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 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 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

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 off- peak 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” 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 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 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 –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 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: –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 voic 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