Chapter 1. Introduction to PON Technology

02.12.2015, 11:51

1.1. General View on PON Technology

Since 2000, information technologies have been substantially changing that has expectedly resulted in large-scale implementation of Ethernet and widening the subscriber access channel to World Wide Web. This has resulted to a race of speed and service quality: at the beginning - copper networks with active repeaters on a way from the provider to the subscriber, then - almost complete shift to FOL and FTTX (FTTC, FTTB, FTTH) technologies.

To date, absolutely nobody will be surprised with "home optical connection" and Internet access speeds of 30-100 MB/s, and the low cost of connection and low monthly fee do wired Internet technologies popular among all layers of the population.

Historically, Internet technology firstly originated from cities and still they are the "spike" of the IT industry: coaxial TV networks with Internet connection (DOCSIS), copper and optical FTTB networks with IPTV and IP telephony as a bonus, CWDM and DWDM backbones between separate city districts and between cities, wireless mobile Internet technology - a potential urban subscriber has a variety of options.

There are so many offers from Internet Service Providers in the city that the new ISP-community participant simply could not "squeeze" into the already formed "conclave" of network service providers. In turn, the already existing urban ISP are seriously competing for each five-storey building and for each subscriber in it (at least, it is true for some CIS countries).

Of course, the days when unfair competitors cut kilometers of opponents' cables, broke main lines and pierced needles in coaxial cables have passed - at the time being the competition is fair (increase service and speed quality, decreasing subscription fees, creation of local media portals etc.). But, as practice shows, any city has already been divided into filed of influence of ISP (with at least two of them in every city) and it is hard to find new subscribers in conventional territories, especially taking into account almost the same range and proposed quality of services.

It would seem - why to change something? ISPs exist and regularly earn their crust by regularly collecting tribute from its subscribers and periodically introducing "something of a sort" allowed under the current legislation and that is immediately repeated by the competitors.

As Ukrainian experience showed, it is possible to earn not only crust, but a thick layer of butter on it - it is just necessary to look toward the origin of this butter - country settlements (countrysides, villages, townships and urban private housing!).

At the time, Ukrainian ISPs were pleasantly surprised by a large number of potential subscribers in rural areas and, as nature abhors a vacuum, began vigorously build a classic FTTX in the absence of civilization. But, as usual, the pilgrims did not consider the presence problems in the countryside, which were almost at every step:

  • absence of canalization (for convenient cable installation);
  • poor electricity supply (and resulting problems with active ISP equipment);
  • absence of telecommunication buildings and the inability to accommodate active equipment on poles;
  • problems with thunderstorms (no lightning discharger, no voltage regulators, twisted pair is hanging from the nearest power box in the open air) ...

And most important - too small number of potential subscribers per square kilometer compared with the city (as a result - a huge cost for installing multi-fiber cables over long distances or headache with calculations in order to save  the cable).

And there is a desire to receive new subscribers and the subscribers seem to be ready to pay  the high cost for connection and even to buy equipment and power it on at their expense - but maintenance of FTTX in rural area is too expensive.

And in this difficult period, when many ISPs refused and and did not want to even hear about subscribers in private sector and villages, the market launched a still quite unknown PON technology, which is now beyond competition in so tough environment for data transmission systems.

PON (eng. Passive Optical Network) – is a fast growing, most promising technology of broadband multiservice multiple access over the optical fiber using the wave division of receive-transmit channels and allowing to realize a single-fiber tree topology "point-to-multipoint" without the use of active network elements in branch point.

In other words, PON - is a completely passive network built on optical fiber and having nothing but a "glass" on an Internet path from the provider to the subscriber. All active equipment is made in a relative safety of residential (and not) buildings, namely:

  • the provider, on its side, installs a main station that controls all passive network including subscribers' devices and "fills" network with traffic;
  • the subscriber, on its side, install receive/transmit converters that, peculiarly, "outflow" subscribers' traffic.

