E-carrier

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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 (CEPT) originally standardized the E-carrier system, which revised and improved the earlier American T-carrier technology, and this has now been adopted by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). It was widely adopted in almost all countries outside the US, Canada, and Japan. E-carrier deployments have steadily been replaced by Ethernet as telecommunication networks transitions towards all IP.

Contents

E1 frame structure

An E1 link operates over two separate sets of wires, usually unshielded twisted pair (balanced cable) or using coaxial (unbalanced cable). 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 (full duplex, i.e. 2.048 Mbit/s downstream and 2.048 Mbit/s upstream) which is split into 32 timeslots, each being allocated 8  bits in turn. Thus each timeslot sends and receives an 8-bit PCM sample, usually encoded according to A-law algorithm, 8,000 times per second (8 × 8,000 × 32 = 2,048,000). 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 management and time slot 16 for signaling; the rest were assigned originally for voice/data transport. [1]

The main characteristics of the 2-Mbit/s frame are described in the following.

E1 frame format ITU-T.png

Special timeslots

One timeslot (TS0) is reserved for framing purposes, and alternately transmits a fixed pattern. This allows the receiver to lock onto the start of each frame and 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 (information), but this is not always used. An alarm signal may also be transmitted using timeslot TS0. Finally, some bits are reserved for national use. [2]

One timeslot (TS16) is often reserved for signalling purposes, to control call setup and teardown according to one of several standard telecommunications protocols. This includes channel-associated signaling (CAS) where a set of bits is used to replicate opening and closing the circuit (as if picking up the telephone receiver and pulsing digits on a rotary phone), or using tone signalling which is passed through on the voice circuits themselves. More recent systems use common-channel signaling (CCS) such Signalling System 7 (SS7) where no particular timeslot is reserved for signalling purposes, the signalling protocol being transmitted on a freely chosen set of timeslots or on a different physical channel. [3]

When using E1 frames for data communication, some systems do use those timeslots slightly different, either a) TS0: Framing, TS1 -TS32: Data traffic This is named Channelized E1, and is used where the framing is required, it allows any of the 32 timeslots to be identified and extracted.

b) TS0 -TS32: Data traffic. Often referred to as ClearChannel E1 or Unchannelized, it is used where no framing is required, timeslot extraction is not required and the full bandwidth (2 Mb/s) is required.

Frame alignment

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 then established, and it is known as frame alignment.

Frame-alignment signal

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 (FAS). In the 2 Mbit/s frame system, the FAS is a combination of seven fixed bits ("0011011") transmitted in the first time slot in the frame (time slot zero or TS0). 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 the first, in the third, in the fifth, and so on). 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.

Multiframe CRC-4

In the TS0 of frames with FAS, the first bit is dedicated to carrying the cyclic redundancy checksum (CRC). 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 CRC-4 procedure

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 then grouped in two blocks of eight frames called submultiframes, over which a CRC checksum or word of four bits (CRC-4) is put in the positions Ci (bits #1, frames with FAS) 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, and an alarm is sent back to the transmitter, indicating that the block received at the far end contains errors.

CRC-4 multiframe alignment

The receiving end has to know which is the first bit of the CRC-4 word (C1). For this reason, a CRC-4 multiframe alignment word is needed. Obviously, the receiver has to be told where the multiframe begins (synchronization). The CRC-4 multiframe alignment word is the set combination "0011011", which is introduced in the first bits of the frames that do not contain the FAS signal. [4]

Advantages of the CRC-4 method

The CRC-4 method is mainly used to protect the communication against a wrong frame alignment word, and also to provide a certain degree of monitoring of the bit error rate (BER), when this has low values (around 10−6). This method is not suitable for cases in which the BER is around 10−3 (where each block contains at least one errored bit).

Another advantage in using the CRC is that all the bits transmitted are checked, unlike those systems that only check seven bits (those of the FAS, which are the only ones known in advance) out of every 51 bits (those between one FAS and the next). However, the CRC-4 code is not completely infallible, since there is a probability of around 116 that an error may occur and not be detected; that is, that 6.25% of the blocks may contain errors that are not detected by the code.

