Shulou(Shulou.com)11/24 Report--

You may not know what "PON" is (usually pronounced "fat"), but you must have heard of "fiber optic home access".

You may not know what "ONU" is, but when you open the weak power box at home, you must see a "small box" connected to the optical fiber.

This "small box" is ONU (Optical Network Unit, optical network unit), commonly known as "optical cat".

Today's protagonist, "PON" (Passive Optical Network), is the key to the implementation of optical fiber into the home, and it is the "last kilometer" optical fiber transmission technology between the network and users.

What is the optical fiber of PON every household, there is a huge network connected behind it. Let's follow the optical fiber, trace the source, and explore together.

The optical fiber connecting the optical cat comes from the splitter in the weak well, which can divide the optical signal into multiple channels in a certain proportion. The optical signal entering the splitter comes from the optical line terminal OLT (Optical Line Terminal).

OLT, ONU and the "transmission channel" ODN (Optical Distribution Network) between them constitute the PON network. Where:

OLT is a carrier-side device, which is responsible for allocating data from the upper business network to users.

ONU is a user-side device, which is responsible for receiving the data sent by OLT and providing services to users directly.

We can take a brief look at the architecture of the PON network according to the following figure.

PON core technology if the PON network as a company, then OLT equivalent to headquarters, ONU equivalent to local offices, splitters equivalent to post stations. Let's first take a look at the three core technologies of PON-encryption, ranging and dynamic bandwidth allocation.

Encrypting the headquarters OLT requires sending a large number of messages to the local office ONU, but the information between different offices needs to be kept secret. In addition, spies (rogue ONU) often sabotage the road. In order to ensure that the information is not disclosed, OLT will regularly negotiate different keys with ONU to ensure the security of the information.

Ranging is divided into areas throughout the country, each with several offices and post stations (splitters). The roads from ONU to the post stations in different offices are different and the time spent is also different. In order to avoid unnecessary conflicts caused by the clerks of each office at the post stations, these routes need to be known in advance.

Therefore, the headquarters OLT will start the ranging function when the ONU is registered for the first time, calculate the physical distance of each ONU, and specify the appropriate equalization delay parameters according to the physical distance of the ONU.

Dynamic bandwidth allocation in order to better manage and guide the work of the office, the headquarters OLT needs to collect the information reported by the office ONU in time, and the more the better. In other words, local offices need sufficient time to report, and do not waste time at the same time. However, each time the information reported by the local office ONU is more or less combined with the local situation, and the importance is also different, so the headquarters OLT will dynamically adjust their reporting time and reporting order according to the specific conditions of each place.

Based on the three core technologies, PON has produced EPON, GPON, 10G-EPON and XG (S) PON in the process of development and evolution.

In the process of evolution, the two families of PON technology are divided into two families-EPON (Ethernet PON) and GPON (Gigabit-capable PON), which are proposed by two different organizations, ITU-T (Telecommunication Standards Branch of the International Telecommunication Union) and IEEE (Institute of Electrical and Electronic Engineers).

The difference between EPON and GPON is mainly in the data link layer, that is, the way data is encapsulated into frames.

EPON adopts Ethernet encapsulation mode, which is suitable for Ethernet services.

GPON adopts GEM (GPON Encapsulation Mode,GPON Encapsulation Mode) encapsulation mode, the technology is relatively complex, and can carry more types of services, including Ethernet services, ATM (Asynchronous Transfer Mode) services and TDM (time division multiplexing) services.

GPON has higher bandwidth and wider coverage than EPON, but it is also more complex and expensive to implement.

Traditional EPON and GPON technologies can provide users with a rate of 100 megabytes. However, with the rapid development of large video and cloud services such as 4K / 8K and AR / VR, people urgently need more advanced PON technology.

EPON and GPON also began technological upgrades, and 10G-EPON and XG (S) GPON gradually replaced EPON and GPON, ushering in the gigabit fiber era.

EPON → 10G-EPON starts with the EPON family.

10G-EPON perfectly inherits the working mechanism of EPON:

OLT sends Ethernet data to ONU by broadcast, initiates and controls the ranging process, and records ranging information. Allocate bandwidth to ONU, control the start time and window size of data sent by ONU, and manage ONU through OAM (Operation Administration and Maintenance) protocol.

The ONU chooses to receive the broadcast data sent by the OLT, responds to the ranging command issued by the OLT, caches the user data, and sends the data to the upstream direction in the sending window assigned by the OLT.

10G-EPON uses the EPON protocol to the maximum extent, meets the high bandwidth needs of users, and supports voice, video, Ethernet and other services. Its inheritance is mainly reflected in two aspects.

Expand the uplink and downlink bandwidth of the EPON standard at a rate of 10Gbps.

10G-EPON supports both symmetric and asymmetric modes, that is, the bandwidth of the upstream and uplink is the same. In actual use, choose one of the two to meet the bearing needs of different customers.

With super compatibility, 10G-EPON 's ONU and EPON's ONU can coexist in an optical distribution network.

10G-EPON downlink wavelength is independent, broadcast mode is used to send information, and ONU takes what it needs; uplink wavelength overlap, using TDMA (Time division multiple access, time division multiple access) mode, to achieve the coexistence of EPON ONU and 10G EPON ONU, supporting the smooth evolution of EPON to 10G-EPON.

Use a table to simply compare EPON and 10G-EPON.

GPON → XG (S)-GPON, let's take a look at the GPON family.

XG (S)-PON, including XGS-PON and XG-PON, is a pair of division brothers.

Symmetrical XGS-PON, uplink and downlink support 10G to meet the needs of government, enterprise, base station bearer and other symmetrical services.

Asymmetric XG-PON, uplink and downlink support 2.5G / 10G, to meet the needs of asymmetric services dominated by home broadband.

Symmetrical XGS-PON is fully compatible with asymmetric XG-PON. It can access symmetric terminal XGS-PON ONU or asymmetric terminal XG-PON ONU under XGS-PON port.

XG (S)-PON inherits the working mechanism of the GPON system:

OLT sends data to ONU by broadcast, initiates and controls the ranging process and records ranging information. Allocate T-CONT (Transmission Container) upstream bandwidth, and manage ONU through OMCI (ONU Management and Control Interface) protocol.

ONU chooses to receive the broadcast data sent by OLT and responds to the ranging command issued by OLT. User data is cached and data is sent upstream in a T-CONT allocated by OLT.

However, XG (S)-PON does not do well in terms of compatibility with GPON.

When the GPON family trained the next generation, it focused on its ability to undertake, distance, safety and so on, but did not consider the family inheritance thoroughly.

XG (S)-PON uplink and downlink wavelengths are different from GPON uplink and downlink wavelengths. XG (S)-PON needs external combiner WDM1r to be compatible with GPON terminals and meet customer upgrade needs.

Use a table to simply compare GPON, XG-PON, and XGS-PON.

Conclusion compared with EPON and GPON,10G-EPON and XG (S)-PON, it can better meet the growing needs of customers in terms of transmission bandwidth, splitter ratio, efficiency, security and so on.

However, the development of the PON family from EPON and GPON to 10G-EPON and XG (S)-PON is not over. PON technology is still developing and improving, how to solve the compatibility between XG (S)-PON and GPON? In the 5G era, how does PON technology play its role?

This article comes from the official account of Wechat: ZTE documents (ID:ztedoc)

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