gpon next gen.docx

7/27/2019 GPON Next gen.docx

1/16

GPON - The Next Generation Access NetworkKumar Shakti Singh

AbstractPassive Optical Network (PON) provides high band-width, low maintenance

optical access technology. A PON is a point to multipoint network. It usesoptical splitting in the downstream to broadcast traffic to all the end nodes, termed as

Optical Network Terminals or Optical Network Units from Optical Line Terminal. In

upstream direction the Optical Line Terminal, or the aggregating node, controls the

transmission of the traffic from the individual ONU/ONT on to the shared fibre. The

earliest version of PON was the BPON (Broadband PON) mainly used for transfer of ATM and

TDM traffic.The relatively new technologies are EPON, GPON and WDM-PON.

Why PON?The demand for higher bandwidth at the customer premises is fuelling the development of

Access Network. As distance and bandwidth pose a major limitation to the

usefulness of copper as medium, optical technology is the alternate choice. Since theaccess networks are much cost sensitive, a passive and shared out-door net-work

suitably addresses both the CAPEX and OPEX. Another important development

of recent times has been the growth of data traffic in the network. The large volume of

data traffic in metro network has resulted in carrying of Ethernet frames right across

the network instead of the IP packets. EPON and GPON address the above requirements

and provide for solutions, though each has a different protocol and variation in

technology. Figure 1.show EPON and GPON networks.

GPON OverviewGPON network reference model is shown in Figure 2. Each component of the network is

looked upon in next few sections.


2/16

The GPON provides varying rates of transmission in both the upstream and the downstream

directions. In the downstream direction the transmission rate can be either 2.488Gbps or

1.244Gbps, whereas in the upstream direction the rate can be selected f r o m 622Mbps or

1.244Gbps. GPON provides support to different types of client services. The ATM client is

mapped transparently into the GEM frame in both the directions. The TDM client is mapped

using GPON Encapsula tion Method (GEM) procedure. This is identical to GFP framingis described later in the article. Fixed numbers of bytes in each frame are reserved for the

TDM transportation in both the directions. Data packets including Ethernet frames are

also mapped using GEM procedure. Fragmentation and Reassembly, i.e. dividing the large

frames into smaller fragments and combining them again at receiver, facilitates reducing

the delay variations of real time traffic and TDM traffic in case of large data packets.

The data traffic can comprise Ethernet frames, IP packets, IP-TV, VoIP and any other type,

making the GEM frame trans-port capability attractive, efficient and simple.

GPON DistributionThe medium for transport is fibre. Fibres and splitters are referred to as Optical Distribution

Network (ODN). On the downstream side (traffic flowing from OLT to ONU/ONT or away

from the network towards the sub-scribers) a single fibre connects to the first splitter. This

then forms the trunk of a tree. The tree is the ODN, with splitters being joints where a branch

offshoots. The ONU/ONT forms the leaves. Each branch ends up at a splitter or at an

ONU/ONT. In the upstream direction multiple branches carrying traffic, are combined into a

single branch using optical couplers, which have directional properties. Full duplex

connection is achieved by having separate fibre network for downstream and upstream

directions or using a single fibre networks with WDM access, i.e. using separate wavelength

fordownstream and upstream traffic. All the components ofthe ODN viz., fibre, splitter &

coupler are passive fulfilling the requirements of a PON.


3/16

GPON ONU/ONTONU/ONT, the nodes at the subscriber end provide for connectivity between customers

(users) and network. The ONU adapts the incoming traffic into GPON protocol before

transmitting to OLT during the assigned period. In downstream direction, the ONUs

continuously listens to all the traffic. They extract and forward the one destined to one of

their ports. Since ONT normally resides at customer premises, it needs to be cost sensitive.ONTs and ONUs act as slave nodes in most of the scenarios in PON. ONU/ONTs are

managed by the network manager via the OLT, where as an ONT provides the User Port

Functions required by customer in Fibre To The Home (FTTH) (residential user), ONU does

not provide these and instead provides the interface to standard transport medium like E1/T1,

Ethernet, DSL cables etc.

