Hello everyone!
Just as we have learned about GPON, which is the most mainstream technology in the access network, today we begin to understand the network architecture, protocol analysis, key technologies, system management, service provisioning modes, and networking protection of GPON in detail.
GPON Network Architecture
PON: a point-to-multipoint (P2MP) passive optical network.
GPON: Gigabit-Capable Passive Optical Network.
A PON consists of an optical line terminal (OLT), an optical network unit/ optical network Terminal (ONU/ONT), and an optical distribution network (ODN).
An optical distribution network (ODN) consists of a passive optical splitter (POS) and optical fibers.
Single Family Unit (SFU)
Single Business Unit (SBU)
Multi-dwelling unit (MDU)
Multi-Tenant Unit (MTU)
Cell Base Unit (CBU)
Optical Access Network Description
As shown in the preceding figure, the optical access network consists of the following modes:
1. FTTB (Fiber to The Business)
FTTB is an access model for commercial subscribers. Based on capacity, FTTB is classified into a single business unit (SBU) and a multi-subscriber lease unit (MTU). The SBU provides only a few POTS, 10/100/1000BASE-T, RF (33 dBmV), and DS1/T1/E1 interfaces. The MTU provides a large number of POTS, 10/100/1000BASE-T, RF, and DS1/T1/E1 interfaces.
2. FTTC&FTTCab (Fiber to The Curb& Fiber to The Cabinet)
The FTTC&FTTCab provides optical fiber-to-the-roadside or FDT access solutions for multi-dwelling units (MDUs) for multi-residential users. The main interfaces are 10/100/1000BASE-T, RF (33 dBmV), and VDSL2. There are a large number of interfaces.
3. FTTH (Fiber to The Home)
FTTH mainly provides the FTTH solution, which is the single-family unit (SFU). The FTTH solution provides a small number of ports such as POTS, 10/100/1000BASE-T, and RF (18 dBmV).
Basic Performance Parameters of the GPON Network
GPON Data Multiplexing Modes
GPON implements single-fiber bidirectional transmission, and the System Uses the Wavelength Division Multiplexing (WDM) technology.
To separate the signals of multiple users on the same optical fiber, the following two technologies are used:
The broadcast technology is used to downlink data streams.
The Time Division Multiple Access (TDMA) technology is used for uplink data streams.
GPON Standard Protocol
GPON Service Bearer Model
The operations for ATM and GEM services are as follows:
ATM in GTC
In the downstream flow, cells are loaded in the ATM block and reach all ONUs. The ONU framing sub-layer extracts cells, and the ATM TC adaptation layer selects correct received cells according to their VPI values. Only the cells with proper VPI values can enter the ATM client function.
In the upstream stream, the ATM service is carried by one or more T-CONTs. Each T- CONT is related to the ATM or GEM service only and is not multiplexed. The OLT identifies the T- CONT according to the Alloc_ID, receives the related transmission cells, and then transmits the cells to the ATM TC adaptation layer and ATM client.
GEM in GTC
In a downstream flow, a GEM frame is loaded in a GEM block and reaches all ONUs. The ONU framing sublayer extracts the corresponding frame, and the GEM TC adapter filters cells according to the 12-bit Port_ID. The GEM client function can be enabled only when the frame with a proper Port_ID value is allowed.
In the upstream stream, the GEM service is carried by one or more T- CONTs. Each T-CONT is related to only the ATM or GEM service and is not multiplexed. The OLT identifies the T-CONT to receive the related transmission data, and then enters the GEM TC adaptation layer and GEM client.
GPON Service Mapping- Upstream Multiplexing
The GPON encapsulation mode (GEM) frame is the smallest service bearer unit in the GPON technology and is the most basic encapsulation structure. All services are encapsulated in GEM frames and transmitted on GPON lines. The services are identified by GEM ports. Each GEM port is identified by a unique port ID, which is globally allocated by the OLT. That is, each ONU/ONT connected to the OLT cannot use a GEM port with the same port ID. A GEM port identifies the service virtual channel between the OLT and the ONU/ONT, that is, the channel that carries the service stream. The GEM port is similar to the VPI/VCI in the ATM virtual connection.
T-CONT: It is a carrier for carrying services in the GPON upstream direction. All GEM ports need to be mapped to T-CONTs. The OLT transmits services upstream in DBA scheduling mode. A T-CONT is the basic control unit of upstream service flows in a GPON system. Each T-CONT is uniquely identified by an Alloc-ID. The Alloc-ID is globally allocated by the OLT. That is, each ONU/ONT connected to the OLT cannot use the T-CONT with the same Alloc-ID.
Figure 1 shows the service multiplexing principle in the GPON system. Different services are mapped to different GEM ports on the ONT. The GEM ports carry the services and then map the services to different types of T-CONTs for upstream transmission. A T-CONT is a basic bearer unit in the upstream direction of a GPON line. On the OLT side, the T-CONT first demodulates the GEM port unit, and then sends the demodulated GEM port payload to the GPON MAC chip for processing. Other processing steps are the same as those on the switch or access network.
