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What is the Fibre to the Home and Passive Optical Network? | FTTH and PON

Fibre to the Home and Passive Optical Network (FTTH and PON):

Optical access services as access systems have grown widespread in recent years. Today, fiber networks come in many varieties, depending on the termination point: premise (FTTP), home (FTTH), curb (FTTC) or node (FTTN). For simplicity, most people have begun to refer to the fiber network as FTTx, in which x stands for the termination point. As telecommunications providers consider the best method for delivering fiber to their subscribers, they have a variety of FTTx architectures to consider.

1.0 Introduction

Since the long back, telecommunications providers have dreamed of an all-fiber network. and for good reason a Fiber provides substantially more bandwidth, carries signals farther, is more reliable and secure, and has a longer life span than any other transmission medium. Additionally, providers view fiber’s bandwidth capacity as a competitive weapon, particularly in the access network. Never before has the access network been as important to telecommunications providers as they look for ways to deliver new high-bandwidth services to their subscribers—services that generate new revenues, help them retain existing customers, attract new ones and increase profits.

Fiber is seen as the preeminent long-term alternative to today’s broadband access technologies, one that not only allows providers to generate new services, but also provides them with significant and sustainable reductions in operating expenses and shifts their capital spending from older technologies to newer, less costly technologies. The single greatest driver for fiber in the access network is “multi-play” services, the opportunity to offer subscribers high-speed data, voice, and video as one of a variety of potential bundled services.

The subscriber market for multi-play is large and growing and includes both residences and businesses. Businesses need more bandwidth and many of the advanced services that only fiber can deliver, and Multi-play offers homeowners the convenience of voice, data and video from a single vendor and on a single bill. All view Multi- Play as a strong competitive service offering now and into the future and are looking at fiber as the way to deliver.

As traditional telecommunications providers explore their fiber network options, many municipalities and utilities are taking the lead, building greenfield fiber networks to serve their communities and to attract new business. Today, fiber networks come in many varieties, depending on the termination point: premise (FTTP), home (FTTH), curb (FTTC) or node (FTTN). For simplicity, most people have begun to refer to the fiber network as FTTx, in which x stands for the termination point. As telecommunications providers consider the best method for delivering fiber to their subscribers, they have a variety of FTTx architectures to consider.

Currently, there is not a one-size-solves-all architecture, so providers must make a series of technology decisions based on their service goals. A primary consideration for providers is to decide whether to deploy an active (point-to-point) or passive (point-to-multipoint) fiber network. Optical fiber cables have conventionally been used for long-distance communications. However, with the growing use of the Internet by businesses and general households in recent years, coupled with demands for increased capacity such as for the distribution of images, the need for optical fiber cable for the last mile has increased.

2.0 What is FTTx?

The FTT in FTTx stands for Fiber To The. How the fiber cable is to be used determines what will replace the letter x. e.g. x-H (Home), x-B (Building), x-C (Curb) etc. FTTH, FTTB, and FTTC each have different configurations and characteristics.

2.1 FTTH (Fiber To The Home):

A method of installing optical fiber cable to the home. FTTH is the final configuration of access networks using optical fiber cable. FTTH consists of a single optical fiber cable from the base station to the home. The optical/electrical signals are converted and connected to the user’s PC via an Ethernet card.

 

2.2 FTTB (Fiber To The Building):

Optical fiber cable is installed up to the metallic cable installed within the building. A LAN or existing telephone metallic cable is then used to connect to the user.

 

2.3 FTTC (Fiber To The Curb):

A method of installing optical fiber cable by the curb near the user’s home.  An optical communications system is then used between the remote unit (optical signal/electrical conversion unit) installed outside (such as near the curb or on a telephone pole) from the installation center. Finally, coaxial or other similar cable is used between the remote unit and user.

 

3.0 FTTx Architectures:

When deciding which architecture to select a provider has many things to consider including the existing outside plant, network location, the cost of deploying the network, subscriber density and the return on investment (ROI). Active architectures sometimes referred to as Home Run Fiber and/or Active Star Ethernet, and passive architectures, which include Passive Optical Networks (PONs), are the current choices. Each has its own pros and cons, and the final selection will depend on the provider’s unique requirements.

