E&T-Carrier

The T-Carrier
The North American DS1 consists of 24 DS0 channels that are multiplexed. The signal is referred to as DS1, whereas the transmission channel over the copper-based facility is called a T1 circuit. The T-carrier is used in the United States, Canada, Korea, Hong Kong, and Taiwan.

TDM circuits typically use multiplexers, such as channel service units/digital service units (CSUs/DSUs) or channel banks at the CPE (customer premises equipment) side, and they use larger programmable multiplexers, such as DACS and channel banks, at the carrier end. The T-carrier system is entirely digital, using PCM and TDM. The system uses four wires and provides duplex capability. The four-wire facility was originally a pair of twisted-pair copper wires, but can now also include coaxial cable, optical fiber, digital microwave, and other media. A number of variations on the number and use of channels is possible. The T-carrier hierarchy used in North America is shown in Table 2-1 and illustrated in Figure 2-9. The DS1C, DS2, and DS4 levels are not commercially used. The SONET Synchronous Transport Signal (STS) levels have largely replaced the DS levels above DS3.


Figure 2-9. T-Carrier Multiplexed Hierarchy

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Table 2-1. T-Carrier Hierarchy Digital Signal Level
Number of 64 kbps Channels
Equivalent
Bandwidth

DS0
1
1 * DS0
64 kbps

DS1
24
24 * DS0
1.544 Mbps

DS1C
48
2 * DS1
3.152 Mbps

DS2
96
4 * DS1
6.312 Mbps

DS3
672
28 * DS1
44.736 Mbps

DS4
4032
6 * DS3
274.176 Mbps





NOTE

Some TDM systems use 8 kbps for in-band signaling. This results in a net bandwidth of only 56 kbps per channel. Japan uses the North American standards for DS0 through DS2, but the Japanese DS5 has roughly the circuit capacity of a U.S. DS4.



DS Framing
The DS1 frame of Figure 2-10 is composed of 24 DS0 (8-bit) channels, plus 1 framing bit, which adds up to 193 bits. The DS1 signal transports 8000 frames per second, which results in 193 * 8000 bits per second or 1,544,000 bps (1.544 Mbps). The first bit (bit 1), or F bit, is used for frame alignment, performance-monitoring cyclic redundancy check (CRC), and data linkage. The remaining 192 bits provide 24 8-bit time slots numbered from 1 to 24.


Figure 2-10. DS Frame





DS systems use alternate mark inversion (AMI) or binary 8 zero substitution (B8ZS) for line encoding. In AMI, every other 1 is a different polarity, and the encoding mechanism does not maintain a "1s density." In B8ZS, the encoding mechanism uses intentional bipolar violation to maintain a "1s density." Bipolar violations are two "1s" of the same polarity. T1 physical delivery is over two-pair copper wires—one pair for RX (1+2) and one pair for TX (4+5). For the CPE, RX means data from the network, whereas TX means data to the network.

DS Multiframing Formats
Two kinds of multiframing techniques are used for DS-level transmissions:

D4 or superframe (SF)

D5 or extended superframe (ESF)

D4 multiframing typically uses AMI encoding, whereas ESF uses B8ZS encoding. However, B8ZS line coding could be used with D4 framing as well as ESF. The multiplexer (mux) terminating the T1 usually determines the multiframing option.

D4 Superframe
In the original D4 (SF) standard, the framing bits continuously repeated the sequence 110111001000. In voice telephony, errors are acceptable, and early standards allow as much as one frame in six to be missing entirely. As shown in Figure 2-11, the SF (D4) frame has 12 frames and uses the least significant bit (LSB) in frames 6 and 12 for signaling (A, B bits). This method of in-band signaling is called robbed-bit signaling. Each frame has 24 channels of 64 kbps. Within an SF, F bits delineate the basic frames within the multiframe. In channel-associated signaling, bits are robbed from time slots to carry signaling messages. Figure 2-11 shows the D4 SF format.


