Easing the last mile?
synergy of PON and XDSL seems to be a panacea for
the last-mile broadband access problem. Here, is
a description of PON techniques for audio, video
and data transmission
recent developments in fiber optic communication
have enhanced the transmission bitrate from megabit/sec
range to terabit/sec range in the core network.
This is equivalent to six-order (10 to the power
of 6) increase in transmission capacity over a single
fiber, during the last decade. However, there is
this last-mile problem of extending the seamless
broadband connectivity to a remote subscriber.
Traditionally, copper pairs have been used to provide
connectivity between the local switching exchange
premises. Every subscriber is connected via a dedicated
pair to the serving exchange in a star configuration.
This copper-pair was sufficient to provide 3.4 KHz
speech telephony over several kilometers.
Recently, the transmission capacity of copper-pair
has been enhanced by several orders due to the deployment
of XDSL technologies. However, XDSL-based services
are limited by bandwidth
or physical constraints such as distance from the
serving switching exchange. Further, the fault liability
of copper-based access network is still too high.
A logical solution is to extend a dedicated fiber
from the serving exchange to a subscriber. However,
the cost of extending a dedicated fiber to a remote
subscriber is prohibitively very high.
A promising alternative is to use a mix of fiber
optic and DSL technologies in the access network.
Here, fiber optic connectivity is extended from
exchange to remote area using shared fiber-optic
network. Thus, a three-tier network is evolved to
extend the seamless broadband connectivity to a
Using the three-tier approach, the access network
is divided into feeder network, distribution network
and the copper-based XDSL network. The feeder network
uses SDH technologies in ring topology to extend
the broadband capability to tens of kilometers from
the serving exchange.
A distribution network extends the fiber reach from
one to two kilometers from the subscriber premises.
PON (Passive optical network) is a promising technology
in a distribution network. Here, like cable television
network, the fiber network is shared among
hundreds of subscribers using bus/tree topology.
This significantly reduces the cost. Further, no
active electronics is used in PON technology.
Finally, copper-based XDSL technology
is used to extend the broadband connectivity within
one to two kilometers range. Thus, the combination
of PON and DSL technologies extend the broadband
connectivity to masses in a cost-effective manner.
Passive optical network (PON): (see Figure-2) It
is a shared tree network; the root (headend) is
located on the feeder networks. The transmission
path from the head-end to the farthest node is transparent.
This implies that the optical network does not contain
any active electronics or power source.
Each intermediate node uses optical splitter that
divides the optical signal into multiple paths.
The signal splitting process also attenuates the
signal level. This necessitates optical transmission
at higher power level at the headend.
PON can be used for multi-service delivery from
the headend to a curb or to the home directly. The
PON can be used both for broadcasting entertainment
programs and interactive communication. The two-way
interactive communication can be realized using
single fiber or two fibers.
When the fiber is extended to the curb then such
a system is known as 'Fiber-To-the Curb' (FTTC).
From curb, the copper-based XDSL technology extends
broadband connectivity to multiple subscribers.
The interface unit between PON and XDSL network
is known as 'Optical Network Unit' (ONU). It is
necessary to make power supply arrangements at the
curb. The interface unit located at the subscriber
end is known as 'Network Termination' (NT). The
NT links copper-based XDSL network and the home
Alternatively, fiber can be extended to the home
directly. Such a system is known as 'Fiber-To-The-Home'
(FTTH). However, the deployment cost of such a network
is high. The interface unit located at the subscriber
end is known as 'Optical Network Termination' (ONT).
This unit acts as interface between home network
Broadcast using single fiber PON: It is similar
to cable television distribution. From the headend,
using tree network, the optical signal is distributed
to remote nodes. At a remote node, optical network
unit converts optical signal into electrical signal
for distribution to customers. Using copper-based
network, it is transmitted to subscribers.
Power loss in PON: In addition to splitter loss,
there is signal attenuation due to fiber splicing
and fiber loss.
For a splitter with two outputs, the input power
is equally divided at the two output ports. In other
words, power on each output port is 3db less than
the input power. Further to the splitter loss, there
is excess loss in the range of 0.2 to 0.4 db at
splitter. Thus, typical power loss for a two-port
splitter is 3.4 db.
