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Introduction Chapter 1

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Outlines

• What is a WDM-enabled optical network?

• Why IP over WDM?

• What is IP over WDM?

• Next-generation Internet

• IP/WDM standardisation

• Summary and subject overview

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1.1 What is a WDM-enabled Optical Network?

• Conventional copper cables can only provide a bandwidth of 100 Mbps (106) over a 1 Km distance before signal regeneration is required.

• In contrast, an optical fibre using wavelength division multiplexing (WDM) technology can support a number of wavelength channels, each of which can support a connection rate of 10 Gbps (109).

• Long-reach WDM transmitters and receivers can deliver good quality optical signals without regeneration over a distance of several tens of kilometres. Hence, optical fibre can easily offer bandwidths of tens of Tbps (1012).

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What is a WDM-enabled Optical Network?

• In addition to high bandwidth, fibre, made of glass (which is in turn made mainly from silica sand), is cheaper than other conventional transmission mediums such as coaxial cables.

• Glass fibre transmission has low attenuation. • Fibre also has the advantage of not being affected by electrom

agnetic interference and power surges or failures. • In terms of installation, fibre is thin and lightweight, so it is ea

sy to operate. • An existing copper-based transmission infrastructure can be (a

nd has been) replaced with fibre cables.

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What is a WDM-enabled Optical Network?

• In the fibre infrastructure, WDM is considered as a parallel transmission technology to exploit the fibre bandwidth using non-overlapping wavelength channels.

• An individual optical transmission system consists of three components:– the optical transmitter– the transmission medium– the optical receiver.

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WDM• The transmitter uses a pulse of light to indicate the ‘1’ bit and the abse

nce of light to represent the ‘0’ bit. • The receiver can generate an electrical pulse once light is detected. • A single-mode fibre transmission requires the light to propagate in a st

raight line along the centre of the fibre. – a good quality signal, – used for long-distance transmission.

• multimode fibre – A light ray may enter the fibre at a particular angle and go throug

h the fibre through internal reflections. – The basic optical transmission system is used in an optical network,

which can be a local access network (LAN), – a metropolitan local exchange network (MAN), or a longhaul inte

r-exchange network (also known as Wide Area Network, WAN).

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TDM vs. WDM• There is a continuous demand for bandwidth in the construction of the

Internet. • It is also relatively expensive to lay new fibres and furthermore to mai

ntain them.• Time Division Multiplexing (TDM)

– is achieved through multiplexing many lower speed data streams into a higher speed stream at a higher bit rate by means of nonoverlapping time slots allocated to the original data streams.

• Wavelength Division Multiplexing (WDM) – is used to transmit data simultaneouslyat multiple carrier wavelen

gths through a single fibre, which is analogous to using Frequency Division Multiplexing (FDM) to carry multiple radio and TV channels over air or cable.

• TDM and WDM can be used together in such a way that TDM provides time-sharing of a wavelength channel, for example, through aggregating access network traffic for backbone network transport.

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TDM vs. WDM

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Cost of WDM

• Splitting the useable wavelength bandwidth into a number of slots (wavelength channels) not only demands sophisticated equipment but also increases the likelihood of inter-channel interference.

• As such, WDM equipment cost may dominate the total cost in a LAN and MAN environment.

• As of November 2001, commercial WDM optical switches are able to support 256 wavelength channels, each of which can support a data rate of OC-192 (10 Gbps).

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A point-to-point WDM-enabled optical transmission

• Wavelength Add/Drop Multiplexer (WADM)

• Wavelength Amplifier (WAMP)

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Double-ring WDM network• A WDM-enabled optical network employs point-t

o-point WDM transmission systems and requires – Wavelength Selective Crossconnect (WSXC) that is ab

le to switch the incoming signal unto a different fibre possibly a different optical frequency.

