1 Introduction
PON technology is another ideal access platform after DSL technology and Cable technology. 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-multipoint structure and passive optical transmission to provide multiple services over Ethernet. It uses PON technology at the physical layer, Ethernet protocol at the link layer, and Ethernet access using the PON topology. Therefore, it combines the advantages of PON technology and Ethernet technology: low cost; high bandwidth; strong scalability, flexible and rapid service reorganization; compatibility with existing Ethernet; convenient management. The test of EPON is very different from the test of traditional Ethernet equipment. This article focuses on the EPON test technology.
2 Introduction to EPON technology and the test challenges it faces
The EPON system consists of multiple optical network units (ONU), an 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 ONUs. In the upstream direction, TDMA multiple access technology is used, and the upstream information of multiple ONUs forms a TDM information stream to be transmitted to the OLT. 802.3ah modified the Ethernet frame format, redefined the Preamble part, and added timestamp and logical link identification (LLID). The LLID identifies each ONU in the PON system, and the LLID is specified during the discovery process.
Figure 1 EPON system composition
2.1 Key technologies in PON system
(1) Ranging
In the EPON system, the physical distance between each ONU and the OLT in the upstream information transmission direction is not equal. The general EPON system stipulates that the longest distance between ONU and OLT is 20km, and the closest distance is 0km. This difference in distance will cause the delay to vary from 0 to 200us. If there is not enough isolation gap, signals from different ONUs may reach the receiving end of the OLT at the same time, which will cause collision of upstream signals. The conflict will cause a lot of error codes and synchronization loss, etc., causing the system to not work properly. Using the ranging method, first measure the physical distance, then adjust all ONUs to the same logical distance as the OLT and then perform the TDMA method to achieve conflict avoidance. At present, there are several ranging methods: spread spectrum method ranging, out-of-band method ranging and in-band windowing method. For example, using the time label ranging method, the signal loop delay time from each ONU to the OLT is first measured, and then a specific equalization delay Td value is inserted for each ONU, so that the loop delay time of all ONUs after inserting Td ( The equalized loop delay values ​​(Tequ) are all equal, and the result is similar to making each ONU move to the same logical distance as the OLT. In the future, the frame can be correctly transmitted according to the TDMA technology without collision. .
(2) Discovery process
The OLT discovers that the ONU in the PON system is sending Gate MPCP messages regularly. Upon receiving the Gate message, ONUs that have not registered will wait for a random time (to avoid multiple ONUs registering at the same time), and then send a Register message to the OLT. After successful registration, the OLT assigns an LLID to the ONU.
(3) Ethernet OAM
After the ONU registers with the OLT, the Ethernet OAM on the ONU starts the discovery process and establishes a connection with the OLT. Ethernet OAM is used on the ONU / OLT link to find remote errors, trigger remote loopback and detect link quality. However, Ethernet OAM provides customized OAM PDUs, information units and time reports. Many ONU / OLT manufacturers use OAM extensions to set ONU special functions. A typical application is to control the amount of bandwidth of the end user with the configuration bandwidth model extended in the ONU. This non-standard application is the key to the test and becomes an obstacle to the interconnection between ONU and OLT.
(4) Downstream flow
When the OLT sends traffic to the ONU, it will carry the LLID information of the destination ONU in the traffic. Because of the broadcast characteristics of PON, the data sent by the OLT will be broadcast to all ONUs. We have to give special consideration to the situation in which downstream traffic transmits video traffic. Due to the broadcast nature of the EPON system, when a user customizes a video program, it will be broadcast to all users, which consumes downstream bandwidth very much. OLT usually supports IGMP Snooping, which can snoop IGMP Join Request messages and send multicast data to users related to this group, instead of broadcasting to all users, reducing traffic in this way.
(5) Upstream flow
Only one ONU can send traffic at a time. The ONU has multiple priority queues (each queue corresponds to a QoS level. The ONU sends a Report message to the OLT to request an opportunity to send, detailing the situation of each queue. The OLT sends a Gate message to respond to the ONU, telling the ONU the start time of the next transmission And continuous sending time. The upstream traffic has many problems for the OLT. The OLT must be able to manage the bandwidth requirements for all ONUs, and the transmission permission must be prioritized, and the requests of multiple ONUs must be balanced according to the priority of the queue. Dynamically allocate upstream bandwidth (ie DBA algorithm).
2.2 According to the technical characteristics of the EPON system, the test challenges faced by the EPON system
(1) Considering the scale of EPON system
Although IEEE802.3ah does not define the maximum number in an EPON system, the maximum number supported by an EPON system is from 16 to 128. Each ONU joining an EPON system requires an MPCP session and an OAM session. As more sites join EPON, the risk of system errors will increase. For example, each ONU needs to rediscover the process, log in and start an OAM session. Therefore, the recovery time of the entire system will increase with the number of ONUs.
(2) Interoperability of equipment
For the interworking of equipment, the following aspects are mainly considered:
â— The dynamic bandwidth algorithm (DBA) provided by different manufacturers is different.
â— Some manufacturers use OAM's "OrganizaTIon Specific Elements" to set specific behaviors.
â— Whether the development of MPCP protocol is completely consistent.
â— Whether the ranging methods and clocks developed by different manufacturers are consistent.
(3) Hidden dangers of EPON system transmitting triple-play service
Due to the transmission characteristics of EPON, some hidden dangers will also be introduced when transmitting triple-play services:
â— Downstream wastes a lot of bandwidth: EPON system uses broadcast transmission in the downlink: each ONU will receive a lot of traffic to other ONUs, wasting a lot of downlink bandwidth.
â— The upstream delay is relatively large: When the ONU sends data to the OLT, it must wait for the transmission opportunity allocated by the OLT, so the ONU must buffer a large amount of upstream traffic, which will cause delay, jitter, and packet loss.
