Planning of Optical Transport Networks Layered Architecture

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1 FEATURE ARTICLES Feature Articles: Telecom Operation & Management Planning of Optical Transport Networks Layered Architecture A. Jajszczyk A. Lason, J. Rzasa, R. Stankiewicz AGH University of Science and Technology, Department of Telecommunications, Al. Mickiewicza 30, Krakow, Poland {jajszczyk, lason, rzasa, M. Jaeger T-Systems, Goslarer Ufer 35, Berlin, Germany S. Spadaro, J. Sole-Pareta Universitat Politecnica de Catalunya (UPC), C/ Jordi Girona 1-3, Barcelona, Spain ABSTRACT This paper discusses the evolution path for core and metropolitan networks taking into account the current economic recovery as well as the changing telecommunications environment. At the beginning of the paper, the current status of core and metropolitan networks is presented, including a brief presentation of such networking technologies as: MPLS, Ethernet, Resilient Packet Ring (RPR), SDH/ SONET, and OTN. Then, the evolution scenarios are provided in three stages: short, medium and long term. The following factors are taken into account and referred to at each step of the evolution scenario: available services, quality of service/traffic engineering, connection provisioning/connection set-up control methods, network resilience and other functions. The short term scenario involves the in- troduction of reconfigurable WDM networks, while in the medium term the Generic Framing Procedure (GFP), enhanced SDH/SONET technologies, and Optical Transport Network (OTN) will be adopted. The long term scenario deals with the addition of a control plane, either ASON or GMPLS based. Key words: OTN, SDH/SONET, ethernet, RPR, IP, network planning, ASON/GMPLS, GFP, LCAS, DWDM/CWDM INTRODUCTION The late 90s were boom times in the telecommunications industry. Thousands of kilometers of fiber were installed, many transport systems implemented, dozens of new companies, both operators and vendors, started their fight for profit. However, the subsequent slowdown in the world economy has brought some 72

2 companies at the edge of catastrophe. Recently, the mood seems to start not to be so pessimistic. However, the question how to develop the most effective and cost efficient telecommunications infrastructure is of utmost importance. At the beginning of the paper, we present the architecture of current core and metropolitan networks and point out their main drawbacks. Next, we propose an evolution path which meets the increasing requirements for performance, functionality and cost efficiency, and is applicable to both core and metropolitan network operators. The paper shows a feasible way to solve difficult task of building an evolution path for transport networks. The path proposed here, presents one of possible ways which the core and metropolitan networks may follow. Certainly, depending on the type of networks, business model, technical constraints, etc., a different evolution path can be drawn. For example, some technologies may be implemented faster by a newcomer than by an incumbent core operator while metropolitan carriers can omit such technology at all. However, our evolution path seems to be representative for current core and metropolitan networks. During the process of building the evolution path, a wide range of issues has to be taken into account, ranging from technical, economic, organizational to social ones. Nevertheless, our paper is mainly focused on technical and, in some parts, economic issues. The following factors are taken into account at each step of the evolution scenario: services, e.g., Bandwidth on Demand Service, Provisioned Bandwidth Service and Optical Vir tual Private Network, quality of service/traffic engineering, connection provisioning/connection set-up con trol methods (permanent connections (PC), soft-permanent connections (SPC), switched connections (SC)), resilience functions, such as protection and restoration, control plane functions, such as routing and signaling, drivers behind each step, etc., All these factors will allow us to evaluate current as well as future steps of the evolution path in a way which permits network operators to offer services to customers. Provisioned Bandwidth Service (PBS) denotes here static near-real-time provisioning through management interfaces via a network management system (NMS) or an operations support system (OSS) with a client-server relationship between clients and the optical network. In contrast, Bandwidth on Demand Service (BDS) denotes dynamic and real-time provisioning in seconds or sub-seconds with signaled connection requests via a User to Network Interface (UNI). Optical Virtual Private Network (OVPN) specifies a set of provided network resources, e.g., link bandwidth, wavelength, and/or optical connection ports that may be used. For clients belonging to an OVPN, a Closed User Group (CUG) and a virtual network are defined, where optical connections may be based on static or dynamic (signaled) provisioning. The resource visibility and its control vary depending on the service contract. The organization of the remainder of this paper is as follows. We start with a description of the current status of transport and metropolitan networks. Then, an evolution scenario for a short term time scale is provided. Subsequently, medium and long term scenarios are presented. Throughout the article, the authors refer to the standardization process which is carried out by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), the Internet Engineering Task Force (IETF) as well as the Institute of Electrical and Electronics Engineers (IEEE). Moreover, the authors refer to other bodies, e.g., the Optical Internetworking Forum (OIF), the Metro Ethernet Forum (MEF) and the Resilient Packet Ring Alliance (RPRA). CURRENT STATUS OF TRANSPORT NETWORKS We start this chapter with an overview of main networking technologies used in modern transport networks. Next, we present the layered structure of 73

