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Telephone line
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Telephone line
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Utility pole with electric lines (top) and telephone cables.
Fixed telephone lines per 100 inhabitants, 1997–2007.
Cross section of telephone cable of 1,800 twisted pairs, 1922.
A newer telephone cable used to carry telephone lines from several customers

A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system.[1] It is designed to reproduce speech of a quality that is understandable.[citation needed] It is the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, and usually also implies a single telephone number for billing purposes reserved for that user.

Telephone lines are used to deliver consistent landline telephone service and digital subscriber line (DSL) phone cable service to the premises.[2] Telephone overhead lines are connected to the public switched telephone network.[3][4] The voltage at a subscriber's network interface is typically 48 V between the ring and tip wires, with tip near ground and ring at –48 V.

In the United States

[edit]

In 1878, the Bell Telephone Company began using two-wire circuits, called the local loop, from each user's telephone to end offices, which performed any necessary electrical switching to allow voice signals to be transmitted to more distant telephones.

These wires were typically copper, although aluminium has also been used, and were carried in balanced pairs of open wire, separated by about 25 cm (10″) on poles above the ground, and later as twisted pair cables. Modern lines may run underground and may carry analog or digital signals to the exchange. They may also have a device that converts the analog signal to digital for transmission on a carrier system. Often, the customer end of that wire pair is connected to a data access arrangement, and the telephone company end of that wire pair is connected to a telephone hybrid.

In most cases, two copper wires (tip and ring) for each telephone line run from a home or other small building to a local telephone exchange. There is a central junction box for the building where the wires that go to telephone jacks throughout the building and wires that go to the exchange meet and can be connected in different configurations depending upon the subscribed telephone service. The wires between the junction box and the exchange are known as the local loop, and the network of wires going to an exchange is known as the access network.

The vast majority of houses in the U.S. are wired with 6-position modular jacks with four conductors (6P4C) wired to the house's junction box with copper wires. Those copper wires may be connected back to two telephone overhead lines at the local telephone exchange, thus making those jacks RJ14 jacks. More often, only two of the wires are connected to the exchange as one telephone line, and the others are unconnected. In that case, the jacks in the house are RJ11.

Older houses often have 4-conductor telephone station cable in the walls color coded with Bell System colors: red, green, yellow, and black as 2-pairs of 22 AWG (0.33 mm2) solid copper; "line 1" uses the red/green pair and "line 2" uses the yellow/black pair. Inside the walls of the house—between the house's outside junction box and the interior wall jacks—the most common telephone cable in new houses is Category 5 cable—4 pairs of 24 AWG (0.205 mm2) solid copper.[5]

Inside large buildings, and in the outdoor cables that run to the telephone company POP, many telephone lines are bundled together in a single cable using the 25-pair color code.[6] Outside plant cables can have up to 3,600 or 3,800 pairs, used at the entrances of telephone exchanges.[7][8]

References

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Telephone line

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from Grokipedia
A telephone line, also known as a fixed telephone line, is an active connection linking a subscriber's telephone equipment to the (PSTN), enabling voice and sometimes data transmission over dedicated circuits. Typically composed of twisted-pair wires insulated to minimize , these lines carry analog signals in traditional setups, with each pair consisting of two insulated conductors twisted together to reduce and . Standard specifications often include 22- or 24-gauge wires, supporting transmission distances up to several kilometers before requiring signal amplification via . The development of telephone lines traces back to the invention of the telephone by in 1876, which built upon earlier telegraph systems to create the first practical voice communication circuits. By the late , networks of these lines expanded rapidly, with the first long-distance connections established in the 1880s, forming the foundation of modern telecommunications infrastructure under companies like the . Over the , innovations such as loading coils for extended range, multi-pair cables for efficiency, and eventual digitization improved reliability and capacity, supporting not only voice calls but also early services like dial-up . In the , telephone lines remain integral to certain applications, including services and rural connectivity, though their global prevalence has declined sharply with the proliferation of mobile networks and (VoIP) technologies. As of 2024, fixed telephone subscriptions stand at approximately 10.3 per 100 people worldwide, reflecting a steady decrease since the early due to shifts toward and alternatives. Despite this, regulatory frameworks continue to support legacy lines for number portability and obligations in many regions.