Monitoring errors

The aim of monitoring errors is to continuously check transmission quality without disturbing the information traffic and, when this quality is not of the required standard, taking the necessary steps to improve it. Telephone traffic is two way, which means that information is transmitted in both directions between the ends of the communication. This in its turn means that two 2 Mbit/s channels and two directions for transmission must be considered.

The CRC-4 multiframe alignment word only takes up six of the first eight bits of the TS0 without FAS. There are two bits in every second block or submultiframe, whose task is to indicate block errors in the far end of the communication. The mechanism is as follows: Both bits (called E-bits) have "1" as their default value. When the far end of the communication receives a 2 Mbit/s frame and detects an erroneous block, it puts a "0" in the E-bit that corresponds to the block in the frame being sent along the return path to the transmitter. This way, the near end of the communication is informed that an erroneous block has been detected, and both ends have the same information: one from the CRC-4 procedure and the other from the E bits. If we number the frames in the multiframe from 0 to 15, the E-bit of frame 13 refers to the submultiframe I (block I) received at the far end, and the E-bit of frame 15 refers to the submultiframe II (block II).

Supervision bits

The bits that are in position 2 of the TS0 in the frame that does not contain the FAS are called supervision bits and are set to “1,” to avoid simulations of the FAS signal.

NFASs – spare bits

The bits of the TS0 that do not contain the FAS in positions 3–8 make up what is known as the non-frame alignment signal or NFAS. This signal is sent in alternate frames (frame 1, frame 3, frame 5, etc.). The first bit of the NFAS (bit 3 of the TS0) is used to indicate that an alarm has occurred at the far end of the communication. When operating normally, it is set to "0", while a value of "1" indicates an alarm.

The bits in positions 4–8 are spare bits, and they do not have one single application, but can be used in a number of ways, as decided by the telecommunications carrier. In accordance with the ITU-T Rec. G.704, these bits can be used in specific point-to-point applications, or to establish a data link based on messages for operations management, maintenance or monitoring of the transmission quality, and so on. If these spare bits in the NFAS are not used, they must be set to "1" in international links.

NFAS – alarm bit

The method used to transmit the alarm makes use of the fact that in telephone systems, transmission is always two way). Multiplexing/demultiplexing devices (known generically as multiplex devices) are installed at both ends of the communication for the transmission and reception of frames. An alarm must be sent to the transmitter when a device detects either a power failure or a failure of the coder/decoder, in its multiplexer; or any of the following in its demultiplexer: loss of the signal (LOS), loss of frame alignment (LOF), or a BER greater than 10−3.

The remote alarm indication (RAI) is sent in the NFAS of the return frames, with bit 3 set to "1". The transmitter then considers how serious the alarm is, and goes on generating a series of operations, depending on the type of alarm condition detected.

Signaling channel

As well as transmitting information generated by the users of a telephone network, it is also necessary to transmit signaling information. Signaling refers to the protocols that must be established between exchanges so that the users can exchange information between them.

There are signals that indicate when a subscriber has picked up the telephone, when he or she can start to dial a number, and when another subscriber calls, as well as signals that let the communication link be maintained, and so on. In the E1 PCM system, signaling information can be transmitted by two different methods: the common channel signaling (CCS) method and the channel associated signaling (CAS) method. In both cases, the time slot TS16 of the basic 2 Mbit/s frame is used to transmit the signaling information.

For CCS signaling, messages of several bytes are transmitted through the 64 kbit/s channel provided by the TS16 of the frame, with these messages providing the signaling for all the channels in the frame. Each message contains information that determines the channel that is signaling. The signaling circuits access the 64 kbit/s channel of the TS16, and they are also common to all the channels signaled. There are different CCS systems that constitute complex protocols. In the following section and by way of example, channel associated signaling will be looked.

