VoIP: Closing in on TDM

VoicenData Bureau
New Update

Today, networks need to be seamless in providing the newer services using the same infrastructure. VoIP takes care of that need, in addition to supporting apps like prepaid, postpaid, and even calling cards


The demand for packet telephony solutions continues to grow as both incumbent and new carriers seeks ways to increase revenues, create differentiated services at reduced costs. As statistics would have it, about 12 percent of the international traffic was carried by VoIP networks during 2002. The number of minutes of traffic carried on VoIP networks is steadily increasing because of significant techno-economic advantages. While new operators are doing greenfield deployments of VoIP networks, existing operators are migrating to soft switch-based packet architectures. The ability to merge seamlessly with legacy systems has accelerated the pace of VoIP’s growth. That IP networks provide more services, flexibility and maneuverability is a forgone conclusion now. 

The various services that VoIP networks can offer include ILD services, NLD services, exchange carrier services, trunk replacement, rural telephony, VoIP VPN, IP centrex / hosted PBX, conferencing, global call center, and calling card.

Cost Advantages of VoIP 

Before any product is introduced incumbent carriers need to carry out cost analysis between the legacy equipment facility’s cost and customer requirements on one side and the cost of new technology equipment, interface, and workability on the other. However, for the purpose of comparison some reference point has to be worked out. That reference point is cost per port.


VoIP Needs Less Ports

In traditional Class-4 switches, each long distance call occupies two ports (incoming and outgoing). Thus, actual number of concurrent calls is equal to half the total number of ports per switch. In contrast, a call being carried by a VoIP network requires only one port at the VoIP gateway.

So the price per port (PPP) of a VoIP gateway is arrived at by dividing the price of the gateway by the number of telephony interfaces, since all these ports share the same outgoing IP link. However, in TDM PPP calculation, the cost of the switch is divided by total number of ports, thus reducing the PPP by a factor of two. 


Packet Tandem: System should be able to support transit traffic service for ILD and NLD traffic. It should also be such that the system support transit traffic from any media gateway and be able to route calls based on routing plans. It should also be possible to offload calls from the switch to the packet tandem and be able to route as per routing plan.
Integration with

This is accomplished through SS7.
SIP Protocol Support: Session Initiation Protocol (SIP) offers a large flexibility for creating and deploying new services and for inter-working with commercially available platforms.

It is necessary that the SIP server is large enough to supply BHCA of around 1 million. Modular expansion should be possible. Similarly media gateway controllers should also be able to match traffic requirements with necessary reliabilities. Hardware configurations in a multi server configuration enable desired performance levels.

Call Routing is the central role of a SIP server in a packet tandem network. Routing is handled by packet tandem service on a SIP server. The packetÂ

tandem service provides a rule based decision engine for flexible routingÂ

with an extensive set of functions for controlling the dialing plan are part ofÂ

the system.
End-to-end Signaling: Success of all call delivery is signal dependant. It is necessary that signaling is transparent to the TDM software also.

CDRs: CDRs are generated by media gateway controllers. It should be possible to collect them via FTP so that remote-billing systems could be used.

Cost of Carrying ILD Calls 

In a large country like ours, invariably calls originated for an international destination is passed through multiple switches before they are handed over to the international gateway. For example, a call originating in Jalandhar will first enter the respective subscriber’s switch, then the TAX switch at Jalandhar, followed by the TAX switch at Delhi, before finally arriving at the VSNL switch at the international gateway. A scenario could arise when the number of switches involved could be more than six. It is significant to note that more the number of switches more the number of ports that the call has to pass through. Contrast this with an IP network where the maximum number of ports per call will be two only.

Entry-level Cost

In small PoPs, with relatively low traffic, VoIP equipment is reportedly cost effective. TDM switches

are generally large.

Dimensioning them to a lower size does not give the requisite cost advantage. The

infrastructure required for TDM switch is high compared to IP PoPs. Thus capital cost is the issue when

it comes to TDM switches. A fine growth granularity is available in VoIP networks with adequate scalability. Return on investment

ratio becomes better and is likely to happen within shorter timeframes as compared to TDM switch-based networks.


