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HIGH CAPACITY NETWORKS
Upgrading Transmission Capacity
There are three approaches towards increasing the
transmission capacity on a link.
1. The space division multiplexing ( SDM) approach
: Keep the bit rate the same but use more fibers.
2. The TDM approach: increases the transmission
bit rate on the fiber.
3. The WDM approach: keep the bit rate the same
but add more wavelengths, each operating at
the original bit rate over the same fiber.
The SDM approach
It has two main drawbacks
• It requires more fibers, which may not be
available along the desired route. The cost of
laying new fiber varies widely.
• If there is space in existing conduits, fiber can
be pulled through relatively inexpensively. If
new conduits must be laid, the cost can be
very expensive, even over short distances.
• If there are no fibers along the desired route
,the link has to be built from the scratch.
• The second drawback of SDM is that even if
the fibers are available, a separate set of
optical amplifiers or repeaters must be
installed for each fiber, which becomes a
significant expense over long distances.
• SDM approach can be used when fibers are
available and distances are relatively short so
that no repeaters or amplifiers are needed.
The TDM Approach
• TDM approach requires the bit rate on the
fiber to be increased to 10Gb/s or beyond.
• Two major system impairments that these
systems face are chromatic dispersion and
polarization mode dispersion (PMD).
• Chromatic dispersion is not a problem if the
link uses dispersion shifted fiber.
• With standard single mode fiber, the
chromatic dispersion limit is about 60km at
10Gb/s and about 1000 km at 2.5Gb/s.
• With practical transmitters, the distances are
• The 10Gb/s limit can be further increased in
the presence of self phase modulation.
• Beyond these distances, the signal must be
electronically regenerated, or some form of
chromatic dispersion compensation must be
• The distance limit due to PMD at 10Gb/s is 16
times less than that at 2.5Gb/s.
• The distance limit is about 25km at 10Gb/s.
• Electronic regeneration is required for longer
• The PMD induced distance limit may be even
lower because of additional PMD caused by
splices, connectors, and other components
along the transmission path.
• TDM is limited by the speeds that can be
achieved by the electronics.
• Higher bit rates can be achieved using optical
• TDM is a competitive approach for short to
medium distance links or for links operating
over dispersion shifted fiber.
The WDM Approach
The WDM approach is to keep the bit rate same,
say, at 2.5Gb/s, and add more wavelengths,
each carrying data at this bit rate. The
advantages of WDM over TDM are the
1. Because of the lower bit rates, the distance
limit due to chromatic dispersion is much
larger for WDM systems than for equivalent
• The transmission capacity can be increased in a
modular manner by adding additional wavelengths as
and when capacity increases are required, as opposed
to a large up front installation in the case of TDM.
• WDM systems can be designed to be transparent
systems. This allows different wavelengths to carry
data at different bit rates and protocol formats.
• WDM may be preferred to TDM in designing more
complicated networks. For example, if there is a
network node at which most of the traffic is to be
passed through and a small fraction is to be dropped
and added, it may be more cost effective to use a
WDM optical add/drop element than a full blown set
of TDM terminals.
Disadvantages of WDM relative to TDM
1. WDM systems are not suitable for deployment
over dispersion shifted fiber because of the
limitation imposed by four wave mixing.
2. WDM systems require specially designed optical
amplifiers that provide flat gain profiles.
3. WDM systems requires separate terminating
equipment for each channel, including an
expensive laser and a receiver. A TDM system on
the other hand, requires one piece of
4. Transparent WDM systems offer less
monitoring and network management
capability than TDM systems because they are
unaware of the actual format and data rate on
the individual channels. Thus they cannot
monitor parameters like bit rate or frame
errors in the data.
Unidirectional versus Bidirectional Systems
• A unidirectional WDM system uses two fibers,
one for each direction of traffic .
• A bidirectional system, on the other hand
requires only one fiber, and typically uses half
the wavelength for transmitting data in one
direction and the other half for transmitting
data in the opposite direction on the same
1. A unidirectional system is capable of handling
W full duplex channels over two fibers. A
bidirectional system handles w/2 full duplex
channels over one fiber. The bidirectional
system, has half the total capacity, but allows
a user to build capacity more gradually than
a unidirectional system.
