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8/3/2019 College Report of Optical Burst Switching
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INTRODUCTION
The current Internet is suffering from its own success. The number of users and
the variety of applications demanding more and more bandwidth keeps on
increasing day by day. These ever-increasing demands need ever-increasing
bandwidth. Here optical communication comes into the picture. It provides huge
amount of bandwidth and leads to the popular concept of optical Internet. The
potential of optical fiber was realized fully when wavelength division
multiplexing (WDM) was invented. It was determined that with wavelengths and
values typically used in optical networks today it is theoretically possible to
transmit data rates of up to 1 Tb/s. Recent Wavelength Division Multiplexing
(WDM) experimental results show successful transmission of data over a single
optical fiber at an aggregate speed of 1 Tb/s, spread over more than 256
independent wavelengths. As the number of wavelengths per fiber increases,
converting data between the optical and electronic domains becomes a critical
bottleneck in terms of cost, size, processing speed and power consumption.
In order to realize potential fiber bandwidth and WDM gains fully, the number of
such conversions must be minimized. Optical Burst Switching (OBS) has recently
been proposed as a future high-speed switching technology that may be able to
efficiently utilize extremely high capacity links without the need for data
buffering or optical-electronic conversions at intermediate nodes. However,
contention between bursts may cause loss within the network. Proposals to date
for OBS have yielded very high loss rates even for moderate network loads.
CURRENT HIGH SPEED NETWORKS
SONET
Synchronous Optical Network (SONET) and the closely related Synchronous
Digital Hierarchy (SDH) standards are the predominant technologies in todays
carrier networks. All-optical networks are transparent and are therefore data
format independent. While the data carried inside optical streams in an all-optical
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network may indeed be SONET formatted, the associated SONET protocols are
restricted to nodes at the edge of an all optical network and therefore do not affect
the operation of the all optical network. SONET is an example of a network
protocol that carries critical control information inside the framing format, control
information that needs to be read at each node in a SONET network. SONET
employs sophisticated multiplexing techniques to interleave synchronous streams
of electronic data from the basic signal rate of approximately 51.84Mb/s (STS-1)
up to a maximum theoretical rate of approximately 40Gb/s (STS-768). All other
SONET rates are integral multiple of this rate, so that an STS-N signal has a bit
rate equal to N times 51.84 Mb/s. SONET is a synchronous system with frames
sent every 125s. To achieve higher speeds, individual STS-1 frames are
aggregated together using byte-interleaving or through the use of larger frame
sizes, usually referred to as concatenated frames. The two main node types in a
SONET network are Add/Drop Multiplexers (ADMs) and Digital Cross Connects
(DCCs). ADMs are designed to pick out one or more low-speed streams from a
high-speed stream and also similarly insert one or more low-speed streams into a
high-speed stream. A DCC is a more advanced node that, in addition to ADM
functionality, can groom traffic. Grooming allows composite low-speed streams to
be individually switched, resulting in fine grained control at the expense of
increased complexity and port count.
SHORTCOMINGS OF SONET
The success of SONET has been largely due to the comprehensive functionality
of the additional control information carried along with the frame. This overhead
includes functions to manage performance, faults, configuration and security but
has a significant drawback: to control the network, this overhead and therefore
each frame needs to be read at each node. This means that each frame must be
received in the optical domain, converted to electrical form and then retransmitted
in the optical domain. This process is called Optical-Electronic-Optical (OEO)
conversion. In addition to these conversions at every node, several electrical
regenerators may need to be placed between each node to restore the output signal
level, reshape the pulses and retime the signal. As a consequence, high speed
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SONET, such as STS-768, is prohibitively expensive due to the large number of
high speed OEO converters required. An important side-effect of OEO conversion
is that the process is code, protocol and timing sensitive.
. The combination of these characteristics result in provisioning and upgrading
being extremely complicated and lengthy processes, often taking up several weeks
or even several months. The coarse granularity of SONET also may cause
significant inefficiency . For example, a customer can only upgrade from an STS-
48 (2.5Gb/s) to an STS-192 (10Gb/s) if more capacity is required. Another
significant inefficiency is due to the Time Division Multiplexing (TDM) nature of
SONET. Even if a fraction of the capacity is being used to transmit useful data,
the excess capacity is not available to other users. Each connection is logically
circuit switched and therefore aggregating many connections gives no
multiplexing gain. As a consequence, SONET networks must be dimensioned
with respect to peak load for eachof its composite streams. Furthermore, SONET
is a single wavelength technology. Given that more than 256 wavelengths can be
used simultaneously on a single fiber, this limitation has forced the rapid
development of alternative network infrastructures and protocols.
