Tabriz university school of new science and technology

Optical switching networks

Designed by : N.pasyar

Optical switching network

optical switching networks refers to all the various types of

flexible and reconfigurable optical networks that use any of

the multiplexing , tuning , and switching techniques.

optical switching networks are single-channel or multichannel

(WDM) networks whose configuration can be changed

dynamically in response to varying traffic loads and network

failures.

End-to-end optical networks Optical switching networks have been used in backbone

networks in order to cope with the ever-increasing amount of traffic and bandwidth-hungry applications.

They are also used in metro(politan), access, and local area networks.

Fig 2.1 Hierarchy of optical switching networks.

Long-haul backbone Networks

Core Ring

Edge RingEdge Ring

Wide Area

Metro Area

Access/Local Area

Last/first mile

phone companies deploy digital subscriber line (DSL) while cable

providers deploy cable modems .so, copper is used on the final

network segment to connect subscribers.

copper-based network segment forms a bandwidth bottleneck.

The bandwidth bottleneck is referred to as the so-called last mile

bottleneck, or also as the first mile bottleneck in order to

emphasize its importance to the end users.

To remove the first/last mile bottleneck, fiber is brought close or even all the way to the subscribers, who might be either business or residential users.

Depending on the demarcation point of fiber, these optical access networks are called fiber to the X (FTTX) networks, where X denotes the various possible demarcation points of fiber.

PON To build cost-effective optical access networks FTTX networks

are based on passive optical components without using amplifiers or any other powered devices. these FTTX networks are called passive optical networks (PONs).

They simplify their operation, administration, and maintenance (OAM) as well as their management due to the completely passive nature of their underlying components.

The transport of data traffic over PONs may be based on asynchronous transfer mode (ATM), resulting in the so-called ATM PON (APON).

Ethernet frames may be transported in their native format in the so-called Ethernet PON (EPON),

whether APON or EPONs are suitable for cost-effective broadband optical access networks?

Given the low costs of Ethernet equipment and by capitalizing on the

technology and products of the Ethernet LAN market, EPON appears to

be better than ATM-based PONs.

the use of WDM on the already installed fiber infrastructure provides an

multichannel upgrade path of EPON access networks, leading to WDM

EPONs.

The 10-Gigabit Ethernet (10GbE) provides an extended maximum

transmission range of 40 kilometers, compared to (GbE) 5 kilometers,.

Ethernet is the predominant technology in local area networks (LANs).and

low-cost solution also for optical high-speed MANs and WANs.

10GbE equipment costs are about 80% lower than that of SONET

equipment and 30–60% lower than other managed network services.

The use of Ethernet technology not only in today’s LANs but also in future

access, metropolitan, and wide area networks will potentially lead to end-

to-end Ethernet optical networks.

future access networks are likely to be bimodal .So-called radio-over-fiber

(RoF) networks may be viewed as the final frontier of optical networks

which interface optical access networks with their wireless counterparts

Applications

comprises small- to medium size file transfers

broadcast television

video conferencing

interactive games

Throughput-Critical

includes large-size file transfers

require much bandwidth

video & image email attachments

backup of files

file downloadingebooks

Latency-Critical

Depending on the application, traffic loads and throughput-delay performance requirements may change over time.

web browsing has led to rather asymmetric network traffic loads. it is based on the client–server paradigm, (each client sends a short request message to a server for downloading data files).

In future (optical) networks, the traffic load is expected to become less asymmetric via so-called peer-to-peer (P2P) applications.In p2p each client also acts as a server from which other clients may download files

Services Connection-Oriented Services: Connection-oriented services

require a connection to be established before any data can be sent by executing a specific handshake procedure between sender and destination.

sender and destination and possibly also intermediate nodes need to maintain state information for each established connection.

Examples are transmission control protocol (TCP), ATM, and multi protocol label switching (MPLS).

By using the state information, connection-oriented services are able to recover from data loss and provide quality of service (QoS) to applications.

Services Connectionless Services: Connectionless services do not

require the establishment of any connection prior to sending

data.

Connectionless services are less reliable than their connection-

oriented counterparts, but they are well suited for the transfer

of best-effort traffic. As an example is the Internet protocol

(IP) which provides best-effort service to higher protocol

layers and applications.

Some of today’s most important services triple-play services :Triple-play offerings comprise

bidirectional voice, bidirectional data, and unidirectional video services. all three services are delivered to the subscriber via FTTX networks .

virtual private network (VPN) services:A VPN supports a closed community of authorized users, allowing them to access various network-related services and sources. a VPN is a virtual topology that is built on a physical network infrastructure whose resources may be shared by multiple other VPNs.

cost-effective provide privacy by isolating traffic belonging to different VPNs from each

other. offer services that enable a mobile to remotely access the intranet and/or

extranet of a company.

