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Feature Article
November 2002

Making Voice A Broadband Reality Today:
TCMoIP Delivers


In order to meet their overall communications needs, businesses typically employ several parallel networking technologies. Data services based on packet switched IP transport are used on the LAN. On the WAN, time division multiplexing (TDM)-based services (at T1/E1, DS3 or higher rates) for voice as well as packet over TDM for data communications are deployed. This results in a disjointed state of complexity and high cost.

Smart companies are actively seeking ways to move to a common IP-based infrastructure capable of meeting all their communications requirements. Packet-based services are cheaper, more flexible, and more efficient in carrying a given payload than dedicated TDM circuits. Facing such strong economic incentive to migrate, businesses are evaluating ways to provide voice as well as data services over a common packet-switched IP based transport. And service providers, such as cable operators, are responding by evaluating new ways to simply emulate and extend T1 or E1 circuits over their high-speed packet networks based on IP or Gigabit Ethernet. This technology option is called Time Division Multiplexing over IP (TDMoIP)

The Case: Why TDMoIP?
While delivering data services over IP is relatively straightforward and currently offered universally, there are two major impediments to the adoption of a pure IP-based infrastructure for delivering voice services. First, voice over IP (VoIP) services are still in their infancy, with a number of competing signaling standards and vendor products that do not always interoperate. TDM-based voice networks, which have evolved over several decades, have a number of advanced calling features that are not readily available with VoIP. These features include, but are not limited to voice mail, unified messaging and call forwarding. Work must be done before VoIP will be able to offer these features on a comparable level. 

Second, most businesses have a significant investment in communications equipment (e.g., PBXs, T1/E1 channel banks, message centers, etc.) that support voice and data services over TDM access. Migrating to VoIP services will render much of this equipment obsolete and will require businesses to replace it with VoIP equipment. Businesses are understandably reluctant to take this step because VoIP services are still in a state of evolution while the availability of capital budgets is very limited in the current economic climate. 

TDMoIP represents a clever and inexpensive way out of this dilemma, and an evolutionary step in migrating to IP-based communications. TDMoIP offers a scheme for carrying TDM traffic transparently over an IP infrastructure so that terminal equipment used for TDM transport can continue to be used even when the TDM-based communications links are replaced with IP-based transport. This means that the full-featured voice services available over TDM networks (voice mail, conferencing, etc.) continue to be available (with no change) even when the TDM-based infrastructure is replaced with an IP infrastructure. Also, there is no need for businesses to replace their expensive communications gear designed for TDM transport even when they switch over to a common IP-based infrastructure.

The Definition
But just what is TDMoIP? What are its capabilities and how does it relate to VoIP? 

A readily available, cost-effective technology, TDMoIP combines features of TDM and Internet protocol (IP). TDMoIP is a protocol for extending circuits over IP networks. TDMoIP enables the transport of circuit-based services, at T1/E1 to T3/E3 speeds over IP-based networks. It works by chopping the T1/E1 synchronous bit stream into Ethernet packets, adding IP headers and then forwarding those packets to their destination over the IP network. At the destination, the original bit stream is reconstructed by removing packet headers, concatenating (or arranging) the packets, and regenerating the clock. The most important requirement for the IP transport network is that it needs to have high symmetrical bandwidth at speeds approaching 100 Mbps and an access network that supports quality of service (QoS) with guaranteed bandwidth and low delay (few milliseconds).

The Difference
TDMoIP is simpler than VoIP because TDMoIP is transparent to voice and data signaling and protocols, even when they are proprietary. Thus, TDMoIP takes advantage of existing (sometimes proprietary) PBX features. TDMoIP technology can be used to offer virtual leased line services over IP irrespective of the data/voice content. More complex, VoIP technology has issues with new protocols and translation between signaling formats. 

And, proponents say that TDMoIP beats VoIP with lower latency and higher quality voice. But, it is important to note that some of these TDMoIP benefits are a result of the technology�s not having to compress voice or use silence suppression (unlike VoIP). While the benefits are great, TDMoIP does require more bandwidth than VoIP. Once the size of VoIP headers is taken into account, industry experts estimate that TDMoIP requires about three to four times the bandwidth of VoIP, depending on the latency permitted. Even with the extra bandwidth requirements, TDMoIP is a here-today broadband solution.