As seen from the above disadvantages SONET is unsuitable for future, high speed
networks. Instead, what is required is a set of protocols and associated network
infrastructure that both overcome the problems with SONET, yet do not introduce
significant new problems themselves. More precisely, the ultimate research goal is
the development of a new scheme that does not require extensive OEO
conversions, can be rapidly provisioned and upgraded, is independent of payload
data formats, uses bandwidth efficiently and most importantly, can scale to large
numbers of wavelengths per fiber. This scheme will be most useful in the cases of
high levels of aggregation of users and therefore of greatest importance in the
network core.
Current research focuses on three main technologies that solve most or all of the
above problems: Optical Circuit Switching (OCS), Optical Burst Switching (OBS)
and Optical Packet Switching (OPS).
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OPTICAL SWITCHING
The appearance of enhanced multimedia services requiring huge bandwidths, such
as broadcasting of high definition television(HDTV), video on demand, online
gaming created a need for transitioning to high switching speeds. As the network
traffic volume rises, particularly from the desire to have high bandwidth
multimedia services, the number of wavelengths per fibre will increase. This
means that changes are needed for the earlier switching methods in which optical
signals are converted to electronic signals then processed, regenerated, switched
electronically and then converted back to optical format. The main reason is that
performance of electronic equipments used in this OEO conversion process isstrongly dependent on the data rate and protocol and also for long distance fibers,
the cost also increases.
The development of the Erbium-Doped Fiber Amplifier (EDFA) in the late 80s
drastically reduced the need for electronic regenerations. This device is capable of
amplifying many wavelengths simultaneously, yet is insensitive to bit-rates,
modulation formats and power levels.
ALL-OPTICAL SWITCHES
Now that regenerators could be removed from optical links, switching nodes
became the electronic bottleneck. If no conversion to electronic form of a data
stream occurs within a switching element, this element is called an all-optical
switch. Furthermore, due to optical technology constraints, data within optical
signals cannot be read in the optical domain. Therefore all-optical switching from
input wavelength to output wavelength is called transparent switching, in contrast
to opaqueswitching where conversion to the electronic domain is required for the
switching process. Assuming control information can be separated from the main
data signal and received electronically at each node, then the required
functionality of an all-optical switch is simply being able to transparently switch a
given input wavelength on a given input fiber to a desired output wavelength on a
desired output link. Several technologies that achieve this goal have been recently
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developed, including micro-electro-mechanical switches (MEMS). This
technology is already employed in commercially available switches such as
Lucents Lambda Router and was recently sold to Japan Telecom to connect
major metropolitan areas across Japan1. MEMS consists of an array of tiny
mirrors that move when an electrical current is applied. By adjusting the tilting
angle of one or more mirrors, optical signals can be switched from input to output
fibers. 3D MEMS is an extension of this technique in which mirrors are positioned
in a three dimensional matrix and rotate on two axes, enabling mappings between
a much greater number of input and output ports . Calient Networks Diamond
Wave PXC photonic switch is an example of currently available switches utilizing
3D MEMS technology to achieve 256x256 switching capacity. Researchers at
Lucent believe that multithousand port fabrics appear to be physically realizable,
with the potential of switching capacity 2000 times greater than that of currently
struggling electronic fabrics. Furthermore, the average loss experienced by a
MEMS switch is extremely low. There are also several other all-optical switching
of fluid, Semiconductor Optical Amplifiers (SOAs) and electro optic lithium
niobate (LiNbO3). The latter two are capable of switching times in the
nanosecond range however, SOAs add significant amounts of noise to optical
signals, while LiNbO3 switches cause approximately 8dB of loss, limiting their
scalability. In addition, both of these fast technologies are polarization sensitive.
OPTICAL CIRCUIT SWITCHING
To send information quickly and reliably across a network, service providers use
various techniques to establish a circuit switched lightpath i.e. a temporary point
to point optical connection between the two communicating ends. An
OXC(optical crossconnect) is a key element to set up express paths through
intermediate nodes for this process. Since an OXC is a large complex switch it is
used in networks where there is a heavy volume of traffic between nodes. In such
networks, the lightpath normally is setup for long periods of time. Depending on
the desired service running between the distant nodes, this time connection can
range from minutes to months and even longer.
