Modern Communication Systems
-Optical Fibre Networks-
En. Mohd Nazri Mahmud
MPhil (Cambridge, UK)
BEng (Essex, UK)
Room 2.14
Calculation of OSNR for a point-to-point link
A Design Example
Design a 4X25 span WDM link with an optical amplifier gain of 22 dB and NF
equal to 5 dB.
Calculate the final OSNR if the input power in 0 dB.
Calculate the signal power at the receiver.
Will the system work if the receiver sensitivity is a minimum of –25dB?
Will the system work if the input power is 10 dB?
Calculation of OSNR for a point-to-point link
A Design Example
Calculation of OSNR for a point-to-point link
A Design Exercise
1) Design a 4X25 span WDM link with an optical amplifier gain of 18 dB and NF
equal to 6 dB.
2) Calculate the final OSNR if the input power in 0 dB.
3) Calculate the signal power at the receiver.
4) Will the system work if the receiver sensitivity is a minimum of –25dB?
5) Will the system work if the input power is 7 dB and amplifier gain = 22 dB?
Calculation of OSNR for a point-to-point link
A Design Exercise 2
Find the number of spans in this link, given Pin = 0 dB; OSNRfinal = 20 dB;
total length = 300 km; NF = 5 dB; Fibre loss = 0.2 db/km.
Optical Network Topologies
Network Architecture
Standardised optical networks such as SONET and SDH are
usually configured as a ring architecture called self-healing ring.
There are 3 main features
Each feature has 2 alternatives.
1. Either 2 or 4 fibres to link the nodes together
2. Either unidirectional (clockwise only) or bidirectional ring
3. Either line switching or path switching for protection switch.
2 architectures have become popular for SONET and SDH networks.
1. Two-fibre, unidirectional, path-switched ring (UPSR) architecture
2. Two-fibre or four-fibre, bidirectional, line-switched ring (BLSR)
Two-fibre UPSR
Two-fibre UPSR
Four-fibre BLSR
Four-fibre BLSR
Four-fibre BLSR - reconfiguration
Commercially available SONET/SDH
• In fibre-optic communications,
wavelength-division multiplexing
(WDM) is a technology which multiplexes
multiple optical carrier signals on a single
optical fibre by using different wavelengths
(colours) of laser light to carry different
signals. This allows for a multiplication in
capacity, in addition to making it possible
to perform bidirectional communications
over one strand of fibre.
• A WDM system uses a multiplexer at the transmitter to
join the signals together, and a demultiplexer at the
receiver to split them apart. With the right type of fibre it
is possible to have a device that does both
simultaneously, and can function as an optical add-drop
• The concept was first published in 1970, and by 1978
WDM systems were being realized in the laboratory. The
first WDM systems only combined two signals. Modern
systems can handle up to 160 signals and can thus
expand a basic 10 Gbit/s fibre system to a theoretical
total capacity of over 1.6 Tbit/s over a single fibre pair.
• WDM systems are popular with
telecommunications companies because they
allow them to expand the capacity of the network
without laying more fibre.
• By using WDM and optical amplifiers, they can
accommodate several generations of technology
development in their optical infrastructure
without having to overhaul the backbone
• Capacity of a given link can be expanded by
simply upgrading the multiplexers and
demultiplexers at each end.
• Most WDM systems operate on single
mode fibre optical cables, which have a
core diameter of 9 µm.
• Early WDM systems were expensive and
complicated to run. However, recent
standardization and better understanding
of the dynamics of WDM systems have
made WDM much cheaper to deploy..
• WDM systems are divided into different
wavelength patterns: Conventional, dense and
coarse WDM.
• WDM, DWDM and CWDM are based on the
same concept of using multiple wavelengths of
light on a single fibre, but differ in the spacing of
the wavelengths, number of channels, and the
ability to amplify the multiplexed signals in the
optical space
• Conventional WDM systems provide up to 16 channels
in the 3rd transmission window (C-band) of silica fibres
around 1550 nm with a channel spacing of 100 GHz.
