Int. J. Communications, Network and System Sciences, 2009, 7, 636-640
doi:10.4236/ijcns.2009.27071 Published Online October 2009 (
Copyright © 2009 SciRes. IJCNS
Next Generation Optical Access Network Using
CWDM Technology
Wireless Networks & Communications Centre, School of Engineering & Design,
Brunel University, London, UK
Received July 20, 2009; revised August 29, 2009; accepted September 23, 2009
We are developing a novel technology for the next generation optical access network. The proposed archi-
tecture provides FTTX high bandwidth which enables to give out 10Gbit/s per end-user. Increasing the sub-
scribers in the future will cause massive congestion in the data transferred along the optical network. Our
solution is using the wavelength division multiplexing PON (CWDM-PON) technology to achieve high
bandwidth and enormous data transmission at the network access. Physical layer modifications are used in
our model to provide satisfactory solution for the bandwidth needs. Thus high data rates can be achieved
throughout the network using low cost technologies. Framework estimations are evaluated to prove the in-
tended model success and reliability. Our argument that: this modification will submit a wide bandwidth
suitable for the future Internet.
Keywords: Passive Optical Network (WDM-PON), Fibre-to-the-Home (FTTH), Optical Access Network,
Next Generation
1. Introduction
Optical access network has attractive much attention [1],
this is because of the low loss and enormous bandwidth
of optical fibre, the increasing demand for capacity, cov-
erage, and the benefits it offers in terms of low cost op-
tical system ,all of which make it an ideal candidate for
future access network.
The network and service providers are seeking to re-
duce their operational costs. The concept of using a pas-
sive optical network (PON) is an attractive option. In a
PON there are no active components between the central
office and customer’s premises, which can eliminate the
need to power and manage active components in the ca-
ble system of the access network, and usually the PON
has a tree topology in order to maximize their coverage
with minimum network splits, thus reducing optical
power loss [2].
Each PON terminates on an Optical Line Termination
(OLT) in the head-end, or hub facility. The OLT con-
nects through a Wave Division Multiplexing (WDM)
coupler with a single fibre strand to the optical distribu-
tion network (ODN), and broadcasts an optical signal at
1490 nm that reaches each subscriber connected to that
fibre through passive optical splitters. The OLT also re-
ceives signals at 1310 nm from each customer optical
network user (ONU). OLTs are housed in a shelf that
typically supports multiple OLTs, common control cards,
and interfaces to voice and data services equipment [3].
Basically, fibre can deliver the information such as data,
voice, and video from central office CO to the end of
subscribers Figure 1. According to Heron [4], both
FSAN-ITU and the IEEE have initiated projects to stan-
dardize a next generation of PON with 10Gbps band-
width. Numerous options are under consideration. In
anticipation of some of the potential options, FSAN-ITU
is proposing a wavelength blocking filter for Gigabit
PON (GPON ONT)s in order to allow for the potential
coexistence of GPON ONTs with other wavelengths on
the PON.
Dinan [5] argued that there are two alternatives for
WDM metro networks dense WDM (DWDM) and
CWDM. In high capacity environments, DWDM is used.
In DWDM, the channel separation can be as small as 0.8
or 0.4 nm, for up to 80 optical channels at line rates up to
10Gbps. DWDM technologies is very expensive, so its
application to access networks is difficult. Instead,
CWDM is emerging as a robust and economical solution.
The advantage of CWDM technology lies in its low-cost
optical components. CWDM offers solutions for 850,
Figure1. Optical access network architecture.
1,300, and 1,500 nm applications at 10 and 40Gbps on
up to 15 optical channels spaced 20 nm a part. Further-
more, the wavelength multiplexer with low channel
crosstalk can be implemented easily for CWDM. It has
been argued that the total system cost is 40% cheaper for
the CWDM-PON [6].
2. WDM-Based Optical Access
2.1. Requirements of Next-Generation FTTH
A set of performance objectives was established by the
members of Full Service Access Network (FSAN) for
next-generation PONs that increase bandwidth and cost-
effectively while safeguarding previous investments.
These performance requirements can be summarized as
follows [5]:
Increase bandwidth by 4x.
Respect similar optical distribution network pa-
Respect wavelength allocations of GPON.
Keep changes of the media access control (MAC)
layer to a minimum.
Enable coexistence with GPON.
In addition, the IEEE has set like targets, but its focus
is centred on the 10G time division multiplexing (TDM)
Ethernet (EPON) solution. Any coexistence issues with
EPON are addressed by using a different wavelength for
the 10G EPON.
Hybrid four-wavelength PON is an approach that
places four logical PONs on a single fibre using four
discrete wavelengths. At 10 G bps, this increases the
downstream bandwidth of GPON four fold. The existing
downstream waveband of 1480-1500nm could be easily
sub-divided into four bands permitting the cost effective
use of four inexpensive medium-density lasers.
