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62-429: In ADSL, the data throughput in the upstream direction (the direction to the service provider) is lower, hence the designation of asymmetric service. In symmetric digital subscriber line (SDSL) services, the downstream and upstream data rates are equal. DSL service can be delivered simultaneously with wired telephone service on the same telephone line since DSL uses higher frequency bands for data transmission. On

124-440: A digital subscriber line access multiplexer (DSLAM) at one end and a DSL modem at the other end. It is possible to set up a DSL connection over an existing cable. Such deployment, even including equipment, is much cheaper than installing a new, high-bandwidth fiber-optic cable over the same route and distance. This is true both for ADSL and SDSL variations. The commercial success of DSL and similar technologies largely reflects

186-427: A subscriber can connect to a service such as an Internet service provider or other network services, like a corporate MPLS network. The underlying technology of transport across DSL facilities uses modulation of high-frequency carrier waves , an analog signal transmission. A DSL circuit terminates at each end in a modem which modulates patterns of bits into certain high-frequency impulses for transmission to

248-515: A certain distance without such coils. Therefore, some areas that are within range for DSL service are disqualified from eligibility because of loading coil placement. Because of this, phone companies endeavor to remove loading coils on copper loops that can operate without them. Longer lines that require them can be replaced with fiber to the neighborhood or node ( FTTN ). Most residential and small-office DSL implementations reserve low frequencies for POTS, so that (with suitable filters and/or splitters)

310-610: 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." 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

372-476: A filter or splitter integrated in the DSLAM or by specialized filtering equipment installed before it. Load coils in phone lines, used for extending their range in rural areas, must be removed to allow DSL to operate as they only allow frequencies of up to 4000 Hz to pass through phone cables. The customer end of the connection consists of a DSL modem . This converts data between the digital signals used by computers and

434-543: A node A on the Internet is closer (fewer hops away) to the Internet backbone than a node B, then A is said to be upstream of B or conversely, B is downstream of A. Related to this is the idea of upstream providers . An upstream provider is usually a large ISP that provides Internet access to a local ISP. Hence, the word upstream also refers to the data connection between two ISPs. This article related to telecommunications

496-560: A premium. Whereas downstream speed is important to the average home user for purposes of downloading content, uploads are used mainly for web server applications and similar processes where the sending of data is critical. Upstream speeds are also important to users of peer-to-peer software . ADSL and cable modems are asymmetric , with the upstream data rate much lower than that of its downstream. Symmetric connections such as Symmetric Digital Subscriber Line (SDSL) and T1 , however, offer identical upstream and downstream rates. If

558-428: A request for response. 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

620-613: A single subscriber to receive two separate services from two separate providers on one cable pair. The DSL service provider's equipment is co-located in the same telephone exchange as that of the ILEC supplying the customer's pre-existing voice service. The subscriber's circuit is rewired to interface with hardware supplied by the ILEC which combines a DSL frequency and POTS signals on a single copper pair. Since 1999, certain ISPs have been offering microfilters. These devices are installed indoors and serve

682-400: 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

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744-570: A unconditioned local loop (ULL); in Belgium it is known as "raw copper" and in the UK it is known as Single Order GEA (SoGEA). It started making a comeback in the United States in 2004 when Qwest started offering it, closely followed by Speakeasy . As a result of AT&T 's merger with SBC , and Verizon 's merger with MCI , those telephone companies have an obligation to offer naked DSL to consumers. On

806-458: A user ID and password. Transmission methods vary by market, region, carrier, and equipment. DSL technologies (sometimes collectively summarized as xDSL ) include: The line-length limitations from telephone exchange to subscriber impose severe limits on data transmission rates. Technologies such as VDSL provide very high-speed but short-range links. VDSL is used as a method of delivering triple play services (typically implemented in fiber to

868-580: 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 . 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

930-479: Is a stub . You can help Misplaced Pages by expanding it . Digital Signal 1 Digital Signal 1 ( DS1 , sometimes DS-1 ) is a T-carrier signaling scheme devised by Bell Labs . 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

