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. The first version, the Transmission System 1, was introduced in 1962 in the Bell System, could transmit up to 24 telephone calls over a single transmission line of copper wire. Subsequent specifications carried multiples of the basic T1 data rates, such as T2 with 96 channels, T3 with 672 channels, others; the T-carrier is a hardware specification for carrying multiple time-division multiplexed telecommunications channels over a single four-wire transmission circuit. It was developed by AT&T at Bell Laboratories ca. 1957 and first employed by 1962 for long-haul pulse-code modulation digital voice transmission with the D1 channel bank. The T-carriers are used for trunking between switching centers in a telephone network, including to private branch exchange interconnect points, it uses the same twisted pair copper wire that analog trunks used, employing one pair for transmitting, another pair for receiving.
Signal repeaters may be used for extended distance requirements. Before the digital T-carrier system, carrier wave systems such as 12-channel carrier systems worked by frequency division multiplexing. A T1 trunk could transmit 24 telephone calls at a time, because it used a digital carrier signal called Digital Signal 1. DS-1 is a communications protocol for multiplexing the bitstreams of up to 24 telephone calls, along with two special bits: a framing bit and a maintenance-signaling bit. T1's maximum data transmission rate is 1.544 megabits per second. Europe and most of the rest of the world, except Japan, have standardized the E-carrier system, a similar transmission system with higher capacity, not directly compatible with the T-carrier. Existing frequency-division multiplexing carrier systems worked well for connections between distant cities, but required expensive modulators and filters for every voice channel. For connections within metropolitan areas, Bell Labs in the late 1950s sought cheaper terminal equipment.
Pulse-code modulation allowed sharing a coder and decoder among several voice trunks, so this method was chosen for the T1 system introduced into local use in 1961. In decades, the cost of digital electronics declined to the point that an individual codec per voice channel became commonplace, but by the other advantages of digital transmission had become entrenched; the most common legacy of this system is the line rate speeds. "T1" now means any data circuit line rate. The T1 format carried 24 pulse-code modulated, time-division multiplexed speech signals each encoded in 64 kbit/s streams, leaving 8 kbit/s of framing information which facilitates the synchronization and demultiplexing at the receiver; the T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in transmission rates of 6.312 and 44.736 Mbit/s, respectively. A T3 line comprises each operating at total signaling rate of 1.544 Mbit/s. It is possible to get a fractional T3 line, meaning a T3 line with some of the 28 lines turned off, resulting in a slower transfer rate but at reduced cost.
The 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago were conducted underground. The test site was typical of Bell System outside plant of the time in that, to accommodate loading coils, cable vault manholes were physically 2,000 meters apart, which determined the repeater spacing; the optimum bit rate was chosen empirically—the capacity was increased until the failure rate was unacceptable reduced to leave a margin. Companding allowed acceptable audio performance with only seven bits per PCM sample in this original T1/D1 system; the D3 and D4 channel banks had an extended frame format, allowing eight bits per sample, reduced to seven every sixth sample or frame when one bit was "robbed" for signaling the state of the channel. The standard does not allow an all zero sample which would produce a long string of binary zeros and cause the repeaters to lose bit sync. However, when carrying data there could be long strings of zeros, so one bit per sample is set to "1" leaving 7 bits × 8,000 frames per second for data.
A more detailed understanding of how the rate of 1.544 Mbit/s was divided into channels is as follows. Given that the telephone system nominal voiceband is 4,000 Hz, the required digital sampling rate is 8,000 Hz. Since each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs 8,000 frames per second to maintain those 24 simultaneous voice channels; because each frame of a T1 is 193 bits in length, 8,000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s. T1 used Alternate Mark Inversion to reduce frequency bandwidth and eliminate the DC component of the signal. B8ZS became common practice. For AMI, each mark pulse had the opposite polarity of the previous one and each space was at a level of zero, resulting in a three level signal which however only carried binary data. Similar British 23 channel systems at 1.536 megabaud in the 1970s were equipped with ternary signal repeaters, in anticipation of using a 3B2T or 4B3T code to increase the number of voice channels in future, but in the 1980s the systems were replaced with European standard ones.
