Why Optical Access Network

Published Date: 02 Nov 2017

Introduction

Access network is the region in a public switched network that connects central office (CO) to individual subscribers or end users or to be simplified; it is the last mile in a network. The last mile is the most expensive part in the network because there are a lot of end users outside there than the backbone nodes. Some example of access networks are twisted copper pairs connecting to every household and residential coaxial cable drops from community antenna TV (CATV) service providers. For your information, optical fibers have been widely used in backbone networks due to their huge available bandwidth and low probability of loss. However, up to this time, the access network is mainly consists of passive, twisted-pair copper wires. As a basic bearer network, the access network has to bear multiple services such as voice, data, video, enterprise access and backhaul services of the radio base station. That is the reason why it has consistently been regarded as a bottleneck in the provisioning of data communications services due to the bandwidth available will be lagged behind in the copper-based access. Therefore, optical fiber in term of concentration factors and economies of scale has been considered and attempting to boost up the significant bandwidth capacity. Optical access network is the part of the access network that is implemented using optical fiber. Optical access network offers greatly increased bandwidth by up to several gigabits per second (Gbps) and most likely will be more as the technology are advance through the time.

Figure 1 show the switching network that covers access network, metro network and backbone network.

Backbone

Access

Metro

Figure 1

Discussion

Why optical access network

Broadband services nowadays are mainly offered by using digital subscriber line (DSL) technologies via the installed twisted pair copper wires which already used for delivering telephone services. The main inconvenience of copper wire is that the transfer rates depend directly on the distance between the hub and the end user. Therefore, the data rates delivered by DSL systems are essentially limited. To solve this bandwidth limitation, a technology is needed which can leverage the existing network as well as increase the economic viability of new network applications. Access network technology tends to take the advantage of fiber communication where it has the potential of huge capabilities to deliver faster and more various services to end users.

In today’s rapidly changing economic environment and the exponent growth of internet bandwidth, fiber optic infrastructure also has been proving to be one of an important part in the network. As a result, it leads to the adoption of optical access solutions since it helps both end users and service providers to connect to the information in a form of superhighway. However, to operate the optical access network, several enabling factors had to be addressed and take into consideration. First is the availability of affordable multiplexing equipment where multiplexing is bandwidth sharing among multiple traffic sources. If multiplexing equipment is not being used where the bandwidths do not share among multiple users then there will a lot of cable in the network. Next is the the deployment of fiber cables in sufficient quantity to create a critical mass for service offerings. Fiber may is a new thing for some service providers and they need to plan well in the designation in the deploying of fiber cables due to fiber is costly.

Benefits

The main benefit introduced by optical access network is that the fiber optic can provide a very huge bandwidth which up to 10 GB/s. Some form of multiplexing can be implemented to effectively utilize the bandwidth of fiber such as wavelength division multiplexing (WDM). Optical access network offers a great performance in term of manageability, flexibility, and affordability especially in the case of physical layer solutions. Optical fiber offers bit rate transparency and service transparency given that the optical switching technology is implemented do not undergo the electronic processing along the chosen path and the data can ideally pass through the intermediate nodes without any limitation on bit rate or protocol format. The need for protocol conversion is eliminated where the service providers can provide smooth transition and thus making the whole process simpler and more reliable.

Besides that, by using wavelength division technologies (WDM) in optical access network allow more users go through each port and high bandwidth lightpaths are provided for users. It is one of the factors to satisfy the service providers of hand off traffic on a per-port basis.

However, in short-term, optical access network will be costly as optical fiber is a very expensive equipment compare to copper wire but in long-term, optical access network will bring a lot of benefits to the service providers and for sure the end users. This is because optical access network has much larger quantities of data across the link can be accommodated, routed, switched and processed which means reduced the average operational expense.

Low attenuation and high bandwidth is the superior transmission characteristics of optical fiber and this is the main reason access network is now deploying optical fiber. However, reduction in cost is more important than capacity and speed in the perspective of service providers. Due to the superior transmission characteristics, the network operator can establish larger service areas served from a single access network node. This simplify the network structure and reduce the number of access nodes, which is expected to enhance the overall cost efficiency of the network.