992-402: Is a schematic of a simple DSL connection (in blue). The right side shows a DSLAM residing in the telephone company's telephone exchange. The left side shows the customer premises equipment with an optional router. The router manages a local area network which connects PCs and other local devices. The customer may opt for a modem that contains both a router and wireless access. This option (within

1054-742: 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, and 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 commercial successes and disappeared quickly. Signals that would have been carried on these systems, called DS1 , DS2 , DS3 , and DS4 , are now carried on T1 infrastructure. DS-1 means Digital Service – Level 1 and has to do with

1116-499: Is a way of providing only DSL services over a local loop . It is useful when the customer does not need the traditional telephony voice service because voice service is received either on top of the DSL services (usually VoIP ) or through another network (E.g., mobile telephony ). It is also commonly called an unbundled network element (UNE) in the United States; in Australia it is known as

1178-403: Is depending on technology, line conditions, and service-level implementation. Researchers at Bell Labs have reached SDSL speeds over 1  Gbit/s using traditional copper telephone lines, though such speeds have not been made available for the end customers yet. Initially, it was believed that ordinary phone lines could only be used at modest speeds, usually less than 9600 bits per second. In

1240-735: Is preferred for streaming multimedia , where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where the delivered data must be error-free but latency (time delay) incurred by the retransmission of error-containing packets is acceptable. Consumer-oriented ADSL was designed to operate on existing lines already conditioned for Basic Rate Interface ISDN services. Engineers developed high speed DSL facilities such as high bit rate digital subscriber line (HDSL) and symmetric digital subscriber line (SDSL) to provision traditional Digital Signal 1 (DS1) services over standard copper pair facilities. Older ADSL standards delivered 8  Mbit/s to

1302-522: Is then split into two different frequency bands for upstream and downstream traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether they are usable. Once upstream and downstream circuits are established,

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1364-571: 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. Connectivity refers to

1426-425: The telephone exchange via a local loop , which is a physical pair of wires. The local loop was originally intended mostly for the transmission of speech, encompassing an audio frequency range of 300 to 3400 hertz ( commercial bandwidth ). However, as long-distance trunks were gradually converted from analog to digital operation, the idea of being able to pass data through the local loop (by using frequencies above

1488-684: The 1950s, ordinary twisted-pair telephone cable often carried 4 MHz television signals between studios, suggesting that such lines would allow transmitting many megabits per second. One such circuit in the United Kingdom ran some 10 miles (16 km) between the BBC studios in Newcastle-upon-Tyne and the Pontop Pike transmitting station . However, these cables had other impairments besides Gaussian noise , preventing such rates from becoming practical in

1550-429: The 23 actual usable telephone lines (known as B channels for bearer ) but also a 24th line (known as the D channel for data ) 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

1612-447: 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

1674-471: The CCITT (now ITU-T ) as part of Recommendation I.120 , later reused as ISDN digital subscriber line (IDSL). Employees at Bellcore (now Telcordia Technologies ) developed asymmetric digital subscriber line (ADSL) by placing wide-band digital signals at frequencies above the existing baseband analog voice signal carried on conventional twisted pair cabling between telephone exchanges and customers. A patent

1736-399: The ESF data link, but may be interrupted for a period not to exceed 100-ms per interruption." 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

1798-513: 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. 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

1860-461: 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

1922-437: 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 circuits, 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

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1984-664: The advances made in electronics over the decades that have increased performance and reduced costs even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive. These advantages made ADSL a better proposition for customers requiring Internet access than metered dial up, while also allowing voice calls to be received at the same time as a data connection. Telephone companies were also under pressure to move to ADSL owing to competition from cable companies, which use DOCSIS cable modem technology to achieve similar speeds. Demand for high bandwidth applications, such as video and file sharing, also contributed to

