A Passive Optical Network (PON) is a group of technologies originally created by the Full Service Access Network (FSAN) working group and now standards of ITU-T and IEEE, allowing fiber as the first mile (or last mile) to the customer premises. A PON consists of a central office node Optical Line Termination (OLT) at the service providers office and a number of Optical Network Units (ONUs) near end users, and the fibers and splitters between them, called the optical distribution network (ODN). The OLT provides the interface between the PON and the backbone network, while the ONT provides the service interface to the end user. PON is a converged infrastructure that can carry multiple services such as voice (plain old telephony service or voice over IP), data, video, and/or telemetry, in that all of these services are converted and encapsulated in a single packet type for transmission over the PON fiber.
In a PON configuration, downstream signals are broadcast to each premises sharing a fiber. Encryption is used to prevent eavesdropping. Upstream signals are combined using a multiple access protocol, invariably time division multiple access (TDMA). This allows for two-way traffic on a single fiber optic cable. The main fiber run on a PON network can operate at 155 Mps, 622 Mbps, 1.25 Gbps or 2.5 Gbps using APON/ BPON, EPON or the emerging GPON standards. Bandwidth allocated to each customer from this aggregate bandwidth can be static or dynamically assigned in order to support voice, data and video applications.
There are various flavors of PON technologies including APON, BPON, EPON and GPON.
APON/BPON (ITU-T G.983): APON, ATM PON, is the initial PON specifications defined by the FSAN committee used ATM as their layer 2 signaling protocol. Use of the term APON led users to believe that only ATM services could be provided to end-users, so the FSAN decided to broaden the name to Broadband PON (BPON). BPON systems offer numerous broadband services including Ethernet access and video distribution.
APON systems are based upon ATM as the bearer protocol. Downstream transmission is a continuous ATM stream at a bitrate of 155.52 Mb/s or 622.08 Mb/s with dedicated Physical Layer OAM (PLOAM) cells inserted into the data stream. Upstream transmission is in the form of bursts of ATM cells, with a 3 byte physical overhead appended to each 53 byte cell in order to allow for burst transmission and reception.
GPON (ITU-T G.984): Gigabit PON is a PON technology operating at bitrates of above 1 Gb/s. Apart from the need to support higher bitrates, the overall protocol has been opened for re-consideration and the sought solution should be the most optimal and efficient in terms of support for multiple services, OAM&P functionality and scalability.
The main GPON requirements are:
- Full Service Support C including voice (TDM, both SONET and SDH), Ethernet (10/100 BaseT), ATM, leased lines and more.
- Physical reach of at least 20 km with a logical reach support within the protocol of 60 km.
- Support for various bitrate options using the same protocol, including symmetrical 622 Mb/s, symmetrical 1.25 Gb/s, 2.5 Gb/s Downstream and 1.25 Gb/s Upstream and more.
- Strong Operation Administration and Maintenance (OAM&P) capabilities offering end to end service management.
- Security at the protocol level for downstream traffic due to the multicast nature of PON.
EPON (IEEE 802.3ah) C Ethernet Passive Optical Network (EPON) is a point to multipoint (Pt-MPt) network topology implemented with passive optical splitters, along with optical fiber PMDs that support this topology. EPON is based upon a mechanism named MPCP (Multi-Point Control Protocol), which uses messages, state machines, and timers, to control access to a P2MP topology. Each ONU in the P2MP topology contains an instance of the MPCP protocol, which communicates with an instance of MPCP in the OLT.At the basis of the EPON/MPCP protocol lies the P2P Emulation Sublayer, which makes an underlying P2MP network appear as a collection of point to point links to the higher protocol layers (at and above the MAC Client). It achieves this by prepending a Logical Link Identification (LLID) to the beginning of each packet, replacing two octets of the preamble. In addition, a mechanism for network Operations, Administration and Maintenance (OAM) is included to facilitate network operation and troubleshooting.
PON: Passive Optic Network Technologies (APON/BPON, EPON and GPON)
Related Terms: PON, APON, BPON, GPON, EPON