In July 1995, Group 13 of the International Telecommunication Union (ITU-T) adopted the new Recommendation G.902 on the framework structure of the access network, which defines the access network as follows:

The access network is composed of line equipment and transmission facilities between the service node interface (SNI) and the user-network interface (UNI). The implementation system that provides the required transmission and bearer capacity for the provision of telecommunication services can be configured and implemented through the management interface. manage. In principle, there are no restrictions on the type and number of UNIs and SNIs that can be implemented by the access network.

SNI (Service Node Interface) is the interface between the access network and the service node. It is independent of the service node and switch, and connects the SNs of different services to the access network through different SNIs. Users provide a variety of different business services. It can be divided into SNI that supports single access and SNI that supports integrated access. UNI (User-Network Interface) is the interface between the access network and the user terminal. It can support various access types and services that the current network can provide. The development of the access network should not limit the existing services and access types. . For different services, corresponding to different interface types.

The PON without light source is considered to be the development direction of the future access network. Because the bandwidth it provides can meet the needs of various broadband services now and in the future, it is generally optimistic about solving the problem of broadband access; on the other hand, its cost is relatively low in terms of equipment cost and operation and maintenance management overhead. .

The PON system structure is mainly composed of the optical line terminal (OLT: OpTical Line Terminal) of the central office, the optical distribution network (ODN: OpTical DistribuTIon Network) including passive optical components, and the optical network unit/optical network at the user end. It is composed of a network terminal (ONU/ONT Optical Network Unit / Optical Network Terminal), the difference is that the ONT is directly located at the user end, and there are other networks between the ONU and the user, such as Ethernet) and a network element management system (EMS) composition, Usually a point-to-multipoint tree topology is used. The outstanding advantage of the PON network is that the outdoor active equipment is eliminated, and all signal processing functions are completed in the switch and the equipment in the user's house.

Working principle:

OLT assembles the downlink services delivered to each ONU into frames, and sends them to multiple ONUs by broadcasting, that is, through optical The splitter is divided into N independent signals, each signal contains frames sent to all specific ONUs, each ONU only extracts the frames sent to itself, and discards the frames of other ONUs; the upstream direction is from each ONU to the upstream of the OLT Data is transmitted through a shared channel in a time division multiple access (TDMA) manner, and the OLT allocates a transmission time slot to each ONU. These time slots are synchronized so that when data packets are coupled into a fiber, there are no collisions between data packets from different ONUs.

Types of PON technology:

1. APON technology: In 1995, the full service network alliance FSAN (Full Service Access Network) was established to jointly Define a general PON standard. In 1998, the ITU-T working group of the International Telecommunication Union, based on 155Mbps ATM technology, released the G.983 series APON (ATM PON) standard. The transmission based on ATM cells is realized on the PON, that is, the APON (APON for short) technology.

2. EPON technology: Almost at the same time as the APON system, IEEE also established an EFM research group to launch an Ethernet-based EPON (Ethernet Passive Optical Network) in the optical access network. Its research scope is also limited to the architecture, and must conform to the existing 802.3 medium access control (MAC) layer function. The research group launched the EPON standard IEEE 802.3ah in April 2004, with an uplink and downlink rate of 1Gbit/s (after using 8B/10B encoding, the line rate is 1.25Gbit/s), which ended the use of proprietary protocols by various EPON manufacturers to develop equipment. Standard status. EPON is a broadband access system based on Ethernet technology, which utilizes PON topology to realize Ethernet access. The key technologies of the data link layer mainly include: the multiple access control protocol (MPCP) of the uplink channel, the plug-and-play problem of the ONU, the ranging and delay compensation protocol of the OLT and the compatibility of the protocol.

3. GPON technology: GPON is a gigabit passive optical network or a gigabit passive optical network. GPON technology is the latest generation of broadband passive optical synthesis based on the ITU-TG. 984.x standard. The access standard has many advantages such as high bandwidth, high efficiency, large coverage, and rich user interfaces. It is regarded by most operators as an ideal technology for realizing broadband and comprehensive transformation of access network services.

Advantages of PON technology:

1. Multi-service: PON system requires voice, data, video and other service access, with good service transparency, Realize full-service access in the true sense.

2. High bandwidth: EPON can currently provide a symmetrical 1.25Gb/s bandwidth for upstream and downstream, and can be upgraded to 10Gb/s with the development of Ethernet technology. GPON is up to 2.5Gb/s bandwidth.

3. Long-distance access: The transmission distance of optical fibers is as high as hundreds of kilometers, so in fact, the distance bottleneck of the physical transmission layer lies in the optical devices of the equipment that receive and receive optical signals. The current PON standard stipulates that the distance is 20km. One is very redundant;

4. Relatively low cost: Since the ODN part of the PON system has no electronic components and no power supply, it is easy to lay, basically requires no maintenance, and has low construction and maintenance costs.

5. Good scalability: PON network, as a point-to-multipoint network, saves fiber resources with a fan-out structure, and at the same time, this shared bandwidth network structure can provide flexible bandwidth distribute. The access to the terminal does not need to increase the lines of the trunk part, so when the system needs to be expanded, the part that needs to be changed is minimal, and the project implementation is relatively simple.

6. Good QoS guarantee; In the design of PON system, it is a technical solution designed to provide multi-service access for telecom operators. In terms of technical measures such as service quality and priority assurance of different businesses, a variety of application solutions are provided. Therefore, a timely and reliable response mechanism can be set, thereby realizing an integrated access system with multiple services and different service levels.


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