Optical amplifier: To compensate for splitter loss,
splicing loss and fiber attenuation, an optical
amplifier is used at the headed (root). An optical
amplifier accepts weak optical signal as input and
without electrical conversion, amplifies the signal
within optical domain. The typical transmitted power
is in the range of 25-30 db.
Receiver sensitivity: The sensitivity of distant
optical receiver is dependent on the transmission
bitrate. A typical value at 155 Mb/s transmission
is 40 db.
Typical power budget available for distribution
network planning is in the range of 65 to 70 db.
This may be enough for 16 branch-outs using two
port splitters. Thus, the distant 'Optical Network
Unit' (ONU) can be located within the 20km range
Bi-drectional transmission using dual fibers
for upstream and downstream transmission dedicated
tree networks exist. The downstream transmission
deploys 'Time Division Multiplexing' (TDM) and the
upstream transmission uses 'Time-Division Multiple
In downstream direction, the headend broadcasts
the time-multiplexed signal to all ONUs. Here, each
ONU in the distribution network is allotted a pre-defined
time-slot. Thus, the headend puts pertinent information
in assigned time-slot and the same is accessed by
OLT. However, this simple scheme lacks privacy because
every ONU has access to data pertaining to other
ONUs. To address this security problem, it is necessary
to use signal encryption techniques.
In upstream direction, to avoid collision of optical
signals from different ONUs, TDMA technique is used.
Here, each ONU restricts its transmission to predefined
time-slot only. Thus, it is easier to identify the
data of an individual ONU at the headend.
In general, the distance between various ONUs and
headend is different. Thus, it is necessary to ensure
strict bit synchronization, stable burst transmission
and wide dynamic range signal recovery, etc.
Bi-directional transmission using single fiber
such schemes, a single fiber is used both for upstream
and downstream transmission. Both 'time compression
multiplexing' (TCM) and 'wave division multiplexing'
(WDM) techniques are used extensively.
Time compression multiplexing: Here, a single fiber
is used alternately manner for downstream and upstream
transmission. The wavelength for each direction
of transmission is the same. This method is also
known as optical ping-pong transmission. It is an
optical burst-mode, time division multiplexing technique.
Here, information for each direction is first time
compressed and is stored in a buffer. The compression
by a factor of 2 enables the information of 'T'
sec duration to be represented by 0.5T sec. This
time-compressed information at the destination end
is decompressed to recover the original signal.
The time compression-multiplexing scheme can be
symmetrical or asymmetrical. In the symmetrical
scheme, the time-slots for upstream and downstream
direction have equal time duration. In asymmetrical
scheme, upstream and downstream time-slots are unequal.
In some cases the time-slots are programmable as
WDM: WDM is similar to frequency division multiplexing
of electrical domain. WDM transforms a fiber into
hundreds of virtual fibers/wavelengths. These virtual
fibers have non-interfering nature. All sorts of
data patterns/formats including analog and video
traffic can be transported simultaneously without
WDM combines the different optical carrier's output
on a single fiber. The light signals combine in
a manner similar to frequency division multiplexing.
There are two incoming fibers, fiber-1 and fiber-2.
These fibers carry coherent optical signals at wavelength
'l1' and 'l2' respectively. These signals appear
as input to a prism or diffraction grating.
These two optical signals are combined in prism
and the composite optical signal containing optical
signals at wavelength 'l1' and 'l2' respectively
travel on a shared fiber. Here, the prism is combining
(multiplexing) two optical wavelengths and acts
as an optical combiner.
At the receiving end, the combined optical signal
is applied to another prism. Since the prism has
different refractive indices for different wavelengths,
the two beams emerge. Thus, prism-2 has split/demultiplexed
the composite optical signal into two separate 3
and 4 fibers
Thus, here one fiber is used to carry two distinct
optical signals simultaneously and results in transmission-capacity
For two-way duplex communications another shared
fiber carries the composite signal in reverse direction.
In practice, the optical signals at wavelengths
' l1' and 'l2' can be two optical signals at 1300
nm and 1500 nm respectively.
In general, a typical WDM system multiplexes and
de-multiplexes four optical channels or less. Thus
widely spaced laser devices can be used. Further,
such a WDM system also has lower cost.
Duplex WDM (see Figure-4): Normally, with WDM technology,
two fibers are needed to carry composite optical
signals in trans and receive directions respectively.
