Next-Generation Metropolitan Area Networks

by Kari Seppänen, Sami Lallukka and Riikka Lemminkäinen

Because of the emergence of new services, the requirements for metropolitan area networks (MANs) have increased and diversified. Consequently, they are harder to meet - at least with the existing SONET/SDH networks - and new solutions are needed. VTT Information Technology has been researching next-generation SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy) networks, which could meet the diverse requirements of MAN while utilising the existing SONET/SDH network infrastructure.

Ideally, network operators would only have to manage a single type of network. One multi-purpose MAN (Figure 1) would connect all the various access networks and provide everything from real-time services to traditional data-transfer services. The network would also provide quality of service (QoS) and handle any kind of traffic, from constant bit-rate traffic to packet- or cell-based traffic. While such a multi-service network would minimise overall operating costs, the existing SONET/SDH infrastructure is unfortunately unable to meet these requirements. The worst-case scenario is that the operators must maintain their legacy networks for legacy services while the new services need overlapping network infrastructure.

Figure 1: Metropolitan area network connects various access networks to the core network.

In recent years, SONET/SDH-based transport networks have come to be considered as too inflexible, inefficient, and overly complex for the purposes of data communication. As the importance of data communication has increased, a search has begun for a replacement for SONET/SDH. However, developing such a replacement is neither easy nor straightforward. On the contrary, many useful functions provided by the SONET/SDH appear to be too complicated to reinvent in other way. Furthermore, long-established telecommunications companies have already invested billions of euros in their SDH networks, and would therefore prefer to utilise the existing infrastructure.

Fortunately, a new solution based on proven SDH/SONET technology is evolving, and promises to turn SONET/SDH into an efficient multi-service transport network that is easy to manage and provision. 'Data over SONET/SDH' (DoS) is based on three new features in SONET/SDH networks: VC (Virtual Concatenation), LCAS (Link Capacity Adjustment Scheme), and GFP (Generic Framing Procedure). VC and LCAS together enable fine-grained capacity allocation and management. Efficient framing and link-layer statistical multiplexing is achieved using GFP, which provides a unified method to multiplex packets from diverse sources. Furthermore, DoS-capable equipment can be mixed with older, inflexible SDH equipment to provide a reasonable evolutionary upgrade path for 'traditional' network operators.

The ability to provide optical connections rapidly and dynamically while making optimal use of network resources is also important. This can be achieved by adding intelligence to a traditional optical transport network and updating it to an Automatic Switched Transport Network (ASTN).

Optical Access Networking Project
The concept of the next-generation SDH network was studied in the Optical Access Networking (OAN) project, a three-year project which commenced early in 2001. Project collaborators included VTT Information Technology, VTT Microelectronics and Networking laboratory of HUT (Helsinki University of Technology). The project was funded by the National Technology Agency of Finland (TEKES) and supported by two industrial parties - the Nokia Research Center and Elisa.

Through the OAN project, a network evaluation platform was developed. The objective was to design and implement the electrical parts of the feeder network, including all the networking activities starting from the link layer.

OAN Platform
The OAN network acts as a feeder network, connecting multiple subscribers to a core network. The test network prototype (Figures 2 and 3) exploits two counter-rotating 2.5 Gbit/s SDH rings in the transport network side, and two Gigabit Ethernet links together with one 2.5 Gbit/s SDH link in the access network side. The network functionality is implemented in Field Programmable Gate Arrays (FPGAs), which enable flexible network design and system testing. The physical equipment of the OAN node is fitted into a standard CompactPCI–frame, making the node easy to move and install. The CompactPCI-frame includes a Central Processing Unit (CPU) card, which is used by the management software.

Figure 2: Access node architecture.

Figure 3: An OAN prototype node is fitted into a standard CompactPCI-frame.

The access network and transport network interfaces are implemented into separate interface and router cards respectively. Data between the interface and router cards is transported using a dedicated bus system and specific transport control. Flexible design of the system allows the interconnection of multiple router and interface cards. By developing new interface card versions, multiple technologies can be supported in the access network side.

To obtain both efficient optical protection and a cost-effective structure, the OAN network is physically a ring and logically a star. The OAN prototype network consists only of three to four nodes, providing low network complexity. One node acts as a hub, connecting other nodes to the core network and to each other. The connections are established using WDM (Wavelength Division Multiplexing) technology.

Furthermore, a network-monitoring system for the OAN network was designed. In the design the usability of Simple Network Management Protocol (SNMP), standardised Management Information Bases (MIBs), and ready-to-use management software was considered.

Next-Generation SONET/SDH in Use
The next-generation SONET/SDH enables new types of services with more efficient network usage to be easily implemented by utilising existing infrastructure.

Corporations require diverse services (eg voice, VPN, data storage, and Internet connection services) from operators. Traditionally the different services are provided through technology-specific transport pipes. However, the next-generation SDH enables the simultaneous transport of heterogeneous services over one wavelength, thereby saving network-building and maintenance costs.

Usually a virtual private connection (VPN) is used to bridge operators' access points. In some applications however, it is desirable to transport the native network signal without extracting packets or frames. Normally the datacom protocols rely on 8B/10B coding, which causes a 25 percent increase in bandwidth. Using the next-generation SDH, which maps 8B/10B-coded data into 64B/65B-coded sequences, the required bandwidth is substantially decreased.

The ability to dynamically reallocate bandwidth allows Bandwidth on Demand (BoD) services. This will revolutionise the network service industry, since the users are able to specify their bandwidth requirements according to the time of day.

Please contact:
Kari Seppänen, VTT Information Technology, Finland
Tel: +358 9 456 5610
E-mail: Kari.Seppanenvtt.fi