2. If only one fiber is available, then there is no
alternative but to deploy bidirectional
3. Unlike unidirectional system, bidirectional
system do not require automatic switching
protocol between the two ends of the link,
since both ends detect a fiber cut
4. BLSR/4 four fiber SONET rings are better
supported using bidirectional systems than
5. Bidirectional systems provide better support
for deploying point to point SONET systems in
1+1 or 1:1 configurations.
6. Bidirectional system can be configured to handle
asymmetric traffic. For ex, a large number of
wavelengths could be used in one direction and a
smaller number in the other direction.
7. It is more difficult to design the transmission
system in bidirectional system since more
impairments must be taken into account, in
8. Although amplifiers for bidirectional system may
employ more complicated structures than
unidirectional systems, they need to handle only
half as many channels as unidirectional systems,
which means that they can produce higher
output power per channel and provide more gain
9. Bidirectional systems usually require a guard
band between the two sets of wavelengths
traveling in opposite directions to avoid cross
penalties, which may not be needed for
10. Bidirectional system can also be operated as
a unidirectional system by simply reversing
the direction of propagation of half the
wavelengths and the corresponding
• Interexchange carriers in U.S have links
spanning several hundred to a few thousand
• Among the major carriers, AT&T and sprint
have primarily installed standard single mode
fiber. Thus WDM is an attractive option for
them, and they are actively deploying WDM
systems on many of their routes.
• Another major carrier MCI has a number of
links using dispersion shifted fiber and is
looking to deploy high speed TDM systems on
these routes and WDM systems on other
routes that use standard single mode fiber.
• In Europe and Japan the links tend to be
shorter, several European carriers are
considering the deployment of WDM.
• Undersea links tend to be longer and may
span distances of several thousand km.
• Shorter distance repeater less undersea links
are also being deployed in many places.
• In long links, the case for WDM over standard
single mode fiber becomes quiet strong.
• Reliability is a very important consideration
for undersea links, which are usually designed
to operate over a 25 year period.
• Passive WDM multiplexing and
demultiplexing technology, combined with 2.5
Gb/s systems, give WDM an edge over higher
speed TDM systems.
• WDM also allows segregation of traffic
Local Exchange Networks
• These networks tend to operate over shorter
distances, typically a few tens of km between
• Local exchange carriers will need to provide
high capacity connections to their customers
such as large enterprises.
• These customers may want to transmit
different types of protocols at different bit
• The bit rates tend to be somewhat lower
compared to interexchange networks,
typically a few hundred to several hundred
• Thus it may be useful to have several lower
speed channels than a few high speed
channels in the network.
• Several carriers in the U.S are considering the
deployment of WDM on selected routes
where they need higher capacities.
• Large enterprises face an increasing need for a
number of high bandwidth connection
between their sites.
• Many enterprises maintain two data centers
that are separated by a few tens of km. All
transactions are mirrored at both sites.
• This allows the enterprise to recover quickly
from a disaster where one of the centers fails.
• There may be other reasons for this as well,
such as lower real estate costs at one location
than at the other.
• This allows peripheral equipment to be
placed at the cheaper site.
• The large mainframes at these data centers
need to be interconnected by several hundred
channels, each at up to a few hundred Mb/s.
• These data centers tend to be located in dense
metropolitan areas where most of the
installed fiber is already in use.
• These networks use a variety of protocols and
• These two factors make WDM an attractive
option for these types of networks.
• WDM systems are being installed in large
enterprise networks in the united states and
all over Europe as well.
• Since the links tend to be short, there is
usually no need to use optical amplifiers or
• This makes the WDM option highly
competitive provided multiple fibers are
available. Or when the fiber is being installed
• Optical signals representing data streams from
multiple sources are interleaved in time to
produce a single data stream. The interleaving
can be done on a bit by bit basis.
• It can also be done on a packet by packet
• In both bit interleaved and packet interleaved
case, framing pulse can be used.