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Lightpaths running from a source node to a destination node may traverse many
fiber links segments along the route. At intermediate points along the connection
route, the lightpaths may be switched between different links and sometimes the
lightpath wavelength may need to change when entering another link segment.
This wavelength conversion is necessary if two lightpaths entering some segment
happen to have same wavelength.
This process of establishing lightpath is called wavelength routing or ligthpath
switching.
However, as the number of wavelengths per fiber and the associated number of
lightpaths required to be managed grows, the ability of circuit switching to scale is
questionable. Given that once the circuit, or lightpath, has been established it is
very difficult to change either the routing or the wavelengths used along the path
without significant disruption, it is very important to choose initial values
carefully. In todays large networks, this system optimization is largely done by
human traffic engineers due to fear among network operators that automated
solutions will possibly be unstable in practice, yielding both sub-optimal
performance and poor reliability, a fear grounded in unsuccessful experiments
with adaptive routing in the ARPAnet. Guaranteeing stability for complex ASON-
style networks may prove to be particularly difficult. Furthermore, circuit
switching is burdened by a fundamental problem. Circuits, by definition, need to
be provisioned for peak traffic intensity levels if loss is to be bounded over short
to medium time scales. Therefore in non-peak periods, much of this allocated
capacity may be unused, yet unavailable for other circuits in the network. To
achieve useful levels of statistical multiplexing through capacity sharing, some
type of packet switching must be used.
OPTICAL PACKET SWITCHING
Circuit switching is inherently inefficient given time-varying traffic intensity as
the capacity reserved by the circuit is not shared. This loss in statistical
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multiplexing capacity was the main motivation behind the development of packet
based networks in the electrical domain and may cause a similar paradigm shift
within the optical domain. The success of electronic packet switched network lies
in their ability to achieve reliable high packet throughputs and to adapt easily to
traffic congestion and transmission link or node failures. Various research have
been carried out to extend this ability to all optical networks in which no OEO
conversion takes place along a lightpath. In an OPS network, user traffic is routed
and transmitted through the network in form of optical packets along with in-band
control information that is contained in a specially formatted header or label. In
OPS the header processing is carried out electronically and the switching of the
optical payload is done in the optical domain for each packet. This decoupling
between header or label processing and payload switching allows the packet to be
routed independent of payload bit rate, coding format and packet length.
OLS(optical label swapping) is a technique for realizing a practical OPS
implementation. In this procedure, optically formatted, packets which contain a
standard IP header and an information payload first have an optical label attached
to them before they enter the OPS network. When the payload plus label packet
travels through an OPS network, the optical packet switches at intermediate nodes
process only the optical header electronically. This is done to extract routing
information for the packet and to determine other factors such as the wavelength
on which packet is being transmitted and the bit rate of encapsulated payload. The
payload remains in optical format as it moves through the network.
Ultimately, cost is the determining factor in the choice of network protocols.
Adding computing to the network in the form of packet switching functionality
was seen to be economically advantageous. The key difference between packet
switching and circuit switching is that in the former, the routing of data is
determined by the label or header of a discrete group of bits, while the latter
simply maps an input port to an output port. As packets are routed individually,
many packets from different sources, going to different destinations can share a
common wavelength, leading to potentially high levels of statistical multiplexing
and associated efficiency gains. There are three main limitations in optical packet
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switching that are not present in the electronic equivalent: the lack of Random
Access Memory (RAM) for buffering, the lack of sophisticated optical processing,
and relatively slow switching speeds.
OPTICAL BUFFERING
It is currently impossible both to store an optical signal indefinitely and randomly
access stored optical signals. In electrical packet switches, to avoid contention
between packets arriving at similar times and destined for the same output link,
packets can be queued for later transmission when the corresponding output link
becomes free. In optical packet switches, such queuing of packets is not currently
possible. Although there have been some promising discoveries, such as the
chiropticene switch, optical RAM is still in the early stages of development and
may never be achievable. A limited form of buffering is achievable in the optical
domain; optical signals can be delayed by a fixed time period by sending them
down an optical fiber that loops back to the input port. Such loops are called Fiber
Delay Lines (FDLs). Delay times are simply the length of the loop multiplied by
the speed of light, for example, 3km of fiber would give an approximate delay of
10s, or approximately the time taken for 10 packets of 1.5kB to be transmitted on
a 10Gb/s link. However, 3km is quite a lot of fiber to install on every output port
and to achieve variable-delays many different length FDLs must be used, adding
to the complexity. Maintaining temperature stability is also difficult across long
sections of fibre.