In 'duplex WDM', the single fiber carries and receives
In the duplex WDM, directional couplers are deployed
at trans and receive ends. The directional coupler
is a three-port device like a hybrid transformer,
used in a copper-based access network. A brief description
of 'duplex WDM' is given below:
Let wavelengths 'l1' is trans direction wavelength
and 'l2' is receive direction wavelengths at
The directional coupler is a three-port device.
When trans signal at wavelengths 'l1' appears
at port-1 then at the output port-2 optical
signal l1 appears. This signal is blocked toward
When the incoming optical signal at wavelength
l2 appears at port-2 then the same is transmitted
to port-3. In the direction of port-1 the optical
signal l2 is blocked. Similar action occurs
at station 'B'.
Such 'duplex WDM' system is used in strategic
applications. The respective pairs of wavelengths
commonly used are 1300 nm / 1500 nm, 980 nm
/ 1550 nm and 1480 nm / 1550 nm.
In practice, the minimum separation between trans
and receive direction wavelengths should be 50 nanometers.
Standardization in distribution network
June 1995, a consortium of major telecommunication
operators collaborated to define specifications
for a multi-service communication system. Known
as the 'Full Service Access Network' (FSAN), it
defines a basic set of communication requirements
for a flexible broadband access network.
In the year 1998, the FSAN developed specifications
were adopted by the 'International Telecommunication
Union' (ITU) as standard G.983. This standard defines
the broadband optical access network using 'ATM-based
Passive Optical Network' (APON). It also defines
the use of WDM technology for two-way interactive
communication and video broadcast using single fiber.
APON can be used to extend fiber to curb or alternatively
to home. The basics of optical transmission between
ONUs / ONTs and OLT remains same.
APON A typical home APON system comprises
an 'optical line transmission unit' (OLT), an 'optical
network termination unit' (ONT), passive optical
network and a network management system. The OLT
is located at the headend (service node) and an
ONT resides at the subscriber premises. Some features
A single fiber carries 3 different wavelengths.
1550 nm wavelength is used to transport video
to ONTs. For downstream interactive communication
1490 nm wavelength is used to broadcast TDM
signals to multiple ONTs. For upstream communication,
1310 nm wavelength in conjunction with TDMA
protocol transmits multi-point-to-point signals.
A single APON can be equipped with up to 64
ONTs. The OLT can be located 20 Km away from
The APON can operate both in symmetric and asymmetric
mode. In symmetric mode, 155 Mbps transmission
is used for both upstream and downstream communication.
In asymmetric mode, the downstream transmission
rate is 622 Mbps and upstream communication
uses 155 Mbps transmission. Here, bandwidth
can be allotted to individual ONT with granularity
of down to 4K bbps.
The downstream transmission is point-to-multi-point
communication. Each ONT monitors the TDM broadcast
and extracts the predestined cell, by unique
address. This addressing field is called the
'Virtual Path Indicator'/'Virtual Channel Identifier'
The downstream TDM broadcast is encrypted using
churning function. This function scrambles i.e.
encrypts the data between the OLT and an individual
ONT using the specific key supplied by the corresponding
ONT. The ONT transmits the key during upstream
communication This churning function provides
low level of protection for data confidentiality.
The upstream communication from multiple ONTs
is controlled by the OLT. For cell transmission,
the OLT grants permission to an individual ONT.
This permission specifies the time-slot to be
used by each ONU.
While granting the permission, the OLT measures
the logical distance between every ONT and OLT.
This process is known as 'ranging'. It enables
an OLT to allot the time-slots to various ONTs
so as to ensure collision-free cell transmission.
Thus, transmission from any two ONTs never interferes
over passive optical network.
The necessary network management function is
achieved by using
industry standard 'Telecommunication Management
Network' defined by I.T.U.
Advantages of APON
APON deploys passive optical networks. This
requires less maintenance than copper plant.
Further, the fiber has longer longevity and
unlimited bandwidth potential.
PON between OLT and ONTs provides higher reliability.
APON is a point-to-multipoint technology. Compared
to point-to-point system, the point-to-multi-point
system is comparatively cheap.
APON uses TDMA technique for upstream communication.
Normally, the data traffic has statistical distribution.
