GPON Technology: How Does the Fiber Optic FTTH Network Work?

We are going to start by explaining a series of concepts that will help us to better understand how this technology works.

GPON network terminology

The GPON networks are made up of different equipment to carry the connection to the network and the Internet through fiber optics, knowing what each equipment is and what it is for is very important, then you can read in detail all the equipment with which it is used. it works when we talk about GPON.

• Gigabit Capable Passive Optical Network (GPON) : Standard for Passive Optical Networks (PON) published by the ITU-T.
• Optical distribution network (ODN) : They are the physical fiber optic devices that distribute signals to users in a telecommunications network. The ODN is made up of passive optical components (POS), such as optical fibers, and one or more passive optical splitters.
• Optical network termination (ONT) / Optical network units (ONU) : These are the equipment that is installed in the end users (desktops, telephones, etc.) to connect to the GPON network. They provide conversion from optical to electrical signal. The ONT’s also provide AES encryption through the ONT key.
• Splitters – Used to add or multiplex fiber optic signals to a single upstream fiber optic cable. In general, the most used ratio is 1:32.
• Optical Line Terminal (OLT) : A device that aggregates all the optical signals from the ONTs into a single multiplexed beam of light that is then converted to an electrical signal, formatted according to TPE standards of Ethernet packets for Layer 2 forwarding or Layer 3.
• Wavelength Division Multiplexing (WDM) : Wavelength Division Multiplexing (WDM) is a technology that multiplexes several optical carrier signals onto a single optical fiber using different wavelengths (i.e. colors) of light To be.
• GEM G-PON (GEM) encapsulation method : It is a data frame transport scheme used in gigabit-capable passive optical network (G-PON) systems that is connection-oriented and supports the fragmentation of data frames. user data in stream chunks of varying size.
• Fiber to the X (FTTX) : FTTX is a generalization for various fiber deployment configurations, organized into two groups: FTTP / FTTH / FTTB (Fiber laid to the end of the premises / home / building) and FTTC / N (fiber laid to the cabinet / node, with copper wires to complete the connection).
• T-CONT / TCONT : It is the transmission container.
• OMCC : It is the control and management channel for optical network units.
• OMCI : It is the control and management interface of the optical network unit.
• PCBd : It is the downstream physical control block.
• TDM : It is time division multiplexing.
• TDMA : Time Division Multiple Access.

Network diagram

In the popular Ethernet design we have three main levels, the core or Core where the L3 equipment is located, they are interconnected between them 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 telephones and others.

In the GPON design we will find a total of two levels, the OLT is one of the most important parts because it will be the one used to interconnect the different equipment, we also have 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 devices are passive, as the name “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 to the network thanks to the fiber, in addition, it is very cheap because the electricity consumption is minimal.

Technology overview

First the OLT is connected to the optical splitter through a single optical fiber, and then the optical splitter will be connected to the GPON ONU / ONT. Then GPON will adopt WDM to transmit data of different upstream / downstream wavelengths on the same ODN. The wavelengths will range from 1290-1330 nm in the upstream direction and 1480-1500 nm in the discharge direction. It will start data transmission in the download direction and in turn in burst mode upload in TDMA mode (time interval based). Finally, point-to-multipoint multicast (P2MP) transmission will be supported.

GPON technology features

GPON technology has been around for years, giving us really high download and upload speeds in our homes, even if we are a long way 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.

GPON limits

• Maximum logical range: 60 km (this is the maximum distance managed by the upper layers of the system (MAC, TC, Ranging), in view of a future specification dependent on physical media (PMD)).
• Maximum fiber distance between sending / receiving (S / R) and receiving / sending (R / S) points: 20 km.
• Maximum differential fiber distance: 20 km.
• Split ratio: route loss restricted, PON with passive splitters (16, 32 or 64 way split).
• Rate: 1.24416 Gbps upload, 2.48832 Gbps download.

Energy budget

As part of GPON, the loss of optical power must be taken into account. This loss can be introduced in various ways, such as:

• Loss within dividers.
• Loss per km of fiber (approximately 0.35 dB per km for 1310 and 1490 nm).
• Splice loss (> 0.2 dB).
• Fiber bending.

As the picture shows, the amount of loss incurred from using multiple dividers:

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 for another, eg. ex. the 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 a loss of approximately 3 dB.

How packet transfer works in GPON technology

Downstream packet path (from OLT to ONT)

Downstream packet ride. As shown in the picture, the packets go downstream from the OLT to various ONUs or ONTs.

Tip to understand the diagram: The downstream flow 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 transmissions, 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 the discharge is 1480 – 1500 nm. Continuous mode operation in discharge direction – even when there is no user traffic through GPON, there is a constant signal, except when the laser is administratively off.

As shown in the picture, the downstream packet forwarding procedure.