The User Port Function is responsible for providing necessary interface processes to

individual end-users that connect to the ONU allowing them access to the Network system.

An ONT is considered as a part of the CPE. The ONU can support traffic through more than

one CPE(s) and is considered a part of operators network. The logical structure of the

ONU/ONT is shown in Figure 3.

GPON OLT

The OLTS is the root of the GPON (PON in general) tree. It provides for the data plane

functions shown in Figure 4. It also controls and manages the ONU/ONTs connected to the

network. For the downstream direction, client traffic is encapsulated with the correct

identification tag and transmitted. In the upstream direction, based on the Service Level

Agreement (SLAs) and Dynamic Bandwidth Requirement report (DBR) from ONUs/ONTs,

it allocates specific periods within theframe for transmission of traffic. For management ofthe network, the OLT measures the power received from individual ONUs/ONTs, allocates


4/16

Ids to ONUs/ONTs, discovers new ONUs added to the network and collects performance

parameters from each ONU/ONT. Just as the ONT performs the User Port Functions for

management, OLT performs the Service Port Functions for the ports attached to the OLT.

GPON Transmission

For downstream traffic the network appears as broadcast medium. Thus all ONU(s) are able

to listen to incoming traffic. The frame comprises of Physical channel message meant and

used by all ONUs, and Payload sections. In the payload section data traffic and TDM traffic

are encapsulated using a GFP like header, called GEM header. Within this header is

embedded the information of destination port of ONU. Thus each port as seen by PON has a

unique identification and up to 4095 such identifications can be supported.

Therefore, using the above port identification, only the ONU, for which the traffic is

destined, extracts the payload. For ATM cells which are directly mapped into the payload

section, VPI / VCI are used for segregating the traffic at the ONUs. In upstream directionbased on the bandwidth requirement of each port, slots within the upstream frames are

allocated to individual ONUs.


5/16

The upstream traffic is transferred in transmission containers (T-CONTs). These T-CONTs

can carry identical traffic from one or manyports of the ONU. There can be up to five

different types of T-ONT.

GPON Protocol

Down StreamThe protocol for the transfer of information has been designed to address the requirements of

point to multi-point system and to utilize the available bandwidth in the network to the

maximum. The downstream frame formats are shown in and Figure 5.The downstream frame

repeats every 125us, very much like the other TDM transport technologies. It also carries the

8Khz synchronization information to the ONUs for synchronizing upstream clocks and any

TDM traffic payload. The downstream frames comprise of GPON Transmission Convergence

(GTC) Physical Control Block downstream [PCBd] and a Payload area. The first field in

PCBd, PSync, refers to the frame pattern used for frame synchronization. It is 4 octets long. It

carries the pattern 0xB6-AB-31-E0 and is not scrambled. Within the ONUs a state machine,

similar to HEC framing is used for frame synchronization and it looks for the above pattern.

The next field, Ident, provides information whether Forward Error Correction (FEC) has been

used in downstream frame or not. It is static information. In PON addition of new ONUs is

not ruled out. So as the new ONUs require this information when admitted into the system, it

is conveyed in every frame. The rest of the Ident field carries a 30 bit super-frame counter

value, which increments every frame and rolls over to 0. It is used for ranging process and for

Advanced Encryption System (AES). The third field, 13bytes long, carries the Physical Layer

OAM (PLOAM) messages required for configuration and management ofthe ONUs and

PON in general. One byte is used by theBIP-8 process, which covers all bytes in frame. BIP-

8 is calculated over scrambled bytes and inserted prior to the scrambling process. The fifth

and sixth fields are Payload Length Indicator (PLend), transmitted twice for robustness. The

Upstream Bandwidth field indicates the length of the upstream bandwidth allocation map andlength of ATM partition with in the payload bytes. The last field carries the upstream


6/16

bandwidth allocation map indicating the start and stop position of each transmit container in

the upstream direction. The control bytes within the ATM partition and upstream bandwidth

map are also protected using CRC-8.

