T1
T1
Existing frequency-division multiplexing carrier systems worked well for connections between distant cities, but required expensive modulators, demodulators 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 later decades, the cost of digital electronics declined to the point that an individual codec per voice channel became commonplace, but by then 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 that runs at the original 1.544 Mbit/s line rate. Originally 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. T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in transmission rates of 6.312 and 44.736 Mbit/s, respectively.
Supposedly, the 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago were conducted underground. To accommodate loading coils, cable vault manholes were physically 2000 meter (6,600 ft) apart, and so the optimum bit rate was chosenempirically — the capacity was increased until the failure rate was unacceptable, then reduced to leave a margin. Companding allowed acceptable audio performance with only seven bits per PCM sample in this original T1/D1 system. The later 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 (Switched 56) there could be long strings of zeroes, so one bit per sample is set to "1" (jam bit 7) leaving 7 bits x 8,000 frames per second for data.
A more detailed understanding of how the rate of 1.544 Mbit/s was derived is as follows. (This explanation glosses over T1 voice communications, and deals mainly with the numbers involved.) Given that the telephone system nominal voiceband (including guardband) is 4,000 Hz, the required digital sampling rate is 8,000 Hz (see Nyquist rate). Since each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs then 8,000 frames per second to maintain those 24 simultaneous voice channels. Because each frame of a T1 is 193 bits in length (24 channels X 8 bits per channel + 1 framing bit = 193 bits), 8,000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s (8,000 X 193 = 1,544,000).
Initially, T1 used Alternate Mark Inversion (AMI) to reduce frequency bandwidth and eliminate the DC component of the signal. Later B8ZSbecame 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 Mbaud 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 merely replaced with European standard ones. American T-carriers could only work in AMI or B8ZS mode.
The AMI or B8ZS signal allowed a simple error rate measurement. The D bank in the central office could detect a bit with the wrong polarity, or "bipolarity violation" and sound an alarm. Later systems could count the number of violations and reframes and otherwise measure signal quality and allow a more sophisticated alarm indication signal system.
[edit]Historical note on the 193-bit T1 frame
The decision to use a 193-bit frame was made in 1958. To allow for the identification of information bits within a frame, two alternatives were considered. Assign (a) just one extra bit, or (b) additional 8 bits per frame. The 8-bit choice is cleaner, resulting in a 200-bit frame, 25 8-bitchannels, of which 24 are traffic and 1 8-bit channel available for operations, administration, and maintenance (OA&M). AT&T chose the single bit per frame not to reduce the required bit rate (1.544 vs 1.6 Mbit/s), but because AT&T Marketing worried that "if 8 bits were chosen for OA&M function, someone would then try to sell this as a voice channel and you wind up with nothing."
Soon after commercial success of T1 in 1962, the T1 engineering team realized the mistake of having only one bit to serve the increasing demand for housekeeping functions. They petitioned AT&T management to change to 8-bit framing. This was flatly turned down because it would make installed systems obsolete.
Higher T
In the late 1960s and early 1970s Bell Labs developed higher rate systems. T-1C with a more sophisticated modulation scheme carried 3 Mbit/s, on those balanced pair cables that could support it. T-2 carried 6.312 Mbit/s, requiring a special low-capacitance cable with foam insulation. This was standard for Picturephone. T-4 and T-5 used coaxial cables, similar to the old L-carriers used by AT&T Long Lines. TD microwave radio relaysystems were also fitted with high rate modems to allow them to carry a DS1 signal in a portion of their FM spectrum that had too poor quality for voice service. Later they carried DS3 and DS4 signals. During the 80's companies such as RLH Industries, Inc. developed T1 over optical fiber. The industry soon developed and evolved with multiplexed T1 transmission schemes.
[edit]Digital signal crossconnect
DS1 signals are interconnected typically at Central Office locations at a common metallic cross-connect point known as a DSX-1. A DS1 signal at a DSX-1 is measured typically at 6 Volts Peak-to-peak (0dBdsx signal level at 772 kHz Nyquist) at plus or minus 1.2 volts to permit easy interconnection of DS1 equipment NCI Code=04DS9/ /). When a DS1 is transported over metallic outside plant cable, the signal travels over conditioned cable pairs known as a T1 span. A T1 span can have up to -130 Volts of DC power superimposed on the associated four wire cable pairs to line or "Span" power line repeaters, and T1 NIU's (T1 Smartjacks). T1 span repeaters are typically engineered up to 6,000 feet (1,800 m) apart, depending on cable gauge, and at no more than 36 dB of loss before requiring a repeated span. There can be no cable bridge taps across any pairs.
