Transcript The public network

Lecture 7
The Public Network
Chapter Outline
• Switched Services
• Dedicated Services
• Signaling
Public Switched Network
• The public switched network is:
– crucial to a countries national security
– central to a healthy, growing economy
– essential in times of public emergencies and
natural disasters
Switched Services
• Dial-up: users dial a number to create a
temporary network connection
• Anyone on the network can be reached by
anyone by dialing a number
• Pay-for-use service for voice, image and
data traffic, analog or digital
• When call completed, network free for next
person
Dedicated Services
• Dedicated, private lines fixed configuration
• Clearly defined boundaries, users fixed
• Used for voice, video, and data, analog or
digital
• Users pay flat monthly fee
• Dedicated to an exclusive set of users
Definition Switching
• Premises equipment which performs the
functions of establishing and releasing
connections on a per call basis between two
or more circuits, services or communication
systems
ITU Definition Switching
• The establishment on demand, of an
individual connection from a desired inlet
to a desired outlet within a set of inlet and
outlet for as long as is required for the
transfer of information
Dialing
• Anytime dialing is done switching occurs
• Switched services are completed in a central
office (CO)
– where calls are routed based on the number
dialed
• Switched services have brand names:
UniPlan OneRate, Real Solutions
Attributes Real Time Switching
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Addressing
DTMF
Pay-as-you-go Postalized Rates
On Demand
Immediate
Analog or digital
Incoming, outgoing, two-way
Addressing-Flexibility
• The telephone number is an address
• Numbers are sent in dual tone multifrequency tones (DTMF)
• Tones are decoded at CO to address signals
– area codes are assigned to metropolitan areas
– exchanges are assigned to a CO
– last four numbers assigned to individual users
Number Portability
• Numbers within an exchange assigned to
different CO’s if users moves
– if a customer changes from their local
telephone company to a CLEC, their phone
number will be assigned to a particular central
office, the last four digit of their number will be
assigned to equipment associated with their
carrier
Transmitting Data w/ Switched Service
• Flexible
• Can be used with multiple locations and
offers many choices of medium
– ISDN
– Switched 56
– POTS
• Frequently used in teleconferencing
Switched 56
• Switched data service which lets you dial
someone else and transmit at 56kilobits p/s
• circuit switched service
• full duplex transmission of data
• digital synchronous 56kbps for price of
telephone call
• most widely used switched digital service in
North America
DTMF
• 1963: Touch tone or Dual Tone
• Prior to 1963: Pulse tone utilized
– Touch tone: dials 10 digits in 1 seconds
– Pulse tone: dials 10 digits in 11.3 seconds
• Adds efficiency to the network
• Speeds up processing time for CO switches
during set up
DTMF
• Used to:
– access voice mail
– bank accounts
– entering orders for home shopping
• from telephones, once connected..
• DTMF tones passed along in system to
interface with voice mail, etc..
DTMF Signals
• are a standard established by ATT
• Provides consistent functionality and format
• Continues to be expanded upon
Pay-as-you-go Service
• Dialing addresses a call in the network
• DTMF tones establish a temporary private
connection
• Services are charged on the amount of time
calls are connected
• Time-of-day rates may vary, peak hours
• Network based on some average usage
Batch or Bulk Usage
• To save on usage charges
• Retailers sent days transactions during offpeak hours, midnight
Usage Costs
• Used to be based on distance of call
• Long distance used to be the most
expensive
• No longer distance sensitive
• Flat rate, postal rate for service today in LD
• Now based on time
Immediacy/Urgency
• Given capacity, usage is instantaneous
• Capacity is influenced by extreme weather
conditions and power sources
• Safety:
– remote alarm monitoring systems
– back-up systems
The “Last Mile”
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Copper
ISDN
DSL
Cable Modem Service
The final cable connection to a home
ISDN
• Switched digital service
• Basic rate ISDN with two paths supporting
voice or data and a third slower path
dedicated to signaling
DSL
• Digital subscriber line services
• Works on copper cabling
• Not a switched service, is always available
in the “last mile” on a dedicated path from
the telephone company to the customer
• Connection always on!
Cable Modem Service
• Non-switched connection
• Always on data communications and
Internet connection
• High-speed services
• One-way being converted to two-way
– Huge investment
Design Assumptions PSTN
• Public Switched Telephone Network
– Assumptions:
• Based on voice traffic where at any given time not
every telephone user would be on a call
• The average call used to be 3 minutes, today the
average call lasts 20 minutes, some last 12-24 hours
• Today there are more frequent calls and longer calls
• Increased demand on public switched network will
need expensive upgrading
Inefficiency of Circuit Switching
• Switching of circuits results in use of a
connection for the duration of the call
• A path is established between callers
• Not shared
• Capacity dedicated to the call
• Wasteful, can’t share usage when there are
pauses in conversation, etc.
