FBG filters for FTTx applications are reflective filters integrated in the LC or SC adapters, which reflect the 1625 nm wavelength and transmit all other (i.e. 1310, 1490 and 1550 nm). They can be used to monitor live network utilizing OTDR operating at 1625 or 1650 nm. End-to-end OTDR measurements from OLT to ONT are typically difficult due to the high point insertion loss introduced by the splitter and due to the required very good spatial resolution. The use of the 1625 nm reflectors is an efficient and cost-effective way of reducing the required dynamic range of the OTDR. The 1625 reflectors are currently considered the best way of implementing real time end-to-end (OLT to ONT) monitoring of the optical layer in live FTTx networks.
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With the continuous progress of science and technology, the Internet has gradually gone into the homes of the ordinary people, and the speed of broadband has increasingly become the topic of people in the entertainment and work often, from narrowband dial-up to broadband Internet, and then the fiber access Internet, broadband network, the rapid pace of PON technology gradually come to the front. Currently, there are two quite compelling PON standard has been officially released, which are GPON standard developed by the ITU / FSAN and EPON standard developed by IEEE 802.3ah working group. PON technology has been no doubt the ultimate solution for the future FTTH era. EPON and GPON who will the dominant FTTH tide has become a new hot debate. What’s the difference between EPON and GPON?
GPON and EPON Differences
Perhaps the most dramatic distinction between the two protocols is a marked difference in architectural approach. GPON provides three Layer 2 networks: ATM for voice, Ethernet for data, and proprietary encapsulation for voice. EPON, on the other hand, employs a single Layer 2 network that uses IP to carry data, voice, and video.
A multiprotocol transport solution supports the GPON structure (Figure 1). Using ATM technology, virtual circuits are provisioned for different types of services sent from a central office location primarily to business end users. This type of transport provides high-quality service, but involves significant overhead because virtual circuits need to be provisioned for each type of service. Additionally, GPON equipment requires multiple protocol conversions, segmentation and reassembly (SAR), virtual channel (VC) termination and point-to-point protocol (PPP).
Figure 1: Diagram showing a typical GPON network.
EPON provides seamless connectivity for any type of IP-based or other “packetized” “communications” (Figure 2). Since Ethernet devices are ubiquitous from the home network all the way through to regional, national and worldwide backbone networks, implementation of EPONs can be highly cost-effective. Furthermore, based on continuing advances in the transfer rate of Ethernet-based transport — now up to 10 Gigabit Ethernet — EPON service levels for customers are scalable from T1 (1.5 Mbit/s) up through 1 Gbit/s.
Figure 2: Diagram showing a typical EPON network.
Upstream Bandwidth
Subtracting the various system run overhead from the total bandwidth of the system uplink transmission is the upstream available bandwidth. It has a great relationship with the number of the ONU contained in the system, DBA (Dynamic Bandwidth Allocation) algorithm polling cycle, the type of bearer services, as well as the various business proportion. EPON and GPON are broadband access technology, hosted business IP data services. Below we will calculate the uplink the beared pure IP services available bandwidth of EPON and GPON that contain 32 ONUs, fiber optic coupler,the case of polling period 750s.
EPON
EPON upstream rate is 1.25 Gbit/s. Because the 8B/10B line coding, each 10bit are 8bit valid data, so its effective upstream transmission bandwidth is 1 Gbit/s. EPON upstream overhead of running the system and its proportion of the total bandwidth are as following:
1. Used for the the burst reception of physical layer overhead: about 3.5%;
2. Ethernet frame encapsulation overhead: about 7.4%;
3. MPCP (Multi-Point Control Protocol) and OAM operation and management of maintenance protocol overhead: about 2.9%;
4. DBA algorithm resulting in the remaining time slots (that is not sufficient to transfer a complete Ethernet frame time slot) wasted: about 0.6%;
