PDF of this tutorialEPON vendors focus on developing the “active” electronic components—such as the CO chassis and ONUs—that are located at both ends of the PON. The CO chassis is located at the service provider’s CO, head end, or POP and houses OLTs, network interface modules (NIM), and the switch card module (SCM). The PON connects an OLT card to 64 ONUs, each located at a home, business, or MTU. The ONU provides customer interfaces for data, video, and voice services, as well network interfaces for transmitting traffic back to the OLT.
CO Chassis
The CO chassis provides the interface between the EPON system and the service provider’s core data, video, and telephony networks. The chassis also links to the service provider’s core operations networks through an element management system (EMS). WAN interfaces on the CO chassis will typically interface with the following types of equipment:
- DCSs, which transport nonswitched and non–locally switched TDM traffic to the telephony network. Common DCS interfaces include digital signal (DS)–1, DS–3, synchronous transport signal (STS)–1, and OC–3.
- Voice gateways, which transport locally switched TDM/voice traffic to the public-switched telephone network (PSTN)
- IP routers or ATM edge switches, which direct data traffic to the core data network
- Video network devices, which transport video traffic to the core video network
Key functions and features of the CO chassis include the following:
- Multiservice interface to the core WAN
- Gigabit Ethernet interface to the PON
- Layer-2 and -3 switching and routing
- Quality of service (QoS) issues and service-level agreements (SLA)
- Traffic aggregation
- Houses OLTs and SCM
Optical Network Unit
The ONU provides the interface between the customer’s data, video, and telephony networks and the PON. The primary function of the ONU is to receive traffic in an optical format and convert it to the customer’s desired format (Ethernet, IP multicast, POTS, T1, etc.). A unique feature of EPONs is that, in addition to terminating and converting the optical signal, the ONUs provide Layer-2 and -3 switching functionality, which allows internal routing of enterprise traffic at the ONU. EPONs are also well suited to delivering video services in either analog CATV format, using a third wavelength, or IP video.
Because an ONU is located at every customer location in FTTB and FTTH applications and the costs are not shared over multiple subscribers, the design and cost of the ONU is a key factor in the acceptance and deployment of EPON systems. Typically, the ONUs account for more than 70 percent of the system cost in FTTB deployments, and in FTTH deployments they account for approximately 80 percent.
Key features and functions of the ONU include the following:
- Customer interfaces for POTS, T1, DS–3, 10/100BASE-T, IP multicast, and dedicated wavelength services
- Layer-2 and -3 switching and routing capabilities
- Provisioning of data in 64 kbps increments up to 1 Gbps
- Low start-up costs and plug-and-play expansion
- Standard Ethernet interfaces eliminate the need for additional DSL or cable modems
EMS
The EMS manages the different elements of the PON and provides the interface into the service provider’s core operations network. Its management responsibilities include the full range of fault, configuration, accounting, performance, and security (FCAPS) functions.
Key features and functions of the EMS include the following:
- Full FCAPS functionality via a modern graphical user interface (GUI)
- Capable of managing dozens of fully equipped PON systems
- Supports hundreds of simultaneous GUI users
- Standard interfaces, such as common object request broker architecture (CORBA), to core operations networks
