Chapter-12 Basic Optical Network

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Transcript Chapter-12 Basic Optical Network

Chapter 12 Basic Optical Network
 12.1 Basic Networks
 12.1.1 Network Topologies
 12.1.2 Performance of Passive Linear Buses
 12.1.3 Performance of Star Architectures
 12.3 Broadcast-and-Select WDM Networks
 12.3.1 Broadcast-and-Select Single-Hop
Networks
 12.3.2 Broadcast-and-Select Multihop
Networks
 12.3.3 The ShuffleNet Multihop Network 8.3
Demodulation Schemes
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Chapter 12 Basic Optical Network
 12.4 Wavelength-Routed Networks
 12.4.1 Optical Cross-Connects
 12.4.2 Performance of Wavelength Conversion
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12.1 Basic Networks
 Figure 12-1 shows the three common topologies
used for fiber optic networks: the linear-bus, ring,
and star configurations.
 Access to an optical data bus is achieved by means
of a coupling element.
 An active coupler converts the optical signal on
the data bus (Fig. 12-1(a)) to its electric baseband
counterpart before any data processing is carried
out.
 A passive coupler employs no electronic elements.
It is used passively to tap off a portion of the optical
power from the bus.
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12.1 Basic Networks
Figure 12-1(a). Linear bus topology used for fiber
optic networks.
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12.1 Basic Networks
 In a ring topology, consecutive nodes are connected
by point-to-point links that are arranged to form a
single closed path.
 Information in the form of data packets is
transmitted from node to node around the ring.
 The interface at each node is an active device that
has the ability to recognize its own address in a
data packet in order to accept messages.
 The active node forwards those messages that are
not addressed to itself on to its next neighbor.
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12.1 Basic Networks
Figure 12-1(b). Ring topology for fiber optic networks.
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12.1 Basic Networks
 In a star architecture, all nodes are joined at a
single point called the central node or hub.
Using an active hub, one can control all routing of
messages in the network from the central node.
 In a star network with a passive central node, a
power splitter is used at the hub to divide the
incoming optical signals among all the outgoing
lines to the attached stations.
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12.1 Basic Networks
Figure 12-1(c). Star topology for fiber optic networks.
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12.1.2 Performance of Passive
Linear Buses
 Over an optical fiber of length x (in km), the ratio
Ao of received power P(x) to transmitted power P(0)
is given by
Ao = P(x)/P(0) = 10-ax/10
(12-1)
where a is the fiber attenuation in units of dB/km.
 The losses encountered in a passive coupler in a
linear bus are shown in Fig. 12-2.
 The coupler has four functioning ports: two for
connecting the coupler onto the fiber bus, one for
receiving tapped-off light, and one for inserting
optical signal onto the line after the tap-off to keep
the signal out of the local receiver.
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12.1.2 Performance of Passive
Linear Buses
Figure 12-2. Losses in a passive linear-bus coupler
consisting of cascaded directional couplers.
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12.1.2 Performance of Passive
Linear Buses
 If a fraction Fc of optical power is lost at each port
of the coupler, then the connecting loss Lc is
Lc = -10 log(1-Fc)
(12-2)
 For example, if we take this fraction to be 20%,
then Lc is about 1 dB; that is, the optical power gets
reduced by 1 dB at any coupling junction.
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12.1.2 Performance of Passive
Linear Buses
 Let CT represent the fraction of power that is removed
from the bus and delivered to the detector port. The
power extracted from the bus is called a tap loss and is
given by
Ltap = 10 log CT
(12-3)
 For a symmetric coupler, CT is also the fraction of
power that is coupled from the transmitting input port
to the bus.
 If Po is the optical power launched from a source flylead,
the power coupled to the bus is CTPo. The throughput
coupling loss Lthru is then given by
Lthru = -10 log(1 - CT)2
= -20 log(1 - CT)
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(12-4)
12.1.2 Performance of Passive
Linear Buses
 In addition to connection and tapping losses, there is an
intrinsic transmission loss Li associated with each bus
coupler.
 If the fraction of power lost in the coupler is Fi, then the
intrinsic transmission loss Li is
Li = -10 log(1 - Fi )
(12-5)
 Consider a simplex linear bus of N stations uniformly
separated by a distance L, as shown in Fig. 12-3.