OPTICAL BURST SWITCHING
Optical burst switching (OBS) was first proposed in the late 1990s as a new means
of providing telecommunications transport services. Optical burst
switching (OBS) is an optical networking technique that allows dynamic sub-
wavelength switching of data. OBS is viewed as a compromise between the yet
unfeasible full optical packet switching (OPS) and the mostly static optical circuit
switching (OCS).
http://en.wikipedia.org/wiki/Optical_networkinghttp://en.wikipedia.org/wiki/Optical_networking8/3/2019 College Report of Optical Burst Switching
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To support bursty traffic on the Internet efficiently, optical burst switching (OBS)
is proposed as a way to streamline both protocol and hardware in building the
future generation Optical Internet. By leveraging the attractive properties of
optical communications and at the same time, taking into account its limitations,
OBS combines the best of optical circuit switching and packet switching.
The central concept of OBS is that rather than switching individual packets, the
source should group packets up into a burst and switch the burst as a unit.
This is the main advantage of OBS, it provides short time-scale statistical
multiplexing that gives benefits to both network operators and users, whilst
providing significantly higher efficiency than OPS for current optical device
technologies.
FUNDAMENTAL OBS CONCEPTS AND ARCHITECTURE
OBS network architecture
Telecommunications networks are often organised in a three-stage hierarchicy:
users connect through an access network; their traffic is then aggregated and
groomed onto a higher capacity intra-city metro network; traffic bound for another
city or country is then further aggregated onto the highest capacity backbone or
core network. OBS is considered a candidate technology for backbone and metro
network implementation. We consider an OBS network as providing data
transport services to the next lower level of the hierarchy, whichever that happens
to be. The only restriction is that the client network is assumed to be a packet
switching network, and submits packets to the OBS network for transmission.
Figure 1 shows client networks gaining access to the OBS networks transport
services via edge routers.
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Figure 1: An OBS network, showing key components: burst assembling, edge
routers and core crossconnects.
The edge routers have the job of grooming and routing the client network trafficinto the OBS network. The OBS network itself consists of optical burst switches
connected by WDM fiber links. When a host in one client network, say A, wishes
to send data to another host, sayB, in a different client network, the client network
routes As packets to the local edge router, X, based on its eventual destination
address (i.e.,B). The edge routerXthen uses the packets destination to determine
how to route the packet through the OBS network. It will use the OBS network to
transmit the packet to edge router Yin the client network to which B is attached.
When the packet reaches Y, Ywill route the packet on towardsB, the destination.
Nevertheless, it is possible to identify several key characteristics that distiguish
OBS from traditional switching techniques. In an OBS network the gateways at
the edge of the network are replaced with burst assembling edge routers, and the
core switching elements are replaced with optical burst switches. The difference in
the dynamic operation of OBS networks compared to OCS and OPS is firstly that
the edge routers assemble packets bound for the same destination edge router (Y)
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into bursts, and secondly that the OBS switches treat bursts as single entities for
switching purposes, amortising switch setup overhead over many packets. A third
difference is that each assembled burst is sent into the network according to a
reservation protocol. The reservation protocol is similar to the circuit setup
protocol of circuit switching. The header information (which in packet switching
would be transmitted in-band and immediately ahead of the payload) is
transmitted out-of-band on a separate control channel, and precedes the burst
payload by an offset time. The OBS nodes then make resource reservations for the
burst in advance, so that when the burst arrives, the nodes OXC is already pre-
configured to switch it onto the correct output fibre and wavelength. The OXC
connection is maintained only as long as the bursts holding time. This is a key
difference to both packet switching and circuit switching. There are numerous
different reservation protocols.
Edge routers
The term burst switching refers to the key concept of OBS, which is that the
edge router, instead of forwarding the packets one at a time through the OBS
network, assembles many packets headed for the same destination edge router into
a much larger super-packet, known as a burst. The reason for doing this is to
gain higher efficiency with slower switching technologies.