In the case of GPON and FSAN, the use of hybrid
splitters is proposed. This would allow only one of the
four GPON signals to pass to any particular ONT. Op-
portunely, the overall loss of a hybrid splitter would stay
put similar or improved to that of a power splitter. A
special dual-use splitter is being proposed that could be
used firstly as a power splitter and later as a hybrid split-
ter, thus avoiding its replacement cost. In the upstream
route, ONTs would contribute to the existing 1310nm
wavelength and the upstream bandwidth would remain
unaffected, resulting in an 8:1 ratio between downstream
and upstream bandwidth.
2.2. Wavelength Options
Coarse (WDM) one of the next generation solution, in
addition, require of opportunity networks to increase
bandwidth with low cost available in WDM. Wavelength
spacing of extra than 20 nm is generally called coarse
WDM (CWDM). Optical interfaces, which have been
standardized for CWDM, can be found in ITU G.695, at
the same time as the spectral grid for CWDM is defined
in ITU G.694.2. If the inclusive wavelength range of
1271 nm to 1611 nm, as defined in ITU G.694.2, is used
with 20 nm spacing, then a total of 18 CWDM channels
are accessible, as can be seen in Figure 2. [7,8].
3. Next Generation WDM-PON Networks
Model Architecture
In our model, we assume a four channel C WDM (1490-
1550nm), 2.5 GB/s directly modulated light wave system
over a passive optical network. The source is DFB-LD
Copyright © 2009 SciRes. IJCNS
Figure 2. Metro CWDM wavelength grid as specified by
ITU-T G.694.2.
modules with (1270 nm ~ 1610 nm) wavelength, and the
bandwidth is 2.5Gbps.
The passive optical network utilizes one 2:2 splitter
and four 1:16 splitters. Atypical topology for a CWDM
metro network is shown in Figure 3. Metro network is
linked via Central Office (CO) by PON. On the other
hand, the CO consists of transmitter and receiver each of
them has four lasers. These lasers have different wave-
lengths: 1490nm (λ1), 1510nm (λ2), 1530nm (λ3), 1550
nm (λ4) respectively for the upstream transmission. The
receivers are consisted of four wavelengths: 1290(λ1) nm,
1310(λ2) nm, 1330(λ3) nm, 1350(λ4) nm respectively as
the downstream transmission.
In this architecture, a single-mode optical fibre (SMF)
connects the CO and the subscribers’ site. The suggested
distance for our estimations is 60 km. Four channels each
of 2.5Gbps are multiplexed using OLT to achieve the
suggested 10Gbit/s bandwidth. In addition long haul
reach and narrow channel spacing are to be verified us-
ing the new arrangements.
4. Model Evaluation
4.1. BER versus SNR
The bit error rate (BER) is defined as the probability that
a bit is inaccurately detected by the receiver, i.e., that a
transmitted (0) is detected as a (1), or a (1) is detected as
a (0). A theoretical bit error rate, as a function of signal
to noise ratio (S/N), is known as a result of formula
BER1erf ()
In our CWDM system, we suppose the episode optical
power on photodiode detector is Pr W, and the responsiv-
ity of the detector is l A/W, the signal current in photo-
diode is could be written:
(I )=P,R (2)
The noise originating in the detector is thermal noise
current and generated within the photo detectors load
resistor RL, the thermal noise current is given by:
(I )=R (3)
where k is the Boltzman constant (1.3805*10-23 J/K), T
is the temperature is 300K f signal bandwidth is
2.5Gbps, RL is load resistor in .
Therefore the total current noise is:
I=I +I (4)
The signal to noise ratio(S/N) is given by:
 
 
Figure 3. Model of next generation CWDM network architecture.
Copyright © 2009 SciRes. IJCNS
The signal to noise ratio of this model is about 41.4dB,
as shown in Figure 4, then BER for the proposed system
is about 5*10-8.
Noticeably, the bit error rate (BER) decreases, as the
signal to noise ratio (S/N) increases. Hence, data can be
transmitted with high superiority as the expectation of
error decreases.
4.2. Coverage of WDM-PON
For different splitter ratio the insert loss is different, with
the insert loss of splitter is given by [2]:
splitter 10
splitter_ratio (6)
The maximum coverage distance of the N remote node
is given by:
Tx Rx-Min)TFFTFF-otherTFF
{(P -P-[N*(2*L+L)+L]-+Lspliter-+Lothers}
D= Fattenuation
where l is the average transmit pow is th
. Conclusions
n innovative solution is presented to increase t
er, PRX-1
minimum receive optical power with error free in
ONU/ONT, or it can be describe as the receive sensitiv-
ity of ONU/ONT, LT is the insert loss of TFF, LTFF–ot is
some other attenuation connected to TFF, Loth is other
attenuations, such as the interface loss, Fattenuat in differ-
ent CWDM wavelengths have different attenuation.
As we show in the Figure 5. The relation between
nce and nodes with different number of splitter, with
increase in the splitter value the PON coverage it become
bandwidth for optical access networks. The projected
broadband access network is the key solution for
point-to-multipoint optical communications. High data
Figure 5. Illustrate the coverage distance of remote nod
tes are achieved using low price infrastructures. In this
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