 From Eq. (12-1) the fiber attenuation between any two
adjacent stations is
Lfiber = -10 logAo = aL
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(12-6)
12.1.2 Performance of Passive
Linear Buses
Figure 12-3. Topology of a simplex linear bus
consisting of N uniformly spaced stations.
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12.1.2 Performance of Passive
Linear Buses
 NEAREST-NEIGHBOR POWER BUDGET.
 The smallest distance in transmitted and received
power occurs for adjacent stations, such as between
stations 1 and 2 in Fig. 12-3.
 If Po is the optical power launched from a source at
station 1, then the power detected at station 2 is
P1,2 = AoCT2(1-Fc)4(1–Fi)2Po
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(12-7)
12.1.2 Performance of Passive
Linear Buses
 The optical power flow encounters the following
loss-inducing mechanisms:
 One fiber path with attenuation Ao.
 Tap points at both the transmitter and the receiver,
each with coupling efficiencies CT.
 Four connecting points, each of which passes a
fraction (1 - Fc) of the power entering them.
 Two couplers which pass only the fraction (1 – Fi) of
the incident power owing to intrinsic losses.
 Using Eqs. (12-2) ~ (12-4) and Eq. (12-6), the losses
between stations 1 and 2 can be expressed as
10 log(Po/P1,2) = aL+2Ltap+4Lc+2Li.
(12-8)
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12.1.2 Performance of Passive
Linear Buses
 LARGEST-DISTANCE POWER BUDGET.
 The largest distance occurs between stations 1
and N. The fractional power level coupled into
the cable from the bus coupler at station 1 is
F1 = (1-Fc)2 CT (1–Fi)
(12-9a)
 At station N the fraction of power from the buscoupler input port that emerges from the
detector port is
FN = (1-Fc)2 CT (1–Fi)
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(12-9b)
12.1.2 Performance of Passive
Linear Buses
 For each of the (N - 2) intermediate stations, the
fraction of power passing through each coupling
module (shown in Fig. 12-2) is
Fcoup = (1-Fc)2(1-CT)2(1–Fi),
(12-10)
since the power flow encounters two connector
losses, two tap losses, and one intrinsic loss.
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12.1.2 Performance of Passive
Linear Buses
 Combining the expressions from Eqs. (12-9a), (129b), and (12-10), and the transmission losses of the
N – 1 intervening fibers,
 we find that the power received at station N from
station 1 is
P1,N = AoN-1F1FcoupN-1FNPo
= AoN-1(1-Fc)2N(1-CT)2(N-2)CT2(1-Fi)NPo
(12-11)
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12.1.2 Performance of Passive
Linear Buses
 Using Eqs. (12-2) ~ (12-6), the power budget for this
link is
10.log(Po/P1,N) = (N-1)aL +2NLc +(N-2)Lthru +2Ltap +Nli
= N(aL +2Lc +Lthru+Li) –aL -2Lthru +2Ltap
(12-12)
 The losses of the linear bus increase linearly with the
number of stations N.
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12.1.2 Performance of Passive
Linear Buses
Example 12-1 :
 Compare the power budgets of three linear buses, having
5, 10, and 50 stations, respectively.
 Assume that CT = 10 %, so that Ltap = 10 dB and Lthru =
0.9 dB. Let Li = 0.5 dB and Lc = 1.0 dB.
 If the stations are relatively close together say 500 m, then
for an attenuation of 0.4 dB/km at 1300 nm the fiber loss
is 0.2 dB.
 Using Eq. (12-12), the power budgets for these three cases
can be calculated as shown in Table 12-1.
 The total loss values given in Table 12-1 are plotted in Fig.
12-4, which shows that the loss increases linearly with the
number of stations.
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12.1.2 Performance of Passive
Linear Buses
Table 12-1. Comparison of the power budgets of three
linear buses that have 5, 10, and 50 stations, repectively.
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12.1.2 Performance of Passive
Linear Buses
Figure 12-4. Total loss as a function of the
number of attached stations for linear-bus
and star architectures.
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12.1.2 Performance of Passive
Linear Buses
Example 12-2 :
 For the Example 12-1, suppose that for
implementing a 10-Mb/s bus we gave a choice of
an LED that emits -10 dBm or a LD capable of
emitting +3 dBm of optical power.
