Optical Networks: A Practical Perspective - Part 73 pps

The first step in the design process is to route the end-to-end traffic and determine the amount of working and protection capacity required.. For 1 + 1 protection, we have to calculate

690 DEPLOYMENT CONSIDERATIONS

Table 13.1 Traffic matrix for the long-haul mesh network case study The fiber topology is shown

in Figure 13.8(a) The traffic is shown in terms of the number of 10 Gb/s wavelengths between pairs

of nodes in the upper-right triangle of this matrix

Number Name 1 2 3 4 5 6 78 9 1 0 1 1 1213 1415 1 6 1 7 1 8 19 Traffic

0 2 2 2 2 1 2 3 3 1 3 3 2 1 3 3 2 3 3 41

0 0 3 2 3 3 3 3 2 1 1 1 1 1 1 2 3 2 1 35

0 0 0 1 1 2 3 1 1 3 1 2 1 3 1 1 3 1 3 33

0 0 0 0 2 1 1 3 2 1 2 3 1 2 1 2 2 3 1 32

0 0 0 0 0 1 2 3 2 2 3 2 1 1 3 2 1 2 1 34

0 0 0 0 0 0 2 2 3 2 1 3 2 2 3 1 1 2 2 34

0 0 0 0 0 0 0 1 3 2 2 3 3 3 3 2 3 2 1 41

0 0 0 0 0 0 0 0 1 2 1 3 1 2 1 3 1 1 1 33

0 0 0 0 0 0 0 0 0 1 2 3 3 1 1 2 3 1 1 35

0 0 0 0 0 0 0 0 0 0 1 3 2 2 3 3 3 3 1 36

0 0 0 0 0 0 0 0 0 0 0 2 3 3 1 2 2 3 1 34

0 0 0 0 0 0 0 0 0 0 0 0 2 1 2 3 1 1 2 40

0 0 0 0 0 0 0 0 0 0 0 0 0 3 2 1 2 2 3 35

0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 1 2 2 1 33

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 2 32

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 1 1 33

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 3 37

0 0 0 0 1 3 0 0 0 0 0 0 0 0 0 0 0 0 0 2 34

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 30

as ultra-long-haul (ULH) systems We also look at the benefits of different types of protection architectures

The network of Figure 13.8(a) has 19 nodes and 28 links interconnecting the nodes Table 13.1 shows the assumed traffic matrix between the various nodes in terms of 10 Gb/s channels The total end-to-end traffic amounts to 3.31 Tb/s and represents a fairly realistic network in the 2 0 0 2 - 2 0 0 3 time frame

The first step in the design process is to route the end-to-end traffic and determine the amount of working and protection capacity required Sophisticated algorithms are used to perform this function in practice, but we use fairly simple algorithms for this study For 1 + 1 protection, we have to calculate a pair of working and protection paths which are node disjoint, that is, do not have any intermediate nodes (and links)

in common This ensures that the protection path will be available in case a node or link along the working path fails We choose the working path as the shortest-length path between the end nodes To calculate the protection path for a given pair of

end nodes, we delete the intermediate nodes in the working path between those two nodes, and calculate the shortest-length path in the resulting topology

For shared mesh protection, we use the same working and protection paths as in the 1 + 1 protection case However, we do not need to allocate protection capacity for each path separately Instead we provide only as much protect capacity as is needed

to reroute the working paths affected by a single link failure To do this, we calculate the protection capacity required on the links for every possible link failure and take the maximum over all possible link failures

Table 13.2 shows the assumed link distances and the number of 10 Gb/s wave- lengths required on each link as a result of the routing and capacity allocation discussed above Even though the end-to-end traffic requirement between any pair

of nodes is no more than 30 Gb/s (three 10 Gb/s wavelengths), there are several links that carry more than 100 wavelengths (or equivalently over i Tb/s of capacity) For example, the Denver-Kansas City link carries 77 working wavelengths and 78 pro- tection wavelengths (in the case of 1 + 1 protection), or 41 protection wavelengths (in the case of shared mesh protection) In many of these links, we will end up using multiple W D M systems in parallel to meet the capacity demand