3 transport networks and outline their crucial characteristics. 1. Wavelength Division Multiplexing Today s transport networks widely use the wavelength division multiplexing (WDM), i.e., circuit switching technology and, in some cases, singlewavelength optical fibers, referred to as Traditional Fiber Optics (TFO). Wavelength division multiplexing is deployed for point-to-point communications with manually configured links. The spectacular increase of the capacity aggregated by the fiber optics removed the bandwidth bottleneck in the core, regional and metropolitan networks. WDM systems already installed by some network operators offer up to 40 Gb/s data rate per optical carrier and 160 carriers per fiber. The ITU-T recommendations specify a broad range of aspects related to optical networks including, e.g., the physical layer and WDM. Recommendations on the fiber optics physical layer are already in place (G.65x series of Recommendations), coarse WDM (CWDM) wavelength and dense WDM (DWDM) frequency grids are available as well (G and G.694.1, respectively) [1], [2]. The recent advances in optical layer technology enable the architecture optimization of telecommunication transport networks. Currently, the main effort of equipment vendors and network operators concerns the architectural aspects of the optical layer. The introduction of optical cross-connects OXC and optical add-drop multiplexers (OADM) are expected to lead to major cost reductions in overall networking due to reduced electronic signal processing and limited use of expensive optoelectronic conversion. Besides WDM, single-wavelength fiber optics systems are in use as well, however, usually at much shorter distances (less than 50 km). Hence, the TFO is typically applicable to some metropolitan networks. Nevertheless, it is broadly believed that the number of traditional fiber optics systems in transport networks will decrease. 2. SDH/SONET and Virtual Concatenation On top of WDM or TFO, the Synchronous Digital Hierarchy (SDH) or Synchronous Optical Network (SONET) is extensively used. The SDH/SONET is a circuit switching technology and is applicable to both metropolitan and core networks. It is wellunderstood, mature and standardized [3], [4], [5]. Since it was initially designed to optimize transport of 64-kb/s-based TDM services, a rigid capacity of payload as well as a coarse fixed-rate multiplexing hierarchy was defined. Today, SDH/SONET systems are built with bit rates as high as 10 Gb/s (STM-64/OC-192), with 40 Gb/s (STM-256/OC- 768) on the horizon. Current SDH/SONET core networks have a switching granularity of VC-4/ STS-3. A majority of all client networks are set up on top of SDH/SONET. By the use of Virtual Concatenation (VC) procedure, SDH/SONET may be improved to better meet today s requirements, e.g., various switching granularities. Virtual Concatenation [3] allows flexible concatenation of several SDH/SONET payloads. It assures an effective use of SDH/SONET capacity. Virtually concatenated payloads constitute a Virtual Concatenation Group (VCG). Members of a VCG, as opposed to contiguous concatenation, may not reside in the same STM-N/OC-N contiguously. They may even reside at different STM-N/OC-N interfaces and are treated within the network separately and independently. As a consequence, members of a VCG may reach the destination through various routes. Intermediate nodes do not need to handle virtual concatenation. The VC functionality must be implemented only at path termination nodes. This feature makes it possible to deploy virtual concatenation on legacy SDH/SONET equipment of existing networks, thus to smooth transition to enhanced networks. On the other hand, it should be noted that differences in the delay of an individual concatenated signal may occur due to pointer processing at intermediate nodes. Compensation of differential delays is handled at the destination node. Another advantage of virtual concatenation is its 74

4 ability to divide STM-N/OC-N bandwidth into several subrates. Each of the subrates may be used for accommodation of a different service. The bandwidth of STM-N/OC-N may be shared, for example, by both telephone service and data signals. An often-mentioned example [6] of a practical use of virtual concatenation is Gigabit Ethernet. VC-4-16c (STM-16) is required to accommodate Gigabit Ethernet signals at full speed under conventional SDH. However, the capacity of 1.4 Gb/s is then wasted. On the other hand, contiguous concatenation of four VC-4 containers (VC-4-4c) provides too small capacity to fully accommodate Gigabit Ethernet signals. The best solution would be concatenation of seven VC-4 payloads. It is possible with virtual concatenation. Bandwidth of 1.05 Gb/s provided by a VC-4-7v VCG is suitable for Gigabit Ethernet. More examples of bandwidth efficiency in carrying Ethernet, Fast Ethernet and Gigabit Ethernet data signals in SDH with and without VC are shown in Table 1. transferred by fiber links only. In this case, two types of physical interfaces were defined, the first one is suitable for local and metro area networks operation (LAN PHY: 10GBase-X, 10GBase-R), the second for wide area networks (WAN PHY: 10GBase-W). The 10 Gigabit Ethernet standard proposes physical interfaces based both on singleand multi-mode fibers. 10 Gigabit Ethernet LAN PHY offers an extended reach compared to Gigabit Ethernet, i.e., over a 40 km long single-mode fiber link. WAN PHY differs from the LAN PHY implementation by the use of the SDH/SONET framing with reduced functionality. The framing for WAN interfaces takes place at the WAN Interface Sublayer (WIS). The output from the WAN PHY is compatible with the synchronous frame format (VC-4-64c or STS-192c) and can be easily transported over an Optical Transport Network (OTN). The output from the LAN PHY interface of 10 Gigabit Ethernet has to be adapted before entering the OTN. The newly proposed Generic Table 1 Bandwidth efficiency of virtual concatenation Data signal SDH without VC SDH with VC payload mapping bandwidth efficiency payload mapping bandwidth efficiency Ethernet (10 Mb/s) VC-3 21% VC-11-7v 89% Fast Ethernet (100 Mb/s) VC-4 67% VC-11-64v 98% Gigabit Ethernet (1 Gb/s) VC-4-16c 42% VC-4-7v 95% 3. Ethernet Ethernet networks are important clients of the transport layer. We use the term Ethernet in the meaning of traditional Ethernet, Fast Ethernet, Gigabit Ethernet as well as 10 Gigabit Ethernet [7], [8]. The Ethernet technology is well understood and robust, its applicability to local computer networks cannot be questioned. Since years, 10 and 100 Mb/s Ethernets have been used for building cost effective, high speed data networks. In recent years, Gigabit Ethernet widely found its way into the metropolitan, regional and even wide area networks. 10 Gigabit Ethernet continues the evolution towards higher bit rates and an extended range, although, data are Framing Procedure format promises to provide this function. The Ethernet technology was also proposed as the base for new high speed access networks. The Ethernet in the First Mile working group, 802.3ah, was formed within the IEEE CSMA/CD working group. The scope of the work is the adaptation of the Ethernet technology to point-to-point and point-to-multipoint (E-PON) access networks [9]. A successful standardization process will extend the Ethernet coverage so that end-to-end Ethernet services can be offered to both business and residential customers. Future improvement of the quality of service functions offered in Ethernet networks can be achieved through the use of 802.1D (Class of Services) and 802.1Q (Virtual 75