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1.1.2 WDM Optical Network Evolution

• The first generation of WDM provides – only point-to-point physical links that are confined to WAN trunks. – configurations are either static or use manual configurations. – only supports relatively low-speed end-to-end connectivity. – The technical issues include design and development of WDM lasers a

nd amplifiers, and static wavelength routing and medium access protocols.

• The WADM can also be deployed in MANs, • To interconnect WADM rings, Digital Cross Connects (DCX)

are introduced to provide narrowband and broadband connections.

• Generally these systems are used to manage voice switching trunks and T1 links.

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1.1.2 WDM Optical Network Evolution

• The second generation of WDM– is capable of establishing connection-orientated end-to-end lightpaths i

n the optical layer by introducing WSXC. – The lightpaths forma virtual topology over the physical fibre topology. – The virtual wavelength topology can be reconfigured dynamically in re

sponse to traffic changes and/or network planning.• The technical issues include

– the introduction of wavelength add/drop and cross-connect devices, – wavelength conversion capability at cross-connects, and – dynamic routing and wavelength assignment.

• network architecture begins to receive attention, • In particular the interface for interconnection with other networ

ks.• Their cost efficiency in long-haul networks has been widely ac

cepted.

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WDM Optical Network Evolution

• The third-generation of WDM – offers a connectionless packet-switched optical network, in which optic

al headers or labels are attached to the data, transmitted with the payload, and processed at each WDM optical switch. Based on the ratio of packet header processing time to packet transmission cost, switched WDM can be efficiently implemented using label switching or burst switching.

– Pure photonic packet switching in all optical networks is still under research.

• The bufferless, all-optical packet router brings a new set of technical issues for network planning:– contention resolution;– traffic engineering;– over-provisioning;– over-subscription;– interoperability with conventional IP (Internet Protocol) routers destina

tion-based routing).

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WDM Optical Network Evolution

• Examples of third-generation WDM devices are:– optical label switch routers– optical Gigabit routers– fast optical switches.

• Interoperability between WDM networks and IP networks becomes a major concern in third-generation WDM networks.

• Integrated routing and wavelength assignment based on MultiProtocol Label Switching (MPLS), also known as Generalised MPLs (GMPLS), starts to emerge.

• bandwidth management, path reconfiguration and restoration, and Quality of Service (QoS) support.

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Evolution

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Switching method

• Optical circuit switching – is used for large-sized aggregated traffic (such as data trunks), so that

once circuits are set up, the formed topology does not change often. – This provides cost-efficiency in the long haul network because a few

add/drop points are needed by the traffic and only physical transport link services are required.

• Optical packet switching – is used for small-sized data packets. It offers efficient, flexible resource

sharing by introducing complexity to the control system.

• Optical burst switching– A compromise between packet and circuit switching,– which switches traffic bursts over a packet-orientated network.

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cut-through paths

– Packet switching offers cut-through paths, known as ‘layer 2 switching’.

– The cut-through paths lower the network latency by avoiding intermediate node layer 3 functions.

– A packet routed network employs a store-and-forward paradigm, where each node maintains a routing table and a forwarding table.

– Once a packet arrives at the node, by comparing the packet header with the local forwarding table, the packet is sent to the next hop on the routing path.

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1.2 Why IP over WDM?

• IP provides the only convergence layer in the global and ubiquitous Internet.

• IP, a layer 3 protocol, is designed to address network level interoperability and routing over different subnets with different layer 2 technologies.

• Above the IP layer, there are a great variety of IP-based services and appliances that are still evolving from its infancy.

• Hence, the inevitable dominance of IP traffic makes apparent the engineering practices that the network infrastructure should be optimised for IP.

• Below the IP layer, optical fibre using WDM is the most promising wireline technology, offering an enormous network capacity required to sustain the continuous Internet growth.

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WDM• WDM-based optical networks have been deployed not only in the backbon

e but also in metro, regional, and access networks. • In addition, WDM optical networks are no longer just point-to-point pipes

providing physical link services, but blend well with any new level of network flexibility requirements.