We assume each of the 19 nodes has one or more electrical core crossconnects The crossconnects terminate all the traffic at the node, including both traffic pass- ing through the node as well as traffic being added/dropped at the node Thus, there is no optical passthrough at the nodes Table 13.3 shows the number of crossconnect ports required for the 1 + 1 and shared mesh protection cases Each node requires a few hundred such ports For 1 + 1 protection, the largest node is Nashville, which has 566 ports and handles 5.66 Tb/s of traffic For shared mesh pro- tection, the largest node is Kansas City, which has 413 ports and handles 4.13 Tb/s

of traffic

The next step in the design is to cost out the network, based on the type and quantity of equipment deployed at all the sites Table 13.4 shows the capabilities and costs of the LH and ULH systems assumed for this study, as well as the crossconnects Table 13.5 shows the quantity of different types of LH and ULH equipment and crossconnects required to support the link distances and capacities shown in Table 13.2 Figure 13.12 shows the corresponding network costs in graphical form and illustrates how the network cost varies with the different options as well as the cost breakdown among the various components Observe that both ULH and mesh protection provide cost savings Also, with this model, the amplifier cost is relatively small compared to the cost of transponders/regenerators and crossconnects

Note that we have assumed the use of crossconnects for both the 1 + 1 case and the shared mesh case Crossconnects are essential in the shared mesh scenario, as they are the ones that provide this capability However, 1 + 1 protection can be implemented directly by the transponders, and we do not need crossconnects for this purpose

692 DEPLOYMENT CONSIDERATIONS

Table 13.2 Link distances in the network topology of Figure 13.8(a) Also shown are the number

of wavelengths required on each link to support the working traffic and the protection traffic for the cases of 1 + 1 and shared mesh protection, assuming the traffic matrix of Table 13.1

(km) Capacity Capacity Capacity

1 + 1 Shared Mesh

At the intermediate nodes, passthrough connections can be patched through using manual patch panels However, if full flexibility is desired in provisioning end-to-end connections, then crossconnects will be needed in both cases

The outcome of the study depends critically on the relative cost and capabilities of different types of equipment, and the routing algorithm used For instance, we have assumed that there is a small premium in cost for ULH amplifiers and transponders relative to their LH counterparts, and a small decrease in number of wavelengths

Table 13.3 Number of crossconnect ports required at each of the 19 nodes in the case of 1+1 and shared mesh protection In 1 + 1 protection, each add/drop wavelength consumes three crosscon- nect ports, one for the local add/drop, one on the working path, and one on the protection path The passthrough traffic consists of both working and protection traffic not terminating at the local node In the shared mesh case, each add/drop wavelength consumes one port for the local add/drop and one additional port for the working path The passthrough ports include ports to carry all the working traffic passing through the node, as well as all the ports reserved for shared protection

1 + 1 Protection Shared Mesh Protection

per system If the relative cost changes, the study conclusions can change quite substantially Figure 13.13 plots the relative cost of LH and ULH options as a function of the relative cost of transponders (and regenerators) and amplifiers

We have only touched some of the issues affecting network design A number

of additional factors need to be taken into account while designing a more realistic network:

9 We can use LH systems on shorter links and ULH systems on longer links to optimize the cost further

9 M a n y ULH systems include optical add/drop capability to pass through signals at intermediate nodes in the optical domain, rather than requiring all wavelengths to

694 DEPLOYMENT CONSIDERATIONS

Table 13.4 Characteristics of the equipment used in the backbone network

study All costs are in thousands of U.S dollars The ULH amplifier and

transponder costs are somewhat higher compared to their LH counterparts,

and the ULH system has fewer wavelengths than the LH system For terminals

(including transponders), regenerators, and crossconnects, there is a common

equipment cost, and in addition a cost per port equipped For example, an

LH terminal equipped with 10 transponders would cost $800,000, and a

crossconnect equipped with two ports would cost $380,000

LH System

Number of wavelengths per system

Spans between regeneration

Terminal common equipment cost

10 Gb/s transponder cost

Regenerator common equipment cost

10 Gb/s regenerator cost

Amplifier cost

80

6 x 80 km (640 km total)