Figure 2-11. D4 SF Format





D5 Extended Superframe
To promote error-free transmission, an alternative called the D5 or extended superframe (ESF) of 24 frames was developed. As shown in Figure 2-12, the ESF frame has 24 frames and uses the LSB in frames 6, 12, 18, and 24 for signaling (A, B, C, D bits). Each frame has 24 channels of 64 kbps. In this standard, 6 of the 24 framing bits provide a 6-bit cyclic redundancy check (CRC-6), and 6 provide the actual framing. The other 12 form a VC of 4096 bps for use by the transmission equipment, for call progress signals such as busy, idle, and ringing. DS1 signals using ESF equipment are nearly error free, because the CRC detects errors and allows automatic rerouting of connections. Within an ESF, the F bits provide basic frame and multiframe delineation, performance monitoring through CRC-6-based error detection, a 4-kbps data link to transfer priority operations messages, and other maintenance or operations messages. The F bits also provide periodic terminal performance reports, or an idle sequence. In CAS, bits are robbed from time slots to carry signaling messages. Figure 2-12 shows the ESF format.


Figure 2-12. ESF Format

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SF and ESF Alarms
It is important to understand D4 and ESF alarm conditions, in order to interpret the behavior of a TDM transmission system on the CPE as well as on the network side. The alarms listed here are commonly used with CPE equipment, such as CSUs/DSUs, T1 repeaters, DACS devices, and multiplexers.

AIS (alarm indication signal)— The AIS is also known as a "Keep Alive" or "Blue Alarm" signal. This consists of an unframed, all-1s signal sent to maintain transmission continuity. The AIS carrier failure alarm (CFA) signal is declared when both the AIS state and red CFA persist simultaneously.

OOF (out-of-frame)— The OOF condition occurs whenever network or DTE equipment senses errors in the incoming framing pattern. Depending upon the equipment, this can occur when 2 of 4, 2 of 5, or 3 of 5 framing bits are in error. A reframe clears the OOF condition.

Red CFA (carrier failure alarm)— This CFA occurs after detection of a continuous OOF condition for 2.5 seconds. This alarm state is cleared when no OOF conditions occur for at least 1000 milliseconds. Some applications (certain DACS services) might not clear the CFA state for up to 15 seconds of no OOF occurrences.

Yellow CFA (carrier failure alarm)— When a device enters the red CFA state, it transmits a "yellow alarm" in the opposite direction. A yellow alarm is transmitted by setting bit 2 of each time slot to a 0 (zero) space state for D4-framed facilities. For ESF facilities, a yellow alarm is transmitted by sending a repetitive 16-bit pattern consisting of 8 marks (1) followed by 8 spaces (0) in the data-link bits. This is transmitted for a minimum of 1 second.

LOS (loss of signal)— A LOS condition is declared when no pulses have been detected in a 175 +/– 75 pulse window (100 to 250 bit times).


The E-Carrier
The basic unit of the E-carrier system is the 64-kbps DS0, which is multiplexed to form transmis-sion formats with higher speeds. The E1 consists of 32 DS0 channels. The E-carrier is a European digital transmission format devised by the International Telecommunication Union Telecommu-nication Standardization Sector (ITU-T) and given the name by the Conference of European Postal and Telecommunication Administration (CEPT). E2 through E5 are carriers in increasing multiples of the E1 format. The E1 signal format carries data at a rate of 2.048 Mbps and can carry 32 channels of 64 kbps each. Unlike T1, it does not bit-rob and all 8 bits per channel are used to code the signal. E1 and T1 can be interconnected for international use. The E-carrier hierarchy used in EMEA, Latin America, South Asia, and the Asia Pacific region is shown in Table 2-2 and illustrated in Figure 2-13. The E2, E4, and E5 levels are not commercially used. The Synchronous Digital Hierarchy (SDH) levels have largely replaced the DS levels above E4.


Figure 2-13. E-Carrier Multiplexed Hierarchy

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Table 2-2. E-Carrier Hierarchy Digital Signal Level
Number of 64 kbps Channels
Equivalent
Bandwidth

E1
32
32 * DS0
2.048 Mbps

E2
128
4 * E1
8.448 Mbps

E3
512
4 * E2
34.368 Mbps

E4
2048
4 * E3
139.264 Mbps

E5
8192
4 * E4
565.148 Mbps





As depicted in Figure 2-14, a 2.048-Mbps basic frame is comprised of 256 bits numbered from 1 to 256. These bits provide 32 8-bit time slots numbered from 0 to 31. The first time slot is a framing time slot used for frame alignment, performance monitoring (CRC), and data linkage. Time slot 0 carries framing information in a frame alignment signal as well as remote alarm notification, 5 national bits, and optional CRC bits. Time slot 16 is a signaling time slot and carries signaling information out of band. However, time slot 16 could carry data as well.