In telecommunication, frame synchronization or framing is the process by which, while receiving a stream of framed data, incoming frame alignment signals are identified, permitting the data bits within the frame to be extracted for decoding or retransmission. If the transmission is temporarily interrupted, or a bit slip event occurs, the receiver must re-synchronize; the transmitter and the receiver must agree ahead of time on which frame synchronization scheme they will use. Common frame synchronization schemes are: Framing bit A common practice in telecommunications, for example in T-carrier, is to insert, in a dedicated time slot within the frame, a noninformation bit or framing bit, used for synchronization of the incoming data with the receiver. In a bit stream, framing bits indicate the end of a frame, they occur at specified positions in the frame, do not carry information, are repetitive. Syncword framing Some systems use a special syncword at the beginning of every frame. CRC-based framing Some telecommunications hardware uses CRC-based framing.
In telemetry applications, a frame synchronizer is used to frame-align a serial pulse code-modulated binary stream. The frame synchronizer follows the bit synchronizer in most telemetry applications. Without frame synchronization, decommutation is impossible; the frame synchronization pattern is a known binary pattern which repeats at a regular interval within the PCM stream. The frame synchronizer aligns the data into minor frames or sub-frames; the frame sync pattern is followed by a counter which dictates which minor or sub-frame in the series is being transmitted. This becomes important in the decommutation stage where all data is deciphered as to what attribute was sampled. Different commutations require a constant awareness of which section of the major frame is being decoded. Asynchronous start-stop Phase synchronization Self-synchronizing code Superframe This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C". J. L. Massey.
"Optimum frame synchronization ". IEEE trans. Comm. com-20:115-119, April 1972. R Scholtz. "Frame synchronization techniques", IEEE Transactions on Communications, 1980. P. Robertson. "Optimal Frame Synchronization for Continuous and Packet Data Transmission", PhD Dissertation, 1995, Fortschrittberichte VDI Reihe 10, Nr. 376 PDF
General Services Administration
The General Services Administration, an independent agency of the United States government, was established in 1949 to help manage and support the basic functioning of federal agencies. GSA supplies products and communications for U. S. government offices, provides transportation and office space to federal employees, develops government-wide cost-minimizing policies and other management tasks. GSA employs about 12,000 federal workers and has an annual operating budget of $20.9 billion. GSA oversees $66 billion of procurement annually, it contributes to the management of about $500 billion in U. S. federal property, divided chiefly among 8,700 owned and leased buildings and a 215,000 vehicle motor pool. Among the real estate assets managed by GSA are the Ronald Reagan Building and International Trade Center in Washington, D. C. – the largest U. S. federal building after the Pentagon – and the Hart-Dole-Inouye Federal Center. GSA's business lines include the Federal Acquisition Service and the Public Buildings Service, as well as several Staff Offices including the Office of Government-wide Policy, the Office of Small Business Utilization, the Office of Mission Assurance.
As part of FAS, GSA's Technology Transformation Services helps federal agencies improve delivery of information and services to the public. Key initiatives include FedRAMP, Cloud.gov, the USAGov platform, Data.gov, Performance.gov, Challenge.gov. GSA is a member of the Procurement G6, an informal group leading the use of framework agreements and e-procurement instruments in public procurement. In 1947 President Harry Truman asked former President Herbert Hoover to lead what became known as the Hoover Commission to make recommendations to reorganize the operations of the federal government. One of the recommendations of the commission was the establishment of an "Office of the General Services." This proposed office would combine the responsibilities of the following organizations: U. S. Treasury Department's Bureau of Federal Supply U. S. Treasury Department's Office of Contract Settlement National Archives Establishment All functions of the Federal Works Agency, including the Public Buildings Administration and the Public Roads Administration War Assets AdministrationGSA became an independent agency on July 1, 1949, after the passage of the Federal Property and Administrative Services Act.
General Jess Larson, Administrator of the War Assets Administration, was named GSA's first Administrator. The first job awaiting Administrator Larson and the newly formed GSA was a complete renovation of the White House; the structure had fallen into such a state of disrepair by 1949 that one inspector of the time said the historic structure was standing "purely from habit." Larson explained the nature of the total renovation in depth by saying, "In order to make the White House structurally sound, it was necessary to dismantle, I mean dismantle, everything from the White House except the four walls, which were constructed of stone. Everything, except the four walls without a roof, was stripped down, that's where the work started." GSA worked with President Truman and First Lady Bess Truman to ensure that the new agency's first major project would be a success. GSA completed the renovation in 1952. In 1986 GSA headquarters, U. S. General Services Administration Building, located at Eighteenth and F Streets, NW, was listed on the National Register of Historic Places, at the time serving as Interior Department offices.
