Digital signal 1 (DS1, also known as T1, sometimes "DS-1") is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside of North America and Japan. Technically, DS1 is the transmission protocol used over a physical T1 line; however, the terms "DS1" and "T1" are often used interchangeably.
A DS1 circuit is made up of twenty-four 8-bit channels (also known as timeslots and DS0's), each channel being a 64 kbit/s DS0 multiplexed pseudo-circuit. A DS1 is also a full-duplex circuit, meaning, in theory, the circuit can send 1.544 Mbit/s and receive 1.544 Mbit/s concurrently. A total of 1.536 Mbit/s of [1] bandwidth is achieved by sampling each of the twenty-four 8-bit DS0's 8000 times per second. This sampling is referred to as 8-kHz sampling (See Pulse-code modulation). An additional 8 kbit/s is obtained from the placement of a framing bit, for a total of 1.544 Mbit/s, calculated as follows:


DS1 frame synchronization
In SF Framing, the framing channel is divided into two channels of 4 kbit/s each. One channel is for terminal frame alignment; the second is used to align the signaling frames. The terminal frame and signaling frame bits are interleaved, rather than consecutive (they are switched in Figure 2). (correction per ANSI T1.403 Section 7.2 "A frame is a set of 192 digit time-slots for the information payload preceded by one digit time-slot containing the framing (F) bit, for a total of 193 digit time-slots." Meaning the first bit of the frame is framing bit and not the last bit.)
The terminal frame alignment channel is carried in odd-numbered frames inside the super frame and occurs with the DS0 channel synchronization. Since the framing bits occur only once per frame, in the 193rd position, the bit placement of each DS0 can be calculated. After the framing bit is sensed, the first DS0 timeslot is taken as the next 1-8 bits. Timeslot 2 is bits 9-16, timeslot 3 is 17-24, through to timeslot 24. See Figure 1. The Terminal frame alignment pattern is carried in odd-numbered frames, inside the super frame, and consists of alternating 1s and 0s: 1–0–1–0–1–0.
Signaling frame alignment channel is carried in even-numbered frames inside the super frame and is used for signaling frame alignment. The signaling frame alignment pattern consists of a 0–0–1–1–1–0. Signaling frames are identified by the framing signal's transition from 1 to 0 and from 0 to 1; thereby frames six and twelve carry signaling information. See Figure 2.
The SF format uses bit robbing to pass signaling information. Bit robbing modifies the least significant bit in each user data timeslot twice per Super Frame. (See also A&B). The two modified frames are the sixth (A) and the twelfth (B). Using two bits, four possible signaling states can be passed in each direction (0–0, 0–1, 1–0, 1–1). In order for A/B signaling to work, the exact placement of the bits must be known by both sides. Information on the frame sequence is necessary to "pick out" the A and B bits. Channel information must also be known in order to pick out the last bit of each channel. If the proper alignment (timing) did not occur, the wrong bit could be modified or read as the robbed bit. This method of signaling is also commonly referred to as Channel Associated Signaling or CAS. See Figure 2.
The SF format is also known as D4 framing and D3/D4 framing format.
Note: The legend at the bottom right of Figure 2 is incorrect. The Pink square is in fact Terminal Frame alignment bits and the Orange square is the Signal Frame alignment. The RBS is however correct.(correction per ANSI T1.403 Section 7.2 "A frame is a set of 192 digit time-slots for the information payload preceded by one digit time-slot containing the framing (F) bit, for a total of 193 digit time-slots." Meaning the first bit of the frame is framing bit and not the last bit.)

SF framing
In ESF, twenty-four frames make up the (extended) super frame. ESF divides the 8 kbit/s framing channel into three segments. The frame pattern uses 2 kbit/s, and a Cyclic redundancy check (CRC) uses 2 kbit/s. The remaining 4 kbit/s make up an administrative data link (DL) channel. The framing pattern occupies the 4th, 8th, 12th, 16th, 20th and 24th frames. The pattern consists of a 0–0–1–0–1–1 sequence. This is the only pattern repeated in the ESF format. See Figure 3.
The CRC algorithm checks a known segment of data and adds the computed value to it. The combined data and CRC blocks are both transmitted. The receive circuitry will run the same CRC algorithm against the data portion and compare the calculation to the transmitter's CRC value. In this manner, corrupted data can be flagged as "CRC errors". The CRC checksum is passed in the 2nd, 6th, 10th, 14th, 18th, and 22nd frames. (See also Error-correcting code).
The administrative channel provides a means to communicate within the DS1 stream (sub-channel). Statistics on CRC errors can be requested and sent from one end to another. The data channel occupies the twelve odd numbered frames. Signaling and other information passes over this channel. Provisions in the ESF standard would allow the normal A/B bit robbed signal to be enhanced. The A/B bits can be extended to four bits (ABCD). This provides 16 distinct states. An improvement from A/B, which provides 4. To overcome incompatibility with A/B signaling, equipment repeats the A&B bits (e.g. C = A and D = B). These additional signaling bits will offer new features as equipment is built to support it.
CRC errors can be detected and counted in at least one of four different registers. The registers are for transmit (in and out) and receive (in and out). Using recovered CRC data, it is possible to segment and isolate the direction of problems.

