Introduction
The Need for Speed
Telecommunications for local access has evolved slowly: telephones and televisions,
the primary terminal types in today’s access networks, have remained essentially
unchanged in function and bandwidth for half a century or more. The limited
capacity of these terminals, and the fixed nature of the services they support,
has allowed them to smoothly evolve into efficient carriers of their respective
services.
The last decade has shown that this situation
is no longer stable. Although services such as fax and Internet can be delivered
over telephone lines today, it is clear that this is patchwork adaptation.
Even high-speed cable modems will not permanently solve the problem of telecommunications
for access in future decades, since the rapid advances of personal computer
processing power has changed the likely trajectory of telecommunications evolution.
Recent experiences with the World-Wide Web suggest that as soon as subscribers
view images, they desire video clips. Hyperlinks and low cost memory suggest
that many of us will become servers, and video sources are likely to pop up
in practically anyone’s backyard. Thus, the demand for a variety of service
options and the supply of inexpensive computer processing speed and memory
seem likely to equilibrate at a point that is far away from the capabilities
of today’s networks: the networks will have to be upgraded far beyond today’s
capabilities. Today, the tendency is towards optical communications.
While
there are several contenders for the protocol and architecture standards (e.g.
ATM), what is clear is the need for faster physical layer technology, to 1
Gbps and beyond. One key difficulty is that the most commonly installed fiber
in local area networks does not support this bandwidth over distances of 500
meters due to modal dispersion, which limits the effective bandwidth distance
product.
WDM
offers an attractive solution to increasing LAN bandwidth without disturbing
the existing embedded fiber, which populates most buildings and campuses,
and continue to be the cable of choice for the near future. By multiplexing
several relatively coarsely spaced wavelengths over a single, installed multimode
network, the aggregate bandwidth can be increased by the multiplexing factor.
Multichannel
optical systems were relatively unknown in 1980, but much technological progress
has been achieved since then. The applications can include a multiplexed high-bandwidth
library resource system, simultaneous information sharing, supercomputer data
and processor interaction, a myriad of multimedia services, video applications,
and many undreamed-of services. As demands for more network bandwidth increase,
the need will become apparent for multiuser optical networks, with issues
such as functionality, compatibility, and cost determining which systems will
eventually be implemented.
Fiber
Bandwidth
The driving
force motivating the use of multichannel optical systems is the enormous bandwidth
available in optical fiber. The high-bandwidth characteristic of the optical
fiber implies that a single optical carrier can be baseband modulated at ~25,000
Gbps, occupying 25,000 GHz surrounding 1.55 nano-meter, before transmission
losses of the optical fiber would limit transmission. Obviously, this bit
rate is impossible for present-day optical devices to achieve, given that
heroic lasers, external modulators, switches or detectors have bandwidths
< 100 GHz. As such, a single high-speed channel takes advantage of an extremely
small portion of the available fiber bandwidth.
Popular
Multiplexing Methods
The 100-Gbps channel mentioned in the previous section probably will be a
combination of many lower-speed signals, since very few individual applications
today utilize this high bandwidth. These lower-speed channels are multiplexed
together in time to form a higher-speed channel. This time-division multiplexing
(TDM) can be accomplished in the electrical or optical domain, with each lower-speed
channel transmitting a bit (or allocation of bits known as a packet) in a
given time slot and the waiting its turn to transmit another bit (or packet)
after all the other channels have had their opportunity to transmit (
Figure 1 ). TDM is quite popular with today’s electrical networks,
and is fairly straightforward to implement in an optical network at < 100-Gbps
speeds. This scheme by itself cannot hope to utilize the available bandwidth
because it is limited by the speed of the time-multiplexing and -demultiplexing
components.
To exploit more of the fiber’s THz bandwidth we seek solutions that complement
of replace TDM. One obvious choice is WDM (wavelength division multiplexing),
in which several baseband-modulated channels are transmitted along a single
fiber but with each channel located at a different wavelength (
Figure 2 ). Each of N different wavelength lasers is operating
at the slower Gbps speeds, but the aggregate system is transmitting at N times
the individual laser speed, providing a significant capacity enhancement.
