. A multiple access protocol, often Time Division Multiplexing (TDM) is used to combine upstream signals. There are three main PON protocols deployed around the world today. While they all use the same fiber technology, there are several key differences. Each of them has its own merits and limitations. The following table summarizes the main attributes of these three PON standards.  

The BPON standard has gone through many revisions since its standardization in 1998. While it is asymmetrical 622 Mbps downstream/155 Mbps upstream data rate does not meet the requirements of triple-play (voice, data, video) applications, it is still prevalent in the market today. The EPON standard was developed as part of the IEEE 802.3 Ethernet-in-the-First-Mile group. EPON uses standard Ethernet frames and symmetric 1 Gbps upstream and downstream rates. Due to increases in symmetric traffic needs for applications such as VoIP, peer-to-peer file sharing, remote collaboration, and email, the need for symmetric downstream and upstream rates are becoming more necessary. EPONs are becoming the technology of choice in many parts of the world.

The GPON standard was introduced to replace BPON. It has better security, faster asymmetrical data rates of 2.4 Gbps downstream/1.2 Gbps upstream, and a choice of Layer 2 protocol (ATM, TDM, or Ethernet).

PON Fiber Access Networks consist of the following components:

• OLT (Optical Line Terminal)--The OLT resides in the Central Office (CO) and transmits and receives optical signals. The OLT converts the incoming traffic from the network into the PON transport layer and distributes it to customer premises ONU devices.
• PON Splitters--Using a PON passive optical splitter, a single trunk fiber from the OLT device at the central office is split and shared by a number of ONUs at the customer premises. Thus, in the downstream direction (from the OLT to the ONUs), the PON is a point-to-multipoint network; while in the upstream direction (from the ONUs to the OLT) it acts as a multipoint-to-point network. The passive splitters used to share the signal with a number of subscribers are limited by splitter attenuation and the working bit rate of the OLT and ONUs.
• ONU (Optical Network Unit)--ONU devices are the interface between the customer equipment and the PON. It receives optical signals from the PON after the PON splitter and converts the signal into electrical signals. On the transmit side, it converts electrical signals into optical signals and transmits the signal back across the PON.
• MDU ONU (Multi-Dwelling Unit ONU)--While standard ONU devices connect fiber directly to each residence or office, a MDU ONU interfaces the fiber network to the existing short runs of copper wire that pre-exist within buildings or across clusters of buildings reducing costs of installation and maintenance.

This method becomes much more difficult in an MDU (Multi-Dwelling Unit). An MDU is an apartment building, condominium, residence hotel, or other building that is subdivided into multiple residences. MDUs house a significant percentage of the population and have considerable implications for local loop deployments, particularly with respect to PONs.

Installing ONTs for each unit in an MDU building requires running a fiber cable to each apartment and installing an individual ONT at each of the units in the building complicating installation and maintenance. This requires the tedious process of obtaining permission from the building owner and from each apartment dweller for the installation of fiber. Once permission is obtained, the physical drilling, drywall removal and replacement, and other construction activities are costly, time-consuming, and disruptive.

Maintenance of this equipment involves the same access problems. Appointments for repair would have to be set up, and the sheer number of ONTs to maintain would grow with the size of the MDU in question, thus increasing maintenance costs over time.

A single fiber cable to the MDU ONU in an apartment building or office complex allows far simpler and less expensive deployment. An MDU ONU device resides in the building and delivers services to individual residences over the existing copper wiring within the building. Copper wiring runs in MDUs are typically short (less than 500 ft), and VDSL2 technology supports up to 100 Mbps of bandwidth over such short distances, enabling triple-play services. The building wiring is still owned and maintained by the building owner and residents. Access carries have a single point of service for maintaining several subscribers through the MDU ONU.

Features of a MDU Device
As the MDU device manages multiple users in an MDU setting, several features become critically important. The device must provision the traffic flow from a user to a service provided by the access carrier. It should allow the providers to monitor, police, and shape the bandwidth of each user depending on the subscribed service. Due to the unique network topology of PONs and its high bandwidth rates, it is essential for a service provider to ensure a scalable and efficient end-to-end Quality of Service (QoS) for its subscribers. The MDU device must provide adequate traffic performance and prioritization guarantees to a customer's voice, video, and data traffic transported over the PON. In addition, to ensure continual service to the users, the device should support 1:1 or 1+1 protection on the uplink connection to the CO.

Conclusion
Service provider subscribers are deploying fiber-based PON networks to enable their higher bandwidth, offering subscribers more broadband internet access. Such deployments are expected to grow dramatically worldwide over the next decade. Most of the newly deployed fiber loop is expected to service multiple subscribers by leveraging the existing copper-based access infrastructure using MDU systems.

About the Author
Bala Velmurugan is product line manager for Access products at Cortina Systems. He received a B.S. degree in Computer Engineering from Madurai Kamaraj University, India and Post Graduate degree in Communications Engineering from Air Force Technical College. He has architected and built QoS mechanisms for many generations of ATM switches and subsequently for the GSR Core Router at Cisco Systems. After Cisco, he joined Azanda Networks; a company specialized in building multi service, traffic management silicon solutions for the Edge and Access network devices. At Azanda he architected as well as successfully marketed the chip to leading equipment vendors such as Huawei, Cisco, Alcatel etc. Azanda was later acquired by Cortina Systems and he is currently focused on managing Cortina's Access product line that includes EPON, GPON and aggregation devices.