gpon fundamentals vvvip

8/3/2019 GPON Fundamentals VVVIP

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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 networkconsists 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

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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 activationvia 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, andmay 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 toestablish 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. Themap 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, andeach 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 controlprotocol, 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 mustbe 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 aGEM 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 ONUbuffers 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.




GPON: VLANs and GEM Ports

This article discusses traffic flows in GPON. As discussed in GPON Fundamentals

article, the OLT is the first aggregation point in GPON access network. The OLTterminates the GPON Transmission Convergence (GTC) layer on the user side and

forwards Ethernet frames to Ethernet layer on the network side. Figure 1 shows the

termination points for ONU/ONT scenario.

The U reference point represents the customer-facing interface of the ONU/ONT. It is

possible that the U reference point can be within the ONU/ONT device when ONT andRG devices are combined into a single device.

The R/S reference point represents the OLT facing ONT interface. The S/R reference

point represents the Optical Distribution Network (ODN) connecting GPON interface

on the OLT. The S/R and R/S interfaces contain all the protocol elements necessary

to allow communication between OLT and one or more ONTs over ODN.

The V reference point represents the network-facing interface of the OLT.

Relationship between T-CONT and GEM Ports

Definitions:

T-CONT: A traffic bearing object within an ONU/ONT that represents a group of

logical connections, and is treated as a single entity for the purpose of upstream

bandwidth assignment on the PON. In the upstream direction, it is used to bear the

service traffic. Each T-CONT corresponds to a service traffic of one bandwidth type.

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Each bandwidth type has its own QoS feature.

ALLOC_ID: Each T-CONT is identified by the ALLOC_ID uniquely. The ALLOC_ID

ranges from 0 to 4095. It is allocated by OLT i.e. a T-CONT can only be used by one

ONU/ONT per PON interface on the OLT.

GEM Port: A GPON Encapsulation Method (GEM) port is a virtual port for performing

GEM encapsulation for transmitting frames between the OLT and the

ONU/ONT. Each different traffic-class (TC) per UNI is assigned a different

GEM Port. Each T-CONT consists of one or more GEM Ports. Each GEM port bears

one kind of service traffic i.e. a T-CONT type.

GEM Port ID: Each GEM Port is identified by a port ID uniquely. The Port ID ranges

from 0 to 4095. It is allocated by the OLT i.e a GEM port can only be used by a single

ONU/ONT per PON interface on the OLT.

Figure 2 shows the relationship between T-CONT and GEM Ports.

Between the ONT and OLT is the ODN, and Ethernet frames are carried over it

through the use of GEM Channels. GPON has GEM channels as part of its GTC layer.

The GEM channels carry variable-length Ethernet frames. GEM channels are

identified by GEM Port IDs. This identifier is assigned by OLT upon creation of a new

channel and is only valid during the entire life-cycle of the channel. Each GPON

interface for a given ONT can have several GEM Ports. A GEM Port ID is unique per




GPON interface and represents a specific traffic or group of flows between the OLT

and the ONT.

There are 2 types of GEM Channels:

Downstream-only GEM Channels - These channels are used to transmitdownstream broadcast/multicast traffic from OLT to all ONTs. The ONTs

identify traffic meant for them based on GEM Port ID.

Bi-directional GEM Channels - These channels are used for upstream and

downstream traffic between the OLT and the ONT. The frames are transmitted

from the OLT into the GPON interface and are forwarded only on the U

interface of the ONT on which that GEM Port has been assigned.

GEM Ports are used to differentiate among traffic classes (TCs). A U interface may

have several GEM Ports associated with it that support different TCs. Thus, within a

GPON interface, each GEM Port carries one or more traffic flows associated with a

specific TC.

On U interface, traffic is classified into VLANs with various Ethernet priorities based

on: Physical Port, VLAN ID, 802.1p bits, &/or DSCP. Once the traffic has been

assigned a VLAN and COS (802.1p) values, these two values are used to select an

upstream GEM Port so that QoS can be applied to the flows carried by the GEM Port.

A GEM Port always belongs to a single T-CONT. In downstream direction, the ONT

forwards the traffic received by GEM Ports to appropriate U interface.

1:1 VLAN

In a 1:1 VLAN architecture, the ONT maps each 1:1 VLAN into a unique U interface.

There are 2 variations on tag assignment at V interface in upstream direction - the

traffic at V interface could be double-tagged or single-tagged.

For double-tagged VLANs at V, the ONT can either assign a C-VLAN ID or

translate a C-VLAN ID. The OLT adds the S-VLAN ID. (Subscriber 1 in Figure

3)

For double-tagged VLANs at V, the ONT can assign S-C VLAN IDs to incoming

traffic, and the OLT passes through the traffic. (Subscriber 2 in Figure 3) For single-tagged VLAN at V, the ONT adds the S-VLAN ID or translates an

incoming tag to S-VLAN ID, and the OLT passes through the traffic.

(Subscriber 3 in Figure 3)

In the downstream direction, the OLT removes the outer tag or passes through the



traffic to proper GEM port based on the tag value and priority bits. The ONT removes

the tags and forwards frames from the GEM port to its associated U interface.

N:1 VLAN

For N:1 VLAN model, the ONT always adds the S-VLAN ID or translate an incoming

tag to S-VLAN ID for upstream traffic. The OLT will pass-through any upstream

traffic with S-VLAN ID on them. In the downstream direction, the OLT will pass-

through traffic with S-VLAN ID to ONT by determining GEM Port based on MAC

address and priority bits. If the GEM Port cannot be determined, then the frame is

flooded using the unidirectional GEM Port associated with the S-VLAN ID. The ONT

will remove the tag and forward frames from the GEM Port to appropriate U

interface. For N:1 model, traffic is always single-tagged at V interface.





QoS and Traffic Management

As seen from figure 1, the GPON link connects the OLT and ONTs to transport

Ethernet services. Please note that GPON can also encapsulate ATM and TDM (E1,E3) services. The GTC Adaptation sublayer maps Ethernet frames into GPON GEM

frames. A QoS mechanism is required in GEM to support Ethernet QoS (i.e. 802.1p

bits). In order to provide QoS, two mechanisms are employed-

Classification of traffic into traffic classes

Forwarding the traffic classes into GEM Ports and T-CONTs configured to

emulate Ethernet QoS service

Upstream Traffic Management

Figure 5 shows a sample model of upstream traffic management. It shows 4 T-

CONTs per PON interface where each T-CONT represents a specific traffic class (TC).

The Classifier receives traffic from U interface and maps to queues as per

configuration using associated GEM Ports. If a second UNI interface is present on the

same ONT, it would also perform classification and map the traffic to TC(s). As

mentioned above, a GEM Port must bear one or more T-CONTs. Other upstream

traffic from other ONTs is mapped to other 4 T-CONTs according to the TC.

At the OLT, each TC is mapped into a separate queue. T-CONTs from various ONTs

that share the same TC are mapped to the same queue, and a scheduler is used

among the queues towards the network-facing port i.e. V interface.



Downstream Traffic Management

Figure 6 shows a sample model of downstream traffic management. In downstream

direction, T-CONT is not used. Traffic received from the V interface at the OLT is

assigned to queues according to the TCs. It is then transmitted in the downstream

direction to the PON interface by using a scheduler. At the ONT, the traffic is




classified again and placed into appropriate queues for each U interface. A scheduler

is used to transmit frames to the U interface.


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