In 1960 GSA created the Federal Telecommunications System, a government-wide intercity telephone system. In 1962 the Ad Hoc Committee on Federal Office Space created a new building program to address obsolete office buildings in Washington, D. C. resulting in the construction of many of the offices that now line Independence Avenue. In 1970 the Nixon administration created the Consumer Product Information Coordinating Center, now part of USAGov. In 1974 the Federal Buildings Fund was initiated, allowing GSA to issue rent bills to federal agencies. In 1972 GSA established the Automated Data and Telecommunications Service, which became the Office of Information Resources Management. In 1973 GSA created the Office of Federal Management Policy. GSA's Office of Acquisition Policy centralized procurement policy in 1978. GSA was responsible for emergency preparedness and stockpiling strategic materials to be used in wartime until these functions were transferred to the newly-created Federal Emergency Management Agency in 1979.
In 1984 GSA introduced the federal government to the use of charge cards, known as the GMA SmartPay system. The National Archives and Records Administration was spun off into an independent agency in 1985; the same year, GSA began to provide governmentwide policy oversight and guidance for federal real property management as a result of an Executive Order signed by President Ronald Reagan. In 2003 the Federal Protective Service was moved to the Department of Homeland Security. In 2005 GSA reorganized to merge the Federal Supply Service and Federal Technology Service business lines into the Federal Acquisition Service. On April 3, 2009, President Barack Obama nominated Martha N. Johnson to serve as GSA Administrator. After a nine-month delay, the United States Senate confirmed her nomination on February 4, 2010. On April 2, 2012, Johnson resigned in the wake of a management-deficiency report that detailed improper payments for a 2010 "Western Regions" training conference put on by the Public Buildings Service in Las Vegas.
In July 1991 GSA contractors began the excavation of what is now the Ted Weiss Federal Building in New York City. The planning for that buildin
Caller ID spoofing
Caller ID spoofing is the practice of causing the telephone network to indicate to the receiver of a call that the originator of the call is a station other than the true originating station. For example, a caller ID display might display a phone number different from that of the telephone from which the call was placed; the term is used to describe situations in which the motivation is considered malicious by the originator. Caller ID spoofing has been available for years to people with a specialized digital connection to the telephone company, called an ISDN PRI circuit. Collection agencies, law-enforcement officials, private investigators have used the practice, with varying degrees of legality; the first mainstream caller ID spoofing service, Star38.com, was launched in September 2004. Star38.com was the first service to allow spoofed calls to be placed from a web interface. It stopped offering service in 2005. In August 2006, Paris Hilton was accused of using caller ID spoofing to break into a voicemail system that used caller ID for authentication.
Caller ID spoofing has been used in purchase scams on web sites such as Craigslist and eBay. The scamming caller claims to be calling from Canada into the U. S. with a legitimate interest in purchasing advertised items. The sellers are asked for personal information such as a copy of a registration title, etc. before the purchaser invests the time and effort to come see the for-sale items. In the 2010 election, fake caller IDs of ambulance companies and hospitals were used in Missouri to get potential voters to answer the phone. In 2009, a vindictive Brooklyn wife spoofed the doctor’s office of her husband’s lover in an attempt to trick the other woman into taking medication which would make her miscarry. Caller ID spoofing is used for prank calls. For example, someone might call a friend and arrange for "The White House" to appear on the recipient's caller display. In December 2007, a hacker used a caller ID spoofing service and was arrested for sending a SWAT team to a house of an unsuspecting victim.