ESF framing
Connectivity refers to the ability of the digital carrier to carry customer data from either end to the other. In some cases, the connectivity may be lost in one direction and maintained in the other. In all cases, the terminal equipment, i.e., the equipment that marks the endpoints of the DS1, defines the connection by the quality of the received framing pattern.
Alarms are normally produced by the receiving terminal equipment when the framing is compromised. There are three defined alarm states, identified by a legacy color scheme: red, yellow and blue.
Red alarm indicates the alarming equipment is unable to recover the framing reliably. Corruption or loss of the signal will produce "red alarm." Connectivity has been lost toward the alarming equipment. There is no knowledge of connectivity toward the far end.
Yellow alarm indicates reception from the far end of a data or framing pattern that reports the far end is in "red alarm." Red alarm and yellow alarm states cannot exist simultaneously on a single piece of equipment because the "yellow alarm" pattern must be received within a framed signal. For ESF framed signals, all bits of the Data Link channel within the framing are set to data "0"; the customer data is undisturbed. For D4 framed signals, the pattern sent to indicate to the far end that inbound framing has been lost is a coercion of the framed data so that bit 2 of each timeslot is set to data "0" for three consecutive frames. Although this works well for voice circuits, the data pattern can occur frequently when carrying digital data and will produce transient "yellow alarm" states, making ESF a better alternative for data circuits.
Blue alarm indicates a disruption in the communication path between the terminal equipment. Communication devices, such as repeaters and multiplexers must see and produce line activity at the DS1 rate. If no signal is received that fills those requirements, the communications device produces a series of pulses on its output side to maintain the required activity. Those pulses represent data "1" in all data and all framing time slots. This signal maintains communication integrity while providing no framing to the terminal equipment. The receiving equipment displays a "red alarm" and sends the signal for "yellow alarm" to the far end because it has no framing, but at maintenance interfaces the equipment will report "AIS" or Alarm Indication Signal. AIS is also called "all ones" because of the data and framing pattern.
These alarm states are also lumped under the term Carrier Group Alarm (CGA). The meaning of CGA is that connectivity on the digital carrier has failed. The result of the CGA condition varies depending on the equipment function. Voice equipment typically coerces the robbed bits for signaling to a state that will result in the far end properly handling the condition, while applying an often different state to the customer equipment connected to the alarmed equipment. Simultaneously, the customer data is often coerced to a 0x7F pattern, signifying a zero-voltage condition on voice equipment. Data equipment usually passes whatever data may be present, if any, leaving it to the customer equipment to deal with the condition.

Connectivity and Alarms
Before the jump in Internet traffic in the mid 1990s, DS1s were found almost exclusively in telephone company central offices as a means to transport voice traffic between locations. DS1s have been and still are the primary way cellular phone carriers connect their central office switches (MSCs) to the cell sites deployed throughout a city.
Today, companies often use an entire DS1 for Internet traffic, providing 1.544 Mbit/s of connectivity (allowing for 1.536 Mbit/s of usable traffic, and 8 kbit/s of framing overhead). However, DS1 can be ordered as a channeled circuit, and any number of channels can be reserved for non-data (for example, voice) traffic.

Real world use
Additionally, for voice T1s there are two types: so-called "plain" or Inband T1s and PRI (Primary Rate Interface). While both carry voice telephone calls in similar fashion, PRIs are commonly used in call centers and provide not only the 23 actual usable telephone lines (Known as "B" channels) but also a 24th line that carries signaling information (Known as the "D" channel for Data.
Inband T1s are also capable of carrying CID and ANI information if they are configured by the carrier to do so but PRI's handle this as a standard and thus the PRI's CID and ANI information has a much better chance of getting through to the destination. While an Inband T1 seemingly has a slight advantage due to 24 lines being available to make calls (as opposed to a PRI that has 23), each channel in an Inband T1 must perform its own set up and teardown of each call. A PRI uses the 24th channel as a data channel to perform all the overhead operations of the other 23 channels (including CID and ANI). So even though an Inband T1 has 24 channels, the PRI can actually dial more calls faster because of the dedicated data (also called "D") channel.