 

1.2. Types of PON

In the early 90's, when the world's attention was focused on the events in the territory of the former Soviet Union, a group of several European telecommunication companies created a consortium to implement the idea of multiple access over a single fiber, named FSAN (Full Service Access Network). The aim of FSAN was to develop common recommendations and requirements for PON equipment  in order for equipment manufacturers and operators to coexist in a competitive market of PON access systems. It resulted in creation of a range of PON standards:

  • ITU-TG.983
    APON (ATM Passive Optical Network);
    BPON (Broadband PON);
  • ITU-T G.984
    GPON (Gigabit PON);
  • IEEE 802.3ah
    EPON/GEPON (Ethernet PON);
  • IEEE 802.3av
    10GEPON (10 Gigabit Ethernet PON);

 

APON and BPON were outdated it their creation, GPON was not very developed because its high (comparing to GEPON) cost as well as due to the reluctance to works with 2.5G speeds, 10GEPON is still under development / debug / test.

As a result, there are only GEPON and GPON, which currently meet the requirements of most providers to connect remote subscribes: "down" and "up" transmission rate  is 1/1 Gb/s or 2,5/1 Gbit/s (for GEPON and GPON, respectively), while the same fiber can host up to 64 terminal devices in the network (for GEPON) and up to 128 (for GPON). However, for less demanding rural subscriber the rate provided by GEPON even during peak periods is sufficient, and the price of equipment (and consequently the connection) significantly below, up to several times. Therefore, at present GEPON technology is the most promising for expansion of ISP towards small/medium-sized towns located in suburbs and at a considerable distance from the city.

* Of course, GPON provides bigger margin rate for each subscriber, but by the time when the rate will be in demand, 10GPON will be widely circulated, so to make extra payments for dubious redundancy at the moment does not make sense *

1.3. GEPON Operation Philosophy

As mentioned earlier, GEPON is a tree network, built on passive optical components on all distance from provider to subscriber.

On its side, the provider installs OLT (eng. Optical Linear Terminal) – L2 or L3 switch with all functionality equipped with Uplink Ports (usually Ethernet standard) and Downlink Ports (working in terms of IEEE 802.3ah standards).

Recently GEPON equipment manufacturers have a wide range of head-end stations(OLT), which mainly differ in number Downlink Ports (for direct connection of passive trees), quantity and speed of Uplink Ports (for example, 1 Gbit/s or 10Gbit/s) and firmware functionality (L2 or L3).

* for example, the Chinese company entitled BDCOM has 3 lines head-end stations:

  • Low-level (P33XX) - OLTs for a small number of subscribers (256) with 4 Uplink and 4 Downlink Ports;
  • Mid-level (P36XX) – OLTs for a middle number of subscribers (512…1024), with 8…16 Downlink and Uplink Ports and 2х10Gbit/s additional Uplink;
  • Tol-level (P69XX, P85XX) - giant GEPON traffic factories with more than 16 GEPON ports and other advantages;*

OLT management is carried out both via a terminal port  and with a help of all your favorite protocols such as SNMP, SSH and TELNET.

On the client side, it is necessary to install ONU (eng. Optical Network Unit), which is also sometimes referred to as ONT (eng. Optical Network Terminal) is a specialized VLAN  small size switch. ONU made by, e.g., BDCOM normally has one optical Gigabit port and 4 Copper (100Mbps or 1Gbps). There are ONU models with a combined optical port for TV and data, with ports for telephony (SIP), with varying amounts of copper ports, with Wi-Fi-adapter, as well as combinations of the above. Each ONU has a built-in MAC-addresses filter; while receiving the ONU package, it determines package affiliation and if the packet is not affiliated to it, drops it. ONU management comes directly from OLT, while OLT considers ONU as its own "remote port".