Figure 2 shows the mapping between services and GEM ports and between services and T-CONTs. A GEM port is the smallest service unit in a GPON system. One GEM port can carry one or more types of services. After a GEM port carries services, the GEM port needs to be mapped to the T-CONT for upstream service scheduling. Each ONT supports multiple T-CONTs and can be configured with different service types. A T-CONT can carry multiple GEM ports or one GEM port, depending on the actual configuration of the user. After the data carried by the T-CONT is transmitted upstream to the OLT, the data is demodulated from the GEM port, and then the service payload of the GEM port is demodulated for service processing.
GPON Service Mapping- Downstream Multiplexing
In the downstream direction, all services are encapsulated into the CEM port in the CPON service processing unit and broadcast to all the ONUs connected to the GPON port. An ONU filters the data according to the GEM port ID, retains only the GEM port belonging to the ONU, decapsulates the service, and then sends the service from the service interface of the ONU to the user equipment.
A shared GEM port is a multicast GEM port.
GPON frame structure
The OLT uses the Flag field in the Bwmap to indicate whether the PLSu, PLOAMu, or DBRu information is transmitted in each allocation. When setting the transmission period, the OLT scheduler also needs to consider the bandwidth and delay requirements of these auxiliary channels.
The status information of the PLOu is included in the allocation arrangement. Each time an ONU takes over a PON medium from another ONU, a copy of the new PLOu data must be sent. When an ONU is allocated with two consecutive IDs (the StopTime of one ID is 1 less than the StartTime of the other), the ONU suppresses the sending of the PLOu data to the second Alloc_ID. When the OLT authorizes multiple consecutive Alloc_ID to the ONU, the suppression can occur multiple times. Note that the continuous OLT allocations to the same ONU must have intervals. The allocations must be strictly continuous or for 2 different ONUs.
User payload data is followed by these overheads during transmission until the position indicated by the StopTime pointer is reached.
GPON Upstream Data
GPON Upstream frame structure
The GPON upstream frame consists of the PLOu, PLOAMu, PLSu, DBRu, and Payload fields. The specific meanings are as follows:
PLOu: physical control header, which is used for frame location, synchronization, and identification of the ONU data of the frame.
PLOAMu: indicates the PLOAM message of upstream data, which is used to report management messages such as ONU maintenance and management status. (Not every frame can be sent, but negotiation is required.)
DBRu: reports the T-CONT status and dynamically allocates bandwidth to the ONU for the next bandwidth application. (Not every frame can be sent, but negotiation is required.)
Payload: data payload, which can be a DBA status report or a data frame. If the frame is a data frame, the frame may be classified into a GEM header and a frame.
GEM header: indicates the GEM frame header, which is used to distinguish data in different GEM ports. The GEM port is the smallest unit for data transmission in GPON. It is similar to the concept of PVC in ATM. Each upstream service must be mapped to the GEM port, and the GEM port is mapped to the T-CONT for transmission.
The GEM header fields are classified into PLI, Port ID, PTI, and HEC. The meanings are as follows:
PLI: indicates the length of the data payload.
Port ID: uniquely identifies a GEM port.
PTI: payload type identifier, which is mainly used to identify the status and type of the currently transmitted data, for example, whether the data is an OAM message and whether the data has been transmitted.
HEC: provides the forward error correction (FEC) function to improve transmission quality.
GPON Downstream Data
Broadcast mode: The downstream frame length of the CPON is fixed at 125 us. In this mode, all ONUSs can receive the same data. The data of different ONUs is distinguished by GEM port ID. The ONUs receive their own data through filtering.
GPON downstream frame structure
A GPON downstream frame consists of PCBd and Payload. The PCBd is a physical control block that implements frame synchronization, positioning, and bandwidth allocation. The payload carries downlink data. Each frame may consist of many units.
PCBd: includes the synchronization field, which indicates whether to enable the FEC algorithm. It mainly allocates uplink bandwidth to solve the bandwidth utilization problem.
In the US BW Map field, Alloc ID indicates the sequence number of the T-CONT. SStart and SStop indicate the number of timeslots where the upload of the T-CONT starts and stops. CRC is a check bit.
Mapping mode of TDM services in GPON
TDM services are queued in the buffer and multiplexed into GEM frames for transmission based on a fixed number of bytes.
In this mode, specific TDM services are not sensed and only transparently transmitted.
The GEM frame has a fixed length, which is advantageous for TDM service transmission.
Mapping mode of Ethernet services in GPON
The GPON system parses the Ethernet frame and maps the data to the GEM payload for transmission.
The GEM frame automatically encapsulates header information.
The mapping format is clear, the device is easy to implement, and the compatibility is good.
Thanks for reading!
