This chapter describes channelized narrowband transmission facilities including the following information:

• Transmission types
• Line coding, framing, and signaling
• Time-division multiplexing
• T1 characteristics
• E1 characteristics
• J1 and Y1 characteristics

Transmission Types 

Four types of narrowband transmission facilities are available, including the following:

• T1 format, used primarily in North America and parts of Asia
• E1 format, used in most of the rest of the world
• J1 format, used exclusively in Japan, usually between a PBX and a switch
• Y1 format, used exclusively in Japan, usually between switches in the WAN

The following table shows the similarities and differences between these four formats. A timeslot (DS0 in T1 systems) is 64 kbps and can carry one digital PCM voice signal.

Cable Connectors 

Narrowband transmission facilities can be attached using a variety of different cable connectors depending on the type of equipment that is being installed. The following graphic show the different cable connectors used with narrowband transmission facilities. 

T1 is most often attached using DB-15 connectors, although RJ-48 connectors are used when a high density of interfaces is required. E1 systems can also use DB-15 or RJ-48 connectors although a pair (one for transmit, one for receive) of BNC connectors is more common. Y1 and J1 facilities, like T1 facilities, most often use DB-15 connectors. 

Time-division multiplexing (TDM) was originally designed to reduce the amount of cabling necessary to carry multiple voice channels. Rather than have 20 or 30 sets of cables, TDM combines the information from different sources onto a single cable. 

The foundation of TDM is built on voice traffic. When an analog voice signal is converted into a digital PCM signal, a 64-kbps channel is created. Although many multiplexed facilities carry a variety of traffic (voice, data, Frame Relay, and video), the 64-kbps channel still remains. 
 
TDM basically converts multiple parallel traffic streams into a single serial traffic stream as shown in the preceding graphic. This is accomplished by taking an 8-bit byte from one 64-kbps input and following it serially with another 8-bit byte from a second 64-kbps input. This process continues until one 8-bit byte from each input has been sent onto the multiplexed line. The number of inputs varies by line type (see the preceding table). The portion of the multiplexed line that contains one 8-bit byte from each of the 64-kbps inputs is called a frame. The frame is the building block for all of the line types as you will see later in this chapter.

Each type of narrowband transmission facility has defined coding, framing, and signaling associated with it. The details are described under each line type later in this chapter. You should understand the significance of the coding, framing, and signaling on a line.

Line coding describes the way the logical bits (1 and 0) are represented on the transmission facility. For example, on a T1 line a 0 bit is represented by a zero voltage on the wire, but with J1 a 0 bit is represented by a transition from a negative voltage to a positive voltage on the wire.

Ones density is also part of the coding definition of a line. Most transmission systems define a maximum number of consecutive 0 bits (therefore, a minimum density of 1 bits) that can occur on the line. The ones density is necessary to maintain synchronization of the signal. Without creating bit errors, the facility must have a means of creating an acceptable ones density. Both T1 and E1 systems have defined ones density enforcement methods. Y1 and J1 do not require a ones density method because the signal coding eliminates the need for additional ones density enforcement.

When errors occur on a line, they can be the result of an error in the line coding, for example, a bipolar or line-code violation. It is important to understand how the facility is coded in order to more easily determine the cause of these types of errors.

Line framing describes how the logical bits are organized on the transmission facility. The framing on a line does the following:

• Line synchronization
• Establishes byte boundaries for multiplexing and demultiplexing
• Defines signaling bit locations
• Defines alarm notification mechanisms

Each transmission facility has one or more defined framing structures. Some line types have very similar framing structures, such as T1 and Y1. The preceding graphic summarizes the number of channels or timeslots and the transmission rate of each of the four facility types—T1, E1, Y1, and J1. Regardless of the number of bits in a frame, frames occur on all transmission facilities at a rate of 8000 frames per second or one frame every 125 microseconds.