GPON Protocol Up Stream

In the upstream direction the frames from different ONUs are spaced using the guard time.The different fields of the upstream are shown in Figure 6.The first field, Physical Layer

Overhead upstream (PLOu) is a variable length field, its size is governed by upstream rate. It

is used for frame markings. Preamble and Delimiter bytes within the PLOu are rate

dependent and mark the start of transmission from the said ONU. The length of the field

increases with higher transmission rate to facilitate clock recovery and frame synchronization

in a multi-clock environment (each ONU/ONT can have different clock parameters). BIP is

the upstream BIP-8 covering all the bytes of the transmitted frame. The ONU-ID indicates

the ONU from which traffic is trans-mitted. This unique ID is provided by OLT to ONU

during the ONU registration phase. The indication field provides information on usage of

FEC

Remote Defect Indication (RDI) input and status on the traffic and messages waiting in

queues at the ONU. The second field of the upstream frame is the PLOAMu, carrying OAM

messages from ONU to OLT. This field is only sent in response to OLT request, present in

upstream bandwidth map of the downstream frame. The Power Level Sequence (PLSu) field

is used by ONU to send power measurement information to OLT. The information is used by

OLT to adjust the ONU power levels to reduce the power level dynamic range visible at theOLT. It is 120 byte long message and is unstructured. The 2,3 or 5 byte DBRu field carries


7/16

the dynamic bandwidth allocation report of the T-CONT for the payload is being sent in the

following payload fields of the current frame from ONU. The number of bytes in the report

depend on the reporting option negotiated between ONU and OLT. The default is 2 bytes,

where one byte being CRC-8. The last field is the payloadfield. It is important to note that

DBRu and Payload form multiple T-CONTs can be packed in a single frame sent by the

ONU, depending on the bandwidth allocation provided by the OLT. The payload can carryone of the three types of information viz. an ATM payload, a GEM encapsulated traffic or a

DBA report. The DBA report is comprehensive and contains information of all the T-CONTs

supported by the ONU. It is different from the one carried in DBR where information of the

concerned T-CONT is only forwarded by the ONU. The three different payload options are

shown in Figure 7 toFigure 9.

GPON Transmission Convergence (GTC) functionThe GTC function adapts user and control information to GPON protocol for transmission.

The user plane and control plane funct ional blocks for GTC are shown in Figure 10.

GTC is the Transmission Convergence (TC) shown for ONU and OLT in Figure 3 and Figure

4. The PLOAM messages are sent out in defined location in the GPON frames. They are

formatted as octets by the PLOAM client of the GTC. ONU Management and Control

Information (OMCI) messages are carried over ONU Management and Control Channel

(OMCC). OMCI messages are adapted using GEM in case of its association with non-ATM

traffic. In case of ONU transporting ATM cells the OMCI are mapped into GPON frame

using ATM HECadaptation mechanism. The OMCI adaptor formats the messages as per the

requirement of the OMCC. ATM TC adaptor transparently maps the ATM cells into ATM

partition of the downstream frames or into the payload area assigned for ATM cells in theupstream direction. The GEM TC adaptor maps data and TDM payload, i.e. non-ATM traffic


8/16

into GPON frames. The GEM TC uses GEM frames which comprise a header an d

p a yl o a d p o r t io n sho wn i n Figure 11 .Th e fi rs t header field is Payload Length

Indicator (PLI) indicating the number of bytes being encapsulated in current GEM

frame. The Port-Id or GEM-Id identifies the userport for which the traffic is des tin ed

or originated at ONU. It is 12 bit field and is able to support up to 4096 user

port s. Pa yload Type Indi cator (PTI) , thi rd field , indicates if the payload has anend of client frame or not, has any congestion occurred for the said client and

whether the client frame is an OAM frame. The 13 bit HEC provides error

detection and correction capability over the headers. The generator polynomial

used in GEM header is then exclusive monitored with a fixed pattern of

0xB6AB31E055. An idle GEM frame comprising only header and no payload is

inserted whenever the OLT or the ONU does not have e nough client frames to fil l

up the slot in GPON frame payload area.

