FTTX access networks

By Nawfel Azami, fiber optic expert in Morocco

Transmission technologies have evolved to integrate optical technologies even in access networks, and as close as possible to the subscriber. Fiber optic networks have established themselves as the transmission medium par excellence, due to their ability to propagate over long distances without signal regeneration, and because of their low latency and their very high bandwidth.

Optical fiber, initially deployed in very long distance and very high-speed networks, is now tending to be generalized to offer more consuming services in terms of bandwidth. These are Fiber to the Home (FTTH) technologies.

With the explosion of the Internet and the opening and the communicating multimedia society, services offered by providers are more and more numerous and varied, integrating video, image as well as speech. The general trend is reflected in Nielsen’s law, which shows a 50% annual increase in Internet speed at the access point. The extrapolation of this law over the next decade reveals the high speeds that will be used, i.e. several hundred Megabits per second, or even Gigabits per second.

Currently, deployed ADSL networks are unable to meet such speed demands due to the limited transmission properties inherent in the physical medium used in this technology, that is the copper pair of phone services  

Businesses

Companies generally have a LAN (Local Area Network) for the interconnection of computers and various peripheral devices. Telephone telecommunications are traditionally managed by a PABX. A server connected with the outside network(s) handles access to the Internet through the LAN network. An equally widespread practice is the WAN (Wide Area Network) which will allow a wider coverage, allowing the implementation of applications such as the intranet or the extranet as well as the interconnection with remote teleworkers. The WAN is based on leased lines. The need of the majority of companies is growing and tends to become symmetrical between uploads and downloads due to the growing number of nomadic collaborators. 

Individuals

For residential users, the main goal is to be able to have an infrastructure that can support the “Triple-Play” service, which includes bandwidth-hungry video, telephony and Internet access services. The needs associated with each household are relatively well defined and their demand for bandwidth is changing day by day with the arrival in homes of 3D televisions and with programs that are using online user applications intensively (personalized medicine, financial activities, etc.). It is thus necessary to specify that the asymmetric character, which presently foresees the data transfers, will fade shortly due to the growing importance of applications requiring the implementation of higher rates in the upstream direction. Indeed, cloud computing encourages the transfer of data and applications in hosting centers connected to the Internet.This brief introduction highlights the importance of fiber optic technologies in the access network. Even if the entire community is convinced of the need to see optical fiber massively implemented down to the subscriber, the economical aspects of investment are holding back its deployment. Experts agree that optical fiber is not in itself the bulk of the investment, but the civil engineering part is quite more important than that of optical components. Also, “Fiber to the Home” (FTTH) is hybridized to reduce costs, knowing that no one doubts that the most capable network to meet future needs cannot be based on anything other than end-to-end optical fiber.

The different FTTx configurations

FTTx is a fiber optic access network topology where the fiber does not reach the subscriber. In most countries, the evolution of the ADSL network is mainly based on these topologies for cost reasons. However, operators and service providers are well aware that they remain in the same philosophy as ADSL where technologies are developed to push back the day when optical fiber will have to enter into homes and offices. Indeed, the key word of the development of ADSL has been to continue developing modulation and multiplexing techniques in order to be able to approach and to intelligently manage the copper cable and to use it in an effective way. This has been conducted by dividing the bandwidth into several individually managed channels to pass an Internet access. The noise is greater than the distance between the subscriber and the operator, reducing the quality of transmission.

Optical fibers can be deployed according to various FTTx topologies where the variable x declines more or less deep the level of depth with regards to the optical fiber establishment relatively to the end user: FTTN (N for Node) up to a network node (equivalent to a under copper distributor), FTTC (C for Curb) or FTTLA (LA for Last Amplifier) ​​up to about ten meters from the subscriber, FTTB / O (B for Building or O for Office) at the ground floor of the building, and FTTH (H for Home) to the subscriber.

Intermediate topologies of FTTH (FTTx)

The intermediate topologies described previously, where the optical fiber does not reach the subscriber, are now preferred for reasons of technical and in term of investment.

A first approach of FTTx networks is to establish a fiber optic link for the remote connection of the subscriber connection node (NRA), and thus reduce the distance between the subscriber and the NRA. The rest of the network remains in its traditional version; in copper pair. Reducing the distance between the subscriber and the NRA, in ADSL engineering, increases the signal-to-noise ratio at the subscriber and thus promotes better speeds.