The WDM channels are separated in wavelength to avoid cross-talk when they
are (de)multiplexed by a non-ideal optical fiber. The wavelengths can be individually
routed through a network or individually recovered by wavelength-selective
components. WDM allows us to use much of the fiber bandwidth, although various
device, system, and network issues will limit the utilization of the full
fiber bandwidth. Note that each WDM channel may contain a set of even slower
time-multiplexed channels.
Another method conceptually related to WDM is subcarrier multiplexing (SCM).
Instead of directly modulating a ~terahertz optical carrier wave with ~100s
Mbps baseband data, the baseband data are impressed on a ~gigahertz subcarrier
wave that is subsequently impressed on the THz optical carrier.
Figure 3 illustrates
the situation in which each channel is located at a different subcarrier frequency,
thereby occupying a different portion of the spectrum surrounding the optical
carrier. SCM is similar to commercial radio, in which many stations are placed
at different RF (Radio Frequency) such that a radio receiver can tune its
filter to the appropriate subcarrier RF. The multiplexing and demultiplexing
of the SCM channels is accomplished electronically, not optically. The obvious
advantage of cost-conscious users is that several channels can share the same
expensive optical components; electrical components are typically less expensive
than optical ones. Just as with TDM, SCM is limited in maximum subcarrier
frequencies and data rates by the available bandwidth of the electrical and
optical components. Therefore, SCM must be used in conjunction with WDM if
we want to utilize any significant fraction of the fiber bandwidth, but it
can be used effectively for lower-speed, lower-cost multiuser systems.
Additional
method is the code-division multiplexing (CDM) (
Figure 4 ). Instead of each channel occupying a given wavelength,
frequency or time slot, each channel transmits its bits as a coded channel-specific
sequence of pulses. This coded transmission typically is accomplished by transmitting
a unique time-dependent series of short pulses. These short pulses are placed
within chip times within the larger bit time. All channels, each with a different
code, can be transmitted on the same fiber and asynchronously demultiplxed.
One effect of coding is that the frequency bandwidth of each channel is broadbanded,
or “spread”. If ultra-short (<100 fs) optical pulses can be successfully
generated and modulated, then a significant fraction of the fiber bandwidth
can be used. Unfortunately, it is difficult for the entire system to operate
at these speeds without incurring enormous cost and complexity.
Yet another optical multiplexing scheme is called space-division multiplexing
(SDM), in which the channel-routing path is determined by different spatial
position (i.e., a different output fiber). A simple example of this is shown
in Figure 5, in which
the optical output of a fiber is split into N different and parallel optical
beam paths. Each of the N output beams is passed through a light-modulating
switch and then coupled to a different output fiber. By controlling the transmissivity
of each optical modulator, a signal on the input fiber can be routed to any
fiber output port. By extending this scenario, N input fiber ports can be
fully interconnected with N output fiber ports by an array of N2
optical switches. The technology for implementing moderate-speed systems is
already commercially available. In contrast to all other methods, however,
each channel occupies its own spatial coordinate, and all other channels cannot
be transmitted simultaneously on the same fiber. In other words, we are not
more fully utilizing the high bandwidth of the fiber, but we are creating
a high-bandwidth space-switching matrix, with the result that a high overall
switching capacity can be realized.
Wavelength
Division Multiplexing
Until the late 1980s, optical fiber communications was mainly confined to
transmitting a single optical channel. Because fiber attenuation was involved,
this channel required periodic regeneration, which included detection, electronic
processing, and optical retransmission. Such regeneration causes a high-speed
optoelectronic bottleneck and can handle only a single wavelength. After the
new generation amplifiers were developed, it enabled us to accomplish high-speed
repeaterless single-channel transmission. We can think of single ~Gbps channel
as a single high-speed lane in a highway in which the cars are packets of
optical data and the highway is the optical fiber. However, the ~25 THz optical
fiber can accommodate much more bandwidth than the traffic from a single lane.
