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T1 GUIDETHE HIGH CAPACITY DIGITAL NETWORK
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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 2
T1 GUIDETHE HIGH CAPACITY DIGITAL NETWORK
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.
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: 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.
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: Internet Service Provider (ISP) access ISDN Primary Rate Interface (ISDN PRI) access Channel Service for multiple applications Local Area Network/Wide Area Network
(LAN/WAN) connectivity for data transfer and
sharing Medical data transfer (i.e., X ray, CAT scan) Mainframe computer links Videoconferencing Private Branch Exchange (PBX) connectivity 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: 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.
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 1s and 0s, the symbols of the binary
system; therefore computer data is already compatible with T1s
digital format. Voice presents another challenge. Voice signals
actually comprise of complex analog waveforms.
Sine waves are all we have to work with in transmitting over
the analog telephone channel because it doesnt 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.
Customer Premise
Equipment (CPE)
Customer Premise
Equipment (CPE)
Central Office
Customer Location A
Customer Location Z
Carrier Facilities
Figure 1: DS1 Hi-Cap, T1 T1 GUIDETHE HIGH CAPACITY DIGITAL NETWORK
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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 actualit