• In the packet interleaved case, framing pulse mark the
boundary between packets.
• In the bit interleaved case, if n input data streams are
to be multiplexed, a framing pulse is used every n bits.
• OTDM networks can be based on a broadcast topology
or incorporate optical switching.
• In case of broadcast OTDM networks the topology used
is either star or a bus.
• Those photonic packet switching networks that
incorporate photonic switching and routing will be
referred to as switch based networks.
• In broadcast networks, there is no routing and
switching within the network.
• Switching occurs only at the periphery of the
network by means of tunable transmitter and
• Tunability in case of TDM networks refers to
the ability to select one of several time
• The switch based networks perform routing
and switching functions optically within the
network in order to provide packet switched
service at very high bit rates.
• The broadcast networks suffers from large
splitting losses, are not scalable and are
generally suitable only for LAN applications.
• The switch based networks on the other hand
are scalable and suited for WDM applications.
Multiplexing and Demultiplexing
• At the inputs to the network, lower stream data
streams are multiplexed optically into a higher
speed stream and at the outputs of the network
the lower speed streams must be extracted from
the higher speed stream optically by means of a
• Functionally optical TDM is identical to electronic
TDM. The only difference is that multiplexing and
demultiplexing operations are performed entirely
optically at high speeds.
• Very short pulses much shorter than the bit
interval of each of the multiplexed streams must
be used in OTDM systems.
• A periodic train of very short pulses with widths
of the order of a few ps can be generated using a
mode locked laser.
• Since the pulses are very short, their frequency
spectrum will be large.
• Therefore unless some special care is take, there
will be significant pulse broadening due to the
effects of chromatic dispersion. For this purpose
soliton pulses are used in OTDM systems.
• Solitons can propagate for long distances
without dispersion induced broadening.
• Almost any pulse shape gets transformed into
the soliton pulse shape after propagating
some distance through optical fiber.
• The only requirement is that the pulse should
have the right peak power.
• The required peak power is inversely
proportional to the pulse width and thus
directly proportional to the bit rate of
• shape maintenance properties of soliton pulse
are true only if the soliton pulses are
sufficiently isolated from each other.
• The periodic pulse train generated by a mode
locked laser is split, and one copy is created
for each data streams to be multiplexed.
• The pulse train for the ith data stream i =
1,2..n is delayed by iτ. This delay can be
achieved by passing the pulse train through
the appropriate length of optical fiber.
• Since the velocity of light in silica fiber is about
2 * 108 m/s, one meter of fiber provides a
delay of about 5 ns. Thus the delayed pulse
streams are nonoverlapping in time.
• The undelayed pulse stream is used for the
• Each data stream is used to externally
modulate the appropriately delayed periodic
• The outputs of the external modulator and
the framing pulse streams are combined to
obtain the bit interleaved optical TDM stream.
• The power level of the framing pulses is
chosen to be distinctly higher than that of the
• The multiplexed input is split into two streams
using say a 3 db coupler. If the jth stream from
the multiplexed stream is to be extracted, one
of these streams is delayed by jτ.
• A thresholding operation is performed on the
delayed stream to extract the framing pulses.
• The reason the framing pulses were
multiplexed with higher power than the other
pulses was to facilitate this thresholding
• A logical AND operation between the framing
pulse stream and multiplexed pulse stream is
used to extract jth stream.
• The output of the logical AND gate is a pulse if
during a pulse interval, both inputs have
pulses, the output has no pulse otherwise.
• A periodic stream of narrow pulses is externally
modulated by the data stream. If the bit interval
is T, the seperation between the successive pulses
is also T.
• We must somehow devise a scheme to reduce
the interval between successive pulses to τ,
corresponding to the higher rate multiplexed
• This can be done by passing the output of the
external modulator through a series of
• If the size of each packet is l bits, the output goes
through k = [log2l] compression stages.
• In the first compression stage bits 1,3,5,7 are
delayed by T – τ.
• In the second compression stage, the pairs of bits
(1,2), (5,6), (9,10).. Are delayed by 2(T – τ).
