lecture 7 the public network. chapter outline switched services dedicated services signaling
TRANSCRIPT
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Lecture 7
The Public Network
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Chapter Outline
• Switched Services
• Dedicated Services
• Signaling
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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
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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
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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
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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
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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
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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
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Attributes Real Time Switching
• Addressing
• DTMF
• Pay-as-you-go Postalized Rates
• On Demand
• Immediate
• Analog or digital
• Incoming, outgoing, two-way
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Addressing-Flexibility
• The telephone number is an address
• Numbers are sent in dual tone multi-frequency 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
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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
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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
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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
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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
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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..
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DTMF Signals
• are a standard established by ATT
• Provides consistent functionality and format
• Continues to be expanded upon
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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
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Batch or Bulk Usage
• To save on usage charges
• Retailers sent days transactions during off-peak hours, midnight
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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
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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
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The “Last Mile”
• Copper
• ISDN
• DSL
• Cable Modem Service
• The final cable connection to a home
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ISDN
• Switched digital service
• Basic rate ISDN with two paths supporting voice or data and a third slower path dedicated to signaling
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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!
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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
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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
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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.
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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
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Attributes of Dedicated Lines
• Fixed monthly fee
• Fixed routes
• Exclusive use
• 24-hour per day availability
• Voice, video, and data
• Fixed capacity
• Analog or digital
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Fixed Routes
• Dedicated circuits are not shared
• Put into place to exclusively transport traffic from one location to the next– Video-conferencing– Retail stores to warehouse– Manufacturing plants to dealers– Global companies
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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
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Security
• No problem transmitting proprietary or financial information
• Added encryption may be added
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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
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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
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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
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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
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Virtual Private Networks
• Easy way to link various sites
• Acts like a private network
• Software defined
• Can be mixed with usage of PSTN
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Signaling
• Tones in the network have special meanings:– dial tone– ringing– busy signal– DTMF tones
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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
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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
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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
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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
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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
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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
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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
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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
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SS7 Glue between Carriers
• Enables all carriers to work with each other
• Standard protocol approved by ITU
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SS7 Components
• Signal transfer points
• Service control points
• Service switching points
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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
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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
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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
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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