5 LAN) specifications. Unlike SDH/SONET, the Ethernet technology does not provide any fast protection mechanism. Ethernet generally relies on the spanning tree protocol to eliminate all loops from a switched network. Even though the spanning tree protocol can be used to achieve path redundancy, it recovers comparatively slowly from a fiber cut, as the recovery mechanism requires the failure condition to be propagated serially to each upstream node. IEEE 802.1D Rapid Spanning Tree Protocol (RSTP) improves resiliency of the Ethernet [10]. However, SDH/SONET-like services still cannot be guaranteed, hence, the Ethernet suffers from inability to provide carrier class services. Although, some early works have been done by the Metro Ethernet Forum in its specification [11], it seems that it is still too early to fully introduce Ethernet based carrier-class services in the metro. Metro Service Model Phase 1 proposes service building blocks or service attributes and specifies how to build an Ethernet service. Such services, described as Ethernet Line, i.e., point-to-point services and Ethernet LAN, i.e., multipoint-to-multipoint service, may be offered over fiber, SDH/SONET or WDM technology. 4. Resilient Packet Ring Resilient Packet Ring is a new technology for ringbased metropolitan area networks that enables an efficient transfer of data traffic as well as fast protection mechanisms. RPR technology, which was standardized as IEEE RPR, is based on two symmetric counter-rotating rings that carry data and control information [12]. Additionally, the ring topology based on RPR is also studied by ITU-T. Specifically, ITU-T Recommendation X.87 specifies Multiple Services Ring (MSR) based on RPR and a way of multi-service provision over RPR [13]. RPR is designed to operate over a variety of physical layers, including SDH/SONET, Gigabit Ethernet, DWDM and dark fiber, and is expected to work over higher-speed physical layers. Some RPR technology features (distributed control, scalability in speed and number of nodes, plug-andplay operation, support various classes of traffic, advanced protection mechanism, etc.) triggered many pre-standard installations by some players in the telecommunications market (e.g., Sprint, Luminous, Bell Canada, MCI and SUNET). The first major pre-ieee RPR standard deployments were Dynamic Packet Transport (Cisco Systems proprietary solution) networks introduced by Sprint in 1999 and Macedonia Telecom and China Telecom in RPR technology implements the spatial reuse, which increases the overall aggregate bandwidth of the ring. Unicast frames are removed from the ring at their destination, which means that they occupy bandwidth on the links from source to destination only. RPR networks support three class of traffic. Specifically, IEEE RPR supports three types of services, namely Class A, Class B and Class C. The Class A service is designed to support real-time applications that require a guaranteed bandwidth and low jitter while the Class B service is dedicated to near real-time applications that are less delay-sensitive but that still require some bandwidth guarantees. Finally, The Class C service implements the best-effort traffic class. This service is subject to weighted fairness mechanisms, which ensure that each station gets its fair share of the bandwidth available. Two protection mechanisms may be used: steering and wrapping, both of which provide fast protection switching comparable with that of SDH/SONET networks. Neither of these mechanisms requires dedicated protection resources. RPR protection mechanisms have been designed and optimized to maintain the network connectivity and to minimize the packet losses in case of fiber cuts or node failures. RPR seems to be a promising technology, since most of the major carriers have actively participated in the standardization process and have shown much interest in the evolution of the standard. RPR systems are seen by many carriers as the inevitable successors to SDH/SONET ADM-based rings. Indeed, RPR network may provide performance-monitoring features similar to those of SDH and, at the same 76