Line signaling describes how control or signaling information is passed between two devices, typically between PBXs transporting voice traffic. A typical application for signaling is to signal the seizure or release (go off hook or on hook) of a voice channel. The transmission facility, depending on the type, reserves or borrows particular bits for signaling.

Line Coding for T1 

Alternate mark inversion (AMI) is the type of line coding used for T1 lines. Electrically, the signal transmitted on a T1 line is a bipolar, return-to-zero (RZ) signal. This simply means that each logical 1 bit is transmitted as a positive or a negative pulse, after which the line voltage always returns to zero. A logical 0 bit is transmitted as a zero voltage on the line.

This format is known as AMI because each logical 1 bit (pulse or mark) is of opposite polarity from the previous one. The following graphic depicts AMI. 

The AMI format provides two essential benefits.

• There is no DC current flowing through the circuit.
• It results in a fundamental frequency of half of the line bit rate.

One additional benefit of the AMI bipolar format is that it permits detection of line errors. If a line problem causes a pulse to be deleted or an unintended pulse to be transmitted, two consecutive pulses with the same polarity on the line will result. The following graphic shows this occurrence, called a bipolar violation (BPV). 

The major disadvantage of the AMI format is that the transmission of a long sequence of zeros:

• Provides no activity on the line.
• Is indistinguishable from a total loss of signal.
• Appears as a DC signal on the line.

A long sequence of zeros on a line that uses AMI will result in a long period of inactivity on the line. Consequently, special care must be taken with lines that use AMI to ensure an adequate density of pulses on the line.

There are two basic ways to ensure an adequate density of ones. The first is to ensure that the user equipment is unable to generate a long sequence of zeros. If that is impractical or impossible, the equipment can ensure a sufficient ones density by carefully replacing long sequences of zeros with a special pattern that:

• Includes both zeros and ones.
• Is distinctive enough to be unmistakably recognized at the remote end of the line where it is removed and the original sequence of zeros replaced.

A widely used method of enforcing ones density on a T1 facility is bipolar 8-zero substitution (B8ZS). In the B8ZS technique, any sequence of eight consecutive zeros is replaced on the line by four zeros and four pulses in the following order:

• Three zeros
• An intentional BPV
• A valid pulse
• A zero
• Another BPV
• A valid pulse

The B8ZS technique requires that this particular sequence of pulses and bipolar violations is unlikely to naturally occur due to random transmission line errors. If the B8ZS substituted pattern does occur, the data on the line will be corrupted because the pattern will be assumed to be a string of eight 0 bits. The following graphic shows a signal using B8ZS.

A T1 frame comprises 24 octets (8-bit bytes) and one additional bit for framing, making a total of 193 bits. The 24 octets are available to carry user traffic, the framing bit cannot be used for any other purpose. The specific sequence of framing bits in successive frames allows the receiving equipment to locate the beginning of each frame by locking on or synchronizing to the pattern. 

There are two slightly different T1 framing bit sequences in widespread use today. They are known as the D4 format and the Extended Superframe (ESF) format.

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NOTE: In most systems, D4 framing is used without any ones density enforcement; ESF framing is used with B8ZS. 

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In the D4 framing format, the framing bits in 12 consecutive frames form a fixed pattern that receiving equipment is able to detect. This sequence constitutes a superframe. The detection of this pattern locates the beginning of each individual frame and allows the receiver to distinguish frame 1 from frame 2, and so on. 

The ESF format differs from standard D4 format only in the total length of the superframe and its definition and use of the framing bits. The 24-frame ESF format uses:

• One-fourth of the framing bits (2000 bits per second) for actual framing using a frame bit pattern of 001011 in the frame bit positions in frames 4, 8, 12, 16, 20, and 24.
• One-fourth of the framing bits (2000 bits per second) for CRC line error detection in the frame bit positions in frames 2, 6, 10, 14, 18, and 22.
• Half of the framing bits (4000 bits per second) offer a nondisruptive diagnostic data channel for line maintenance and repair purposes. This data channel uses the frame bit positions in all the odd-numbered frames.