Forward Error CorrectionApart from the BIP-8 which provides limited error detection capability, forward

error correction (FEC) is defined for use in GPON. The use of FEC results in an increase in link budget by 3-4dB (coding gain) and therefore higher bit rates,

larger distances between OLT and ONUs and higher split-ratios support in the

network. The use of FEC is optional both in upstream and downstream direction.

The FEC is defined in a way to facilitate the inter-operability of FEC encoded

transmission and non-FEC encoded reception and vice-versa. The FEC used is

defined in ITU-T G.975. It uses Reed Solomon code represented as

RS(255,239,8). The code is non-binary, operates on byte symbols and belongs to

family of systematic linear cyclic block codes. The block length is 255 out of

which 239 are the data symbols (bytes) and 16 are the parity symbols added by

theen coder. The generator polynomial G(x) for the RS(255,239) satisfies the

equation

()

( )


9/16

Where is root of polynomial

The FEC code group can be represented by polynomial

where C(x)is the encoded word, D(x) is the information bytes and P(x) is the

parity code.


10/16

The encoding and decoding is done on the scrambled data. The data is divided into

groups of 239 symbols as demanded by the process. To this a block of 16 parity

symbols are added. The groupings and addition of parity is sequential. Thus after encoding

the next code group starts with the original 240 symbols after the 16th parity symbol of

previous group, which means no synchronization is needed while decod in g. Th e last

code group which may have data symbols less than the required 239 information byt es isfirst padded up with 0x00 to 239.The parity is calculated over the 239 symbols, the pad-

ding is discarded and parity inserted after the information symbols. At receiver the decoder

first pads up the information bits and does the decoding process. The encoder and

decoder are shown in Figure 12 and Figure 13 res pectively. The Hamming di stance

of the code is 17, and thus it can detect up to 16 errors in the FEC code word and

correct up to 8 error symbols.

SecurityIn the downstream direction, the GPON (or any other PON) is a point-to-

multipoint network. The data then can be listened to by the entire ONUs. The one (or

the port of the ONU) for which it is destined, extracts it and forwards it on to correct port. Asecurity threat exists if a malicious ONU extracts information not intended for it. This th re at

model is at the physical layer and against which defense mechanism needs to exist.

Other threat models also exist both for upstream and downstream traffic, but are

more relevant at higher layers. In the GPON downstream frame, the user traffic is

protected using AES encryption scheme. Only the payload portion of the frame is

encrypted. The counter mode of the AES i s us ed . In t he up s tr ea m di r e ct i on si n ce

t h e GPON behaves as point-to-point link, no security threat exists. Thus it is used for

distribution of keys to be used in the downstream traffic encryption. The keys are transmitted

multiple times by ONU to OLT. OLT acknowledges the correct reception of the keys. This is

followed by a message from OLT to ONU, indicating the frame in which switch over to

new key is planned. The frame is indicated by the Super Frame count numbermentioned earlier in section GPON Protocol - Downstream.


11/16

Figure15 shows the encryption and decryption methods. The AES encryption logic is shown

in Figure 14.The main functions of AES are Column transformation, S-Box

multiplication, Round key addition and round key generation. The details of AES can

be found in AES standard [Advanced Encryption Standard - 2001]. The above processes

cause enough diffusion and confusion, so that recreation of key using plain text andciphered text is very unlikely and takes infinite amount of time.


12/16

ONU ActivationIn GPON and other PONs, in built mechanisms are provided to activate a newly added

ONU to the network. This process is called Activation method. The full activation

process also controls the power measurement at ONU and the adjustment of the power level

at ONU to reduce the dynamic bandwidth of the OLT receiver. The Activation process also

measures the phase relationship for each ONU with respect to the downstream frames. Thisinformation of phase relationship is sent across to the relevant ONUs by OLT, to avoid

collision during upstream transmission. The activation process starts with the

discovery of new ONUs, by operation system command, periodic polling for new

ONUs or periodic reactivation of ONU during long alarm intervals. During this phase the

exchange of client data is temporarily suspended. OLT asks for Serial-Ids of all ONUs by

opening a window for upstream transmission called ranging window. In this window, new

and unregistered ONUs, send identification and physical information to OLT. Since there can

be more than one ONU which need activation, collision(s) can occur during this process. To

avoid collision, each ONU waits a random period of time before transmission. On receipt of

the Serial-Id, OLT binds it with ONU-Id and sends the information to corresponding ONU

for use in future transactions. The OLT also measures the power and phase during thisoperation and sending the related control information back to the respective


13/16

ONUs. This completes the registration process of ONU which then can begin receiving and

transmitting information.