In February 2008, a Collegeville, Pennsylvania man was arrested for making threatening phone calls to women and having their home numbers appear "on their caller ID to make it look like the call was coming from inside the house."In March 2008, several residents in Wilmington, Delaware reported receiving telemarketing calls during the early morning hours, when the caller had spoofed the caller ID to evoke the 1982 Tommy Tutone song "867-5309/Jenny." By 2014, an increase in illegal telemarketers displaying the victim's own number, either verbatim or with a few digits randomised, was observed as an attempt to evade caller ID-based blacklists. In the Canadian federal election of May 2, 2011, both live calls and robocalls are alleged to have been placed with false caller ID, either to replace the caller's identity with that of a fictitious person or to disguise calls from an Ohio call centre as Peterborough, Ontario domestic calls. See Robocall scandal. In June 2012, a search on Google returned nearly 50,000 consumer complaints by individuals receiving multiple continuing spoofed voice over IP calls on lines leased / originating from “Pacific Telecom Communications Group” located in Los Angeles, CA, in apparent violation of FCC rules.
Companies such as these lease out thousands of phone numbers to anonymous voice-mail providers who, in combination with dubious companies like “Phone Broadcast Club”, allow phone spam to become an widespread and pervasive problem. In 2013, the misleading caller name "Teachers Phone" was reported on a large quantity of robocalls advertising credit card services as a ruse to trick students' families into answering the unwanted calls in the mistaken belief they were from local schools. On January 7, 2013, the Internet Crime Complaint Center issued a scam alert for various telephony denial of service attacks by which fraudsters were using spoofed caller ID to impersonate police in an attempt to collect bogus payday loans placing repeated harassing calls to police with the victim's number displayed. While impersonation of police is common, other scams involved impersonating utility companies to threaten businesses or householders with disconnection as a means to extort money, impersonating immigration officials or impersonating medical insurers to obtain personal data for use in theft of identity.
Bogus caller ID has been used in grandparent scams which target the elderly by impersonating family members and requesting wire transfer of money. Caller ID is spoofed through a variety of different technology; the most popular ways of spoofing caller ID are through the use of PRI lines. In the past, caller ID spoofing required an advanced knowledge of telephony equipment that could be quite expensive. However, with open source software, one can spoof calls with minimal costs and effort; some VoIP providers allow the user to configure their displayed number as part of the configuration page on the provider's web interface. No additional software is required. If the caller name is sent with the call it may be configured as part of the settings on a client-owned analog telephone adapter or SIP phone; the level of flexibility is provider-dependent. A provider which allows users to bring their own device and unbundles service so that direct inward dial numbers may be purchased separately from outbound calling minutes will be more flexible.
A carrier which doesn't follow established hardware standards or locks subscribers out of configuratio
Business telephone system
A business telephone system is a multiline telephone system used in business environments, encompassing systems ranging from the small key telephone system to the large private branch exchange. A business telephone system differs from an installation of several telephones with multiple central office lines in that the CO lines used are directly controllable in key telephone systems from multiple telephone stations, that such a system provides additional features related to call handling. Business telephone systems are broadly classified into key telephone systems, private branch exchanges, but many hybrid systems exist. A key telephone system was distinguished from a private branch exchange in that it did not require an operator or attendant at the switchboard to establish connections between the central office trunks and stations, or between stations. Technologically, private branch exchanges share lineage with central office telephone systems, in larger or more complex systems, may rival a central office system in capacity and features.
With a key telephone system, a station user could control the connections directly using line buttons, which indicated the status of lines with built-in lamps. Key telephone systems are defined by arrangements with individual line selection buttons for each available telephone line; the earliest systems were known as wiring plans and consisted of telephone sets, keys and wiring. Key was a Bell System term of art for a customer-controlled switching system such as the line-buttons on the phones associated with such systems; the wiring plans evolved into modular hardware building blocks with a variety of functionality and services in the 1A key telephone system developed in the Bell System in the 1930s. Key systems can be built using three principal architectures: electromechanical shared-control, electronic shared-control, or independent key sets. New installations of key telephone systems have become less common, as hybrid systems and private branch exchanges of comparable size have similar cost and greater functionality.
Before the advent of large-scale integrated circuits, key systems were composed of electromechanical components as were larger telephone switching systems. The systems marketed in North America as the 1A, 6A, 1A1 and the 1A2 Key System are typical examples and sold for many decades; the 1A family of Western Electric Company key telephone units were introduced in the late 1930s and remained in use to the 1950s. 1A equipment required at least two KTUs per line. The telephone instrument used by 1A systems was the WECo 300-series telephone. Introduced in 1953, 1A1 key systems simplified wiring with a single KTU for both line and station termination, increased the features available; as the 1A1 systems became commonplace, requirements for intercom features grew. The original intercom KTUs, WECo Model 207, were wired for a single talk link, that is, a single conversation on the intercom at a time; the WECo 6A dial intercom system provided two talk links and was installed as the dial intercom in a 1A1 or 1A2 key system.
