A Review of Passive Optical Networks

International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013
ISSN 2319 - 4847
A Review of Passive Optical
Er. Satish Sharma
Vpo-jabli tehsil-kasauli,district–solan(H.P)173209
This paper discusses about the evolution and basic structure of Passive Optical Networks. In this paper we discussed about the
Access networks, their classifications and its types. How copper networks differs from optical network The Difference between
active and passive optical networks and basic system of Passive Optical Networks. We show through analysis that Passive
Optical Networks are very cheap and can provide better services as compare to copper access networks. PON is also better
option for access networks because it is very cheaper than active optical networks
Keywords: Optical Line Termination (OLT), Optical Network Units (ONU), Passive Optical Networks (PON).
1. Introduction
Telecommunications today is perhaps the fastest evolving field of study. It is continuously offering new challenges and
opportunities to telecommunications network planners. The subscriber part of the telecommunications network or the
network connecting the subscribers to the central office or the access network that has been traditionally simple twisted
copper pair based, point to point, passive network is now becoming increasingly complex. In the present scenario it
becomes imperative for the access network planner to be familiar with both traditional and new technologies, structures
and methods as their plans would have a profound long term impact on how the network shapes up and meets the
desired objectives [1].
The basic idea of telecommunication is the exchange of information. The information may include voice, text, data,
image and video. A telecommunications network is therefore a system which can provide these services to a number of
end users. It is very important for network planners to pay attention to the technical evolution of telecommunication
systems. This would to enable proven new technologies to provide high quality telephone service and meet demands of
new telecommunication services. Owing to prospective development of these technologies and increasing demand for
new services other than telephony, telecommunication networks are changing from partly analog to fully digital.
Demand and traffic patterns will change faster in the future than they do today. To cope with this, one important
property a network should have is flexibility. Flexibility in simple term implies being able to provide bandwidth on
demand. If bandwidth can be provided on demand then the network becomes capable of deploying and supporting a
wide variety of services and with greater ease and speed [2]
2. Access Networks
It is that part of a telecommunications network which connects subscribers to their immediate service provider. The
term access network refers to the network between the local exchange and the subscriber. In many countries this
network is still predominantly made up of the copper cable based point to point connections [3]. The technology has not
changed much during the last many decades even though considerable changes have been introduced in the field of
switching and transport.
Fig 1: Access Network [3]
With the advent of digital technology, the process of installation, maintenance has become less cumbersome and quality
of services has improved. It is therefore felt that the any cause for dissatisfaction, among customers about present
services, is predominantly due to the frequent failures in the access network and the time taken for restoring them. One
Volume 2, Issue 5, May 2013
Page 181
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013
ISSN 2319 - 4847
of the most fundamental and remarkable of the driving technologies of today is the optical fibre. Without it the current
telecommunications revolution would have been a non-event. Supporting the high growth telecommunications demand
with copper, radio and satellite alone would have stalled the information revolution. These would not have made
bandwidth and connectivity for all possible. Increasingly, therefore twisted pair copper cable is being replaced by
optical fibre cable with new transmission technologies. The term access network has gained popularity after the advent
of new technologies in the local loop. Another change which is now becoming evident is change of character of the
access network from passive to active. These changes hold the promise of removing the limitations of the copper cable
2.1 Copper network
In the foreseeable future the subscribers in all sectors: government, residence, business, education and military will
demand more and more enhanced services [4]. Demand for an integrated voice, video and data services is building up
and very soon customers are not going to be satisfied even with the bandwidth the narrow band ISDN offers. The
present network suffers from several limitations that make it inadequate to offer the customers all modern
telecommunication services. Some of the significant limitations include: Limited bandwidth, inflexibility, limited
reliability, long installation time, maintenance, passive network, losses, applicability, interference, and security.
2.2 Copper access networks
A family of technologies that have begun to transform the narrow band copper access network into broadband network
is the x DSL family of technologies. The term DSL, or digital subscriber line, refers to the modem which when
connected at either ends of a normal twisted wire pair line, converts it into a digital line capable of handling data rates
well into broad band [5]. The letter "x" indicates that there are many variants of DSL technology.
Table 1: DSL Technologies
Digital subscriber
High Data Rate Digital
Subscriber Line(HDSL)
Single Line Digital
Subscriber Line(SDSL)
Asymmetric Digital
Subscriber Line(ADSL)
Data Rate
ISDN series, voice and data comm.
No POTS, E1 LAN/WAN, Service
access feeder plant.