History

Early Invention and Development

The , patented by , marked a pivotal moment in communication history, though the patent was disputed by inventors such as and . Bell's U.S. Patent No. 174,465, titled "Improvement in ," was granted on March 7, 1876. This patent described a device that transmitted speech electrically using a variable resistance transmitter and electromagnetic receiver, building on earlier principles. Bell's breakthrough enabled the conversion of sound waves into electrical signals, laying the foundation for wired voice communication over distances. The first practical demonstration of a telephone line occurred on October 9, 1876, when Bell and his assistant conducted a two-way over a 2-mile (3.2 km) telegraph line between and Cambridgeport, . This test, using existing telegraph infrastructure, confirmed the viability of transmitting intelligible speech outdoors, with Bell in speaking to Watson in Cambridge and vice versa for about 30 minutes. Initial implementations relied on single-wire ground-return systems, where one conductor carried the signal and the earth served as the return path, adapted directly from telegraph lines for simplicity and cost. However, by 1878, the transitioned to two-wire balanced lines, which used a dedicated return conductor to minimize and improve signal quality. Early telephone lines faced significant challenges, including signal that weakened voice clarity over distances beyond a few miles, exacerbated by the higher frequencies of speech compared to telegraph signals. To address infrastructure needs, open-wire construction emerged, with bare or iron wires strung on wooden poles equipped with insulators to prevent shorting and support aerial spans. These setups, while economical, were vulnerable to weather-induced noise and from adjacent lines. In the 1880s, the introduction of manual switchboards revolutionized connectivity; the world's first commercial opened on January 28, 1878, in , under George W. Coy, using a switchboard to connect 21 subscribers via plugs and jacks. This innovation enabled multi-user networks, allowing operators to route calls dynamically and scale beyond point-to-point lines.

Global Expansion and Standardization

The rapid growth of telephone lines in the early transformed communication, with the leading the expansion as the number of telephones reached approximately 9.4 million by 1915, surpassing 10 million by 1916. This surge was fueled by aggressive commercial investments from companies like , which extended networks across urban and rural areas, while in , the number of household telephones reached approximately 3 million by 1910, driven by similar private sector initiatives in countries such as and the . Globally, networks proliferated through commercial ventures and colonial administrations, which prioritized infrastructure in territories like British India and to support administrative control and economic ties. Technological advancements further enabled this scaling, including the introduction of loading coils by the in 1899, placed at intervals of about 4,500 to 6,000 feet, which compensated for signal attenuation and enabled reliable voice transmission over much longer distances, such as up to several thousand miles with repeaters. By the 1920s and 1930s, urban areas saw a significant shift from vulnerable open-wire lines strung on poles to more reliable underground cables, reducing interference and weather-related disruptions while accommodating denser wiring in growing cities like New York. These improvements, combined with vacuum tube repeaters developed around 1915, allowed for longer-distance calls and supported the burgeoning transcontinental networks in . The World Wars profoundly influenced telephone infrastructure, accelerating development in Europe and Asia as military demands spurred innovations in wiring and switching systems during , with post-war reconstruction in the expanding civilian access. further intensified this, as wartime needs in regions like Britain and led to hardened networks and international coordination, paving the way for global standardization. Following the war, the (ITU) reorganized in 1947 as a United Nations specialized agency, establishing protocols for international telephone interconnectivity to facilitate seamless cross-border calls. A landmark in this era was the 1956 completion of , the first transatlantic submarine telephone cable, which used coaxial technology to support 36 simultaneous voice circuits between and Newfoundland, marking a new phase in reliable global connectivity.