To increase the system capacity we can transmit several different independent
wavelengths simultaneously down a fiber to fully utilize this enormous fiber
bandwidth. Therefore, the intent was to develop a multiple-lane highway, with
each lane representing data traveling on a different wavelength. Thus, a WDM
system enables the fiber to carry more throughput. By using wavelength-selective
devices, independent signal routing also can be accomplished. The highway
principle is illustrated in Figure
6.
It is expected that WDM will be one of the methods of choice for future
ultra-high bandwidth multichannel systems. Of course, this could be changed
as the technology evolves.
Basic Operation
As explained before, WDM enables the utilization of a significant portion
of the available fiber bandwidth by allowing many independent signals to be
transmitted simultaneously on one fiber, with each signal located at a different
wavelength. Routing and detection of these signals can be accomplished independently,
with the wavelength determining the communication path by acting as the signature
address of the origin, destination or routing. Components are therefore required
that are wavelength selective, allowing for the transmission, recovery, or
routing of specific wavelengths.
In a simple WDM system (
Figure 7 ), each laser must emit light at a different wavelength, with all the lasers’
light multiplexed together onto a single optical fiber. After being transmitted
through a high-bandwidth optical fiber, the combined optical signals must
be demultiplexed at the receiving end by distributing the total optical power
to each output port and then requiring that each receiver selectively recover
only one wavelength by using a tunable optical filter. Each laser is modulated
at a given speed, and the total aggregate capacity being transmitted along
the high-bandwidth fiber is the sum total of the bit rates of the individual
lasers. An example of the system capacity enhancement is the situation in
which ten 2.5-Gbps signals can be transmitted on one fiber, producing a system
capacity of 25 Gbps. This wavelength-parallelism circumvents the problem of
typical optoelectronic devices, which do not have bandwidths exceeding a few
gigahertz unless they are exotic and expensive. The speed requirements for
the individual optoelectronic components are, therefore, relaxed, even though
a significant amount of total fiber bandwidth is still being utilized.
The concept of wavelength demultiplexing using an optical filter is illustrated
in
Figure 8. In the figure, four channels are input to an optical filter that has
a nonideal transmission filtering function. The filter transmission peak is
centered over the desired channel, in this case, l3, thereby transmitting that channel and blocking all other channels. Because
of the nonideal filter transmission function, some optical energy of the neighboring
channels leaks through the filter, causing interchannel, interwavelength cross-talk.
This cross-talk has the effect of reducing the selected signal’s contrast
ratio and can be minimized by increasing the spectral separation between channels.
Although there is no set definition, a nonstandardized convention exists for
defining optical WDM as encompassing a system for which the channel spacing
is approximately 10 nm.
Topologies and Architectures
Let us consider a simple point-to-point WDM system (
Figure 9(a) ) in which several channels are multiplexed at one node, the combined signals
are transmitted across some distance of fiber, and the channels are demultiplexed
at a destination node. This facilitates high-bandwidth fiber transmission.
Additionally, high-bandwidth routing can be facilitated through a multiuser
network (
Figure 9(b) ). The wavelength becomes the signature address for either path through
an optical network. Because nodes will want to communicate with each other,
either the transmitters or the receivers must be wavelength tunable to facilitate
the proper link set-up (in this example, the transmitters were chosen to be
tunable).
Two
common network topologies can use WDM, namely, the star and the ring networks
( Figure 10 ). Each node
in the star has a transmiter and a receiver, with the transmitter connected
to one of the central passive star’s inputs and the receiver connected to
one of the star’s outputs. WDM networks can also be of the ring variety. Rings
are popular because so many electrical networks use this topology and because
rings are easy to implement for any network geographical configuration. In
this example, each node in the unidirectional ring can transmit on a specific
signature wavelength, and each node can recover any other node’s wavelength
signal by means of a wavelength-tunable receiver.