5. EPON upstream total overhead is all of the above about 144 Mbit/s, the available bandwidth is about 856 Mbit/s.
GPON
GPON supports a variety of rate levels, has asymmetric rate that downlink is 2.5Gbps or 1.25Gbps, the upgoing is 1.25Gbps or 622 Mbps. NRZ encoding the uplink total bandwidth for 1.244 Gbit/s, GPON upstream overhead of running the system as following:
1. The proportion of its total bandwidth is used for the the burst reception of physical layer overhead: about 2.0%;
2. GEM (GPON encapsulation method) frame and the Ethernet frame encapsulation overhead: about 5.8%;
3. The PLOAM (physical layer operation, management and maintenance) protocol overhead: about 2.1%;
4. Remaining slots of the DBA algorithm introduced the additional encapsulation overhead: about 0.8%.
5. GPON upstream total overhead is all of the above about 133 Mbit/s, the available bandwidth about 1111 Mbit/s.
ODN is a FTTH fiber optic cable network based on PON equipment. Its role is to provide optical transmission channel between the OLT and ONU. Accroding the function, ODN from the central office to the client can be divided into four parts: feeder fiber optic subsystems, cable wiring subsystem, home line of fiber optic subsystems and fiber terminal subsystems. The main components in ODN include optical fibers, optical connectors, optical splitters and corresponding equipments for installing them.
OLT (Optical line terminal)
OLT is a terminal equipment connected to the fiber backbone. It sends Ethernet data to the ONU, initiates and controls the ranging process, and records the ranging information. OLT allocates bandwidth to the ONU and controls the starting time and the transmission window size of the ONU transmission data.
ONU (Optical network unit)
ONU is a generic term denoting a device that terminates any one of the endpoints of a fiber to the premises network, implements a passive optical network (PON) protocol, and adapts PON PDUs to subscriber service interfaces. In some contexts, ONU implies a multiple subscriber device. Optical Network Terminal (ONT) is a special case of ONU that serves a single subscriber.
APON / BPON
APON (ATM PON) is the first PON system that achieved significant commercial deployment with an electrical layer built on Asynchronous Transfer Mode (ATM). BPON (Broadband PON) is the enhanced subsequence of APON, with the transmission speed up to 622Mb/s. At the same time, it added the dynamic bandwidth distribution, protection and other functions. APON/BPON systems typically have downstream capacity of 155 Mbps or 622 Mbps, with the latter now the most common.
GPON
GPON (Gigabit PON) is based on the TU-TG.984.x standard for the new generations of broadband passive optical access. Compared with the other PON standards, GPON provides the unprecedented high bandwidth downlink rate of up to 2.5 Gbit/s, the asymmetric features better adapt to the broadband data services market. It provides the QoS full business protection, at the same time carries ATM cells and (or) GEM frame, the good service level, the ability to support QoS assurance and service access. Carrying GEM frame, TDM traffic can be mapped to the GEM frame, 8kHz using a standard frame able to support TDM services. As a carrier-grade technology standards, GPON also provides access network level protection mechanism and full OAM functions. GPON is widely deployed in FTTH networks. It can develop into two directions which is 10 GPON and WDM-PON.
WDM-PON
WDM-PON uses wavelength division multiplexing technology to access to the passive optical network. It has four programs as following:
1. Each ONU is assigned with a pair of wavelength, for uplink and downlink transmission, thereby providing the OLT to each ONU fixed virtual point-to-point bidirectional connections.
2. ONU uses tunable lasers, according to the needs of the ONU to dynamically allocate the wavelength, and each ONU can be shared the wavelength, the network are reconfigurable.
3. Using colorless ONUs, the ONU are independent from the wavelength.
4. Using a combination of TDM and WDM technology, Composite PON (CPON). CPON uses WDM technology in the downstream, and TDMA technology in the upstream.
EPON / GEPON
EPON (Ethernet PON) is the rival activity to GPON which uses Ethernet packets instead of ATM cells. GEPON uses 1 gigabit per second upstream and downstream rates. It is a fast Ethernet over PONs which are point to multipoint to the premises (FTTP) or FTTH architecture in which single optical fiber is used to serve multiple premises or users. EPON is an emerging broadband access technology, through a single fiber-optic access system, to access the data, voice and video service, and it has a good economy.