Technical Fundamentals

Physical Components and Wiring

Telephone lines primarily utilize twisted-pair copper wires as the physical medium for , with the twisting of the two conductors serving to minimize (EMI) and by balancing . This design, patented by in 1881, became a standard for metallic circuits by the late 1880s, replacing earlier open-wire configurations that were prone to noise from unbalanced fields. For local loops connecting customer premises to the central office, 24-gauge (AWG) wire is commonly employed due to its balance of conductivity and flexibility, supporting voice-grade performance up to 16 MHz in Category 3 (Cat-3) rated cables suitable for telephony. The historical progression of telephone wiring began with open-wire pairs in the 1870s, using iron or conductors with a ground return, but transitioned to insulated twisted pairs by the to address interference issues in growing urban networks. In 1888, issued the first specification for twisted-pair telephone cables, featuring 18-gauge wires insulated with paraffin-impregnated paper and sheathed in lead. Modern implementations bundle multiple twisted pairs—typically ranging from 25 to 3,800 pairs—into aerial, buried, or underground cables for efficient distribution, with color-coding systems identifying individual pairs; for instance, pair 1 consists of a white conductor with a band (tip) and a conductor with a white band (ring). These cables are deployed aerially on poles, directly buried in trenches, or pulled through underground conduits to protect against environmental factors. Connectors for terminating telephone lines include RJ11 jacks, which accommodate two conductors for single-line service, and RJ14 jacks for two-line configurations using four conductors, both adhering to the registered jack standards for modular interfacing. The demarcation point, often implemented as a network interface device (NID), marks the boundary between the carrier's external wiring and the customer's internal premises wiring, ensuring clear responsibility for maintenance and troubleshooting. Core materials in telephone lines consist of solid annealed conductors for optimal electrical conductivity, paired with insulation such as (PVC) for indoor applications or (PE) for outdoor durability against moisture and abrasion. Conductor resistance varies by gauge, typically ranging from 8.21 ohms per 1,000 feet for 19 AWG (used in longer-haul applications) to 66.2 ohms per 1,000 feet for finer 28 AWG wires in short subscriber drops, influencing signal over distance.

Electrical and Signaling Characteristics

Traditional telephone lines operate using specific direct current (DC) voltages to maintain supervision and power the telephone equipment. In the on-hook state, the line provides a nominal DC voltage of -48 V, with the ring conductor at -48 V relative to the tip conductor at 0 V, ensuring no current flows through the connected device. When the telephone goes off-hook, this voltage drops to between -6 V and -12 V to supply operating power while limiting current draw. For incoming calls, a ringing signal superimposes 75 to 90 V AC at 20 Hz on the DC voltage to activate the ringer in the telephone set. The audio bandwidth of voice-grade telephone lines is limited to 300 Hz to 3,400 Hz, which captures the essential frequencies of human speech while minimizing bandwidth requirements and susceptibility. Signal attenuation over the line depends on wire gauge and length; for 24-gauge twisted-pair wire, it is approximately 0.5 dB per 1,000 feet at 1 kHz, affecting longer loops by reducing signal strength and necessitating amplification at the central . The of telephone lines is 600 ohms, designed to match the of telephone equipment and central interfaces for efficient power transfer and minimal reflections. Off-hook, the loop current typically ranges from 20 to 50 mA, providing sufficient power for the telephone's , speaker, and dialing circuitry while preventing excessive drain on the battery supply. Signaling on telephone lines uses distinct methods to indicate call states and transmit dialed digits. Loop-start signaling, common in residential lines, seizes the line by closing the circuit between tip and ring conductors to signal off-hook, drawing loop current to alert the central office. Ground-start signaling, often used in business PBX systems, improves reliability by first grounding the ring conductor to request service, avoiding simultaneous seizure (glare) on shared trunks, before completing the loop. For dialing, dual-tone multi-frequency (DTMF) tones generate pairs of sinusoidal frequencies in the voice band; for example, the digit '1' uses 697 Hz and 1,209 Hz simultaneously, enabling reliable decoding at the exchange.