$200

$60

$200

$100

$200 ULH System

Number of wavelengths per system

Spans between regeneration

Terminal common equipment cost

10 Gb/s transponder cost

Regenerator common equipment cost

10 Gb/s regenerator cost

Amplifier cost

60

25 x 80 km (2000 km total)

$200

$7s

$200

$125

$240

Crossconnect

Number of 10 Gb/s ports

Common equipment cost

Cost per 10 Gb/s port

128

$300

$40

be terminated This capability can be used to reduce the nodal costs by eliminating some of the transponders required to terminate the passthrough traffic In this case, we also have to deal with the routing and wavelength assignment problem discussed in Chapter 8, as signals being passed through optically cannot be converted to other wavelengths

,, Using more sophisticated routing and capacity allocation algorithms will bring the cost down for both 1 + 1 and shared mesh protection

Table 13.5 Number of amplifiers, transponders, regenerators, and crossconnects

required for LH and ULH systems to realize the capacities and link distances shown

in Table 13.2, for both 1 + 1 and shared mesh protection

Amplifiers

Transponders

Terminal common equipment

Regenerators

Regenerator common equipment

Crossconnect ports

Crossconnect common equipment

Figure 13.12 Breakdown of network costs for LH and ULH systems with 1 -t- 1 and shared mesh protection

696 DEPLOYMENT CONSIDERATIONS

1.3 1.2

r ~

O

o 1.1

1

"~ 0.9

0.8

0.7

ULH ampli ULH amplifier cost = LH a m ~

~ ~ ULH amplifier cost=LH amplifier cost

Baseline costs used in the text

Relative ULH transponder and regenerator cost

Figure 13.13 Sensitivity of study results to the relative cost of ULH and LH transpon- ders (and regenerators) and amplifiers The x axis indicates the ULH transponder and regenerator cost relative to the LH transponder and regenerator cost The y axis indicates the relative network cost for ULH and LH systems assuming 1 + 1 protection

9 We have decoupled the network costing from the routing and capacity allocation However, further cost optimization is possible by considering the two parts together For example, in the LH case, we might choose slightly longer paths

if it means using fewer regenerators on some of the links in the path

9 We have not taken into account the cost of blocking when considering cross- connects Observe that many nodes require more than one crossconnect, given our assumption of a 1.28 Tb/s crossconnect In this analysis, we have simply used as many crossconnects as needed to obtain the desired port counts, without considering the cost of scaling the crossconnect or the cost of blocking

9 We have implicitly assumed that there is no protection between the client equip- ment (for example, routers) and the optical layer equipment (such as crosscon- nects) In practice, we'll need to have some protection here as well and factor its cost into account

9 Traffic demands are at 10 Gb/s We haven't dealt with aggregating and grooming lower-speed demands

13.2.7 Long-Haul Undersea Networks

The economics of long-haul undersea links is similar to that of the long-haul terres- trial links, but with a few subtle differences First, there are several types of undersea links commonly deployed One type spans several thousands of kilometers across the Atlantic or Pacific oceans to interconnect North America with Europe or Asia, as shown in Figure 13.14 Another type tends to be relatively shorter haul (a few hun- dred kilometers), interconnecting countries either in a festoon type of arrangement

or by direct links across short stretches of water The term festoon means a string suspended in a loop between two points In this context, it refers to an undersea cable used to connect two locations that are not separated by a body of water, usu- ally neighboring countries A trunk-and-branch configuration is also popular, where

an undersea trunk cable serves several countries Each country is connected to the trunk cable by a branching cable, with passive optical components used to perform the branching at the branching units If a branch cable is cut, access to a particular country is lost, but other countries continue to communicate via the trunk cable WDM is widely deployed in all these types of links

The long-haul undersea systems tend to operate at the leading edge of technol- ogy and have to overcome significant impairments to attain the distances involved The links use the dispersion management technique described in Section 5.8.6 by having alternating spans with positive and negative dispersion fiber to realize a total chromatic dispersion of zero but at the same time have finite chromatic dispersion

at all points along the link

The shorter-distance undersea links also stretch design objectives but in a different way The main objective with these links is to eliminate any undersea amplifiers or repeater stations, due to their relatively higher cost of installation and maintenance

