P-P or Mesh

Point-to-Point or Mesh Topologies in

the Metro Optical Network

Definition According to point-to-point topology, one node connects directly to another node. Mesh is a network architecture that improves on point-to-point topology by providing each node with a dedicated connection to every other node.

Overview This tutorial highlights the key advantages of adopting a point-to-point strategy and eventual mesh topology, a new approach in transport technology.

Topics 1. Introduction 2. DWDM Mesh Designs Enabled by OXCs 3. Adopting DWDM Point-to-Point Solutions 4. Advantages of DWDM Point-to-Point Solutions 5. Conclusion Self-Test Correct Answers Glossary

1. Introduction According to Ryan, Hankin, Kent, Inc. (RHK), the San Francisco-based market research and consulting firm, Internet traffic will have reached 350,000 Terabytes per month as we pass into the new millennium. This is a significant milestone, as it indicates that data has already surpassed the voice network. To keep pace with seemingly insatiable demand for higher-speed access, a huge, complex, network-building process is beginning. Decisions made by network

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architects today will have an immense impact on the future profitability, flexibility, and competitiveness of network operators. Despite the dominance of synchronous optical network (SONET), a transport technology based on time division multiplexing (TDM), more and more operators consider adopting a point-to-point strategy and eventual mesh topology. This article highlights the key advantages of this new approach.

With such strong demand for wideband access1.5 million households already have cable or digital subscriber line (DSL) modems capable of operating at 1 Mbpsthere is no doubt that the future for service providers is extremely bright. However, there are a number of more immediate challenges that must be addressed. At the top of the list is the fact that network investments must be made before revenues are realized. As a result, there is a need for less complex and more efficient network builds. In an effort to cut network costs, action is being taken across several fronts: consolidating network elements, boosting reliability, reducing component system costs, and slashing operational costs. As far as optical networks are concerned, the action likely to make the most positive impact is the deployment of new network architectures, such as point-to-point/mesh designs. Ring architectures will still be supported, but new Internet protocol (IP) and asynchronous transfer mode (ATM) networks will find that mesh, with its well-defined optical nodes, lends itself to robust optical rerouting schemes (see Figure 1).

Figure 1. Ring, Point-to-Point, and Mesh Topologies

2. DWDM Mesh Designs Enabled by Optical Cross-Connects (OXCs) The emergence of OXCs represents a watershed in optical networking, as it enables point-to-point wavelength division multiplexing (WDM) technology to evolve into a next-generation transport platform. High-capacity, point-to-point trunks can now be deployed strategically as part of a true optical-networking solution. In addition to providing advanced routing capabilities, OXCs enable this functionality to be controlled by software and managed centrally. By leveraging the capabilities that OXCs provide, a mesh network can be built utilizing myriad



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point-to-point solutions and managed entirely at the optical layer. Metropolitan dense wavelength division multiplexing (DWDM) may easily be deployed in advance of the OXC, in anticipation of its arrival in the network as the first wave of deployment. This will allow the operator to build where capacity is needed and add the OXC later, as revenue and complexity demand (see Figure 2).

Figure 2. DWDM Mesh Enabled by OXC

3. Adopting DWDM Point-to-Point Solutions The key advantages of adopting point-to-point are pay-as-you-grow capability, provisioning flexibility, digitized signal transparency, and the ability to evolve seamlessly into a full-fledged mesh network. This enables network planners to tackle todays key challengefiber exhaustwith simple point-to-point solutions, while at the same time putting in place the building blocks of a sophisticated future optical network. They may still, of course, support their legacy rings with metropolitan DWDM as required, or even create ring-on-ring and ring-to-mesh topologies.

