PON (Passive Optical Network)
A Passive Optical Network (PON) refers to an optical distribution network (ODN) between the OLT and ONU that contains no active electronic devices. It includes ATM-based Passive Optical Network (APON) and IP-based PON.
APON service development is implemented in phases, with the initial phase primarily offering VP leased line services. Compared with ordinary leased line services, the VP leased line services provided by APON offer lower equipment costs, smaller size, lower power consumption, reliable and stable system performance, and a certain advantage in cost-effectiveness. The second phase implements primary rate and secondary rate circuit emulation services, providing enterprise intranet connectivity as well as enterprise telephone and data services. The third phase implements Ethernet interfaces, providing Internet access services and VLAN services. Subsequently, other services will be gradually expanded to make it a truly full-service access network system. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
APON adopts a cell-based transmission system, allowing multiple users in the access network to share the entire bandwidth. This statistical multiplexing approach enables more efficient utilization of network resources. A key factor in the large-scale deployment of APON is pricing. Currently, first-generation actual APON products have limited service provisioning capabilities and excessively high costs. Their market prospects are uncertain due to the global setback of ATM, but their technical advantages are evident. Particularly when considering overall operation and maintenance costs, deploying a PON system in newly developed areas, highly competitive areas, or areas requiring replacement of old copper cable systems—whether via FTTC or FTTB—is a forward-looking choice. Whether the cost-performance ratio can be improved to a market-acceptable level in the coming years is the key to the survival and development of APON technology.
The upper layer of IP PON is IP, which allows more efficient utilization of network resources, facilitates dynamic bandwidth allocation of the system, and simplifies the complex equipment in the intermediate layer. PON-based OAN does not require the installation of expensive active electronic equipment in external stations, enabling service providers to deliver the required bandwidth to enterprise users cost-effectively.
A Passive Optical Network (PON) is a purely passive medium network that avoids electromagnetic interference and lightning effects on external equipment, reduces the failure rate of lines and external equipment, improves system reliability, and saves maintenance costs. It is a technology long awaited by telecommunications maintenance departments. The advantages of passive optical access networks are specifically reflected in the following aspects:
(1) The passive optical network has a small footprint, simple equipment, low installation and maintenance costs, and relatively low investment.
(2) Passive optical equipment offers flexible networking, with topology structures supporting tree, star, bus, hybrid, and redundant network topologies.
(3) Installation is convenient, with both indoor and outdoor types available. The outdoor type can be directly mounted on walls or placed on "H" poles without the need to lease or construct equipment rooms. In contrast, active systems require optical-electrical and electrical-optical conversion, high equipment manufacturing costs, dedicated sites and equipment rooms, difficult remote power supply solutions, and heavy routine maintenance workloads.
(4) PON is suitable for point-to-multipoint communication, utilizing only passive optical splitters to achieve optical power distribution.
(5) PON is a purely passive medium network that completely avoids electromagnetic interference and lightning effects, making it highly suitable for use in areas with harsh natural conditions.
(6) From a technological development perspective, PON expansion is relatively simple, requiring no equipment modification—only software upgrades. Hardware equipment is purchased once for long-term use, laying the foundation for fiber-to-the-home and ensuring the protection of user investment.
PON is a solution for the lack of bandwidth in the so-called "last mile." Home users have extremely limited options for obtaining fast Internet access (telephone or cable systems). Similarly, enterprises are limited to the performance provided by T1 and T3 carriers, although current wireless, fiber, and satellite services have become more mature. PON provides an alternative solution in the metropolitan area. It is primarily used to solve the problem of broadband end-user access to the central office. Because this access technology requires only passive optical components such as optical fiber and optical splitters between the central office (OLT) and users (ONU) of the access network—without the need to lease equipment rooms or provide power—it is called a passive optical network. It is used for FTTH (Fiber to the Home). Hybrid PON systems extend fiber optic cables to the remote terminal of the communications company, then use copper-based DSL services to reach homes.
