GPON stands for Gigabit Passive Optical Network, the alternative to Ethernet switching in campus networks.GPON replaces the traditional three-tier Ethernet design with a two-tier optical network by eliminating access and distribution Ethernet switches with passive optical devices.Today at RedesZone we are going to tell you all the technical secrets of this technology, which is used by all fiber optic operators in Spain to bring fiber Internet connection to the homes of all their customers.We are going to start by explaining a series of concepts that will help us to better understand how this technology works.GPON networks are made up of different pieces of equipment to bring the connection to the network and to the Internet through fiber optics. Knowing what each piece of equipment is and what it is for is something very important. Next, you can read in detail all the pieces of equipment with which works when we talk about GPON.In the popular Ethernet design we have three main levels, the nucleus or Core where the L3 devices are, they are interconnected with each other and provide redundancy using internal gateway dynamic routing protocols such as OSPF, and also protocols such as VRRP.The distribution level is also made up of L3 and L2 equipment, and finally we have the access layer, which are the equipment to which the final equipment will be connected, such as computers, WiFi access points, IP phones, and others.In the GPON design we will find a total of two levels, the OLT (Optical Line Terminal) is one of the most important parts because it will be the one used to interconnect the different equipment, we also have the 1:32 splitters that allow us to subdivide the fiber to connect more users simultaneously, and, finally, we have an ONT for each of the users.Of course, all these equipments are passive, as the very name of «GPON» indicates.As you have seen, the GPON design is very simple but powerful. A good example of this is that it will allow us to achieve high speeds on the network thanks to fiber. In addition, it is very cheap because electricity consumption is minimal.First the OLT (Optical Line Terminal) is connected to the optical splitter through a single optical fiber, and then the optical splitter will be connected to the ONUs/ONTs.Later GPON will adopt WDM to transmit data of different wavelengths upstream/downstream over the same ODN.Wavelengths will range from 1290-1330nm in the up direction and 1480-1500nm in the down direction.It will start the data transmission in the download direction and in turn in the upload direction in burst mode in TDMA mode (based on time slots).Lastly, point-to-multipoint (P2MP) multicast transmission will be supported.GPON technology has been with us for years, giving us really high download and upload speeds in our homes, even if we are very far from the main OLT to which our encryption branch connects.Now we are going to see the main characteristics of GPON, so that you can see its limits and electrical consumption.As part of GPON, optical power loss must be taken into account.This loss can be introduced in various ways, such as:As shown in the image, the amount of loss incurred by using multiple splitters:As shown in the picture, the minimum and maximum optical path loss per class:NOTE: The requirements of a particular class may be more stringent for one type of system than another, e.g.egthe class C attenuation range is inherently more stringent for the TCM system due to the use of a 1:2 splitter/combiner on each side of the ODN, each with about 3 dB loss.Next, we are going to explain how the different packages are transferred using GPON technology, these packages travel from the OLT of our operator to the ONT that we have in our house, from start to finish, but in between we have different devices such as Splitters to serve more customers.Packet walk downstream.As shown in the image, the packets go downstream from the OLT to various ONUs or ONTs (Optical Node Terminal).Tip to understand the diagram: The downstream is from the perspective of the splitter, we can think of it as traffic directed towards the ONU / ONT, or the end users.Downstream packets are forwarded as broadcasts, with the same data sent to the same ONU/ONT with different data identified by the GEM port ID.Allows an ONU/ONT to receive the desired data by ONU ID.The wavelength range for discharge is 1480 – 1500nm.Continuous mode operation in downstream direction – even when there is no user traffic through GPON, there is a constant signal, except when the laser is administratively turned off.As shown in the picture, the downstream packet forwarding procedure.A downstream GPON frame has a fixed length of 125 s, composed of two components: downstream physical control block (PCBd) and payload.The OLT transmits PCBd to all ONUs/ONTs.The ONUs / ONTs receive the PCBd and carry out operations based on the information received.PCBd consists of the GTC header and BWmapAs shown in the image, upstream packet flow from various ONUs to the OLT.Tip to understand the diagram: You can think of the upstream from the perspective of the splitter, or the traffic sent from the ONU/ONT, the end users towards the OLT.Upstream packet transmission occurs via TDMA (Time Division Multiple Access).The distance between OLT and ONT/ONU is measured.Time slots are allocated based on distance ONT/ONU sends upstream traffic based on granted time slot.Dynamic Bandwidth Allocation (DBA) allows the OLT to monitor real-time congestion, bandwidth usage, and configuration.Detect and prevent collisions across the range.The upstream wavelength varies from 1290 to 1330 nm.As shown in the picture, the upstream packet forwarding procedure.Each upstream GPON frame has a fixed duration of 125 s.Each upstream frame contains the data carried by one or more T-CONT/TCONTs.All ONUs connected to a GPON port share the upstream bandwidth.All ONUs send their data upstream in their own time slots based on the bandwidth map (BWmap) requirements.Each ONU reports the status of the data to be sent to the OLT through the use of uplink frames.OLT uses DBA to allocate upstream time slots to ONUs and sends updates every frame.Note: Upstream frames are sent as bursts, which are made up of upstream physical layer overhead (PLOu) and one or more bandwidth allocation slots associated with a specific Alloc-ID.As shown in the picture, the difference between a downstream and upstream plot.An OLT consists of three main parts:PON TC function – Responsibilities include framing, media access control, OAM, DBA, and protocol data unit (PDU) delineation for cross-connect function and ONU management.The functional blocks are similar to the OLT.In the case that the ONU/OLT works with a single PON interface (maximum 2 for protection purposes), the cross-connection function is omitted.Instead of this function, the MUX and DEMUX service are now responsible