• In the third compression stage, the bits (1,2,3,4),
(9,10,11,12) .. Are delayed by 4(T- τ).
• Each compression stage consists of a pair of 3 dB
couplers, two semiconductor optical
amplifiers(SOAs) used as on – off switches and a
• The demultiplexing operation is equivalent to
“decompressing” the packet.
• This can be achieved by passing the
compressed packet through a set of
decompression stages that are similar to the
• The on – off switches required in this
approach must have switching times of the
order of the pulse width τ.
• A more practical approach is to use a bank of
• It converts the single high speed data stream
into multiple lower speed data streams that
can be processed electronically.
• A bank of AND gates is used to break up the
incoming high speed stream into five parallel
streams each with five times the pulse spacing
of the multiplexed stream.
• One input to each AND gate is the incoming
data stream, and the other input is a control
pulse stream where the pulses are spaced five
• The control pulse streams to each AND gate
are appropriately offset from each other so
that they select different pulses.
Optical AND gates
• The logical AND operation between two
signals can be performed by an on – off switch
if one of the signals is input to the switch and
the other is used to control it.
Nonlinear Optical Loop mirror
• The nonlinear optical loop mirror (NOLM)
consists of a 3 dB directional coupler, a fiber
loop connecting both outputs of the coupler
and a nonlinear element (NLE) located
asymmetrically in the fiber loop.
• First ignore the non linear element and
assume that a signal is present at one of the
inputs shown as arm A of the directional
• Then the two output signals are equal and
undergo exactly the same phase shift on
traversing the fiber loop.
• In this case both the clockwise and the
counterclockwise signals from the loop are
completely reflected onto input A: specifically,
no output pulse emerges from arm B.
• However if one of the signals were to undergo
a different phase shift compared to the other,
then an output pulse emerges from arm B.
• In nonlinear amplifying loop mirror (NALM)
the pulse traversing the fiber loop clockwise is
amplified by an EDFA shortly after it leaves the
• The amplified pulse has higher intensity and
undergoes a larger phase shift on traversing
the loop compared to an unamplified pulse.
• These configurations are not convenient for
using the NOLM as a high speed
• First the intensity dependent phase change in
silica fiber is a weak nonlinearity and a few
hundred meters of fiber are required in the loop
to exploit this effect for pulse switching.
• It would be desirable to use a nonlinear effect
that works with shorter lengths of fiber.
• Second to realize an AND gate, we require NLE
whose nonlinear properties can be conveniently
controlled by the use of control pulses.
• NALM has both these properties and is called
terahertz optical asymmetric demultiplexer.
• TOAD has another directional coupler spliced
into the fiber loop for the purpose of injecting
the control pulses.
• The control pulses carry sufficiently high
power and energy so that the optical
properties of NLE are significantly altered by
the control pulse for a short time interval after
the control pulse passes through it.
• In particular the phase shift undergone by
another pulse passing through the NLE during
this interval is altered.
• For the proper operation of the TOAD as a
demultiplexer, the timing between the control
and signal pulses is critical.
• Assuming the NLE is located such that the
clockwise signal pulse reaches it first, the
control pulse must pass through the NLE after
the clockwise signal pulse but before the
counterclockwise signal pulse.
• If this happens the clockwise signal pulse
experiences the unsaturated gain of the
amplifier, whereas the counterclockwise pulse
sees the saturated gain.
• The latter also experiences an additional phase
shift that arises due to gain saturation.
• Owing to this asymmetry, the two halves of the
signal pulse do not completely destructively
interfere with each other, and a part of the signal
pulse emerges arm arm B of the input coupler.
• Along with the signal pulse, the control pulse will
also be present at the output. This can be
eliminated by using different wavelengths for the
signal and control pulses and placing an optical
filter at the output to select only the signal pulse.
Soliton Trapping AND Gate
• The soliton trapping AND gate uses some
properties of soliton pulses propagating in a
• In a birefringent fiber, these two modes
propagate with different group velocities.
