Optical Networks: A Practical Perspective - Part 47 potx

a Write a computer program that takes as its input the set of lightpaths in the network and their wavelengths, the loss between each pair of adjacent nodes, and determines whether each l

430 W D M NETWORK ELEMENTS

to using OLTs interconnected back to back, or relying on other equipment to handle the passthrough traffic OADMs are typically deployed in linear or ring topologies Several types of OADMs are possible with a range of capabilities based on the number of wavelengths they can add and drop, the ease of dropping and adding additional wavelengths, static or reconfigurable, and so on We studied the basic architectural flavors of OADMs: parallel, serial, and band drop Each of these has its pros and cons We also looked at reconfigurable OADM architectures, which use tunable filters and/or multiplexers, as well as tunable lasers, in order to provide the maximum possible flexibility in the network

An optical crossconnect (OXC) is the other key network element in the optical layer OXCs are large switches used to provision services dynamically as well as provide network restoration OXCs are typically deployed in a mesh network con- figuration As with OADMs, several variants of OXCs exist, ranging from OXCs with electrical switch cores capable of grooming traffic at STS-1 rates to all-optical OXCs that can switch wavelengths, bands of wavelengths, and entire fibers Op- tical core crossconnects can also be surrounded by optical-to-electrical-to-optical converters to provide some of the grooming and wavelength conversion capabilities offered by electrical core crossconnects, but are not suited for grooming traffic at fine granularities such as STS-1 rates Each has its role in the network

Further Reading

Information regarding the various types of OLTs, OADMs, and OXCs is not easy

to come by because many of the commercial implementations are proprietary in nature Browsing through network equipment vendors' Web pages will provide some illustration of the capabilities of the different products in this space How- ever, there are several early testbeds that explored various forms of these network elements For instance, [Ale93, Kam96] used a static all-optical OXC that pro- vided a fixed interconnection pattern without any switching or wavelength con- version [Cha94, CEG§ explored a parallel WADM architecture as well as

an OXC with an electrical switch core [Hi193] developed an all-optical OXC without wavelength conversion [WASG96, Gar98] developed a parallel WADM

as well as a small all-optical OXC without wavelength conversion See also [HH96, OWS96, Der95, MS96, Ber96a, Ber96b, Bac96, RS95, Chb98, KWK+98] for other relevant testbeds and architectures The use of wavelength bands has been discussed in various contexts in [Ste90, GRW00, SS99] For a discussion of optical crossconnects and a comparison of them to electrical crossconnects, see [GR00, GRL00]

Problems 431

7.1

7.2

7.3

Problems

Consider a ring network with two intermediate adjacent nodes A and B, each with

an OADM

(a) Consider the case where the OADM at node A adds wavelength )~1 and the OADM at node B drops the adjacent wavelength k2 Suppose the minimum received power is set at - 3 0 dBm and the transmit power is set at 0 dBm Adjacent channel crosstalk at the receiver must be less than 15 dB Assume that signals are added and dropped by the OADMs with no loss What is the crosstalk suppression required at the OADM for the adjacent channel? How does this change with the link loss between the two nodes?

(b) Next consider the case where both OADMs drop and add wavelength )~1

We are worried about the case where some of the )~1 power, instead of being dropped at the node, "leaks" through The intrachannel crosstalk

at the receiver must be at least - 3 0 dB below the desired signal For the same assumptions as above, what is the intrachannel crosstalk suppression required at the OADM? How does this change with the link loss between the two nodes?

This problem illustrates some of the difficulties facing network planners when they have to use OADMs that are constrained in what channels they can add and drop Consider a four-node linear network with nodes A, B, C, and D in that order We have three wavelengths )~1, ~.2, )~3 available and are given OADMs that drop two fixed channels That is, we can put in OADMs that drop either )~1, )~2, or )~2, k3, or )~1, )~3 Now consider the situation where we need to set up the following lightpaths: AB,

BC, CD, AC, BD What OADMs would you deploy at each of the nodes? Suppose

at a later point the lightpath traffic changes and now we need to replace lightpaths

AC and BD by AD and BC What changes would you have to make to support this new traffic?

