To address the increasing customer demand for high-bandwidth applications and converged voice and data services, and to address the shortcomings of traditional metro access and transport equipment, a wide array of solutions has been proposed by equipment vendors. These solutions tend to fall into four main categories: WDM and DWDM, virtual path ATM, passive optical networking (PON), and Gigabit/Terabit Ethernet routers that handle both data and voice traffic.

While each has propelled the market toward simplification and cost improvements, they all have their shortcomings for the metro edge.

WDM and DWDM

WDM and DWDM technology is the new de facto standard for core transport applications. By multiplexing many wavelengths of light onto fiber-optic cable, service providers are able to transport mass amounts of data over very long distances. Because metro DWDM solutions are derived from their more expensive upstream cousins in the core networks, they are still very expensive to deploy and very difficult to manage. Additionally, there are still no standards for DWDM, making every vendor’s solution proprietary and non-interoperable with other DWDM systems, let alone SONET or SDH. While there is no doubt that WDM technologies are the end game for metropolitan access and transport networks, the economics and technology are simply not there at this point in time.

VP ATM

Virtual path ATM (VP ATM) solutions try to solve the access and transport efficiency problem by overlaying the ATM protocol on top of existing SONET/SDH physical infrastructures. The key problem with this technology is that of interoperability. VP ATM sits on top of (or overlays) existing SONET infrastructures utilizing only the physical characteristics of the protocol. In this type of network, an existing SONET ADM would be configured in a pass-through mode; in other words; traffic originating from an existing ADM cannot talk with the new VP ATM device and vice versa.

Passive Optical Networking

PON technology operates by basically shooting a single high-bandwidth pipe to a densely populated area, then splitting that pipe out into smaller pipes for access and transport. While these solutions bring bandwidth closer to the user and are protocol-insensitive, they are basic Layer-1 devices, with no switching or grooming intelligence. As there are no standards as of yet, every PON system is proprietary and requires both optical networking units (ONUs) to access the transport network and optical modems for the actual local access. Finally, PON systems are not interoperable with existing SONET/SDH infrastructures, making them unattractive for more established networks.

Gigabit/Terabit Ethernet

Gigabit Ethernet (GigE) for metro networks is an extension of basic LAN access technology. While GigE is well suited for moving massive amounts of data over mesh-oriented networks, it overlooks the complexities of grooming and transporting bundled voice and latency-sensitive data services. Until these vendors can standardize network-wide quality-of-service (QoS) provisioning and better control jitter and delay, GigE will most likely remain in the domain of core data networks.