T1 copper spans are being replaced by optical transport systems, but if a copper (Metallic) span is used, the T1 is typically carried over an HDSLencoded copper line. Four wire HDSL does not require as many repeaters as conventional T1 spans. Newer two wire HDSL (HDSL-2) equipment transports a full 1.544 Mbit/s T1 over a single copper wire pair up to approximately twelve thousand (12,000) feet (3.5 km), if all 24 gauge cable is used. HDSL-2 does not employ repeaters as does conventional four wire HDSL, or newer HDSL-4 systems.
One advantage of HDSL is its ability to operate with a limited number of bridge taps, with no tap being closer than 500 feet (150 m) from any HDSL transceiver. Both two or four wire HDSL equipment transmits and receives over the same cable wire pair, as compared to conventional T1 service that utilizes individual cable pairs for transmit or receive.
DS3 signals are rare except within buildings, where they are used for interconnections and as an intermediate step before being muxed onto aSONET circuit. This is because a T3 circuit can only go about 600 feet (180m) between repeaters. A customer who orders a DS3 usually receives a SONET circuit run into the building and a multiplexer mounted in a utility box. The DS3 is delivered in its familiar form, two coaxcables (1 for send and 1 for receive) with BNC connectors on the ends.
Bit robbing
Twelve DS1 frames make up a single T1 Superframe (T1 SF). Each T1 Superframe is composed of two signaling frames. All T1 DS0 channels that employ in-band signaling will have its eighth bit over written, or "robbed" from the full 64 kbit/s DS0 payload, by either a logical ZERO or ONE bit to signify a circuit signaling state or condition. Hence robbed bit signaling will restrict a DS0 channel to a rate of only 56 kbit/s during two of the twelve DS1 frames that make up a T1 SF framed circuit. T1 SF framed circuits yield two independent signaling channels (A&B) T1 ESF framed circuits four signaling frames in a twenty four frame extended frame format that yield four independent signaling channels (A, B,C,& D).
Main article: Robbed bit signaling
NOTE: 56 kbit/s DS0 channels are associated with digital data service (DDS) services typically do not utilize the eighth bit of the DS0 as voice circuits that employ A&B out of band signaling. One exception is Switched 56 kbit/s DDS. In DDS, bit eight is used to identify DTE request to send (RTS) condition. With Switched 56 DDS, bit eight is pulsed (alternately set to logical ZERO and ONE) to transmit two state dial pulse signaling information between a SW56 DDS CSU/DSU, and a digital end office switch.
The incident use of robbed-bit signaling in North America has decreased significantly as a result of Signaling System Seven (SS7) on inter-office dial trunks. With SS7, the full 64 kbit/s DS0 channel is available for use on a connection, and allows 64 kbit/s, and 128 kbit/s ISDN data calls to exist over a switched trunk network connection if the supporting T1 carrier entity is optioned B8ZS (Clear Channel Capable).
REFERENCES: ANSI T1.403//The Book On ESF, Verilink Corporation, 1986//D4 Digital Channel Bank Family, Bell System Technical Journal, Nov 1982
[edit]Carrier pricing
Carriers price DS1 lines in many different ways. However, most boil down to two simple components; local loop (the cost the local incumbent charges to transport the signal from the end user's central office, otherwise known as a CO, to the point of presence, otherwise known as a POP, of the carrier) and the port (the cost to access the telephone network or the Internet through the carrier's network). Typically, the port price is based upon access speed and yearly commitment level while the loop is based on geography. The farther the CO and POP, the more the loop costs.
The loop price has several components built into it, including the mileage calculation (performed in V/H coordinates, not standard GPS coordinates) and the telco piece. Each local Bell operating company - namely Verizon, AT&T, and Qwest - charge T-carriers different price per mile rates. Therefore, the price calculation has two distance steps: geomapping and the determination of local price arrangements.