Communication process

1. The OLT sends Ethernet frames from the uplink ports to the rules-based GPON service processing module configured for the PON ports.
2. The GPON service processing module encapsulates Ethernet frames in GEM port data packets for downstream transmission.
3. GPON Transmission Convergence (GTC) frames containing GEM PDUs are transmitted to all ONT / ONUs connected to the GPON port.
4. The ONT / ONU filters the received data based on the GEM port ID contained in the header of the GEM PDU and retains the data only significant for the GEM ports in this ONT / ONU.
5. The ONT decapsulates the data and sends the Ethernet frames to the end users through the service ports.

Downstream packet frame structure

A GPON frame in the discharge direction has a fixed length of 125 s, made up of two components: the physical control block in the discharge direction (PCBd) and the 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 GTC header and BWmap

• GTC header – Used for frame delineation, timing, and forward error correction (FEC).
• BWmap: the field notifies the ONU itself of the upstream bandwidth allocation. Specify the ascending start and end time intervals for the T-CONTs of each ONU, this ensures that all ONUs send data based on the time intervals specified by the OLT to avoid data conflicts.
  As shown in the picture, an enlarged view of the PCBd and what the GTC payload contains.

Key terms

• Psync (4 bytes long): physical synchronization field, indicates the start of each PCBd.
• Ident (4 bytes long): used to indicate larger frame structures, contains the super frame counter used by the encryption system.
• PLOAMd (13 bytes long) – OAM Physical Layer (PLOAM) downstream field, think of this as a message-based management and operation channel between OLT and ONU / ONT.
• BIP (1 byte in length): bit interleaved parity, by the receiver to measure the number of errors in the link.
• Plend (4 bytes long): Descending field of the length of the payload.

Upstream packet path (from ONT to OLT)

As shown in the picture, upstream packet flow from multiple ONUs to OLT.

Tip to understand the diagram: You can think of upstream from the splitter perspective, or the traffic sent from the ONU / ONT, the end users to the OLT.

Upstream packet transmission occurs via TDMA (Time Division Multiple Access). The distance between OLT and ONT / ONU is measured. Slots are allocated based on the distance ONT / ONU sends upstream traffic based on the time slot granted. Dynamic Bandwidth Allocation (DBA) enables the OLT to monitor congestion, bandwidth usage, and configuration in real time. Detects and prevents collisions across the range. The upstream wavelength ranges from 1290 to 1330 nm. As shown in the picture, the upstream packet forwarding procedure.

Communication process

1. ONT / ONU sends Ethernet frames to GEM ports according to configured rules that map service ports and GEM ports.
2. GEM ports encapsulate Ethernet frames in GEM PDUs and add these PDUs to TCONT queues according to the rules that assign GEM ports and TCONT queues.
3. TCONT queues use DBA-based time slots and then transmit the upstream GEM PDUs to the OLT.
4. OLT decapsulates the GEM PDU, now the original Ethernet frame is visible.
5. OLT sends Ethernet frames from a specified uplink port according to the rules that assign service ports and uplink ports.

Upstream packet frame structure

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 intervals according to the requirements of the bandwidth map (BWmap). Each ONU reports the status of the data to be sent to the OLT using upstream frames. OLT uses DBA to assign upstream time slots to ONUs and sends updates on every frame.

Note: Upstream frames are sent as bursts, which are made up of the upstream physical layer overhead (PLOu) and one or more bandwidth allocation slots associated with a specific Alloc-ID.

As the picture shows, the difference between a descending and ascending frame.

Key terms

• Upstream Physical Layer Overload (PLOu) – Upstream Physical Layer Overload.
• Upstream OAM Physical Layer (PLOAMu) – Upstream data PLOAM messages. Think of this as a message-based operation and management channel between the OLT and the ONUs / ONTs.
• Up Power Level Sequence (PLSu) – Up Power Level Sequence.
• Dynamic Upstream Bandwidth Report (DBRu) – Dynamic Upstream Bandwidth Report.
• Payload: user data.

Functional blocks of GPON technology

OLT function blocks

An OLT consists of three main parts:

• Service port interface function – Provides translation between the service interfaces and the TC frame interface of the PON section.
• Cross-connect functionality – Provides a communication path between the PON shell and the service shell, as well as cross-connect functionality.
• Optical Distribution Network Interface (ODN) – Further subdivided into two parts:

PON interface function

PON TC function – Responsibilities include framing, media access control, OAM, DBA, and protocol data unit (PDU) delineation for the cross-connect function and ONU management.

ONU / OLT functional blocks

The functional blocks are similar to the OLT. In case the ONU / OLT works with a single PON interface (maximum 2 for protection purposes), the crossover function is omitted. Instead of this feature, the MUX and DEMUX service are now responsible for the traffic.

Stacking protocol

The GPON protocol has its own stack, just Ethernet or IP. As shown in the image, this is the stacking protocol for GPON:

Key terms

• PMD Layer – Equivalent to the GPON interfaces found between OLTs and ONUs.
• GTC Layer – Responsible for encapsulating payloads through the use of ATM cells or GEM frames. GEM frames can carry Ethernet, POTS, E1 and T1 cells.

Traffic mapping: Ethernet

• It resolves Ethernet frames and directly maps the Ethernet frame data to the GEM payload.
• GEM frameworks automatically encapsulate the header information.
• 1: 1 alignment between an Ethernet frame and a GEM frame.