Solution to last mile bottleneck

Today most widely deployed broadband solutions are Digital Subscriber Line (DSL) and Cable Modem networks. Although broadband copper-based access networks provide much higher data rate than 56 Kbps dial-up lines, they are unable to provide enough bandwidth for the tremendous growth of Internet traffic, emerging applications such as Internet Protocol TV (IPTV), video-on-demand (VoD), peer-to-peer applications, and interactive games.

The emerging web applications require unprecedented bandwidth, exceeding the capacity of traditional VDSL or CATV technologies. The explosive demand for bandwidth is leading to new access network architectures which are bringing the high-capacity optical fiber closer to the small businesses and residential subscribers. This leads to so called fiber to the X (FTTx) networks. The FTTx models are Fiber to the Building (FTTB), Fiber to the Home (FTTH), Fiber to the Curb (FTTC), Fiber to the Premises (FTTP), Fiber to the Node (FTTN). They offer the potential for unprecedented access bandwidth to end users up to 100 Mbps per user. These technologies aim at providing fiber directly to the home, or very near the home, from where technologies such as VDSL or wireless can take over. FTTx is considered an ideal solution for access networks because of the inherent advantages of optical fiber in terms of low cost, huge capacity, small size and weight, and its immunity to electromagnetic interference and crosstalk. FTTx solutions are mainly based on the Passive Optical Network (PON). The FTTN/B describes a PON where the fiber arrives directly from the Central Office (CO) to the building and the signal is converted and carried to the user dependencies by using copper or coaxial cables. Developments in PON in recent years come in different flavours include Ethernet PON (EPON), ATM-based PON (APON), Broadband-PON (BPON), and Gigabit-PON (GPON). APON is the first PON system but later it is improved and replaced by BPON. BPON is based on APON with the adding support for WDM and higher bandwidth. EPON is IP efficient and low cost which based on Ethernet technology. It support voice and data transport but for voice transfer is not so efficient. The wide deployment of Ethernet LAN technology make the EPON appears to be advantage over ATM-based PON. GPON provide high bit-rate support enabling multiple services transport in a way of high efficiency.

The principle of PON is to share equipment and the feeder fibers among as many optical transport networks as possible. PON reduces the network cost by eliminating the operational cost along the fiber path from Central Office (CO) to end users, and by sharing the significant portion of the network cost among multiple users. Besides, it also simplifies network management.

The most common FTTx network types in the market are FTTN/B and FTTH. FTTH refers to the reach of the fiber cables till the household, providing more and cheaper bandwidth for the users. It consists of an Optical Line Termination (OLT) at the central office (CO) which is the service provider site, and a number of Optical Network Unit (ONU) near the end-user’s device which also can be called Optical Network Terminal (ONT). PON can use single or multiple fibers for upstream and downstream traffic, with or without WDM, being a single fiber-single channel tree the most common topology. The downstream channel is a broadcast channel where traffic broadcast by the OLT to ONU, and the upstream channel is often shared amongst users’ devices. Thus, their access must be separated in order to avoid collisions. For that reason, users’ bursts are separated by a configurable guard-time. Such fiber solutions are commonly referred to as TDM-PON where is under control of the OLT and assigns time slots to each ONU for synchronized transmission of its data bursts.

Construction, operation and maintenance issue in the start-up

Operators can reinforce the effectiveness of overall planning for access optical fiber, OLT, ODN and system devices during FTTx network construction by substituting fiber with copper cable. To strengthen the linkage between front-end and back-end devices, power supply assurance, IT support, project construction, operation and maintenance is necessary and transform the access capability into service capabilities in a short time. However, the actual situation indicates that FTTx deployment faces several problems that caused by the lack of adequate capability and experience on the part of the operator in the construction, operation and maintenance of the optical fiber access network.

First, the deployment, configuration, and upgrade of devices in the optical network are a complicated and troublesome affair. The types of devices and the quantity of network elements are increasing and their management and maintenance are laborious. The operator in charge in the area of construction, operation and maintenance have to learn and adapt to the new technology environment which takes time and thus become one of the difficulty in deploying optical access network. Besides, the number of remote nodes requiring maintenance and management has doubled along with the gradual devolution of optical fiber. The fault rate will be rise and this leads to a significant amount of troubleshooting workloads. This requires a person that have very experience to handle it due to access network cover all the end users outside there and if does not handle well, all the subscribers network will be down since if the fiber being bend then the signal transferred will be lost. These issues have imposed very high requirements for FTTx device management, device deployment, service issuance, fault monitoring, and control capability on network security. 