• As a result, if two pulses at the same
wavelength but with orthogonal polarizations
are launched in a birefringent fiber, they
would walk off, or spread apart in time, owing
to this difference in group velocities.
• However, soliton pulses are an exception to
this walk off phenomenon.
• A pair of orthogonally polarized soliton pulses
propagate in birefringent fiber without walk-
• What occurs is that the two pulses undergo
wavelength shifts in opposite directions so
that the group velocity difference due to the
wavelength shift exactly compensates the
group velocity difference due to birefringence.
• Since the two soliton pulses travel together,
this phenomenon is called soliton trapping.
• The logical AND operation between two pulse
streams can be achieved using this
phenomenon if the two pulse streams
correspond to orthogonally polarized soliton
• Most high speed TDM systems use soliton
pulses to minimize the effects of group
velocity dispersion so that the soliton pulse
requirement is not a problem.
• The orthogonal polarization of the two pulse
streams can be achieved by appropriately
• The logical AND operation is achieved by using
an optical filter at the output of the
• Fig shows the block diagram of such a soliton
trapping AND gate. It consists of a piece of
birefringent fiber followed by an optical fiber.
• When pulses of both polarization are present at
the wavelength λ, one of them gets shifted in
wavelength to λ + δλ, and the other to λ – δλ.
• The filter is chosen so that it passes the signal at λ
+ δλ and rejects the signal at λ.
• Thus the passband of the filter is such that one of
the wavelength shifted pulses lies within it. But
the same pulse, if it does not undergo a
wavelength shift, will not be selected by the filter.
• Thus the filter output has a pulse only if both
pulses are present at the input, and no pulse
(logical Zero) otherwise.
• Synchronization is the process of aligning two
pulse streams in time. In photonic packet
switching networks, it can refer either to the
alignment of an incoming pulse stream and a
locally available clock pulse stream or to the
relative alignment of two incoming pulse
• The two periodic pulse streams with period T are
not synchronized because the top stream is
ahead in time by ΔT. To achieve synchronization,
the top stream must be delayed by ΔT with
respect to the bottom stream.
• A fixed delay can be achieved by using a fiber
of the appropriate length.
• In case of a synchronizer, and in some other
applications in photonic packet switching
networks, a tunable delay element is required
since the amount of delay that has to be
introduced is not known a priori.
• A tunable optical delay line capable of realizing
any delay from 0 to T – 2-k in steps of 2-k .
• The parameter k controls the resolution of the
• The delay line consists of k-1 fixed delays with the
values T/2, T/4 …. T/ 2k-1 interconnected by k 2*2
• By appropriately setting the switches in the cross
or bar state, an input pulse stream can be made
to encounter or avoid each of these fixed delays.
• If all the fixed delays are encountered, the
total delay suffered by the input pulse stream
is T/2 + T/4 +…. T/ 2k-1 = T/ 2k.
• This structure can be viewed as consisting of
k- 1 stages followed by an output switch.
• The output switch is used to ensure that the
output pulse stream always exits the same
output of this switch.
• Thus with tunable delay, the two pulse
streams can be synchronized.
Optical Phase Lock Loop
• Consider a NOLM that does not a separate
nonlinear element bur rather uses the
intensity dependent refractive index of silica
fiber itself as the nonlinearity.
• Thus if a low power pulse stream, say stream
1, is injected into the loop from arm A of the
directional coupler, the fiber non linearity is
not excited, and both the clockwise and the
counterclockwise propagating pulses undergo
the same phase shift in traversing the loop.
• As a consequence, no power emerges from
the output in this case . If a high power pulse
stream, say stream 2, is injected in phase with,
say, the clockwise propagating pulse stream.
• owing to the intensity dependence of the
refractive index of the silica fiber, the
refractive index seen by the clockwise
pulse,and hence the phase shift undergone by
it, is different from that of the counter
• This mismatch in the phase shift causes an
output to emerge from arm B.
• If the high power pulse stream is not in phase
with the clockwise propagating pulse stream,
the clockwise and counterclockwise pulses
undergo the same phase shift and no output
emerges from arm B of the directional
• To achieve snchronization between pulse
streams 1 and 2, a tunable delay element can
be used to adjust their relative delays till there
is no output of stream 1 from the NOLM.
