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Telephone Switching
Basics and Back to the Future (Voice
over Internet Protocol, VoIP)
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Why is VoIP important?
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Data Growth Relative Volume vs. Revenue
Bits per second per dollar invested
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History of Telephone
Switching
Users would ring an operator and askto be connected to a given line
The operator would then run a cableconnecting his/her line to the called-partys line
Operators may barge-in to let youknow of and incoming call (VERY earlycall-waiting signal)
Limited number of lines able toconnect
Challenging calling queue foroperators
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Fairness of Attention to Callers
Allmon Strowgers switch
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Dialing to reach a party
A pulse advances the relay one step
Train of pulses constitute the phone
number of the party
Beginnings of a dialing plan (P6-5000)
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Switch bank
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Call Aggregation Trunking
Impractical to build, house and maintain
bigger switches
Need to interconnect switches within a region
(urban sprawl)
Scale-up the volume of calls (make more
revenue)
Tap into the Long Distance market
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DS0 Voice Sampling
Voice band: 20Hz to 4000HzNyquist sampling rate: sample at
least twice as fast as the highest
frequency in the band
Hence, sample at 8000Hz
But, do it for several
conversations (channels), somultiplex
Add framing information (In-
band signaling)
8000*193=1.54MHz
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Time Division Multiplexing (TDM)
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E1 T1 Signaling Comparison
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8000*256=2.048 MHz
8000*193=1.544 MHz
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Digital Signal Hierarchy
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Meanwhile, at the Central Office (CO)
Telcos (Telephone companies) started selling newservices, suitable for small business start-ups,banks and financial institutions, temporarylocations, to name a few.
It wasnt necessary for any switching equipmentto be installed on the customer premises (noPremises Equipment, PE).
Stations inside the service group could call each
other with 3, 4 or 5 digits, depending on howlarge the group, instead of an entire telephonenumber.
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Kinds of Centrex Services
Call Transfer
Call Divert on no reply /on busy
Call Waiting
Three Party Conference
Call Pick Up (Group) Ring Back
Reminder / Alarm call
Last Number Redial
Centrex Hotline (non-dialed connection)
Centrex Warm Line (delayed Hotline) Centrex Hunt Groups, with optional bypass numbers,
AND AND
Automatic Call Distribution (ACD)
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Miniaturization
In the early seventies, the advent ofthe microprocessor unveiled a
plethora of switches targeted to
Government offices, hotels,
universities and Fortune 500
companies.
Depending on the customers
requirements, they were connected by
one or more T1 trunks.
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For Smaller Customers
A key system or key telephone system is amultiline telephone system typically used in small
office environments. They usually offered the
following functions:
Answering machine functions
Remote supervision of the entire system
Automatic call accounting
Speed dialing
Caller ID
Station-specific limitations (such as no long
distance access or no paging)
Selection of signaling sounds
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Key Service Unit (KSU)
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TELEPHONE SWITCHING PART 2
Now back to our regular programming
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Circuit Switching vs.
Packet Switching
Circuit switching is part of an ordinaryphone call
Each user has sole access of a circuit
during network use
3 Phases: Set-up, use, disconnect
Set-up is achieved by dialing
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Inefficient use of resources when
circuit is used to transfer information
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Packet switching
Breaks messages into packetsPackets can traverse the network
individually
Packet == Datagram == PDU
Protocol Data Unit (PDU). More on
PDUs later
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Role of Network Protocols
Protocols establish actions on processes occurringin different network devices
The format or structure of the message
The method by which networking devices shareinformation about pathways (trunks, links) withother networks
How and when error and system messages arepassed between devices
The setup and termination of data transfersessions
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Layering and Models
Using a layered model to describenetwork operations can aid in
Protocol design.
Allowing products from different
vendors work together.
Preventing any changes in one layer
from affecting other layers above and
below.Providing a common language to
describe networking functions and
services.
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Internet Protocol (IP)
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Message Encapsulation
As application data is passed down theprotocol layers on its way to be
transmitted across the network various
protocols add information to it at each
level.
On the receiving computer, the
opposite process reassembles thePDUs into the necessary data for the
peer application to run.
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A brief digression
The Internet Protocol grew up as part of theinter-networking efforts of the labs working forthe Defense Advanced Research Program Agency(DARPA).
At the same time the InternationalTelecommunications Union (ITU) were developinga set of protocols and interfaces definitions forthe Integrated Services Digital Network (ISDN) so
digitized voice and packet data would seamlesslyintegrate within the Public Switched TelephoneNetwork (PSTN)
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Integrated Services Digital
Network
A circuit-switched telephone networksystem.
Also provides access to packet
switched networks.
Designed to allow digital transmission
of voice and data over ordinary
telephone copper wires.
Bearer (B) channels carry voice ordata streams at DS0 rate.
Delta (D) channel carries signaling
and control information.
Out-of-band signaling
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ISDN Promises
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Virtual Circuits
For packet switching networks (X.25, Frame
Relay), even though the packets are
independent units, routing is assured for them
Those constitute Virtual Channels
In Asynchronous Transfer Mode (ATM) packets
are reduced in size down to 53 bytes (48
payload)
Virtual Channels now become Virtual Circuits
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Demise of Broadband ISDN (B-ISDN)
Fiber optic deployment among Telcospromised to provide greater throughput tocustomers
The problem with ATM was in porting servicesand applications into their own services
ATM is, nevertheless, used as the premier
packet transport mechanism, for instance inDigital Subscriber Loop (DSL) and othernetworks
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TELEPHONE SWITCHING PART 3
Once again, back to our regular programming
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What is a Carrier?
The 1996 Telecommunications Actsought to open up markets to
competition.
Key fact: Interconnectedness
entrants could interconnect their
networks with those of the incumbent
carriers.
Competing Local Exchange Carriers
(CLECs) connect their equipment at
the main distribution frame where the
plain old telephone systems (POTS)
exchange equipment connects with
the outside plant (user telephone
lines).
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Main Distribution Frame (MDF)
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A much briefer digression
CLECs connect their networks at thelocal exchange (LE) or class 5 of the
Switching Hierarchy.
Local Number Portability (LNP) was
mandated by the 1996 Act, allowing
subscribers to change service to a
different provider and keep the same
phone number.
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Switching Hierarchy and Numbering Plan
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Success factors in VoIP
Standards
For interconnection to the PSTN
For the interconnection of IP-based components
that emulate and replace the functionality of thelegacy equipment: signaling, call setup and tear
down, number portability, etc.
A CLEC doesnt have to spend millions on PE,but can provide some of the telephone
network functions at competitive prices
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Ral Hazas
VoIP Topology
The Media Gateway is a router withtrunk capabilities; converts PSTN DS0
streams into IP packets.
A class 5 Service Switching Point (SSP)
is responsible for call setup,
management, and termination with
other SSPs (SSP==CO). Provide dial
tone to subscribers.
In order to provide LNP, phone
companies subscribe to the services of
an independent administrative entity
who holds the management rights for
up to 5 years.
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