As shown in the image, an Ethernet frame is mapped to a GEM frame:

OMCI

The ONU Management and Control Interface (OMCI) messages are used to discover ONT / ONUs 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:

• Establish and release connections with the ONT.
• Manage the UNIs in the ONT.
• Request configuration information and performance statistics.
• Autonomous alert of events, such as a link failure.

Key points:

• The protocol runs through a GEM connection between OLT and ONT.
• The GEM connection is established while the ONT is initializing.
• Protocol operation is asynchronous – OLT controller works as master, ONT controller as slave.

Important techniques

Rank

To avoid data conflicts (collisions), the OLT must be able to accurately measure the distance between itself and each ONU / ONT to provide a suitable time interval to provide data upstream. This allows ONUs to send data at specified time intervals to avoid upstream problems. This process is accomplished through a technique called rank.

Rank process

The OLT initiates the process at an ONU when the ONU first registers with the OLT and obtains the round-trip delay (RTD) from the ONU.

Based on the RTD, the other key components are identified

Calculation of the physical scope of that specific ONU, since this OLT requires an adequate equalization delay (EqD) for each ONU based on the physical scope. 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 ONU / OLTs at the same virtual distance from the OLT.

Burst technology

Upstream packet flow is achieved through bursts, and each ONU / ONT is responsible for data transmission within its assigned time slots. When an ONU / ONT is not within its time slot, the device disables transmission from its optical transceiver to avoid other ONU / ONT hits.

• The burst transmission function is supported by ONU / ONT modules.
• The burst reception function is compatible with OLT modules.
• The varying distance between each ONU / ONT and OLT results in an attenuation of the optical signal. As a result, the power and level of the packets received by an OLT vary at different time intervals.
• Dynamic Threshold Adjustment allows the OLT to dynamically adjust the threshold for optical power levels. This ensures that all ONU signals can be recovered.

As shown in the image, a demonstration of different data streamed in bursts and then recovered:

Dynamic Bandwidth Allocation (DBA)

DBA enables an OLT module to monitor PON network congestion in real time. This allows the OLT to adjust bandwidth based on a variety of factors, including congestion, bandwidth usage, and configuration.

DBA key points

The DBA module built into the OLT constantly collects DBA reports, performs calculations, and notifies the ONU through the BWMap field within the downstream frame. As a result of the BWMap information, the ONU sends data upstream in the time slots allocated to occupy the upstream bandwidth. Bandwidth can also be allocated in static / fixed mode.

Using the DBA allows

Improved use of upstream bandwidth on a PON port. Higher bandwidth for users and support for more users on one PON port. Forward Error Correction (FEC). Transmission of digital signals can introduce bit errors and jitters, which can degrade the transmission quality of the signal. GPON can take advantage of FEC, which allows the RX endpoint to check for error bits in the transmission.

Note: FEC is one-way and does not support error information comments.

Key points of FEC:

• It does not require data retransmission.
• It supports FEC only in the downstream direction.
• Improved PCBd transmission quality and payload processing.

Line encryption

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.

Key line encryption points

• Using line encryption does not increase overhead or decrease bandwidth usage.
• Using line encryption does not prolong transmission delays.

Key exchange and switching

• The OLT initiates a key exchange request to the ONU. The UN responds to the request with a new key.
• After receiving the key, the OLT uses the new key to encrypt the data.
• OLT sends the frame number that the users of the new key send to the ONU.
• The ONU receives the frame number and switches the verification key on incoming data frames.

As shown in the image, the key exchange process:

Network protection modes in GPON technology

There are several different types of network protection modes that GPON can use.

Type A

• It does not require an additional OLT PON port.
• When the primary fiber fails, services are transferred to the secondary fiber.
• The duration of the outage depends on the recovery time of the line.
• If the failure occurs on the splitter line to the UN, there is no backup.

Type B

• OLT provides two GPON ports as valid and protection OLTs.
• Protection is restricted to the fiber from the OLT to the splitter and the OLT boards.
• Equipment redundancy is not provided on the ONU or power fibers.
• No ONU or full ODN protection.
• It uses a 2 x N splitter and without any additional optical loss.

Type C

Redundancy for OLT, ODN and ONU (s)

• Provides 2 fully redundant links to subscriber facilities.
• Two options: 1 + 1 linear and 1: 1 linear protection.

1 + 1 protection:

• PON protection is dedicated to valid PON.
• Normal traffic is copied and sent to both PONs, with a permanent bridge between the two OLTs.
• Traffic is sent to an ONU simultaneously, the selection between the two signals is based on predetermined criteria.

1: 1 protection:

• Normal traffic is carried on the valid or protection PON.
• Automatic protection switches between PONs.
• More expensive, but offers maximum availability.

Finally, 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 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 above 1 Gbit / s. Two new recommendations have subsequently been edited: G.984.6 (Extension of scope) and G.984.7 (Long scope). With all this information we hope that you can now fully understand GPON technology.