Ever since the World Wide Web started to get popular, T1 (or T-1) circuits have been a topic of conversation. T1 circuits continue to be part of technology conversations even today. The reason for the continued popularity, is the demand for bandwidth by the consumers and the continuously dropping prices. A T1 is enough bandwidth for an entire office, full of people, to have a constant (always on) and high-speed internet connection. Today, I read about T1 circuits costing as little as $350.00 per month.
With all the discussion about the T1 many people may be confused about what all a T1 circuit has to offer. In this post, I’m going to attempt to make all the T-talk a little more clear. To do this, I’m going to start with a little history.
The T-carrier, (as used in telecommunications; sometimes abbreviated as T-CXR), is the generic designator for any of several digitally multiplexed telecommunications carrier systems. These systems were originally developed by Bell Labs and are used in North America, Japan and Korea.
You may be more familiar with the basic unit of the T-carrier system. It is known as the DS0 and has a transmission rate of 64 kbit/s. The DS0 is commonly used for a single voice circuit, like a home phone. A 64 kbit/s DSO is commonly referred to as a channel.
A T1 circuit has 24 of these 64 kbit/s channels. This provides a total of 1,536 kbit/s (or 1.544 Mbit/s – pronounced as Megabits. For the purpose of this article you’ll just have to trust me on the math. If you don’t want to take my word for it, go online and look up the numerical conversion for yourself, to see how I came up with 1.544 Mbit/s!)
A T1 will generally come in to a location in the same area designated for Telco terminations. This space would be where all the telephone lines come in from the telephone company.
Most T1 circuits will be terminated in a Network Interface Device (NID). A NID is simply a device serving as the demarcation point between the carrier’s local loop and the customer’s premises wiring.
NIDs may also be called a Network Interface Unit (NIU) or telephone network box. Some T1 Circuits will first be punched down on a 66 Block.
A smartjack is a type of NID, with capabilities beyond simple electrical connection (I.E. diagnostics). (Shown below with a 66 Block)
Smartjacks are typically used for more complicated types of telecommunications service, such as the T1 circuits we are discussing. A smartjack may provide signal conversion. It may buffer and/or regenerate the signal, similar to what a repeater does.
A Smartjack will also typically provide basic diagnostic capabilities, such as a loopback. A loopback is where a signal from the telco is transmitted back to the telco through the smartjack. This provides the telco with the ability to test the line from the central office, without the need to have test equipment on site. In some cases, the telco may be able to remotely activate loopback. (When looped back, the customer equipment is disconnected from the line.)
Other smartjack diagnostic capabilities include an alarm indication signal, to report trouble at one end of the line, to the far end. This helps the telco know if trouble is present in the line, the smartjack, or customer equipment. Lights may also indicate the configuration, status, and any alarms. Such smartjacks are generally powered from the telephone line, rather than relying on premises electrical power, although this is not a universal rule.
Some smartjacks require manual manipulation for the telco to test the lines. These types of smartjacks generally don’t have lights or indicators of any kind.
There are several different T1 options to accommodate your business needs. A FULL T1 is perfect for small and medium sized businesses needing internet access for 20 to 50 users. A T1 connection is a 24-channel circuit providing an internet connection straight from the providers backbone. A FRACTIONAL T1 is a portion of a 24-channel circuit providing a connection straight from the providers backbone. A smaller portion can help a business save money. An INTEGRATED T1 (or channelized T1) can provide internet access and a series of voice lines for your company.
If you subscribe to a Full or Fractional T1 and only use Data services, there will be a T1 modem connected directly to the NID, NIU or Smartjack.
If you subscribe to an Integrated T1 (could be full or fractional) to supply Data and Voice services, and if your T1 is digital, you’ll probably use a piece of equipment called a CSU (Channel Service Unit) or DSU (Data Service Unit”) to connect to the DTE (Data Terminal Equipment), such as a modem or router. A CSU/DSU is normally much less expensive than a channel bank.
If your Integrated T1 circuit is not digital, a piece of equipment called a channel bank will be connected to the NID, NIU or Smartjack. A channel bank changes normal analog voice signals into digital data and multiplexes the conversations to flow together to the other end, where the data bits are then decoded and conversations are separated though a similar piece of equipment. All this complex processing happens lightning-fast, and phone calls sound just fine.
The CSU/DSU or the Channel Bank, will separate the Voice portion of your T service from the Data portion. Such equipment will usually provide a connection to any number of different phone systems, usually via a RJ21 Amphenol connector/cable, to a 66 block.
The Data portion of a T1 circuit will always connect to a DTE (Data Terminal Equipment) of some sort. Various manufacturers such as Adtran, Cisco, and others make modems and routers for this specific purpose. The DTE will provide the uplink to your internal network, thus giving your business the always on, high-speed, internet connection it needs.
Some special Data Terminal Equipment can be connected directly to an Integrated T1 from the NID, NIU or Smartjack and still provide Voice services. An Adtran Total Access 908e, from the Total Access 900e Series provides a connection to your telephone system via the built in RJ21 Amphenol Connector, while also providing the data uplink to your network.
I hope this article helps you grasp basic concepts of T1 circuits, what services it provides and what equipment is used. T1s have re-entered IT conversations because of major deployments of VoIP service, which leads directly to the VoIP topic I will be writing about very soon!