Types and Variants

Analog Lines

Analog telephone lines, also known as (POTS), transmit continuous electrical signals that directly mimic the acoustic waveforms of human speech over twisted-pair copper wires, providing a single voice channel per wire pair. This analog transmission operates within a voice-frequency bandwidth of approximately 300 to 3400 Hz, ensuring compatibility with standard telephone handsets without the need for digital conversion at the local loop. These lines have been primarily used for residential and business voice communications, enabling reliable point-to-point calls over the public switched telephone network. They also support analog fax machines, which modulate document images into audio tones for transmission, and low-speed dial-up modems achieving data rates up to 56 kbps under the ITU-T V.92 standard, suitable for early internet access and file transfers. Basic wiring involves a single twisted-pair with off-hook voltages around -48 V DC and ring signals up to 90 V AC, as referenced in core network specifications. A key limitation of analog lines is their vulnerability to electromagnetic and , where unwanted signals from adjacent pairs degrade audio quality, particularly over longer distances or in bundled cables. The practical maximum length for the local loop is about 18,000 feet (5.5 km) using 24-gauge wire, beyond which signal exceeds acceptable levels without or loading coils, limiting deployment in rural areas. Additionally, each line supports only one simultaneous conversation, as it lacks for multiple channels. Notable features include , which alerts users to incoming calls via brief in-band tones during an active conversation, and , delivered as (FSK) data bursts in the voice band just before the first ring. Deployment of analog lines expanded significantly in the late , with over 435 million main fixed telephone lines worldwide by 1988, predominantly analog. Total fixed lines peaked at around 1.26 billion in 2006 before declining with the rise of mobile and digital alternatives. As of 2024, fixed telephone subscriptions (largely legacy analog) stand at approximately 828 million worldwide, reflecting ongoing decline.

Digital and Broadband Lines

Digital telephone lines represent an evolution from traditional analog systems, enabling the transmission of digitized voice, data, and other services over existing infrastructure through (PCM) encoding, which samples signals at 8 kHz to produce 64 kbps channels using μ-law in or A-law internationally. This digitization occurs primarily at central office switches, where analog signals from subscriber loops are converted to digital format for and routing across the network. Integrated Services Digital Network (ISDN) provides a foundational digital line service, with the (BRI) offering two 64 kbps B-channels for voice or data transmission alongside a 16 kbps D-channel for signaling, as specified in Recommendation I.430. (PRI) variants support higher capacities, such as 23 B-channels in or 30 in , bundled into T1 or E1 frames. For carrier-grade applications, T1 lines deliver 1.544 Mbps aggregate capacity across 24 channels using μ-law PCM, while E1 lines provide 2.048 Mbps over 30 channels with A-law PCM, both adhering to ITU-T G.702 and G.703 for hierarchical digital interfaces. These configurations allow multiple simultaneous connections, enhancing efficiency over single analog lines. Broadband services extend digital lines by overlaying high-speed data on telephone infrastructure, primarily through (DSL) technologies that separate voice and data frequencies using low-pass splitters at the customer premises. Asymmetric DSL (ADSL), defined in G.992.1, employs discrete multitone (DMT) modulation to achieve downstream speeds up to 8 Mbps initially, with ADSL2+ (G.992.5) extending this to 24 Mbps over distances up to 5 km. Very-high-bit-rate DSL (VDSL), per G.993.1 and enhanced in G.993.2 (VDSL2), supports over 100 Mbps downstream on shorter loops (under 1 km), enabling video streaming and other bandwidth-intensive applications while preserving POTS voice on the same pair. The shift to digital lines involves upgrading central office switches from analog electromechanical systems to digital stored-program control switches, which perform PCM conversion and for efficient . This transition, widespread since the 1980s, facilitates integrated voice and data services but excludes pure IP-based (VoIP) implementations over DSL, which rely on packet-switched rather than circuit-switched telephone lines. Key advantages of digital lines include inherent support for detection via framing bits in T1/E1 structures and the ability to incorporate in higher-layer protocols, reducing bit rates compared to analog transmission. Multiple channels enable of 24 voice/data streams in a single T1 facility, optimizing bandwidth utilization and scalability for business applications. Global adoption accelerated in the 1980s with ISDN and deployments, followed by DSL, which peaked at over 330 million lines worldwide around 2010, though declining with fiber and cable alternatives to approximately 200 million by the early 2020s. As of 2023, DSL represents about 15-20% of global fixed subscriptions.

