Saturday, August 26, 2006

T1 Lines to Support Enterprise VoIP

T1 lines are often associated with their original use of aggregating multiple telephone calls on a single digital trunk by using TDM or Time Division Multiplexing. But T1 lines are also good conduits for voice over IP telephony between company locations or to remote gateways that interface with the PSTN (Public Switched Telephone Network).

The original T1 framing was designed to accommodate 24 toll quality telephone calls digitally on a dual copper pair. Each call is digitized and assigned to one of 24 eight bit time slots rigidly synchronized to atomic clocks. Interfaces called channel banks provide the multiplexing and demultiplexing. At each end of the T1 line, you have basically 24 independent telephone lines. A T1 PRI line works similarly, but reserves one channel for signaling and data such as Caller ID.

T1 and T1 PRI lines are still attractive options for standard switched circuit PBX business telephone systems. The interface characteristics remain the same, but line prices have dropped dramatically over the past few years. Most PBX and larger Key systems have interface boards available that connect directly to the T1 / PRI line.

VoIP works somewhat differently from circuit switched telephony. Voice is still converted from analog to digital format, sometimes using the same CODEC (Coder / Decoder). But instead of transmitting the voice bits in a rigid round-robin time sequence, several samples of digital voice are formatted into a voice packet. The packet follows the same IP or Internet Protocol as data packets so that voice and data can be mingled on a single digital network. The packets are transmitted from source to destination as fast as the network will accommodate.

For VoIP applications, you want to use the entire 1.5 Mbps capacity of the T1 line as a WAN or Wide Area Network. A T1 line set up this way is called "unchannelized." Think of it as a big data pipe, similar to a local area network but going from point to point. The two ends of the T1 line can both be company sites, or one end can be connected to the Internet.

One important difference between TDM based T1 phone lines and VoIP is that your network takes over the job of managing bandwidth for a VoIP system. The routers or IP PBX system you are using need to ensure quality of service by prioritizing voice packets over less time sensitive data packets. You also need to make sure you have enough network bandwidth to accommodate the maximum amount of voice traffic needed. If you outgrow a single T1 line, you can easily bond additional lines to get 2x, 3x, 4x, or more bandwidth as your business grows.

I mentioned that unchannelized T1 data lines can accommodate both voice and data traffic, since the packets both use the IP format and will go through switches and routers similarly. This principle is often used to provide what's called integrated T1 line service which is both telephone and Internet access on the same T1 line. A device called an IAD converts phone conversations to VoIP format, intermingles voice and Internet data packets over the T1 line, and then send the voice packets to the phone network and the data packets to the Internet at the terminating end.

Enterprise VoIP users have the opportunity to use T1 lines for both voice and data connections between office and plant sites. An IP PBX or IP Phones convert the telephone conversations to voice packets so a separate IAD is not required. A voice gateway may be needed to provide interface to the telephone company for calls that leave the organization.

You should also know that some WAN connections are now available in native Ethernet protocol. By ordering Ethernet, Fast Ethernet or Gigabit Ethernet WAN connections, the enterprise network can be interconnected efficiently regardless of how far-flung remote locations happen to be.

Are you looking for reliable, low cost WAN connections for your enterprise VoIP and/or data network? If so, discuss your needs with our consultants at no cost and get competitive quotes for the bandwidth you need at

Click to check pricing and features or get support from a Telarus product specialist.

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