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VoIP vs. TDM Voice

TDM (Time Division Multiplexing), a phone technology based on circuits switched by PBXs (Private Branch Exchanges) provided important services for a long time. Everybody got their own extension, a fancy corporate phone, voice mail and sometimes extra features such as  conferencing, multiple call appearances and caller ID.  This was kept well maintained by well trained experts who knew and understood the switches thoroughly.

When VoIP came along, promising to run voice calls over IP networks for less money, customers were intrigued. Rather than send out a technician to move a phone extension from one office to another, the user just had to move their phone to a new jack and it would receive calls meant for the person it was assigned to, no technician required.

Rather than supporting two separate networks, VoIP riding on the data network would allow them to get by with one. If voice was just an application running on the network, the telecom staff wouldn’t be necessary or at least wouldn’t need to be so large.

The cost savings proved to be a red herring, but in the cases where business adapted VoIP, they found other reasons to like the technology. If a customer help line in one office was swamped, more agents from anywhere on the network could quickly and painlessly be routed into the call-distribution list and start fielding calls.

VoIP supports services TDM doesn’t, such as popping up account information about customers on call agents’ computer screens based on the caller IDs or enabling users to place calls by clicking on phone numbers within applications or finding out who is available to take a call  transfer by using presence information that VoIP supports.

Are you wondering about the great divide in enterprise telephony? Should you stay with your trusty TDM based PBX phone system or toss everything out in favor of VoIP? Stick with the  tried and true and you fear that you may be missing out on services that could improve productivity. Scrap it all for a brand new VoIP solution and you may have an eye-popping capital expense to swallow. What to do?

Regardless of the impression you get from technical news reports and vendor ads, you have more options than you think. You don’t have to make an all-or-nothing decision either way.

You can decide whether to have analog, digital, or IP phones on each desk and whether to go with TDM or VoIP telephone services to connect with the outside world. Some systems will even allow you to mix and match. TDM? VoIP? Both? Why not?

Most phone systems that handle more than a few telephone handsets give you an option on how to connect to the PSTN or Public Switched Telephone Network. Perhaps you’ve accumulated analog phone lines to the point where you have a dozen or two plugged into FXO (Foreign eXchange Office) interface cards. In this case, a move to TDM or Time Division Multiplexing would make economic sense. TDM is the digital standard for standard telephone trunking. A T1 voice line or ISDN PRI is an example of a TDM connection that multiplexes up to a couple of dozen phone lines onto a single TDM trunk line.

T1 lines using the TDM protocol are the mainstay of enterprise PBX telephone systems. Most all offer slots for one or more T1 TDM-based telephone trunk lines. An alternative is the T1 PRI or ISDN PRI line that offers the same trunkline service plus additional services such as Caller ID.

Well, what about VoIP? A pure VoIP or Voice over Internet Protocol phone system operates completely in the IP world from handset to call termination. The PBX equivalent is the IP PBX which is may be a dedicated system or software, such as Asterisk, running on a standard Linux server. The handsets are referred to as SIP phones. SIP or Session Initiation Protocol is the switching protocol that controls the phone calls on packet based networks.

Even your phone service can be brought in on a SIP Trunk, which is a private line or Internet connection to your VoIP service provider. Calls between subscribers to the same VoIP service provider stay completely on the network and never enter the PSTN or Public Switched Telephone Network. Calls to and from non-subscribers are terminated to the PSTN at the provider’s location.

As you might guess, there are purely Analog phone systems, purely digital and TDM phone
systems and purely VoIP phone systems. Most of the time, though, you can mix and match. A
VoIP system may well have FXS (Foreign Exchange Subscriber or Station) ports to connect to
analog handsets, as do traditional PBX systems. The phone service connection may be analog,
T1 or ISDN, or SIP Trunking.

In general the newer IP PBX telephone systems offer a wider range of handset and phone service connection options than legacy PBX systems. But before you make any big purchases or plans to scrap you current system, why not review your options with one of our expert voice  and data service consultants. You may have better options than you think.

Culled from T1 Rex and NetworkWorld.

For commentaries or suggestions please write to Ikenna@ccsnl.com.

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