for the traffic.The GPON protocol has its own stack, just Ethernet or IP.As shown in the image, this is the stack protocol for GPON:As shown in the image, an Ethernet frame is mapped to a GEM frame:ONU Management and Control Interface (OMCI) messages are used to discover ONT/ONO for management and control.These specialized messages are sent through dedicated GEM ports established between an OLT and an ONT/ONU.The OMCI protocol allows an OLT to:To avoid data conflicts (collision), the OLT must be able to accurately measure the distance between itself and each ONU/ONT to provide an adequate time slot to facilitate upstream data.This allows ONUs to send data at specified time intervals to avoid upstream problems.This process is accomplished through a technique called ranging.The OLT starts the process on an ONU when the ONU first registers with the OLT and obtains the round trip delay (RTD) from the ONU.Calculation of the physical reach of that specific ONU, since this OLT requires an appropriate equalization delay (EqD) for each ONU based on the physical reach.RTC and EqD synchronize the data frames sent by all ONUs.As shown in the image, a demonstration of what the process achieves, to place all ONUs/OLTs at the same virtual distance from the OLT.Upstream packet flow is achieved through bursts, with each ONU/ONT responsible for transmitting data within their assigned time slots.When an ONU/ONT is not within its time slot, the device disables its optical transceiver transmission to avoid other ONU/ONT hits.As shown in the image, a demonstration of different data transmitted by bursts and then retrieved:DBA enables an OLT module to monitor congestion in the PON network in real time.This allows the OLT to adjust bandwidth based on a variety of factors, including congestion, bandwidth usage, and configuration.The DBA module embedded within the OLT constantly collects DBA reports, performs calculations and notifies the ONU via the BWMap field within the downlink frame.As a result of the BWMap information, the ONU sends data upstream in the allotted time slots to occupy the upstream bandwidth.Bandwidth can also be assigned in static/fixed mode.Improved upstream bandwidth usage on a PON port.Higher bandwidth for users and support for more users on one PON port.Forward Error Correction (FEC).The transmission of digital signals can introduce bit errors and jitter, which can degrade the transmission quality of the signal.GPON can take advantage of FEC, which allows the RX end to check for error bits in the transmission.Note: FEC is one-way and does not support error information feedback.All downstream data is transmitted to all ONUs.One risk is that it is not authorized.ONUs receive downstream data destined for authorized ONUs.To combat this, GPON uses the AES128 algorithm to encrypt data packets.As shown in the picture, the key exchange process:There are several different types of network protection modes that GPON can use.Comment that GPON technology is a telecommunications access technology that, as we have seen, uses fiber optics to reach the end customer.Its technical standards were approved in 2003-2004 by ITU-T in the recommendations G.984.1, G.984.2, G.984.3, G.984.4 and G.984.5.All equipment manufacturers must comply with it to ensure interoperability.These are the standardizations of PON networks at speeds greater than 1 Gbit/s.Two new recommendations have subsequently been edited: G.984.6 (Range extension) and G.984.7 (Long range).With all this information we hope that now you can fully understand GPON technology.We have already seen in detail how GPON technology works, now we are going to talk about the advantages and disadvantages of this standard.As strong points, we must take into account that the GPON standard allows us to multiply by four the maximum link distance with respect to copper connections used by ADSL and VDSL2 technologies, being able to have links of up to 20Km between the OLT and the ONT without loss. of signal, in addition, we must take into account that the bandwidth is much greater, because it allows us to achieve links of up to 2.4Gbps speed.Another aspect that we must highlight is that GPON works with voice, data and video without the need to deploy anything else, it also has a QoS to guarantee quality of service, and AES encryption to keep our data protected from the OLT to the ONT.However, not all are advantages in terms of the GPON standard.As disadvantages, we can talk about the fact that this technology implies being very careful when making the cable splices, it must have maximum accuracy, and the connectors as well, because at the slightest bit of dirt or if the splice is poorly made, we could lose a lot of speed and even directly not connect correctly.Another negative point is that we cannot use the ONT that we want, depending on the OLT and how the operator has it configured internally, we will need to use an ONT allowed by said operator, otherwise we will not be able to authenticate.Operators generally do not help customers who want to buy an ONT that is not their own, so you will have to make a living in the vast majority of cases.The GPON standard today is already considered almost old, because the operators are already deploying the XGS-PON standard based on the GPON that we have talked about, but they are capable of providing a speed of up to symmetrical 10Gbps, both in download and upload.This will allow operators to provide FTTH connections at real speeds of around 8.5Gbps, much higher than the current 2.5Gbps of the GPON standard.In addition, another aspect that we must highlight is that the XGS-PON standard allows there to be no "collapse" during peak hours, that is, that all customers who have contracted 2Gbps speed always achieve these speeds.Currently all fiber optic operators to the home (FTTH) use the GPON standard for their Internet connections, but they have been working on XGS-PON for years, which is the future of GPON networks, a much more efficient, faster standard that allows us to It will allow you to level up in terms of speed and connection quality.Operators such as Digi have already announced their 10Gbps speed, although the speed that customers could actually achieve is approximately 8.5Gbps, Orange has also announced this type of technology and it will arrive in the coming months.It is clear that all operators are betting on XGS-PON, which is the evolution of the GPON that we know so far.As you have seen, the GPON standard is what is currently used for FTTH networks, a standard that has been with us for many years and that has allowed us to enjoy real speeds of up to 1Gbps by operators.Now the XGS-PON standard will be the priority for the operators, because it is much faster and will avoid any kind of bottleneck, especially in urban centers where there is a high density of connected clients and transferring a lot of traffic.