GPON Fundamentals

GPON stands for Gigabit Passive Optical Networks. GPON is defined by ITU-T recommendation series G.984.1 through G.984.6. GPON can transport not only Ethernet, but also ATM and TDM (PSTN, ISDN, E1 and E3) traffic. GPON network consists of mainly two active transmission equipments, namely- Optical Line Termination (OLT) and Optical Network Unit (ONU) or Optical Network Termination (ONT). GPON supports triple-play services, high-bandwidth, long reach (upto 20km), etc.

Figure 1 shows various FTTx network architectures.

A single fibre from the OLT runs to a passive Optical Splitter (passive means, it does not require any power to operate) which is located near the users' locations. The Optical Splitter merely divides the optical power into N separate paths to the users. The optical paths can vary between 2 to 128. From the Optical Splitter, a single mode (SM) fibre strand run to each user. This is shown in figure 2. GPON adopts two multiplexing mechanisms- a) in downstream direction (i.e. from OLT to users), data packets are transmitted in an broadcast manner, but encryption (AES) is used to prevent eavesdropping, b) in upstream direction (i.e. from users to OLT), data packets are transmitted in a TDMA manner.

The next section describes GPON key technologies.

ONU Identifier (ONU-ID)

ONU-ID is an 8-bit identifier that an OLT assigns to an ONU during ONU activation via PLOAM messages. The ONU-ID is unique across the PON and remains until the ONU is powered off or deactivated by the OLT.

Allocation Identifier (ALLOC_ID)

ALLOC_ID is a 12-bit number that the OLT assigns to an ONU to identify a traffic-bearing entity that is a recipient of upstream bandwidth allocations within that ONU. This traffic-bearing entity is also called T-CONT.

Each ONU is assigned a default ALLOC_ID which is equal to that ONU's ONU-ID, and may be assigned additional ALLOC_IDs as per OLT's discretion.

Transmission Containers (T-CONT)

A Transmission Container (T-CONT) is an ONU object representing a group of logical connections that appear as a single entity for the purpose of upstream bandwidth assignment on the PON. For a given ONU, the number of supported T-CONTs is fixed. The ONU autonomously creates all the supported T-CONT instances during ONU activation. The OLT discovers the number of T-CONT instances supported by a given ONU.

To activate a T-CONT instance to carry upstream user traffic, the OLT has to establish a mapping between T-CONT instance and an ALLOC_ID, which has been previously assigned to the ONU via PLOAM messages. Any ALLOC_ID assigned to the ONU, including the default ALLOC_ID, can be associated with single user traffic T-CONT.

There are 5 types of T-CONTs which can be allocated to the user-
  1. Type 1: This T-CONT is of fixed bandwidth type and mainly used for services sensitive to delay and high priority like VOIP.
  2. Type 2 and Type 3: Both T-CONT are of guaranteed bandwidth types and mainly used for video services and data services of higher priorities.
  3. Type 4: This T-CONT is of best-effort type and mainly used for data services such as Internet and services of low priority which do not require high bandwidth.
  4. Type 5: This T-CONT is of mixed type, involving all bandwidth types and bearing all services.

Dynamic Bandwidth Allocation (DBA)

The OLT is responsible for allocating upstream bandwidth to the ONUs. Because the access network is shared, ONU upstream transmissions could collide if they were transmitted at random times. ONUs can be located at varying distances from the OLT, and hence the transmission delay from each ONU is unique. The OLT measures delay and sets a register in each ONU via PLOAM (Physical Layer Operations, Administration and Maintenance) messages to equalize its delay with respect to all other ONUs on the access network. This is called Ranging.

Once the delay of all ONUs have been set, the OLT transmits grants to individual ONUs. A grant is permission to use a defined interval of time for upstream transmission. The grant map is dynamically re-calculated every few milliseconds. The map allocates bandwidth to all ONUs such that each ONU receives timely bandwidth for its needs.

DBA is a methodology that allows quick adoption of users' bandwidth allocation based on current traffic requirements and it is especially good for dealing with bursty upstream traffic. GPON uses TDMA for managing upstream access by ONUs, and at any one point in time, TDMA provides unshared timeslots (upstream bandwidth over time) to each ONU for upstream transmission.

DBA allows upstream timeslots to shrink and grow based on the distribution of upstream traffic loads. DBA functions on T-CONTs, which are upstream timeslots, and each is identified by a particular ALLOC_ID. An ONU must have atleast one T-CONT, but most have several T-CONTs, each with its own priority or traffic class, and each corresponds to a particular upstream timeslot on the PON. Without DBA support on the OLT, upstream bandwidth is statically assigned to T-CONTs, which cannot be shared, and can be changed only through a management system.

There are two forms of DBA - Status Reporting DBA (SR-DBA) and Non-Status Reporting DBA (NSR-DBA).

In NSR-DBA, an OLT constantly allocates a small amount of extra bandwidth to each ONU. If the ONU has no traffic to send, it transmits idle frames. If the OLT observes that an ONU is not sending idle frames, it increases the bandwidth allocation to that ONU. Once that ONU starts sending idle frames, the OLT reduces its allocation accordingly. NSR-DBA has the advantage that the ONUs need not be aware of DBA, however, its disadvantage is that there is no way for the OLT to know how to allocate bandwidth to several ONUs in the most efficient way.

