Implementation of TDM service in Epon system

Beacon Communication Technology Co., Ltd. domestic market headquarters Zhang Yong

1 Introduction

In recent years, the proliferation of data traffic and the rapid development of IP service have put forward many new problems for the current network construction. On the one hand, the traditional telephone service and leased line business and other basic telecommunications business is still the main source of revenue for telecom operators, on the other hand, the rapid development of data services to force the telecommunications network IP become an irreversible trend. The integration of data, voice and video services has become an inevitable trend of the next Generation Network (NGN) development. How to let the data network carry TDM business and how to balance the existing business and the future network development has become the focus of the telecom operators. and end users always want to be at the lowest possible cost within the enterprise or home to achieve the telephone, video and data and other services integrated access. Epon as a new generation of broadband access technology in order to meet the objective requirements of network development and integration, it is necessary to realize a variety of services including TDM service integrated access.

How to implement TDM service in 2EPON

Ethernet's encapsulation means that epon technology is very suitable for hosting IP services while also facing a major problem-difficult to host voice or circuit data such as TDM business. Epon is an asynchronous transmission network based on Ethernet, it does not have the high precision clock with full network synchronization, and can not meet the timing and synchronization requirements of TDM service. In order to solve the problem of timing synchronization of TDM service and ensure the QoS of TDM service, we should not only improve the design of epon system, but also adopt some specific techniques.

At present, one of the most important methods to realize TDM service transmission on EPON is the circuit simulation technology based on packet switching network (cesop,circuitemulation over Packet switched Net).

2.1 Basic ideas of switching from circuit to packet switching

Cesop technology is a circuit simulation on a non-TDM network, which realizes the transmission of TDM services such as E1/T1,E3/DS3 or STM-1 on a packet-switched network. The basic principle is to set up a "channel" on the packet switching network, by adding the header, encapsulating each T1 or E1 frame with IP packet, through the packet switching network (PSN) to transmit to the end-to-end. The destination receives the data packet, regenerates the synchronous clock signal, removes the IP header from the packet, and converts the other data into the original TDM data stream, so that the TDM device at both ends of the network does not care about whether its connected network is a TDM network. Cesop is a transparent transmission for E1, so it is very good for the traditional telecommunications network compatibility, all the traditional protocols, signaling, data, voice, image and other services can be used unchanged, and the relevant equipment does not need to make any changes, so that the telecommunications operators to make full use of existing resources, The traditional TDM service is applied to the IP network.

Figure 1: Circuit emulation service implemented by packet switching network

2.2 Implementation of Circuit simulation

CESOP Circuit simulation requires an interactive connection at both ends of the packet switching network. At the entrance of the packet switching network, the interactive connection function converts the TDM data into a series of groupings, while at the exit, the TDM circuit is regenerated using this series of groupings. At present, there are two methods of structured simulation and unstructured simulation to realize this interactive function module.

The structure simulation uses the time slot structure inherent in the TDM circuit. First, the frame structure (such as f-bit in DS1) is extracted from the data stream, and then each time slot is added sequentially to the grouped payload, followed by the same time slot in the next frame, and so on. When the payload is fully populated and a group header is added, the packet is sent to the packet switched network. Payloads typically contain approximately eight frames of TDM data (256 eight-bit bits for E1 circuits). At the exit of the packet network, the TDM data stream is restarted and the new frame structure is used.

Unstructured transmission ignores any structure that may exist in the TDM circuit and considers the data as a pure bit stream for a given data rate. A series of eight-bit groups are intercepted sequentially from the TDM bit stream to form the payload of the packet. Therefore, the number of eight-bit groups that make up each packet payload is random. In general, the length of the payload is selected to make the grouping time around 1ms, for the T1 circuit, the length is 193 eight-bit group (see Figure 2). For the E1 circuit, the length is 256 eight bits. In this way, the signaling in the TDM business is transparently transmitted and any type of TDM business can be implemented without any signaling protocol conversion device.

Figure 2: Structured circuit emulation service and unstructured circuit emulation service