As shown , optical switching networks offer services to applications.

Depending on the traffic demands of various clients optical switching networks need to be able to provide different types of connections.

Fig 2.2 Optical switching networks offering services to applications.

Optical switching networksopOptical switching networks

services

services

Applications

Applications

Switching granularity connections between source and destination may be switched

at different granularities. Major switching granularities, ranging from coarse granularity

at the top to fine granularity at the bottom of the table:

Switching granularity

Fiber switching

Waveband switching

Subwavelength switching

Optical circuit switching (OCS)

Optical burst switching (OBS)

Optical packet switching(OPS)

Switching granularity Fiber switching : is done by

switching all data arriving on an incoming fiber to another outgoing fiber without making any difference among the various wavelength channels.

waveband switching : the set of wavelength channels carried on the fiber is divided into multiple adjacent wavebands. Wavebands are switched independently from each other; waveband switching provides a finer switching granularity.

Wavelength switching : wavelength switching an input WDM comb signal is first demultiplexed into its individual wavelength channels and each wavelength channel is then switched independently. A given wavelength channel may carry the data of a single client . In OTDM networks, each time slot carries the data of a different client and may be switched individually.

OCS: all switching techniques discussed so far are optical circuit switching (OCS) techniques. circuits – fibers, wavebands, wavelengths, or time slots –are dedicated to source–destination node pairs .

OPS: OPS networks is an example for subwavelength switching optical networks, where packets on a given wavelength channel are switched independently from each other.

packet-switched networks require buffers to resolve contention. Fiber delay lines (FDLs) are used.FDLs are fiber loops where the optical signal continues to circulate until it is forwarded.

the reading/writing of the information is restricted to integer multiples of the round-trip time of the loop FDLs must be designed such that the largest packet size can be stored on the loop.

OPS networks favor cell switching, where each cell has the same fixed size.

OCS networks provide circuits with a high level of QoS. At the downside, they require a two-way reservation to set up optical circuits.

OPS networks are connectionless and thus avoid any delay due to circuit set-up. But OPS networks do not necessarily meet QoS requirements.

OBS: in OBS networks client data is aggregated at the network ingress and sent as bursts across the network. For each burst a reservation control packet is sent on a dedicated control wavelength channel prior to sending the burst on one of the data Wavelength channels after an offset time.

The control packet is used to configure intermediate OBS nodes between the ingress node and egress node of the OBS network.

The offset time is set such that the burst can be all-optically switched at intermediate nodes. so, the burst does not need to be OEO converted, buffered, and processed at intermediate nodes, thus avoiding the need for optical RAM and FDL.

OBS networks deploy a one-way reservation, as is done in OCS networks .Also, by controlling the offset time, service differentiation and various QoS levels can be achieved.

Interlayer networking The data plane encompasses all switching mechanisms required

to send data from source to destination. the data plane needs to be controlled by means of a control plane.

the control plane is responsible for setting up and making the various switching techniques work in a coordinated and efficient manner throughout the optical network.

The control plane can be implemented at the medium access control (MAC) layer of optical switching networks to avoid collisions of data frames on each wavelength channel. Also to route and establish optical connections from source to destination.

Interlayer networking Two approaches for control plane: new control protocols are designed for the optical switching

networks, taking their respective properties into account. already existing control protocols which were successfully

deployed in electronic data networks ,are used.for example IP centric control plane .

interlayer networking between AONs and IP clients became the next evolutionary step in designing flexible optical switching networks, leading to so-called IP/WDM networks.

interconnection models of IP-over-optical networks: Peer Model: IP networks and optical networks are considered an

integrated network with a unified control plane .peer model implies that optical network nodes need to provide IP routers with full routing information .

Overlay Model: IP networks and optical networks operate completely independently from each other and run their own set of routing and signaling protocols. In order to interconnect geographically distributed IP routers across optical networks appropriate interfaces between both types of network need to be standardized.

Interdomain (Augmented) Model: both IP and optical networks have their own routing instances, but reachability information of geographically distributed IP routers is passed by the underlying optical networks onward to IP clients.

The standardization will allow end users and network operators to dynamically control end-to-end connections.

International Telecommunication Union (ITU-T). ASTN

Internet Engineering Task Force (IETF). GMPLS Optical Internet working Forum (OIF).

The ASON and GMPLS protocols are well suited for conventional centrally managed optical switching networks .

in customer-controlled and optical networks customers acquire their own dark fibers and/or point-to-point wavelengths and are responsible for protection and restoration, without requiring any service from a central management.

Buying one’s own optical network may result in significant cost savings since ongoing monthly expenditures are replaced with a one-time initial CAPEX shared by the customers.

user-controlled optical networks support wavelength switching and burst switching.

security Many of the security mechanisms used in electronic networks

can also be applied in optical networks.