• The control plane is responsible for transporting control messages to exchange reachability and availability information and computing and setting up the data forwarding paths.

• The data plane is responsible for the transmission of user and• application traffic. An example function of the data plane is packet bufferin

g and forwarding. • IP does not separate the data plane from the control plane, and this in turn r

equires QoS mechanisms at routers to differentiate control messages from data packets.

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IP over WDM• A conventional WDM network control system uses a separate control chan

nel, also known as a data communication network (DCN), for transporting control messages.

• A conventional WDM network control and management system, e.g.• according to the TMN framework, is implemented in a centralised fashion. • To address scalability, these systems employ a management hierarchy. • Combining IP and WDM means, in the data plane, one can assign WDM o

ptical network resources to forward IP traffic efficiently, and in the control plane, one can construct a unified

• control plane, presumably IP-centric, across IP and WDM networks. IP over WDM will also address all levels of interoperability issues on intra- and inter-WDM optical networks and IP networks.

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The motivation behind IP over WDM

• WDM optical networks can address the continuous growth of the Internet traffic by exploiting the existing fibre infrastructure. The use of WDM technology can significantly increase the use of the fibre bandwidth.

• Most of the data traffic across networks is IP. Nearly all the end-user data application uses IP. Conventional voice traffic can also be packetised with voice-over-IP techniques.

• IP/WDM inherits the flexibility and the adaptability offered in the IP control protocols.

• IP/WDM can achieve or aims to achieve dynamic on-demand bandwidth allocation (or real-time provisioning) in optical networks.– By developing the conventional, centralised controlled optical networks into a

distributed, self-controlled network, the integrated IP/WDM network can not only reduce the network operation cost, but can also provide dynamic resource allocation and on-demand service provisioning.

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The motivation behind IP over WDM

• IP/WDM hopes to address WDM or optical Network Element (NE) vendor interoperability and service interoperability with the help of IP protocols.

• IP/WDM can achieve dynamic restoration by leveraging the distributed control mechanisms implemented in the network.

• From a service point of view, IP/WDM networks can take advantage of the QoS frameworks, models, policies, and mechanisms proposed for and developed in the IP network.

• Given the lessons learned from IP and ATM integration, IP and WDM need a closer integration for efficiency and flexibility. For example, classical IP over ATM is static and complex, and IP to ATM address resolution is mandatory to translate between IP addresses and ATM addresses.

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What is IP over WDM?

• IP/WDM network is designated to transmit IP traffic in a WDM-enabled optical network to leverage both IP universal connectivity and massive WDM bandwidth capacity.

• IP, as a network layer technology, relies on a data link layer to provide:– framing (such as in SONET or Ethernet);

– error detection (such as cyclic redundancy check, CRC);

– error recovery (such as automatic repeat request, ARQ).

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All-optical network• An objective of optical networking is to provide optical transparency frome

nd to end so that the network latency is minimised. • This requires all-optical interfaces and all-optical switching fabric for the e

dge and intermediate network elements. • Transponders are used to strengthen the optical signal.

– all-optical transponders (tunable lasers) and – Optical-Electrical-Optical (O-E-O) transponders.

• The figure shows two types of traffic, IP (e.g. Gigabit Ethernet) and SONET/SDH, which in turn requires Gigabit Ethernet and SONET/SDH interfaces.

• In the case of multiple access links, a sublayer of the data link layer is the Media Access Protocol (MAC) that mediates access to a shared link so that all nodes eventually have a chance to transmit their data.

• The definition of a protocol model to efficiently and effectively implement an IP/WDM network is still an active research area.

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Possible approaches for IP over WDM

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IP/ATM/SONET/WDM

• transports IP over ATM (Asynchronous Transfer Mode), then over SONET/SDH and WDM fibre.

• WDM is employed as a physical layer parallel transmission technology.

