This chapter describes channelized narrowband transmission facilities
including the following information:
Four types of narrowband transmission facilities are available, including
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.
||Number of Timeslots
||Bit Rate (Mbps)
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.
Framing, and Signaling
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:
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
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
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).
Ones Density Enforcement
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
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:
An intentional BPV
A valid pulse
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
NOTE: In most systems, D4 framing is used
without any ones density enforcement; ESF framing is used with B8ZS.
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.
Extended Superframe Format
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
Ones Density Enforcement
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
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
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).
Channel Associated Signaling
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.
Common Channel Signaling
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
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