6 time, maintain Ethernet s advantages (e.g., low equipment cost, high bandwidth granularity and statistical multiplexing capability). We can note that inability to operate over multiple rings may impede implementation of RPR in some areas. 5. Multiprotocol Label Switching Multiprotocol Label Switching (MPLS) is a connection oriented packet switching technique providing mechanisms for engineering network traffic patterns independently of routing tables. MPLS assigns short, fixed-length (20-bit) labels to network packets that describe how to forward them through the network. In an MPLS environment, the analysis of the packet header is performed just once, when the packet enters the MPLS domain. Label forwarding tables in routers store information on where to forward the packets. Additional information can be assigned with a label, such as class-of-service (CoS) values that can be used to prioritize packet forwarding. Usage of MPLS is not limited to IP networks. It may peer with ATM or Frame Relay networks. Appropriate standards were defined by IETF [14], [15], [16]. Label switched path may be tunneled (extended) in such networks. This functionality extends capabilities of IP services. Currently, the two main roles of MPLS are traffic engineering and Virtual Private Network support. MPLS provides functional traffic engineering capabilities required to implement policies that facilitate efficient and reliable network operations in an MPLS domain. MPLS decouples the routing and forwarding functionality. Finding an optimal routing scenario in presence of constraints imposed by limited capacity of connections and network topology is facilitated. These capabilities can be used to optimize the utilization of network resources and to enhance traffic oriented performance characteristics. MPLS TE (MPLS Traffic Engineering) provides capabilities for traffic tunneling, load balancing and explicit routing. Moreover, it eliminates the need for manual setting up of explicit routes. TE functionality encompasses also resilience issues. MPLS provides fast protection and restoration mechanisms. The network recovers dynamically from a failure by adapting its topology to a new set of constraints. MPLS VPNs do not need a predefined logical or virtual channel provisioned between two endpoints to establish a connection between the two endpoints. Traffic of various users is treated separately within the MPLS network without the need for encryption or tunneling at lower layers. MPLS VPNs are scalable (as opposed to connection oriented Frame Relay (FR) or ATM VPNs requiring hundreds of virtual channels for each closed group of users). Moreover, MPLS provides a capability for consolidation of data, voice and video services. Each VPN may use its own independent addressing plan. An incumbent operator does not need to change its addressing plan while deploying an MPLS VPN. MPLS also facilitate Quality of Service (QoS) assurance but it must be remembered that putting it on a par with QoS architectures such as IntServ and DiffServ is a misconception [17]. Its role is different. IntServ and DiffServ network models are not dependent on OSI/ISO layer 2 technologies and define a general QoS architecture for IP networks, which can integrate different transmission technologies in one IP network. MPLS is another networking technique, like ATM and Frame Relay, defined in layers 2 and 3. MPLS was originally intended to simplify packet forwarding in routers rather than to address service quality. Some features of MPLS can facilitate the QoS assurance. It can extend IntServ and DiffServ capabilities to a wider range of platforms beyond the IP environment. It facilitates offering IP QoS services via FR or ATM networks. Other MPLS features, such as capabilities for load balancing, flow control, explicit routing and tunneling are also important from the QoS perspective [17]. 6. The architecture of current core and metropolitan networks The circuit switched voice traffic was traditionally a major part of the traffic in core and metropolitan networks. Recently, however, besides the voice traffic, leased lines service has become an 77

7 important source of profits for networks operators. Additionally, it usually consumes a large part of the network s capacity. As the data, which may be identified with IP traffic, proceeds from narrowband towards broadband connections, it starts to play the dominant role in transport networks. Usually, IP routers are simply connected by SDH/SONET links with STM-16/OC-48 or STM-64/OC-192 interfaces. If protection is needed, the connections are transported over SDH/SONET. Otherwise, if it is sufficient to provide resilience purely at the IP layer, the IP router connections are directly mapped into static WDM wavelengthbased connections. The MPLS technology, which additionally improves the functionality of the IP layer, is installed in IP networks today. Resilience functions are possible at the MPLS layer and may be implemented on a per service basis in future networks. Broadband Leased Lines Services are based on SDH/SONET as well as on WDM technologies. At the same time, the position of dark fiber services is continuously decreasing. The transport of voice traffic is mainly performed by SDH/ SONET networks, however, the role of packet switching technologies, mainly the IP protocol, is growing. It can be noted that voice traffic may be conveyed by IP protocol over SDH/SONET technology or by IP over MPLS over SDH/SONET or by MPLS without usage of the IP protocol. A way in which voice payload may be directly encapsulated is defined in [18]. In this paper we use the term voice over IP to indicate that voice is transported Fig.1 Current transport networks - protocol stack over packet switching technologies. The protocol stack for the current transport networks is shown in Figure 1. Services Currently, the core and metropolitan networks provide Provisioned Bandwidth Service at the IP and the SDH/SONET layer. Additionally, it is possible to offer VPN services at the IP, WDM and SDH/ SONET layers. At both, the SDH/SONET and WDM layers, service provisioning may be very time consuming, particularly at the WDM layer, where provisioning of a service is a mostly manual process. QoS/TE Referring to the QoS and traffic engineering (TE) features the situation is nearly the same in the core and metropolitan networks. At the IP layer, MPLS supports traffic engineering, but quality of service parameters are still insufficient for a majority of service providers. QoS at the SDH/SONET usually meets expectations of users. Connection provisioning It can be noted that at the IP layer switched connections may be provisioned while the SDH/SONET technology allows only permanent and soft-permanent connections. At the WDM layer, permanent manually configured connections are feasible only. Hence, connection provisioning may be very laborious, time consuming and expensive. It seems that such a functionality, in most cases, is sufficient for operators of core networks. In contrast, in the metropolitan networks there is a growing demand for fast connection provisioning. Resilience Protection relies on pre-provisioned backup resources, whereas restoration, in principle, assigns backup resources only after the occurrence of a failure. Currently, both protection and restoration are possible in the core and metropolitan networks at the MPLS/IP level, while only protection mechanisms are provided at the SDH/SONET layer. Drivers Several factors play a significant role in the evolution of current transport networks. The growing volume of data traffic to be transported over networ 78