Same as HDSL + POTS
1.5-8Mb/s ↓
128kb-768kb ↑
Interest access, video on demand,
simplex video, remote LAN access,
interactive multimedia.
Fig 2: General Model of DSL systems[5]
3. Optical networks
Fiber optics is a technology that uses glass (or plastic) threads, called fibers to transmit data. A fiber optic cable consists
of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves. Over metal
cables, fiber optics provide a much greater bandwidth to carry more data, they are are less susceptible than metal cables
to interference, they are thinner and lighter than metal wires, and most importantly data can be transmitted digitally
(the natural form for computer data) through the use of fiber optics. Like all technologies, fiber optics has its downside.
The cost of fiber optic cable is more than metal cable, making the cost for installation much greater. In addition they
are more fragile than wire and are difficult to split. Fiber optics is a particularly popular technology for local-area
networks. Telephone companies are steadily replacing traditional telephone lines with fiber optic cables. In the future,
almost all communications will employ fiber optics [1].
Volume 2, Issue 5, May 2013
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
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ISSN 2319 - 4847
3.1 Active optical networks
An active optical system uses electrically powered switching equipment, such as a router or a switch aggregator, to
manage signal distribution and direct signals to specific customers. This switch opens and closes in various ways to
direct the incoming and outgoing signals to the proper place. In such a system, a customer may have a dedicated fiber
running to his or her house [2].
3.2 Passive Optical Network
A Passive Optical Network is a single, shared optical fiber that uses inexpensive optical splitters to divide the single
fiber into separate strands feeding individual subscribers. OPTICAL NETWORKS are called "passive" because, other
than at the CO and subscriber endpoints, there are no active electronics within the access network. Eliminating the
need for electrical equipment in the first mile network is a key facet of the EFMP topology. Another advantage is that
much less fiber is required than in point-to-point topologies. To visualize the lower fiber requirements, it is useful to
look at the topologies of point to point Ethernet and “curb switched” Ethernet along with Passive Optical Network.
Figure 1 illustrates all of these options. OPTICAL NETWORK is based on the Ethernet standard, unlike other Passive
Optical Network technologies, which are based on the ATM standard.
3.3 FTT-X Technologies
The three methods, that are normally used, get their name from the location of the remote terminal equipment.
Accordingly we have:
(a) Fibre to the Curb (FTTC), in which the terminal equipment is located on the curb, from where it would be
convenient to serve a suitable service area. Since the distribution would still be copper, suitable location for the
terminal would be one which optimizes the cost, reduces back-feeding, reduces distribution cost and takes safety factors
into consideration [4].
(b) Fibre to the building (FTTB), in which the terminal equipment is located inside a multi-storeyed building, this
brings higher bandwidth closer to the subscriber. The distribution part is still copper [4].
(c) Fibre to the home/Office (FTTH/FTTO) in this method the fibre goes up to the subscriber premises.
Fig 3:Point to Point Ethernet, Curb-Switched Ethernet and Passive Optical Network [3]
4. Passive Optical Network
A Passive Optical Network includes an optical line terminal (OLT) and an optical network unit (ONU). The OLT
resides in the CO (POP or local exchange). This would typically be an Ethernet switch or Media Converter platform.
The ONU resides at or near the customer premise. It can be located at the subscriber residence, in a building,
or on the curb outside. The ONU typically has an 802.3ah WAN interface, and an 802.3 subscriber interface. The OLT
is on the left and several ONU’s are shown below [5].
4.1 Passive Optical Network Systems
Passive Optical Networks is configured in full duplex mode (no CSMA/CD) in a single fibre point-to- multipoint
(P2MP) topology. Subscribers, or ONUs, see traffic only from the head end; each subscriber cannot see traffic
transmitted by other subscribers, and peer-to-peer communication is done through the head end, or OLT. As Figure 4
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
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shows, the head end allows only one subscriber at a time to transmit using a Time Division Multiple Access (TDMA)
Passive Optical Network systems use optical splitter architecture, multiplexing signals with different wavelengths for
downstream and upstream as such:
Fig 4: OLT and ONU [5]
Though configured as point to multipoint, Ethernet Passive Optical Network can be deployed in an Ethernet access
platform, with both point-to-point and point-to-multipoint access cards [5]
4.2 Passive Optical Network Protocol
To control the P2MP fiber network, Passive Optical Network uses the Multi-Point Control Protocol (MPCP). MPCP
performs bandwidth assignment, bandwidth polling, auto-discovery, and ranging. It is implemented in the MAC Layer,
introducing new 64-byte control messages:
• GATE and REPORT are used to assign and request bandwidth
• REGISTER is used to control the auto-discovery process
MPCP provides hooks for network resource optimization. Ranging is performed to reduce slack, and bandwidth
reporting satisfies requirements by ONUs for DBA. Optical parameters are negotiated to optimize performance [5].