Differences: for high-speed data rates on multiple WDM channels infrequent

attacks may lead to large amounts of lost information.

eavesdropping by means of tapping may comprise the privacy of optical communications.

malicious signals are harder to detect in transparent optical networks than in opaque ones with OEO conversion.

In (EDFAs), a malicious high-power optical signal uses more of the upper-state photons and reduces the gain of the other user signals.

the limited crosstalk of optical devices ,(OXCs) and (OADMs), can lead to a reduced QoS on one or more wavelength channels if the power of a malicious optical signal is sufficiently high.

the security of optical networks can be improved by means of preventive measures. bandlimiting optical filters discard optical signals outside the operational spectrum and mitigate the gain competition of EDFAs.

Traffic grooming Traffic grooming :is the process of grouping many small

telecomunications flows into larger units, which can be processed as single entities.

In a network using both TDM and WDM , two flows which are destined for a common node can be placed on the same wavelength, allowing them to be dropped by a single OADM.

traffic grooming was done in the area of SONET/ (SDH) ring networks. it can be extended to optical mesh WDM networks, where low-rate circuits and data flows are assigned to wavelength channels which optically bypass OADMs.

objective of grooming

Often the objective of grooming is minimizing the cost of the network and line terminating equipment (LTE).

Thus grooming typically involves minimizing the usage of ADMs.This is similar to the use of virtual networking and virtual paths in ATM networks.

Effective grooming requires consideration of the topology of the network and the different routes in use. This is especially useful when dealing with mesh networkes

Components Combiner:It collects wavelength channels from all S input

ports and combines them onto the common output port.

Splitter: It equally distributes all incoming wavelength channels to all S output ports.

S x 1 1XS

λ

λ

λ

λ λ

λ

λ

λCombiner Splitter

Waveband partitioner: It partitions an incoming set of contiguous wavelength channels into two wavebands A and B, where 1 ≤ ΛA, Λ B ≤ Λ − 1, and Λ = Λ A + Λ B.

Waveband departitioner:It collects two different wavebands consisting of ΛA and Λ B contiguous wavelength channels from the upper and lower input port, respectively.

λ

λ

λ λ

λ

λ

Λ

ΛB

Λ

ΛA

ΛB

ΛA

Π Σ

Passive star coupler (PSC): . it collects wavelength channels from all D input ports and equally distributes them to all D output ports.

Arrayed waveguide grating (AWG):The AWG routes every second wavelength to the same output port. This period of the wavelength response is called free spectral range (FSR).

the FSR of a D × D AWG consists of D contiguous wavelengths, the physical degree of an AWG is identical to the number of wavelengths per FSR.

DxDpsc

DxDAWG

λλ

λλ λ

λ

λ

FSR FSR

The arrayed waveguides

introduce wavelength-dependent

phase delays such that only

frequencies with a phase

difference of integer times2π

interfere constructively in the

output slab waveguide. Thus,

each output port carries periodic

pass frequencies. The spacing of

these periodic pass frequencies is

called FSR.

In put / out put waveguides

N inputports

WaveplateArray of M waveguides

N output ports

Slab waveguide

Schematic layout of an NxN AWG

In Fig. 3.3 the routing connectivity of an 8 × 8 AWG is illustrated. λk ’s

routing information is to exit the output port that is (k − 1) ports below the

corresponding input port; that is, λ1 goes from input port 1 to exit port 1 and

from input port 5 to exit port 5.

λ1…λ4 …λ8

λ1…λ4 …λ8

λ1…λ4 …λ8

1

2

3

4

5

6

7

8

2

1

3

8

7

6

5

4

λ1

λ1

λ4

λ4

λ8

λ8

λ8

INPUT

OUTPUT

Fig. 3.3

Transmitters A transmitter comprises a light source, a modulator ,and

supporting electronics. Broadband light sources: broad spectrum in the range of 10-100 nm. small bandwidth-distance product LEDs are mainly applied for low data

rates and/or short distances. output powers −10 dBm. superluminescent diodes with an output power of 18.0 dBm and a 3-dB

bandwidth of 35 nm .

Lasers: To achieve a significantly increased bandwidth-distance product lasers are

deployed. laser is an optical amplifier enclosed within a reflective cavity that causes the light to oscillate via positive feedback. output powers, between 0 and 10 dBm.

Lasers can be either fixed tuned or

tunable.

a laser can be tuned by controlling the

cavity length and/or the refractive

index of the lasing medium.

instead of tunable lasers one might

use an array of fixed-tuned lasers, or

multi frequency lasers

Tranmitter tuning tuning

Type range time

Mechanically 500nm 1-10ms

Tunable

Acousto-optic 100nm 10μs

Electro-optic 10-15nm 1-10ns

Injection current 30nm 15ns

Receivers (optical filters)

A receiver is composed of an optical filter, a photodetector,

a demodulator, and supporting electronics.