2046-524: 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

2108-483: The analog voltage signal of a suitable frequency range which is then applied to the phone line. In some DSL variations (for example, HDSL ), the modem connects directly to the computer via a serial interface, using protocols such as Ethernet or V.35 . In other cases (particularly ADSL), it is common for the customer equipment to be integrated with higher-level functionality, such as routing, firewalling, or other application-specific hardware and software. In this case,

2170-413: The carrier system. 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

2232-419: The curb network architectures). Terabit DSL, is a technology that proposes the use of the space between the dielectrics (insulators) on copper twisted pair lines in telephone cables, as waveguides for 300 GHz signals that can offer speeds of up to 1 terabit per second at distances of up to 100 meters, 100 gigabits per second for 300 meters, and 10 gigabits per second for 500 meters. The first experiment for this

2294-427: The customer over about 2 km (1.2 mi) of unshielded twisted-pair copper wire. Newer variants improved these rates. Distances greater than 2 km (1.2 mi) significantly reduce the bandwidth usable on the wires, thus reducing the data rate. But ADSL loop extenders increase these distances by repeating the signal, allowing the local exchange carrier (LEC) to deliver DSL speeds to any distance. Until

2356-433: The customer premises, a DSL filter is installed on each telephone to prevent undesirable interaction between DSL and telephone service. The bit rate of consumer ADSL services typically ranges from 256 kbit/s up to 25 Mbit/s, while the later VDSL+ technology delivers between 16 Mbit/s and 250 Mbit/s in the direction to the customer ( downstream ), with up to 40 Mbit/s upstream. The exact performance

2418-428: The customer side, a DSL modem is hooked up to a phone line. The telephone company connects the other end of the line to a DSLAM , which concentrates a large number of individual DSL connections into a single box. The DSLAM cannot be located too far from the customer because of attenuation between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM. The above figure

2480-475: The customer's premises. The theoretical foundations of DSL, like much of communication technology, can be traced back to Claude Shannon 's seminal 1948 paper, " A Mathematical Theory of Communication ". Generally, higher bit rate transmissions require a wider frequency band, though the ratio of bit rate to symbol rate and thus to bandwidth are not linear due to significant innovations in digital signal processing and digital modulation methods . Naked DSL

2542-403: The dashed bubble) often simplifies the connection. At the exchange, a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off to other networking transports. The DSLAM terminates all connections and recovers the original digital information. In the case of ADSL, the voice component is also separated at this step, either by

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2604-484: The early days of digital and data transmission, the three-megabit-per-second data rate was used to link mainframe computers together. The physical side of this circuit is called T1C. 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

2666-405: The equipment is referred to as a gateway. Most DSL technologies require the installation of appropriate DSL filters at the customer's premises to separate the DSL signal from the low-frequency voice signal. The separation can take place either at the demarcation point , or with filters installed at the telephone outlets inside the customer premises. It is possible for a DSL gateway to integrate

2728-443: The existing voice service continues to operate independently of the DSL service. Thus POTS-based communications, including fax machines and dial-up modems , can share the wires with DSL. Only one DSL modem can use the subscriber line at a time. The standard way to let multiple computers share a DSL connection uses a router that establishes a connection between the DSL modem and a local Ethernet , powerline , or Wi-Fi network on

2790-534: The field. The 1980s saw the development of techniques for broadband communications that allowed the limit to be greatly extended. A patent was filed in 1979 for the use of existing telephone wires for both telephones and data terminals that were connected to a remote computer via a digital data carrier system. The motivation for digital subscriber line technology was the Integrated Services Digital Network (ISDN) specification proposed in 1984 by

2852-431: The filter, and allow telephones to connect through the gateway. Modern DSL gateways often integrate routing and other functionality. The system boots, synchronizes the DSL connection and finally establishes the internet IP services and connection between the local network and the service provider, using protocols such as DHCP or PPPoE . Many DSL technologies implement an Asynchronous Transfer Mode (ATM) layer over

2914-495: 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

2976-405: 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

3038-429: The late 1990s, the cost of digital signal processors for DSL was prohibitive. All types of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Due to the advancements of very-large-scale integration (VLSI) technology, the cost of the equipment associated with a DSL deployment lowered significantly. The two main pieces of equipment are

