Transcript Week 3 - Cochise College

Week 3
Things you really want to know
Things to know
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Describe functions/services of a central office
Describe trunk
Describe local loop
Describe the NANP, how does the CO fit into this plan.
Describe pulse dial and DTMF
Difference between in-band & out of band signaling
Describe the concept of grade of service
What is the busiest telephone day of the year? Day
with the most collect calls?
An Overview of the Public Network
• Public Switched Telephone Network (PSTN) - consists of all
the facilities and connections maintained by all local and long
distance providers.
• In telecommunications, line is used frequently to refer to one
of two things:
– the physical connection between a subscriber and the telephone
company’s facilities
– a single communications channel between a subscriber and the
central office
An Overview of the Public Network
• Termination - the place where a wire is connected to another
part of the public telephone network (for example, a switch or
a customer’s home).
• Point of presence (POP) - refers to a carrier’s facilities that
allow it or its customers access to the public network.
Common Carriers
• Common carriers - entities directly involved in supplying
regulated telecommunications services to the public.
• Reseller - a common carrier, or a company that leases
another company’s facilities, and then sells services over
those facilities under its own name.
Local Exchange Carriers (LECs)
• Currently, two types of common carriers provide local phone
service:
– Incumbent local exchange carriers (ILECs): companies that have been
providing local phone service since before competition was allowed for
intraLATA traffic
– Competitive local exchange carriers (CLECs): companies that began
offering local phone service after the Telecommunications Act of 1996
introduced competition.
• Facilities-based - CLECs that build their own facilities in addition to
leasing and using ILEC facilities to provide service under their name.
Interexchange Carriers (IXCs)
Interexchange Carriers (IXCs)
Demarcation Point
Demarcation Point
Demarcation Point
Local Loop
The portion of a business or residential telephone network that
connects the demarcation point to the local phone company’s nearest
central office is called the local loop.
• Local loop (last mile) portion of a connection is the most expensive for
a carrier to provide because separate lines must be installed for each
individual subscriber.
• The local loop is the part of a connection most likely to have the
lowest throughput and, further, be the most susceptible to damage or
noise.
Local Loop
Local Loop
Serving Area Concepts (SAC)
• Drop wire - the cable that runs from a subscriber’s demarcation point
to a telephone pole or underground conduit.
– The drop wire connects the subscriber’s home or business line to a
distribution cable, which gathers multiple drop wires from a
neighborhood.
• Conduit - the thick tube (usually made of PVC plastic) that surrounds
a distribution cable.
– The conduit protects the wires within the cable from environmental
damage.
Serving Area Concepts (SAC)
Serving Area Concepts (SAC)
Serving Area Concepts (SAC)
Serving Area Concepts (SAC)
Cable Vaults
Cable Vaults
Distributing Frames
• Main distributing frame (MDF) - a piece of equipment where incoming
wires terminate and their circuits are connected to another set of wires
that lead to central office equipment.
• Punch-down block - a row of metallic clips (or receptors) that accept a
wire termination.
• Jumper wires - used to connect incoming lines’ punch-down blocks with
the outgoing lines’ punch downblocks.
• Cross-connect - wires terminating at two sets of punch-down blocks are
interconnected.
Distributing Frames
Distributing Frames
Distributing Frames
Switching Equipment
Switching Equipment
• Major functions of switching equipment at a central office:
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Dial tone
Customer and phone number identification
Call setup
Call routing
Call supervision
Line testing and maintenance
Billing
Figure 18–35
The three sources of power available in a central office.
Power Equipment
Central Office Hierarchy
• Serving area (of a local office) - the geographical boundary
that includes all its subscribers. It extends roughly three
miles in all directions from the central office (CO).
• Trunk - a transmission route between switches that
typically has a great deal more capacity than a feeder.
• Regional offices - Class 1 central offices.
Central Office Hierarchy
LEC-to-LEC Connections
LEC-to-LEC Connections
LEC-to-LEC Connections
LEC-to-IXC Connections
LEC-to-IXC Connections
Billing Between Carriers
The fees charged by ILECs are based on the leasing carrier’s:
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Grade of service received
Number of trunks used
Amount of traffic transmitted
Placement of equipment in ILEC’s facility, also called collocation
Facilities and circuit installation
Maintenance and support agreement
North American Numbering Plan
A scheme for assigning unique phone numbers to every line in
the country.
• For numbering purposes, North America is divided into
several smaller geographic regions called Numbering Plan
Areas (NPAs), more commonly known as area codes.
North American Numbering Plan
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NPAs reserved for special purposes include:
– Easily Recognizable Codes (ERCs) - When the second and third digit of
the NPA are identical (for example, the numbers 888 or 411), the NPA is
known as an Easily Recognizable Code (ERC).
– N9b - these 80 NPA codes have been reserved for use when the current
NANP numbering scheme undergoes further expansion.
– 37b and 96b - these 20 NPA codes have been reserved in case a
previously unanticipated need for blocks of 10 contiguous NPAs arises.
