As the voice network continued to expand into the 1960's two significant problems continued to trouble the carriers:
T1 is a digital data stream capable of handling 24 independent connections simultaneously. T1 utilizes two wire pairs (one in each direction) running at 1,544,000 bits per second.
The first problem, with the quality of the signal, was eliminated with digital transmission techniques. Analog signals degrade quickly as a function of distance. In order to transmit signals over long distances, repeaters are used to amplify the signals at regular intervals. Analog repeaters tend to introduce and amplify noise picked up during transmission.
By using digital transmission techniques, it is possible to virtually eliminate this noise. With digital links, it is only necessary to differentiate between a 1 and a 0 in order to regenerate the signal at each stage. At each stage, the regenerated signal is 100% accurate and accumulated errors are not passed down the link.
According to sampling theory, a signal can be accurately reproduced if it is sampled at a rate of not less than 2 times the highest frequency of the signal. The highest frequency of significance in a voice signal is about 4,000 Hz.
Thus, an accurate representation of a voice signal can be created by sampling the signal 8,000 times per second. Each sample can then be converted into an 8-bit byte, with the value of the byte being representative of the voltage level of the sample.
Using these assumptions, a voice signal can be converted into a digital data stream running at 64,000 bits per second (8,000 samples per second X 8 bits per sample).
The second problem, the number of lines required, was solved by using time-division multiplexing techniques to combine multiple voice channels onto a single link.
T1 combines 24 separate voice channels onto a single link. The T1 data stream is broken into frames consisting of a single framing bit plus 24 channels of 8-bit bytes (1 framing bit per frame + 24 channels per frame X 8 bits per channel = 193 bits per frame).
The frames must repeat 8,000 times per second in order to properly recreate the voice signal. Thus, the required bit rate for T1 is 1.544 Mbps (8,000 frames per second X 193 bits per frame).
During the 1970's AT&T began to offer high-speed data services utilizing its T1 backbone network. The first service to see widespread use was the DDS or DATAPHONE Digital Service (Trademark AT&T) introduced during the mid-1970's. This service provided the users with a digital 56,000 bps interface.
By the late 1970's AT&T was beginning to offer 1.544 Mbps services. The advantages of these services were the obvious increased bandwidth as well as quality objectives, which far exceeded the quality of existing analog lines. With these services AT&T actually leased fixed T1 lines to the user.
There were very few restrictions placed on these services. One requirement was that the user's data must meet the 1's density required to maintain timing. This requirement effectively reduces the bandwidth of the user's data to 1.344 Mbps. Other than this, the user was free to use any available channelization or framing techniques.
In the early 1980's AT&T renamed the service High-Capacity Terrestrial Service and began to require that the user's equipment employ D4 framing. (This will be discussed in detail later).
This requirement caused many T1 equipment suppliers to redesign their products in order to continue to operate over AT&T lines. By 1986 use of D4 framing was mandatory on AT&T T1 lines.
In the mid-1980's AT&T renamed its service ACCUNET and began to offer new functions to their users. Among these new functions were multiplexing and network cross-connect reconfiguration.
T1 utilizes bipolar electrical pulses. Where most digital signals are either a ONE or a ZERO (unipolar operation), T1 signals can be one of three states. The ZERO voltage level is 0 volts, while the ONE voltage level can be either a positive or a negative voltage.
AMI encoding causes the line to alternate between positive and negative pulses for successive 1's. The 0's code is no pulse at all. Thus, a data pattern of 11001011 would cause the following pattern on an AMI line: - +,-,0,0,+,0,-,+.
With this encoding technique there is a problem with long strings of 0's in the user's data which produce no transitions on the line. The receiving equipment needs to see transitions in order to maintain synchronization. Because of this problem, DS-1 specifications require that users limit the number of consecutive 0's in their data steam to less than 15.
With this scheme of encoding there should never be consecutive positive or negative pulses on the line (i.e., the following pattern should never occur: 0,+,-,+,+,-). If two successive positive or two successive negative pulses appear on the line, it is called a Bipolar Violation (BPV). Most T1 systems watch for this event and flag it as an error when it occurs.
B8ZS and HDB3 are both methods which permit the user to send any pattern of data without affecting the operation of the T1 line. Both of these encoding schemes make use of BPVs to indicate that the users data contains a long string of 0's.
B8ZS looks for a sequence of eight successive 0's and substitutes a pattern of two successive BPVs. The receiving station watches for this particular pattern of BPV's and removes them to recreate the original user data stream.
HDB3 is the scheme recommended by the CCITT. This scheme watches for a string of four successive 0's and substitutes a single BPV on the line.
D4 defines a 12-bit framing sequence which is sent as the 193rd bit in 12 consecutive frames. These 12 frames together are referred to as a superframe.
The framing pattern is defined as 100011011100. This pattern repeats continuously and the receiving equipment locks onto it in order to properly synchronize with the incoming data.
In order to send supervisory information over a D4 link "bit robbing" is used. A voice signal is not significantly affected if the low-order bit in a byte is occasionally wrong. D4 framing makes use of this characteristic of voice and uses the least-significant bits in each channel of the 6th (A Bit) and 12th (B Bit) frames to send signalling information; on-hook, off-hook, dialing and busy status.
D4 framing requires that the 8th bit of every byte of every frame be set to a 1 when data is transmitted. This requirement guarantees the required 1's density on the link, regardless of the contents of the user data. This requirement reduces the bandwidth available to the user from 64 Kbps to 56 Kbps (7 bits/frame X 8,000 frames/second).
In ESF the 193rd bit location serves three different purposes:
In D4 framing, 12 bits are used per superframe for synchronization. In ESF framing, 6 bits are used per superframe for synchronization.
There is no link-level error checking available with D4 framing (except for bipolar violations). ESF framing utilizes a 6-bit Cyclic Redundancy Check (CRC) sequence to verify that the frame has been received without any bit errors. As a superframe is transmitted, a 6-bit CRC character is calculated for the frame. This character is then sent in the six CRC bit locations of the next superframe.
The receiving equipment uses the same algorithm to calculate the CRC on the received superframe and then compares the CRC value that it calculated with the CRC received in the next superframe. If the two compare, then there is a very high probability that there were no bit errors in transmission.
As was stated earlier, 12 bits are used for maintenance communications. These 12 bits give the maintenance communications channel a capacity of 4,000 bits per second. This function enables the operators at the network control center to interrogate the remote equipment for information on the performance of the link.
As with D4 framing ESF utilizes "robbed bits" for in-band signalling. ESF utilizes 4 frames per superframe for this signalling. The 6th (A bit), 12th (B bit), 18th (C bit), and 24th (D bit) frames are used for the robbed bits. The function of the robbed bits is the same as in D4 framing.
There are two defined loopback signals: the loop-up command and the loop-down command. The loop-up command sets the link into loopback mode and consists of the following pattern:
....100001000010000100...sent within normal D4 framing for 5 seconds.
The loop-down command resets the link to its normal mode and consists of the following pattern:
....100100100100100100....again sent within normal D4 framing for 5 seconds.
While the link is in loopback, the operator can insert test equipment onto the line to test its operation.