A tree shaped passive optic network and its derivatives are located between client and provider. The main components of the passive optical network is an optical fiber and optical splitters operating in the "branching" mode in the direction of provider-client and in the "mixer" mode in the opposite direction. The undoubted advantages of the passive equipment are its independence from power and ease of use: being once installed, passive equipment requires only periodic preventive maintenance (often only in the form of visual inspection).

Figure 1 - Functional diagram of PON connection

Since the passive optical networks are physically compound with multiple-access (point-to-multipoint), they must separate upstream and downstream data, and coordinate communication between a range of subscriber units and the head-end stations. For this purpose one simultaneously uses two technologies to transmit data in the environment shared among many subscribers: time division and frequency division multiplexing.

Time division multiplexing (eng. TDM - TimeDivisionMultiplexing) acts from the side of OLT, which determines at what times a particular subscriber unit is allowed to broadcast in the common data transmission environment. On the side of ONU, TDMA is operative (eng. TimeDivisionMultipleAccess - division multiple access in time), according to which the subscriber unit is subject to OLT.

At the same time, the entire Passive Optical Network Technology operates WDM (eng. WavelengthDivisionMultiplexing), which distributes direct (downstream from the OLT to ONU) and reverse (upstream from ONU to OLT) data streams to different wave lengths (frequencies). Herewith, the downstream is transferred at a wavelength of 1490 nm , while the upstream - at a wavelength of 1310 nm. This is done in order to avoid collisions ("collision" of upstream and downstream on the same wavelength), and leave place for CATV (analog TV), which also can be sent via PON tree to subscriber. CATV transmitters broadcast at a wavelength of 1550 nm or 1310 nm, but GEPON equipment manufacturers took 1310 nm for Upstream to maximally reduce the cost of client device (emitting lasers at a wavelength of 1310nm are much cheaper than emitting lasers at a wavelength of 1550 nm).

The cost of GEPON laser GEPON transceiver is quite high in relation to their Ethernet counterparts, and it is not fortuitous: they are very powerful. Their power is enough to "break through" more than 100 km of standard optic fiber on the straight! However, PON-trees usually reach only 10-15 km in depth, with a depth limit about 20km. This is due to the fact that passive optical splitters introduce  huge line attenuation  while providing branching and saving optical fiber.

It is worth noting that GEPON standard is somewhat different from the usual Ethernet by its frame structure, so "no-GEPON" devices in PON network will not work.   Moreover, the IEEE 802.3ah standard has been adopted relatively recently, and almost none of the manufacturers has 100% compatibility (and many of them do not particularly want to). Because of this, there is no complete cross-platform compatibility of the equipment (for example, OLT by D-Link will not work with ONU by ZTE, or OLT by HUAWEI will not reveal its full potential when working with ONU by BDCOM).

* In fact, compatibility of different manufacturers is possible, but not for 100%; traffic between OLT and ONU, possibly, will "be transferred", but full OLT control of "non-native" ONU will not be guaranteed.*

 

It is specifically necessary to consider the data exchange technology between ONU and OLT:

  • any ONU broadcasts only at time allotted for it by OLT (TDMA);
  • for any ONU in network, OLT determines the time interval during which ONU can broadcast (TDM);
  • newly connected ONU communicates with OLT via MPCP protocol (eng. Multi-PointControlProtocol);
  • any ONU can not communicate with other ONU without participation of OLT in such communication. All packages for any destination is centrally processed by one device in the network - OLT.

 

 

Распределение временных промежутков между ONU

Figure 2 - Distribution of time intervals between ONU

To support assignment of temporary domains using OLT, a group of IEEE 802.3ah  developed MPCP protocol. This protocol is based on two Ethernet messages: GATE and REPORT.  GATE message is sent from OLT to ONU and is used to assign temporary domain. REPORT message is used by ONU to inform OLT about its condition (the buffer is full, etc.) to help making the right decision on allocation of temporary domain. Both GATE and REPORT messages are MAC management frames (type 88-08).