The 6A systems were complex and expensive, never became popular. The advent of 1A2 technology in the 1964 simplified key system maintenance; these continued to be used throughout the 1980s, when the arrival of electronic key systems with their easier installation and greater features signaled the end of electromechanical key systems. Two lesser-known key systems were used at airports for air traffic control communications, the 102 and 302 key systems; these were uniquely designed for communications between the air traffic control tower and radar approach control or ground control approach, included radio line connections. Automatic Electric Company produced a family of key telephone equipment, some of it compatible with Western Electric equipment, but it did not gain the widespread use enjoyed by Western Electric equipment. With the advent of LSI ICs, the same architecture could be implemented much less expensively than was possible using relays. In addition, it was possible to eliminate the many-wire cabling and replace it with much simpler cable similar to that used by non-key systems.
Electronic shared-control systems led to the modern hybrid telephone system, as the features of PBX and key system merged. One of the most recognized such systems is the AT&T Merlin. Additionally, these more modern systems allowed a diverse set of features including: Answering machine functions Automatic call accounting Caller ID Remote supervision of the entire system Selection of signaling sounds Speed dialing Station-specific limitations Features could be added or modified using software, allowing easy customization of these systems; the stations were easier to maintain than the previous electromechanical key systems, as they used efficient LEDs instead of incandescent light bulbs for line status indication. LSI allowed smaller systems to distribute the control into individual telephone sets that don't require any single shared control unit; these systems are used with a few telephone sets and it is more difficult to keep the feature set in synchrony between the various sets. Into the 21st century, the distinction between key systems and PBX systems has become blurred.
Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations. The modern key system now supports SIP, ISDN, analog handsets (in addition to it
Telecommunication is the transmission of signs, messages, writings and sounds or information of any nature by wire, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology, it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is used in its plural form because it involves many different technologies. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, optical heliographs. Other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, loud whistles. 20th- and 21st-century technologies for long-distance communication involve electrical and electromagnetic technologies, such as telegraph and teleprinter, radio, microwave transmission, fiber optics, communications satellites.
A revolution in wireless communication began in the first decade of the 20th century with the pioneering developments in radio communications by Guglielmo Marconi, who won the Nobel Prize in Physics in 1909, other notable pioneering inventors and developers in the field of electrical and electronic telecommunications. These included Charles Wheatstone and Samuel Morse, Alexander Graham Bell, Edwin Armstrong and Lee de Forest, as well as Vladimir K. Zworykin, John Logie Baird and Philo Farnsworth; the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, the Latin communicare, meaning to share. Its modern use is adapted from the French, because its written use was recorded in 1904 by the French engineer and novelist Édouard Estaunié. Communication was first used as an English word in the late 14th century, it comes from Old French comunicacion, from Latin communicationem, noun of action from past participle stem of communicare "to share, divide out.
Homing pigeons have been used throughout history by different cultures. Pigeon post had Persian roots, was used by the Romans to aid their military. Frontinus said; the Greeks conveyed the names of the victors at the Olympic Games to various cities using homing pigeons. In the early 19th century, the Dutch government used the system in Sumatra, and in 1849, Paul Julius Reuter started a pigeon service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. In the Middle Ages, chains of beacons were used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London. In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris.
However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres. As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. On 25 July 1837 the first commercial electrical telegraph was demonstrated by English inventor Sir William Fothergill Cooke, English scientist Sir Charles Wheatstone. Both inventors viewed their device as "an improvement to the electromagnetic telegraph" not as a new device. Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837, his code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was completed on 27 July 1866, allowing transatlantic telecommunication for the first time; the conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876. Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849.