• Since pulses from stream 2 will always be
present at the output, in order to detect the
absence of pulses from stream 1, the two
streams must use different wvelengths or
• When different wavelengths are used, owing
to the chromatic dispersion of the fiber, the
two pulses will tend to walk away from each
other, and the effect of non linearity will be
• To overcome this effect, the wavelengths can
be chosen to lie symmetrically on either side
of the zero dispersion wavelength of the fibre
so that the group velocities of the two pulse
streams are equal.
Broadcast OTDM Networks
• For a header of fixed size, the time taken for
demultiplexing and processing the header is
fixed, and the remainder of the packet is
buffered optically using a delay line of
• The processing of header bits may be done
electronically or optically, depending on the
kind of control input required by the switch.
• The packet destination information from the
header is used to determine the outgoing link
from the switch for this packet, using a look
• If the switch has two inputs and outputs, as
shown in fig, the lookup table determines, for
each input packet, whether the switch should
be set in the cross state or bar state.
• Of course, this leads to a conflict if both inputs
have a packet destined for the same output.
• This is one of the reasons for having buffers in
the routing node.
• Several techniques have been proposed to
simplify the task of header recognition.
• One common technique is to transmit the
header at a much lower bit rate than the
packet itself, allowing the header to be
received and processed relatively easily within
the routing node.
• The packet header could also be transmitted
on a wavelength that is different from the
• It could also be transmitted on a separate
subcarrier channel on the same wavelength.
• A routing node contains buffer to store the
packets from the incoming links before they
can be transmitted or forwarded on the
outgoing links. Hence the name store and
forward for these networks.
• In these networks, the buffer may be present
at the inputs only, at the outputs only or at
both the inputs and the outputs.
• There are 3 reasons for having to store or
buffer a packet before it is forwarded on its
• First, the incoming packet must be buffered
while the packet header id processed to
determine how the packet must be routed.
• Second, the required switch input and/or
output port may not be free, causing the
packet to be queued at its input buffer.
• The switch input may not be free other
packets that arrived on the same link have to
be served earlier.
• The switch output port may not be free
because packets from other input ports are
being switched to it.
• Third, after the packet has been switched to
the required output port, the outgoing link
from this port may be busy transmitting other
packets, thus making this packet wait for its
• In photonic packet switching networks buffers
are scarce resources.
• The best way to construct an optical buffer is
to use a piece of optical fiber and delay the
signal within it.
• Thus usually very small buffers are used in
photonic packet switching networks.
• Unlike an electronic buffer, a packet cannot be
accessed at an arbitrary point of time, it can
exit the buffer only after a fixed time interval
after entering it. This is the time taken by a
packet to traverse the fiber length.
• In photonic packet switching networks, the
switch operates at the speed of the input and
output links but usually not faster.
• This means that the packet arrive to the
output links from the switch no faster than the
rate at which they can be transmitted on the
• Thus the packets are transmitted immediately
on arrival and no buffering at the output link is
• In a 2* 2 switch, if both the inputs have a
packet for the same output, only one of them
can be transmitted by the switch immediately.
• There are 3 options
• First, option is for the packet to wait, which
necessitates an input buffer beyond what is
needed merely for packet header recognition.
This is expensive.
• The second option is for the packet to be
dropped. This is not attractive as the source
has to retransmit the packet.
• The third option is for the packet to be
misrouted by the switch, that is, transferred
by the switch to the wrong output.
• This option is termed as deflection routing.
• Deflection routing is also sometimes called
hot potato routing.
• Misrouting packets rather than storing them
will cause packets to take longer paths on
average to get to their destinations, and thus
will lead to increased delays and lesser
throughput in the network.
• Due to deflection routing the average delay
experienced by the packets in the network is
larger than in store and forward networks.