• The main advantage of this approach by using ATM is – to be able to carry different types of traffic onto the same pipe with diff

erent QoS requirements.– its traffic engineering capability and the flexibility in network provisio

ning, which complements the conventional IP best effort traffic routing.

• Disadvantage– offset by complexity, as IP over ATM is more complex to manage and

control than an IP-leased line network.

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IP/ATM

• ATM uses a cell switching technology. Each ATM cell has a fixed 53-byte (5-byte header and 48-byte user data) length, so application traffic has to be packetised into cells for transport and reassembled at destination.

• ATM cell packetisation is the responsibility of the ATM SAR (Segmentation and Reassembly) sublayer.

• SAR becomes technically difficult above OC-48. • Having an ATM circuit layer between IP packet and the WDM

circuit seems superfluous. • The statement is strengthened by the emergence of the MPLS t

echnique of the IP layer.

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key features of MPLS

• Use of a simple, fixed-length label to identify flows/paths.• Separating control from data forwarding, control is used to set

up the initial path, but packets are shipped to next hop according to the label in the forwarding table.

• A simplified and unified forwarding paradigm, IP headers are processed and examined only at the edge of MPLS networks and then MPLS packets are forwarded according to the ‘label’ (instead of analysing the encapsulated IP packet header).

• MPLS provides multiservice. For example, a Virtual Private Network (VPN) set up by MPLS has a specific level of priority indicated by the Forwarding Equivalence Class (FEC).

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key features of MPLS (count.)

• Classification of packets is policy-based, with packets being aggregated into FEC by the use of a label. The packet-to-FEC mapping is conducted at the edge, for example, based on the class of service or the destination address in the packet header.

• Providing enabling mechanisms for traffic engineering, which can be employed to balance the link load by monitoring traffic and making flow adjustments actively or proactively. In the current IP network, traffic engineering is difficult if not impossible because traffic redirection is not effective by indirect routing adjustment and it may cause more congestion elsewhere in the network. MPLS provides explicit path routing so it is highly focused and offers class-based forwarding. In addition to explicit path routing, MPLS offers tools of tunneling, loop prevention and avoidance, and streams merging for traffic control.

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IP/SONET/WDM

• IP/MPLS over SONET/SDH and WDM.• SONET/SDH provides several attractive

features to this approach:– SONET provides a standard optical signal

multiplexing hierarchy by which low-speed signals can be multiplexed into high-speed signals.

– SONET provides a transmission frame standard.– the SONET network protection/restoration

capability, which is completely transparent to upper layers such as the IP layer.

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IP/SONET/WDM

• SONET networks usually employ a ring topology. SONET protection scheme can be provided:– as 1 + 1 meaning data are transferred in two paths in the opposite direct

ion and the better signal is selected at the destination;– as 1:1 indicating there is a separate signalled protection path for the pri

mary path;– or as n:1 representing where primary paths share the same protection p

ath.

• The design of SONET also enhances OAM&P (Operations, Administration, Maintenance, and Provisioning) to communicate alarms, controls, and performance information at both system and network levels.

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SONET/SDH

• However, SONET carries substantial overhead information, which is encoded in several levels. – Path overhead (POH) is carried from end-to-end.

– Line overhead (LOH) is used for the signal between the line terminating equipment, such as OC-n multiplexers.

– Section overhead (SOH) is used for communication between adjacent network elements, such as regenerators.

• For an OC-1 pipe with 51.84 Mbps transmission rate, its payload has the capacity to transport a DS-3 with 44.736 Mbps digital bit rate.

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3G

• IP/MPLS directly over WDM, • the most efficient solution among the possible approaches. • It requires that the IP layer looks after path protection and rest

oration. • It also needs a simplified framing format for transmission erro

r handling. – Several companies are developing a new framing standard known as Sl

im SONET/SDH, which provides similar functionality as in SONET/SDH but with modern techniques for header placement and matching frame size to packet size.