A queue exists for each class for each destination. The process is depicted in the
dashed ellipse in Figure 1. An edge router that assembles packets into bursts in
this manner is known as a burst assembling edge router or more simply as a burst
assembler(BA). Each BA will have one queue for each possible destination edge
router. If the OBS core network supports service differentiation based on class of
service (CoS) labels, then rather than one queue per destination there may be K
queues per destination, given that there are Ksupported service classes. This is the
situation depicted in Figure 2.
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Figure 2: Architecture of a burst assembler. A queue exists for each class for each
destination.
Incoming packets are routed to a particular queue based on their destination and
CoS. When a queue satisfies a certain trigger condition, all packets in that queue
are grouped into a burst and scheduled for transmission into the OBS core
network. The BA sends a message into the network that notifies each OBS node
along the intended path through the network of the imminent arrival of the burst,and requests transmission resources. This message is known as a control packet.
The control packet specifies both the length of the burst in seconds (its holding
time), and the offset time, as well as any other information about the burst that the
OBS core nodes require (such as CoS). The offset time is the difference in time
between the arrival of the control packet at an OBS node and the arrival of the
first bit of the burst. This is illustrated in Figure 3.
Figure 3: Burst data preceded in time by the control packet.
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Control packets are request messages, similar to the setup, tear-down and
acknowledgement messages of circuit switching networks. Supposing that the bit
rate of the source transmitter isR bits per second and that the burst containsx bits
of data, then we have
h = x/R seconds.
Thus the holding time h is determined by the amount of data in the burst, which
depends on the trigger condition used to decide when the queue contains enough
data to send as a burst. This condition is the concern of burst assembly algorithms.
Burst assembly algorithms fall into three main groups: timerbased algorithms,
threshold-based algorithms, and hybrids of the two To give a concrete example of
burst holding time, let us assume that a burst contains 100 packets and
that the average packet size is 500 bytes. The size of the burst in bits is then
5008100 = 400, 000 bits.
If the line rate is 10 Gbps, then the holding time is h = 400, 000/10109 = 40 s.
In reality this would be augmented slightly by receiver synchronization and
framing overhead and guard times.
OBS cross-connect architecture
Once the control packet is sent by the burst assembling edge router, in turn each
optical burst switch decides whether the burst should be forwarded in its
transmission or dropped. This is controlled partly by the reservation protocol used
by the nodes. We consider the architecture of the individual nodes, which is
illustrated in Figure 4.
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Figure 4: Architecture of an optical burst switch.
The links of an OBS network are optical fibres bearing WDM optical data signals.
The OBS node in Figure 4 consists of N input fibers and Noutput fibers, each
carrying k+ 1 WDM channels, {W1, . . . ,Wk,Wc}. The first kwavelengths on
each fibre are de-multiplexed by NWDM demultiplexers, and the resulting N k
distinct optical signals are switched to output ports by the OXC. The cross
connected signals are then re-multiplexed onto the Noutput fibers. Meanwhile,
wavelength Wc is tapped off to the Electronic Control Unit (ECU) and
demodulated into electrical form (i.e. O/E conversion). This wavelength is called
the control wavelength or control channel, and it is the transmission channel forthe control packets. The control channel line rate may be significantly lower than
the data channels line rates, since control packets are designed to have negligible
length compared to the burst and have a one-to-one correspondence to bursts.
Once the information in the control packet has been read, the first step taken by
the ECU is to make a forwarding decision, i.e. which output fiber to switch the
burst to. It then determines if the burst can be transmitted on the chosen output
fiber by comparing the requested transmission interval with its current list of
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reserved intervals on the wavelengths of that fiber and executing a channel
allocation algorithm. If there is a free interval that fits the new request, the ECU
records the new reservation on the chosen data wavelength and retransmits the
control packet on the control wavelength of the chosen output fiber. The control
wavelength is multiplexed back together with the data-bearing wavelengths. If no
suitable free interval is found, the control packet is discarded and the data burst
will be dropped when it arrives at the switch. For successful reservations, the ECU
then uses a signalling interface to the OXC (shown in Figure 4) to set up a
connection for each burst between its input and output fibre and wavelength. The
connection is short lived, its lifetime depending on the reservation protocolused
by the ECU and the burst holding time.
Burst Assembly
Semiconductor Optical Amplifier (SOA) and electro-optic lithium niobate
(LiNbO3) all-optical switches are capable of switching times in the nanosecond
range but have serious problems. Assuming these problems will not be quickly
overcome, the time required to reconfigure an all-optical switch matrix is a
significant fraction of, or even more than, the time taken to transmit an IP packet.