3.1 Home Run Fiber (Point-to-Point)

A Home Run Fiber architecture is one in which a dedicated fiber from an Optical Line Terminal (OLT) unit located in the Central Office (CO) connects to an Optical Network Terminal (ONT) at each premise. Both OLTs and ONTs are active, or powered, devices, and each is equipped with an optical laser. Subscribers can be located as far away from the CO or OLT as 80km, and each subscriber is provided a dedicated “pipe” that provides full bi-directional bandwidth.

Over the long term Home Run Fiber is the most flexible architecture; however, it may be less attractive when the physical layer costs are considered. Because a dedicated fiber is deployed to each premise, Home Run Fiber requires the installation of much more fiber than other options, with each fiber running the entire distance between the subscriber and the CO. The fiber cost and size of the fiber bundle at the OLT can make this network expensive and inconvenient in many service areas.

 

3.2 Active Star Ethernet (Point-to-Point)

An Active Star Ethernet (ASE) architecture is a point-to-point architecture in which multiple premises share one feeder fiber through a remote node located between the CO and the served premises. Environmentally hardened optical Ethernet electronics—switches or Broadband Loop Carriers—are installed at the remote node to provide fiber access aggregation. The remote node can be shared between four to a thousand homes via dedicated distribution links from the remote node. Like Home Run Fiber, subscribers can be located as far away from the remote node as 80km, and each subscriber is provided a dedicated “pipe” that provides full bidirectional bandwidth. Active Star Ethernet reduces the amount of fiber deployed; lowering costs through the sharing of fiber. ASE also offers the benefits of standard optical Ethernet technology, much simpler network topologies and supports a wide range of CPE solutions. And, most importantly, it provides broad flexibility for future growth.

 

4.0  Passive Optical Network (Point-to-Multipoint)

Passive Optical Network is essentially a cost-effective optical fiber-based access system for providing multi-play (voice, video, data etc) services, being rolled out by BSNL shortly, to both business and residential customers. Passive Optical Networks (PON) use optical fiber and optical power splitters to connect the Optical Line Terminal (OLT) at the local exchange to the subscriber’s Optical Network Unit (ONU) on his premises. No electrical or electronic components are used between these points. This approach greatly simplifies network operation & maintenance and reduces the cost. Another advantage is that much less fiber is required than in point-to-point topologies.

 

Using Ethernet technology to create a passive optical infrastructure, PONs builds a point-to-multi-point fiber topology that supports a speed of Gbps for up to 20 km. While subscribers are connected via dedicated distribution fibers to the site, they share the Optical Distribution Network (ODN) trunk fiber back to the Central Office.

The figure 7 shows the less fiber requirement for PON (EPON & GPON) as compared to the topologies of point-to-point Ethernet and point-to-multipoint switched Ethernet.

Point-to-point Ethernet might use either N or 2N fibers, and would have 2N optical transceivers. Point-to-multipoint switched Ethernet uses one trunk fiber and thus would save fiber and space in the Central Office (CO). But it would use 2N+2 optical transceivers and would require electrical power in the field. PON also uses only one trunk fiber and thus minimizes fibers and space in the CO, and it also uses only N+1 optical transceivers. It requires no electrical power in the field. The drop throughput can be up to the line rate on the trunk link. EPON can support downstream broadcast such as video. EPON is typically deployed as a tree or tree-and-branch topology, using passive 1:N optical splitters.

 

Time Division Multiplexed (TDM) data is broadcast downstream from the OLT towards each ONU where the appropriate portion is extracted for local use. In the Upstream direction a Time Domain Multiple Access (TDMA) protocol allocates slots for data transmitted from each ONU to communicate back to the OLT without any contention between different subscribers.

