Optical Services
Optical networks can provide
circuit, packet, and cell services at the UNI. Each optical service
can be categorized as analog or digital. We consider four major
categories of optical services:
Analog Circuit Service
Analog circuit service provides a
dedicated band-limited optical channel between ONAs. Point-to-point,
multi-point, or broadcast connections may be supported. The network
guarantees delivery with minimal distortion of any signal conforming
to the channel bandwidth and input power specifications. Analog
circuits may be full-bandwidth, occupying an entire waveband, or
time-shared on a slotted basis.
To establish an analog circuit
between ONAs, a circuit is assigned a waveband which is
transparently routed through the network. This process is called
wavelength routing. If wavelength changers are used within the
network, the circuit may terminate on a different waveband than it
originated on. i
Digital Circuit Service
This service provides a dedicated
logical digital channel between the ONAs. The network guarantees
delivery with minimal errors of any digital signal conforming to the
channel modulation specifications (e.g., solitons). Digital circuits
may be full-rate or interleaved on a bit, byte, or slotted basis.
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Analog Packet/Cell Service
This provides a packet/ cell
service where the packet/cell body is a time slot which is
transparently routed through the network. The time slot may contain
any analog signal conforming to the channel bandwidth and input
power specifications. The packet/cell header is a digital address
which is sent out-of-band (e.g., in time or frequency) and travels
along with the body on the same path and in synchronization. The
header must be interpretable by network nodes and ONAs. There are a
multitude of methods for encoding the header. i
Digital Packet/Cell Service
Provides a digital packet/cell
service analogous to conventional electrical packet/cell services.
If ON user traffic is bursty it
might be beneficial to offer packet/cell services in the optical
layer (at the UNI) in order to more efficiently utilize optical
network resources. The ONAs could then utilize these optical
packet/cell services to provide electronic packet/cell services to
the ON users. Such an ONA would likely contain buffering (optical or
electronic) and might be responsible for such functions as traffic
shaping and flow control.
Current technology severely limits
the range of optical packet/cell services that can be offered.
Memory is perhaps one of the most difficult functions to implement
optically.
There seems to be little motivation
for digital packet/cell services at conventional electronic rates.
Today current technological
limitations allow only analog circuit services (Fig. 3).
Figure 3 - Possible future trends in
optical network technology
Note that that there is an
important distinction between the optical services (which are
provided by the optical network to the ONAs), and the transport
services (which are provided by the ONAs to the ON users). For
example, to be specific, ONAs may provide SONET/SDH transport
circuits (e.g., OC-48/STM-16) over optical analog circuits by
conventional modulation and demodulation of an optical carrier;
similarly, an analog transport service can be provided over a
digital optical circuit via sampling and reconstruction.
For the most part, transport
circuits are carried by optical circuits, and transport packet/cell
services are supported by optical packet/cell services. A much
researched and demonstrated exception to the latter occurs in LANs
and MANs where the distances are short enough to implement a fast
circuit-switched optical layer via a medium access control (MAC)
protocol. This enables transport packet/cell services over optical
broadcast circuits. In addition, it is theoretically possible, but
technically difficult, to support transport circuits over optical
packets/cells.
In the optical networking
community, analog channels are often called fully transparent, or
simply transparent, implying that any modulation format may be used
on the channel. Very often, analog channels are only transparent to
certain modulation types (e.g. amplitude modulation), while
nontransparent to others (e.g., phase modulation). Therefore, to
fully describe a channel its "level of transparency" must be
determined, which is simply a specification of the modulation
formats that can be transmitted over the channel with acceptable
quality.
There are two fundamental reasons
channels may be nontransparent to certain modulations:
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First, the fidelity
of the optical channel usually degrades with distance and with
the number of signals utilizing a fiber due to additive noise,
multi-access interference, and nonlinearities of the optical
amplifiers and the fiber itself; therefore, long links which are
highly utilized may not support certain modulation types.
Because of these difficulties, a significant amount of effort
has gone into investigating long-haul WDM links for on-off
keying (OOK) and the results have been very successful with 1
Tb/s links demonstrated over ~150 km and 100 Gb/s links
demonstrated over 9100 km. Other modulation techniques, the
mixing of different modulation techniques on different
wavelengths, and the system implications of coding are not fully
understood at this time.
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The second reason a
channel might be nontransparent to a modulation type is that
certain useful network elements may not support certain
modulation types. An important example of this is a "gain
saturating wavelength changer" which changes the wavelength of
an optical signal. These devices use the amplitude of the input
signal to amplitude-modulate light on a desired output
wavelength. Therefore, all phase information is lost and only
amplitude modulated signals are passed. In fact, only OOK
signals will pass with sufficient fidelity. Note that the OOK
signals can be of any bit rate, but this wavelength conversion
process becomes increasingly difficult at rates faster than ~10
Gb/s. When a channel will pass any bit rate of a certain
modulation type, it is called bit rate transparent.
Below we describe each service in
more detail with an emphasis on the implementation and technical
difficulties which make certain services impractical in certain
situations.
Before entering that discussion, we
first note that each optical service is offered on a waveband (at
the UNI) and that there are trade-offs between
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network capability,
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upgradability,
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ONA complexity.
In particular, ONA hardware can be
simplified by offering an optical service over a fixed, standardized
waveband. The network would then be required to use wavelength
converters to convert this waveband to another waveband in order to
use WDM within the network. An alternative solution is for ONAs to
use wavelength-agile transmitters and receivers, in which case the
network would not be required to perform wavelength conversion. The
latter is currently far more economical. i
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