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- Optical networks – review
- Components for optical networks
- Wavelength-division multiplexing (WDM)
- Long-distance, metro- and access networks
- Components
- Switches
- Optical cross connects (OXCs)
- Optical add-drop multiplexers (OADMs)
- Splitters and couplers
- Repeaters and optical amplifiers
- Textbook: Djafar K. Mynbaev and Lowell L. Scheiner, Fiber-Optic
Communications Technology, Prentice Hall, 2001, ISBN 0-13-962069-9.
- Notes:
- The figure numbers in these modules are the same as in the textbook. New
figures are not numbered.
- Always see examples in the textbook.
- Key words
- Wavelength-division multiplexing (WDM)
- Long-distance networks
- Metro-are networks
- Access networks
- Switches
- Optical cross connects (OXCs)
- Optical add-drop multiplexers (OADMs)
- Splitters and couplers
- Repeaters (regenerators
- Optical amplifiers
- Erbium-doped fiber amplifier (EDFA)
- Raman amplifier
- Semiconductor optical amplifier
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- Optical networks: point-to-point link and network. Physical and
intelligent levels.
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- First, the network must provide physical connections among all
communicating parties; secondly, communication is possible if and only
if the communicating parties follow the certain rules (protocols). But connections are something
physical, tangible while protocols are something logical, intelligent.
Therefore,
- telecommunications network are built on two aspects (sides): physical
and logical (intelligent).
- Clearly, physical side is given by the network’s physical facilities
such as transmitters, receivers, switches, links, etc. What devices
support logical (intelligent) work? The general answer is, computers.
Specifically, we refer to electronic machines that are able to perform
arithmetic and logic operations, such as processors.
- What if the nodes of a telecommunications network would not be able to
perform switching? There would be no network, just a set of
point-to-point links. Therefore,
- switching (routing) ability is the main feature of a logical
(intelligent) aspect of a telecommunications network.
- A word about terminology: Switching is a general term that refers to
operation of relaying messages through a network. However, in
telecommunications we use term switching to describe transferring
signals from one circuit to another; in other words, switching refers to
circuit-to-circuit connections within a network. Routing is also a
general term that refers to switching among networks; however, in
regards to the Internet communications, routing means forwarding packets
through network.
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- Control plane is a set of
software and/or hardware residing in a network node that executes
control and management functions. Implementation of control plane
depends on protocols. An example of hardware is a router. Examples of
control plane protocols include signaling system seven (SS7) protocol
stack in voice transmission, open shortest path first (OSPF) routing
protocol in data transmission and generalized multiprotocol label
switching (GMPLS) protocol.
- Data (information, or forwarding) plane is a set of hardware and
software that provides transportation of voice, data, and video traffic.
An example of hardware is an optical cross-connect (OXC) and an example
of protocols is IP suite.
- Control and data plane interaction: Control plane at a node generates
routing and label tables and exchange this information with peers. This
information is used by data (forwarding in IP routers) plane for
transportation [2]. In other words, control plane protocols (OSPF and
others) enable IP to forward traffic correctly [3]. Separation of
control and data planes makes data plane protocol-independent.
- Today, control plane interact with data (forwarding) plane through open
interface, which constitute the third (current) generation of the
network element architecture.
- Control planes residing in nodes of any given subnetwork make up a
control domain of this subnetwork. Control planes enable traffic
transportation within and between their subnetworks.
- The main functions of an optical control plane are targeted solving the
problem of ”find, route, and connect,” which requires the follows:
- •Naming and addressing scheme (find)
- •A routing process to handle the network resources usage and route
calculation (route), including routing and wavelength assignment (RWA)
and topology and resources discovery
- •A signaling network that provides communication between entities
requesting services and those provision these services
- •A signaling protocol for the setup, maintenance, and tear down of
optical trails, including lightpath signaling and maintenance
- In addition, control plane has to support network survivability based on
fault monitoring and protection and restoration.
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- In conclusion, we need to stress where optical networks were yesterday
and where they are today:
- Initially optical fibers were used as pipes to transport large volume of
traffic while all processing (intelligent) work was relegated to
electronics. Thus, multiplexing, switching and routing were done in
electronic domain. Optical transport was simply the sets of
point-to-point links.
- Today optical networks have reached the point where the need arise for
execution of all transport tasks in optical domain.
- Now we are in the transition stage.
- This is the trend to watch in the development of optical networks.
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- References:
- 1. Djafar K. Mynbaev, “Next-generation optical networks from network
layer and physical layer perspectives,” Tutorial presented at the 11th
International Conference on Telecommunications, Fortaleza, Brazil,
August 2004.
- 2. Manasi Deval et al, “Distributed Control Plane Architecture for
Network Elements,” Intel Technology Journal, November 14, 2003,
pp.51-63.
- 3. Uyless Black, Optical Networks, Prentice Hall, 2002.
- Yinghua Ye and Sudhir Dixit, “Surviavibility in IP-over-WDM Networks,”
in IP over WDM edited by Sudhir Dixit, Hoboken, N.J.: Wiley –
Interscience, 2003.
