EPON Testing Technology (1)

1 Introduction

With the rapid development of broadband access technology, various emerging broadband access technologies have sprung up. PON is another ideal access platform following DSL and Cable technologies. PON can directly provide optical services or FTTH services. EPON Ethernet + Passive Optical Network) is a new type of optical fiber access network technology. It uses a point-to-multiple-point structure and passive optical transmission, providing a variety of services over Ethernet. It uses PON technology in the physical layer, Ethernet protocol in the link layer, and Ethernet access using the PON topology. Therefore, it integrates the advantages of PON and Ethernet technologies: low cost, high bandwidth, high scalability, flexible and fast service restructuring, compatibility with existing Ethernet, and convenient management. EPON testing is very different from traditional Ethernet equipment testing. This article focuses on the testing technology of EPON.

2. Introduction of EPON technology and test challenges

The EPON system consists of multiple optical network units (ONU), one optical line terminal (OLT), and one or more optical splitters (see figure 1 ). In the downstream direction, the signal sent by the OLT is broadcast to all ONU. In the uplink direction, TDMA multi-access technology is used to form a TDM information stream to be transmitted to the OLT. 802.3ah modified the Ethernet frame format, redefined the Preamble part, and added the timestamp and LLID of the Logical Link identifier ). LLID identifies each ONU Of The PON system. LLID is specified during the discovery process.

Figure 1 EPON system composition

2.1 Key Technologies in PON Systems

1) ranging

In the EPON system, the physical distance between ONU and OLT In the uplink information transmission direction is not equal. Generally, the maximum distance between ONU and OLT is 20 km, and the closest distance is 0 km. This distance difference will cause the latency to be between 0 and ~ Between us. If there is not enough isolation gap, different ONU signals may reach the OLT receiver at the same time, which will cause the uplink signal conflict. Conflicts will cause a large number of errors and synchronization loss, and the system will not work properly. Using the ranging method, first measure the physical distance, and then adjust all ONU to the same logical distance as the OLT before TDMA to avoid conflicts. Currently, the following ranging methods are used: Spread-spectrum ranging, out-of-band ranging, and in-band Window ranging. For example, the Time Label ranging method is used to first measure the signal loop delay time from each ONU to the OLT, and then insert a specific Td value for each ONU, make the loop delay time of all ONU after inserting Td called the balanced loop delay value Tequ) equal, the result is similar to moving each ONU to the same logical distance as the OLT, in the future, frames can be correctly sent based on the TDMA technology without conflict.

2) discovery process

OLT finds that ONU in The PON system regularly sends Gate MPCP messages. After receiving the Gate message, the unregistered ONU will wait for a random time to avoid multiple ONU registration at the same time) and then send the Register message to the OLT. After successful registration, the OLT assigns an LLID to ONU.

3) Ethernet OAM

After ONU registers with the OLT, the Ethernet OAM on The ONU starts to discover the process and establish a connection with the OLT. Ethernet OAM is used on the ONU/OLT link to detect remote errors, trigger remote loop back and detect link quality. However, Ethernet OAM provides customized oam pdus, information units, and time reports. Many ONU/OLT vendors use OAM extensions to set special functions of ONU. A typical application is to use an extended ONU configuration Bandwidth Model to control the bandwidth of end users. This non-standard application is the key to testing and becomes a barrier for intercommunication between ONU and OLT.

4) downstream traffic

When an OLT sends an ONU packet, it carries the LLID of the destination ONU in the traffic. Due to the broadcast feature of PON, the data sent by the OLT will be broadcast to all ONU. We need to consider the downstream Traffic Transmission video business flow. Due to the broadcast nature of the EPON system, when a user customizes a video program, it broadcasts it to all users, which consumes a lot of downstream bandwidth. OLT usually supports IGMP Snooping. It can listen to IGMP Join Request messages and send multicast data to users related to this group, instead of broadcasting data to all users. In this way, traffic is reduced.

5) upstream traffic

At a time, only one ONU can send traffic. ONU Queues with multiple priorities correspond to one QoS level for each queue. ONU sends a Report message to the OLT request sending opportunity, detailing the situation of each queue. The OLT sends a Gate message to the ONU, telling the ONU the start time and the duration of the next sending. Managing upstream traffic has many problems with olts. The OLT must be able to manage bandwidth requirements for all ONU instances. The sending permission must be given priority. Based on the priority of the queue and the balance between multiple ONU requests, the upstream bandwidth is dynamically allocated, that is, the DBA algorithm ).

2.2 test challenges faced by the EPON system based on the technical characteristics of the EPON system

1) considering the scale of the EPON system

Although 802.3ah does not define the maximum number in an EPON system, the maximum number supported by an EPON system is from 16 ~ 128. An MPCP session and an OAM session are required for each ONU to be added to the EPON system. When more sites are added to EPON, the risk of system errors increases. For example, each ONU needs to rediscover the process, log on to the process, and start the OAM session. Therefore, the recovery time of the entire system will increase with the number of ONU.

2) device interconnection Problems

Consider the following aspects for device intercommunication:

◆ Different manufacturers provide different dynamic bandwidth algorithm DBAs.

◆ Some manufacturers use OAM's "Organization Specific Elements" to set Specific behaviors.

◆ Is the development of MPCP protocol completely consistent.

◆ Whether the ranging method and clock processing developed by different manufacturers are consistent.

3) potential risks in the transmission of triple playback services in the EPON system

Due to the transmission characteristics of EPON, there are also some hidden risks in the transmission of triple playback services:

◆ A large amount of downstream bandwidth is wasted: the EPON system uses a broadcast transfer method in the downstream mode: Each ONU will receive a large amount of traffic sent to other ONU, wasting a lot of downstream bandwidth.

◆ Large uplink latency: when sending data to the OLT, The ONU must wait for the transfer opportunity allocated by the OLT. Therefore, the ONU must cache a large amount of upstream traffic, this results in latency, jitter, and packet loss.

3. EPON Testing Technology

Tests on EPON mainly include interoperability testing, protocol testing, system transmission performance testing, business and function verification. The standard test topology is shown in figure 2. IXIA IxN2X provides a dedicated EPON test card and an EPON test interface to capture and analyze MPCP and OAM protocols, send EPON traffic, and provide automatic test programs, it helps you test the DBA algorithm.

Figure 2 EPON system test Topology