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Digital signal 1 (DS1, also known as a T1) is a T-carrier signaling scheme devised by Bell Labs. It is a widely used standard in telecommunications to transmit voice and data between devices.
A DS1 circuit is made up of 24 eight-bit channels (a.k.a. timeslots and DS0's), each channel being a 64 Kb/s DS0 multiplexed pseudo-circuit. A DS1 is a also a full-duplex circuit, meaning you can (in theory) send 1.536 Mbit/s and receive 1.536 Mbit/s simultaneously. A total of 1.536 Mb/s of bandwidth is achieved by sampling each of the 24 8-bit DS0's 8000 times per second. This sampling is referred to as 8KHz sampling. See Pulse-code modulation. An additional 8 Kbit/s is obtained from the sampling of a final framing bit, for a total of 1.544 Mb/s:
(8 bits/channel * 24 channels/frame + 1 framing bit) * 8000 frames/s = 1.544 Mb/s
DS1 Frame Synchronization
Frame synchronization is necessary to identify the timeslots within each 24-channel frame. Synchronization takes place by allocating a framing, or 193rd bit. This results in 8 Kb/s of framing data, for each DS1. Because this 8Kb/s channel is used by the transmitting equipment as overhead, only 1.536 Mb/s is actually passed on to the user. Two types of framing schemes are Super Frame (SF) and Extended Super Frame (ESF). A Super Frame consists of twelve consecutive 193-bit frames, where as, an Extended Super Frame consists of twenty four consecutive 193-bit frames of data. Due to the unique bit sequences exchanged, they are not compatible with each other. These two types of framing (SF and ESF) use their 8Kb/s framing channel in different ways.
SF Framing
In SF Framing, the framing channel is divided into two channels of 4 Kb/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.
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, 3 is 17–24, through to timeslot 24. See Figure 1. Terminal frame pattern consists of alternating 1s and 0s: 1–0–1–0–1–0.
 Figure 1. SF Framing bit. A single bit is added to the end of the DS1 frame to signal the start of a new frame. 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.
Figure 2. SF Framing Format. The SF format is also known as D4 framing and D3/D4 framing format.
ESF Framing
In ESF, twenty-four frames make up the (extended) super frame. ESF divides the 8Kb/s framing channel into three segments. The frame pattern uses 2 Kb/s, and a Cyclic redundancy check (CRC) uses 2 Kb/s. The remaining 4 Kb/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.
Figure 3. ESF Framing Format. 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.
Real world use
Before the jump in Internet traffic in the mid 1990's, DS1's were found almost exclusively in the telephone company central office as a means to transport voice traffic between locations. DS1's have been and still are the primary way cellular phone carriers connect their central office switches (MSC's) to the cell sites deployed throughout a city.
Today, companies often use an entire DS1 for Internet traffic, giving you 1.544 million bits per second of connectivity (actually, it's 1.536 Mbit/s; the other 8 kbit/s goes to framing overhead.) However, if you so desire, you can order the DS1 as a channelize circuit, and can reserver channels for non-data (i.e., voice) traffic.
Trivia
Originally, T-1 meant "Transmission - Level 1", and had to do with the media that the signal was passed over. DS-1 meant "Digital Service - Level 1", and had to do with the service to be sent(originally 24 digitized voice channels over the T1). The terms T1 and DS1 have become synonymous and include a plethora of different services from voice to data to clear channel pipes. The line speed of them is always consistent at 1.544 Mb/s, but the payload can vary greatly.
See also: T1 History ("All You Wanted to Know About T1 But Were Afraid to Ask") (http://www.dcbnet.com/notes/9611t1.html)
Examples
The global telephone network (also known as the Public Switched Telephone Network or PSTN)
See also
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