- Rajiv Ramaswami and Kumar N. Sivarajan, Optical Networks – A Practical
Perspective, 2nd ed., San Francisco: Morgan Kaufmann, 2002.
- Vivek Alwayn, Optical Network Design and Implementation, San Jose, CA:
Cisco Press, 2004.
- Arun K. Somani, Survivability and Traffic Grooming in WDM optical
Networks, New York: Cambridge University Press, 2006.
- John R. Vacca, Optical Networking Best Practices Handbook, Hoboken,
N.J.: Wiley – Interscience, 2007.
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- In the middle of the 1990s,
network operators started to experience a lack of fiber capacity (“fiber
exhaust”). WDM has become the solution to this problem.
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- By the end of the 1990s, new long-haul (backbone) optical networks had
been massively deployed and the use of WDM technology had dramatically
increased the capacity of these networks.
- These developments have placed major demands on metro networks. In
response, major research and development efforts in the optical
communications industry have been concentrated in this area As a result,
metro networks today are able to carry all required traffic.
- Because of a continuous increase in traffic growth, access networks have
become the bottleneck in the global communications infrastructure.
- All these advances are reflected in the progress of components
development and the equipment requirements for these networks.
- Long-haul č
metro č
access components
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- Components for different types of networks--Example: Transmitters
- Long-haul networksč Dense WDM
(DWDM) čexpensive cooled lasers.
- 160 channels within 1530-1625 nm (EDFA) bandwidth requires 0.4 nm
spacingčminimal
wavelength drift.
- Metro networksčCoarse
WDM. (CWDM)čless expensive uncooled lasers.
- 18 wavelengths within 1281-1611 nm bandwidth allows 20 nm spacingčno strict
requirements on wavelength
drift.
- PONčVery
coarse WDMčinexpensive uncooled lasers
- Three wavelengths: 1310 nm, 1490 nm and 1550mč Wavelength drift is not the issue.
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- Switches - introduction
- Two types:
- Optical/Electrical/Optical (O/E/O)
- Optical/Optical/Optical (O/O/O)
- Current situation:
- Most switches are O/E/O types.
- Migration to 40 Gbit/s and higher channel count will require O/O/O
switches.
- IP over WDM requires new approaches: burst and packet switching and
label switching.
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- MEMS optical switch: discussion
- Advantages:
- Easy manufacturing.
- Protocol independent.
- Wavelength independent č covers entire spectrum from 1280 nm to 1625 nm.
- Can be made in 2D and 3D configurations.
- Scalability: small-, intermediate-, and large-port switches can be
built.
- Problems:
- Conceptual: Digital operation vs. analog implementation.
- Specific:
- Precise analog electronic control is needed.
- Aging č
regular tuning is required.
- Relatively big insertion loss (from 3 dB).
- Relatively slow switching time
(several ms).
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- We need regeneration to support high bit rate. However, regeneration
presents the following main problems:
- It is the most costly operation in optical transport network.
- It causes delay in transmitting traffic.
- Regeneration sites have the highest failure rate in the network because
of heavy concentration of high-speed electronic and optical components.
- Regeneration – solutions: Avoid regeneration by all means. Specifically:
- Employ dispersion management strategy (the most practical)
- Make use of hybrid Raman/EDFA amplification (the most practical)
- Put into practice the new coding techniques (FEC, DPSK, etc.)
- Use new pre-compensation [6] and electronic compensation[7], [8]
techniques
- Example: One of the new ULH network will have up to 2,000 km of
unregenerated links with 40 Gb/s x 80 channels (scalable to 160
channels).
- All these solutions translate into the need for development of new
components. Examples: inexpensive E/0 converters and
dispersion-compensation modules.
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- EDFA: basic block diagram
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- Raman amplification: components
- Active medium: Transmission fiber for distributed amplification. Raman
gain coefficient varies for different fibers (SMF, NZDSF, DSF, etc.)
within 20%.
- Pump sources:Laser diodes (most common), Raman fiber lasers (noisy).
- WDM multiplexer: Must keep up to 1 W optical power. Dielectric (bulk
optics) and fused-fiber coupler.
- Optical monitoring: Provides control of the whole operation and
eye-safety control.
- .Polarization-beam splitter: Provides mixing of two polarized beams č depolarized pump
source.
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- See reading assignment and homework problems in the course’s outline.
- After study this module you must be able to:
- Explain the operation of point-to-point and WDM links.
- Describe local, metro and long-distance networks and the volume of
their traffic.
- Describe why different types of network require different types of
components.
- Explain operation of a coupler/splitter and its main characteristics.
- Explain the optical switch operation and list the main types of optical
switches.
- Describe operation of MEMS switches and their applications.
- Explain operation of OXC and OADM and their applications in optical
networks.
- Explain the difference between repeater and optical amplifier and list
drwabacks of a repeater.
- Discuss classification of optical amplifiers and their bandwidths.
- Explain operation of EDFA and its application.
- Explain operation of Raman amplifier and its application.
- Explain operation of SOA and its application.
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Notes
