Digital Signal 1 (DS1, sometimes DS-1) is a T-carrier signaling scheme devised by Bell Labs. [1] DS1 is the primary digital telephone standard used in the United States, Canada and Japan and is able to transmit up to 24 multiplexed voice and data calls over telephone lines. E-carrier is used in place of T-carrier outside the United States, Canada, Japan, and South Korea. DS1 is the logical bit pattern used over a physical T1 line; in practice, the terms DS1 and T1 are often used interchangeably. [lower-alpha 1]
T1 refers to the primary digital telephone carrier system used in North America. T1 is one line type of the PCM T-carrier hierarchy. T1 describes the cabling, signal type, and signal regeneration requirements of the carrier system.
| Digital Signal Designation | Line rate | Channels (DS0s) | Line |
|---|---|---|---|
| DS0 | 64 kbit/s | 1 | |
| DS1 | 1.544 Mbit/s | 24 | T1 |
| DS1C | 3.152 Mbit/s | 48 | T1C |
| DS2 | 6.312 Mbit/s | 96 | T2 |
| DS3 | 44.736 Mbit/s | 672 | T3 |
| DS4 | 274.176 Mbit/s | 4032 | T4 |
| DS5 | 400.352 Mbit/s | 5760 | T5 |
The signal transmitted on a T1 line, referred to as the DS1 signal, consists of serial bits transmitted at the rate of 1.544 Mbit/s. The type of line code used is called Alternate Mark Inversion (AMI). Digital Signal Designation is the classification of digital bit rates in the digital multiplex hierarchy used in transport of telephone signals from one location to another. DS-1 is a communications protocol for multiplexing the bitstreams of up to 24 telephone calls, along with two special bits: a framing bit (for frame synchronization) and a maintenance-signaling bit, transmitted over a digital circuit called T1. T1's maximum data transmission rate is 1.544 megabits per second.
A DS1 telecommunication circuit multiplexes 24 DS0s. [1] The twenty-four DS0s sampled 8,000 times per second (one 8bit PCM sample from each DSO per DS1 frame) consume 1.536 Mbit/s of bandwidth. One framing bit adds 8 kbit/s of overhead, for a total of 1.544 Mbit/s, calculated as follows:
DS1 is a full-duplex circuit, concurrently transmitting and receiving 1.544 Mbit/s.
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 superframe (SF) and extended superframe (ESF). A superframe consists of twelve consecutive 193-bit frames, whereas an extended superframe 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.
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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 indication signal 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, also known as remote alarm indication (RAI), indicates reception of a data or framing pattern that reports the far end is in "red alarm". The alarm is carried differently in SF (D4) and ESF (D5) framing. For SF framed signals, the user bandwidth is manipulated and "bit two in every DS0 channel shall be a zero." [5] The resulting loss of payload data while transmitting a yellow alarm is undesirable, and was resolved in ESF framed signals by using the data link layer. "A repeating 16-bit pattern consisting of eight 'ones' followed by eight 'zeros' shall be transmitted continuously on the ESF data link, but may be interrupted for a period not to exceed 100-ms per interruption." [5] Both types of alarms are transmitted for the duration of the alarm condition, but for at least one second.
Blue alarm, also known as alarm indication signal (AIS) indicates a disruption in the communication path between the terminal equipment and line repeaters or DCS. If no signal is received by the intermediary equipment, it produces an unframed, all-ones signal. 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 intermediary 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.
Additionally, for voice T1s there are two main 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 for bearer) but also a 24th line (Known as the "D" channel for data [6] ) that carries line signaling information. This special "D" channel carries: Caller ID (CID) and automatic number identification (ANI) data, required channel type (usually a B, or Bearer channel), call handle, Dialed Number Identification Service (DNIS) info, requested channel number and a request for response. [7]
Inband T1s are also capable of carrying CID and ANI information if they are configured by the carrier by sending DTMF *ANI*DNIS*. However, PRIs handle this more efficiently. 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 setup and tear-down 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). Although an inband T1 has 24 channels, the 23 channel PRI can set up more calls faster due to the dedicated 24th signalling channel (D Channel).