Unlike some topologies, the point-to-point strategy requires little upfront planning; random growth is quite acceptable. Even more importantly, network operators can pay as they grow, adding channels and equipment as they add customers. Furthermore, point-to-point enables service providers to receive revenue from an initial low-cost investment as soon as the first customer is activated. This is in stark contrast to the majority of network solutions that essentially require the entire network to be planned, built, and paid for before the first potential customer is even contacted. As a result, point-to-point is an ideal solution for competitive local exchange carriers (CLECs) seeking to enter new markets while minimizing start-up expenditures. Also, by utilizing the digitized signal transparency, exiting traffic can be handled on one wavelength while allowing the network to expand one channel at a time and allowing customers to be signed up without regard to traffic types (IP, ATM, video, SONET/synchronous digital digital hierarchy (SDH), enterprise systems connection (ESCON), ISC, Gigabit Ethernet, etc).



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By adding an OXC to point-to-point trunks, the entire nature of the system evolves from a simple growth requirement to a sophisticated, all-optical network (AON) that has DWDM as its transport technology. The increased sophistication provided by optical routing dramatically increases capabilities. Some of this routing capability may initially be in the metropolitan DWDM in the form of dynamic optical add/drop (OAD) modules (see Figure 3). Even though capabilities are extensive, however, the core network is extremely simple to plan and expand.

Figure 3. Adding an OXC

Existing DWDM network management systems already have the ability to provision point-to-point circuits, one channel at a time. Furthermore, it is possible to plan wavelengths on the fly and use wavelengths independently on each hop. For example, hop one might contain four channels while hop two might contain eight channels (see Figure 4). Eventually, with an OXC installed between each hop, system-level wavelength planning becomes unnecessary, and more flexible utilization of previously installed DWDM becomes available. There is absolutely no need to reserve a wavelength through the entire network just to have the ability to provide a channel to a specific site sometime in the future, as is now required in a TDM or OAD multiplexer (OADM)based system.

Figure 4. On-the-Fly Expansion per Hop



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4. Advantages of DWDM Point-to-Point Systems

Less Complex, More Efficient

The DWDM point-to-point architecture is inherently simple to build and troubleshoot. Unlike some systems, planners do not have to determine the ultimate capacity of the trunk prior to construction. Furthermore, they need not live with their capacity decision for years to come. With its fundamentally different multiplexing architecture, point-to-point DWDM enables protocol transparency, incremental growth, and capacity expansion over time, while dramatically reducing start-up costs.

Because DWDM is not TDM based, all channels are discrete and essentially stand independent of each other. Each channel card represents an individual customer or protocol. These can be added one at a time as colors are added; channel assignments need not be on adjacent channels, and different kinds of data or different speeds can be added over each channel. In the TDM world, the entire backbone must be in place at the outset for the total number of customers that are going to be on the circuit.

Point-to-point solutions are also extremely efficient. Every possible wavelength can be utilized without regard to the rest of the network. Furthermore, as long as the light can originate and terminate end to end, amplifiers are rarely needed. A typical system will transport effectively for up to 100 kilometers without an amplifier, which is a significant distance inside a metro area and more than enough to link central offices and facilities. With no amplifiers or additional equipment required, point-to-point represents a simple, cost-effective, and extremely efficient solution.

Incorporates Both an Add/Drop Multiplexer (ADM) and Digital Cross-Connect System (DCS)

The role of the ADM is to determine which channels remain at the site and which pass through. A point-to-point system with an OXC offers the same functionality as a discrete ADM. Even though specific add/drop nodes are used, the OXC can compliment the add/drop function. When the network sophistication requires such complexity, the OXC may be used in place of an OAD. Because additional equipment is not required, costs are reduced. There is also no electronic latency or point-of-component failure when one eliminates the need for a TDMbased DCS.



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Amplifiers are not standard equipment in every DWDM network and are only necessary if there is significant decibel loss in the span. If the light that is being generated at the origination site with substantial power sustains enough loss through the fiber route so that the light arrives below the specified receive sensitivity specification at the receiving end, only then are amplifiers required.