The plesiochronous digital hierarchy

Based on the E1 signal, the ITU defined a hierarchy of plesiochronous signals that enables signals to be transported at rates of up to 140 Mbit/s. [1] This section describes the characteristics of this hierarchy and the mechanism for dealing with fluctuations in respect to the nominal values of these rates, which are produced as a consequence of the tolerances of the system.

Higher hierarchical levels

As is the case with level 1 of the plesiochronous digital hierarchy (2 Mbit/s), the higher levels of multiplexing are carried out bit by bit (unlike the multiplexing of 64 kbit/s channels in a 2 Mbit/s signal, which is byte by byte), thus making it impossible to identify the lower level frames inside a higher level frame. Recovering the tributary frames requires the signal to be fully demultiplexed.

The higher hierarchical levels (8,448, 34,368, and 139,264 kbit/s, etc.; referred to as 8, 34, and 140 Mbit/s for simplicity) are obtained by multiplexing four lower level frames within a frame whose nominal transmission rate is more than four times that of the lower level (see Table 3), in order to leave room for the permitted variations in rate (justification bits), as well as the corresponding FAS, alarm, and spare bits.

E2: multiplexing level 2: 8 Mbit/s

The 8 Mbit/s frame structure is defined in the ITU-T Rec. G.742. The frame is divided into four groups, each of length 212 bits:

E2 | 212 Bits/Row
GroupBits
IFAS 1111010000 (10 bits)AS200 Tributary Bits
IIJ1 | J2 | J3 | J4208 Tributary Bits
IIIJ1 | J2 | J3 | J4208 Tributary Bits
IVJ1 | J2 | J3 | J4R1 | R2 | R3 | R4204 Tributary Bits

E3: multiplexing level 3: 34 Mbit/s

The structure of this frame is described in the ITU-T Rec. G.751 (see Figure 20). As in the previous case, the frame is divided into four groups, each of length 384 bits:

E3 | 384 Bits/Row
GroupBits
IFAS 1111010000 (10 bits)AS372 Tributary Bits
IIJ1 | J2 | J3 | J4380 Tributary Bits
IIIJ1 | J2 | J3 | J4380 Tributary Bits
IVJ1 | J2 | J3 | J4R1 | R2 | R3 | R4376 Tributary Bits

E4: multiplexing level 4: 140 Mbit/s

The structure of this frame is described in the ITU-T Rec. G.751 (see Figure 20). In this case, the frame is divided into six groups, each of length 488 bits:

E4 | 488 Bits/Row
GroupBits
IFAS 111110100000 (12 bits)A3 S Bits472 Tributary Bits
IIJ1 | J2 | J3 | J4484 Tributary Bits
IIIJ1 | J2 | J3 | J4484 Tributary Bits
IVJ1 | J2 | J3 | J4484 Tributary Bits
VJ1 | J2 | J3 | J4484 Tributary Bits
VIJ1 | J2 | J3 | J4R1 | R2 | R3 | R4480 Tributary Bits
The PDH hierarchy, with four levels from 2 to 140 Mbit/s. Higher rates are not standard.
LevelStandardRateSizeFrame/sCodeAmplitudeAttenuation
E1G.704/7322.048 Mbit/s ± 50 ppm256 bits8,000HDB32.37-3.00 V6 dB
E2G.7428.448 Mbit/s ± 30 ppm848 bits9,962.2HDB32.37 V6 dB
E3G.75134.368 Mbit/s ± 20 ppm1536 bits22,375.0HDB31.00 V12 dB
E4G.751139.264 Mbit/s ± 15 ppm2928 bits47,562.8CMI1.00 V12 dB

Service bits in higher-level frames

In any of the groups containing the FAS in the 8, 34, and 140 Mbit/s frames, alarm bits and spare bits are also to be found. These are known as service bits. The A-bits (alarm) carry an alarm indication to the remote multiplexing device, when certain breakdown conditions are detected in the near-end device. The spare bits are designed for national use, and must be set to "1" in digital paths that cross international boundaries.

