STOLL LAYOUT

FIELD TRIAL: ALL-OPTICAL ADD/DROP MULTIPLEXER

Metropolitan DWDM: A Dynamically Configurable Ring for the KomNet Field Trial in Berlin Detlef Stoll, Patrick Leisching, Harald Bock, and Alexander Richter, SIEMENS AG

ABSTRACT

Editorial Liaison: T. Vasilakos.

106

We present the system concept of a dynamically configurable all-optical add/drop multiplexer for metropolitan wavelength division multiplex rings using dense channel spacing (DWDM). We first outline the essential network requirements. Subsequently, an optical system concept is presented that meets these requirements. In the network elements, innovative all-optical switching technology is employed. Tunable fiber Bragg gratings that perform routing functions by wavelength filtering are discussed in more detail. The system is laboratory-tested. Recently, it has been installed in Berlin for a multi-vendor field trial within the German research program “KomNet.” Three optical add/drop multiplexers form a dynamically configurable Metro DWDM ring. Future communication networks will face three major changes: • A strong increase in traffic flow, mainly driven by Internet applications. • The merging of telecommunication and data networks. • Switching granularities getting coarser corresponding to an increase in bit rates of single data streams. At the same time the requirements for reliability and availability will remain high and pressure will increase to operate networks cost-efficiently. These requirements can hardly be met alone by the currently deployed synchronous network technology (SONET/SDH) in a cost-efficient manner. In many networks that provide path lengths up to 300 km, DWDM solutions are ideally suited for this new situation [1]. Metro DWDM accommodates high bandwidths: a metropolitan ring that provides 0.8 terabit/s ring capacity has been demonstrated recently [2]. Furthermore, Metro DWDM can provide optically transparent channels enabling the transport of various data

0163-6804/01/$10.00 © 2001 IEEE

formats simultaneously, e.g., SONET/SDH and Gigabit Ethernet. Conversion to a standard format becomes obsolete. A third benefit is the granularity at wavelength channel level. It is better suited for future network demands than the fine granularity that SONET/SDH network elements provide. Lowrate streams requiring fine switching granularities are losing importance. Metro DWDM networks can be a viable solution if new fiber installation is too expensive or impossible. This is known from long-haul systems and can apply in some city or regional networks as well. To reduce operational costs, automatic switching of DWDM channels can be performed by remote control (dynamic configuration). This requires optical switching matrices or alternative techniques that can be controlled electronically. However, dynamic configuration causes higher installation costs. Manual configuration of DWDM channels is less expensive. It is performed by hand-plugging cables to optical connector patch boards (distribution frames). The cost-efficiency of dynamic configuration depends on the average lifetimes of the optical connections. The alloptical add/drop multiplexer (OADM) presented in this article provides both manual and dynamic configuration. In this article, the system concept of the OADM is developed starting with basic metropolitan DWDM network requirements. These requirements are treated in the first two sections. The next section describes the features of the DWDM ring, followed by a presentation of the system concept of the flexible OADM. Our focus is on dynamic configuration techniques. Since tunable fiber Bragg gratings are promising components for dynamic optical channel switching [3], this technology is treated in more detail in the last section.

IEEE Communications Magazine • February 2001

The Berlin ATM switch (concatenated SDH/SONET)

metropolitan backbone Access network

of two

Vendor domain 1 Transparent long-haul WDM link Berlin ´ Frankfurt

Add/drop multiplexers WDM terminal

network consists interconnected DWDM rings of

Ring interconnection

different vendors. Each ring is

Vendor domain 2

IP-router

connected to the transparent longhaul link. Three

High end user (Gigabit Ethernet, SDH/SONET…)

City Network Berlin 16…80 WDM channels Ring circumference: 60 km

■ Figure 1. The KomNet DWDM metropolitan area network in Berlin.

THE KOMNET NETWORK The optical add/drop multiplexer presented here is installed in the experimental network “KomNet.” The KomNet network is an optical network all over Germany. The field trials cover a variety of realistic network scenarios. Results that emerge from its operation will be considered for future DWDM products. The KomNet network consists of long distance lines (i.e., 4 x 40 Gb/s) between Frankfurt and Stuttgart, a transparent 750 km WDM longhaul system between Berlin and Frankfurt, a metropolitan area network in Berlin (circumference: 60 km each ring) and various access applications. One of these applications is Internet traffic using signal formats that are optimized for WDM systems (IP over WDM). This experimental network is built by Siemens, Alcatel, Bosch, Lucent Technologies, Deutsche Telekom, and several German research institutes [4]. The metropolitan part of the KomNet network, Fig. 1, is situated in Berlin. The Berlin metropolitan backbone network consists of two interconnected DWDM rings of different vendors. Each ring is connected to the transparent long-haul link. Three of the add/drop multiplexers presented here will form one of the rings. In Fig. 1 they are shown in red. The ring interconnection is realized by optically transparent interfaces. It connects equipment of different vendors. Such a situation is expected to become important in optical networks due to alliances, acquisitions, or mergers between network operators. To enable multivendor interconnections, some interface parameters must be standardized. One goal of KomNet is to identify essential parameters and to determine optimum values. They will be contributed for standardization by all partners after verification by field trials. A further goal is the optimization of Metro DWDM systems to network demands. Promising