Dedicated Services
• Available for exclusive use of owner
• Placed at predetermined locations
• Cheaper than switched circuit use when
volume is high
• Flat fee per month, usage not charged by
volume
• Very secure transmission of data
Attributes of Dedicated Lines
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Fixed monthly fee
Fixed routes
Exclusive use
24-hour per day availability
Voice, video, and data
Fixed capacity
Analog or digital
Fixed Routes
• Dedicated circuits are not shared
• Put into place to exclusively transport traffic
from one location to the next
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Video-conferencing
Retail stores to warehouse
Manufacturing plants to dealers
Global companies
Voice, Video and Data
• Firms often lease T-1 lines to have 24
channels to tie two locations together
• May use 10 data and 14 for voice or video
Security
• No problem transmitting proprietary or
financial information
• Added encryption may be added
Applications Dedicated Services
• Video transmission to multiple sites
• Transfer of customer calls between sites
within a company
• Transmission of orders to factory
• Bulk transmission of x-ray images
• Database access between different sites
Topologies Dedicated Lines
• Point-to-point: one line, two locations
• Multi-point: more lines connecting several
sites
• Star configuration: all locations connect to
a central site
• Mesh design: all points on network connect
to each other
Pricing
• Based on distance and speed of medium
• Must add in cost of employing technical
staff to maintain network
• Organizational expertise needed to design,
implement and maintain the system
Outsourcing
• Become a hot business
• Companies hire outside expertise to manage
network and equipment
– need expertise to select: multiplexers, routers,
modems that connect to computer networks
– Vendor then becomes responsible for problems
with the network
Virtual Private Networks
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Easy way to link various sites
Acts like a private network
Software defined
Can be mixed with usage of PSTN
Signaling
• Tones in the network have special
meanings:
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dial tone
ringing
busy signal
DTMF tones
Signaling
• Process of sending information between two
parts of the network to control, route and
maintain a telephone call
• Pertains to the transmission of electrical
signals to and from the user’s premises and
the telephone company central office
Types of Signals
• Supervisory signals: monitor busy or idle
condition of the phone
• Alerting signals: bell signals, tones,
strobes, lights alerting that a call has arrived
• Addressing signals: touch tones or pulse
tones telling network where to send a call
In-band Signaling
• Signal sent over the same channel as the
voice channel
• Used extensively prior to 1976
• Inefficient use of phone line
• Ties up line to set up a call
– 20-30% of attempted calls never reach
destination due to busies or no answers
• Slow call set up
Out-of-band Signaling
• Common channel interoffice signaling
• A separate network laid over the switching
network
• Signaling done on a separate channel
– Includes:
• routing instructions
• data base information
• specialized programs
Signaling System 7 (SS7)
• Signaling system based on CCIS
• Instrumental in lowering barriers to entry
• Routing intelligence migrated from
expensive switching equipment to lower
cost computer based peripherals
• Most significant innovation in public
switched network
Advantages SS7
• Problems can be detected remotely
• Network information sent to centralized
network maintenance location
• Network can be dynamically reconfigured
• Central offices not required to maintain
sophisticated routing information
Added Features
• Voice-activated dialing for calling cards
• Automated roaming on cellular telephone
networks, roaming locations in database
• Custom calling features
• Load balancing by call volume
• Detailed record keeping per call
End and Tandem Offices
• Local phone company has 2 types offices:
– Tandem Office:
• connect central office to central office
• central office to inter exchange carrier
• volume high
– End Office:
• connect homes to central office equipment
• volume low, last to get switch upgrade
• caller id information sent on this last connection
SS7 Glue between Carriers
• Enables all carriers to work with each other
• Standard protocol approved by ITU
SS7 Components
• Signal transfer points
• Service control points
• Service switching points
Signal Transfer Points
• Packet switches that route signals between
central offices and databases
• One link can support multiple call paths
• Function of transferring signaling messages
from one signaling link to another and
considered exclusively from the viewpoint
of the transferer
Service Switching Points
• Enable CO’s to initiate queries to databases
and specialized computers
• Software capable of sending specialized
messages
– 900 # dialed, query sent to 900 database, which
has information on how to route the call
• Encodes and decodes switching information
to switches from databases
Service Control Points
• Hold specialized databases with routing
instructions for each call based on calling
party
• In cellular networks holds roaming
information
• Network based voice-mail and fax
applications are located on service control
points
SS7 in IP-Based Networks
• Networks are being built to support Voice
over IP based networks
• Gateways are required to translate and
deliver SS7 signaling messages between
PSTN and IP based networks