FIBER OPTIC MULTIPLEXING METHODS
Methods that are currently being applied in fiber optic networking include Time Division
Multiplexing [57-59], here the data streams are interleaved into time slots, Wavelength
Division Multiplexing (WDM) in which separate streams are assigned to different optical
wavelengths [60-62] and SubCarrier Multiplexing (SCM), also called Frequency Division
Multiplexing (FDM) in which the different data streams are modulated into slightly
different radio frequencies and all are impressed onto the same optical carrier. Dense
Wavelength Multiplexing (DWDM) is a variant of WDM using several closely spaced
laser wavelengths, named this way to distinguish it from earlier forms of WDM that used
as few as two very different wavelengths. Each of these multiplexing methods has
advantages and disadvantages and different technical challenges.
1. Time Division Multiplexing (TDM)
Time Division Multiplexing is a technique where a short time sample of each channel is
inserted into the multiplexed data stream. The sample period has to be fast enough to
sample each channel according to the Nyquist theory (i.e 2 times highest frequency), and
to be able to sample all the other channels within that same time period. It can be thought
of as a very fast mechanical switch, selecting each channel for a very short time, then
going on to the next channel. Each channel has a time slice assigned to it (whether the
terminal is being used or not). TDM is more efficient, easier to operate, less complex and
less expensive than FDM.
To expand a TDM, system’s capacity by a small amount can require either the overlay of
an entirely new network or the upgrade of electronics throughout the entire network to
the speed of the aggregate data rate of all the channels. TDM multiplexers come in
discrete sizes and thus may require additional sub-rate multiplexers to add low-speed
channels to a higher-speed multiplexer.
2. Frequency Division Multiplexing (FDM)
Frequency Division Multiplexing (FDM) is an analog technique where each
communication channel is assigned with a carrier frequency. To separate the channels, a
guard-band would be used. This is to ensure that the channels do not interfere with each
other. FDM does not require all channels to terminate at a single location. Channels can
be extracted using a multi-drop technique; the terminals can be stationed at different
locations within a building or a city. FDM is an analog and a slightly historical
multiplexing technique. It is prone to noise problems, and has been overtaken by Time
Division Multiplexing (better suited for digital data).
FDM or SCM (Subcarrier multiplexing) requires the optoelectronic detection,
demultiplexing, regeneration, remultiplexing and retransmission of all traffic including
express traffic at any add or drop point. Since it departs from a simple on-off keyed
digital modulation format to use more sophisticated multilevel analog signals, the noise
and linearity of the optoelectronic components become more critical. Modulation still
needs to occur at high center frequency to accommodate the aggregate data rate.
3. Wavelength Division Multiplexing (WDM)
Wavelength Division Multiplexing is the technique that allows several different signals to
be carried along a single fiber at the same time. It achieves this by using different
wavelengths for each transmission and can be employed on single mode or multimode
fibers using lasers. Usually WDM uses multiple coherent lasers, modulators, detectors
and filters, all tuned to different wavelengths, resulting in network planning, provisioning
and operations nightmare in the rapidly changing metro environment. When
amplification is used, it needs to be sophisticated and adaptive to accommodate varying
numbers and power levels of signals at different wavelengths.
WDM is the latest multiplexing technology adopted worldwide. Although WDM much
better than TDM and FDM, WDM possesses many disadvantages as well. The
comparison between WDM/DWDM and OCDM are shown in Table 2.1.
Code Division Multiple Access (CDMA) is the latest development in optical
multiplexing/multiple accessing technique. It was initially proposed in the optical domain
due to its success in wireless communication. This thesis focuses on the development and
application of a family of OCDMA codes.
Table 2.1: Comparison between WDM/DWDM and OCDM.
No. OCDM
1.
Multiple carriers from a single
broadband source.
2.
Total broadcasting system which
reduces the equipments
significantly
3.
No need for sophisticated
equipment at each point at which
data enters or exits the fiber
4.
No accidental reception of
unwanted channel
WDM/DWDM
Single carriers from a single
source
Point-to-point link
5
Highly tolerant to noises
Sensitive to noise
6
Less sensitive to power and
wavelength fluctuations
Highly sensitive to source’s power
and wavelength fluctuations
Need of sophisticated equipment
at each drop point
Higher probabilities of accidental
reception of unwanted channel
4. Optical CDMA Technology
A common feature to each of these multiplexing techniques (TDM, FDM and WDM) is
the physical topology they impose on network design. They all require sophisticated
multiplexing and demultiplexing equipment at every point at which revenue traffic enters
or exits the fiber. This results in a network topology consisting of point-to-point links
wherein changes or additions to the network require considerable advance planning. In
order to add or change a customer location, the service provider must deploy backhaul
fibers to the nearest Central Office (CO), Point of Presence (POP) or establish a new POP
at the customer premise to house the multiplexing hardware. The lead-time associated
with acquiring and equipping CO and POP is often lengthy and expensive.
OCDM differs from traditional multiplexing approaches in that revenue connections to
the network are made through passive power taps. Multiplexing and demultiplexing take
place by simply adding and subtracting power from the fiber "bus," and performing the
required filtering (tuning) operations in a transceiver located within the CO, POP, or
customer premises. In fact, fiber transmission itself serves as the multiplexer. Attachment
of a new POP or user to the network does not require breaking the ring and routing fibers
through the new node, but is as simple as running a spur of fiber from the nearest splice
case to the new user as shown in Figure 2.1
RECEIVER
DECODER
TRANSMITTER
Balanced Receiver
(RX)
ENCODER
EXTERNAL
MODULATOR
(A) OCDM Coupler: All Channels “Express” Through
RECEIVER
MUX/DEMUX
TRANSMITTER
(B) All Other Technologies: Must Break Fiber, Demultiplex
Data
Figure 2.1: Adding a Node.