Optical filters : are used to select a slice of a broadband signal or one wavelength out of the (WDM) comb. The selected wavelength is subsequently optoelectrically converted by a photodetector.

Fiber bragg gratings(FBGS)

Fixed tuned filters

Diffraction gratingsDielectric thin film filters

tunable optical filters

Mach-Zehnder interferometer (MZI)

acousto-optic tunable filters (AOTFs)

Electro-optic tunable filters (EOTFs)

Transmission impairments Attenuation. Dispersion Nonlinearities

Attenuation : The peak in loss in the 1400-nm region is due to hydroxyl

ion (OH−) impurities in the fiber. this peak is reduced In Lucent’s AllWave fiber. among

0.85, 1.3, and 1.55 µm, the latter band provides the smallest attenuation of ∼0.25 dB/km.

dispersion different components of the transmitted signal travel at

different velocities in the fiber, arriving at different times at the receiver. As a result, the pulse widens and causes inter symbol interference (ISI).

dispersion limits the minimum bit spacing (maximum transmission rate).

Modal dispersion: arises only in multimode fiber where different modes travel at different velocities. Clearly, in single-mode fibers (SMFs) modal dispersion is not a problem.

Waveguide dispersion : After entering an SMF, major portion of an information-carrying light pulse travels within the core, the rest within the cladding. Both portions propagate at different velocities since the core and the cladding have different refractive indices.

Chromatic (material) dispersion: different frequency components of a pulse travel at different velocities due to the fact that the refractive index of the fiber is a function of the wavelength. Standard (SMFs) have a chromatic dispersion of 17 ps/(nm·km) at 1550 nm.

By controlling the waveguide dispersion accordingly, NZ-DSFs have a chromatic dispersion between 1 and 8 ps/(nm·km).

Alcatel’s TeraLight Metro Fiber has a dispersion of 8 ps/(nm·km). low negative dispersion of Corning’s MetroCor fiber enables the use of

low-cost directly modulated DFB lasers.

Polarization mode dispersion (PMD) :PMD arises because the fiber core is not perfectly circular, particularly in older installations.Thus, different polarizations of the signal travel at different velocities. PMD is proving to be a serious impediment in very-high-speed systems operating at 10 Gb/s and beyond.

Nonlinearities

when the power levels get fairly high the nonlinearities can place significant limitations on high-speed systems as well as WDM systems.

Nonlinearities owing to the dependence of refractive index on the optical power : (SPM), (XPM), and four-wave mixing (FWM).

Nonlinearities owing to scattering effects in the fiber medium due to the interaction of light waves with phonons (molecular vibrations) in the silica medium. stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS).

Self-phase modulation :variations in the power of an optical signal changes the phase of the signal. It leads to spectral broadening. instantaneous variations of the frequency around the signal’s central frequency.

Cross-phase modulation :XPM is a shift in the phase of a signal caused by the change in intensity of a signal propagating at a different wavelength. XPM can lead to asymmetric spectral broadening.

Four-wave mixing :FWM occurs when two wavelengths, operating at frequencies f1 and f2, respectively ,mix to cause signals at frequencies such as 2 f1 − f2 and 2 f2 − f1. These extra signals can cause interference if they overlap with frequencies used for data transmission.

Stimulated Raman scattering :SRS is caused by the interaction of light with molecular vibrations. Light incident on the molecules creates scattered light at a longer wavelength than that of the incident light. The light generated at the lower frequencies is called the Stokes wave. To reduce the amount of loss, the power on each channel needs to be below a certain level.

Stimulated Brillouin scattering : SBS is similar to SRS, except that the frequency shift is caused by sound waves rather than molecular vibrations .in SBS the Stokes wave propagates in the opposite direction of the input light.

Crosstalk Crosstalk decreases the signal-to-noise ratio (SNR) leading to

an increased BER.

interchannel crosstalk : is caused by signals on different wavelengths . It may be removed through the use of appropriate narrowband filters.

intrachannel crosstalk: is caused by signals on the same wavelength on another fiber due to imperfect transmission characteristics of components (AWG). it occurs in switching/routing nodes and it cannot be removed through filtering.

Noise Amplified spontaneous emission (ASE) :In optical EDFA

the spontaneous emission is amplified in addition to the incident light signal.

Shot noise :Is the main complication in recovering the transmitted bit of photodetector. Shot noise current occurs due to the random distribution of the electrons generated by the photodetection even when the input light intensity is constant.

Thermal noise : the photocurrent is amplified by an electrical amplifier. The electrical amplifier introduces thermal noise current due to the random motion of electrons at typical temperatures.

Thank you for your attention

The End