However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London. Starting in 1894, Italian inventor Guglielmo Marconi began developing a wireless communication using the newly discovered phenomenon of radio waves, showing by 1901 that they could be transmitted across the Atlantic Ocean; this was the start of wireless telegraphy by radio. Voice and music had little early success. World War I accelerated the development of radio for military communications. After the war, commercial radio AM broadcasting began in the 1920s and became an important mass medium for entertainment and news. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. Development of stereo FM broadcasting of radio
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 standardized the E-carrier system, which revised and improved the earlier American T-carrier technology, this has now been adopted by the International Telecommunication Union Telecommunication Standardization Sector, it was adopted in all countries outside the US, Japan. E-carrier deployments have been replaced by Ethernet as telecommunication networks transitions towards all IP. An E1 link operates over two separate sets of wires unshielded twisted pair or using coaxial. A nominal 3 volt peak signal is encoded with pulses using a method avoiding long periods without polarity changes; the line data rate is 2.048 Mbit/s, split into 32 timeslots, each being allocated 8 bits in turn. Thus each timeslot sends and receives an 8-bit PCM sample encoded according to A-law algorithm, 8,000 times per second.
This is ideal for voice telephone calls where the voice is sampled at that data rate and reconstructed at the other end. The timeslots are numbered from 0 to 31; the E1 frame defines a cyclical set of 32 time slots of 8 bits. The time slot 0 is devoted to transmission time slot 16 for signaling; the main characteristics of the 2-Mbit/s frame are described in the following. One timeslot is reserved for framing purposes, alternately transmits a fixed pattern; this allows the receiver to match up each channel in turn. The standards allow for a full cyclic redundancy check to be performed across all bits transmitted in each frame, to detect if the circuit is losing bits, but this is not always used. An alarm signal may be transmitted using timeslot TS0; some bits are reserved for national use. One timeslot is reserved for signalling purposes, to control call setup and teardown according to one of several standard telecommunications protocols; this includes channel-associated signaling where a set of bits is used to replicate opening and closing the circuit, or using tone signalling, passed through on the voice circuits themselves.
More recent systems use common-channel signaling such Signalling System 7 where no particular timeslot is reserved for signalling purposes, the signalling protocol being transmitted on a chosen set of timeslots or on a different physical channel. In an E1 channel, communication consists of sending consecutive frames from the transmitter to the receiver; the receiver must receive an indication showing when the first interval of each frame begins, so that, since it knows to which channel the information in each time slot corresponds, it can demultiplex correctly. This way, the bytes received in each slot are assigned to the correct channel. A synchronization process is established, it is known as frame alignment. In order to implement the frame alignment system so that the receiver of the frame can tell where it begins, there is so called a frame alignment signal. In the 2 Mbit/s frame system, the FAS is a combination of seven fixed bits transmitted in the first time slot in the frame. For the alignment mechanism to be maintained, the FAS does not need to be transmitted in every frame.
Instead, this signal can be sent in alternate frames. In this case, TS0 is used as the synchronization slot; the TS0 of the rest of the frames is therefore available for other functions, such as the transmission of the alarms. In the TS0 of frames with FAS, the first bit is dedicated to carrying the cyclic redundancy checksum, it tells us whether there are one or more bit errors in a specific group of data received in the previous block of eight frames known as submultiframe. The aim of this system is to avoid loss of synchronization due to the coincidental appearance of the sequence "0011011" in a time slot other than the TS0 of a frame with FAS. To implement the CRC code in the transmission of 2 Mbit/s frames, a CRC-4 multiframe is built, made up of 16 frames; these are grouped in two blocks of eight frames called submultiframes, over which a CRC checksum or word of four bits is put in the positions Ci of the next submultiframe. At the receiving end, the CRC of each submultiframe is calculated locally and compared to the CRC value received in the next submultiframe.
If these do not coincide, one or more bit errors is determined to have been found in the block, an alarm is sent back to the transmitter, indicating that the block received at the far end contains errors. The receiving end has to know, the first bit of the CRC-4 word. For this reason, a CRC-4 multiframe alignment word is needed; the receiver has to be told where the multiframe begins. The CRC-4 multiframe alignment word is the set combination "0011011", introduced in the first bits of the frames that do not contain the FAS signal; the CRC-4 method is used to protect the communication against a wrong frame alignment word, to provide a certain degree of monitoring of the bit error rate, when this has low values. This method is not suitable for cases in which the BER is around 10−3. Another advantage in using the CRC