Figure 2-14. E1 Frame Structure





Like all basic frames used in telecommunications, the E1 basic frame lasts 125 microseconds. The full E1 bit rate is 2.048 Mbps. We calculate this bit rate by multiplying the 32-octet E1 frame by 8000 frames per second. Subtracting time slots 0 and 16, we see that E1 lines offer 30 time slots to carry user data or a payload-carrying capacity of 1.920 Mbps.

E1 uses AMI or high-density bipolar 3 (HDB3) for line encoding. In AMI, every other 1 is a different polarity, and the encoding mechanism does not maintain a "1s density." AMI is used to represent successive 1s' values in a bit stream with alternating positive and negative pulses to eliminate any direct current (DC) offset.

NOTE

AMI is not used in most 2.048-Mbps transmission systems because synchronization loss can occur during long strings of 0s, because there are no pulses.



In HDB3, every other 1 is a different polarity and the encoding mechanism uses a bipolar violation to maintain a "1s density." The HDB3 coded signal does not have a DC component. Therefore, the signal can be transmitted through balanced transformer-coupled circuits. The clock recovery circuits of the receivers can operate well, even though the data contains long strings of 0s.

Unbalanced E1 physical delivery is over two-pair copper wires with 120-ohm line impedance—one pair for RX (1+2) and one pair for TX (4+5). For the CPE, RX means data from the network, whereas TX means data to the network. Balanced E1 physical delivery is over a pair of 75-ohm coaxial cables. One coax is used for TX, whereas the other one is for RX.

E1 Frame Alignment Signal (FAS)
Framing is necessary so that any equipment receiving the E1 signal can synchronize, identify, and extract the individual channels. The 2.048-Mbps E1 frame consists of 32 individual time slots (numbered 0 through 31). Each time slot consists of individual 64-kbps channels of data. Time slot 0 of every even frame is reserved for the FAS. As shown in Figure 2-15, odd frames have the NFAS word that contains the distant alarm indication bit and other bits reserved for national and International use. Thirty-one time slots remain for bearer channels, into which customer data can be placed.


Figure 2-15. E1 Frame Alignment Signal

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E1 MultiFrame Alignment Signal (MFAS)
Sixteen E1 consecutive frames form a new structure called an E1 multiframe. The frames in a multiframe are numbered 0 to 15. Multiframe structure is used for two purposes: CAS signaling and CRC. Each of these modes is independent from the use of the other. CAS is carried in time slot 16, and CRC is carried in time slot 0. The purpose of the multiframe is to have sufficient overhead bits to support two key functions in time slot 16, which carries signaling information when an E1 is transmitting digital voice streams. MFAS framing is used for CAS to transmit ABCD bit information for each of the 30 channels, as illustrated in Figure 2-16.


Figure 2-16. E1 Multiframe Alignment Signal

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This method uses the 32 time slot frame format with time slot 0 for the FAS and time slot 16 for the MFAS and CAS. When a PCM-30 multiframe is transmitted, 16 FAS frames are assembled together. Time slot 16 of the first frame is dedicated to MFAS bits, and time slot 16 of the remaining 15 frames is dedicated to ABCD bits.

E1 CRC Error Checking
A cyclic redundancy check-4 (CRC-4) is often used in E1 transmission to identify possible bit errors during in-service error monitoring. CRC-4 is a checksum calculation that allows for the detection of errors within the 2.048-Mbps signal while it is in service. A discrepancy indicates at least one bit error in the received signal. The equipment that originates the E1 data calculates the CRC-4 bits for one submultiframe. It inserts the CRC-4 bits in the CRC-4 positions in the next submultiframe.

The receiving equipment performs the reverse mathematical computation on the submultiframe. It examines the CRC-4 bits that were transmitted in the next submultiframe. It then compares the transmitted CRC-4 bits to the calculated value. If there is a discrepancy in the two values, a CRC-4 error is reported via E-bits indication. Each individual CRC-4 error does not necessarily correspond to a single bit error, which is a drawback. Multiple bit errors within the same submultiframe will lead to only one CRC-4 error for the block. Thirty-one time slots remain for bearer channels, into which customer data can be placed.