Consider a linear network with serial OADMs Assume that the transmit power

is 0 dBm; the minimum received power is - 3 0 dBm; and each OADM has a passthrough loss of 2 dB, a loss of I dB for the drop path, and a loss of i dB for the add path Assume that the adjacent channel suppression offered by each OADM is

20 dB and that at the receiver the adjacent channel power must be at least 15 dB less than the desired signal power

(a) Write a computer program that takes as its input the set of lightpaths in the network and their wavelengths, the loss between each pair of adjacent nodes, and determines whether each lightpath is feasible or not The program should also determine any wavelength conflicts, that is, if two lightpaths overlap and are assigned the same wavelength

4 3 2 WDM NEXWORK ELEMENTS

7.4

7.5

7.6

(b) What is the maximum number of OADMs that a lightpath can pass through before it needs to be regenerated? Plot this number as a function of the total link loss in the network

(c) Plot the output of your program for a network with five nodes numbered sequentially from 1 to 5, link loss of 5 dB between nodes, and the following sets of lightpaths and wavelength assignments: (1, 5, ~.1), (2, 4, )~2), (3, 5, ~.3), (1, 4, )~4)

This problem explores architectures for constructing fully reconfigurable OADMs Consider the parallel architecture shown in Figure 7.7(d) How would you build

a fully reconfigurable parallel OADM like this one, wihtout using a large optical switch? You are allowed to use tunable filters, passive splitters and combiners, and small (2 x 2) optical switches These solutions need to meet properties (1) and (2) specified for the ideal OADM described in Section 7.3 With respect to property (3), you still need to keep the loss fixed, regardless of how many channels are dropped

or added, but are allowed to have a reasonably high value for this loss Compare the pros and cons of your solution versus the one in Figure 7.7(d)

You have to design a five-node ring network with a hub node and four remote nodes Each remote node needs two wavelengths of traffic to/from the hub node on both sides of the ring; that is, you will need to dedicate two wavelengths to each remote node and terminate all the wavelengths at the hub node You have to pick between two systems

The first system uses eight channels in two bands, each with four channels It provides band OADMs, which can drop one out of the two bands Once a band is dropped, all four wavelengths in the band have to be regenerated A band OADM costs $20,000 and a single-channel regenerator costs $10,000 No optical amplifiers are required with this system

The second system also uses eight channels but has no bands It provides SC-OADMs, which can drop any single wavelength Each SC-OADM costs $10,000 For this system, two optical line amplifiers are required, each costing $30,000 Whose system would you select based on just equipment cost?

This problem illustrates the need for large OXCs and also illustrates the value of using wavelength bands

Consider an all-optical OXC with 256 ports deployed in the configuration shown

in Figure 7.9(d) Each W D M line system carries 32 wavelengths; 75% of the light- paths pass through the node while the remaining 25% are dropped and added onto routers attached to the OXC Each lightpath added and dropped onto a router takes

up two OXC ports

(a) How many W D M line systems can the OXC support?

References 433

7.7

(b) Next suppose that 25% of the lightpaths passing through need to be con- verted from one wavelength to another This is done by sending the lightpath

to one of a pool of regenerators/wavelength converters attached to the OXC Each such regenerator uses two ports in the OXC Thus a lightpath needing

to be converted uses four OXC ports Now, how many WDM line systems can the OXC support?

(c) Now suppose the WDM line systems are designed with eight bands, each with four wavelengths Assume that all the lightpaths passing through can be passed through at the band level without having to be demultiplexed down to the individual channel For lightpaths that are dropped and added, the entire band is dropped and demultiplexed after the bands are passed through the OXC No wavelength conversion is needed How many WDM line systems can the OXC support?

Consider the wavelength plane switch architecture of Figure 7.12 Consider the situation where we have a total of four fibers and 40 wavelengths on each fiber

We must design the node such that any four signals can be dropped (Note that this implies we could potentially drop all the wavelengths on a particular fiber while passing through all the wavelengths on the other fibers.) The wavelengths are dropped onto transponders, which have tunable lasers The 40 wavelengths are split into five bands of 8 wavelengths each, and a tunable laser can tune over a single band

(a) Draw a block diagram of this node and indicate the minimum number of transponders needed Compare this against an approach using large non- blocking switches

(b) Now suppose we have tunable lasers that can tune over two bands instead

of one How does the situation change?

References

[Ale93] S.B Alexander et al A precompetitive consortium on wide-band all-optical

networks IEEE/OSA Journal on Lightwave Technology, 11:714-735, May/June

1993

[Bac96] E.-J Bachus et al Coherent optical systems implemented for business traffic routing

and access: The RACE COBRA project IEEE/OSA JLT/JSAC Special Issue on Multiwavelength Optical Technology and Networks, 14(6):1309-1319, June 1996

[Ber96a] L Berthelon et al Experimental assessment of node cascadability in a

reconfigurable survivable WDM ring network In Proceedings of Topical Meeting

on Broadband Optical Networks, 1996

434 WDM NExwo~I( ELEMENTS

[Ber96b] L Berthelon et al Over 40,000 km across a layered network by recirculation

through an experimental WDM ring network In Proceedings of European

Conference on Optical Communication, 1996

[CEG+96] G.K Chang, G Ellinas, J K Gamelin, M Z Iqbal, and C A Brackett

Multiwavelength reconfigurable WDM/ATM/SONET network testbed IEEE/OSA JLT/JSA C Special Issue on Multiwavelength Optical Technology and Networks,

