T1's are an outdated, overpriced, bandwidth limited, "legacy" techonolgy circa 1960's. Dump your over priced unreliable T1 and get True unlimited broadband speeds at a fraction of the cost for your business that supports both Data and Voice.

Existing Frequency-division multiplexing carrier systems worked well for connections between distant cities, but required expensive modulators, demodulators and filters for every voice channel. For connections within metropolitan areas, Bell Labs in the late 1950s sought cheaper terminal equipment. Pulse-code modulation allowed sharing a coder and decoder among several voice trunks, so this method was chosen for the T1 system introduced into local use in 1961. In later decades, the cost of digital electronics declined to the point that an individual codec per voice channel became commonplace, but by then the other advantages of digital transmission had become entrenched.

The most common legacy of this system is the line rate speeds. "T1" now seems to mean any data circuit that runs at the original 1.544 Mbit/s line rate. Originally the T1 format carried 24 pulse-code modulated, time-division multiplexed speech signals each encoded in 64 kbit/s streams, leaving 8 kbit/s of framing information which facilitates the synchronization and demultiplexing at the receiver. T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in transmission rates of 6.312 and 44.736 Mbit/s, respectively.

What this means to you as a business owner is that a T1 is nothing but a glorified phone line…It is still a physical copper line that has a single point of failure and can go down if it gets cut or wet. What's more is a T1 is limited to a MAXIMUM bandwidth of 1.544 Mbs (not fast at all by todays standards). If you ever wanted any more bandwidth than this you would have to bring in a whole other T1 with the extra up fron capital expenditure…. Most T1's deployments can have an up front cost from $1,000 to $5,000 with an on going monthly fee of $320 to $1200 per T1 per Month….And this is for a maximum of 1.544 Mbs each!…YUCK

Supposedly, the 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago were conducted underground. To accommodate loading coils, cable vault manholes were physically 6600 feet apart, and so the optimum bit rate was chosen empirically–the capacity was increased until the failure rate was unacceptable, then reduced to leave a margin. Companding allowed acceptable audio performance with only seven bits per PCM sample in this original T1/D1 system. The later D3 and D4 channel banks had an extended frame format, allowing eight bits per sample, reduced to seven every sixth sample or frame when one bit was "robbed" for signaling the state of the channel. The standard does not allow an all zero sample which would produce a long string of binary zeros and cause the repeaters to lose bit sync. However, when carrying data (Switched 56) there could be long strings of zeroes, so one bit per sample is set to "1" (jam bit 7) leaving 7 bits x 8000 frames per second for data.

A more common understanding of how the rate of 1.544 Mbit/s was achieved is as follows. (This explanation glosses over T1 voice communications, and deals mainly with the numbers involved.) Given that the highest voice frequency which the telephone system transmits is 4000 Hz, the required digital sampling rate is 8000 Hz (see Nyquist rate). Since each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs then 8000 frames per second to maintain those 24 simultaneous voice channels. Because each frame of a T1 is 193 bits in length (24 channels X 8 bits per channel + 1 framing bit = 193 bits), 8000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s (8000 X 193 = 1544000)

(Note that communications rates are often quoted in literal kilo- and megabits, rather than the kibibits and mebibits often used to describe computer parts)

Initially, T1 used Alternate Mark Inversion (AMI) to reduce bandwidth and eliminate the DC component of the signal. Later B8ZS became common practice. For AMI, each pulse had the opposite polarity of the previous one, resulting in a three level signal which however only carried binary data. Similar British 23 channel systems at 1.536 Mbaud in the 1970s were equipped with ternary signal repeaters, in anticipation of using a 3B2T or 4B3T code to increase the number of voice channels in future, but in the 1980s the systems were merely replaced with European standard ones. American T-carriers could only work in AMI or B8ZS mode.

The AMI or B8ZS signal allowed a simple error rate measurement. The D bank in the central office could detect a bit with the wrong polarity, or "bipolarity violation" and sound an alarm. Later systems could count the number of violations and reframes and otherwise measure signal quality.

[edit] Historical Note on the 193-bit T1 frame The decision to use a 193-bit frame was made in 1958, during the early stages of T1 system design. To allow for the identification of information bits within a frame, two alternatives were considered. Assign (a) just one extra bit, or (b) additional 8 bits per frame. The 8-bit choice is cleaner, resulting in a 200-bit frame, 25 8-bit channels, of which 24 are traffic and 1 8-bit channel available for operations, administration, and maintenance (OA&M). The single bit per frame was chosen, not because a single bit saves bandwidth (by a trivial amount 1.544 vs 1.6 Mbit/s), but from a comment from AT&T Marketing. They claim that "if 8 bits were chosen for OA&M function, someone would then try to sell this as a voice channel and you wind up with nothing."

Soon after commercial success of T1 in 1962, the T1 engineering team realized the mistake of having only one bit to serve the increasing demand for housekeeping functions. They petitioned AT&T management to change to 8-bit framing. This was flatly turned down because it would make installed systems obsolete.

Having this hindsight, some ten years later, CEPT chose 8 bits for framing the European E1. And sure enough, those 8 bits these days are often sold to customers, resulting in losses on OA&M functions