Talking about the Structure and Key Technology of GPON System

GPON is a flexible Gigabit-level fiber access network proposed by ITU-T. It carries multi-services with ATM cells and GEM frames, and supports broadband full-service access for commercial and residential services. GEM (GPON EncapsulaTIonMethod) is similar to the general framing procedure GFP and is a GPON-specific package. It not only has the gigabit transmission capacity per second reflected in the name, but also a new PON technology that is fundamentally different from existing PON systems.

With the continuous rapid development of the Internet and the popularity of personal computers, various value-added multimedia and IP services have proliferated. The existing xDSL access technology cannot meet its huge bandwidth requirements, and the access network has become the "bottleneck" for the development of the entire network. In view of this reality, the Full Service Access Network (FSAN) started the standard study of PON systems above lGb/s from January 2001.

FSAN is trying to find a full-service access solution that is flexible, planable, high-rate, high-efficiency, low-cost, and has rich business and bandwidth management capabilities to effectively support existing and future services. GPON series standard specifications.

The GPON network structure consists of the GPON OLT (OpTIcal Line TerminaTIon), the remote access device ONT (OpTIcal Network Unit), and the ODN (Optical Distribution Network). The ODN does not contain any active electronic devices. The devices are made up of passive components such as passive splitters (Splitter) and do not require expensive active devices. In the GPON network structure, an OLT can have multiple GPON modules, each of which can lead to an independent GPON access network, and is connected to multiple different types of ONUs by passive optical splitters and optical fibers, as shown in the figure. 1 shows [l].

Figure l GPON system reference configuration

The GPON central office equipment OLT is mainly responsible for high-speed connection with the WAN or the backbone metropolitan area network, and transmits the high-speed connection data information to the remote access device ONU through the ODN. As a high-performance optical network platform, it supports packet data and TDM applications in transmission formats such as IP/SONET/SDH/ATM/CWDM on the GPON architecture, and transparently transmits the above application services to meet the real-time and jitter of TDM. The strict requirements, complete service guarantee mechanism and fault handling capability, the downlink rate is as high as 1.244Gb/s or 2.488Gb/S.

The ONU of the remote access device is mainly responsible for the connection with the client device, and functions to aggregate the multi-service data stream of the client. It can be transparently transmitted in the original format of the packet data and the TDM service flow, ensuring the integrity and unification of the service performance. Sex, and provide users with a rich network interface type and standard rate to meet the needs of users for multiple services.

ODN is responsible for data signal transmission of passive optical access lines. Its transmission distance is shorter than that of active optical fiber access systems. The coverage is within 40km. The networking structure is flexible, no need to set up another equipment room, and the maintenance is relatively simple. Therefore, the access network structure can economically provide full service access services for various types of users.

Dynamic Bandwidth Allocation (DBA) technology refers to a technology that adapts various changes in user rates through real-time dynamic changes in the uplink usage bandwidth of each ONU in a GPON system to improve the bandwidth utilization of the system. The uplink access of the PON system generally adopts a combination of central control on-demand allocation and fixed allocation, that is, static bandwidth allocation and dynamic bandwidth allocation according to ITU-T G983.4, and priority allocation according to service level. For the variable rate service of data communication in GPON, static bandwidth allocation is not suitable, and the bandwidth utilization of the system needs to be greatly improved by dynamic bandwidth allocation. That is, the DBA technology passes the ONU according to the requirements of the ONU burst service. Dynamically adjust bandwidth to improve upstream bandwidth utilization efficiency. According to the characteristics of GPON and the G983 recommendation, it can be known that the specific requirements of dynamic bandwidth allocation are: service transparency, bandwidth utilization as high as possible, low jitter and delay characteristics, fair bandwidth allocation, robust signal, strong real-time performance, and guaranteed different services. QoS and so on.

3.1.1 Bandwidth Type

The GPON standard follows the provisions of QoS support in BPON. ITU-T clearly proposes four priority levels of bandwidth in BPON standard G.983.4 and GPON standard G984.3 [2]. They are the four types of bandwidth, Fixed, Assured, Non-assured, and Best-effort, which are fixed type, guaranteed type, non-guaranteed type, and best-effort type of bandwidth. At the same time, the transmission container (T-CONT) of the basic unit of bandwidth allocation in the GPON uplink system is divided into five types according to the type of bandwidth type used. The service level-based priority allocation scheme can provide good QoS guarantee for the service while achieving efficient DBA. Therefore, it is important for the DBA to clearly recognize the characteristics of various bandwidth resources and understand the scope of application of T-CONT.

3.1.2 Transfer Container T-CONT

It is pointed out earlier that GPON has four types of bandwidth, each type of bandwidth can support certain QoS requirements: T_CONT indirectly guarantees the QoS requirements of the service by providing a specific type of bandwidth for the service. Whether a T-CONT can support a certain QOS-required service depends entirely on whether the T-CONT can provide bandwidth that satisfies the QoS requirements of the service. If the T-CONT provides bandwidth that meets the QoS requirements of a service, it can carry the service. In the five types of T-CONT, the type l type T-CONT only provides the fixed type bandwidth; the type 2 type only provides the Assured type bandwidth: the type 3 type provides the Assured type and the Non-assured type bandwidth; the tyPe4 type only provides Best-effort type bandwidth: The type 5 type is the first four types of convergence, which can provide all types of bandwidth and support all types of services. These five T-CONTs are sometimes referred to herein as Type 1, Type 2, Type 3, Type 4, and Type 5 T-CONT, respectively.

