Hi to everyone
This post continues with the GPON theme
GPON PRINCIPLES
Key GPON Technologies
Key GPON Technologies
Why Ranging Is Required
· The distances between ONUs and OLTs are different. The transmission time of optical signals on the optical fiber is different, and the time when the optical signal arrives at each ONU is also different.
· The OLT allocates different timeslots to each ONU to transmit upstream data. How to ensure each ONU can accurately locate timeslots?
· How to avoid conflicts between upstream data from multiple ONUs, and implement frame synchronization?
Raging Principles
· The raging principles is as follows:
§ The OLT obtains the Round Trip Delay (RTD) of each ONU through ranging, and calculates the physical distance of each ONU.
§ Then, the OLT specifies a proper equalization delay (EqD)
§ In the ranging process, a window needs to be opened, that is, quit zone to suspend the upstream transmission channels of other ONUs. The OLT opens the window by setting BWmap to null without authorizing any timeslot
· The PON upstream transmission adopts the TDMA mode. One OLT can be connected to multiple ONUs. The distance between the ONU and the OLT can range from dozens of meters to 20 km. Optical fibers are used to transmit optical signals with a delay of 5 ps per kilometer. Due to the change of the ambient temperature and the aging of the components, the transmission delay is also changing. To realize the TDMA access and ensure that the upstream data of each ONU is inserted into the specified timeslots after data is converted on the public fiber without collision between ONUs, the OLT must accurately measure the distance between each ONU and the OLT, so as to control the moment at which each ONU sends upstream data.
· The OLT obtains the round trip delay (RTD) of each ONU through ranging, and specifies equalization delay (EqD) to ensure that no conflict occurs on the optical splitter when each ONU sends data.
· In the ranging process, a window needs to be opened, that is, Quiet Zone to suspend the upstream transmission channels of other ONUs. The OLT opens the window by setting BWmapto null without authorizing any timeslot
Ranging Result
· Using RDT and EqD, the OLT can ensure that data frames sent by different ONUs are synchronized and do not conflict on an optical splitter.
· All ONUs are at the same logical distance and send data in the corresponding timeslots
Why DBA Is Required?
· What Is DBA?
§ Dynamic Bandwidth Assignment (DBA)
§ DBA can dynamically allocate upstream bandwidth within microseconds or milliseconds.
· Why DBA Is required?
§ Improve the upstream line bandwidth utilization of the PN ports.
§ Connect more users to the PON ports.
§ Provide users with better services at higher bandwidths, especially for services that have bandwidth bursts.
· DBA Working Mechanims
§ SR-DBA: status report-DBA
§ NSR-DBA: non status report-DBA
Implementation Process of SR-DBA
· The DBA module of the OLT continuously collects DBA report information, performs calculation, and sends the calculation result to each ONU in the form of BW Map.
· Each ONU sends upstream burst data in their respective timeslot according to the BW Map information to occupy the upstream bandwidth.
· The GPON system uses the SBA+DBA mode to achieve effective bandwidth utilization.
§ TDM services are configured with a bandwidth through SBA to ensure high QoS.
§ Bandwidths for other services are dynamically allocated through DBA.
§ The implementation of DBA is closely related to the QoSguarantee mechanism.
§ The GPON system supports dynamic bandwidth allocation through status reporting and OLT service monitoring (non-status reporting).
· There are three mechanisms for the GPON to report DBA status:
§ PLOu-State IndDBA
§ DBRu-Piggy-back DBA
§ Payload DBA
· The implementation mechanism of the DBA function consists of the following parts:
§ The OLT or ONU performs congestion detection.
§ Report the congestion status to the OLT.
§ Update the bandwidth to be allocated by the OLT according to the specified parameters.
§ The OLT sends authorization according to the newly allocated bandwidth and T-CONT type.
§ Manage DBA operations.
DBA Basics-T-CONT
· Transmission Containers (T-CONTs): dynamically receives the authorization from the OLT, manages the uostream bandwidth allocation at the transmission convergence layer of the PON system, and improves the upstream bandwidth in the PON system.
· Bandwidth type:
§ FB, AB, NAB, BE
· T-CONT type
§ Types 1 to 5
Relationship between T-CONT Types and Bandwidth Types
· According to service priorities, the system sets the SLA for each ONU and limits the bandwidth of the services.
· The maximum and minimum bandwidths limit the bandwidth of each ONU at extreme conditions to ensure that the bandwidth varies according to the service priorities. Generally, the voice service has the highest priority, followed by video and data services.
· The OLT allocates the bandwidth according to the service, SLA and actual ONU conditions. The higher the priority is, higher bandwidth is.
· T-CONTs are classified into five types. In the upstream service scheduling process, different types of T-CONTs are selected for different types of services. Each T-CONT bandwidth type has specific QoS characteristics. QoS characteristics mainly indicate the bandwidth guarantee, including fixed bandwidth, assured bandwidth, assured/maximum bandwidth, maximum bandwidth, and hybrid mode (corresponding to Type 1 to Type 5 of five T-CONT types).
Why Encryption
· The broadcast technology is used in the GPON downstream direction. On the same PON port, data is the same on the backbone optical fiber and after optical splitting. The data received by each ONU is the same. How to ensure that the data intended for one ONU is by parsed by other ONUs?
· The GPON supports AES128 encryption for downstream broadcast data.
§ Advanced encrypt system (AES) is an international encryption algorithm.
ü Only the Payload in a GEM frame is encrypted.
ü The GPON system periodically exchanges and updates the AES key to improve the reliability of the line data.
AES Encryption in the GPON System
· The OLT initiates a key replacement request. The ONU responds to request, generates a new key, and sends the new key to the OLT for three times.
· After receiving the new key, the OLT starts key switching, and notifies the ONU (also three times) of the frame number of the new key by using related commands. The ONU switches the check key in the corresponding data frame.
Here ends the post. To complete your knowledge you can visit the following links:
https://forum.huawei.com/enterprise/en/huaweis-access-network-gpon-principles/thread/603932-100181