optical network technologies and components are evaluated in a field environment in cooperation with a network operator. Some requirements of Metro DWDM rings are different from those of electro-optical ring network solutions, e.g., SONET/SDH or data network formats such as FDDI. This is due to the physical characteristics and costs of optical switching technology. These requirements will be discussed in the following section.

of the add/drop multiplexers presented here will form one of the rings.

DWDM NETWORK REQUIREMENTS Static Traffic/Dynamic Traffic —The traffic transported over a Metro DWDM network can be divided into static traffic and dynamic traffic. Static traffic is related to optical links (paths) that, once established, will be in service for such a long time that manual configuration of the necessary optical connections are the most costeffective solution. First results indicate that the lifetimes of static optical paths are longer than three months in typical network situations, but this is a subject of detailed studies within KomNet. Dynamic traffic corresponds to shorter optical path lifetimes. Most economically, this traffic is dynamically configured using optical switching matrices or alternative technologies. Our system concept is based on this different treatment of dynamic and static traffic. Since optical switching technology is expensive and components still need to be improved, this separation yields as much flexibility and performance as necessary at minimum cost. This is important, especially in the Metro DWDM market, which is highly cost driven. Network Concept — Rings impose low requirements on the optical hardware and on the network management system. Therefore, the ring topology was chosen by the KomNet consortium for the all-optical metropolitan backbone. Actually, in dynamically configurable

107

(a)

(b)

Different cables/ducts

Working traffic

Protection traffic

■ Figure 2. a) Protected optical link in a bi-directional ring; b) protected optical link in a unidirectional ring.

networks, optical path lengths of up to 100 km can be achieved without regeneration using 2.5 Gbit/s directly modulated laser sources. This imposes a maximum geographic ring circumference of about 30 km, considering a realistic cable installation. Traffic Protection — Protected optical links between any two ring nodes can be realized by establishing a working path and a protection path. The routing of protected traffic is depicted in Fig. 2 for unidirectional and for bi-directional rings. From the network management perspective, bi-directional traffic is preferred since both transmission directions of one path are routed over the same network elements. In bi-directional rings, unprotected traffic uses less network capacity since both transmission directions of a path take the shortest path through the network. However, the optical hardware of bi-directional rings can be more complex than for unidirectional rings. The OADM presented here supports both kinds of traffic.

C

A

D

B

■ Figure 3. Wavelength blocking: A link between node B and node C shall be established. The only wavelength that is still available at node B for transmission is drawn in red. Despite free ring capacity, the link cannot be established since “red” is already occupied for a link between node A and node D.

108

Figure 2 shows another advantage of bi-directional ring traffic: both fibers between two nodes can be installed in the same duct or cable. In case of a cable break — one of the most frequent reasons for network failures — both directions of the same path are interrupted. In unidirectional rings, one direction of the working path and the other direction of the protection path would be interrupted. Therefore, in unidirectional rings both fibers should be installed in separated ducts. Wavelength Blocking — This kind of blocking can only occur in optical networks. It happens when there is no capability to assign an optical signal to an unused ring wavelength. Figure 3 shows such a situation. Therefore, every node must be able to add and to drop any wavelength that can be transported in the ring (flexible wavelength assignment) to avoid wavelength blocking. If the path goes over several spans, this wavelength should preferably be available in every span to avoid changing its wavelength from span to span. However, there are situations when each span provides a set of wavelengths that are still available for new paths, but there may be no wavelength commonly available in every span. To avoid blocking, wavelength conversion must be provided for channels that are already in the ring. Without flexible wavelength assignment and wavelength conversion capabilities in every node, it might not be possible to establish new links in spite of free network capacity. In the section entitled “System Concept of a Flexible OADM,” it is shown how these two features can be implemented. However, avoiding wavelength blocking in this manner is very expensive. Mainly, the costs are driven by the large number of wavelength transponder elements. If the number of channels that must be added and dropped is low compared to the ring capacity, it is less expensive only to install the capability to insert a subset of the ring wavelength channels. If these subsets are different for each node, each wavelength can only be used by one node for transmission. No wavelength blocking is possible in this case. This solution requires