OADM nodes built from DWDM filters cause different optical channel-power levels for pass-through and inserted light. This all-optical, pass-through approach requires attenuators and amplifiers at each node to balance individual channel levels. Telcordias OADM standards stipulate amplifiers at each node for this reason. Point-to-point systems, however, contain O-E-O conversion at each end and therefore always regenerate the power into balanced channel levels on each hop, thus eliminating the need for custom attenuation and amplifiers. Ultimately, this method eliminates a costly single-point failure from the system while also eliminating the complex step of level balance planning.

Allows for Multiple Customers to Share a Wavelength

Placing an electronic photonic concentrator (EPC) at all nodes improves the economics at lower speeds. An EPC is a combination of optical and electrical technology that allows the division of DWDM channels into point-to-point subrate channels of varying bandwidth while maintaining total protocol and format transparency. As a result, low-speed channels that cannot economically justify their own wavelengths can be combined and placed on one wavelength. In addition to subrate multiplexing, EPCs provide direct optical connections to customer premises equipment (CPE). This technique elegantly provides subrate traffic to the mesh network (see Figure 5).

Figure 5. Allows Multiple Customers to Share



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Programmable Regenerators at Point-to-Point Transponders Support Usage-Based Circuit Pricing

Through regeneration, every receiver would reclock, retime, and reamplify the signal, cleaning, the pulse on every point-to-point hop. In addition to improving the performance of the signal, the regeneration feature enables the provisioning of speedif it is selectable at a specific site. As a result, network operators can offer different tiers of service and pricing to their customers. This also provides an excellent control mechanism, ensuring that customers receive exactly the quality of service (QoS) for which they are paying.

Regeneration is an electrical feature built inside each DWDM channel module. As a result, it is extremely cost-effective and requires no additional space. In the TDM world, regeneration typically requires the installation of a brand-new node. Essentially, the service provider must purchase a whole core multiplexer to regenerate the signal. However, no revenue-generating customers are added.

Open Systems and Interoperability, OXC Used at the Core

Differences exist between the light characteristics of DWDM channels as a result of the quality needed for different channel densities. A four-channel system's light used poorer tolerances than a 32-channel system. Frequency accuracy and channel width are just some of the important parameters that vary between systems. Thus, true end-to-end interoperability from different systems DWDM light signals will not be achieved unless all systems use the most dense and expensive specifications. This is not economical, and it does not allow for future systems of greater density to interoperate with current systems within the DWDM layer. If all systems accept and emit a standard light such as 1310 nm at their terminal interfaces, however, interoperability between nodes of different types and vendors will become simplified. Point-to-point systems of virtually all vendors can accept and deliver 1310 nm and thus can use a common standard for interconnection. Point-to-point DWDM with optical cross-connection allows for lower-density systems to connect to higher-density systems and supports a mixed-vendor environment for end-to-end circuits.

Fewer Nodes Are Needed

The pay-as-you-grow nature of point-to-point dictates that nodes must only be installed when fiber exhaust is reached or when a new customer is to be added. This contrasts sharply to planning the entire network and building rings for all sites, even though many sites may have no customers.



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Metropolitan DWDM and Later OXCs Can Be Provisioned at the Time of Installation for the Required Method of Protection

This enables the transition from bidirectional line switched ring (BLSR) to mesh and 1 for n restoration schemes. BLSR is a method of SONET transport in which half of the working network is sent counter-clockwise over one fiber, and the other half is sent clockwise over another fiber. Protection fibers are shared across the entire ring. If any connection between two sites breaks, the protection fibers are used to reheal the link by traveling around the ring in the other direction. This requires complex software, and its function must be planned in advance of building the network. It also requires knowledge of the entire fiber network that may not be available in the new environment of shared, co-opted, and sublet fiber networks.

As a feature of the AON, protection against a fiber break can be provided in a mesh network on a per-channel basis. Providers and customers can choose which circuits to protect and how much protection is needed at the time of installation. Thus, network operators can offer tiered pricing for different levels of protection. With this higher level of flexibility and greater granularity, service providers can arrange multiple protection routes if the customer so desires and is willing to pay. In the TDM world, one-size-fits-all is still the rule, and all decisions regarding circuit protection must be made prior to the initial build out.