8 ks is commonly referred to as an important driver impacting the transformation of the network architecture. However, especially in the developed world, dozens national and international fiber backbones have been installed. Hence, besides some developing countries, there is no need for new fiber links crossing continents. The situation in the metropolitan areas is similar, even though it seems that there is still some room for new installations. Therefore, especially in the short term perspective, drivers different from those purely increasing demand for bandwidth are expected to dominate. Taking into account the worldwide economic slowdown, which we experienced in last years, the huge investments done recently by telecommunications operators and the strong competition on the telecommunications services market, it is obvious that cost reduction will be the predominant design constraint of the future transport networks. Spending of telecommunication operators can be reduced by limiting the necessary capital expenditures (CAPEX) on one side, and by optimizing the network operational costs (OPEX) on the other. Possible savings in operational expenditures together with enhanced network flexibility will be critical for the commercial success of network operators. The first step on the evolution path to cost reduction and increased network flexibility is the introduction of integrated and reconfigurable WDM systems (denoted here as rwdm). This step complements the need for increased bandwidth and the need for cost reduction at the lowest optical layer of the transport network. So far, at the WDM layer there are mostly static cross-connecting elements. It is not possible to allow in-service selection of the optical channel to be switched, added or dropped by the use of software control. Instead of early deployed point-to-point WDM systems, future systems will deploy a wavelengthrouted network. It may be accomplished by the use of flexible or reconfigurable optical add/drop multiplexers, optical cross-connects, as well as tunable lasers and receivers. Hence, Leased Line Service and Ethernet will be increasingly based on the optical layer. Improvement of flexibility seems to be particularly important in metropolitan networks, whereas in the core, usually underutilized links do not have to be equipped with reconfigurable elements. In the metro environment, some (mainly newcomer) operators will deploy Gigabit Ethernet and RPR technology-based networks. Furthermore, single-wavelength fiber optics systems will play a minor role in the transport networks. The layered architecture for the short term scenario with reconfigurable WDM is shown in Figure 2. SHORT TERM SCENARIO - ECONFIGURABLE WDM Fig.2 Network Evolution - reconfigurability at the WDM layer Services At this evolution step, the IP and SDH/SONET layer service provisioning is still time consuming and static in both metropolitan and core networks. However, the introduction of reconfigurable elements at the WDM layer allows network operators to offer PBS at the optical level. In contrast to the current situation, service provisioning at the optical layer may be performed faster. In addition to permanent and soft-permanent connections, which are feasible at the IP and SDH/SONET layers, in WDM networks with reconfigurable elements it will be possible to provision connections via the management plane. Additionally, in the metropoli- 79

9 tan networks flexibility of services may be improved by the increased number of Ethernet and RPR installations. QoS/TE Traffic engineering aspects do not change at this stage. Similarly, the QoS remains the same in the core networks. However, in the metropolitan area quality of service may be slightly augmented by implementation of RSTP and RPR. Connection Provisioning At this stage, rwdm will boost the connection provisioning in the core and metropolitan networks. Additionally, wider, than in the previous phase, implementation of Ethernet and RPR will increase the capability to deliver connections/ services to customers. Resilience At the IP and SDH/SONET layers, resilience remains the same as in the previous scenario, i.e., IP uses its rerouting capability in failure cases and SDH/SONET offers pre-provisioned protection options. At the reconfigurable WDM layer, at this stage, it will be possible to perform pre-provisioned protection, i.e., an rwdm device may detect a Loss of Signal (LOS) and automatically switch traffic from a faulty to a pre-provisioned working link. However, proper procedures to coordinate protection/restoration mechanisms at the electrical and the optical layers to provide a survivable network with QoS support and race conditions avoidance mechanisms have to be implemented. Similarly as for core networks, resilience in some metropolitan networks will be affected by the introduction of reconfigurability at the WDM layer. In other networks, the implementation of RPR may help network operators to ensure efficient protection at the required level. Drivers Strong competition on the market and continuously decreasing profit margins will force telecommunication operators to find new customers and to offer new services. This cannot be done using the business model based on the cost reduction only. We believe that the offer of new services is the key for success. The development of applications assuring fast and reliable access to remote resources - data storage applications, network-wide computation services, virtual reality - will affect network architectures as well. This can be translated into technical requirements as a need for flexible and standard framing methods for a wide range of client signals, starting from Fiber Channel or Enterprise Systems Connection (ESCON) formats to Ethernet or IP protocols. However, at this stage, there is still a problem with adapting the SDH/ SONET layer for transporting data traffic with either block-coded data streams such as Fiber Channel or Fiber Connection (FICON) or packet-oriented data streams, such as IP/PPP or Ethernet. Moreover, the legacy infrastructure in both core and metropolitan networks does not have the ability to adjust already established connections to changing conditions in the network. The equipment and/or control software are needed to allow a network operator fast adaptation to needs of a customer. This is particularly true for metropolitan networks. Hence, the TDM infrastructure, i.e., the voice oriented technology, has to be adapted to the data centric environment with proper flexibility and adaptability for the changing requirements. The need for cost effective solutions is still essential for network operators. The cost reduction may be achieved by transition of data transport and switching from the electrical to optical domain. Therefore, the core as well as metropolitan networks should be continuously transformed towards the optical domain. MEDIUM TERM SCENARIOS 1. Implementation of Generic Framing Procedure In this phase, the Generic Framing Procedure (GFP) will be implemented. The Generic Framing Procedure defines a very effective way of mapping a wide variety of data signals into transport networks [19]. It adapts traffic from higher-layer client signals over SDH/SONET, OTN or dark fiber into a common 80