Ring west

Add/drop stage L

Add/drop stage R

Transponder array

Ring east

Transponder array

Distribution stage

Distribution stage

Working Protection

Working Protection

Dynamic add/drop switching unit Tributary interface cards Working Protection

■ Figure 4. Functional diagram of the flexible KomNet OADM. establishing wavelength assignment plans in the network planning phase. Furthermore, it implies that the go- and return-direction of the optical paths do not need to operate at the same wavelength. The network requirements treated in this and the previous section define the technical features of the experimental Metro DWDM ring. These features are outlined in the following section.

FEATURES OF THE DWDM RING The network requirements of the DWDM ring are taken into account in the following way: • Static/dynamic traffic: The dynamic traffic is routed via optical switching matrices and wavelength-tunable filters. The static traffic is routed manually on dedicated wavelength channels using distribution frames for cost reasons. • Unidirectional/bi-directional traffic: The OADM concept presented here supports both kinds of traffic. Due to the advantages in network planning and management, the KomNet consortium prefers bi-directional ring traffic. • Optical protection: One major objective of the field trials is to investigate the interaction between the optical path protection and protection mechanisms of various kinds of client equipment, e.g., SONET/SDH. • Wavelength blocking: In order to examine cost-efficient solutions, wavelength blocking is avoided here by a wavelength assignment plan, as mentioned in the previous section. Efficient wavelength assignment strategies are the subject of network simulations and the field experiments. However, the system provides flexible wavelength assignment and wavelength conversion for experimental reasons.


The OADM concept that supports these network features is treated in the following section.

SYSTEM CONCEPT OF A FLEXIBLE OADM A flexible OADM system concept is depicted in Fig. 4. It enables the routing and protection functions mentioned above. The functional diagram shows the OADM configured for bi-directional ring traffic. The ring interface amplifiers compensate for fiber loss and internal losses. Furthermore, there are group filters to demultiplex the DWDM channels into groups for separate processing of static and dynamic traffic. These group filters are not shown in the figure. The figure shows the processing of just one dynamic channel group. Each shaded block in the functional diagram represents a separated functional unit within the network element. The add/drop process is performed by a twostage dynamic add/drop switching unit. The add/drop stage (first stage) filters the drop channels out of the DWDM signal and adds new channels into the ring. The distribution stage (second stage) assigns the added and dropped signals to the tributary interface units (cards). The add/drop stage L adds and drops traffic to and from west. The add/drop stage R is related to the east direction. In bi-directional twofiber rings no separation of working traffic and protection traffic exists. Usually the working traffic takes the shortest direction to the destination node. Therefore, a distribution stage is necessary to sort the channels from west/east traffic to working and protection traffic. Working and protection traffic is assigned to separate tributary interface clusters. In the add direction the distribution stage is connected to the add/drop stage via an array of different colored DWDM transponders.

109

Circulator

Tunable fiber Bragg gratings

Add/drop stage Frequency filtering matrix

Demultiplexer

Combiners

…

…

Working path (from west)

Distribution stage Space switching matrix

Transponder arrary

…

Trib-Trib

WWPP

…

NxM

NxM

…

…

Wavelength conversion

… WWPP

Protection path (from east)

Transponder arrary

…

NxM

…

NxM

…

WWPP

WWPP

■ Figure 5. Add/drop matrix (see blue shaded box in Fig. 4). The traffic inside the OADM is fully protected. In case of a breakdown of any one card, either the west or the east add/drop traffic is interrupted. As shown in Fig. 2a, the related protection paths to the interrupted working traffic enter the OADM via the other ring arm. They are processed by the remaining add/drop stage. Thus, all protected links are maintained in case of a single failure. To the optical path termination, such interruptions appear as fiber breaks. The optical path termination is located in the tributary interface cards. The interruption of through-traffic (e.g., by an add/drop stage breakdown) does not interrupt any protected link either; as evident from Fig. 2a, through-nodes do not carry a working and the related protection path together. The viability of this integrated traffic and card protection concept — which is chosen here for cost reasons — is the subject of the field trials. The two stages of the add/drop switching unit (see blue shaded box in Fig. 4) deploy different technologies. The add/drop stage is designed using wavelength filtering devices; the distribution stage is realized using optical switching matrices. Both technologies will play an essential role in future optical networks. The testing of these technologies in practical environments is one subject of the KomNet field trials. The concept of the add/drop switching unit is shown in Fig. 5 in more detail. Add/drop Stage — The add channels are inserted into the ring by wavelength-independent couplers. The drop signals are extracted out of the DWDM ring signal using optical circulators and tunable Bragg gratings. The DWDM signal entering the add/drop