HFC cable networks (FTTN or FTTLA) are a second example where a coaxial cable is used in the last mile. Frequency multiplexing is used in this case. If distance allows, the coaxial network can be passive (without an amplifier) ​​and identifies with the FTTLA.

The third example corresponds to Ethernet architectures with copper termination. The standards are based on the 802.3 Ethernet family.

FTTH architecture

The very high speed FTTH network is made up of a certain number of elements which can be classified into three categories: Infrastructure (composed in particular of ducts, rooms, street cabinets and technical rooms), passive optics (passive components of the network and which include in particular optical fibers, splice boxes and distribution cabinets) and active optics (components that consume electrical current and which include in particular the transmitting and receiving systems).

The FTTH network is made up of three subnetworks: the transport network, the distribution network and the branch network.

An interior optical termination device (IOTD) is placed inside the housing (fig). It constitutes the termination point for optical cabling. The media adapter will be connected to the optical connector of this box and will ensure the opto-electronic conversion of the signal. The IOTD is wired to the Optical Connection Point (OCP), which is the mandatory interface for the actions of connecting individual subscribers from basic cables. In the case of apartment buildings, the optical cables belonging to the vertical cabling can be grouped together at the foot of the building in a transition box, which ensures continuity with the network deployed in the public domain. In buildings, the floor box is an OCP that can serve several DTIOs located on several floors.

Intermediate devices may exist between the optical distribution frame of the subscriber connection node and the OCPs. These intermediate points are necessary for the implementation of regulatory obligations related to pooling.

Most popular architecture: PON “Passive Optical Network” architecture

The FTTH architecture generally adopted by operators is a PON (Passive Optical Network) architecture. PON is a point-to-multipoint architecture based on the following elements:

– A shared optical fiber infrastructure. The use of optical couplers in the network is the basis of the architecture and deployment engineering. The couplers are used to serve several zones or several subscribers.

– Center equipment acting as an optical Line Termination (OLT). The OLT manages the broadcasting and reception of streams through network interfaces and its purpose is to receive signals from clients as well as to broadcast them content based on specific services (for example triple play). Built around cards, it manages a certain number of subscribers and allows them access to specific speeds.

– end equipment:

* ONT (Optical Network Terminations) in the event that the equipment is dedicated to a customer and the fiber reaches the customer. In this case it consists of a FTTH (Fiber To The Home) type of architecture. There is only one fiber per customer (signals are bidirectional)

* ONU (optical network unit) in the case of FTTB networks.

The diagram below shows the principle and the different elements that make up a PON network.

Figure 1: Schematic diagram of a PON architecture

Note that behind the name PON hides a whole series of variants based on different protocols:

– B-PON (Broadband PON) defined in the ITU recommendation G.983 which is based on ATM, B-PON can go up to speeds of 1Gb / s in the downstream direction and 622Mb / s in the upstream direction but its use is usually seen for bit rates of 622Mb / s downstream and 155Mb / s upstream.

– E-PON (Ethernet PON) defined in the IEEE 802.3ah recommendation which is based on Ethernet and has a symmetrical speed of 1Gb / s. –

G-PON (Gigabit PON), presented as the successor to B-PON and defined in recommendation ITU G.984, which relies on GEM (G-PON Encapsulation Method) to transport different protocols. G-PON can achieve speeds of 1 Gb / s or 2Gb / s in the downstream direction and 155Mb / s, 622Mb / s, 1 Gb / s or 2Gb / s in the upstream direction (knowing that the download speed is always ≥ upload speed). But, typically, G-PON is seen to work with combinations of 2Gb / s downstream, 622Mb / s, or 1Gb / s upstream rates.

For example, France Telecom has opted for G-PON type technology.

The future technology, which still needs to be standardized although already implemented in long distance broadband networks, is WDM-PON. It is based on PON with a wavelength-multiplexing layer. WDM will boost speed and multiply it by a factor equal to the number of channels (wavelengths) used.

Sharing

As much the concept of pooling in xDSL was simply resolved by making available to operators or service providers customers’ copper pairs available at the level of the incumbent operator, the concept of pooling in FTTx networks seems to take several options.

Indeed, many configurations can be found depending on the situation. However, all of these different scenarios are based on the block diagram in the figure below. The pooling point can be found both in the optical connection node of the access operator and in an intermediate box.

Figure 2: General diagram of the different pooling configurations

By Nawfel Azami, fiber optic expert in Morocco