Errors could occur such that the new CRC-4 bits are calculated to be the same as the original CRC-4 bits. CRC-4 error checking provides a most convenient method of identifying bit errors within an in-service system, but only provides an approximate measure (93.75 percent accuracy) of the circuit's true performance. Consider the MFAS framing shown in Figure 2-17. Each MFAS frame can be divided into "submultiframes." These are labeled SMF1 and SMF2, and consist of eight frames apiece. We associate 4 bits of CRC information with each submultiframe. The CRC-4 bits are calculated for each submultiframe, buffered, and then inserted into the following submultiframe to be transmitted.


Figure 2-17. E1 CRC Error Checking

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ITU-T specifications G.704 and G.706 define the CRC-4 cyclic redundancy check for enhanced error monitoring on the E1 line.

E1 Errors and Alarms
It is important to understand E1 error and alarm conditions, in order to interpret the behavior of a TDM transmission system on the CPE as well as on the network side. The alarms listed here are commonly used with CPE equipment such as CSUs/DSUs, E1 repeaters, DCS devices, and multiplexers:

Alarm indication signal (AIS)— Alarm indication signal is an unframed, all-1s signal.

Background block error (BBE)— A background block error is an error block (a block is a set of consecutive bits associated with a path) that does not occur as part of a severely errored second (SES).

Bit errors— Bit errors are bits that are in error. Bit errors are not counted during unavailable time.

Bit slip— A bit slip occurs when the synchronized pattern either loses a bit or has an extra bit stuffed into it.

Clock slips— Clock slips occur when the measured frequency deviates from the reference frequency by a one-unit interval.

Code errors— A code error is a violation of the coding rules: two successive pulses with the same polarity. In HDB3 coding, a code error is a bipolar violation that is not part of a valid HDB3 substitution.

Cyclic redundancy check (CRC) errors— CRC-4 block errors. This measurement applies to signals containing a CRC-4 check sequence.

Degraded minutes— A degraded minute (DM) occurs when there is a 10 to 6 or worse bit error rate during 60 available, nonseverely bit-errored seconds.

Errored block— A block in which one or more bits are in error.

E-bit indication— An E-bit is transmitted by the receiving equipment after detecting a CRC-4 error.

Errored second (ES)— An errored second is any second in which one or more bits are in error. An errored second is not counted during an unavailable second. For G.826, an errored second contains one or more blocks with at least one defect.

Frame alarm (FALM)— Frame alarm seconds is a count of seconds that have had far-end frame alarm (FAS remote alarm indication [RAI]), which is when a 1 is transmitted in every third bit of each time slot 0 frame that does not contain the FAS.

Frame alignment signal (FAS)— A count of the bit errors in the frame alignment signal words received. It applies to both PCM-30 and PCM-31 framing.

Frequency— Any variance from 2.048 Mbps in the received frequency is recorded in hertz or parts per million.

Loss of frame seconds (LOFS)— Loss of frame seconds is a count of seconds since the beginning of the test that have experienced a loss of frame.

Loss of signal seconds (LOSS)— Loss of signal seconds is a count of the number of seconds during which the signal has been lost during the test.

Multiframe alarm (MFAL)— Multiframe alarm seconds is a count of seconds that have had far-end multiframe alarm (MFAS RAI).

Multiframe alignment signal (MFAS) distant alarm— In this alarm, a 1 is transmitted in every sixth bit of each time slot 16 in the 0 frame.

Severely errored second (SES)— A severely errored second has an error rate of 10-3 or higher. Severely errored seconds are not counted during unavailable time. For G.826 block measurements, an SES is a 1-second period containing 30 percent or greater errored blocks.

Time slot 16 AIS— In this alarm, all 1s are transmitted in time slot 16 of all frames.

Unavailable seconds (UAS)— Unavailable time begins at the onset of 10 consecutive severely errored seconds. Unavailable seconds also begin at a loss of signal or loss of frame.

Wander— This is the total positive or negative phase difference between the measured frequency and the reference frequency. The +wander value increases whenever the measured frequency is one unnumbered information (UI) frame larger than the reference frequency. The –wander increases whenever the measured frequency is one UI frame less than the reference frequency.

NOTE

The following ITU-T recommendations are commonly used with TDM systems: G.703, physical/electrical characteristics of hierarchical digital interfaces; G.704, synchronous frame structures used at 1544, 6312, 2048, 8488, and 44,736 kbps; G.706, frame alignment and CRC procedures relating to basic frame structures defined in Recommendation G.704; G.711, PCM of voice frequencies.