In both the star and the ring scenarios,
each node has a signature wavelength, and any two nodes can communicate with
each other by transmitting on that wavelength. This implies that we require
N wavelengths to connect N nodes. The obvious advantage is that data transfer
occurs with an uninterrupted optical path between the origin and the destination,
known as a single-hop network. The optical data start at the originating node
and reach the destination node without stopping at any other intermediate
node. A disadvantage of a single-hop WDM network is that the network and all
its components must accommodate N wavelengths, which may be difficult (or
impossible) to achieve in a large network. Current fabrication technology
cannot provide and transmission capability cannot accommodate 1,000 distinct
wavelengths for a 1,000-user network.
An
alternative to requiring N wavelengths to accommodate N nodes is to have a
multihop network, in which two nodes can communicate with each other by sending
through a third node, with many such intermediate hops possible. A dual-bus
multihop eight-node WDM network is shown in Figure
11 for which each node can transmit on two wavelengths and receive
on two other wavelengths. The logical connectivity is also shown. As an example,
if node 1 wants to communicate with node 5, it transmits on wavelength l1 and only a single hop is required. However, if node
1 wants to communicate with node 2, it first must transmit to node 5, which
then transmits to node 2, incurring two hops. Any extra hops are deleterious
in that they:
1)
Increase the transmit
time between two communicating nodes, since a hop typically requires some
form of detection and retransmission
2)
Decrease the throughput,
since a relaying node can transmit its own data while it is in the process
of relaying another node’s data
However, a multihop networks do reduce the required number of wavelengths
and the wavelength tunability range of the components.
Wavelength
Shifting and Wavelength Reuse
In an ideal WDM network, each user would have its own unique signature wavelength.
Routing in such a network would be straightforward. This situation may be
possible in a small network, but it is unlikely in a large network whose number
of users is larger than the number of provided wavelengths. In fact, technologies
that can provide and cope with 20 distinct wavelengths are the state of the
art. There are some technological limitations in providing a large number
of wavelengths, for instance: due
to channel-broadening effects and non-ideal optical filtering, channels must
have minimum wavelength spacing. Wavelength range, accuracy, and stability
are extremely difficult to control.
Therefore, it is quite possible that a given network may have more users than
available wavelengths, which will necessitate the reuse of a given set of
wavelengths at different points in the network.
Passive Wavelength Routing
In case we have a limited number of available wavelengths, a network can use
passive routing of a signal through the network based only on its wavelength.
The routing is designed to reuse wavelengths in non-shared links. For example,
we can see in Figure
12 that user I can use wavelength l1 to establish a link
with user II, while simultaneously user
V can reuse the same wavelength, l1, to establish a connection with user III.
This functionality
is accomplished by the proper arrangement of the cross-connects that route
an input signal to a wavelength-determined output. A simple example of the
operation of a passive WDM cross-connect is shown in Figure
13. The cross-connect is composed of wavelength demultiplexers
for the input stage, wavelength multiplexers for the output stage, and fibers
interconnecting the two stages. In the example, although there are only two
wavelengths, there are four possible non-interfering routing paths based on
both wavelength and origin. In general, instead of N wavelengths and N possible
connection paths, now there are N wavelengths and N2 connections. The same wavelength could be reused by
any of the input ports to access a completely different output port and establish
an additional connection. This technique increases the capacity of a WDM network.
Active Wavelength Shifting
In contrast to passive routing, which is limited to a static network conditions,
active wavelength shifting is dynamically deals with changes of the network
condition. It does that by changing the routing depending on the available
links and wavelengths. This concept of a network requiring active wavelength
shifting is illustrated in Figure
14. In the figure there are two small LANs connected to a larger
WAN, and each LAN can transmit on only two available wavelengths (la and lb). Node I wishes to communicate with node II. When node I wishes to transmit, the only wavelength available is la. However,
when the signal reaches the right LAN, it is revealed that la is already being used by the right LAN. Therefore,
the only way for the signal to reach node II is
to be actively switched onto the available lb.