Summary
• The Public Switched Telephone Network (PSTN) is defined as the collection
of local and long distance providers’ facilities and connections that are
available for public voice (and more recently, data) communications.
• Common carriers are entities directly involved in supplying regulated
telecommunications services to the public.
• The local loop, or "last mile," is the connection between a subscriber and
the nearest central office.
Key Terms in Switching Systems
• Common control systems
– Translation of the telephone number, automatic call
routing, digit conversions, and trunk signaling
• Direct control systems
– Lack alternate routing and digit translation capabilities
• Virtually non-blocking
– Not totally non-blocking but provides enough paths so
users are rarely blocked
• Busy Hour Call Attempts (BHCA)
– The number of calls the system can handle during peak
hour
• Concentration or line-to-trunk ratio
– Determines the probability that a call will be completed
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Telecom Infrastructure Hierarchy
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Telephone Cable Architecture
• Telephone Cable Hierarchy
– Trunks (in North America, that are same as
“Junctions” in Europe)
• High-speed digital carriers that interconnect nodes
– Feeders
– Branch Feeders
– Station Drops (local loops, subscriber lines)
• One pair of UTP wire that is usually analog
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T-Carriers and their
Transmission Capacity
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Optical Carriers and their
Transmission Capacity
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Line Conditioning
• Line Conditioning
– Is used to tighten telephone company parameters so
that they can transfer data at higher speed with
reduced errors
• Propagation delay
– Time taken by a signal to travel from source to
destination and “envelope delay distortion” measures
the variance in propagation delay within the voice
band
• Attenuation distortion
– Gain fluctuations with frequency
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The Telephone
• Telephony
– Science of translating sound into
electrical signals
• Tip and Ring
– Transmit and Receive wire that connect
the instrument to a plug in the wall
using RJ-11 jack
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The Telephone: Tip and Ring
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Outgoing Call
• Pulse Dial
– In general, pulse repetition rate is between
8 and 11 pulses per second (pps)
• Dual Tone Multiple Frequency (DTMF)
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Most commonly used signaling system today
More reliable and faster than “pulse dial”
Transmission rate is 7 digits per second
Consists of a frequency matrix
• Multi-Frequency (MF)
– Used on trunk circuits
– Transmission rate is 7 digits per second
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Incoming Call
• Ringer Equivalence Number (REN)
– Is used to ensure that the local exchange can
provide the correct amount of power required
to ring the telephone
• The Ring voltage is about 90 to 105 volts AC
with a frequency of 20 Hz
• The –48 volts DC that is always on the line
operates the telephone when it is being
used
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Line Signaling: Loop Start
• Current flows only when the phone is off-hook
• Local exchange senses that and provides a dial tone
• No need for accurate ground references between the
local exchange (remote end) and the telephone (local
end)
• Tip and Ring wires may be reverse
• Problem of “glare” (when both the local end and the
remote end attempt to access the circuit at the same
time)
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Line Signaling: Ground Start
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Usually used only on trunks and PBXs
Minimizes the possibility of “glare”
Tip and Ring wires cannot be reversed
Local end and remote end must be at the
same potential
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Trunk Signaling
• Out-of-band
– Separate network to pass call setup, charging, and
supervision information
• In-band
– Carries call setup, charging, and supervision information
over the same circuit
• Advantages of out-of-band over in-band
– Lower susceptibility to fraud
– Lower setup time
– Capable of supporting virtual networks
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In-band Signaling Methods
• Single Frequency
– Most common in-band analog signaling system
– Idle or busy status indicated by the presence or absence of
a 2600 Hz tone in the U.S.
• E&M Signaling (recEive and transMit)
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Used on digital four-wire circuits
Type I: Common in North America
Type II: Usually on Centrex circuits
Type V: Most popular outside North America
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Out-of-band Signaling Method
• Common Channel Signaling
– Most common out-of-band signaling system
• Signaling System Seven (SS7) Standard
– HDLC-based protocol developed by CCITT
– Uses layered protocol that resembles the OSI model
– Message Transfer Part of SS7 (bottom three layers of
OSI) Telephony User Part (top four layers of OSI)
– Components:
• Service Switching Point (SSP) or Action Control Point (ACP)
• Signal Transfer Point (STP)
• Service Control Point (SCP) or Network Control Point (NCP)
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Network Design Parameters
• Grade of Service (GoS)
– Ratio of the number of lost calls to the total
number of attempted calls, same as the
probability of blockage.
– The lower the number the better the system (A
GoS of 0.01 is better than a GoS of 0.05)
Grade of Service = Number of lost calls
Number of attempted calls
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Network Design Parameters
continued…
• Estimated Traffic
– Traffic is the term that quantifies usage. Usage
or total traffic intensity is measured in centicall seconds (CCS) = 100 call seconds of traffic
in one hour. 36 CCS = 100% utilization
• Network Design
– Trade-off between cost and quality of service
– Optimum designs: cost-savings while
maintaining quality
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