 APD receiver with sensitivity of -48 dBm is used
at the destination.
 In the LED case, the power loss allowed up to 5
stations on the bus.
 For the LD, we gave an additional 13 dB of margin,
so we can have a maximum of 8 stations connected
to the bus.
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12.1.2 Performance of Passive
Linear Buses
DYNAMIC RANGE:
 System dynamic range is the maximum optical
power range to which any detector must be able to
respond.
 The worst-case dynamic range (DR) is found from
the ratio of Eq. (12-7) to Eq. (12-11):
(12-13)
 This could be the difference in power levels
received at station N from station (N - 1) and from
station 1 (i.e., P1,2 = PN-1,N).
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12.1.2 Performance of Passive
Linear Buses
Example 12-3 :
 Consider the linear buses described in Example 12-1.
 For N = 5 stations, from Eq. (12-13) the dynamic
range is
DR = 3[0.2 + 2(l.0) + 0.9 + 0.5] dB = 10.8 dB.
 For N = 10 stations,
DR = 8[0.2 + 2(l.0) + 0.9 + 0.5] dB = 28.8 dB.
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12.1.3 Performance of Star
Architectures
 From Eq. (10-25), for a single input power Pin and N
output powers, the excess loss is given by
Excess Loss = Lexcess
= 10.log(Pin/SNi=1Pouti)
(12-14)
 The total loss of the device consists of its splitting loss
plus the excess loss in each path through the star.
 The splitting loss is given by
Splitting Loss = Lsplit
= -10.log(1/N) = 10.logN (12-15)
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12.1.3 Performance of Star
Architectures
 For power-balance equation, the following
parameters are used:
• PS is the fiber-coupled output power from a source
in dBm.
• PR is the minimum optical power in dBm required
at the receiver to achieve a specific BER.
• a is the fiber attenuation.
• All stations are located at the same distance L
from the star coupler.
• Lc is the connector loss in decibels.
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12.1.3 Performance of Star
Architectures
 The power-balance equation for a particular link
between two stations in a star network is
PS - PR = Lexcess + a(2L) + 2Lc + Lsplit
= Lexcess + a(2L) + 2Lc + 10.logN.
(12-16)
 In contrast to a passive linear bus, for a star network
the loss increases much slower as logN.
 Figure 12-4 compares the performance of the two
architectures.
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12.1.3 Performance of Star
Architectures
Figure 12-4. Total loss as a function of the number
of attached stations for linear-bus and star
architectures.
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12.1.3 Performance of Star
Architectures
Example 12-4 :
 Consider two star networks that have 10 and 50 stations,
respectively. Assume each station is located 500 m from the
star coupler and that the fiber attenuation is 0.4 dB/km.
 Assume that the excess loss is 0.75 dB for the 10-station
network and 1.25 dB for the 50-station network. Let the
connector loss be 1.0 dB.
 For N = 10, from Eq. (12-16) the power margin between
the transmitter and the receiver is
PS - PR = [0.75 + 0.4(1.0) + 2(1.0)10log10] dB
 For N = 50, the power margin is
PS - PR = [1.25 + 0.4(1.0) + 2(1.0)10log50] dB
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12.1.3 Performance of Star
Architectures
 Using the transmitter output and receiver sensitivity
values given in Example 12-2, we see that an LED
transmitter can easily accommodate the losses in this
50-station star network.
 In comparison, a laser transmitter could not even
meet the 10-station design in a passive linear bus.
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12.3 Broadcast-and-Select
WDM Networks
 Broadcast-and-select techniques employing passive
optical stars, buses, or wavelength routers are used
for LAN applications.
 Active optical components form the basis for
constructing wide-area wavelength-routing networks.
 Single-hop refers to broadcast-and-select networks
where information transmitted without O/E
conversions at any intermediate point.
 Intermediate E/O conversion can occur in a multihop broadcast-and-select network.
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12.3.1 Broadcast-and-Select
Single-Hop Networks
 Figure 12-14 shows N sets of transmitters and
receivers being attached to a star coupler or a passive
bus.
 Each transmitter sends its information at different
wavelength.
 All transmissions from various nodes are combined in
a passive star coupler or coupled onto a bus and the
result is sent out to all receivers.