Therefore, to achieve useful levels of efficiency, packets must be aggregated at the
edge of an OBS network. The node where packets are aggregated is called an
ingress node. After being switched through the OBS network, successfully
received bursts are disaggregated into packets. The final node is called an egress
node. A sample path in an OBS network is shown in Figure 1.
PATH RESEVATION
In order to achieve statistical multiplexing gains, the entire capacity of a network
must remain unsegmented such that there is a single pool of unused bandwidth
that is universally available. In the case of circuit switching, any unused
bandwidth in a circuit is inaccessible to other circuits and therefore bursty traffic
distributions result in very low utilization of the network. Early burst switching
technologies, called Tell-and-Wait (TaW) and Tell-and-Go (TaG), were
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developed in the early 1990s to reduce this inefficiency. Both Tell-and-Wait and
Tell-and-Go attempt to reserve a short term circuit to deliver a burst of cells such
that network capacity can be shared and subsequent multiplexing gains achieved.
TaW sends a short request message that attempts to reserve bandwidth at each
switch in the path. If the reservation is successful, an acknowledgement (ACK) is
sent from the final node to the origin of the request message and the burst
immediately sent on receipt of this ACK. If a reservation cannot be made at any of
the nodes in the path, a Negative Acknowledgment (NACK) is returned to the
origin of the request message along the reverse path and previously made
reservations are freed. TaG, on the other hand, does not reserve any bandwidth in
advance and sends burst whenever it is ready. Upon arrival of the header at an
intermediate node in the path, capacity on the corresponding output link is
reserved, given that sufficient capacity is available. In the case that sufficient
capacity is not available, the burst is discarded and only the header forwarded to
the final node, which then returns a NACK. The performance of these two
protocols depends on the propagation delay of the path and the size of the burst.
For large propagation delays with respect to the burst size, TaG outperforms TaW
and vice-versa.
RESERVATION PROTOCOLS
As mentioned, the OBS nodes ECU has two tasks, channel allocation and
reservation protocol processing. Reservation protocols are frequently closely
related to the channel allocation algorithm. Together, the two determine whether
and in what manner transmission resources are allocated to a particular burst at
each link in its path. A burst may need to traverse many fiber links in order to
reach its destination, and at each link, there must be sufficient capacity to
accommodate it.
In OBS there are generally two types of reservation protocols-:
1- One-way reservation protocols
2- Two-way reservation protocols
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One way reservation protocols
The most well known basic reservation protocol is the Just Enough Time protocol
(JET). It was proposed by Yoo and Qiao. Rather than using a two-way reservation
protocol, JET is a one-way reservation protocol. Since burst holding times are
much smaller than typical circuit holding times, the delay between sending the
setup message (or control packet) and receiving an acknowledgement is an
appreciable fraction of the holding time and could represent an undesirably long
delay to the packets in the burst. It could also result in low utilisation of the edge
routers access link to the OBS network. Thus, JET instead uses an
unacknowledged one-way reservation algorithm. No acknowledgement is
required. Instead, the source sends the control packet and then simply waits for a
set offset time. Once offset time has elapsed, it sends the data burst itself. ECU
knowswhen the burst is coming because the control packet tellsit. This is one of
the most important functions of the control packet, transmitting this information.
This allows the ECU to implement the second important feature of JET, which is
known as delayed reservation. In JET, the channel is only reserved for that period
of time during which the burst will be traversing the cross-connect. The cross-
connect is free to assign the channel to other bursts from different sources duringthe periods of time between the control packet and burst arrival times, leading to
higher channel utilization.
One alternative, which is also a one-way protocol, is known as Just In Time (JIT).
The JIT protocol is similar to JET, but uses an acknowledgement from only the
first cross-connect. Furthermore, the control packet does not carry timing
information, the channel is reserved from the moment the control packet is
received and processed, hence the offset time, which is determined by the first
cross-connect, must be incorporated into the bursts channel holding time. In this
case, it is important to have as small offset time as possible. Like in JET, once the
source is informed of the correct offset time to use by the first cross-connect, it
simply sends its burst at that offset, without waiting for acknowledgement of
resources, thus the protocol is still a one-way protocol.