There are two MPCP operation modes: auto detection (initialization) and normal mode. Autodetection mode is used to detect the newly connected ONU and determination of RTT (eng. Round Trip Time - the time from sending request to receipt of response) and MAC-address of this ONU. Normal mode is used for temporary allocation of temporary domain for all initialized ONUs.

Standard Ethernet frames in PON are slightly modified for work in shared environment on the basis of TDM, however, OLT modifies outgoing packages so that the standard Ethernet stream is obtained on the output from PON. The same is in the opposite direction. The structure of a standard Ethernet frame (IEEE 802.3), PON frame (IEEE P802.3ah) and control frame IEEE P802.3ah is presented below (Figure 3):

Figure 3 - Comparison of frame fields IEEE 802.3 and IEEE P802.3ah

Preamble of a standard Ethernet frame (Figure 3a), modified by addition of several service fields (Figure 3b):

  • SOP (eng. Start Of Packet) –1 byte, indicates the beginning of the frame;
  • Reserved field, 4 bytes;
  • LLID  (eng. Logical Link Identificator) - 2 bytes, indicates the unique identifier of EPON node. There is a question: how much identifiers can have a subscriber unit ONU - one or several? LLID is required to emulate point-to-point and point-to-multipoint connections in EPON network. The first bit of the field indicates the frame transmission mode (unicast or multicast). The remaining 15 bits contain individual address of EPON node;
  • CRC (eng. Sircle Redundancy Check) - 1 byte, checksum on the preamble (standard P802.3ah).

When frame leaves GEPON network, the frame preamble is converted to a standard form - a tag is eliminated. For example, in the direct flow, OLT modifies the preamble of each frame 802.3 entering PON, in particular, the preamble is added with a special tag LLID. This tag is retrieved by the appropriate sublayer on ONU, where the preamble is recovered. ONU node in normal mode of operation, i.e. when it is registered, processes only those frames, where LLID identifier in preamble coincides with its own LLID. Other fields of EPON frame coincide with standard Ethernet frame fields:

  • DA (eng. Destination Address) - 6 bytes, indicates MAC-address of the destination station. It may be the only physical address (unicast), the group address (multicast) or the broadcast address (broadcast);
  • SA (eng. Source Address) - 6 bytes, indicates MAC-address of the sender station.
  • L/T (eng. Length/Type) - 2 bytes, contains information about length or type of frame;
  • The data field of variable length;
  • PAD (filler) - field is used to supplement the frame to the minimum size;
  • FCS (eng. Frame Check Sequence) - 4 bytes, checksum of the frame, calculated using cyclic redundancy code;
  • OpCode (eng. Optional Code) - 2 bytes, specifies type of control frame. There are two categories of control frames differ by field values: GATE message generated by the OLT, and REPORT messages generated by the ONU;
  • TS (Time Stamp) - 4 bytes, contains a timestamp of the sender;
  • message – 40 bytes, this field contains service information required for MPCP protocol.

More information about PON logic can be found at http://book.itep.ru.

OLT and ONU provide encapsulation of data in the modified Ethernet frames under IEEE P802.3ah standard, however it uses 8B/10B channel coding (8 subscriber's bits are converted into 10 channeled).

The final algorithm of PON network activity after setting up is as follows:

  •  ONU "listening to the line";
  • OLT receives a package under IEEE 802.3 standard from the superior device and modifies it under IEEE P802.3ah standard;
  • OLT sends the package to a particular destination (ONU):
  • All ONU receive the package, but only the addressee leaves it for itself - the remaining packages are discarded;
  • ONU modifies IEEE P802.3ah standard package  for IEEE 802.3 standard and gives it to the client PC;
  •  ONU receives packages from the client PC, modifies them from IEEE 802.3 standard to IEEE P802.3ah standard and buffers it;
  • OLT allows transferring data to a specific ONU;
  • ONU broadcasts a certain amount of time and then stops again and "listens" to the line;
  • OLT receives IEEE P802.3ah standard package  from ONU, modifies it under the IEEE 802.3 standard, then passes it to the next higher unit.