Fig 5: PASSIVE OPTICAL NETWORK Configuration [5]
4.3 ONU and OLT Operation
The ONU performs an auto-discovery process which includes ranging and the assignment of both Logical Link IDs and
bandwidth. Using timestamps on the downstream GATE MAC Control Message, the ONU synchronizes to the OLT
timing. It receives the GATE message and transmits within the permitted time period. The OLT generates time
stamped messages to be used as global time reference. It generates discovery windows for new ONUs, and controls the
registration process. The OLT also assigns bandwidth and performs ranging operations [5].
5. Related Issues
5.1 Dynamic Bandwidth Allocation
Ethernet Passive Optical Networks consist of one optical line terminal (OLT) situated at a central office and multiple
optical network units (ONUs) located at the equipment at a customer’s premises. In the upstream direction as it is a
shared medium by ONUs, scheduling is required to prevent a data collision from different ONUs [6]. Thus dynamic
bandwidth allocation (DBA) scheme is required to support better QOS in Ethernet Passive Optical Networks. Dynamic
band width assignment (DBA) can be on two Basis, inter-ONU scheduling and intra-ONU scheduling.
5.2. Inter–Optical Network Unit (ONU) bandwidth scheduling
A dynamic bandwidth allocation (DBA) scheme, an inter–optical network unit (ONU) bandwidth scheduling, is
presented to provide quality of service to different classes of packets in Ethernet Passive Optical Network [6]. These
scheme are based on efficient threshold reporting from, and adaptive polling order rearranging of, ONUs.
5.3 Intra–Optical Network Unit (ONU) bandwidth scheduling
Dynamic Bandwidth Allocation algorithms and intra-ONU scheduling algorithms, These algorithms can be
implemented in Ethernet Passive Optical Networks in order to efficiently support the transmission of multimedia traffic
and improve the performance of the low priority traffic at the same time. The system model separates the transmission
Volume 2, Issue 5, May 2013
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 5, May 2013
ISSN 2319 - 4847
of high priority traffic from the transmission of lower priority traffic and introduces the implementation of intra-ONU
scheduling algorithms for lower traffic class transmission [6].
PON is called passive because other than at the central office there is no active element within the access network. A
PON enables an service provider to deliver a true triple play offering of voice, video and data. PON are getting more
widespread in rollout of Fiber to the Home (FTTH) infrastructure. Wavelength Division Multiplexing PON, or WDMPON, is a type of passive optical networking, being pioneered by several companies, that uses multiple optical
wavelengths to increase the upstream and/or downstream bandwidth available to end users. This technology looks
forward to a day when optical technology is cheaper and easier to deploy, and end users demand higher bandwidth.
WDM-PON can provide more bandwidth over longer distances by devoting rawer optical bandwidth to each user and
by increasing the link loss budget of each wavelength, making it less sensitive to the optical losses incurred at each
optical splitter. To overcome the problem of increasing number of users DBA , inter and intra scheduling are used.
7. References
[1.] Kramer, Glen, Writing in the field of-Communications Engineering-Ethernet Optical Network, McGraw-Hill,
[2.] G. Keiser, Writing in the field of Communications Engineering -Optical Communications Essentials Third
Edition, Tata McGraw Hill Publishing Company Limited, 2003.
[3.] B. Chen, J. Chen, S. He, “Efficient and Fine Scheduling Algorithm for Bandwidth Allocation in Ethernet Optical
Network,” IEEE Journal of Quantum Electronics, vol. 12, no. 4, pp. 183-187,2006.
[4.] Chang-Hee Lee, Wayne V. Sorin, Byoung Yoon Kim, “Fiber to the Home Using a OPTICAL NETWORK
Infrastructure,” Journel Of Lightwave Technology”, Vol. 24, no. 12, December 2006.
[5.] Cedric Lam, Writing in the field of- Optical Network, Elsevier, 2007.
[6.] S. Jiang, J. Xie, “A Frame Division Method for Prioritized DBA in EOPTICAL NETWORK”, IEEE Journal on
Selected Areas in Communications, vol. 24, no. 4,pp. 83-94,2007.
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