Convergence is commonly understood as the availability of voice and data services including Internet access, multimedia, gaming, video, etc., on the same “platform”. In the context of the end-customer, it refers to the availability of services such as multimedia messaging or data services on mobile phones, or VoIP services on a personal computer. The deployment of both voice and data services in India is occurring at a frenetic pace.

Teledensity, or the availability of voice telephone services, is increasing dramatically and has reached 9 percent with 2 million phones added in February ’05 alone. Much of this increase occurred through the addition of mobile phones, which are also capable of data SMS and Multimedia Messaging Services.

Broadband penetration is steadily increasing with wide adoptions like DSL services to the home for Internet access, again leading to a convergence of voice and data services to the home.

From the service providers’ perspective these converged services also require corresponding networks that can support voice and data services seamlessly. With decreasing cost of services, the service provider is driven to reduce both his capital and operational expenditure. At the same time, the customer expects these services to be as robust as the traditional fixed line telephone service with which he is familiar.

Thus, networks delivering these services need to be robust and highly reliable; the typical requirement being an availability or uptime requirement of 99.99 percent.

Technologically there are three key components of the network in the delivery of voice and data services. The first is the “Switching” network that is usually service specific. For example, core routers handle the switching of Internet traffic and exchanges, voice traffic.

Interconnecting these core routers and exchanges/switches is the “Transport” network, which is invariably based on Synchronous Digital Hierarchy technology. SDH retains its pre-eminent position as the sole transport technology for its superior Operations, Administration and Management, fault isolation and performance monitoring capabilities. These networks have high reliability at least five-nines, typically approaching six-nines and support data rates of 2.5 Gbps or 10 Gbps.

These rates can be further aggregated using Dense Wavelength Division Multiplexing technology to 100s of Gbps.

The third component of all networks is the “Access” portion that connects subscribers to exchanges or routers and which will be the focus of the rest of this article. There are two approaches to building access networks. The first is to build two networks: one for voice and TDM services such as leased lines including Ethernet private line services, and the other is data aggregation services such as Internet access. The TDM network is typically a next-generation SDH network that provides native support for Ethernet transport. This is termed the overlay approach since the voice/SDH network is typically pre-existing, and the data aggregation network is added on as separate network. In this approach, the data aggregation network runs Ethernet directly on a separate Ethernet-over-fiber and provides Internet access services. This architecture increases both capital expenditure and operation expenditure since two separate networks have to be built, operated and maintained, as illustrated in

Figure 1.
The second approach is more cost-effective and technologically superior to building an overlay network. This would be to build a “Switched-SDH” network or in other words a “Converged” access network. These networks address the varied networks infrastructure prevalent today from Copper and Co-axial Cables (PDH) to Fiber (SDH). This network support packet switching technologies such as Resilient Packet ring, Multi Protocol Label Switching and Layer 2 Virtual Private Networks all targeted towards deploying data services seamlessly. Hence, it reduces capital expenditure by eliminating all the external equipment, which is otherwise required for supporting data services using overlay networks.

Today most service providers have started deploying MPLS backbones in addition to their SDH backbones. New carriers will opt for Switched-SDH for supporting both voice and packet aggregation services, using a single network.

These networks enable traffic multiplexing and have an intelligent common control plane for effective provisioning and control of services. Equipments supporting Switched-SDH incorporate intelligence on the same lines of IP/MPLS routers based on GMPLS (generalized Multi-Protocol Label Switching) standards. Thus, switched SDH combines the strengths of both SDH and IP/MPLS networks: fault resiliency, very high reliability, and extensive OAM features of SDH, with the flexibility and control plane intelligence of IP/MPLS networks.