Hello, everyone!
Today, I would like to share with you an article about Access optical network.
Access networks connect business and residential premises to metropolitan area networks (MANs) or wide area networks (WANs). Because of the explosive growth of broadband multimedia applications, such as video streaming, high-definition television (HDTV), video on demand, and interactive games, a huge demand for bandwidth has been imposed on the access infrastructure. As DWDM technology was developed for the long haul network and gigabit Ethernet for the local area network (LAN), access networks tend to be the bottle neck for end-to-end broadband applications.
Today, the two most popular access network solutions are digital subscriber loop (xDSL) technologies deployed by telephone companies, and cable modems from cable companies. These access technologies do not have comparable bandwidth capability with Gigabit Ethernet and have limitations in providing high quality integrated services, including video, voice, and data. Unlike metro and long-haul networks, access networks must serve a more diverse and cost sensitive customer base.
End users may range from individual homes, to corporate premises, to hotels, and services must therefore be provisioned accordingly. Data, voice, and video must be offered over the same high-speed connection with guarantees on quality of service (QoS), and the ability to upgrade bandwidth and purchase content on a needed basis. Therefore, the next generation access solutions have to be cost efficient when providing more bandwidth.
In the so-called FTTx access networks, optical fiber replaces copper in the distribution network. For example, in fiber to the curb (FTTC) or home (FTTH), the capacity of access networks is sufficiently increased to provide broadband services to subscribers.
Because of the cost sensitivity of access networks, passive optical networks (PONs) are considered to be the most promising technology as they can provide reliable yet integrated data, voice, and video services to end users at bandwidths far exceeding current access technologies. Unlike other access networks, PONs are point to multipoint networks capable of transmitting over 20 kilometers of single mode fiber.
PONs can offer symmetrical data transmission on both the upstream and downstream links, allowing the end user to provide Internet services such as music file sharing and Web hosting. In addition to providing a good alternative, PONs represent an excellent evolutionary path for current access technologies such as cable and DSL. By using passive components (such as optical splitters and couplers) and eliminating regenerators and active equipment normally used in fiber networks, PONs reduce the installation and maintenance costs of fiber as well as connector termination space. These costs still require laying fiber, which makes PONs more expensive to install. However, since fiber is not bandwidth limited but loss limited (as opposed to copper wires, cable, and wireless), the potential performance gains and long-term prospects make PONs well-suited for new neighborhoods or installations.
PONs typically fall under 2 groups:
ATM PON (APON) and Gigabit PON (GPON) from Full Service Access Network (FSAN)
International Telecommunication Union - Telecommunication Standardisation Sector (ITU-T); and Ethernet PON (EPON) from IEEE 802.11ah Ethernet in the first mile (EFM) working group.
In the APON and BPON specifications, ATM is used as the native protocol data unit (PDU), which implies protocol conversion between Ethernet and ATM is needed. EPON combines low-cost Ethernet equipment and low cost passive optical components, and has therefore attracted more attention in recent years.
Fig. 1. Distributing optical backbone connectivity to enterprises and homes.
Current access technologies represent a significant bottleneck in bandwidth and service quality between a high-speed residential/enterprise network and a largely overbuilt core backbone network. Backbone networks are provisioned for operation under worst-case scenarios of link failures, and thus backbone links are lightly loaded most of the time. In addition, high capacity routers and ultra-high capacity fiber links have created a true broadband architecture.
However, large backbones are not the whole the equation; distribution of that connectivity to individual enterprises and homes is just as critical for meeting the huge demand for more bandwidth as potrayed in the above figure. Unfortunately, the cost of deploying true broadband access networks with current technologies remains prohibitive. This in turn makes it difficult to support end-to-end Quality of Service (QoS) for a wide variety of applications, particularly non-elastic applications such as voice, video, and multimedia that cannot tolerate variable or excessive delay or data loss.
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