Before T1 PRI existed there was T1 CAS. T1 CAS is not common today but it still exists. CAS is Channel Associated Signaling. It is also referred to as Robbed Bit Signaling. CAS is a technology with roots in the 60's and before.
The name T1 came from the carrier letter assigned by AT&T to the technology in 1957, when digital systems were first proposed and developed, AT&T decided to skip Q, R, S, and to use T, for Time Division. The naming system ended with the letter T, which designated fiber networks. Destined successors of the T1 system of networks, called T1C, T2, T3, and T4 were not a commercial success and disappeared quickly, signals that would have been carried on these systems, called DS1, DS2, DS3, and DS4 are now carried on T1 infrastructure. [8]
DS-1 means "Digital Service – Level 1," and had to do with the signal to be sent—as opposed to the network that delivers it (originally 24 digitized voice channels over the T1). Since the practice of naming networks ended with the letter "T," [8] the terms T1 and DS1 have become synonymous and encompass 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. [9]
Dark fiber: Dark fiber refers to unused fibers, available for use. Dark fiber has been, and still is, available for sale on the wholesale market for both metro and wide area links, but it may not be available in all markets or city pairs.
Dark fiber capacity is typically used by network operators to build SONET and dense wavelength-division multiplexing (DWDM) networks, usually involving meshes of self-healing rings. Now, it is also used by end-user enterprises to expand Ethernet local area networks, especially since the adoption of IEEE standards for gigabit Ethernet and 10 Gigabit Ethernet over single-mode fiber. Running Ethernet networks between geographically separated buildings is a practice known as "WAN elimination".
DS1C is a digital signal equivalent to two Digital Signal 1, with extra bits to conform to a signaling standard of 3.152 Mbit/s. Few (if any) of these circuit capacities are still in use today. In the early days of digital and data transmission, the 3-Mbit/s data rate was used to link mainframe computers together. The physical side of this circuit is called T1C. [10]
The T1/E1 protocol is implemented as a "line interface unit" in silicon. The semiconductor chip contains a decoder/encoder, loop backs, jitter attenuators, receivers, and drivers. Additionally, there are usually multiple interfaces and they are labeled as dual, quad, octal, etc., depending upon the number.
The transceiver chip's primary purpose is to retrieve information from the "line", i.e., the conductive line that transverses distance, by receiving the pulses and converting the signal which has been subjected to noise, jitter, and other interference, to a clean digital pulse on the other interface of the chip.
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work on the standard began in 1980 at Bell Labs and was formally standardized in 1988 in the CCITT "Red Book". By the time the standard was released, newer networking systems with much greater speeds were available, and ISDN saw relatively little uptake in the wider market. One estimate suggests ISDN use peaked at a worldwide total of 25 million subscribers at a time when 1.3 billion analog lines were in use. ISDN has largely been replaced with digital subscriber line (DSL) systems of much higher performance.
Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission rates data can also be transferred via an electrical interface. The method was developed to replace the plesiochronous digital hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber without the problems of synchronization.
In digital transmission, the number of bit errors is the numbers of received bits of a data stream over a communication channel that have been altered due to noise, interference, distortion or bit synchronization errors.
In telecommunications, a channel service unit (CSU) is a line bridging device for use with T-carrier, which
In telecommunications, a digital multiplex hierarchy is a hierarchy consisting of an ordered repetition of tandem digital multiplexers that produce signals of successively higher data rates at each level of the hierarchy.
Digital subscriber line is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology, for Internet access.
Modified AMI codes are a digital telecommunications technique to maintain system synchronization. Alternate mark inversion (AMI) line codes are modified by deliberate insertion of bipolar violations. There are several types of modified AMI codes, used in various T-carrier and E-carrier systems.
