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T1 OVERVIEW
THE HIGH-CAPACITY
DIGITAL NETWORK
MeterCenter
(800) 230-6008
(480) 659-8351 Outside USA
eteM
TABLE OF CONTENTS
THE HIGH-CAPACITY DIGITAL NETWORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
T1 Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
Customer Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
Technology Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
T1 Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
T1 Equipment Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Customer Service Unit (CSU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Demarcation Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Network Interface Device (NID) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Station Wire Color Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Network Interface Unit (NIU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Main Distribution Frame (MDF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Office Repeater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Digital Signal Cross-Connect (DSX) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Testing and Troubleshooting T1 Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Performance Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Stress Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Alarm and Status Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Common Tests Performed With the 20T1 Test Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
20T1 Modes of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
Performing Automatic Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Monitoring Live Traffic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Performing a BERT (Bit Error Rate Test) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Performing Clock-Slip Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Performing a Half Duplex Drop & Insert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Performing a Loop Delay Measurement on a T1 Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Performing a Bridge Tap (BTP) or Multi-pattern (MPT) Test on a T1 Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . .23
BTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
MPT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
T1 GUIDE—THE HIGH CAPACITY DIGITAL NETWORK
1
THE HIGH-CAPACITY DIGITAL NETWORK
High-Capacity, Digital Signal Level 1 (DS1 Hi-Cap) service, also
known as T1, is just one type of Hi-Cap service available to
telephony customers. There is a higher order, or hierarchy, of
T1 such as DS2, also known as T2, or DS3, also known as T3,
that also may be ordered. However, the focus of this
document is on T1 service.
■
Connect cellular sites
■
Interexchange Carrier (IC)
Note: The PRI circuit requires special dial-though testing not
associated with a point-to-point T1.
Customer Benefits
Some of the drivers behind the demand for T1 service are:
T1 is a high speed network developed by AT&T in 1957 and
implemented in the early 1960s. The technology was
developed to support long-haul pulse-code modulation (PCM)
voice transmission. The innovation of T1 was to introduce
digitized voice and to create a communication link that
enables transmission of voice, data and video signals at the
rate of 1.544 million bits per second (Mb/s). The telephone
companies initially used T1 to reduce the number of
telephone cables in a large metropolitan area. A DS1 Hi-Cap,
or T1, is:
■
Flexibility
■
T1 handles voice and/or data services.
Bandwidth can be allocated on demand.
■
Improved quality over analog lines.
■
Increased capacity over conventional lines:
■
1-24 standard voice/data channels (DS0s).
■
Guaranteed Service
■
Most carriers strive to offer restoration in less than 3 hrs.
■
Available 99.99 percent annually.
In the past decade, costs have been reduced from tens of
thousands of dollars per month to around $500 per month
and installation times (customer due dates) have gone from
several weeks to same-day service.
■
A point-to-point service.
■
1.544 Mbps digital pipe —Transport Technology
The T1 may be provided using fiber optic transport devices or
copper cable facilities.
Technology Review
A vast majority of T1 benefits are attributed to the fact that
voice and data share a single digital communication link.
Computer data consists of 1’s and 0’s, the symbols of the binary
system; therefore computer data is already compatible with T1’s
digital format. Voice presents another challenge. Voice signals
actually comprise of complex analog waveforms.
T1 Service
Due to installation costs, early T1 services were used primarily
by phone companies and the federal government. Customers,
particularly those not requiring all of the bandwidth available
on a T1 circuit, would have needed to purchase expensive
multiplexing equipment not required for analog transmission.
The cost of multiplexing equipment, plus the fact that service
charges were still based on the amount of bandwidth
purchased, did not make T1 an economic decision for many
potential customers. T1 was re-tariffed in the early 1980s to
allow substantial cost savings to customers who had multiple
circuits between two locations. With customer requirements
for interlocation connectivity, growth rates have continued to
climb for services such as:
Sine waves are all we have to work with in transmitting over
the analog telephone channel because it doesn’t transmit
pulses. Digital transmission systems will transmit pulses, and
with them we can encode either analog or digital information
by modulating pulses. There are a few ways to modulate a
series of pulses to carry data.
Internet Service Provider (ISP) access
ISDN Primary Rate Interface (ISDN PRI) access
Carrier Facilities
Channel Service for multiple applications
Local Area Network/Wide Area Network
(LAN/WAN) connectivity for data transfer and
sharing
Customer Premise
Equipment (CPE)
Customer Premise
Equipment (CPE)
Central Office
Medical data transfer (i.e., X ray, CAT scan)
Mainframe computer links
Videoconferencing
Customer Location A
Customer Location Z
Figure 1: DS1 Hi-Cap, T1
Private Branch Exchange (PBX) connectivity
2
T1 GUIDE—THE HIGH CAPACITY DIGITAL NETWORK
When the amplitude of the pulses is varied to
represent analog information, the method is
called pulse amplitude modulation (PAM). This
method is very susceptible to electrical noise
interference.
The process of sampling an analog signal as in
Pulse Amplitude Modulation, but where the
amplitudes of the samples are encoded into
binary numbers represented by constant
amplitude pulses is called Pulse Code
Modulation (PCM). This method overcomes the
noise interference problem of Pulse Amplitude
Modulation. The PCM system used by
communication carriers employs a three step
process: sampling, quantization and coding.