Dynamic Bandwidth Assignment- Reporting and AllocationIn the upstream direction, the bandwidth to be used by individual ONU not only depends on

the traffic scenario at concerned ONU, but also on the traffic pattern at other ONUs in

network. As the medium is shared, any self-initiated transfer by any ONU in upstream

direction would result in collision and retransmission causing degraded performance.

Therefore, this shared media is made to behave as multiple point-to-point connectionsbetween ONU and OLT by use of TDMA. The OLT being the central agency is told about

the bandwidth demand at each ONU.

Based on the traffic pattern at allthe ONUs it grants access to the ONUs at fixed slots

with respect to downstream frame. For the upstream traffic, the frame can be


14/16

considered to be divided into different container types. There are five type defined in

GPON. Type-I T-CONT service is based on unsolicited pe riod ic pe rmi ts gran ting fixed

pa yload allocation or catering to fixed bandwidth requirements. This is a static T-CONT

type and is not serviced by DBA. Type-II T-CONT is intended for variable bit rate

wi th bounded delay and jitter requirements like video and voice over IP. Type-III T-CONT

is intended for guaranteed delay. Type-IVis for the best effort traffic. Type-V T-CONT is

combined for two or more of the other four types defined above, and in this case the

individual bandwidth reporting and assignment is done at ONU. As said earlier, three

different mechanisms exist for sending the traffic pattern report to OLT by each ONU.

Whatever the mechanism, the report consists of pattern for each T-CONT types and not for

different clients mapped to the same T-CONT. The client traffic, queues, T-CONT map-pingand reporting is shown in Figure 16.

A Peek At EPON

Most of the recent networks in PON have been E-PONs also kno wn as EPONs. EPON is

the extension of the Ethernet IEEE 802.3 protocol for point-to-multipoint

technology. As a result it has lot in commo n wi th the Ethernet and uses the Ethernet

protocol as transmission medium. Data, control and OAM frames, TDM or ATM traffic all

first need to be adapted to Ethernet protocol before being transmitted.


15/16

An additional sub-layer called Multi-Point MAC control is introduced in IEEE802.3

specification for TDMA access of upstream traffic and handling of bandwidth assignments.

Another modification to the Ethernet standard is the introduction of Logical Link

Identification to bind the ONU with traffic transmission and upstream grants. FEC has been

introduced for resilience to bit error rate and to increase the optical distance. The 8B/10B line

coding, larger channel separation requirements, encoding of the OAM cells as Ethernet frame

reduce the bandwidth capacity of the EPON as compared to GPON. The difference between

EPON and GPON are in Table 1.One of the advantages of GPON over EPON, as suggested

by the table 1, is the ability to carry different payload types across the network in same frame


16/16

in a standard way. Other advantages are the varying rates of the upstream and the

downstream transmission supported by GPON standard. Defined fiber, equipment (ONU and

OLT) and network protection by GPON gives improved reliability as compared to EPON.

Exhaustive OAM provides better management, configuration and fault localization in case of

GPON as compared to EPON. The low Laser On/Off time of GPON increases the utilization

of the available upstream band-width at the cost of expensive laser component.

SummaryGPON provides an option to bring the fiber to the curb, business or residence. With its in

built and versatile adaptation functions for different type of client traffic it provides a

common transport platform. The fragment and reassembly and GEM process provides ideal

TDM mapping interface. The well drafted standards increase the possibility of OLT-ONU

interworking from different vendors. The PLOAM and OMCI provide enhanced and well

defined OAM capabilities within GPON, very much in demand by public networks.

gpon next gen.docx

Documents