Passive optical network-PON is a network that brings optical fiber singal to the end of users by the point-to-multipoint(P2MP) fiber to the premises in which optical splitters are used to “broadcast” signals to many users. A PON consists of an optical line terminal (OLT) at the service provider’s central office and a number of optical network units (ONUs) near end users. PONs also are called fiber to the home (FTTH) networks. Using PON system can reduce. The cost of the system substantially by sharing one set of electronics and an expensive laser with up to 32 homes. The main disadvantage is a shorter range of coverage limited by signal strength. While an active optical network (AON) can cover a range to about 100 km (62 miles), a PON is typically limited to fiber cable runs of up to 20 km (12 miles).
The first PON systems that achieved significant commercial deployment had an electrical layer built on Asynchronous Transfer Mode (ATM, or “cell switching”) and were called “APON.” These are still being used today. APON systems typically have downstream capacity of 155 Mbps or 622 Mbps, with the latter now the most common. Upstream transmission is in the form of cell bursts at 155 Mbps.
BPON, or broadband PON, was the most popular current PON application in the beginning. BPON uses ATM as the protocol. ATM is widely used for telephone networks and the methods of transporting all data types (voice, Internet, video, etc.) are well known. BPON digital signals operate at ATM rates of 155, 622 and 1244 Mb/s.
GPON, or gigabit-capable PON, uses an IP-based protocol and either ATM or GEM (GPON encapsulation method) encoding. which has a variety of speed options ranging from 622 Mbps symmetrical (the same upstream/downstream capacity) to 2.5 Gbps downstream and 1.25 Gbps upstream. From GPON, the future could take two branches: 1) 10 GPON would increase the speed of a single electrical broadband feed to 10G; and 2) WDM-PON would use wavelength-division multiplexing (WDM) to split each signal into 32 branches.
EPON or Ethernet PON is based on the IEEE standard for Ethernet in the First Mile. EPON 802.3ah specifies a similar passive network with a range of up to 20 km. It uses WDM with the same optical frequencies as GPON and TDMA. The raw line data rate is 1.25 Gbits/s in both the downstream and upstream directions. EPON is widely deployed in Asia. The system architecture is the same as GPON but data protocols are differenet.
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