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12.3.1 Broadcast-and-Select
Single-Hop Networks
Figure 12-14. Alternate physical architectures for
a WDM-based local network : (a) star, (b) bus.
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12.3.1 Broadcast-and-Select
Single-Hop Networks
 Figure 12-15 illustrates the concept of multicast or
broadcast for a star network.
 Workstations at nodes 4 and 2 communicate using l2,
whereas a user at node 1 broadcasts information to
workstations at nodes 3 and 5 using l1.
 The same concepts are applicable to bus structures,
although the losses encountered in the star and bus
architectures are different.
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12.3.1 Broadcast-and-Select
Single-Hop Networks
Figure 12-15. A single-hop broadcast-and-select network.
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12.3.1 Broadcast-and-Select
Single-Hop Networks
 The WDM setup in Fig. 12-15 is protocol transparent.
This means that different sets of communicating
nodes can use different information-exchange rules
(protocols) without affecting the other nodes in the
network.
 This is analogous to TDM telephone lines in which
voice, data, or facsimile services are sent in different
time slots without interfering with each other.
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12.3.2 Broadcast-and-Select
Multihop Networks
 Figure 12-16 shows an example of a four-node
broadcast-and-select multihop network where each
node transmits on one set of two fixed wavelengths
and receives on another set of two fixed wavelengths.
 Stations can send information directly only to those
nodes that have a receiver tuned to one of the two
transmit wavelengths.
 Information destined for other nodes will have to be
routed through intermediate stations.
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12.3.2 Broadcast-and-Select
Multihop Networks
Figure 12-16. Architecture and traffic flow of
a multihop broadcast-and-select network.
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12.3.2 Broadcast-and-Select
Multihop Networks
 As shown in Fig. 12-17, at each intermediate node,
the address header is decoded to examine the routing
information field.
 Using this routing information, the packet is
switched electronically to the specific optical
transmitter.
 The specific optical transmitter will appropriately
direct the packet to the next node in the logical path
toward its final destination.
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12.3.2 Broadcast-and-Select
Multihop Networks
Figure 12-17. Representation of the fields contained
in a data packet.
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12.3.2 Broadcast-and-Select
Multihop Networks
 The flow of traffic can be seen from Fig. 12-16.
 If node 1 wants to send a message to node 2, it first
transmits the message to node 3 using l1.
 Then node 3 forwards the message to node 2 using l6.
 With this scheme there are no destination conflicts
or packet collisions in the network, since each
wavelength channel is dedicated to a particular
source-destination link.
 For H hops between nodes, there is a network
throughput penalty of at least 1/H.
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12.3.3 ShuffleNet Multihop Network
 The extension of perfect shuffle to optical networks
consists of a cylindrical arrangement of k columns,
each having pk nodes, where p is the number of
transceiver pairs per node.
 The total number of nodes is then
N = kpk
with k = 1, 2, 3, ... and p = 1, 2, 3, .…
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(12-17)
12.3.3 ShuffleNet Multihop Network
 Given that each node requires p wavelengths to
transmit information, the total number of
wavelengths Nl needed in the network is
Nl = pN = kpk+1
(12-18)
 Figure 12-18 illustrates a (p, k) = (2, 2) ShuffleNet,
where the (k+1)-th column represents the completion
of a trip around the cylinder back to the 1st column,
as indicated by the return arrow.
 In this example, there are eight nodes and sixteen
wavelengths.
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12.3.3 ShuffleNet Multihop Network
Figure 12-18. Logical interconnection pattern and
wavelength assignment of a (p,k) = (2,2) ShuffleNet.
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12.3.3 ShuffleNet Multihop Network
 An important performance parameter for the
ShuffleNet is the average number of hops between
any two randomly chosen nodes.
 Since all nodes have p output wavelengths,
p nodes can be reached from any node in one hop,
p2 additional nodes can be reached in two hops,
and so on, until all the (pk-1) other nodes are visited.
 The maximum number of hops is
Hmax = 2k -1
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(12-19)
12.3.3 ShuffleNet Multihop Network
 In Fig. 12-18, consider the connections between
nodes 1 and 5 and between nodes 1 and 7.
 In the first case, the hop number is one.
 In the second case, three hops are needed with the
routes being either 1-6-4-7 or 1-5-2-7.