As a result, these systems use relatively high-power transmitters

The trunk-and-branch configuration is also evolving The early branching units contained passive splitters and combiners, but optical add/drop multiplexers are now being used to selectively drop and add specific wavelengths at different locations Undersea systems are designed to provide very high levels of reliability and availability due to the high cost of servicing or replacing failed parts of the network Optical amplifiers with redundant pumping arrangements have proven to be highly reliable devices, and their failure rates are much lower than those of electronic regenerators Likewise, optical add/drop multiplexers using passive WDM devices have been qualified for use in undersea branching configurations

Undersea networks are very expensive to build, and the capacity on these net- works is shared among a number of users WDM allows traffic from different users

to be segregated by carrying them on different channelsma useful feature

698 DEPLOYMENT CONSIDERATIONS

Figure 13.14 Different types of undersea networks, showing a couple of ultra-long-haul trans- Atlantic links, shorter-haul direct repeaterless links, a trunk-and-branch configuration, and a festoon

One key difference between undersea links and terrestrial links is that, in most cases, undersea links are deployed from scratch with new fibers rather than over existing fiber plant It is rare to upgrade an existing long-haul amplified undersea link, as the cost of laying a new link is not significantly higher than the cost of upgrading an existing link This provides more flexibility in design choices

13.2.8 Metro Networks

The metro network can be broken up into two parts The first part is the metro access network and extends from the carrier's central office to the carrier's customer locations, serving to collect traffic from them into the carrier's network The second part of this network is the metro interoffice n e t w o r k ~ t h e part of the network

between carrier central offices The access network today typically consists of rings a few kilometers to a few tens of kilometers in diameter, and traffic is primarily hubbed into the central office The interoffice network tends to be several to a few tens of kilometers between sites, and traffic tends to be more distributed

Because of the shorter spans involved, the case for W D M links is less compelling

in metro networks The other alternatives, namely, using multiple fibers or using higher-speed TDM, are quite viable in many situations Despite this, however, there hasn't been widespread deployment of OC-192 in the metro network One reason

is that OC-192 interfaces have only recently appeared on metro systems Another reason is that carriers in this part of the network are interested in delivering low-speed services at DS1 (1.5 Mb/s) or DS3(45 Mb/s) rates and OC-192 equipment is only now becoming a cost-effective alternative for this application

On the other hand, reasons other than pure capacity growth are driving the deployment of W D M in these networks Metro carriers need to provide a variety

of different types of connections to their customers The service mix includes leased private line services; statistical multiplexing types of services such as frame relay, ATM, and IP; Gigabit Ethernet; ESCON; and Fibre Channel In many cases this service mix is supported by having a set of overlay networks, each dedicated to supporting a different service These overlay networks are ideally realized using a single infrastructure Due to its transparent nature, a W D M network provides a better infrastructure than most others, such as SONET/SDH, for this purpose Another factor is that the traffic distribution changes much more rapidly in metro networks than in long-haul networks This drives the need to be able to rearrange network capacity quickly and efficiently as needed Reconfigurable WDM networks allow capacity to be provided as needed in an efficient manner

A big driver for WDM deployment in metro networks has been the need for large enterprises to interconnect their data centers These data centers are separated by several kilometers to a few tens of kilometers All transactions are mirrored at both sites This allows the enterprise to recover quickly from a disaster when one of the centers fails There may be other reasons for this as well, such as lower real estate costs at one location than at the other Peripheral equipment such as disk farms can

be placed at the cheaper site The bandwidth requirement for such applications is large The large mainframes at these data centers need to be interconnected by several hundred channels, each at up to 1 Gb/s For example, IBM mainframes communicate using hundreds of ESCON channels, discussed in Chapter 6, running at 200 Mb/s each, or Fibre Channel at 1 Gb/s, as discussed in Chapter 6 Typically, these data centers tend to be located in dense metropolitan areas where most of the installed fiber

is already in use Moreover, these networks use a large variety of protocols and bit rates These two factors make WDM an attractive option for these types of networks