During the sampling process, the analog signal
is sampled 8,000 times per second.
The resulting samples represent an infinite
number of voltages. Thus, the second step in
the PCM process, called quantization, reduces
the PAM signal to a limited number of discrete
amplitudes. The third step in the PCM process,
known as coding, reduces the number of
unique values of the PAM signal so they can be
coded through the use of an 8-bit byte. For
simplicity, the lower portion of the diagram in
Figure 4 uses 4 bits to represent each PAM
signal; however, in actuality 8 bits are used.
The 8,000 samples per second multiplied by
the 8 bits per sample create the 64 kb/s rate
known as Digital Signal Level Zero (DS0).
Once digitized, voice and/or data signals from
various sources can be combined, or
multiplexed, and transmitted over a single T1
link. This process is known as Time Division
Multiplexing (TDM).
Figure 2: DS1 Service
TDM divides a T1 link into 24 discrete 64 kb/s time slots. An
identical number of DS0 signals, representing 24 separate
voice and/or data calls, are assigned to each time slot for
transmission within the link. PCM and TDM are keys to
understanding the basic T1 rate of 1.544 Mb/s.
In T1, the 8 bit digital samples created in the PCM step, for
voice traffic only, are grouped into the 24 discrete DS0 time
slots created by TDM. Each group of 24 time slots is referred
to as a T1 Frame. Since there are 24 time slots, each
containing 8 bits, the number of bits per frame totals 192. To
mark the end of one frame and the beginning of another
frame, a 193rd bit is added. This additional bit is called the
framing bit. Since DS0 signals are sampled 8,000 times per
second, it means that 8,000 192-bit information frames are
Figure 3: Pulse Amplitude Modulation (PAM)
T1 GUIDE—THE HIGH CAPACITY DIGITAL NETWORK
3
 created during that period. 8,000 samples per second
multiplied by 192 bits totals 1.536 Mb/s. At 8,000 samples
per second, framing bits are created at the rate of 8 kb/s.
The result is a single 1.544 Mb/s signal known as digital
signal level one (DS1).
Since the signal consists of 1’s and 0’s, recreating it is not a
complicated task. Repeaters are typically at 6000-foot
intervals for copper lines.
A DS1 signal is transmitted on the T1 link in a binary format
of 1’s and 0’s. Regeneration repeaters rely on proper DS1
format to recognize the DS-1 signal and distinguish it from
line noise. Alternate Mark Inversion (AMI) is a very common
format that is used over metallic transmission media.
The DS1 signal starts strong when it is newly created, but
degrades rapidly as it travels along the transmission media.
Factors such as attenuation and dispersion attribute to signal
degradation. To compensate for signal loss, regeneration
repeaters are used to sample and recreate the original signal
at periodic intervals along the T1 link.
In the AMI signaling format, the binary value of 1 is
represented by a square wave (pulse); the binary value of 0 is
represented by a straight line (no pulse). A bipolar format is
used to achieve superior signal travel distance and to offer a
built-in method for error detection. If consecutive pulses of
the same polarity are detected a bipolar violation (BPV) will be
created. BPV’s indicate that the signal input has been
disturbed due to environmental conditions or defective
equipment.
10
9
8
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10
3 6 8 9 10 9 8 7 4 1
Regeneration repeaters must know when to sample the
bipolar signal to determine whether a 0 or a 1 is being
transmitted at any given time. To ensure proper sampling,
the repeater relies on a timing method that uses the binary
pulses (ones) to maintain synchronization with the network
equipment that is transmitting the DS1 signal.
3 6 8 9 10 9 8 7 4 1
Pulses are critical for maintaining proper signal timing,
therefore, DS1 signals are required to meet specific ones
density standards. The standards require that at least one
pulse be transmitted within any eight-bit sequence. Since
long strings of consecutive zeros between digital values have
devastating effects on timing, ones density standards prohibit
the transmission of more than 15 zeros in succession.
0011
0110
1000
1001
Meeting ones density requirements can vary depending on
the application. The size and content of the bit patterns
representing human speech are constant, therefore, with
voice applications acceptable ones density is virtually
guaranteed. In data applications, however, the computer data
is highly variable in size and content. Conformance to ones
density cannot always be guaranteed. Bipolar with 8-Zero
Substitution (B8ZS) is a
technique that addresses
this problem.
Figure 4: Pulse Code Modulation
Data Bit
Frame Bit
B8ZS uses intentional BPV’s in
the data stream to break up
long strings of zeros. With
B8ZS coding, each block of 8
consecutive zeros is replaced
with the B8ZS code word.
The BPV’s are placed in bit
positions 4 and 7. Bit
positions 5 and 8 are also
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Time Slot 1 Time Slot 2 Time Slot 3 . . . Time Slot 24
1 Frame = 193 Bits (192 Data Plus 1 Framing)
Figure 5: Typical T1 Frame Created through TDM
4
T1 GUIDE—THE HIGH CAPACITY DIGITAL NETWORK
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