 The average number of hops of a ShuffleNet is
(12-20)
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12.3.3 ShuffleNet Multihop Network
 As a result of multi-hopping, only part of the capacity of
a particular link directly connecting two nodes is actually
utilized for carrying traffic between them.
 The rest of the link capacity is used to forward messages
from other nodes.
 Since the system has Np = kpk+1 links, the total network
capacity C is
C = kpk+1/H
(12-21)
and the per-user throughput S is
S = C/N = p/H
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(12-22)
12.3.3 ShuffleNet Multihop Network
 Different (p, k) combinations result in different
throughputs, so one can make some tradeoffs among
the variables to get a better network performance.
 For example, given that the number of nodes is fixed
at N, one can reduce the average number of hops by
increasing p (which decreases k) to boost the capacity
and the throughput.
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光纖通訊實驗室 黃振發教授 編撰
12.4 Wavelength-Routed Networks
 Two problems in Broadcast-and-Select networks :
1). More wavelengths are needed as the number of
nodes in the network grows.
2). Without optical booster amplifiers, a large
number of users spread over a wide area
cannot be interconnected.
 Wavelength-routed networks can overcome these
limitations through wavelength-reuse, wavelengthconversion, optical-switching.
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光纖通訊實驗室 黃振發教授 編撰
12.4 Wavelength-Routed Networks
 Wavelength-routed network consists of optical
wavelength routers interconnected by pairs of
point-to-point fiber links in a mesh configuration,
as illustrated in Fig. 12-19.
 In Fig. 12-19 the connection from node 1 to node 2
and from node 2 to node 3 can both be on l1,
whereas the connection between nodes 4 and 5
requires a different wavelength l2.
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光纖通訊實驗室 黃振發教授 編撰
12.4 Wavelength-Routed Networks
Figure 12-19. Wavelength reuse on a mesh network.
國立成功大學 電機工程學系
光纖通訊實驗室 黃振發教授 編撰
12.4.1 Optical Cross-Connects
 Consider the OXC architecture shown in Fig. 12-20
that uses space switching without wavelength
conversion.
 The space switches can be cascaded electronically
controlled optical directional-coupler elements or
semiconductor-optical-amplifier switching gates.
 Each of the input fibers carries M wavelengths, any
of which could be added or dropped at a node.
國立成功大學 電機工程學系
光纖通訊實驗室 黃振發教授 編撰
12.4.1 Optical Cross-Connects
Figure 12-20. Optical cross-connect architecture using
optical space switches and no wavelength converters.
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光纖通訊實驗室 黃振發教授 編撰
12.4.1 Optical Cross-Connects
 At the input, the arriving signal wavelengths is
amplified and passively divided into N streams by a
power splitter or AWG demultiplexer.
 Tunable filters then select individual wavelengths,
which are directed to an optical space-switching
matrix.
 The switch matrix directs the channels either to one
of the eight output lines if it is a through-traveling
signal,
 or to a particular receiver attached to the OXC at
output ports 9 through 12 if it has to be dropped to
a user at that node.
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光纖通訊實驗室 黃振發教授 編撰
12.4.1 Optical Cross-Connects
 Signals that are generated locally by a user get
connected electrically via the DXC to an optical
transmitter. The switch matrix directs them to the
appropriate output line.
 The M output lines, each carrying separate
wavelengths, are fed into a wavelength multiplexer
to form a single aggregate output stream.
 Contentions arise in the architecture shown in Fig.
12-20 when channels having the same wavelength
but traveling on different input fibers enter the OXC
and need to be switched simultaneously to the same
output fiber.
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光纖通訊實驗室 黃振發教授 編撰
12.4.1 Optical Cross-Connects
 The contentions could be resolved by assigning a
fixed wavelength to each optical path throughout
the network, or by dropping one of the channels and
retransmitting it at another wavelength.
 In the first case, wavelength reuse and network
scalability are reduced.
 In the second case, the add/drop flexibility of the
OXC is lost.
 These blocking characteristics can be eliminated by
using wavelength conversion at any output of the
OXC.
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光纖通訊實驗室 黃振發教授 編撰
12.4.1 Optical Cross-Connects
Example 12-5 :
 Consider the 4 x 4 OXC shown in Fig.12-21. The OXC
consists of three 2 x 2 switch elements.