110

stage from the left ring input is forwarded clockwise by the optical circulator to the next port. The drop channels are reflected by tunable fiber Bragg gratings. The channels can be dropped or passed by tuning these band reject filters. If their resonance wavelength is tuned to a channel center wavelength, this channel is dropped; if it is tuned to a neutral “park” wavelength between two channels, the optical channels are passed. Each channel to be dropped requires one tunable filter. Today, the loss of one filter is about 0.1 dB. Thus, 16 filters or more can be cascaded without significant transmission loss [5]. Channels to be dropped are reflected via the circulator. A final separation of the dropped channels to single fibers is performed by a static optical demultiplexer. This demultiplexer moreover suppresses crosstalk between the DWDM channels (interchannel crosstalk) significantly [6]. Up to 50 dB have been achieved. Distribution Stage — In the KomNet system the distribution stage is realized using optical switching matrices (see lower part of Fig. 5). Separate optical NxM switching matrices are used for the add and drop directions. In addition to this basic function and to the sorting of west/east traffic to working/protection traffic (W/P), the distribution stage fulfills the following functions: • Flexible wavelength assignment. The NxM switching matrices in add direction assign the add signals to a transponder of a certain wavelength. This function of the switching matrix becomes obsolete if tunable transponders are deployed. • Wavelength conversion. Dropped channels


■ Figure 6. Add/drop stage design using optical switches (left) or wavelength filters (right).

can be re-inserted on another wavelength via the link “wavelength conversion” between the drop and the add NxM switching matrix. The meaning of this feature was mentioned in a previous section. • Tributary I/O selection. Not every signal that is connected to the tributary interface cards has to be inserted into the ring at any time. There may be two or more “part-time” leased lines that share one wavelength channel. The distribution stage selects channels from tributary interface cards to be inserted into the ring. • Tributary-tributary cross connect functionality. The OADM is able to work as an optical cross connect for tributary signals. This means optical signals that enter the OADM via a tributary interface card do not necessarily enter the ring. They can be routed back to another tributary port (hairpinning). This traffic uses the “Trib-trib” link between two NxM switching matrices. These functions are performed by an add/drop switching unit using two different switching technologies. In the following section advantages and disadvantages of space switching and wavelength filtering are discussed. Basic features of tunable fiber Bragg gratings performing wavelength filtering are emphasized [7].

WAVELENGTH FILTERING VS. SPACE SWITCHING An essential advantage of wavelength filtering compared with space switching is evident from Fig. 6. This figure shows an add/drop stage realization using space switching and wavelength filter cascades. In the space switching realization (left hand side) the incoming DWDM signal (from the left) is separated by a wavelength demultiplexer. Each channel is added, dropped, or forwarded separately. Each channel requires its own switch and its own internal fiber connections and splices. The channels are recombined by a DWDM multiplexer to be forwarded in the ring. Complexity — Within wavelength filter cascades (right part of Fig. 6) the DWDM characteristic is maintained as far as possible.


Therefore, wavelength filter cascades need less internal fiber connections. They are easier to handle from a manufacturing perspective. The dashed lines indicate the segmentation of the functional units, as shown in Fig. 5. If this segmentation is used, an integration of optical switches and (de-)multiplexers may be applied to reduce the number of splices. But it will not reduce the number of fiber connections between both functional units. A critical issue of wavelength filters compared to optical switches is their physical transmission characteristics. Figure 7 shows the transmission and reflection characteristics of mechanically tunable fiber Bragg gratings based on [8] designed for DWDM systems using 100 GHz channel spacing (according to ITU-T rec. G.692). Attenuation of Drop Signals — The reflectivity curve in Fig. 7a shows that the attenuation of a reflected channel is less than 5 dB. This means channels that are filtered out of the ring are attenuated by less than 5 dB, compared with the 2 dB that can be achieved using space switches. Attenuation of Through-signals — Figure 7b shows a transmission loss of channels forwarded through an add/drop stage of about 3 dB. This curve relates to a cascade of eight filters. Meanwhile, technology has improved to values of 1 to 2 dB. The transmission loss increases with the number of channels. Today, a cascade of 16 wavelength filters has about the same loss as a space switch solution. Crosstalk Between Drop Signals — The reflectivity curve in Fig. 7a shows a filter bandwidth of 50 GHz. The low selectivity of this filter would lead to high interchannel crosstalk if no further filtering is applied. The demultiplexer in the drop path (Figs. 5 and 6) suppresses the interchannel crosstalk. Values of up to 50 dB have been achieved. In comparison, today a concept based on space switches allows for about 40 dB. Dispersion of Through-signals — To pass a channel through a filter cascade, the related filter is tuned to a frequency between two channels. The minimum of the transmission curve at