Plesiochronous synchronization

As far as synchronization is concerned, the multiplexing of plesiochronous signals is not completely trouble free, especially when it comes to demultiplexing the circuits. In a PCM multiplexer of 30 + 2 channels, a sample of the output signal clock (1/32) is sent to the coders, so that the input channels are synchronized with the output frame. However, higher level multiplexers receive frames from lower level multiplexers with clocks whose value fluctuates around a nominal frequency value within certain margins of tolerance.

The margins are set by the ITU-T recommendations for each hierarchical level. The signals thus formed are almost synchronous, except for differences within the permitted margins of tolerance, and for this reason they are called plesiochronous.

Positive justification

In order to perform bit-by-bit TDM, each higher-order PDH multiplexer has elastic memories in each of its inputs in which the incoming bits from each lower level signal line or tributary are written. Since the tributary signals have different rates, they are asynchronous with respect to each other. To prevent the capacity of the elastic memories from overflowing, the multiplexer reads the incoming bits at the maximum rate permitted within the range of tolerances.

When the rate of the incoming flow in any of the tributary lines is below this reading rate, the multiplexer cannot read any bits from the elastic memory, and so it uses a stuffing bit or justification bit (called justification opportunity) in the output aggregate signal. Its task is that of adapting the signal that enters the multiplexer to the rate at which this signal is transmitted within the output frame (its highest clock value). This type of justification is called positive justification. Justification bits, together with other overhead bits, make the output rate higher than the total of the input signals.

Justification opportunity bits

The task of the justification opportunity bits (R-bits) is to be available as extra bits that can be used when the rate of the incoming tributaries is higher than its nominal value (within the margin specified by ITU-T) by an amount that makes this necessary. In this case, the opportunity bit is no longer mere stuffing, but becomes an information bit instead.

In order for the device that receives the multiplexed signal to be able to determine whether a justification opportunity bit contains useful information (i.e. information from a tributary), justification control bits (J-bits) are included in the frame. Each group of control bits refers to one of the tributaries of the frame. All of them will be set to "0" if the associated opportunity bit is carrying useful information; otherwise they will be set to "1".

Several bits are used instead of just one, to provide protection against possible errors in transmission. On examining the control bits received, if they do not all have the same value, it is decided that they were sent with the majority value (a "1" if there are more 1s than 0s, for instance; it is assumed that there has been an error in the bits that are at 0).

It can be seen that there is a dispersion of the control bits referring to a tributary that causes them to be in separate groups. Spreading out the J-bits (control bits), reduces the probability of errors occurring in them, and a wrong decision being made as to whether or not they have been used as a useful data bit. If the wrong decision is made, there is not only an error in the output data, but also a slip of one bit; that is, the loss or repetition of one bit of information.

Managing alarms in higher-level hierarchies

The A-bit of the FAS in 8, 34, and 140 Mbit/s frames enables the multiplexers that correspond to these hierarchies to transmit alarm indications to the far ends when a multiplexer detects an alarm condition.

In addition, 140 Mbit/s multiplexers also transmit an alarm indication when faced with the loss of frame alignment of the 34-Mbit/s signals received inside the 140 Mbit/s signals, as well as in the NFAS of the 34 Mbit/s signal that has lost its alignment (bit 11 of group I changes from "0" to "1") in the return channel.

Glossary

Hierarchy levels

The PDH based on the E0 signal rate is designed so that each higher level can multiplex a set of lower level signals. Framed E1 is designed to carry 30 or 31 E0 data channels plus 1 or 2 special channels, all other levels are designed to carry 4 signals from the level below. Because of the necessity for overhead bits, and justification bits to account for rate differences between sections of the network, each subsequent level has a capacity greater than would be expected from simply multiplying the lower level signal rate (so for example E2 is 8.448 Mbit/s and not 8.192 Mbit/s as one might expect when multiplying the E1 rate by 4).

Note, because bit interleaving is used, it is very difficult to demultiplex low level tributaries directly, requiring equipment to individually demultiplex every single level down to the one that is required.

See also

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References

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