Another
scenario that would require active wavelength switching is where one set of
wavelengths are used exclusively by each LAN, whereas another set of wavelength
is used exclusively for communication between LANs. The wavelengths that are
used in a LAN can be reused by each LAN since it will not interfere with another
LAN. This situation is demonstrated in Figure
15.
Shifting one wavelength to another wavelength complexes the network functionality.
One method to perform the active wavelength switching is to employ optoelectronic
wavelength shifters. This method necessitates optoelectronic conversions and
will cause an eventual optoelectronic speed bottleneck. In order to overcome
this problem the final goal is to achieve all-optical active wavelength shifting
to retain a high speed data path. All-optical means that all the shifters
are purely optical, i.e. not using optoelectronic conversion of the optical
data. There are several methods for all-optical wavelength shifting. Each
method has its advantages and disadvantages, and it is not clear if any method
will eventually be implemented. There is room for more research in this area.
Switching
We know that networks
establish communication links based on either circuit or packet switching.
For high-speed optical transmission, packet switching holds the promise for
more efficient data transfer.
Network packet
switching can be accomplished in a straightforward manner by requiring a node
to optoelectronically detect and transmit each and every incoming optical
data packet. As for the routing, all the switching functions can occur in
the electrical domain prior to optical retransmission of the signal. Unfortunately,
this approach suffers from an optoelectronic speed bottleneck. Alternatively,
much research is focused toward maintaining an all-optical data path and performing
the switching functions all optically with only some electronic control of
the optical components. However, there are many difficulties with optical
switching, for instance:
1)
A redirection of an optical path is not easy since photons do not have
as strong interaction with their environment as electrons do.
2)
Switching has to be extremely fast due to the high speed of the incoming
signal.
3)
Switching nodes cannot easily tap a signal and acquire information about
the channel.
Contention Resolution
Consider
a situation in which two or more input ports request a communications path
with the same output port, known as output-port contention. Since we are dealing
with a high-speed system, a rapid contention resolution is required, in which
one signal is allowed to reach its destination while the other signal is delayed
or rerouted in some fashion. In our multiplexing scheme, the issue of contention
exists when signals from two different input ports would request routing to
the same output port and contain identical wavelengths.
Several
approaches exist for resolving contention. One of them is buffering:
The
packet is retained locally at the switching node and then it is switched to
the appropriate output port when that port is available. The local buffering
can be implemented either in electrical or optical form. Electronic buffering
is straightforward but requires undesirable optoelectronic conversions and
may require very large buffers. On the other hand, optical buffering is difficult
because many buffering schemes require updating a priority bit (it is difficult
to change a priority bit of an optical data stream), and optical memory is
not an advanced art, consisting mostly of using an optical delay line.
Synchronization
A high-speed network transmitting digital signals must have adequate time
synchronization to recover the data stream. Time synchronization is especially
required with packet switching, asynchronous packet arrival times, and long-distance
transmission.
In
a WDM network, it is also possible that wavelength synchronization will be
required in addition to time synchronization. In such a scenario, a wavelength
standard could be broadcast through the network. However, the hope is that
the network wavelength stability and accuracy will be robust and will not
require its own system overhead and complexity.
Data-Format Conversion
In a large network, it is quite possible that a combination of data formats
will be used. This may occur, for instance, if some links may more efficiently
use TDM signaling, whereas other links may more effectively use WDM. This
explains the need for data-format conversion at network gateways, as illustrated
in Figure 16.
Protocols
A standardized network protocol must be used to ensure that data packets are
all formatted with recognizable routing information so that the packet can
be switched through the network with full global compatibility. There are
two standards that show the most promise of full adoption for a global optical
network:
1)
SONET - Synchronous Optical Network.
2)
ATM - Asynchronous Transfer Mode.
These
two standards can be combined in one network as follows:
Data
and header information are bounced into small ATM packets. These packets arrive
at a switching node at random times and are grouped together into a large
SONET frame ( Figure 17 ),
which makes its way in predetermined synchronous time slots through the network.