VoIP Technology Is Efficient

Deployment of a new TDM PoP in a TDM network would call for a major investment in transportation and infrastructure as compared to VoIP networks. In a VoIP network, adding a PoP means adding a single interconnection between the gateway and the IP cloud. However, in TDM networks, in order to avoid the situation where a single call has to pass through multiple switches to reach the destination, the new switch must be connected in a mesh-like configuration to other switches in the network. This increases transport costs as well as lengthening timetable for implementation. During the operations, circuit verification processes for E1s call for higher manpower and increased downtimes. Network efficiencies become a function of manpower and their abilities and mutual co-ordinations.

Cost of Data Applications

So far, the revenue generated is mainly through voice. Driven by the Internet, growth of revenues from data services like e-mail and chat is also growing to a significant proportion. Music, multimedia, and file movement are putting additional load on the networks. From the cost point of view, it is not economical to run voice services and data services exclusively.


Convergence has become imminent if operators have to stay and survive. It has become a necessity that networks are seamless in providing the newer services using the same infrastructure.

VoIP networks take care of the need of providing all these services, in addition to supporting applications like prepaid, postpaid, and international calling cards. In a TDM switch, IVR cards have to be placed separately, whereas in VoIP these facilities come as part of the main equipment, which results in cost advantage. The revenue generating applications come within the scope of the main network and are not normally required to be procured as additional standards add on.

Open vs Proprietary 

TDM systems demand continuous feature support because the architecture is monolithic and proprietary in terms of technology. However, VoIP supports open standards and uses industry standard hardware to enable multi-vendor, best-of-breed networks and service platforms. The maintenance cost of a VoIP network is comparatively less per port than that of a TDM network.


Bandwidth Cost

As VoIP provides for compression techniques, bandwidth utilization becomes efficient and cost effective. Even though bandwidth costs are dropping, compression techniques make them further cost effective. Thus, TDM networks will become increasingly vulnerable. 

IP networks will become the order of the day. Costs are likely to fall further. 3G mobile networks may also have IP as the main technology. However, the carrier grade and reliability will take sometime to mature. In any case, the threat to TDM-based networks is getting serious day by day.

P Harihar Kumar, former general manager (specialists group), MTNL



SIP Server

This is capable of the full array of call routing that maximizes network performance. An SIP server is part of a soft switch application layer and its internal functions include service creation, routing and addressing over next-generation public or managed corporate networks. An SIP server can deliver up to 1M BHCA 1 box. This is essential for advanced voice networks and other services. The server sits at the core of the SIP network. It routes calls between media gateway controller, media gateways, IP end points and application servers, and supports multiple interfaces.

Media Gateway Controller

A call control function comprises the MGC and signaling gateway. A call control layer performs call processing, signaling and call switching. MGC also interconnects media gateways to circuit switched TDM networks.

An MGC performs real-time call processing and SSF layer functions. It manages trunk resources, alarms, and call signaling, and provides billing information.

Main functional difference between MGC & SIP server

Call control and end to

end signaling
Provides services logic layer, so that the networks and the services could be separated. This enables easy configuration and maintenance.

Signaling Gateways

Signaling gateways are used to terminate signaling system 7 and to pass signaling information to media gateway controller host. These gateways interface both SS7 and IP networks. This lets SS7 information to be carried to and from
MGC, in IP format.

Routing features

  • Source based routing: The service

    provider can configure a specific dialing plan per carrier.

  • Locality based routing: Least

    cost routing.

  • Percentage based routing: One can

    control inter connect agreements example 50 percent of the calls to

    destination X be routed by carrier A, 25 percent via carrier B another 25

    percent by carrier C etc.

  • Resource availability: Calls are

    routed, based on the capacity available on the ports at the end points.

  • Time of the day based routing:

    Calls are routed depending upon time /date.

  • SS7 parameter and SIP parameter

    based routing: Such routings can be based on carrier identification code,

    bearer capabilities, calling port category, from, to etc.

  • Longest prefix match: Calls are

    routed to termination end points that support longest prefix match between

    the B party number and the supported routing prefixes.

  • Network redundancy: These are

    available though:

    1. Link level
    2. Board level
    3. Signaling gateway level
    • Fault Monitor: OSS fault monitor enables to understand the fault zone,

      troubleshooting remedy. The fault re-routes traffic.

    Hunting Groups

    Hunting group is used to select trunks in a sequence. Available options are.

    • Ascending

    • Descending

    • Random

    • Odd Even selection