TDM Services over IP Networks
1. ABSTRACT Time Division Multiplexing (TDM) circuits have been the backbone of communications over the past several decades. These circuits which provide reliable and low-delay services for voice, data and video transport, are migrating towards Internet Protocol (IP) based packet switched networks. The primary reason for the migration ofthese circuits is to reduce the cost of transport and management by having a converged network for all services. Due to the sheer magnitude of the installed legacy TDM equipment, this migration to “end-to-end IP” will go through a transitional phase where some services will continue to use legacy equipment, while the core network moves towards IP. In this transitional phase, there is a need fortechnology allowing seamless transmission of TDM services across the packet switch networks. The Internet Engineering Task Force (IETF) and International Telecommunication Union (ITU) provide specifications for interoperability to emulate TDM circuits over IP networks. These emulated services can be implemented using a gateway device that provides for inter-working function (IWF) between TDM and IPnetworks. The primary challenge of the gateway device is to provide the equivalent level of reliability and security of traditional TDM networks. To that extent, the gateway device must be able to achieve an equivalent level of synchronization of TDM circuits across the IP network and cope with packet impairments of the underlying IP network. In addition, the gateway can provide value addedservices such as echo cancellation, multicasting and IP security. The remainder of this paper explores the architecture of a Gateway device such as Harris’s NetXpress, which can facilitate a smooth transition for legacy TDM applications as the core network is migrating from TDM circuit switched to IP based packet switched networks. 2. NETWORK EVOLUTION Over the years, TDM based Wide Area Networks (WANs)have enabled transport of a multitude of user applications including: basic telephony, trunked radio, low to medium rate data services and studio to transmitter links for broadcast audio and video. Figure 1 shows a basic model for an end to end TDM system.
Figure 1 End to End TDM Network
1-4244-1513-06/07/$25.00 ©2007 IEEE
The TDM network dedicates a circuit with a fixed amount ofbandwidth for the duration of a session, regardless of its actual usage. For voice applications, these networks have performed well, however for emerging data intensive applications, these networks do not scale effectively. As a result, IP based WANs are deployed to allow for cost efficient expansion of capacity and statistical multiplexing gain for new emerging applications. In addition, IP based WANsare built on open standards allowing inter-operability of equipment from different vendors. To allow for a smooth migration of legacy equipment, the IP WANs will be required to support legacy TDM applications in a seamless manner. The convergence of TDM traffic into the IP networks has to be well managed for maintaining the performance level of the TDM applications. For transporting voice acrossIP networks, Voice over IP (VoIP) technology has emerged at the forefront. While this technology provides the most flexible way of routing telephony calls, it is very complex. The complexity of protocol inter-working between TDM and IP call processing may not be suitable and needed for many applications. Alternatively, Circuit Emulation Service (CES) technology has emerged as an option totransport TDM trunks containing legacy applications across managed IP networks. This technology is sometimes referred to as pseudo-wire, as it emulates the TDM circuit across a packet network using virtual IP tunnel or path. The primary benefit of this technology is the cost and simplicity of deployment to support all types of legacy TDM applications without the need for complex protocol inter-working....