– adopt the Gigabit Ethernet framing format. The new 10-Gigabit Ethernet is especially designed for dense WDM systems. Using the Ethernet frame format, hosts (Ethernet) on either side of the connection do not need to map to another protocol format (e.g. ATM) for transmission.

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Signal

• Conventional IP networks use in-band signalling so data and control traffic is transported together over the same link and path.

• A WDM optical network has a separate data communication network for control messages. Hence, it uses out-of band signalling.

• In the control plane, IP over WDM can support several networking architectures, but the architecture selection is subject to constraints on existing network environments, administrative authority, and network ownership.

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Signalling

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1.4 Next-generation Internet

• US Internet-related research and development partnerships include not only entities that are directly focused on Internet development such as IETF but also general standard organisations such as IEEE and ANSI and federal government agencies such as DARPA (Defense Advanced Research Projects Agency) (www.darpa. mil) and NSF (National Science Foundation) (www.nsf.gov).

• The Next Generation Internet (NGI) initiative (www.ngi.gov) was established in 1998 for the period of 5 years, through which government agencies will cooperate to create next generation Internet capabilities to allow for enhanced support for their core missions, as well as to advance the state-of-the-art in advanced networking.

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NGI initiative will

• develop new and more capable networking technologies to support Federal agency missions;

• create a foundation for more powerful and versatile networks in the 21st century;

• Form partnerships with academia and industry that will keep the US at the cutting edge of information and communication technologies;

• enable the introduction of new networking services that will benefit businesses, schools, and homes.

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NGI goals

• conducting research in advanced end-to-end networking technologies, including differentiated services, particularly for digital media, network management, reliability, robustness, and security;

• prototyping and deploying national-scale testbeds that are able to provide 100 to 1000 times current Internet performance;

• developing revolutionary new applications requiring high performance networks.

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SuperNet testbed

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SuperNet streamline networking protocol stacks

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CANARIE

• the Canadian Network for the Advancement of Research, Industry, and Education, is a non-profit corporation supported by its members, project partners and the Canadian government to accelerate Canada’s advanced Internet development and facilitate the widespread adoption of faster, more efficient networks and enable the next generation of advanced products, applications and services.

• In February 1998, the Canadian government provided CANARIE with a $55 million grant towards the $120 million project to develop a national optical R&D Internet, known as CA*net III. Industry members provided the remaining part of the funding.

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Canada

• Using new fibre optic-based technology and Dense Wavelength Division Multiplexing (DWDM), CA*net 3 intends to deliver unrivalled network capability with a potential for OC768 (40 Gbps) to Canadian research institutions and universities.

• Phase I was completed in October 1998, where an optical Internet backbone was set up between Toronto, Ottawa and Montreal. Currently Phase II is in progress,

• through which the optical Internet Backbone will be extended west from Toronto to Vancouver and east from Montreal to Atlantic Canada.

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European &Asia

• European ACTS (Advanced Communications Technologies and Services) program.

• In addition, many European NRNs (National Research Networks) have established national high-performance advanced network infrastructures.

• In Asia Pacific, a number of countries have participated in the APAN (Asia Pacific Advanced Networking) initiative.

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1.5 IP/WDM Standardisation

• the Internet Engineering Task Force (IETF) (www.ietf.org) and

• the International Telecommunication Union, Telecommunication Standardisation Sector (ITU-T) (www.itu.org) respectively.

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• In particular, IETF has been focusing on these IP/WDM-related issues:– MPLS/MPl S (Multiprotocol Lambda Switching)/GMPLS (Generalize

d MPLS).– Layer 2 and layer 3 functionalities within optical networks.– NNI (Network to Network Interface) standard for optical network.

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ITU-T

• In particular, ITU-T has been focusing on these IP/WDM related issues:– Layer 1 features of the OSI model.– Architectures and protocols for next-generation opt

ical networks, also known as optical transport network (OTN), defined in G.872.

– Architecture for the automatic switched optical network, defined in G.ason.