Digital Signal 1
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Digital signal 1 (DS1, also known as T1, sometimes "DS-1") is a T-carrier signaling scheme devised by Bell Labs.[1] 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 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:
(8 bits/channel * 24 channels/frame + 1 framing bit) * 8000 frames/s = 1.544 Mbit/s
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[edit] 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 kbit/s of framing data, for each DS1. Because this 8-kbit/s channel is used by the transmitting equipment as overhead, only 1.536 Mbit/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, whereas an Extended Super Frame consists of twenty-four consecutive 193-bit frames of data. Due to the unique bit sequences exchanged, the framing schemes are not compatible with each other. These two types of framing (SF and ESF) use their 8 kbit/s framing channel in different ways.
[edit] SF framing
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).
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.
[edit] ESF 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.
[edit] Real world use
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 Mbps of connectivity (actually, it is 1.536 Mbit/s; the other 8 Kbit/s goes to 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.
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 (the 24th line carries signaling information) but also Caller ID (CID) and Automatic Number Identification (ANI) data, commonly referred to in industry parlance as 'signalling 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).
[edit] Trivia
Originally, T1 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 is always consistent at 1.544 Mbit/s, but the payload can vary greatly.
The 24 channels of traffic in a T1 line are sometimes called a T-span.
[edit] Examples
The global telephone network (also known as the Public Switched Telephone Network or PSTN).
[edit] See also
- T-carrier
- E-carrier
- Time-division multiple access
- Pulse code modulation
- Federal Standard 1037C
- DS1 Encoding schemes: B8ZS, HDB3, AMI
- Line code
- Time-division multiplexing
- Multiplexing
- Physical layer
- Data frame
