Mr. Mark Welton
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Most telecom infrastructure that exists today
is based on standards more than 100 years
old
Telecom infrastructure is often so reliable
that we expect reliable phone service more
than reliable Internet service or power
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Telecommunications refers to the traditional
circuit-based switching technologies primary
used by voice services
data communications often refers to packetbased switching of the modern Internet
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ACD – automatic call distribution
◦ An ACD is usually found in a call center, where
calls may come in from anywhere and
need to be directed to the next available
operator or queued until one is available
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Add/Drop – used to describe the capability of
peeling off channels from a circuit for
another use
An add/drop CSU/DSU can separate ranges of
channels, thus allowing a T1 to be split for
both voice and data use or as two partial T1s
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analog in telecom refers to a signal that is
continuous in amplitude and time, any small
fluctuation of the signal is important
◦ Radio waves, power waves, and sound waves are
analog
◦ When you speak, you create waves of air that hit
people’s eardrums
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Digital refers to a signal that has discrete
values
If you analyze a sound wave, and then assign
a value to each sample of the wave at specific
time intervals, you will create a digital
representation of the analog wave
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Bandwidth vs. throughput
A range of frequencies is called a band
The width of the band is referred to as
bandwidth
US FM channels run from 87.8 MHz to 107.9
MHz
The bandwidth is 20MHz
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On a digital link what is referred to as
bandwidth is really throughput
Throughput is the number of possible state
transitions per second
This can also be referred to as the data rate
The signal on the right has more bits per
second (bps) then the left (19 possible state
changes vs. 6)
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BERT – Bit Error Rate Test
BERT are disruptive tests run on a T1 to
validate the integrity of the circuit (looks for
errors)
CO – central office
◦ the first hop where phone lines go
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Channel bank – a device that separates a T1
into 24 individual analog phone lines
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CSU/DSU – Channel Service Unit and Data Service
Unit
The CSU is responsible for interfacing with the
WAN service
The DSU is responsible fore interfacing with the
data equipment, such as the router
The CSU typically has an RJ-45 connection from
the demark point and the DSU has a V.35
connection to a router
In a CSU/DSU router card, the RJ-45 connection
from the demark point goes to the router card
and the connection on the card interfaces it to
the router
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CPE – customer premises equipment
Traditionally, the term was used to describe
equipment owned by a telephone service
provider that resided at customer premises,
but it has evolved to include equipment
owned by anyone
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DACCS (pronounced dacks) – Digital Access
Cross-Connect System, allows changes to the
way voice channels are connected between
trunks
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Demarc (pronounced dee-mark) –
demarcation point where the telecom
provider’s responsibilities end and the
customer’s begins
What is an extended demark?
Provider installed location somewhere beyond
the real demarc
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DS hierarchy – Digital Signal, describes the
signaling rates of links
Designator
Carrier
Transmission
rate
Voice channels
DSO
N/A
64 Kbps
1
DS1
T1
1.544 Mbps
24
DS2
T2
6.312 Mbps
96
DS3
T3
44.736 Mbps
672
DS4
T4
274.176 Mbps
4,032
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T-carrier – the generic name of digital
multiplexed carrier systems in North America
◦ T stands for trunk, since they originally designed to
trunk multiple phone lines between central offices
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E-carrier - The European version of digital
multiplexed carrier systems
J-carrier – The Japanese version of digital
multiplexed carrier systems
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SONET – synchronous optical network, fiber
optic system makes use of the optical carrier
levels
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OC1 – 51 Mbps
OC3 – 155 Mbps
OC12 – 622 Mbps
OC48 – 2,488 Mbps
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ISDN – Integrated Services Digital Network
allows for the simultaneous transmission of voice
and data
Uses a separate channel for signaling (Data
channel or D-channel) and two remaining
channels for content (Bearer channel or BChannel)
Each B-Channel is capable of 64kbps or a total of
128kbps and the D-channel capable of 16kbps
on a Basic Rate Interface (BRI) and 64kbps on a
Primary Rate Interface (PRI)
A PRI is an ISDN T1 composed of 23 B-channels
and 1 D-channel
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LATA – local access and transport area,
government defined areas in which a telecom
provider can provide local services
IXC – interexchange carrier providers services
to local exchanges by connecting in between
LATAs
LEC – local exchange carrier, provides local
service within a LATA
Local loop – the last mile connection for a
circuit from a telecom facility to it destination
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Latency – term used to describe the amount
of time it takes for data to be processed or
moved along a network
Not related to throughput, bandwidth, or
speed of a link
Related to distance, speed of light, and
amount of time for hardware processing
Two parts of latency – propagation delay and
processing delay.