While most carriers utilize a geographic pricing model as described above, some Competitive Local Exchange Carriers (CLECs), such as Covad,Integra Telecom, tw telecom, Windstream, Level 3 Communications, and XO Communications offer national pricing. Under this DS1 pricing model, a provider charges the same price in every geography it services. National pricing is an outgrowth of increased competition in the T-carrier market space and the commoditization of T-carrier products.[1] Providers that have adopted a national pricing strategy may experience widely varying margins as their suppliers, the Bell operating companies (e.g., Verizon, AT&T and Qwest), maintain geographic pricing models, albeit at wholesale prices.
For voice DS1 lines, the calculation is mostly the same, except that the port (required for Internet access) is replaced by LDU (otherwise known as Long Distance Usage). Once the price of the loop is determined, only voice-related charges are added to the total. In short, the total price = loop + LDU x minutes used.
On March 29, 2011 the United States Patent Office issued patent no. 7,916,844 to Telarus for software that calculates real-time T-carrier prices.[2]
Notes
Note 1: The designators for T-carrier in the North American Digital multiplex hierarchy correspond to the designators for the digital signal (DS) level hierarchy.
Note 2: T-carrier systems were originally designed to transmit digitized voice signals. Current applications also include digital data transmission.
Note 3: Historically, if an "F" precedes the "T", optical fiber cables are utilized at the same rates.
Note 4: The North American and Japanese hierarchies are based on multiplexing 24 voice-frequency channels and multiples thereof, whereas the European hierarchy is based on multiplexing 32 voice-frequency channels and multiples thereof. See table below,.
Note 5: Will be directed to this table by certain Network+ books. See table below,.
| T-carrier and E-Carrier Systems | North American | Japanese | European (CEPT) |
|---|---|---|---|
| Level zero (Channel data rate) | 64 kbit/s (DS0) | 64 kbit/s | 64 kbit/s |
| First level | 1.544 Mbit/s (DS1) (24 userchannels) (T1) | 1.544 Mbit/s (24 user channels) | 2.048 Mbit/s (32 userchannels) (E1) |
| (Intermediate level, US. hierarchy only) | 3.152 Mbit/s (DS1C) (48 Ch.) | – | – |
| Second level | 6.312 Mbit/s (DS2) (96 Ch.) (T2) | 6.312 Mbit/s (96 Ch.), or 7.786 Mbit/s (120 Ch.) | 8.448 Mbit/s (128 Ch.) (E2) |
| Third level | 44.736 Mbit/s (DS3) (672 Ch.) (T3) | 32.064 Mbit/s (480 Ch.) | 34.368 Mbit/s (512 Ch.) (E3) |
| Fourth level | 274.176 Mbit/s (DS4) (4032 Ch.) | 97.728 Mbit/s (1440 Ch.) | 139.264 Mbit/s (2048 Ch.) (E4) |
| Fifth level | 400.352 Mbit/s (DS5) (5760 Ch.) | 565.148 Mbit/s (8192 Ch.) | 565.148 Mbit/s (8192 Ch.) (E5) |
Note 1: The DS designations are used in connection with the North American hierarchy only. Strictly speaking, a DS1 is the data carried on a T1 circuit, and likewise for a DS3 and a T3, but in practice the terms are used interchangeably.
Note 2: There are other data rates in use, e.g., military systems that operate at six and eight times the DS1 rate. At least one manufacturer has a commercial system[citation needed] that operates at 90 Mbit/s, twice the DS3 rate. New systems, which take advantage of the high data rates offered by optical communications links, are also deployed or are under development. Higher data rates are now often achieved by usingsynchronous optical networking (SONET) or synchronous digital hierarchy (SDH).
Note 3: A DS3 is delivered native on a copper trunk. DS3 may be converted to an optical fiber run when needing longer distances between termination points. When a DS3 is delivered over fiber it is still an analog type trunk connection at the termination points. When delivering data over an OC3 or greater SONET is used. A DS3 transported over SONET is encapsulated in a STS-1 SONET channel. An OC-3 SONET link contains 3 STS-1s, and therefore may carry 3 DS3s. Likewise, OC-12, OC-48, and OC-192 may carry 12, 48, and 192 DS3s respectively.[citation needed]
Komentar
Posting Komentar