The features of different PON standard

Features	BPON	GPON	EPON
Responsible Standard body	FSAN & ITU-T SG15 (G-983 Series)	FSAN & ITU-T SG15 (G-984 Series)	IEEE 802.3ah
Bandwidth	Down Stream up to 622 Mbps Up Stream up to 155.52 Mbps	Down Stream up to 2.5 Gbps Up Stream up to 2.5 Gbps	Down Stream up to 1.25 Gbps Up Stream up to 1.25 Gbps
Downstream ג	1490 nm & 1550 nm	1490 nm & 1550 nm	1490 nm
Upstream  ג	1310 nm	1310 nm	1310 nm
Layer-2 Protocols	ATM	ATM, Ethernet, TDM over GEM	Ethernet
Frame	ATM	GPON Encapsulation Method	Ethernet Frame
Max. Distance (OLT to ONU )	20 km	20 Km(supports logical reach up to 60 Km)	10 and 20 Km.
Split Ratio	1:16, 1:32 and 1:64	1:16, 1:32 and 1:64	1:16 and 1:32
Line Codes	NRZ ( Scrambled )	NRZ ( Scrambled )	8B/10B
Downstream Security	AES: Advanced Encryption Standard -128 bit key	AES: Advanced Encryption Standard ( Counter mode)	Not Defined
FEC	None	Yes	Yes
No. of  fibers	1 or 2	1 or 2	1
Protection Switching	Support multiple protection configuration	Support multiple protection configuration	None

 

5.0 PON Architecture:

The key interface points of PON are in the central office equipment, called the OLT for optical line terminal, and the CPE, called ONU for optical network unit (for EPON) and ONT for optical network terminal (for GPON). Regardless of nomenclature, the important difference between OLT and ONT devices is their purpose. OLT devices support management functions and manage maximum up to 128 downstream links. In practice, it is common for only 8 to 32 ports to be linked to a single OLT in the central office. On the other hand the ONT (or ONU) devices in the CPE support only their own link to the central office. Consequently, the ONT/ONU devices are much less expensive while the OLTs tend to be more capable and therefore more expensive.

1. OLT: The OLT resides in the Central Office (CO). The OLT system provides aggregation and switching functionality between the core network (various network interfaces) and PON interfaces. The network interface of the OLT is typically connected to the IP network and backbone of the network operator. Multiple services are provided to the access network through this interface,.
2. ONU/ONT: This provides access to the users i.e. an External Plant / Customer Premises equipment providing user interface for many/single customer. The access node installed within user premises for network termination is termed as ONT. Whereas access node installed at other locations i.e. curb/cabinet/building, are known as ONU. The ONU/ONT provide, user interfaces (UNI) towards the customers and uplink interfaces to uplink local traffic towards OLT.
3. PON: Distributed or single staged passive optical splitters/combiners provides connectivity between OLT & multiple ONU/ONTs through one or two optical fibers. Optical splitters are capable of providing up to 1:64 optical split, on end to end basis. These are available in various options like 1:4, 1:8, 1:16, 1:32 and 1:64.

 

4. NMS: Management of the complete PON system from OLT.

• One OLT serves multiple ONU/ONTs through PON
• TDM/TDMA protocol between OLT & ONT
• Single Fiber/ Dual Fiber to be used for upstream & downstream
• Provision to support protection for taking care of fiber cuts, card failure etc.
• Maximum Split Ratio of 1:64
• Typical distance between OLT & ONT can be greater than 15Km (with unequal splitting – up-to 35Km)
• Downstream transmission I.e. from OLT to ONU/ONT is usually TDM
• Upstream traffic I.e. from ONU/ONT to OLT is usually TDMA
• PON system may be symmetrical or asymmetrical
• PON and fiber infrastructure can also be used for supporting any one way distributive services e.g. video at a different wavelength

PON is configured in full duplex mode in a single fiber point to multipoint (P2MP) topology. Subscribers see traffic only from the head end, and not from each other. The OLT (head end) allows only one subscriber at a time to transmit using the Time Division Multiplex Access (TDMA) protocol. PON systems use optical splitter architecture, multiplexing signals with different wavelengths for downstream and upstream.

EPON & GPON Applications:

• Residential or Business Services
  • High Speed Internet
  • Transparent LAN Service
  • Broadcast Video
  • Multi-Play (Voice, Video, Data etc.)
  • TDM Telephony
  • Video on Demand
  • On –line Gaming
  • IPTV etc
  • Wireless Services
  • Wireless backhaul over PON

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