14(6):1320-1340, June 1996

[Cha94] G.K Chang et al Experimental demonstration of a reconfigurable

WDM/ATM/SONET multiwavelength network testbed In 0FC'94 Technical

Digest, 1994 Postdeadline paper PD9

[Chb98] M.W Chbat et al Towards wide-scale all-optical networking: The ACTS optical

pan-European network (OPEN) project IEEE JSAC Special Issue on

High-Capacity Optical Transport Networks, 16(7):1226-1244, Sept 1998

[Der95] E Derr Design of an 8 x 8 optical cross-connect switch: Results on subsystems and

first measurements In ECOC'95 Optical Networking Workshop, 1995 Paper

$2.2

[Gar98] L.D Garrett et al The MONET New Jersey demonstration network IEEE JSAC:

Special Issue on High-Capacity Optical Transport Networks, 16(7):1199-1219, Sept 1998

[GR00] J Gruber and R Ramaswami Moving towards all-optical networks Lightwave,

34(8):40-49, Dec 2000

[GRL00] J Gruber, P Roorda, and E Lalonde The photonic switch crossconnect (PSX) its

role in evolving optical networks In Proceedings of National Fiber Optic

Engineers Conference, pages 678-689, 2000

[GRW00] O Gerstel, R Ramaswami, and W-K Wang Making use of a two stage

multiplexing scheme in a WDM network In OFC 2000 Technical Digest, pages

ThD 1-1-ThD 1-3, 2000

[HH96] A.M Hill and A J N Houghton Optical networking in the European ACTS

programme In 0FC'96 Technical Digest, pages 238-239, San Jose, CA, Feb 1996 [Hi193] G.R Hill et al A transport network layer based on optical network elements

IEEE/OSA Journal on Lightwave Technology, 11:667-679, May/June 1993

[Kam96] I.P Kaminow et al A wideband all-optical WDM network IEEE JSAC/JLT

Special Issue on Optical Networks, 14(5):780-799, June 1996

[KWK+98] M Koga, A Watanabe, T Kawai, K Sato, and Y Ohmori Large-capacity optical

path cross-connect system for WDM photonic transport network IEEE JSAC" Special Issue on High-Capacity Optical Transport Networks, 16(7):1260-1269, Sept 1998

References 435

[MS96] W.C Marra and J Schesser Africa ONE: The Africa optical network IEEE

Communications Magazine, 34(2):50-57, Feb 1996

[OWS96] S Okamoto, A Watanabe, and K.-I Sato Optical path cross-connect node

architectures for photonic transport network IEEE/OSA JLT/JSAC Special Issue

on Multiwavelength Optical Technology and Networks, 14(6):1410-1422, June

1996

[RS95] R Ramaswami and K N Sivarajan Routing and wavelength assignment in

all-optical networks IEEE/ACM Transactions on Networking, pages 489-500, Oct 1995 An earlier version appeared in Proceedings of IEEE Infocom'94 [SS99] A.A.M Saleh and J M Simmons Architectural principles for optical regional and

metropolitan access networks IEEE/OSA Journal on Lightwave Technology,

17(12), Dec 1999

[Ste90] T.E Stern Linear lightwave networks: How far can they go? In Proceedings of

IEEE Globecom, pages 1866-1872, 1990

[WASG96] R.E Wagner, R C Alferness, A A M Saleh, and M S Goodman MONET:

Multiwavelength optical networking IEEE/OSA JLT/JSAC Special Issue on Multiwavelength Optical Technology and Networks, 14(6):1349-1355, June 1996

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WDM Network Design

I N T H E P R E V I O U S C H A P T E R , WE L E A R N E D circuit-switched connections, or lightpaths, between pairs of higher-layer equip- that the optical layer provides high-speed ment such as SONET/SDH muxes, IP routers, and ATM switches The optical layer realizes these lightpaths over the physical fiber using elements such as optical line ter- minals (OLTs), optical add/drop multiplexers (OADMs), and optical crossconnects (OXCs) We called a network using such lightpaths a wavelength-routing network

In this chapter, our goal is to study how to design a wavelength-routing network This involves studying not only how to design the optical layer but also how the higher-layer SONET or IP network is to be designed because the design of the two layers is closely coupled We illustrate with an example