The Primary Rate Interface (PRI) is a telecommunications interface standard used on an Integrated Services Digital Network (ISDN) for carrying multiple DS0 voice and data transmissions between the network and a user.
The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls.
Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern. It can be used when the bit rate of the transmission medium exceeds that of the signal to be transmitted. This form of signal multiplexing was developed in telecommunications for telegraphy systems in the late 19th century, but found its most common application in digital telephony in the second half of the 20th century.
The E-carrier is a member of the series of carrier systems developed for digital transmission of many simultaneous telephone calls by time-division multiplexing. The European Conference of Postal and Telecommunications Administrations (CEPT) originally standardised the E-carrier system, which revised and improved the earlier American T-carrier technology, and this has now been adopted by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). It was widely used in almost all countries outside the US, Canada, and Japan. E-carrier deployments have steadily been replaced by Ethernet as telecommunication networks transition towards all IP.
A digital subscriber line access multiplexer is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital communications channel using multiplexing techniques. Its cable internet (DOCSIS) counterpart is the cable modem termination system.
A leased line is a private telecommunications circuit between two or more locations provided according to a commercial contract. It is sometimes also known as a private circuit, and as a data line in the UK. Typically, leased lines are used by businesses to connect geographically distant offices.
In communications systems, robbed-bit signaling (RBS) is a scheme to provide maintenance and line signaling services on many T1 digital carrier circuits using channel-associated signaling (CAS). The T1 carrier circuit is a type of dedicated circuit currently employed in North America and Japan.
In telecommunications, superframe (SF) is a T1 framing standard. In the 1970s it replaced the original T1/D1 framing scheme of the 1960s in which the framing bit simply alternated between 0 and 1.
The media-independent interface (MII) was originally defined as a standard interface to connect a Fast Ethernet media access control (MAC) block to a PHY chip. The MII is standardized by IEEE 802.3u and connects different types of PHYs to MACs. Being media independent means that different types of PHY devices for connecting to different media can be used without redesigning or replacing the MAC hardware. Thus any MAC may be used with any PHY, independent of the network signal transmission media.
Alarm indication signal (AIS) is a signal transmitted by an intermediate element of a multi-node transport circuit that is part of a concatenated telecommunications system to alert the receiving end of the circuit that a segment of the end-to-end link has failed at a logical or physical level, even if the system it is directly connected to is still working. The AIS replaces the failed data, allowing the higher order system in the concatenation to maintain its transmission framing integrity. Downstream intermediate elements of the transport circuit propagate the AIS onwards to the destination element.
A digital cross-connect system is a piece of circuit-switched network equipment, used in telecommunications networks, that allows lower-level TDM bit streams, such as DS0 bit streams, to be rearranged and interconnected among higher-level TDM signals, such as DS1 bit streams. DCS units are available that operate on both older T-carrier/E-carrier bit streams, as well as newer SONET/SDH bit streams.
High-bit-rate digital subscriber line (HDSL) is a telecommunications protocol standardized in 1994. It was the first digital subscriber line (DSL) technology to use a higher frequency spectrum over copper, twisted pair cables. HDSL was developed to transport DS1 services at 1.544 Mbit/s and 2.048 Mbit/s over telephone local loops without a need for repeaters. Successor technology to HDSL includes HDSL2 and HDSL4, proprietary SDSL, and G.SHDSL.
The pulse-code modulation (PCM) technology was patented and developed in France in 1938, but could not be used because suitable technology was not available until World War II. This came about with the arrival of digital systems in the 1960s, when improving the performance of communications networks became a real possibility. However, this technology was not completely adopted until the mid-1970s, due to the large amount of analog systems already in place and the high cost of digital systems, as semiconductors were very expensive. PCM’s initial goal was that of converting an analog voice telephone channel into a digital one based on the sampling theorem.
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