In the PON architecture, a single optical line terminal (OLT) can support multiple Passive Optical Network (PON) units. Each unit can form an independent PON network, connecting various types of ONTs through inexpensive wavelength splitters and fiber distribution. The passive design of the access network reduces the need for electronic components, thereby lowering maintenance costs.
PON is a "revived" fiber optic technology originally designed for cable television networks. Recently, it has gained attention as an architecture capable of providing high-speed access in metropolitan areas. PON is now an ITU specification.
Through PON, a single optical fiber extends from the service provider's equipment to a location near residential areas or business centers. "Passive" means the system requires no power or active electronic components between the service provider and the customer. It consists only of optical fiber, splitters, splices, and connectors. A single fiber can serve multiple customers, whereas previous systems required a dedicated fiber for each customer. PON can be used over long distances, making it ideal for rural areas.
Figure P-5 describes the basic PON architecture. The concept is to radiate fiber trunks from the service provider's headend to users. This system has the following components:
· OLT (Optical Line Terminal) — The termination of the PON fiber at the service provider's facility.
· ONT (Optical Network Terminal) — The termination at the user's location.
· OAS (Optical Access Switch) — A switch located at the service provider that aggregates cells/data packets from all users and provides connectivity to the Internet and PSTN.
· POS (Passive Optical Splitter) — Or "splitter," which separates the trunk and optical signals at any point along the path into a multi-point tree topology.
· ONU (Optical Network Unit) — Provides fan-out connections to users. Each PON trunk can support up to 32 splits and 64 ONUs. User connections to the ONU can use coaxial cable, twisted pair, fiber optic cable, or even wireless connections.
· IOT (Intelligent Optical Terminal) — Primarily refers to ONUs designed for business connections. It provides enterprises with multiple voice and data services, very similar to integrated access devices.
PON trunk bandwidth ranges from 155 Mbit/s to 622 Mbit/s. Each split reduces bandwidth, so the bandwidth available to a user depends on the number of splits between the user and the headend equipment. For example, for a 622 Mbit/s trunk, if it is split to support 32 ONUs, users connected to the ONU can obtain a maximum of 19.5 Mbit/s of bandwidth. This bandwidth is shared by all users. To organize communication on this link, many technologies can be employed, including ATM, Ethernet, FDM (Frequency Division Multiplexing), and WDM (Wavelength Division Multiplexing).
The FSAN (Full Service Access Network) consortium made a decision on ATM PON (APON), and APON became the ITU G.983 standard. APON uses well-known technology and provides guaranteed QoS (because ATM cells have a fixed size and ATM-specific QoS protocol functions). APON is a TDM/TDMA technology based on ATM cells. Due to ATM's flexibility in multiplexing different services and adapting to different bandwidths, APON is an ideal long-term solution that combines ATM's multi-service, multi-bit-rate support capability with the transparent broadband transmission capability of passive optical networks. It represents the development direction of future broadband access technology, following the ITU-T G.983 recommendation, with a maximum rate of 622 Mbit/s. Because APON uses ATM encapsulation and transmission technology at Layer 2, it suffers from insufficient bandwidth, technical complexity, high cost, and low efficiency in carrying IP services. To better adapt to IP services, the Ethernet in the First Mile Alliance (EFMA) standardized it in early 2001 through the IEEE 802.3ah working group. Several companies led by Cisco and Corning are promoting the use of Ethernet PON. They argue that Ethernet is more justified than ATM as a choice for PON, because most enterprises use Ethernet connections. Therefore, the EPON technology, which replaces ATM with Ethernet at Layer 2, was proposed. The IEEE formed the "Ethernet in the First Mile Study Group" to evaluate Ethernet PON and other access technologies. EPON can support 1.25 Gbit/s symmetric rates, and the rate can be upgraded to 10 Gbit/s in the future. EPON products have achieved greater commercial deployment.