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Lecture 7

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

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Lecture 7

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  1. Lecture 7 The Public Network

  2. Chapter Outline • Switched Services • Dedicated Services • Signaling

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. Attributes Real Time Switching • Addressing • DTMF • Pay-as-you-go Postalized Rates • On Demand • Immediate • Analog or digital • Incoming, outgoing, two-way

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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..

  16. DTMF Signals • are a standard established by ATT • Provides consistent functionality and format • Continues to be expanded upon

  17. 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

  18. Batch or Bulk Usage • To save on usage charges • Retailers sent days transactions during off-peak hours, midnight

  19. 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

  20. 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

  21. The “Last Mile” • Copper • ISDN • DSL • Cable Modem Service • The final cable connection to a home

  22. ISDN • Switched digital service • Basic rate ISDN with two paths supporting voice or data and a third slower path dedicated to signaling

  23. 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!

  24. 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

  25. 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

  26. 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.

  27. 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

  28. 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

  29. 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

  30. 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

  31. Security • No problem transmitting proprietary or financial information • Added encryption may be added

  32. 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

  33. 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

  34. 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

  35. 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

  36. Virtual Private Networks • Easy way to link various sites • Acts like a private network • Software defined • Can be mixed with usage of PSTN

  37. Signaling • Tones in the network have special meanings: • dial tone • ringing • busy signal • DTMF tones

  38. 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

  39. 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

  40. 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

  41. 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

  42. 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

  43. 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

  44. 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

  45. 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

  46. SS7 Glue between Carriers • Enables all carriers to work with each other • Standard protocol approved by ITU

  47. SS7 Components • Signal transfer points • Service control points • Service switching points

  48. 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

  49. 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

  50. 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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