Installation and Infrastructure

Local Loop and Connection Methods

The refers to the physical connection, often termed the "last mile," that links a customer's to the nearest telephone central office, carrying voice signals over twisted-pair copper wires or other media. In the United States, the mandated the unbundling of the local loop, requiring incumbent local exchange carriers to provide nondiscriminatory access to this segment for competitive carriers, fostering market entry and infrastructure sharing. This unbundling applies to elements such as loops, enabling competitors to lease and utilize the existing last-mile infrastructure without constructing duplicate facilities. Telephone lines connect to customer premises through various methods, depending on terrain, urban planning, and infrastructure availability. Aerial drops typically involve suspending service wires from utility poles to the building using messengers or lashed cables, a cost-effective approach in rural or suburban areas where poles are prevalent. Buried connections employ direct-buried cables or conduits placed underground to protect against weather and vandalism, commonly used in urban settings or new developments to minimize visual impact. In hybrid modern deployments, fiber-to-the-premises (FTTP) or fiber-to-the-node (FTTN) extends closer to the end-user, transitioning to for the final segment, enhancing capacity for voice and data services. Inside the premises, wiring extends from the network interface device (NID)—a protective demarcation box installed by the carrier—to internal telephones via punch-down blocks, such as 66-type or 110-type, which facilitate organized terminations and extensions using twisted-pair cables. For residential setups, a single-line configuration uses one twisted-pair for basic voice service, supporting one telephone number and extensions across the home. Multi-line setups incorporate additional pairs to enable multiple independent lines, often for , , or secondary numbers, with wiring distributed through a central junction or directly to outlets. Extension wiring within the home, typically using 22- or 24-gauge wire, can span typical home distances from the NID without significant signal loss for voice service, though the total loop length from the central office limits overall performance. In commercial environments, telephone lines integrate with private branch exchange (PBX) systems via trunk lines, which are high-capacity bundles connecting the central office to the PBX for routing internal and external calls. The occurs at the minimum point of entry (MPOE), defined as the closest practicable location where carrier wiring crosses the property line or enters a multiunit building, beyond which the customer assumes responsibility for inside wiring and equipment. This setup allows PBX systems to manage multiple extensions efficiently, with trunks provisioned as flat-rate, measured, or rotary to suit business needs.

Maintenance and Network Integration

Routine maintenance of telephone lines involves periodic testing to ensure and operational reliability. Technicians use butt sets, portable devices that connect directly to the line via alligator clips or modular jacks, to perform tests for continuity, voltage levels, and noise interference on copper-based (POTS) lines. These tests verify that the line can carry a clear and voice signal without excessive or , typically measuring loop resistance under 1700 ohms for standard residential loops. For fiber optic telephone lines, optical time-domain reflectometers (OTDRs) are employed to detect breaks, bends, or by sending pulses and analyzing reflections, achieving resolution down to meters for fault location. Fault isolation often occurs at cross-connect boxes, where technicians divide the line into segments using tone generators and probes to pinpoint issues like open circuits or shorts. Common issues in telephone lines include water ingress, which compromises insulation and leads to signal degradation; corrosion on connectors or conductors due to environmental exposure; and overloads from excessive electrical surges or multiple device connections. ingress is particularly problematic in buried or aerial cables, where it can cause grounding faults and increased , often addressed by sealing splices with moisture-resistant compounds during repairs. manifests as increased resistance or intermittent connectivity, while overloads may trigger protective fuses or degrade line quality. Repairs typically involve splicing damaged sections using gel-filled connectors or to restore continuity and prevent further moisture entry, or full cable replacement for severe or multiple faults. Telephone lines integrate into larger networks by connecting to the (PSTN) through central office switches, which route calls based on dialed numbers and establish end-to-end circuits. In analog systems, (FDM) combines multiple voice channels by assigning distinct frequency bands within a shared medium, such as cables, allowing up to 12 channels per group in early PSTN trunks. For digital lines, (TDM) interleaves signals in fixed time slots, as in the ITU E-1 standard providing 30 channels at 64 kbps each over a 2.048 Mbps link, enabling efficient integration with switches for voice and data traffic. Key tools and standards for maintenance include tests, which verify circuit integrity by looping the signal back to the source for self-diagnosis, often integrated into SS7 protocols for automated continuity checks in modern networks. Signaling System No. 7 (SS7) facilitates network integration by providing control for call setup, , and across PSTN elements, using methods like transponders to confirm line availability without disrupting service. These standards ensure reliable fault detection and seamless in hybrid analog-digital environments.