New Trends

Long-reach PON

An alternative technology, called Long-Reach Optical Access Network, sometimes be term as Passive Optical Network (LR-PON), was proposed as a more cost effective solution for the broadband optical access network. The strength of it is its ability to displace electronics and simplify the network by combining network tiers. Traditionally, the network consists of the access network, the metropolitan network, and the backbone network. However, throughout the time and with the maturing of technologies for long-reach broadband access, the metro network is then combined with the access network. As a result, the telecom network hierarchy can be simplified with the access head end close to the backbone network as shown in the figure below. The need for managing control units will be fewer and this has significantly reduced the network’s Capital Expenditure (CapEx) and Operational Expenditure (OpEx). The access and metro is combined into one through the use of an extended back-haul fiber, possibly 100km in length to incorporate protection path and mechanisms, used with PON. As a summary of LR-PON, general architecture is it is composed by an extended shared fiber connecting the CO and the local user exchange, and users is connecting by optical splitter to the shared fiber. The difference between LR-PON with traditional PON is multiple Optical Line Terminals (OLTs) and the Central Offices (COs) where they are located is consolidated, thus reducing the corresponding Operational Expenditure (OpEx) of the network. In general, the LR-PON can simplify the network, reducing the number of equipment interfaces, network elements, and even nodes.

Figure 2 show how the network is being simplified to only access and backbone area.

Backbone

-100-1000km

-mesh topology

Metro

-10km

-rings

Access

-a few km

-hubbed rings, PON

Users

Backbone

-100-1000km

-mesh topology

Long-reach Access

-100 km

Users

Figure 2

WDM-PON

Current TDM-PONs is migrating to a higher bandwidth, cost-effective, scalable Wavelength Division Multiplexing PON (WDM-PONs) due to some limitation. TDM-PONs have only one wavelength for downstream data and one for upstream data to share among all users, thus limiting the average bandwidth per user to a few tens of Mbit/s. The availability of fiber’s high bandwidth is just mostly wasted. Alternatively, wavelength Division Multiplexing (WDM) technology has been considered as an ideal solution to extend the capacity of TDM-PONs without changing too much of the fiber infrastructure. WDM technologies can be used to increase the throughput of PONs. The multiple wavelengths of a WDM-PON can be used to separate Optical Network Units (ONUs) where each of the ONU uses different wavelength sending data upstream so that can be continuously and will not be congested. The receiver of OLT which normally is splitter is replaced with wavelength demultiplex device such as array waveguide grating (AWG) at the same time. The main difference of WDM-PON and TDM-PON is that there is no sharing among ONUs of OLT bandwidth. Overall, the wavelengths can be used collectively through statistical multiplexing to provide lower delays experienced by the ONUs and efficient wavelength utilization.

However, the cost of tuneable and wavelength-sensitive WDM components is quite high for access networks. Thus, developing network architectures, sub-systems and devices that reduce the cost of WDM-PONs is crucial for their successful deployment yet it still under research to enhance it.

Below is figure 3 about the simple picture of architecture of WDM-PON.

Central Office, CO

ONU

OLT

ONU

OLT

ONU

OLT

Wavelength multiplexer/demultiplexer

Wavelength multiplexer/demultiplexer

Figure 3

Table below shows different types of PONs technologies:

TDM-PON

WDM-PON

EPON

BPON

GPON

Standard

IEEE 802.3ah

ITU G.983

ITU G.984

None

Framing

Ethernet

ATM

GEM/ATM

None

Specified

Maximum

Bandwidth

1 Gbit/s

622 Mbit/s

2.488 Gbit/s

1-10 Gbit/s

per channel

Users/PON

16

32

64

100’s

Average

Bandwidth

per User

60 Mbit/s

20 Mbit/s

40 Mbit/s

1-10 Gbit/s

Video

RF / IP

RF

RF / IP

RF / IP

Estimated

Cost

Lowest

Low

Medium

High

Upgradeability

Difficult

Difficult

Difficult

Easy