1. Introduction

With the increasing number of access network services and the enhancement of interactivity, broadband access network will be an inevitable trend. At present, the commonly used broadband access methods generally include xDSL access, Cable Modem access, Ethernet access, wireless access and optical fiber access.

Compared with other access technologies, the biggest advantage of optical fiber access technology lies in the available bandwidth and huge development potential, which is incomparable to other access technologies. Optical fiber access network also has the characteristics of good transmission quality, long transmission distance, strong anti-interference ability, high network reliability and saving pipeline resources. In addition, the standardization of SDH and APON equipment is high, which is conducive to reducing production and operation costs.

Optical access network (0AN) can be divided into two types: active optical network (AON) and passive optical network (PON). AoN can be divided into SDH-based AoN and PDH-based AoN. PON can be divided into P0N(APON) based on ATM and PoN(EP0N) based on Ethernet.

2. Principles

The optical fiber consists of a single glass fiber, a cladding near the fiber core, a main coating and a plastic protective layer. The core and cladding are composed of two media with different optical properties, and the refractive index of the internal media is higher than that of the surrounding media. Therefore, when light is incident from the high refractive index side to the low refractive index side, as long as the incident angle is greater than a critical value, reflection will occur and energy will not be lost. At this point, the covering layer wrapped around the periphery is like an opaque substance, which prevents light from escaping from the surface during insertion.

The optical signal emitted by light emitting diode (LED) or injection laser diode (ILD) propagates along the optical fiber. At the other end, PIN or APD photodiode is used as a detector to receive the signal. In order to ensure the effective signal transmission, it is necessary to add an optical amplifier in front of the optical transmitter to improve the optical power of the optical fiber, and amplify the micro-signal behind the photodetector at the receiver to improve the receiving ability.

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3. 1 AON based on SDH system

With the increasing transmission capacity of backbone network, the access modes of transmission network are becoming more and more diversified. From a technical point of view, the access layer needs less bandwidth and needs to provide integrated services such as IP, TDM and ATM. A system based on SDH that can provide IP and ATM transmission and processing will be the main method to solve the transmission in access layer. It can provide access, transmission and protection of high-quality private line, ATM, IP and other services in a service provider (POP) at a low price.

The application of SDH technology in access network can bring its huge bandwidth advantage and technical advantage in core network into the field of access network, and provide ideal network performance and service reliability by using SDH synchronous multiplexing, standardized optical interface, powerful network management, flexible network topology and high reliability.

The development trend of access network is to support IP access. At present, at least the mapping of Ethernet interfaces needs to be supported. In addition to carrying voice services, it is also necessary to use part of the payload of SDH to transmit IP services, so that SDH can also support IP access. There are many ways to support it, including the existing point-to-point communication protocol (PPP), link access protocol (LAPS) and general framing protocol (GFP) on SDH.