Regulations and Standards

United States Regulations

The (FCC), established by the , holds primary oversight authority over interstate telephone communications, including regulation of telephone lines to ensure reliable service and promote competition. This Act empowered the FCC to regulate wireline communications for the public interest, fostering nationwide access to telephone service. A landmark development in FCC policy came with the 1968 Carterfone decision, which prohibited telephone companies from restricting customer attachments to their networks, thereby allowing non-Bell System devices like acoustic couplers and modems to connect directly to telephone lines. This ruling dismantled monopolistic control over , spurring innovation in attachments and paving the way for broader market access. The significantly reshaped telephone line regulations by introducing the Universal Service Fund (USF), a mechanism to subsidize access to telephone service in rural and underserved areas through contributions from telecommunications providers. The USF supports deployment and maintenance of lines in high-cost regions, ensuring affordable basic service for all Americans. Additionally, the Act mandated unbundling of (ILEC) networks, including local loops, to enable competitive local exchange carriers (CLECs) to lease these facilities and offer services, fostering local market competition. Safety and compatibility standards for telephone lines are governed by FCC Part 68, which sets requirements for connecting terminal equipment and protective circuitry to the , ensuring and preventing network harm. Complementing this, the (NEC) Article 800, published by the (NFPA), outlines installation methods for communications circuits within buildings, including wiring protections against fire and electrical hazards on the customer side of the . In recent years, the FCC has advanced rules to support the transition from traditional (TDM) analog networks to (IP)-based systems, as outlined in its 2018 Technology Transitions Order, which streamlines approvals for retiring copper-based telephone lines while protecting consumers. These regulations facilitate voluntary network upgrades but include safeguards like notice periods; in late 2025, the FCC proposed to phase out mandatory support for certain legacy analog services, such as TTY-based telecommunications relay services, in favor of IP alternatives in applicable areas, with the proposal under consideration as of November 2025.

International Standards and Practices

The Telecommunication Standardization Sector () plays a central role in establishing global standards for telephone lines, ensuring across national Public Switched Telephone Networks (PSTN). These standards, known as ITU-T Recommendations, cover transmission characteristics, signaling protocols, and performance metrics for analog and digital , with a focus on international connections. The G series addresses transmission systems and media, while the Q series handles switching and signaling, providing guidelines that member states voluntarily adopt to facilitate cross-border communication. For analog telephone lines, Recommendation G.101 outlines the transmission plan for international connections. Recommendation Q.552 specifies characteristics for 2-wire analog interfaces in digital exchanges, including nominal line impedance of ohms (balanced) with input/output impedance tolerances of ±30% to minimize reflections and ensure across circuits. This impedance standard supports bands from 300 to 3400 Hz, with a reference level of -10 dBm0 at 1020 Hz for testing send and receive loss. Additionally, Recommendation Q.552 outlines characteristics for 2-wire analog interfaces in digital exchanges, including input/output impedance tolerances of ±30% around ohms and longitudinal balance requirements exceeding 40 dB to reduce noise coupling. These parameters enable consistent audio quality, with one-way limited to under 150 ms for satisfactory conversation, as per G.114. Signaling and supervision practices are standardized in the Q series to manage call setup, supervision, and disconnection. Recommendation Q.23 defines basic telecommunication access signaling for analog lines, using DC loop supervision where off-hook detection occurs via a line current drop below 20-50 mA, and ringing signals applied as superimposed AC voltage. Internationally, ringing voltage is harmonized around 75-90 Vrms at 16-25 Hz, though exact cadences vary by region while adhering to P.310 for subjective transmission assessments. For international dialing, provides the global numbering plan, assigning country codes and national significant numbers up to 15 digits to route calls seamlessly across networks. In practice, these standards promote uniformity in PSTN infrastructure worldwide, with adaptations for local conditions. For instance, the P series, including P.79 for ratings, ensures perceived speech remains consistent, targeting a rating between 2 and 12 for international calls. Digital variants, such as ISDN basic rate interfaces under I.430, build on analog foundations by specifying 2B1Q line coding for 2-wire loops up to 5.5 km, maintaining compatibility with legacy telephone lines. Adoption is widespread, with over 4,000 Recommendations influencing national regulations, though enforcement remains voluntary to accommodate technological evolution.

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