3 EPON test technology
The testing of EPON mainly includes interoperability testing, protocol testing, system transmission performance testing, business and functional verification, etc. The standard test topology is shown in Figure 2. IXIA's IxN2X products provide dedicated EPON test cards, provide EPON test interfaces, can capture and analyze MPCP and OAM protocols, can send EPON traffic, provide automatic test procedures, and can help users test DBA algorithms. 
Figure 2 EPON system test topology
3.1 Testing of MPCP registration process
Test purpose: Within the correct time window, the ONU registers with the OLT.
Test method: Use IxN2X to analyze MPCP and complete this test. Triggers can be set up to capture other interactive messages when the OLT's Gate message is captured.
â— The OLT sends out Gate messages regularly, and the Discovery Flag in the Gate messages is set.
â— ONU waits for a random time when sending Register message.
â— The key point of the test is to test the time taken by the ONU registration process. You can analyze the captured Gate message to get the Discovery Window, and then compare the time stamp of the received Register message to ensure that it is in the Discovery Window.
3.2 Downstream traffic broadcasting
Test purpose: Downstream traffic can be correctly forwarded to the designated ONU. Although downstream traffic can be broadcast to all ONUs, only designated ONUs can forward traffic to its end users.
Test method: IxN2X can be used to discover the LLID assigned to each ONU, and then receive traffic on the designated ONU, and observe whether the traffic can be received on the designated ONU.
â— Capture MPCP messages and view the LLID assigned to each ONU.
â— Send traffic from the OLT side, the LLID of the traffic points to an ONU.
â— At the designated ONU end, it can receive the traffic forwarded by this ONU.
â— On other ONUs, the traffic forwarded by this ONU cannot be received.
3.3 Upstream transmit queue
Test Purpose: This test is to verify that ONU can send traffic within the transmission window time.
Test method: IxN2X can accurately calculate the time for the ONU to send packets, and can verify whether the ONU is sending traffic in the sending window.
â— Set trigger: When the specified LLID Gate message is captured bidirectionally, the capture will start.
â— Set filter: only capture the upstream traffic of the specified LLID.
â— Send traffic to the ONU with the specified LLID, the ONU sends a Report message to the OLT, and the OLT sends a Gate message to the ONU. Use IxN2X to capture the protocol packet and view the sending window allocated by the OLT for this LLID.
â— Capture the upstream traffic sent by the specified ONU.
â— By comparing timestamps, it can be verified that the ONU should send traffic within the sending window.
3.4 EPON ranging performance test (see Figure 3)
Figure 3 Topology diagram of ranging test
Test purpose: test the minimum distance and maximum distance that the OLT side can measure for the ONU; test whether the newly added ONU affects the normal operation of other online ONUs during the ranging; test and measure the accuracy of the ranging.
testing method:
â— Set up the test configuration to make the system work under the maximum split ratio. The distance between ONU1 ~ ONUn-1 and OLT is 0km (directly connected through a splitter), and the distance between ONUn and OLT is 10km / 20km.
â— Under the condition that all ONUs work normally, each ONU is measured separately on the OLT side.
â— If all ONUs can perform normal ranging, IxN2X can monitor whether all ONUs (ONU1 ~ ONUn) can work normally (for IP services, it is required to test at 90% of throughput without packet loss), indicating that the ranging range is index.
â— Range the ONU3 and record the distance measurement value as b1.
â— Add 3m optical jumper to ONU3.
â— Range the ONU3 again and record the distance measurement value as b2.
â— Remove the optical jumper, and then measure the distance of ONU3, and record the distance measurement value as b3.
◠Calculate the change of the ranging value | b2 -b1 | and | b2 -b3 | should be ≤16ns.
3.5 Performance test of EPON system (see Figure 4)
Figure 4 Performance test topology
The purpose of the test: to better isolate the cause of the degradation of the system forwarding performance.
Test method: IxN2X can count PON traffic at the PON port, calculate delay and packet loss. The test topology is shown in Figure 4. IxN2X 103/2 and N2X 103/3 are Ethernet ports, N2X 101/1 is an EPON interface and counts upstream traffic, and IxN2X 101/2 is an EPON interface and counts downstream traffic.
â— Send bidirectional business flow between 103/2 and 103/3.
â— Statistics on upstream and downstream traffic on ports 101/1 and 101/2 respectively.
â— Users can calculate the delay through ONU and OLT separately.
The test results are shown in Figure 5. Packet loss can also be calculated in the same way.
Figure 5 Performance test results of ONU and OLT
From the above test results, we can calculate:
â— OLT Downstream avg Latency = 10.4us.
â— ONU Upstream avg Latency = 406.4us.
â— OLT Upstream avg Latency = 416.7-406.4 = 10.3us.
â— ONU Downstream avg Latency = 13.4-10.4 = 3us.
4 Conclusion
IXIA IxN2X has always been in the leading position in the field of EPON testing. It provides a wealth of EPON test functions, which can test MPCP, OAM protocol, DBA algorithm, EPON system forwarding performance, EPON system QoS guarantee capability, etc. According to the transmission characteristics of EPON, it is necessary to effectively test the EPON system.
Compabile Models:
For Samsung:
For Galaxy S6, For Galaxy S6 Edge, For Galaxy S6 Edge+,
For Galaxy S6 Active, For Galaxy S6 Duos, For Galaxy Note Edge,
For Galaxy S7, For Galaxy S7 Edge, For Galaxy Note 5
For Galaxy S8, For Galaxy S8 Plus, For Galaxy Note 8
For Galaxy S9,For Galaxy S9 Plus
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For Iphone:
For Iphone8/X/XR/XS Max
For Others:
For YotaPhone 2, For Elephone P9000
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