AONs Provide Provisioning, Including Wavelength (Lambda) Assign and Conversion for Many Routes at a Node Site

Traffic arriving on one point-to-point system at a site can be easily transferred to another system via the cross-connect function, which could be simple fiber jumpers or a modular cross-connect system (see Figure 6). Circuits are built to pass between the DWDM nodes, regardless of channel numbers of each span.

Figure 6. Provisioning Flexibility



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Small Sites Can Start with Basic Metropolitan DWDM Systems until Traffic Justifies an OXC or Combination of the Two

A start-up CLEC, for example, would find it difficult to justify purchasing an OXC with 512 ports if only two circuits were in use. With point-to-point, it is possible to use small DWDM systems at the outset until there are enough customers to warrant an investment in a software-control system. This ability enables service providers to enter small markets more cost-effectively and be extremely price-competitive with existing providers that are recouping revenues to pay for much larger networks.

Shorter Distances Can Nullify Most Common Barriers

Lower launch power can be used to engineer and terminate a point-to-point system. This leads to lower costs and a safer network. In addition, as a result of lower power, the incidents of cross-talk and mixing are reduced considerably. Furthermore, in a point-to-point scenario, there is no need to use a high-power, long-haul system. In a ring topology, this may be required, as optical nodes are subject to high loss. Combating this would require multiple amplifiers or high launch power.

With point-to-point, distances are also shorter between nodes. If the light is being terminated at every mesh site, then dispersion is not a problem. Dispersion is a particular form of distortion that happens to all light signals as they traverse distance, and, as the signal degrades over every kilometer, errors occur. To combat this, the complicated procedure of dispersion calculations must be completed for circuits without regeneration.

Point-to-point also simplifies circuit management and fault isolation. If every point is monitored, it is an easy procedure to conduct segment-by-segment troubleshooting to identify the breaking point. With a point-to-point metropolitan DWDM system and future OXC combination, it is relatively simple to switch service from the problem channel to a working channel from a central console.

Pay-as-You-Grow Defers Costs

As mentioned previously, point-to-point enables providers to pay to upgrade one channel at a time instead of having to install a costly core system with multiple OADMs and amplifiers.



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Investment Protection/Return on Investment

By following a strategy of point-to-point DWDM systems, providers can overcome the immediate challenge of fiber exhaust. Moreover, by adding OXCs to metropolitan DWDM systems, it is possible to evolve seamlessly into a fully optical network, which guarantees interoperability within and also interfaces to other optical networks.

As for reliability, all circuit protection issues can be handled through features of the metropolitan DWDM and the OXC, including wavelength translation between hops. If regeneration is used, a point-to-point circuit that leverages a cross-connect can cover vast distances. For example, a firm in Southern California could build an end-user circuit using cross-connected city metro systems through many communities throughout the state without having to install a specific long-haul route, as long as there is connectivity between the various metro areas along the way. The additional advantage is that the systems many points of presence (PoPs) make it a blended metropolitan and long-distance system so that delivering service to customers along the routes is very convenient. Even alternate routing choices of more than two paths can be built for each channel independently, without resorting to costly ring-layer, protect-all-channels designs.

A mesh topology is also an ideal compliment to the high demand for data networking and packet-based transport. While telecommunications providers have typically built networks based on rings as a result of their self-healing and rerouting properties, data backbones have always been mesh designs. This is also partly a result of the complex interconnection of IP routers. Certainly, a mesh topology shared by both IP and transport networks provides an elegant solution. Moreover, a mesh with optical routing can also provide self-healing capabilities without the complexity associated with ring software. Alternate routing through the mesh either by the router, switch, or optical network becomes an apparent solution.

5. Conclusion With a seamless migration path that leverages existing equipment with digitized signal transparency, mesh topologies beginning with point to point DWDM systems offer an ideal solution to todays fiber exhaust challenge while laying the foundation for tomorrows fully optical mesh network. Featuring network protection, simplicity of construction and incremental growth at reasonable cost, it is apparent why the mesh topology is gaining in popularity among todays network providers.