The basic spectral OCDMA architecture naturally possesses a number of desirable
advantageous features[8]:
a.
b.
c.
d.
e.
f.
g.
The all-optical multiplexing results in a protocol agnostic system in which
channels can be carried at any combination of data rates and formats in an
independent, unsynchronized fashion.
Since there is no need for TDM or temporal encoding, each channel
operates at its native data rate. Since there is no need for repetitive opticalto-electrical-to-optical conversion at each node, there is no accumulation
of temporal jitter, and electronic regeneration is unnecessary.
The use of incoherent sources and the spreading of each channel over
multiple wavelengths affords spectral OCDMA an inherent tolerance to a
variety of imperfections in optical components and the transmission
medium such as center wavelength shifts, slow drop offs at the edges of
filters, polarization dependent loss and fiber nonlinearities.
Sophisticated encryption is not required because OCDMA is already
encoded and does not suffer from the same type of adjacent-channel
crosstalk as DWDM: that is, center wavelength shifts in filters do not
result in the accidental reception of someone else's signal. Similarly,
OCDMA cannot result in the accidental reception of an unwanted channel
as could occur with errors in synchronization in TDM.
The flexibility afforded by the tap-and-insert nature (as in Figure 2.2) of
the optical bus combined with the programmability of the transceivers
enables the assignment of bandwidth and logical connections where they
are needed.
Multiple logical topologies can be supported simultaneously on the same
physical network. For example, a physical ring could be implemented for
optical layer protection on top of which virtual rings, meshes, stars, and
trees can also exist.
The broadcast nature of the system also lends itself to video distribution in
a point-to-multipoint configuration.
RING TOPOLOGY
POP
Central
Office
NEW FIBER
NEW
POP
OLD FIBER
Additional of new POP
requires new fiber route
construction to preserve
ring protection
NEW FIBER
POP
POP
BUS TOPOLOGY
POP
Central
Office
NEW FIBER
OLD FIBER
POP
NEW
POP
Additional of new POP
only requires spur fiber
connection to existing
protected bus
POP
Figure 2.2: Tap-and-Inserts.
2.4 Optical Code Division Multiple Access (OCDMA) Systems
Since mid 1980’s OCDMA has been a topic of academic interest, More than 250 papers
have been written in this area since 1985 [76]. Many overviews have been done in this
OCDMA networks by J. A. Salehi et, al 1989 [3], N. Karafolas et, al [4] and H. Fatallah
et, al [7]. The first OCDMA proposal was introduced by Prucnal [88] in 1985. The first
proposal appeared almost immediately following or concurrently with those in wireless
communications [7]. The motivation was promised to accommodate a high number of
lower bit rate clients to communicate simultaneously through the fiber. Nevertheless,
CDMA, in wireless and in optics has observed completely different evolution speed and
eventually different target.
The root of CDMA is found in Spread Spectrum communication techniques. Spread
spectrum was developed in mid 1950s mainly as a novel form of transmission [4]. It is
based on the idea of spreading the spectrum of the narrowband message over a wider
frequency spectrum by means of a digital code. Due to the spreading action the
transmitted signal arrives at the receiver as a noise-like signal and message recovery is
impossible unless the original code used is known by the intended receiver [7]. Spread
spectrum techniques are widely used in digital transmission system. Depending on the
coding technique, two categories of spread spectrum system exist: direct sequence spread
spectrum and frequency hopping spread spectrum [7].
The Optical CDMA communication system can be all-optical or partly optical. The
information bits may be originally optical or electrical. There are two basic categories of
optical CDMA systems, coherent [63] and incoherent [64-65] OCDMA. The all-optical
CDMA system is usually an incoherent system. Within these two broad categories there
are many different implementations, each with its advantages and disadvantages. The
following discusses their issue.
2.4.1 Optical Code Division Multiple Access Equipments
OCDMA equipment is a subsystem that consists of light source, splitter, encoder,
decoder, external modulator and multiplexer at the transmit side and splitter, decoder and
receiver at the receive side. The configuration is shown in Figure 2.3.
DECODER 1
DATA 1
ENCODER 1
OPTICAL SOURCE
M
U
L
T
EXTERNAL I
MODULATOR P
L
E
X
E
R
SPLITTER
ENCODER 2
DATA 2
RECEIVER 1
FILTER
PIN
FILTER
PIN
SPLITTER
S
P
L
I
T
T
E
R
SUBTRACTOR
RECEIVER 2
DECODER 2
FILTER
PIN
FILTER
PIN
SPLITTER
SUBTRACTOR
Figure 2.3: Optical CDMA Equipments.
Encoder/Decoder is a pair of devices or sub-systems required in an OCDMA
transmission system; encoder at the transmitter, decoder at the receiver. The function of
the encoder is to amplitude-spectrally encode the source according to the specific code it
uses. One unique encoded spectrum represents one channel. While a decoder consists of a
set of filters and/or reflectors arranged in unique configurations with other components,
the decoder has two sets of filters. Encoders and decoders can be implemented using any
types of optical filtering technology, including thin-film filters, fiber Bragg gratings or
free-space diffraction gratings. The same types of components used in WDM systems are
useful, with suitable adjustment of the specifications to optimize their performance in an
OCDMA system.