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PBX – private branch exchange, used by large
organizations as a localized phone company
for its ability to share a limited number of
public phone lines and the ability to call any
internal extension number
POTS – plain old telephone service, your
traditional analog phone line
Smart jack – a device that terminates a digital
circuit, the service provider can perform
remote testing of a circuit with the smart jack
installed
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RBOC – Regional Bell Operating Company
Result of the 1984 breakup of AT&T Bell System,
into seven regional local only carriers
The Telecom Deregulation Act of 1996 allowed
LECs (RBOCs) to sell local and long distance
services.
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Bell Atlantic
Southwestern Bell
Nynex
Pacific Bell
Bell South
Ameritech
US West
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T1 circuits are full-duplex asynchronous
circuits
Two Common types
◦ Channelized T1 – voice circuit with 24 voice
channels, each channel contains is own signaling
information inserted into the data stream (voice) via
in-band signaling
◦ PRI – voice circuit with 24 channels, one of which is
dedicated to signaling (out of band signaling),
leaving 23 available voice channels
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two types of encoding
Alternate Mark Inversion (AMI) – used for
voice only
Binary Eight Zero Substitution (B8ZS) – used
for data but can be used for voice
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There are two “states” (zero and one) for the signal
on the line
The mark state representing a one (+/-5 volts) and
the space representing a zero (zero volts)
The next mark is always the opposite of the
preceding mark
Robbed-bit signaling – every eighth bit is used to
keep signal (always set to 1 regardless of original
value)
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introduced to resolve the shortcomings of
AMI
In voice the eighth bit 1 is not an issue
because you can not hear the change
In data this would change the information
Need to deal with the synchronization issue
of too many zeros in a row without changing
the information
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if eight zeros in a row are detected in a
signal, those eight zeros are converted to a
pattern including intentional BPVs
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Sample audio 8,000 times per second
Each sample is converted in to an eight-bit
value
8th bit is used for signaling (robbed-bit
signaling in AMI)
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In standard voice framing each eight-bit
sample is relayed from each channel in order
each eight-bit sample is relayed from each
channel in order
8 bits X 24 channels = 192 bits
The 193rd bit is used as a framing bit
It will change this bit over 12 frames using
the pattern 110111001000
12 frames are a superframe
Used for voice circuits
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D4/superframe standard is not practical for data
transmission (lack error detection)
Extended Super frame (ESF) was developed to deal
with this
Uses a pattern of 24 frames instead of 12
◦ Frames 4, 8, 12, 16, 20, and 24 (every fourth frame)
 These frames’ framing bits are filled with the pattern 001011
◦ Frames 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, and 23 (every
odd-numbered frame)
 These frames’ framing bits are used for a new, 4,000 bps
virtual data channel. This channel is used for out-of-band
communications between networking devices on the link
◦ Frames 2, 6, 10, 14, 18, and 22 (the remaining even-
numbered frames)
 These frames’ framing bits are used to store a six-bit CRC value
for each superframe.
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Loss of Signal (LOS) – no electrical pulses
detected (line is dead)
Out of Frame (OOF) or Loss of Frame (LOF) – a
number of frames have been received with
errors (synchronization invalid between two
sides)
BiPolar Violation (BPV) – two mark signals
occur in sequence at the same polarity
(usually caused by voltage spike on the line)
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CRC6 - Cyclic Redundancy Check (six-bit)
mechanism for error checking in ESF (check sum
did not match)
Errored Seconds – shows the number of seconds
in a 15-minute window in which errors on the
line have occurred
Red Alarm – a local failure on the circuit or
continuous OOF error
Yellow Alarm – remote alarm indication of
problem down the line
Loopback test – loopback state allowing the
sending of data over the link, data should return
to sender exactly as sent
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Two types of DS3
◦ Channelized
 672 DS0s (28 DS1s), each capable of supporting a
single POTS-line phone call
 Same times referred to as a “channeled T3”
 DS3 is not the same as a T3
◦ Clear-channel
 Has no channels and is used for pure data
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DS3 is actually a group of seven DS2s
multiplexed together
Originally designed to handle phone calls
Each DS1multiplied into a DS3 has its own
clocking, framing, and encoding
Multiplexing them into a DS3 can not affect
this
The DS3 must also have its own clocking,
framing, and encoding
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M13 (pronounced M-one-three, not Mthirteen) is short for Multiplexed DS1 to DS3
Requires two stages of multiplexing
◦ M12 – multiplexed DS1 to DS2
◦ M23 - multiplexed DS2 to DS3
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DS3s were originally used to aggregate T1s