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What is Ethernet over TDM?
How it is practical to transport Carrier Ethernet over T1 lines and other TDM circuits.

By: John Shepler

You’ve heard of Ethernet over Copper and Ethernet over Fiber, but have you heard of Ethernet over TDM? Here’s why it’s good to be familiar with this technology and what it can do for you.

What Does TDM Mean?
TDM stands for Time Division Multiplexing. It’s the basis of T1 lines and SONET fiber optic carrier services. TDM was the chosen protocol of the Public Switched Telephone Network (PSTN) when the decision was made to transition from analog to digital mid-20th century. It is still important is because it is still around and going strong.

How TDM is Organized
TDM is a way to transmit multiple signals on one digital line or wireless channel. The way TDM works is that a digital transmission is divided up into timeslots. Each timeslot contains the digitized information for one signal. All the timeslots are linked together like railroad cars into a frame that contains all of the digital bits, end to end. This frame is then transmitted from source to destination. Once one frame is sent another immediately follows.

Refreshing The Data
Think of TDM as a round-robin polling process. You take a chunk of information from one source, then a chunk from another source, and so on. When you’ve got something from all sources that you wish to multiplex, then you start over with the first source again. That way you always have fresh data from frame to frame.

24 Lines Into 1
What TDM did for the phone companies was to pack 24 separate telephone calls onto one T1 line instead of 24 separate telephone wires. The multiplexing process combines all calls into one bitstream that is sent down the physical copper wires. At the other end, a complementary process called demultiplexing separates the individual calls so they can be used by particular telephones.

Ethernet vs TDM
Ethernet is not a TDM process. Instead, it bundles bits of data into packages called packets. The packets are sent on their way individually when they are ready. There’s no assembling the packets into a multiplexed “train” so that they can all travel together. Each packet is switched and routed individually.

Incompatible Technologies?
From the sound of it, you would think that TDM circuits, like T1 lines, and packet switched circuits, like Ethernet over Copper, would be incompatible. It’s true that they are completely different technologies and don’t exist on the same piece of wire at the same time. That would really create a mess and you’d get completely jumbled up data. However, clever engineering can let one protocol carry another. This is where the idea of Ethernet over TDM comes about.

Transmitting Ethernet on T1 Lines
How Ethernet over TDM works is that the underlying transmission protocol is TDM. A typical example is a T1 line. The T1 frames are transmitted at a rate of 1.5 Mbps from source to destination. Ethernet packets are loaded into those T1 frames. In this case all the claptrap associated with dividing the T1 line into 24 separate channels is ignored. The internal channels are combined so that Ethernet packets can use the entire frame. The T1 timing and waveforms are preserved so that T1 interfaces and signal regenerators will work as-is.

The Advantages of Ethernet over TDM
Why go to all this trouble to piggyback Ethernet on top of a T1 TDM system? It’s to give you more options. Ethernet over Copper is a distance limited technology that works great within a couple of miles from the central office, but loses signal strength and bandwidth fast as you get farther away. T1 has fewer distance limitations and delivers a steady bandwidth of 1.5 Mbps even out into rural areas. As a newer development, Ethernet over Copper is also less available than T1 lines that have been around for 50 years or more.

Is Ethernet over TDM Right For You?
So, if you want the advantages of Ethernet that include bandwidth scalability, easy interfacing to your network, and standardized services such as Ethernet Line and Ethernet LAN, but can’t get Ethernet over Copper or Ethernet over Fiber, consider Ethernet over TDM. This is also known as Ethernet over T1 or Ethernet over DS1. Check pricing and availability on all Copper Ethernet Services now.

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