SR-DBA involves explicit T-CONT buffer status provided by the ONUs when OLT polls them. In this method, the OLT solicits T-CONT buffer status, and the ONUs respond with a separate report for each assigned T-CONT. The report contains the data currently waiting in T-CONTs in the specified time slots. OLT receives the status (DBA) report, re-calculates bandwidth allocation (BW Map) through DBA algorithm and sends new BW Map to the ONUs in-band with downstream traffic. The ONU receives the BW Map from OLT and sends the data in the specified time slots. When an ONU has no information to send, upon receiving a grant from the OLT, it sends an idle cell upstream to indicate that its buffer is empty. This informs the OLT that the grants for that T-CONT can be assigned to other T-CONTs. If an ONU has a long queue waiting in its buffer, the OLT can assign multiple T-CONTs to that ONT.

GPON Transmission Convergence (TC) Layer

ITU-T recommendation G.984.3 describes GPON TC layer which is equivalent to Data Link layer of OSI model. It specifies GPON frame format, the media access control protocol, OAM processes and information encryption method. Figure 3 shows the GTC frame structures for downstream and upstream directions. The downstream GTC frame consists of the physical control block downstream (PCBd) and the GTC payload section. The upstream GTS frame contains multiple transmission bursts. Each upstream bursts consists of the upstream physical layer overhead (PLOu) section and one or more bandwidth allocation intervals associated with a specific ALLOC_ID.

The downstream GTC frame provides the common time reference for the PON and common control signaling for the upstream.

Downstream GPON Frame Format

A downstream GTC frame has a duration of 125us and is 38880 bytes long, which corresponds to downstream data rate of 2.48832 Gbps. Figure 4 shows a detailed downstream GTS frame format.

The OLT sends the PCBd in the broadcast manner, and every ONU receives the entire PCBd. The ONUs then act upon the relevant information contained therein. The Psync field indicates beginning of the frame to the ONUs. The Ident field contains an 8-KHz Superframe Counter field which is employed by the encryption system, and may also be used to provide low rate synchronous reference signals. The PLOAMd field handles functions such as OAM-related alarms or threshold-crossing alerts. BIP field is Bit Interleaved Parity used to estimate bit error rate. The downstream Payload Length indicator (Plend) gives the length of the upstream bandwidth (US BW) map. The Plend is sent twice for redundancy. Each entry in the Upstream Bandwidth (US BW) map field represents a single bandwidth allocation to a particular T-CONT. The number of entries is given in the Plend field.

The Allocation ID (ALLOC_ID) field indicates the recipient of the bandwidth allocation i.e. a particular T-CONT. The lowest 254 allocation ID values are used to address the ONU directly. During the ranging process, the first ALLOC_ID given to the ONU must be in this range. This ALLOC-ID is known as the default Allocation ID. This ALLOC_ID is same as ONU-ID number used in PLOAM messages. If further ALLOC_ID values are required for that ONU, they should be taken from those above 255. ALLOC_ID 254 is the ONU Activation ALLOC_ID- used to discover unknown ONUs. The Flag field allows the upstream transmission of physical layer overhead blocks for a designated ONU. The Slot Start and Stop field indicates the beginning and ending of upstream transmission window. The CRC field provides error detection and correction on bandwidth allocation field.

The GTC payload field contains a series of GEM (GPON Encapsulation Method) frames. The downstream GEM frame stream is filtered at the ONU based upon the 12-bit Port ID field contained in the header of each GEM frame. Each ONU is configured to recognize which Port-IDs belong to it. The Port-ID uniquely identifies a GEM Frame.

Upstream GPON Frame Format

The Upstream GTS frame duration is also 125us and is 19440 Bytes long, which gives an upstream data rate of 1.24416 Gbps. Each upstream frame contains a number of transmission bursts coming from one or more ONUs.  Each upstream transmission burst contains an upstream physical layer overhead (PLOu) section and one or more bandwidth allocation intervals associated with individual ALLOC-IDs. The BW map dictates the arrangement of the bursts within the frame and the allocation intervals within each burst. Each allocation interval is controlled by a specific allocation structure of the BW map. Figure 5 shows upstream GTC frame format.

The physical layer overhead (PLOu) at the start of the ONU upstream burst contains the preamble which ensures proper physical layer operation of the burst-mode upstream link. The PLOu field contains the ONU-ID field which indicates the unique ONU-ID of the ONU that is sending this transmission. The upstream physical layer OAM (PLOAMu) field is responsible for management functions like ranging, activation of an ONT, and alarm notifications. The upstream power leveling sequence (PLSu) field contains information about the laser power levels at the ONUs as seen by the OLT. The dynamic bandwidth report (DBRu) field informs the queue length of each T-CONT at the ONT.

Mapping of GEM Frames into GTC Payload

GEM traffic is carried over the GTC protocol in transparent fashion. In the downstream direction, GEM frames are transmitted from the OLT to the ONUs using the GTC frame payload section. The OLT may allocate as much duration as it needs in the downstream, upto and including all of the downstream frame. The ONU filters the incoming frames based on Port-ID. In the upstream direction, frames are transmitted from ONU to OLT using the configured GEM allocation time. The ONU buffers GEM frames as they arrive, and then sends them in bursts when allocated time to do so by the OLT. The OLT receives the frames and multiplexes them with the frames from other ONUs.

Ethernet over GEM

The Ethernet frames are carried directly in the GEM frame payload. The preamble and SFD bytes are discarded prior to GEM encapsulation. Each Ethernet is mapped to a single or multiple (by fragmenting) GEM frames.