EoDS1 offers IP T1 Service
T1 lines can be used to transport Ethernet over long distances.
By: John Shepler
T1 line service has been available for decades. It originated as a proprietary telephone company technology designed as a trunk for transporting multiple phone calls between switching centers. Later, T1 was made available for businesses to support their PBX phone systems. When the Internet came along, T1 lines were pressed into service as dedicated Internet connections. But now we live in an IP world. Can T1 also act as an IP connection?
The Telco Foundation of T1
Yes, it can. That may seem counterintuitive to those who understand the origins and technology behind T1 service. The T-Carrier specifications, of which T1 is the most popular line service, describe a system organized as multiplexed channels 64 Kbps wide. That channel size was optimized to transport a single independent digitized telephone call using PCM (Pulse Code Modulation). Clearly, T1 is oriented toward the switched circuit based public telephone network.
T1 Can Transport IP
That’s true, but the important thing to remember about digital network technologies is that a bit is a bit. It’s either true or false, one or zero, regardless of the particular physical layer or higher level protocols involved. That means that a time division multiplexing technology like T1 can be used to transport a packet oriented technology like IP or Ethernet with a little protocol conversion.
How It Works
That’s essentially what’s being done when T1 lines are used for data transmission. It’s been a long time since anything but Ethernet has been the standard for local area networks. Ethernet is an IP technology, based on packet switching. Those packets can be queued up and loaded into T1 frames for transmission through T1 lines and even multiplexed into T3 and SONET frames for longer haul transmission. At the far end, the packets are offloaded and sent on their way through an IP edge router connected to the T1 CPE (Customer Premises Equipment.)
Ethernet over T1
If you want to connect your Ethernet networks over metropolitan and wide area networks, one technology you’ll be interested in is called Ethernet over T1 or EoT1. This is also called EoDS1 for Ethernet over DS1.
What is DS1?
DS1 is simply the data service or protocol that runs on T1 lines. The reason there are two designations is that T1 describes a physical line consisting of two twisted pair copper telco loops running at 1.5 Mbps. The DS1 is all the bits formated for transmission just before loading into the actual T1 line. You can takethat same DS1 and feed it into an add/drop multiplexer and transport it along with many others on a T3 circuit or a fiber optic service like OC-3. At the other end, the DS1 can be demultiplexed from the other services on the same line and delivered separately.
In practice T1 and DS1 refer to the same thing to the actual user. What’s important is that Carrier Ethernet service can be delivered on a T1 line, albeit bandwidth limited to 1.5 Mbps. Who would want service that slow? Lots of companies need connectivity between locations, but don’t require high speeds. They may be using the service for retail credit card verification, communication with corporate headquarters, or to upload files to their web server. The big advantage they have with T1 lines is that they are almost universally available. If you can get business phone service, chances are that you can also get a T1 line installed. They are connected over the same telco wiring.
The Ethernet over Copper Alternative
The competing service to Ethernet over T1 is Ethernet over Copper. That may seem like a fine line of distinction, but it is important. Ethernet over Copper (EoC) also uses twisted pair copper but a different modulation scheme that offers higher bandwidths but with distance limitations. If you are within a mile or two of the central office that has Ethernet over Copper equipment, you may be able to get 10 Mbps, 20 Mbps or even higher bandwidths from EoC. Some installations are transporting in excess of 100 Mbps over multiple copper pair.
Bonding to Increase Bandwidth
Ethernet over T1 bandwidth can also beimproved by adding more copper pair. The process of combining the bandwidths of several T1 lines into one larger bandwidth is called bonding. EoT1 bonding can deliver up to 10 or 12 Mbps to both metropolitan and rural areas, providing the equipment is available.
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