• DWDM uses the same transmission window but with
less channel spacing enabling up to 31 channels with 50
GHz spacing, 62 channels with 25 GHz spacing
sometimes called ultra dense WDM.
• CWDM uses increased channel spacing to allow less
sophisticated and thus cheaper transceiver designs. To
again provide 16 channels on a single fibre CWDM uses
the entire frequency band between second and third
transmission window (1310/1550 nm respectively)
WDM deployment
Dense WDM
FTTX refers to several different optical fiber
architectures including:
– Fiber to the node/neighborhood (FTTN)
– Fiber to the exchange (FTTEx)
– Fiber to the cabinet (FTTC) or Fiber to the
– Fiber to the building (FTTB) or Fiber to the
home (FTTH) or Fiber to the premises (FTTP)
ONT = Optical Network Termination
OLT = Optical Line Termination System
ONU = Optical Network Unit
NT = Networking Termination unit
ONT – interfaces the system to customers’ home
OLT - manages the ONTs, aggregates traffics from multiple services and
interface the system to the core transmission networks
ONU - multiplex and demultiplex signals to and from a fiber transmission line.
An ONU terminates an optical fiber line and converts the signal to a format suitable
for distribution to a customer's equipment.
NT – provides services to the end user
Fiber to the node (FTTN) is a broadband
architecture that provides high speed internet
and other services to the home by running fiber
to the node and some form of DSL to the enduser. This architecture is lower cost to deploy
than the competing fiber to the premises (FTTP)
technology because of the ability to use existing
copper plant but in turn does not bring the full
bandwidth capability of fiber to the home. Data
rates are limited by the type of DSL used.
An integrated platform capable of providing telephony, data and video services to
residential customers.
One key network element, the FTTN Remote Node (RN).
Broadband services are provided to this element from the central Office (CO) by
a Gigabit Ethernet Fibre. These are connected to the existing copper line in the
service access interface and transported to the customer using DSL technology
Fiber to the curb or fiber to the cabinet
(FTTC) is a telecommunications system
based on fiber-optic cables run to a
platform that serves several customers.
Each of these customers has a connection
to this platform via coaxial cable or twisted
• Two competing FTTP technologies are Active FTTP, also called
Active Ethernet, and passive optical network (PON) architectures.
• Active FTTP networks utilize powered (i.e. 'active') electronic
equipment in neighbourhoods, usually one equipment cabinet for
every 400-500 subscribers. The IEEE 802.3ah standard enables
service providers to deliver up to 100 Mbit/s full-duplex over one
single-mode optical fiber to the premises depending on the provider.
• A passive optical network (PON) is a point-to-multipoint, fiber to
the premises network architecture in which unpowered optical
splitters are used to enable a single optical fiber to serve multiple
premises, typically 32. A PON consists of an Optical Line
Termination (OLT) at the service provider's central office and a
number of Optical Network Units (ONUs) near end users.
FTTH (Active Ethernet)
• With Passive Optical Networks, all active components between the
central office exchange and the customer premises are eliminated,
and passive optical components are put into the network to guide
traffic based on splitting the power of optical wavelengths to
endpoints along the way. This replacement of active with passive
components provides a cost-savings to the service provider by
eliminating the need to power and service active components in the
transmission loop. The passive splitters or couplers are merely
devices working to pass or restrict light, and as such, have no power
or processing requirements thereby lowering overall maintenance
costs for the service provider.
• In PON architectures, the switching and routing is done at the
carrier's central office.. Optical signals, once received in the home,
are processed and routed to the appropriate component in the
home (voice, video or data).
• A PON consists of a central office node, called an optical
line terminal (OLT), one or more user nodes, called
optical network units (ONU) or optical network terminals
(ONT), and the fibers and splitters between them, called
the optical distribution network (ODN).
• The OLT provides the interface between the PON and
the backbone network. These typically include: standard time division multiplexed (TDM) interfaces such
as SONET/SDH or PDH at various rates - Internet
Protocol (IP) traffic over Gigabit or 100 Mbit/s Ethernet ATM UNI at 155-622 Mbit/s
• A PON is a converged network, in that all of these services are
converted and encapsulated in a single packet type for transmission
over the PON fiber.