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Self-Test 1. Data has already surpassed the voice network.

a. true

b. false

2. Which of the following enables point-to-point WDM technology to evolve into a next-generation transport platform?

a. IP

b. ATM

c. SONET

d. OXC

3. Which of the following may be easily deployed in advance of the OXC?

a. ATM

b. SONET

c. DWDM

d. IP

4. Which of the following does not describe the point-to-point strategy?

a. pay-as-you-grow capability

b. much upfront planning

c. digitized signal transparency

d. provisioning flexibility

5. Existing DWDM network management systems already have the ability to provision point-to-point circuits, one channel at a time.

a. true

b. false



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6. Data backbones have always been ring designs.

a. true

b. false

7. A point-to-point system with an OXC offers the same functionality as a discrete ADM.

a. true

b. false

8. Which of the following is not true of point-to-point systems that contain O-E-O conversion on each end?

a. Power is regenerated into balance channels on each hop.

b. The need for custom attenuation and amplifiers is eliminated.

c. The need for level balance planning is increased.

d. A costly single-point failure is eliminated from the system.

9. Regeneration is an electrical feature that is extremely cost-effective and requires no additional space.

a. true

b. false

10. Which of the following is not true of a point-to-point system?

a. Lower launch power can be used.

b. Distances are longer between nodes.

c. High-power, long-haul systems are not necessary.

d. Costs are lower.

Correct Answers 1. Data has already surpassed the voice network.

a. true



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b. false

See Topic 1.

2. Which of the following enables point-to-point WDM technology to evolve into a next-generation transport platform?

a. IP

b. ATM

c. SONET

d. OXC

See Topic 2.

3. Which of the following may be easily deployed in advance of the OXC?

a. ATM

b. SONET

c. DWDM

d. IP

See Topic 2.

4. Which of the following does not describe the point-to-point strategy?

a. pay-as-you-grow capability

b. much upfront planning

c. digitized signal transparency

d. provisioning flexibility

See Topic 3.

5. Existing DWDM network management systems already have the ability to provision point-to-point circuits, one channel at a time.

a. true

b. false

See Topic 3.



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6. Data backbones have always been ring designs.

a. true

b. false

See Topic 3.

7. A point-to-point system with an OXC offers the same functionality as a discrete ADM.

a. true

b. false

See Topic 4.

8. Which of the following is not true of point-to-point systems that contain O-E-O conversion on each end?

a. Power is regenerated into balance channels on each hop.

b. The need for custom attenuation and amplifiers is eliminated.

c. The need for level balance planning is increased.

d. A costly single-point failure is eliminated from the system.

See Topic 4.

9. Regeneration is an electrical feature that is extremely cost-effective and requires no additional space.

a. true

b. false

See Topic 4.

10. Which of the following is not true of a point-to-point system?

a. Lower launch power can be used.

b. Distances are longer between nodes.

c. High-power, long-haul systems are not necessary.

d. Costs are lower.



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See Topic 4.

Glossary ADM add/drop multiplexer

AON all-optical network

ATM asynchronous transfer mode

BLSR bidirectional line switched ring

CLEC competitive local exchange carrier

CPE customer premises equipment

DCS digital cross-connect system

DSL digital subscriber line

DWDM dense wavelength division multiplexing

EPC electronic photonic concentrator

ESCON enterprise systems connection

IP Internet protocol

OAD optical add/drop

OADM optical add/drop multiplexer



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OXC optical cross-connect

PoP point of presence

QoS quality of service

SDH signal digital hierarchy

SONET synchronous optical network

TDM time division multiplexing

WDM wavelength division multiplexing

DefinitionOverview1. Introduction2. DWDM Mesh Designs Enabled by Optical Cross-Connects (OXCs)3. Adopting DWDM Point-to-Point Solutions4. Advantages of DWDM Point-to-Point Systems5. ConclusionSelf-TestCorrect AnswersGlossary

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