• Here not only the topology of the network is
fixed but the statistics of the packet arrivals
between each source – destination pair is also
• The delay experienced by a packet consists of
• The first is the queuing delay, which is the
time spent waiting in the buffers at each
routing node for transmission. There is no
queuing delay in the case of deflection
• The second component of the delay
experienced by a packet is the propagation
delay, which is the time taken for the packet to
traverse all the links from the source node to
the destination node.
• The propagation delay is often larger for
deflection routing than for routing with
buffers owing to the misdirection of packets
away from their destinations.
• This increased propagation delay almost
always compensates for the lack of queuing
delay in deflection routed networks so that
the total average delay is higher for the same
• In deflection routing the throughput of the
network is decreased compared to routing
• It depends on the interconnection topology of
the network and the data rates on the links.
• In addition it depends on the traffic pattern,
which must remain fixed in defining the
• The traffic pattern specifies the fraction of
new packets for each source destination pairs.
• In all theoretical analysis the throughput is
evaluated for a uniform traffic pattern, which
means that the arrival rates of new packets for
all source destination pairs in the network are
• If all the links run at the same speed, the
throughput can be expressed as a fraction of
the link speed.
• The second property of the network that we
must consider is its deflection index.
• The deflection index is the largest number of
hops that a single deflection adds to the
shortest path between some two nodes in the
• A single deflection adds at most four hops to
the path length so that its deflection index is
• Network topologies with small diameter and
small deflection indices are best suited for
photonic packet switching networks.
• Performance of deflection routing networks
can be further improved by using appropriate
• A deflection rule specifies the manner in
which the packets to be deflected are chosen
among the packets contending for the same
switch output port.
• In the event of a conflict between two
packets, both packets are equally likely to be
deflected. This deflection rule is termed
• Another possible deflection rule called closest
to finish, states that when two packets are
contending for the same output port, the
packet that is farther away from its
destination is deflected.
• This reduces the average number of
deflections suffered by a packet and thus
increasing the throughput.
• Deflection routing, which uses no buffers
except for packet header processing, is also
called hot potato routing.
• If the limited buffer is used and if these
buffers are full, the packet is again deflected.
• Limited buffer deflection routing achieve
higher throughputs than hot potato routing.
• When a network employs deflection routing,
there is the possibility that a packet will be
deflected forever and never reach its destination.
This phenomenon has been called both deadlock
• It can be eliminated by suitably designed
• It is difficult to find a deflection rule that is
• One way to eliminate livelocks is to simply drop
packets that have excedded a certain threshold
on the hop count.
Feed Forward and Feedback Delay Lines
• The throughput and delay performance of a
photonic packet switched network can be
improved by addition of small number of buffers
at each node.
• Two ways in which buffering of a few packets can
be incorporate into an optical routing nodes are
Feed Forward and Feedback Delay Lines.
• In the feed forward architecture a two input, two
output routing node is constructed using three
2*2 switches interconnected by two delay lines.
• If each delay line can store one packet, that is
the propagation time through the delay line is
equal to one slot, the routing node has a
buffering capacity of two packets.
• If two packets destined for the same output
arrive simultaneously at both inputs, one
packet will be routed to its correct output and
the other packet will be stored in delay line 1.
• This is accomplished by setting switch 1 in the
appropriate state. This packet then has
opportunity to be routed to its desired output
in a subsequent slot.
• If no packets arrive in the next slot, this stored
packet can be routed to its desired output in
the next slot by setting switches 2 and 3
• In the feedback architecture the delay lines
connect the outputs of the switch to its
• With two delay lines, the switch is internally a
4*4 switch with two inputs from outside and
two from the delay lines.
• If two packets contend for a single output, one
of them can be stored in a delay line.
• If the delay line has length equal to one slot,
the stored packet has an opportunity to be
routed to its desired output in the next slot.
• If there is contention again, it, or the
contending packet, can be stored for another
slot in a delay line.
• The feed forward architecture is preferred to
the feedback architecture since it attenuates
the signal almost equally, regardless of the
path taken through the routing node.
• This is because almost all the loss is in passing
through the switches, and in this architecture,
every packet passes through the same number
of switches independent of the delay it