The ATM packets are unloaded by the SONET frame when its direction is switched
through the network and it can be placed into a different SONET frame. We
can think of the ATM packets as people randomly boarding a time-scheduled
SONET train.
Experimental
Results
Experimental results on WDM point-to-point links can be divided into two
groups based on whether the transmission distance is ~ 100 km or exceeds
1000 km. Since the 1985 experiment in which ten 2-Gbps channels were transmitted
over 68.3 km, both the number of channels and the bit rate of individual
channels have increased considerably. By 1995, a capacity of 340 Gbps was
demonstrated by transmitting 17 channels, each operating at 20 Gbps, over
150 km. This record was broken within a year by three experiments that used
WDM to realize the total bit rate of 1 Tbps or more. By the end of 1996,
a bit rate of 2.64 Tbps was demonstrated in a 132-channel WDM experiment
using 0.27nm channel spacing. The following table lists several record-setting
WDM transmission experiments performed after 1995.
The second group of WDM experiments worked
on a transmission distance of more than 1000 km. A 1994 experiment realized
transmission of 40 Gbps over 1420 km by multiplexing sixteen 2.5 Gbps channels
while maintaining an amplifier spacing of about 100 km. It was followed
by many experiments that increased either the transmission distance or the
bit rate. In one test-bed experiment, a transmission distance of 6000 km
at 20 Gbps ( 8 channels at 2.5 Gbps ) has been realized with an amplifier
spacing of 75 km. On the high-bit-rate end, a 1996 experiment multiplexed
sixteen 10 Gbps channels to realize transmission at 160 Gbps, but the link
length was only 531 km. Using very sophisticated techniques, 160 Gbps transmission
over a transoceanic distance of 9100 km has been realized.
|
Channels
N
|
Bit rate
B (Gbps)
|
Capacity
NB (Gbps)
|
Distance
L (km)
|
NBL Product
[(Tbps)-km]
|
|
10
|
100
|
1000
|
40
|
40
|
|
16
|
10
|
160
|
531
|
85
|
|
32
|
10
|
320
|
640
|
205
|
|
32
|
5
|
160
|
9300
|
1488
|
|
50
|
20
|
1000
|
55
|
55
|
|
55
|
20
|
1100
|
150
|
165
|
|
132
|
20
|
2640
|
120
|
317
|
Table: Record-setting WDM transmission experiments
The development of WDM fiber links has led to the advent of the fourth generation
of lightwave systems, which make use of the WDM technology to increase the
bit rate and in-line optical amplifiers to increase the transmission distance.
Four-channel WDM links, each channel operating at 2.5 Gbps, became available
commmercially in 1995. By 1996, WDM systems with a capacity of 40 Gbps (
16 channels at 2.5 Gbps or 4 channels at 10 Gbps ) were commercialized.
Recently, the Colt Telecom Group company decided to lay out a new communication
network in Europe with a capacity of 1.6 Tbps ( 160 channels at 10 Gbps
each ). This network will spread across Europe from London on the west,
to Turkey on the east, crossing many major cities like Paris and Amsterdam.
The network will be built by the Nortel company and it will be working by
the end of 2000. Needless to say that this WDM network will be the fastest
network in the world.
Conclusion
Twenty years ago, who could have predicted the success of personal computers,
much less the growth of the Internet and the Web? The next 20 years are likely
to bring surprises, too. If we recognize this and install robust communications
plant, we will be prepared for them.
References
[1] "Fiber-Optic Communication
Systems - Second Edition"
Written
by Govind P. Agrawal
[2] "Photonic Networks - Advances
in Optical Communications"
Written
by Giancarlo Prati (Ed.)
[3]
"Optical Fiber Communication Systems"
Written
by Leonid Kazovsky, Sergio Benedetto & Alan Wilner
Permission was granted to use figures from this book
Thanks to JB & Oded