3.1.3 GPON implementation of DBA process

According to the protocol, the DBA process of GPON includes the following five steps: congestion state detection, reporting congestion status to OLI, OLT re-bandwidth allocation according to provided parameters, OLT transmitting authorization according to updated bandwidth allocation information and T-CONT type, Negotiation of management information is implemented in the DBA. The DBA process is shown in Figure 2 [3].

Figure 2 GPONDBA process diagram

According to the agreement, the DBA of GPON is divided into two modes: one is that the ONU reports its own state and required bandwidth to the OLT, and the OLT performs DBA on the ONU according to the reported data, so-called dynamic bandwidth allocation based on status report ( SR-DBA, Status Reporting DBA): The other is that the ONU does not need to report to the OLT. The OLT implements automatic and dynamic bandwidth allocation by its own traffic monitoring function, that is, dynamic bandwidth allocation based on status reporting (NSR-DBA, Non Status Reporting DBA).

3.2.1 GEM function

From the perspective of frame structure encapsulation, GEM is similar to other data encapsulation methods. However, GEM is embedded inside the PON and has nothing to do with the SNIs on the OLT side and the UNls type on the ONU side (see Figure 3). That is to say, the GEM encapsulation function is terminated within GPON, and systems other than GPON cannot be seen [4].

Figure 3 embedded GEM

3.2.2 GEM frame structure

The GEM frame structure is shown in Figure 4. It consists of two parts: the GEM frame header and the payload field. The GEM frame header consists of PLI (net load length indication), PortID (port ID), PTI (net load type indication), and HEC (header error check). The PLI indicates that the payload field behind the header has a length of L bytes and a length of 12 bits, up to a maximum of 4095 bytes, so user data frames larger than this value must be transmitted using a fragmentation mechanism. The 12-bit PortID can provide 4096 different ports for multi-port multiplexing, which is equivalent to VPI in APON [5].

Figure 4 GEM frame structure

PTI is used to indicate the content type of the payload segment and the corresponding processing method, similar to the application in ATM. The highest bit indication in 3bit is a data frame or a GEM OAM frame. The lowest bit of the data frame indicates whether it is the end of the frame in the fragmentation mechanism, and the next lower bit indicates whether congestion occurs.

PTI reserves some code. The HEC is 13 bits and provides head error detection and error correction. It is a combination of a BCH (39, 12, 2) code and a parity bit.

After the GEM frame header is determined, the transmitter XORs the header with the fixed OxOXb6AB31E055 and sends the header. The receiver uses the same XOR calculation to reply to the header.

The GPON system uses a fixed downlink frame of 125us period to ensure the timing relationship of the entire system. The downlink frame structure is shown in Figure 5 [3].

Figure 5 is a structure diagram of the downlink frame

The downstream frame is composed of PCBd and load. PCBd is a downlink physical control block that provides OAM functions such as frame synchronization, timing, and dynamic bandwidth allocation.

PCBd consists of the following fields:

The physical synchronization field is a fixed 32-bit style. The ONU logic uses this field to find the starting position of the downstream frame for synchronization with the OLT.

The lower 30 bits of the Ident field contain a superframe counter, and the Ident of each frame will be one greater than the previous one. When this counter reaches its maximum value, the next frame counter will be reset to zero. It is primarily used in user data encryption systems and can also be used to provide lower speed synchronous reference signals.

The PLOAMd field is used to transmit physical layer management information. BIP is a bit interleaved parity 8-bit code used for error monitoring.

The Plend (payload length downstream) field specifies the length of the bandwidth map and the ATM portion, which is sent twice to ensure the robustness of the error.

BWmap is a scalar array of 8-byte allocation structure. Each item of the array represents a special T-CONT bandwidth mapping. The number of items N in the mapping array is given in the Plend field. It is mainly used for uplink bandwidth allocation. .

As shown in Figure 6, the upstream frame of the GPON system consists of a number of bursts, each of which should contain at least PLOu. In addition to the load, it may also contain PLOAMu, PLSu, and DBRu fields. The OLT indicates whether the ONU is allowed to send PLOAMu, PLSu, and DBRu information in each allocation through the Flags field in the BWmap. The load of the upstream frame is composed of the ATM load, the GEM load, and the DBA load.

Figure 6 shows the uplink frame structure of GTC

The Uplink Physical Layer Overhead (PLOu) contains preambles and delimiters for system synchronization, which allows proper operation of the upstream burst links.

The PLOAMu field is used to carry uplink physical layer management information. The PLSu is a power measurement sequence with a length of 120 bytes and is used to adjust the optical power. DBRu is used to report the upstream bandwidth requirement of the ONU to the OLT, and the OLT performs appropriate uplink bandwidth allocation according to the report.

Compared with other access network technologies, GPON can support all existing service types and meet the adaptation requirements of future services, and can provide high-efficiency GEM encapsulation with low implementation cost. The industry has recognized the ideal medium for fiber-to-the-home convergence of triple-play, and GPON, as the most advanced access network technology today, is one of the best technologies for achieving fiber-to-the-home.