10 format. The ITU-T recommendation defines two transport modes. The first mode, referred to as Frame-Mapped GFP (GFP-F), is optimized for the adaptation of PDU-oriented streams such as IP, native PPP, MPLS or Ethernet traffic. The second mode, optimized for block-code-oriented streams, is called Transparent GFP (GFP-T). This mode is used for Gigabit Ethernet, Fiber Channel, FICON and ESCON traffic. Both transport modes may coexist within the same transport channel. GFP addresses requirements of delay-sensitive applications such as storage area network (SAN). It is also expected to support the new IEEE RPR standard. Another advantage of GFP is its particular suitability to high-speed transmission links stemming from reduction of processing requirements for data link mappers/demappers as well as simplification of receiver logic [20]. At this stage, it seems that the center of gravity will shift towards services offered through GFP over WDM rather than SDH/SONET over WDM. Functionally, GFP consists of common and client-specific aspects. The former apply to all traffic. It encompasses data link synchronization and scrambling, PDU delineation, PDU multiplexing and client-independent performance monitoring. The client-specific aspects include mapping of particular client PDUs into the GFP frame, client-specific performance monitoring and OA&M functionality. Interrelation between GFP-F, GFP-T, the client-specific and common aspects as well as GFP relationship to client signals is shown in Figure 3. Fig.3 GFP mapping relationships Examples of client payloads that can be mapped on SDH/SONET via GFP are as follows [21] : Fiber Channel (850/ Mb/s); VC-4-6v/STS-3c-6v (900 Mb/s); Gigabit Ethernet (1000/1250 Mb/s); VC-4-7v/ STS-3c-7v (1050 Mb/s); FICON (850/ Mb/s); VC-4-6v/ STS-3c-6v (900 Mb/s). At the same time more efficient use of available network resources will be achieved. For many metropolitan areas it seems that there is still some room for resource usage optimization on the per day basis. Together with the switching capability, capacity of links used by business customers during the day can be re-used for residential users in the evening. Such a resource usage optimization at the medium time scale can be achieved at this stage by the use of the Link Capacity Adjustment Scheme (LCAS) protocol with support of agile management systems. LCAS [22] is an extension to Virtual Concatenation. It allows the dynamic alteration of bandwidth of SDH/SONET transport pipes. This is a key functionality for the transport of data-traffic coming from IP-applications while saving bandwidth. The number of concatenated payloads may be increased or decreased at any time without affecting traffic currently being sent. Moreover, LCAS will automatically decrease the capacity if a member of a VCG experiences a failure in the network, and LCAS will increase the capacity when the network recovers. When one of the constituent channels experiences a failure, the failed channel will be automatically removed while the remaining channels are still working. Thus, the available bandwidth will be lowered but the connection will be maintained. It can be noted that such a solution provides a lower probability of a complete connection failure in the system. The synchronization between endpoints during the addition or deletion of channels to a VCG is done via signaling. Similarly, single-wavelength fiber optics systems will be less used due to still increasing traffic. Their use will be mostly limited to access and metro areas. The development of new telecommunication services will also impose 81

11 more stringent requirements on methods responsible for providing and controlling services with guaranteed quality. Presumably, the growing amount of voice traffic and data traffic with stringent requirements will be conveyed by the IP/ MPLS and Differentiated Services (DiffServ) networks, which will be introduced at this stage. Differentiated Services architecture is a solution for providing different levels of service quality [23]. Independent flows choose one of the limited number of predefined services. Flows (packets) that choose the same service are aggregated and receive the same level of QoS. Aggregated packet processing by a network node is called Per Hop Behavior (PHB). Currently, the DiffServ architecture defines expedited forwarding (EF) [24] and assured forwarding (AF) PHBs [23] beyond the besteffort service. Traffic entering a network is classified and conditioned at the boundaries of the network. Active queue management mechanisms within a DiffServ domain are responsible for intelligent dropping of packets not conforming to a contract between a customer and an operator. In metro environments, it seems that the Ethernet as well as RPR standard systems will be widely deployed at this stage. Dynamic development of the Ethernet networks will probably essentially impact services offered in packet switched networks as well. In the medium term perspective, high speed, widely used and matured IP networks with MPLS support will be used for circuit emulation and for transparent transport of ATM, FR, Ethernet or even SDH data units. Such a network architecture - the architecture enabling transfer of layer two data units (e.g., Ethernet) over layer three (IP or IP/MPLS) may be very interesting for low cost and efficient interconnection of different network domains in highly competitive metropolitan environment. The IETF has already published first RFC standard on architecture of Pseudo Wire Emulation Edge-to-Edge (PWE3) services [25]. Next documents are expected to deal with mapping procedures for encapsulation of specific technologies, set-up and maintenance of the tunnel for data encapsulation, traffic policing, data fragmentation, connection verification and others. Internet drafts on the enumerated issues are already available at the web site of PWE3 working group [26]. Taking into account introduction of PWE3 services, layered architecture of transport network for the medium term scenario is shown in Figure 4. Fig.4 Transport Network Evolution - implementation of GFP Services Under this scenario, at both core and metropolitan networks, at the IP level, Provisioned Bandwidth Service and Bandwidth on Demand Service may be offered while in rwdm networks still the former one only. Due to the introduction of LCAS it is possible to dynamically increase or decrease the bandwidth of a connection at the SDH/SONET layer. Hence, the SDH/SONET better meets user requirements. Non-broadband connections such as STM-1/OC-3, up to now realized by using the SDH/ SONET technology, will be provided by the IP/ MPLS protocol as well. At the optical layer, only high bandwidth connections may be offered. Moreover, it seems that at this stage pseudo wire emulation service will be implemented. QoS/TE In the core networks, the QoS remains the same as in the previous scenario while quality in the metro- 82