E x a m p l e 8.1 In Figure 8.1(a), there are three nodes labeled A, B, and C, con- nected by WDM fiber links For simplicity, assume the traffic generated is in the form of IP packets from routers located at these nodes Similar examples hold if the higher layer consists of SONET/SDH muxes or ATM switches For concreteness, also assume that all router interfaces operate at 10 Gb/s, which

is also the transmission capacity on each wavelength on the W D M links Now suppose, based on estimates of the IP packet traffic, 50 Gb/s of capacity is re- quired between all three pairs of routers: A-B, B-C, and A-C The network can

be designed to handle this traffic in two ways

1 No optical add/drop: In the first method, we set up 10 wavelengths on each

of the links A-B and B-C connecting the routers at the ends of these links

We observe that the traffic flowing on link A-B is 50 Gb/s (traffic from A-B)

4 3 7

438 W D M NETWORK DESIGN

Figure 8.1 (a) A three-node network (b) Nodes A-B and B-C are interconnected by WDM links All wavelengths are dropped and added at node B (c) Half the wavelengths pass through optically at node B, reducing the number of router ports at node B

+ 50 Gb/s (traffic from A-C that must use link A-B) = 100 Gb/s Similarly, the traffic flowing on link B-C is also 100 Gb/s Thus the 10 wavelengths on each of the links A-B and B-C are sufficient to carry this traffic In this case,

we use 10 router ports at node A, 20 router ports at node B, and 10 router ports at node C, for a total of 40 router ports At the optical layer, nodes A and C have OLTs, whereas node B has a pair of OLTs that terminate all the wavelengths passing through node B This is illustrated in Figure 8.1(b)

2 With optical add/drop: In the second design, we set up only five lightpaths each on the routes A-B, B-C, and A-C The five lightpaths on the route A-C pass through the node B within the optical layer, without being converted to

an electrical signal This design requires only 10 router ports at each of the three nodes, A, B, and C, for a total of 30 router ports, compared to 40 router ports in the design without optical add/drop However, this design requires node B to have an OADM node that is capable of adding and dropping 10

of the 20 lightpaths that terminate at the router at node B, while passing the other 10 lightpaths through This is illustrated in Figure 8.1(c)

Thus, in the design with optical add/drop capability, we can trade off the number of IP router ports at node B (10 versus 20) for optical add/drop capability

at the same node In general, as we will see later, the trade-off is between the

WDM Network Design 439

cost of the higher-layer equipment (IP router ports) and the cost of the optical layer equipment (OADMs, or increased number of wavelengths as we will see

in other examples later) Both designs are perfectly valid and will do the job

as far as the user is concerned The choice between them will be made based

on the cost trade-off between the optical and higher-layer equipment In this example, providing optical add/drop capability requires an OADM at node B instead of two OLTs The cost of doing this is cheaper in many scenarios today than providing additional 10 Gb/s IP router ports This situation is likely to prevail over the long run as well fundamentally because passing a wavelength through is a much simpler operation than routing all the packets that have been transmitted on a wavelength at the IP layer

Note that in this example the transponder costs are bundled with the IP router port costs Increasingly, routers are being equipped with optical interfaces with wavelengths on the ITU grid, so that no additional transponders are required Thus, strictly speaking, in this example, we are comparing the cost of IP layer terminations versus keeping things optical

In the same example, if the amount of passthrough traffic at node B was a small fraction of a wavelength, an entire wavelength with a capacity of 10 Gb/s would have

to be used for the passthrough traffic if we used a design with optical passthrough

At the same time, a design without optical passthrough may be able to handle the passthrough traffic without an increase in the number of IP router ports This would lead us to prefer to handle the passthrough packets using the IP router at node B

We will study this effect further in the next section in the context of rings

From the point of view of the IP routers, the topology of the network when all the wavelengths are terminated at node B is shown in Figure 8.2(a) This is the topology seen by the IP layer packet-routing algorithm, such as OSPE This is a linear topology with 10 parallel links between nodes A and B, and 10 parallel links between nodes B and C In the optical add/drop case, the topology of the network seen by IP routers is

a completely connected mesh with 5 parallel links between each of the three pairs of nodes, as shown in Figure 8.2(b) Note that both topologies are capable of meeting the traffic needs at the IP layer, which calls for 50 Gb/s of capacity between each pair

of routers

The topology seen by the IP routers, or the SONET/SDH muxes, is the topology

of the lightpaths provided by the optical layer, and hence we will call it the lightpath topology, It is often called the logical or virtual topology, but we will not employ this terminology In the same vein, the fiber topology upon which the lightpaths are created is called the physical topology, but we will not use this terminology either

We can view the general problem of designing wavelength-routing networks as follows The fiber topology and the traffic requirements (traffic matrix) are specified