Scatmag.com

November 2018

PON NETWORKS


Part-2
By Devchand Haria
Part-1 of This Article, Carried Last Month, Provided An Over View Of A Passive Optical Network (PON). This month’s article provides details related to practical Implementation Of PONs.


Lets take a closer look at each of the identified the 3 key components of a Passive Optical Network(PON):



PON COMPONENTS

THE OPTICAL LINE TERMINATION (OLT)

A PON system consists of an ‘Optical Line Termination’ (OLT) unit at the control room, which is in essence an Optical transmitter, capable of accepting Voice (telephony ) and Data (Internet) inputs.

The OTL is inherently a digital device, and hence it cannot accept analog Video and sound from a satellite receiver, as is used at an analog Cable TV headend. It can accept and distribute television signals if they are converted into IPTV format, as used in many digital headends.

There is also a solution to use & distribute analog TV signals over a PON network, as we shall see later.


OPTICAL FIBRE & SPLITTERS

For distribution of the optical signal from the OTL (Optical Line Termination), optical fibre and optical splitters are used. Functionally, these are same as used in a Cable TV network. However, the PON optical splitter must operate not only at 1310nm & 1550nm but also at 1490nm.

The optical fibre used in a PON network must also operate at all these 3 wavelengths, but that can usually be taken for granted, for most optical fibre.


A PON Optical Splitter Must Operate At 1310, 1550 & 1490nm


OPTICAL NETWORK UNIT (ONU)

The Optical Network Unit (ONU) in a PON network replaces an optical node used in a HFC Cable Network. The ONU receives an optical signal and provides 3 simultaneous electrical outputs.



These are an RF Output to feed a TV Set; an ethernet connection for a LAN Card of a PC for internet delivery; and a telephone jack for connection to a Telephone (for Voice) in the consumer’s home.

(See Box on Specifications of a Typical ONU)





OPERATION WAVELENGTHS

Distribution of internet content requires the delivery of 2 way data. Also telephony requires 2 way communication from each user to the control room. Clearly, the PON network needs to carry signals in both directions – To & From the Control Room. A PON establishes 2 way communication using separate wavelengths for upstream and downstream traffic. The 1490 nanometer (nm) wavelength band is used for Downstream (Control Room to End Users) data transmission.



For upstream (User back to the Control Room) data transmission, the popular CATV wavelength of 1310 nm is used.


ADDING ANALOG TV ON PON

The PONS network is basically a digital network, carrying IP (Internet Protocol) data packets. A PONS can therefore directly carry and distribute IPTV or digital CATV based on an IP data stream, such as those generated by digital headends using IP protocol.

To make PONS deliver analog TV signals, the PON uses a conventional analog Optic fibre transmitter operating at 1550 nm wavelength. The 1550 nm signal carries only analog CATV signals. Since TV signal distribution requires only delivery from the Control Room to the end user, the 1550 nm signal is run only downstream.

Upstream signals required for interactivity, can easily be deployed on the upstream channel at 1310 nm, thus providing full and seamless triple play interactive networks.


WDM

To cater to increased traffic, different carriers are often used, within the same wavelength band. As an example, there could be 50 different carriers all closely spaced around 1310 nm, for the upstream traffic. This is called “Wavelength Division Multiplexing” or WDM.


A TYPICAL PON SYSTEM

Figure 4 explains a typical PON system.

The OLT forms the heart of the system and has separate digital inputs with feeds from the internet, telephone exchange and a Video On Demand (VOD) server.



Instead of the VOD server, an IPTV server providing digital CATV in IPTV format can also be accepted by the OLT.

All the above inputs are received by the OLT as digital, electrical signals.

The OLT also accepts an optical signal at 1550 nm, containing analog CATV signals from upto an 860 MHz analog cable TV headend. The analog cable TV headend’s RF signals are fed to a regular 1550 nm fibre optic transmitter whose output is accepted by the OLT.

The OLT outputs all the digital signals (VOD + Internet + Telephony) at 1490 nm. The analog CATV signals at 1550 nm are also mixed with the 1490 nm signals and put out as a feed for the Passive Optical Network (PON).

Each PON can feed upto 32 optical splitters to 32 ONUs. The PON can have a cascaded optical fibre length of upto 20 kms.

If necessary, the control room can have multiple OLTs as shown in figure 4, to feed even a larger number of ONUs (similar to an optical node).