12 politan networks may be enhanced by DiffServ. Traffic engineering, however, may be improved by implementation of LCAS in both core and metropolitan networks. Connection Provisioning At this stage, the connection provisioning capability remains the same as in the previous scenario. Resilience In the previous evolution steps, protection and/or restoration mechanisms were available not only at the SDH/SONET layer but also realized at the IP layer using MPLS functions, and at the WDM layer. In a network based on IP over rwdm with GFP framing, regardless of type of network, majority of functions of the next generation SDH/SONET (NG- SDH/SONET) technology, including resilience aspects, will be distributed over the IP and rwdm layers. NG-SDH/SONET denotes here SDH/ SONET with the VC and LCAS functionality. The need for proper coordination of protection/restoration mechanisms is still valid. 2. Introduction of Optical Transport Network An Optical Transport Network (OTN) is composed of a set of optical network elements connected by optical fiber links. An OTN is able to provide functionality of transporting, multiplexing, routing, management, supervision and survivability of optical channels carrying client signals. A distinguishing characteristic of the OTN is its provision of transport for any digital signal independently of client-specific aspects, i.e., it provides client independence. As such, according to the general functional modelling described in [27], the OTN boundary is placed across the Optical Channel/Client adaptation, in a way to include the server specific processes and leaving out the client specific processes. The client specific processes related to Optical Channel/Client adaptation are described in Recommendation G.709 [28], [29]. The standardization process of the OTN is conducted by the ITU-T. Namely, ITU-T Study Group 15 has been designated as a Lead Study Group for two important activities - the project on Access Network Transport (ANT) and Optical Transport Networks & Technologies (OTNT). The OTNT Standardization Work Plan describes the activities towards the specification of architectures and technologies for Metropolitan Optical Networks (MON), as well as Long Haul Optical Networks (LHON) [29]. The main difference between these two networking domains is the network requirements posed by telecommunications operators. The main driver forcing the evolution of metropolitan optical networks is low cost connectivity. This drives the adaptation of the local area network technologies (e.g., Ethernet). On the other hand, pervasive ring topologies force the introduction of RPR technology in the metropolitan networks. The issue of service dynamics also has to be considered. An increased demand for fast provisioned data transmission services characterizes rather metropolitan than long haul optical networks. The technologies considered to support MON include SDH/SONET, DWDM/ CWDM, Optical Ethernet, RPR and APON/EPON (ATM/Ethernet PON) [29]. The most promising technologies applicable to LHON implementation include almost the same set of technologies, excluding probably RPR and APON/EPON. The key recommendations on the OTN transport plane are at hand. A framework for OTN as well as references for definitions of high-level characteristics of OTN along with a description of the relevant ITU-T Recommendations is provided in G.871 [30]. The network architecture is characterized in G. 872 [31]. G.709 defines the interfaces of the optical transport network to be used within and between subnetworks of the optical network, particularly the optical transport hierarchy (OTH), functionality of the overhead in support of multiwavelength optical networks, frame structures, bit rates and formats for mapping client signals [28]. G.806, G.798 and G. 805 specify the equipment functionality [32], [33], [27]. At the same time, G.874, G and G.7710 describe equipment management functions of transport network elements [34], [35], [36]. Specifications of protection switching in 83

13 OTNs, e.g., G and G.873.1, are available as well [37], [38]. G.8201 and G.8251 are related to the error performance parameters for multi-operator international links and the control of jitter and wander within OTN [39], [40]. The physical issues, besides [1] and [2], i.e., the physical layer inter-domain interface (IrDI) specifications for optical networks, are provided in G [41]. The introduction of the OTN at the optical layer will enable telecommunications operators to provide digital services of controlled quality to the most important customers, customers requesting high data rate and high quality services. OTN supporting protection at the optical layer is the step at the network evolution path supporting the demand for high quality services, while implementation of restoration protocols at the same time will additionally assure better resource usage and promises cost reduction for offered services. The layered architecture for the medium term scenario with OTN is presented in Figure 5. Fig.5 Transport Network Evolution - OTN implementation Services The implementation of the OTN ensures that digital optical services may be offered, in contrast to the purely analogue WDM technology. OTN guarantees client independence, hence, a wide range of client signals after GFP encapsulation may be transparently conveyed. QoS/TE The introduction of OTN allows network operators to ensure QoS parameters at the optical layer. This can be achieved due to the Reed-Salomon 16 byteinterleaved forward error control (FEC), as described in G.709 [28]. Additionally, some proprietary FEC schemes are allowed and even better parameters of optical signal may be achieved. Moreover, OTN connection monitoring capabilities allow operation in a multicarrier environment. Namely, G defines error performance events, parameters and objectives for Optical Channel Data Unit (ODUk) paths of the OTN [39]. Soft-permanent and permanent connections will be provisioned by using the OTN technology. Moreover, at the OTN layer, connection monitoring will be conducted. Therefore, connection provisioning capabilities will be increased. Additionally, due to introduction of FEC, the strong limitation on some parameters of optical elements may be reduced. Hence, more economical network elements may be used. Furthermore, in contrast to current networks, longer transparent optical paths may be established. Resilience At the optical layer, besides LOS, a link or path degradation may be detected and proper mechanisms will be used to protect data traffic. The process of path selection for protection/restoration will be presumably performed in the management plane. G873.1 defines the APS protocol and protection switching operation for the linear protection schemes for the OTN at the ODUk level [38]. This recommendation defines subnetwork connection protection with a sublayer, inherent and non-intrusive monitoring. Drivers It seems that at this stage three main drivers will force the development of both core and metropolitan networks. Firstly, the voice revenue will dramatically drop. The growing number of mobile telephony users on one hand along with the increasing number of clients using packet voice techniques on the other, will probably dry up today s main revenue stream. Hence, the pressure on carriers to find new sources of revenues will grow. Secondly, the number of broadband users 84