 Suppose that l2 on input fiber 1 needs to be switched to
output fiber 2 and that l1 on input fiber 2 needs to be
switched to output fiber 1.
 This is achieved by having the 1st two switch elements set
in the bar-state and the 3rd elements set in the cross-state,
as indicated in Fig. 12-21.
 Obviously, without wavelength conversion there would be
wavelength contention at both mux output ports.
 By using wavelength converters ahead of the multiplexer,
the cross-connected wavelengths can be converted to
noncontending wavelengths.
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光纖通訊實驗室 黃振發教授 編撰
12.4.1 Optical Cross-Connects
Figure 12-21. Example of a simple 4x4 OXC architecture
using optical space switches and wavelength converters.
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光纖通訊實驗室 黃振發教授 編撰
12.4.2 Performance of Wavelength
Conversion
 Assume that there are H links (or hops) between
nodes A and B.
 Take the number of available wavelengths per fiber
link to be F, and let r be the probability that a
wavelength is used on any fiber link.
 Since rF is the expected number of busy wavelengths
on any link, r is a measure of the fiber utilization
along the path.
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光纖通訊實驗室 黃振發教授 編撰
12.4.2 Performance of Wavelength
Conversion
 In the case of wavelength conversion, a connection
request between nodes A and B is blocked if one of the
H intervening fibers is full.
 The probability Pb’ that the connection request from
A to B is blocked is the probability that there is a
fiber link in this path with all F wavelengths in use,
so that
Pb’ = 1 - (1 - rF)H
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光纖通訊實驗室 黃振發教授 編撰
(12-23)
12.4.2 Performance of Wavelength
Conversion
 Let q be the achievable utilization for a given blocking
probability in a network with wavelength conversion,
q = [1 - (1 - Pb’)1/H]1/F
= (Pb’/H)1/F
(12-24)
where the approximation holds for small values of
Pb’/H.
 Figure 12-22 shows the achievable utilization q for
Pb’ = 10-3 as a function of the number of wavelengths
for H = 5, 10, and 20 hops.
 The effect of path length is small, and q rapidly
approaches 1 as F becomes large.
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光纖通訊實驗室 黃振發教授 編撰
12.4.2 Performance of Wavelength
Conversion
Figure 12-22. Achievable wavelength utilization as a
function of the # of wavelengths for a 10-3 blocking
probability in a network using l-conversion.
國立成功大學 電機工程學系
光纖通訊實驗室 黃振發教授 編撰
12.4.2 Performance of Wavelength
Conversion
 The probability Pb that the connection request from A
to B is blocked is the probability that each wavelength is used on at least one of the H links, so that
Pb = [1 - (1 - r)H]F
(12-25)
 Letting p be the achievable utilization for a given
blocking probability in a network without wavelength
conversion, then
p = 1 - (1 - Pb1/F)1/H
= -(1/H).ln(1 - Pb1/F)
(12-26)
 The achievable utilization is inversely proportional to
the length of the path H between A and B.
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光纖通訊實驗室 黃振發教授 編撰
12.4.2 Performance of Wavelength
Conversion
 Figure 12-23 depicts the achievable utilization p for
Pb=10-3 as a function of the number of wavelengths
for H = 5, 10, and 20 hops.
Figure 12-23. Achievable wavelength utilization as
a function of the # of wavelengths for a 10-3 blocking
probability in a network not using l-conversion.
國立成功大學 電機工程學系
光纖通訊實驗室 黃振發教授 編撰
12.4.2 Performance of Wavelength
Conversion
 Define the gain G=q/p to be the increase in fiber or wavelength utilization for the same blocking probability.
 Setting Pb’=Pb in Eqs. (12-23) and (12-25), we have
(12-27)
 Figure 12-24 shows G as a function of H = 5, 10, and 20
links for a blocking probability of Pb=10-3. The figure
shows that as F increases, the gain increases, and peaks
at about H/2.
 The gain then slowly decreases, since large trunking
networks are more efficient than small ones.
國立成功大學 電機工程學系
光纖通訊實驗室 黃振發教授 編撰
12.4.2 Performance of Wavelength
Conversion
Figure 12-24. Increase in network utilization as a
function of the # of wavelengths for a 10-3 blocking
probability when l-conversion is used.
國立成功大學 電機工程學系
光纖通訊實驗室 黃振發教授 編撰