PON network algorithm to convert packages from one standard to another can be represented as follows (Figure 4):

Алгоритм работы PON по преобразованию пакетов

Figure 4 - PON network algorithm to convert packages

 

1.4 Comparison of PON with Classical FTTH Connection Diagram

Classical FTTH is characterized by a big number of used fibers (one for each optical consumer, whether an end subscriber or a high-rise building), which in turn leads to an inefficient use of the cable according to the principle: the more capacious than the cable, the more it is used inefficiently.

For example, four fiber cable running to a group of closely spaced high-rise buildings via the sewer shaft (one fiber for each) shall be laid into the basement of one of them, and to cut, having branched off one fiber per optic consumer. The remaining three fibers carrying information signal shall be put via a sewerage to the next house. At the same time the cable from the first branching point to the second, still have four fibers, just one fiber remains unused. Etc...

Of course, you can gradually reduce cables fibers, laying in narrower sections less capacious cables, but, in practice, it is not very convenient: to keep several kilometer bays with different fibers is expensive even for a basic work with 8-fiber cable, not to mention the more capacious ones.

Besides, a disadvantage of FTTH even in the city is the large number of intermediary active access devices and aggregation between the provider and the subscribers - they consume electricity, require regular maintenance and are sensitive to voltage drops, strongly dependent on the ambient temperature, humidity ... If to project all these disadvantages on a countryside, with attics and cellars, as well as centralized sewerage and power network is not always available, and take into account issues such as a standard "switch is malfunctioning and do not response - it shall be restarted manually" - it is absolutely uninteresting to develop the private network and to lay the cable to the village.

To solve the above problems GEPON is an ideal technology, which has already been encouraging Internet subscribers the most remote settlements on the map of Ukraine for about five years.

When using GEPON per 64 subscribers, it is necessary to use only one optical waveguide and a four fiber cable will be sufficient for 256 subscribers respectively. At the same time, subscribers may be at a sufficient distance from each other and from the nearest primary cable. When constructing PON network the unused fiber in the cable is practically absent, and main 4 or 8 fibers cable  is enough for effective deployment of passive optical network together with subscriber's «fiberdropcable» protected by patch cords of different lengths.

However, the most desirable advantage of the passive optical network is no demand for power in intermediate nodes between the subscriber and the provider. This immediately removes a number of questions from energy supply companies, firefighters and other troubled agencies. The same advantage can be effectively used in the countryside: intermediate nodes, not attached to power supply, can be placed anywhere, in this case a significant part of funds for maintaining uninterrupted power supply will be saved, as well as the means for prevention and repair of any active equipment in the network.

Another important fact is that adjustment of all active GEPON equipment in a particular passive network is carried out via a single device - the head-end station (OLT). This greatly simplifies work of system administrator allowing the most efficient troubleshooting as well as to perform regular maintenance of the network.

Furthermore, it is easy to launch analog TV via the passive network (Figure 5):

 

Применение PON в качестве среды для использования CATV

Figure 5 - Use of PON as a medium for CATV

Therefore, the advantages of PON as compared with FTTH:

  • Minimal use of active equipment;
  • Minimization of cabling infrastructure;
  • Low maintenance cost;
  • Possibility of integration with cable TV;
  • Good scalability;
  • High density of subscriber ports.

At the same time, when considering GEPON technology, it is necessary to take into account its peculiarities, especially in comparison with the "point to point" lines:

  • bandwidth shared between subscribers (shared medium may not be suitable for the client on security grounds);
  • passive components (splitters) make it difficult to troubleshoot the optical line;
  • equipment failure in one subscriber may impact another (in case of ONU failure, there is an extremely low probability that the transmitter of "failed" ONU will constantly ping, disturbing other transmitters);
  • fewer benefits, if implemented during the construction phase.
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