14 will likely multiply. Ubiquitous broadband service offered to thousands of clients may force operators to transform their infrastructures towards networks with data centric technologies only. Additionally, broadband services may cannibalize the traditional voice service offered by operators by common usage of packet telephony, which may hasten the withdrawal from fixed telephony. Thirdly, it seems that the market for corporate telecom service will grow. Hence, network operators may start to offer new and more intelligent services. They can take part in the growing trend towards outsourcing and offer, for example, not only dumb connections but a whole package of services. It can be hosting the IT equipment, the management of data centers, the backup or disaster recovery service or a new comprehensive service. Such a single integrated service may be combination of knowledge about networks, possessed by network operators, and skills related to software integration, which is performed today by the IT sector. An example of such a convergence is the voice over IP (VoIP) technology. Undoubtedly, the key issue is the ability to offer flexible and customer tailored services. Hence, automated, or so-called intelligent network is a matter of utmost importance. LONG TERM SCENARIOS 1. Implementation of Automatically Switched Optical Network The introduction of intelligence (by means of signaling and routing protocols) in multilayer optical networks will enable network operators to meet emerging requirements, such as: dynamic and rapid provisioning of connections, automatic topology discovery and network inventory, reactive traffic engineering, and faster optical restoration. All these functions and features are important for the implementation of cost optimized, high quality telecommunication services offered in a flexible, high data rate telecommunication network. The Automatically Switched Optical Network (ASON), and its more generic counterpart, i.e., Automatic Switched Transport Networks (ASTN), are a set of control plane components which provide the possibility of setting up, maintaining and releasing connections [42], [43]. By using ASON, networks operators will be able to offer services which may be initiated by a client through the UNI interface [42]. ASON as well as ASTN, which are being developed by Study Group 15 of ITU-T, is the architecture that defines components and a set of reference points and rules which must be applied at the interface between clients and the network as well as between networks. The architecture defined by ITU-T is protocol independent and sufficiently generic to support various business requirements. The control model assumed in the architecture is the overlay model while connections may by signaled or may be provisioned in a hybrid way [42]. ITU-T Recommendation G.7714 describes the specifications for automatic discovery techniques to aid resource management and routing in the ASON networks [44]. G.7713 provides the requirements for the distributed call and connection management for both the User Network Interface (UNI) and the Network Node Interface (NNI) [45]. G is the answer for the requirements provided in [45] and is based on the PNNI/Q.2931 [46]. Meanwhile, G meets the same requirements but is based on the RSVP-TE [47]. In G.7715, the requirements and architecture for the ASON routing functions used for the establishment of switched connections (SC) and soft-permanent connections (SPC) are specified [48]. However, the protocol-neutral requirements for a hierarchical link state routing protocol are provided in a newly proposed G [49]. The transport of distributed call and connection management and signaling messages may be performed by a data communication network (DCN) described in [50]. The Optical Internetworking Forum proposed the User Network Interface (UNI) 1.0 Signaling Specification [51]. OIF is a non-profit organization with the aim to foster development and deployment of interoperable products and services for data switching and routing using optical networking technologies. The organization, which has the official liaisons with ATM Forum, IEEE 802.3, IETF and ITU-T SG 15, has six working groups. The groups cover a wide range of technical issues related 85

15 to optical networks. OIF Implementation Agreement OIF-CDR-01.0 specifies the usage of measurement functions that an Optical Switching System will need to perform in order to enable carriers to bill for OIF UNI 1.0 optical connections using their legacy billing systems [52]. It also specifies three formats for storing these usage records in files for processing by the carrier s billing systems. OIF-SEP-01.1 defines a common Security Extension for securing the protocols used in UNI and NNI [53]. The OIF-E-NNI-01.0 specifies of External NNI (E-NNI) signaling abstract messages, attributes, and flows for end-to-end dynamic establishment of transport connections across multiple control domains and, so far, applies to SDH/SONET connection services only [54]. It can be noted that with ASON, Generalized MPLS (GMPLS) family protocols may be used as well, e.g., Resource Reservation Protocol - Traffic Engineering (RSVP-TE) [55]. On the other hand, there are some differences which can make the process of reusing GMPLS tools in the ASON scenario difficult for networks operators. Firstly, UNI is not a trusted reference point, and hides all routing and addressing information pertaining to the interior of the network from the user. Moreover, a user belongs to a different address space than the internal network nodes. Hence, the ASON scenario may be identified with the overlay model only. Secondly, the ASON concept assumes a distinction between call and connection signaling which is not present in the GMPLS set. Therefore, from today s point of view, in some areas the GMPLS set is well suited to operate over the ASON architecture and at the same time some mechanisms taken from the ITU-T and IETF standardization seem incoherent. Furthermore, ASON focuses merely on SDH/SONET, OTN and PDH while GMPLS embraces packet, time-division, wavelength and spatial switching. However, the authors believe that a solution based on a constructive compromise between ITU, IETF as well as OIF will be found. Essential advances in optical technology will probably enable a new transfer mode of data in optical networks. In the long term scenario, at the edge of next generation optical networks, data addressed to a particular destination will be collected together and formed into an optical data unit, referred to as burst. The burst is to be forwarded towards destination with the use of any available wavelength. All the control information necessary to transfer the burst to the final destination will be sent with by using an out of band control channel (next wavelength, for example). The optical network adapting the presented idea is referred as Optical Burst Switching (OBS) network. Bursts, composed of data of distinct users, can share in an OBS network a single wavelength. It is expected that OBS networks will offer much higher flexibility, will increase network resource utilization ratio and will essentially improve efficiency of the optical and IP network interface. In the perspective of long term scenario it is expected that some open issues specific to OBS networks will be solved, for example, optical routing protocols, QoS assurance, burst assembly procedures, resource reservation, as well as security issues. The layered architecture of future transport networks with possible implementation of the OBS idea in the transport plane is shown in Figure 6. Fig.6 Transport Network Evolution - implementation of ASON or GMPLS Services The implementation of ASON or GMPLS in both core and metropolitan networks significantly changes the spectrum of services offered to customers. Under this scenario, Bandwidth on Demand Service (BDS) is introduced at the optical layer. Therefore, a client may 86