3100-583: The low-level bitstream layer to enable the adaptation of a number of different technologies over the same link. DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnetwork . The earliest implementations used DHCP to provide the IP address to the subscriber equipment, with authentication via MAC address or an assigned hostname . Later implementations often use Point-to-Point Protocol (PPP) to authenticate with

3162-517: The opposing modem. Signals received from the far-end modem are demodulated to yield a corresponding bit pattern that the modem passes on, in digital form, to its interfaced equipment, such as a computer, router, switch, etc. Unlike traditional dial-up modems, which modulate bits into signals in the 300–3400 Hz audio baseband, DSL modems modulate frequencies from 4000 Hz to as high as 4 MHz. This frequency band separation enables DSL service and plain old telephone service (POTS) to coexist on

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3224-443: The popularity of ADSL technology. Some of the first field trials for DSL were carried out in 1996. Early DSL service required a dedicated dry loop , but when the U.S. Federal Communications Commission (FCC) required incumbent local exchange carriers (ILECs) to lease their lines to competing DSL service providers, shared-line DSL became available. Also known as DSL over unbundled network element , this unbundling of services allows

3286-458: The same cables, known as voice-grade cables. On the subscriber's end of the circuit, inline DSL filters are installed on each telephone to pass voice frequencies but filter the high-frequency signals that would otherwise be heard as hiss. Also, nonlinear elements in the phone could otherwise generate audible intermodulation and may impair the operation of the data modem in the absence of these low-pass filters . DSL and RADSL modulations do not use

3348-566: The same purpose as DSL splitters, which are deployed outdoors: they divide the frequencies needed for ADSL and POTS phone calls. These filters originated out of a desire to make self-installation of DSL service possible, and eliminate early outdoor DSL splitters which were installed at or near the demarcation point between the customer and the ISP. By 2012, some carriers in the United States reported that DSL remote terminals with fiber backhaul were replacing older ADSL systems. Telephones are connected to

3410-525: The signal carried—as opposed to the network that delivers it (originally 24 digitized voice channels over a T1). Since the practice of naming networks ended with the letter T , the terms T1 and DS1 have become synonymous and encompass a variety of services including voice, data, and clear-channel pipes . The line speed is always 1.544 Mbit/s, but the payload can vary greatly. Dark fiber : Dark fiber refers to unused fibers available for use. Dark fiber has been, and still is, available for sale on

3472-462: The system is configured. Allocation of channels continues to higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. More usable channels equate to more available bandwidth, which is why distance and line quality are a factor (the higher frequencies used by DSL travel only short distances). The pool of usable channels

3534-453: 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

3596-528: The voice-frequency band so high-pass filters are incorporated in the circuitry of DSL modems filter out voice frequencies. Because DSL operates above the 3.4 kHz voice limit, it cannot pass through a loading coil , which is an inductive coil that is designed to counteract loss caused by shunt capacitance (capacitance between the two wires of the twisted pair). Loading coils are commonly set at regular intervals in POTS lines. Voice service cannot be maintained past

3658-472: The voiceband) took hold, ultimately leading to DSL. The local loop connecting the telephone exchange to most subscribers has the capability of carrying frequencies well beyond the 3400 Hz upper limit of POTS . Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how

3720-408: 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

3782-464: Was carried out with copper lines that were parallel to each other, and not twisted, inside a metal pipe meant to simulate the metal armoring in large telephone cables . Upstream (networking) In computer networking , upstream refers to the direction in which data can be transferred from the client to the server ( uploading ). This differs greatly from downstream not only in theory and usage, but also in that upstream speeds are usually at

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3844-518: Was filed by AT&T Bell Labs on the basic DSL concept in 1988. Joseph W. Lechleider 's contribution to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of symmetric DSL. This allowed Internet service providers to offer efficient service to consumers, who benefited greatly from the ability to download large amounts of data but rarely needed to upload comparable amounts. ADSL supports two modes of transport: fast channel and interleaved channel . Fast channel

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