Most T1 systems transmit voice signaling information by specifying that certain bits, which would normally be used for actual user information, be used instead for signaling. Because these bit positions can no longer carry user information, this signaling technique is called robbed bit signaling. 

Voice signaling is placed in the least significant bit position of every timeslot in the 6th and 12th frame of every D4 superframe. If ESF framing is used, the 18th and 24th frame also carry signaling bits.

The signaling bit in the 6th frame is called the A bit and the signaling bit in the 12th frame is called the B bit. Some devices can support four signaling bits—A, B, C, and D—on an ESF line where the 18th frame carries the C bit and the 24th frame carries the D bit. If only two signaling bits are needed on an ESF line, the A and B bits are simply repeated in frames 18 and 24, respectively.

In most cases, the corruption of a single bit in an 8-bit digital voice sample is negligible—it only represents 125 microseconds of the signal. However, it should be noted that a T1 system that uses robbed bit signaling cannot carry 24 transparent data channels of 64 kbps each because unlike voice, every data bit is significant. Consequently, data transport services in North America historically carry some number of 56-kbps channels (that is, Nx56 kbps), using only seven bits in every octet in order to avoid the data corruption that would result from the insertion of the signaling bits. Most T1 systems today are flexible enough to eliminate the signaling bits if a channel is used for data allowing for a full 64-kbps data rate.

Line Coding for E1 

E1 lines use AMI for line coding as described for T1 in the previous section.

E1 transmission facilities use high-density bipolar 3 (HDB3) for ones density enforcement, similar to the way T1 facilities use B8ZS for the same purpose. Unlike B8ZS, HDB3 is standard for all E1s. It is not optional.

In the HDB3 technique, any sequence of four consecutive zeros is coded on the line as follows:

• The first bit is coded as a valid pulse (according to the AMI rule) or a 0 bit:
• If there has been an even number of pulses (of either polarity) since the last BPV, then the first bit is a pulse.
• If there has been an odd number of pulses (of either polarity) since the last intentional BPV, then the first bit is a 0.
• The next two bits are coded as 0 bits.
• The fourth bit is coded as an intentional BPV.

This technique inserts bipolar violations in the fourth bit position in any sequence of four consecutive zeros in such a way as to guarantee that intentional violations are of alternating polarity so that no DC component is introduced in the signal. The following graphic shows an E1 signal using HDB3. 

An E1 frame comprises 32 channels or timeslots, one of which is reserved for framing. The remaining 31 octets in every frame are available to carry user traffic although often timeslot 16 is reserved for signaling. 

Timeslot 0 also contains bits that support other functions besides framing. It contains framing information in a frame alignment signal (FAS), a remote alarm notification, five national bits, and optional CRC bits. 

To convey multiple frames, E1 uses a multiframe structure. The following graphic shows how this structure implements the FAS and the optional CRC error checking. 

In the E1 format, one entire 64-kbps channel, timeslot 16, is dedicated to carry the necessary signaling information for the other 30 information channels. Timeslot 16 is typically used in one of two signaling formats, channel associated signaling (CAS) or common channel signaling (CCS).

In the CAS format, the available bandwidth in timeslot 16 is allocated to the 30 information channels using the structure shown in the following graphic. In this structure, each channel is allocated a total of 2 kbps used to carry four signaling bits, known as the A, B, C, and D bits. For example, the four signaling bits for channel 23 are in bits 5, 6, 7, and 8 of frame number 7 in timeslot 16. The remaining 4 kbps is used for signaling multiframing and alarm reporting. 

In the CCS format, the available bandwidth in timeslot 16 is not preallocated to the 30 information channels. Instead, it is simply used as a transparent 64-kbps channel between the two end devices, which they can use to exchange signaling information of any type and in any format they choose. Typically, in CCS mode, signaling information is only sent for a particular channel when necessary.

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