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Two-way reservation protocols
In a two-way, acknowledged reservation protocol, the source burst assembler can
easily retain the burst in memory and continue requesting transmission until the
request succeeds. Thus delay has two interesting components in a two-way
protocols, the burst assembly delay and the resource reservation delay. Several
two-way or acknowledgedOBS reservation protocols have been proposed out of
which one is most prominently used. The most prominent is known as
wavelength-routed optical burst switching or WR-OBS. WR-OBS varies from
OBS/JET in two significant ways: first, it presumes much longer bursts; and
second, it uses dynamic, acknowledged lightpath establishment to provide a
dedicated channel for the transmission of each burst. Bursts are longer in WR-
OBS because burst aggregation is assumed to take time T that is of the same order
as the time required to request a lightpath. Given realistic network propagation
delays, this is likely to be on the order of milliseconds; OBS/JET generally
assumes burst lengths and burst assembly delays on the order of microseconds.
A WR-OBS edge router collects packets for a burst until some condition is met
that triggers the source to send a request for a lightpath to a central network
controller. The aggregation then continues until an acknowledgement that the
lightpath was successfully established makes its way back to the source. At this
point the transmission of the burst begins. The condition on which the lightpath
request is sent may either be that the amount of packet data collected exceeds
some threshold, or that some limit on allowable delay has been reached.
CONTENTION RESOLUTION
Once bursts are assembled, they are launched into the network according to a
reservation protocol. It is possible for the reservation protocol to fail if there are
not enough resources for burst transmission. The resources of an OBS network are
the wavelengths supported on each link. When a control packet arrives at a cross-
connect, say at time t, the control unit decodes it to extract information about the
offset time Toand duration (holding time) hof the burst; its destination, and some
other related information. The ECU uses this information to make a routing
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decision. Given the bursts destination, it decides which output fiber it should use
to forward the burst. It then considers whether there is a wavelength that is free
from time (t+ To)until (t+ To+ h)on the chosen output fiber. If so, the burst can
be accommodated. If not, then there is said to be contention. An OBS cross-
connect does not have the luxury enjoyed by electronic packet switches of
delaying or queueing the burst indefinitely in the case of contention, because no
optical technology can yet store data for an indefinitely long period of time
There are four main methods for resolving contention-:
1. Wavelength Conversion: On contention, we try to make a reservation on a
different output wavelength on the desired output link.
2. Fiber Delay Line (FDL): On contention, we try to make a reservation on
the desired output wavelengthon the desired output link but at a different
time.
3. Deflection Routing: On contention, we try to make reservation on desired
output wavelength on a different output link.
4. Preemption: On contention, remove the contending reservation, then make
reservation on the desired output wavelength on th desired output link.
ADVANTAGES
Table 1: Advantages of OBS.
DISADVANTAGES
1.
Faces two technological bottlenecks: Processing speed and buffering.
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2. Noise accumulation.
3. Burst dropped in case of contention.
CONCLUSION
The fundamental concepts of OBS are burst assembly and edge routers;
reservation protocols, control packets and offset times; OBS node architectures;
and contention resolution. Despite the fact that OBS was invented less than ten
years ago, there is already a relatively large body of published OBS research.
Regardless, there remain numerous open issues and challenges still facing
researchers and engineers. The primary challenge is to move OBS from research
into practical realisation, and then on into commercial deployment. Current
research is overwhelmingly theoretical or simulation-based. Significant
investments, possibly funded in part by commercial interests, will be needed to
develop components and testbeds to prove the viability of the ideas behind OBS.
The realities of optical device physics pose significant challenges to realising the
types of switches needed by OBS (fast switching speed, low loss and distortion,scalability), and other technical problems (such as offset time control and receiver
synchronisation) can be envisaged today. Still further problems are likely to arise
as implementations proceed as a lot has still to be done in optical field.
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REFERENCES
1. Optical Fiber communications- Gerd Keiser.
2.
Telecommunication Switching, Traffic and Networks- J.E. Flood.
3. Modeling and Dimensioning of Optical Burst Switching Networks-
Jolyon Ambrose Scoresby White.
4. Optical Burst Switching: Towards Feasibility- Craig Warrington Cameron.
5. Optcal Burst switching: A new paradigm for optical internet- Chunming
Qaio, Mungsik Yoo.
6. http://en.wikipedia.org/wiki/Optical_burst_switching
http://en.wikipedia.org/wiki/Optical_burst_switchinghttp://en.wikipedia.org/wiki/Optical_burst_switchinghttp://en.wikipedia.org/wiki/Optical_burst_switching