PON STANDARDS

A major boost to the use of PON deployment is the evolution of two PON standards IEEE 802.3ah EPON & ITU-T G.984 GPON.


EPON

The EPON is an Ethernet Passive Optical Network. It uses Ethernet as the data transport layer.

EPON supports symmetrical data rates up to 1 Gbps. Ethernet compatibility and support for switched digital video makes EPON a great choice for developing regions where there is little or no existing legacy infrastructure. EPON allows for 32 time slots (32 ONT per OLT) and typically supports link spans up to 20 km


GPON

To cater to speed greater than 1.25 Gigabits per second (Gbps), Giga Bit Passive Optical Networks (GPONS) have been developed. The ITU.T G.984 standard is applicable to GPONs.


UPSTREAM & DOWNSTREAM COMMUNICATION

In a PONS network, the communication modes for downstream (i.e., from OLT to ONU) and upstream (i.e., from ONU to OLT) are different.

For the downstream transmission, the OLT broadcasts an optical signal to all the ONUs in continuous mode (CM), i.e., the downstream channel always receives an optical data signal. This is similar to a CATV Optic transmitter.

However, in the upstream channel (from consumer to the control room), ONUs cannot transmit Continuous Mode (CM) optical signals. It would result in all the signals transmitted from the ONUs adding up into one fiber (through the optical splitters) and overlapping.

To solve this problem, Burst Mode (BM) transmission is adopted for the upstream channel. The given ONU only transmits optical packets in a ‘burst’ when it is allocated a time slot and it needs to transmit. This is called Time Division Multiplexing.


OPTICAL BUDGETS & PON SYSTEM DESIGN

The PON reduces network operating cost by sharing a single fiber line from the central unit to a neighborhood and then splitting the fiber up to 32 or 64 subscribers per passive optical splitter.

The entire distribution is passive. No power is required during distribution. As a result, the optical power launched by the OLT is progressively attenuated by the optical splitters and length of fibre, till it reaches the ONU at the consumer’s building.

In CATV optical system design, calculations are done, commencing with the optical power output from the transmitter. Fibre and splitter losses are added and the design must ensure that the last node receives an optical signal of (preferably) 0dB or more.

In contrast, the system design in a PON network is extremely simple.

The PON standards typically specify an “Optical Budget” of 28 dB.

This simply implies that the an optical loss of upto 28 dB can be introduced between the OTL in the Control Room and the ONU at the furthest building.

28 dB corresponds to about 20 km with a 32-way split !

There is no need to grapple with selection of the OLT’s output power, or calculate total optical loss in distribution.

Simply ensure that you have 32 or less splits and that the fibre length from the Control Room to the furthest subscriber is less than 20 KMs.


ENHANCEMENTS

Forward Error Correction (FEC) is sometimes added as software correction. This permits reception of even smaller signals, and the errors arising during reception of these small signals is compensated by FEC.

This provides another 2-3 dB of loss budget.

As optics improve, the 28 dB budget will likely increase.


PON INSTALLATIONS IN INDIA

India has 401.41 million wireless broadband users (mostly mobile phone based), but only 17.97 million wired broadband users. Wired broadband provides delivery of much faster broadband, at a lower cost to consumers. Delivery of wired broadband is also lucrative for the service provider, and this segment is seeing a renewed interest & price drops, particularly after JIO’s announcement to aggressively address this market segment.

Passive Optical Networks (PONs) readily provide internet connectivity for bandwidths upto 1 Gbps !

Cable networks delivering broadband will still have a reasonable operating margin, even after JIO enters the business, since the current margins are high, and cable operators typically operate with much smaller operating overheads than large corporates.


CONCLUSION

In comparison with active optical networks, passive optical networks have significant advantages. They avoid the complexities involved in keeping electronic equipment operating outdoors.

PONs offer an excellent upgrade path to deliver high speed triple play solutions, including IPTV, simultaneously with 1 GBps internet, digital CATV and telephony, on a passive optical fibre network into consumer’s homes. n



ABOUT THE AUTHOR

Devchand Haria - B.E. (Elect. & Telecom) is Director at Optilink Networks Pvt. Ltd. a leading solution provider for digital headend solutions & distributors for RF, Networking and Fiber Optic Products.

Over the last 26 years, Optilink Networks Pvt. Ltd. has developed a leading market position as a thematic system provider & distributor of high quality satellite receiving equipments, fiber optic equipments & digital headend solutions.