111

0

(a) 10 Reflectivity

Penalty (dB)

6

-20

4 Penalty

Reflectivity (dB)

-10

8

-30

2 0

-40 (b) 10

Transmission

0

-10

6 4

-20 2

Penalty

0

Transmission (dB)

Penalty (dB)

8

-30

-2 -60

-40

-20

0 20 Detuning (GHz)

40

60

80

■ Figure 7. Transmission quality of tunable fiber Bragg gratings versus detuning of its resonance frequency (wavelength) from the channel center frequency. Upper diagram: drop traffic (power penalty and reflectivity); lower diagram: through-traffic (power penalty and transmission) [5].

– 60 GHz below the channel center frequency in Fig. 7b is related to such a filter in “pass” position. The minimum above +80 GHz is related to a filter tuned to the adjacent channel 100 GHz higher. In order not to affect through-channels by dispersion of these filters, their resonance wavelengths (frequencies) must be tuned to a high degree of accuracy. Furthermore, the variation of the resonance wavelength due to temperature changes or slow changes of material parameters must be sufficiently small as well. The sensitivity of the transmission quality due to filter detuning is indicated by the power penalty curves in Fig. 7. The power penalty indicates the additional channel power that is required to compensate for the increase of bit error rate (BER) due to filter detuning. The BER measurements were performed at 9.953 Gb/s using a pseudo random bit sequence. The penalty of drop signals (reflected by the filter) due to dispersion is below 2 dB within a tuning accuracy of ±20 GHz around the channel center frequency (wavelength). However, a tuning accuracy of ±5 GHz can easily be achieved by mechanically tuned gratings without control circuits [9]. Figure 7b shows that a filter that is

112

tuned between two channels does not affect a through-channel even if its frequency (wavelength) deviates about ±20 GHz from the accurate value. In systems using 50 GHz channel spacing, control circuits have to ensure an even finer tuning accuracy. Less than ±1 GHz can be achieved for mechanically tunable gratings using simple control circuits. Simulations show that at least seven spans can bridged by an optical path of 10 Gb/s bit rate using fiber Bragg gratings commercially available today [10]. The maximum path length is 100 km using standard single mode fiber and dispersion compensation. These values are comparable to values that are achievable with space switches. Switching Speed —The switching time of mechanically tuned gratings was measured to be below 1 ms [5]. The switching speed of space switches are in the same order of magnitude. Thus, both technologies are fast enough for performing optical protection switching besides routing. Comparison — The basic characteristics of space switching and wavelength filtering for an add/drop multiplexer as depicted in Fig. 6 (four wavelength channels per group) is summarized in Table 1. Transmission characteristics and switching speed of currently available tunable fiber Bragg gratings fulfill the requirements of all-optical DWDM networks. To build switching networks, tunable Bragg gratings are an important alternative to optical switching matrices. Arrays of circulators and tunable filters can be applied to build optical cross connects [3].

SUMMARY An all-optical add/drop multiplexer concept for dynamically configurable Metro DWDM rings has been presented. Metro DWDM can be a cost-efficient solution in metropolitan area networks for traffic that has to be rearranged frequently, if the installation of new fiber is too expensive or if fiber shortage cannot be overcome. Metro DWDM networks yield enough capacity to cope with the expected strong increase of traffic flow. Due to their optical transparency they provide the opportunity to integrate telecommunication and data networks. Their coarse switching granularities are well suited for existing or future high bit rate data formats. To minimize cost, the dynamically configurable traffic and the static traffic is routed via separate hardware in different DWDM channel groups. For cost reasons, only the dynamic traffic is routed via automatic switching networks. In DWDM metropolitan rings, bi-directional traffic is favored over unidirectional traffic for network management and planning reasons. The system described here supports both kinds of traffic. To reduce realization effort, we abstained from doubling functional units (cards) as much as possible. Instead, a combination of card and traffic protection is employed. Besides dynamic configuration, the OADM offers tributary-tribu-


Basic characteristics

Space switches

Wavelength filter cascades

Number of internal splices

High (26)

Low (17)

Attenuation of drop channels

Average (~2.7dB)

(~3.2dB)

Attenuation of through channels

Average (~3.4dB)

Average (~2dB) (circulator instead of 3dB coupler)

Interchannel crosstalk (drop channels)

Average

Low

Dispersion and group delay ripple of through channels

Average

High

Switching speed

Fast (