• A PON is a shared network, in that the OLT sends a single stream of
downstream traffic that is seen by all ONTs. Each ONT only reads
the content of those packets that are addressed to it. Encryption is
used to prevent unauthorized snooping of downstream traffic. The
OLT also communicates with each ONT in order to allocate
upstream bandwidth to each node. When an ONT has traffic to
send, the OLT assigns a timeslot in which the ONT can send its
packets. Because bandwidth is not explicitly reserved for each ONT
but allocated dynamically, a PON allows statistical multiplexing and
over-subscription of both upstream and downstream bandwidth. This
gives PON yet another advantage over point-to-point networks, in
that not only the fiber but also the bandwidth can be shared across a
large group of users, without sacrificing security.
PON standards
ITU-T G.983
APON (ATM Passive Optical Network).
– This was the first Passive optical network standard. It was used primarily for
business applications, and was based on ATM.
BPON (Broadband PON) is a standard based on APON.
It adds support for WDM, dynamic and higher upstream bandwidth allocation, and
The older ITU-T G.983 standard is based on asynchronous transfer mode
(ATM), and has therefore been referred to as APON (ATM PON). Further
improvements to the original APON standard -- as well as the gradual falling
out of favor of ATM as a protocol -- led to the full, final version of ITU-T
G.983 being referred to more often as Broadband PON, or BPON. A typical
APON/BPON provides 622 Megabits per second (Mbit/s) of downstream
bandwidth and 155 Mbit/s of upstream traffic.
PON standards – ATM PON
PON standards – ATM PON
The network components supporting ATM PON consist of OLT, ONT, and a
passive optical splitter. One fiber is passively split up to 64 times between
multiple ONTs that share the capacity of one fiber. The use of the optical
splitter in the PON architecture allows users to share bandwidth.
The ATM–PON system uses a double-star architecture. The first star is at
the OLT, where the wide-area network interface to services is logically split
and switched to the ATM–PON interface. The second star occurs at th
e splitter where information is passively split and delivered to each ONT.
The OLT is typically located in the carrier's CO. The OLT is the interface
point between the access system and service points within the carrier's
network. When data content from the network reaches the OLT, it is actively
switched to the passive splitter using TDM in the downstream. The OLT
behaves like an ATM edge switch with ATM–PON interfaces on the
subscriber side and ATM–synchronous optical network (SONET) interfaces
on the network side.
PON standards
ITU-T G.984
• GPON (Gigabit PON) is an evolution of the BPON standard. It
supports higher rates, enhanced security, and choice of Layer 2
protocol (ATM, GEM, Ethernet). It also created a standard
management interface between the OLT and ONU/ONT, enabling
mixed-vendor networks.
• The ITU-T G.984 (GPON) standard represents a boost in both the
total bandwidth and bandwidth efficiency through the use of larger,
variable-length packets. A GPON network delivers up to 2,488 Mbits
per second (Mbit/s) of downstream bandwidth, and 1,244 Mbit/s of
upstream bandwidth. GPON Encapsulation Method (GEM) allows
very efficient packaging of user traffic, with frame segmentation to
allow for higher Quality of Service (QoS) for delay-sensitive traffic
such as voice and video communications
PON standards
PON standards
• IEEE 802.3ah
• EPON (Ethernet PON) is an IEEE/EFM standard for using Ethernet
for packet data.
• The IEEE 802.3 Ethernet PON (EPON or GEPON) standard was
completed in 2004 as part of the Ethernet First Mile project.
• EPON uses standard 802.3 Ethernet frames with a symmetrical 1
Gbps upstream and downstream rates. EPON is applicable for datacentric networks, as well as full-service voice, data and video
• Starting in early 2006, work began on a very high-speed 10
Gigabit/second EPON (XEPON or 10-GEPON ) standard
PON standards- GEPON