Thus, this technique allows an individual user
to access more bandwidth compared to fixed bandwidth
of circuit-switched network.
ATM provides requisite quality-of-service (QOS)
capabilities for multi-service broadband delivery
to multiple ONTs. Further, there is provision
to program the QoS using software. This enables
differentiated services to ONTs.
The quantum bridge is a leading optical access
network vendor. The APON is used to deliver
voice and 10 Mbps Ethernet to business-park
users. Redundancy is also planned.
Here, a node on feeder network is known as 'service-point-of
At SPOP, OLTs corresponding to multiple PONs can
be located. The Ethernet traffic from Lotus can
be connected to a LAN switch and can be switched
to the desired OLT.
Terrawave's trial system offers 622 Mbps PON
to 2000 key subscribers. The PON deploys 32
splitters. Thus each subscriber gets 20 Mbps.
The system has provision for additional splitters
to meet the future requirements.
In the near future, Terrawave plans to offer higher
speeds including 1.2 Gbps and 2.4 Gbps. To extend
the reach of PON network beyond 20km, Terrawave
proposes to use repeaters.
Terrawave offers protection switching within 50msec.
The dynamic bandwidth allocation feature will offer
idle bandwidth to any customer on the fly.
Bell system is currently offering a trial APON
to 400 residential users. It offers video and
high-speed data services to users. The first
prototype system has used 2 PONs. First PON
is APON. It uses two levels of splitting. First
level deploys 1: 8 splitters and second level
deploys 1: 4 splitters. Second PON is used exclusively
for transmission of analog and digital video.
Analog video uses 50 - 500 MHz band while digital
video is transmitted on 550-570 MHz band.
Alcatel offers ATM-based PON over a single fiber
to residential customers. A single OLT can support
up to 72 ONTs. It supports both symmetrical
and asymmetrical access as per G. 983.
The ONT contains ' DSL Access Multiplexer ' (DSLAM).
It multiplexes/ demultiplexes voice, Ethernet and
video traffic in to ATM traffic.
Marconi is also a leading supplier of fiber-to
the-curb equipment. It has supplied more than
3 million lines.
It encrypts user data between OLT to a respective
ONT/ONU. The encryption is achieved by data scrambling.
It offers low level of protection for data confidentiality.
Grant: It is the permission granted by the
OLT to an ONT/ONU for the use of specified time-slot
for upstream cell transmission.
Optical Line Termination unit (OLT): It is
the network side interface to PON. It is located
at the headend.
Optical Network Unit (ONU): It is a remote
unit. It interfaces PON and copper network. It is
located at the curb.
Optical Network Termination unit (ONT): It
is the remote unit connected at the user side of
PON. It resides at building / home.
Ranging: It measures the logical distance
between the OLT and an ONU/ONT. This enables collisionfree
cell transmission between various ONTs / ONUs and
Distribution Network: It is passive optical
network between network service node i.e. OLT and
ONTs/ONUs located at building / home or curb.
ATM: The term stands for 'Asynchronous Transfer
Mode'. It allows the integration of bursty data
and continuous stream. It maps incoming asynchronous
traffic into 53 byte cells. In a cell, there are
5 overhead bytes and 48 payload bytes. ATM provides
cell sequence integrity. It offers excellent quality-of-service
for the end-to-end services. ATM is standardized
Narrowband services: nx64 Kbit/s transmission.
Here, n' may take integer values from 1 to 31.
Broadband services: The boundary between
narrowband and broadband transmission is 2 Mbit/s.
Thus, transmission of 2Mbit/s and beyond constitutes
XDSL: A generic term for copper-based digital
subscriber line. Here, 'X' denodes the flavour of
DSL technology. Some of often used DSL technologies
High-bitrate symmetrical Digital Subscriber
Asymmetrical Digital Subscriber Line (ADSL).
Symmetrical Digital subscriber Line(SDSL).
SDH : Synchronous Digital Hierarchy.
For more information about PON visit home page
of FSAN at WWW.fsan.org
Visit for reports on PON deployment at www.light
Visit Quantum Bridge site www.quantum bridge.org
for a white paper entitled 'affordable fiber-to-the
For more information about PON visit www.converge
K. Vanwasi is GM (R&D), ITI LTD. Naini, Allahabad.
He can be reached at Vanwasi_nni@itiltd.co.in.