The Requirements: Delivering TDMoIP
In examining TDMoIP, it is critical to recognize that circuit emulation requires a symmetrical, high-bandwidth network and strong QoS support with delay and bandwidth guarantees. Why? Terminal equipment designed to handle TDM traffic generates and expects to receive synchronous transmissions in accordance with appropriate standards. 

When TDM links are supported over an IP-based infrastructure, the challenge is being able to deliver the payload in a synchronous manner even when the underlying IP infrastructure introduces variable delays and the occasional lost packet. The key to providing TDM services over an IP network is the deployment of an IP multiplexer or gateway. This device collects the payload bytes from one or several frames associated with each TDM link on the source side for replacement with a TDMoIP link. The collected bytes are formatted into a packet with appropriate headers and trailers, and transmitted toward the destination over the packet-switched IP network. At the destination, the IP gateway receives these packets, strips off the headers and trailers, and delivers the payload in a synchronous manner to the corresponding TDM terminal equipment. In order to combat the variable delay, the destination IP multiplexer employs a jitter buffer to overcome the worst packet delay variation expected in the network.

The Opportunity
�The challenges ahead for providers are to find ways to continue to grow the subscriber base, and to add new services for existing customers,� according to Bruce Leichtman, president and principal analyst for Leichtman Research Group, Inc. TDMoIP is one of the best keys to �unlock� new broadband services that will attract subscribers and enhance any broadband service provider�s portfolio of offerings. The cable operator is a prime example of a service provider that can harness the power of its existing network to deliver TDMoIP services.

For cable operators to deliver low-cost TDMoIP services, they must prime their HFC networks. This preparation is the deployment of a broadband, QoS-managed access medium over their existing hybrid fiber-coax (HFC) networks. This broadband access network can be enabled by intelligent switching devices that replace existing analog devices (amplifiers, taps, etc.) and convert the HFC plant into a switched Ethernet network with fully symmetrical (upstream and downstream) gigabits-per-second transmission speeds on the HFC trunks and symmetrical 100 Mbps drops.

Cable operators offering broadband access networking described above can deploy a TDM-to-IP gateway at the subscriber�s premise to interface with the TDM equipment. This network is then ideally suited to support TDMoIP connections with guaranteed bandwidth and low packet delays and delay jitter. Properly engineered, delays for the highest priority traffic can be kept to within 2 to three milliseconds. This is key for potential TDMoIP customers for whom network performance is critical.

The TDMoIP business opportunities for cable service providers can be applied to at least three distinct areas: TDM links between PBXs and central offices or between PBXs at multiple locations; TDM links for data communications; and TDM for backhaul from cellular networks. And, since TDM services (T1/E1 and DS3 links) available to the SMB market are typically expensive and require a lengthy provisioning time, cable operators are well positioned to inexpensively and quickly set up TDMoIP links over existing HFC and provide T1/E1 services to compete with incumbent and competitive local exchange carriers (ILECs and CLECs.)

It�s Time for TDMoIP
TDMoIP is the informed technology choice for delivering synchronous T1 and E1 circuits transparently over IP networks, a key functionality that will help take broadband networking to the next level. TDMoIP provides service providers, like cable operators, with an alternative way to build market share among business customers, quickly and inexpensively with more services. This enabling technology differs from VoIP in that it is totally transparent to myriad signaling systems and protocols used in the PSTN. The fact that TDMoIP supports any T1 or E1 traffic irrespective of whether it�s framed or unframed, fractional or full, provides carriers and operators with a much higher level of flexibility in offering a variety of services. Data, voice, and signaling protocols can now be transparently carried as bit streams over IP.

Sanjiv Nanda is director of systems engineering at Narad Networks. Mark Heslop is director of product marketing at Narad Networks. Narad Networks is an IT services software and broadband access company that�s creating new business models for cable operators through a comprehensive software suite which enables the creation, provisioning, and management of end-to-end broadband services dynamically and on demand. For more information, please visit them online at www.naradnetworks.com.

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