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TDC 461: Basic Telecommunications

Learn the basics of Key Telephone Systems (KSU), PBX, traffic engineering, and voice processing. Explore features, call procedures, and differences between key systems and PBX setups.

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TDC 461: Basic Telecommunications

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  1. TDC 461: Basic Telecommunications DePaul University 17 April, 2001 LoriLee M. Sadler

  2. Agenda • Key Systems • PBX • Centrex • Traffic Engineering • Voice Processing

  3. Does Everyone Talk on the Phone at Once? • Lines vs. Trunks • Why not business lines for large business? • How to share? • PBX • Key Telephone systems

  4. Key Telephone Systems • Key System Unit (KSU) ties many multi-line sets into a set of C.O. lines. (generally assume 3 station sets to 1 business line) • User selects outside line by pressing corresponding line button on key set.

  5. Typical Key System Features • Call Transfer • Call Hold • Conference Calling • Consultation Hold • Put one party on hold while talking to another • Speed dialing • Intercom calling between key sets • Speakerphones / Hands-free calling • Automatic Call Information Recording

  6. A Call using a Key SystemFrank calls Sam in New York • Frank picks up phone, hits LINE #2 button • Key system connects set to C.O. line #2 • Frank hears dial tone from the C.O. • Frank dials 1-212-894-6622.

  7. A Call using a Key SystemFrank calls Sam in New York • C.O. routes call to New York, rings Sam’s phone. Sam answers phone. • Frank and Sam talk for a while

  8. A Call using a Key SystemFrank calls Sam in New York • Frank decides to consult with Jane • Frank hits HOLD button - Key System puts current call to Sam on hold. • Frank hits INTERCOM button and dials Jane’s set number

  9. A Call using a Key SystemFrank calls Sam in New York • Jane hears a special intercom ring • Jane and Frank talk

  10. A Call using a Key SystemFrank calls Sam in New York • Frank hits LINE #2 button again and now talks with Sam again.

  11. A Call into a Key SystemJoe calls Frank • Joe picks up phone, dials number associated with LINE #2 of Frank’s Key System

  12. A Call into a Key SystemJoe calls Frank • C.O. rings LINE #2 connected to KSU • KSU sends ringing signal out to all phones programmed for access to LINE #2

  13. A Call into a Key SystemJoe calls Frank • Frank answers phone and talks to Joe

  14. Private Branch Exchange (PBX) • PBX is a true intelligent switching system. • Provides local switching between station sets or access to C.O. Users dial access code (like ‘9’) to get C.O. service. • Provides advanced intelligent features to users. • Costs approximately $750-$1000 per station set

  15. PBX FeaturesNot available on Key Systems • Private Dialing Plans • 4-digit, special prefixes for WATS, FX, etc. • Automatic Route Selection • PBX collects dialed digits and intelligently decides how to route this call for lowest cost • Over FX line • Over WATS trunk • Over DOD trunk

  16. PBX FeaturesNot available on Key Systems • Voice Mail • Automatic Call Distribution (ACD) • Routes incoming calls (usually INWATS) to the best station set and location • Voice Response Unit (VRU) • Provides recorded messages and responds to touch-tone requests from callers

  17. PBX FeaturesNot available on Key Systems • Class-of-Service • Authentication Codes • Data Services

  18. A Call using a PBXFrank calls Sam in New York • Frank picks up the phone and gets dial tone from the PBX. • Frank dials 9-1-212-894-6622. • PBX consults routing tables, determines that this call should go over the FX trunk to New York

  19. A Call using a PBXFrank calls Sam in New York • PBX sends dialed number over FX trunk, dropping area code (sends “894-6622”). • Call to New York is completed. Frank and Sam talk.

  20. A Call into a PBXJoe calls Frank • Joe picks up phone. • Dials Frank’s public number • 1-312-362-6587 • C.O. chooses trunk to PBX (could choose ANY trunk).

  21. A Call into a PBXJoe calls Frank • C.O. sends OFF-HOOK signal to PBX over the trunk • PBX sends WINK-BACK signal to C.O. • C.O. sends last 4 digits - “6587” (this is a DID trunk).

  22. A Call into a PBXJoe calls Frank • PBX looks into its station address table and determines that “6587” corresponds to Frank’s station set. • PBX rings Frank’s station set.

  23. A Call into a PBXJoe calls Frank • Frank picks up telephone handset. • PBX sends ANSWER signal back to C.O. indicating that the call has been answered • Frank and Joe talk

  24. PBX Trunk Signaling • 1-way trunks: A call can only be initiated from one end (incoming or outgoing). • 2-way trunks: A call can be initiated from either end of the trunk (PBX or C.O.)

  25. PBX Trunk Signaling • Trunk signaling for PBX trunks: • Loop start : Like subscriber loop. • Ground start : Either side can ground a wire to initiate a call. Better control than loop start. • E&M : Separate wire pair used for signaling.

  26. PBX Trunk Types • CO Trunk (1-way outgoing) • Standard switched service. • WATS (1-way outgoing) • Switched service to limited geographic area. • INWATS (1-way incoming) • Trunk for incoming 800 number calls.

  27. PBX Trunk Types • DID (Direct Inward Dial) Trunk • 1-way incoming trunk • CO sends dialed number after call connect so PBX can automatically ring station. • DOD (Direct Outward Dial) Trunk • 1-way outgoing trunk • PBX repeats dialed number to C.O.

  28. Least Cost Routing • => • Rationale • How to get it:

  29. Centrex • Centrex is a service that imitates all PBX features at the C.O. • Private dial plans, special features, etc. are all available at a cost from your local telco.

  30. Centrex AdvantagesCompared with PBX system:

  31. Centrex DisadvantagesCompared with PBX system: • Customer has less direct control • Centrex may be more expensive if most calls are internal (between phones on PBX)

  32. Voice Processing • Automated Attendant • Automated Call Distribution • Voice Mail/Messaging • Interactive Voice Response

  33. Inputs • DTMF • Voice

  34. Automated Attendant • Enables technology to handle repeated requests for same information • Examples • Daytime, weekend, and evening dialogs

  35. Automatic Call Distributor • Purpose • When to use • How it works • What it does

  36. Configuration • Standalone— • As a peripheral • Centrex

  37. Benefits

  38. Messaging • Generally referred to as voice mail • Provides message taking and retrieval services • Broadcast • Many new voicemail systems have similar functions as e-mail: Mailing groups, message forwarding, message reply, etc.

  39. Configuration • Standalone system • PBX Integration • Vendors • Provisioning (trunking) • Storage Capacity

  40. Benefits of Messaging Systems

  41. Interactive Voice Response • Also known as Voice Response Unit (VRU) • Applications • PIN • Other peripheral combinations

  42. Benefits of IVR

  43. How much bandwidth do you need?

  44. Traffic Engineering • The process of modeling call traffic patterns in order to provide facilities and infrastructure to meet call demands. • Goal is to serve the maximum number of callers at a single point in time (generally a 1 hour period)

  45. Metrics • Busy hour • Overbuild

  46. Collecting Data • Telephone bills • 800/888 inbound calls. • Telephone bills for all outgoing calls.

  47. Other Data Sources • Call processing time: • Traffic reports from PBXs, ACDs, and voice mail systems • Busy study:

  48. Tools for Analysis • Erlang B: Developed in early 1900s. Assumes that some alternative to a busy signal is available, even if alternative is more expensive. Effective at calculating call completion and attempts on DOD. No queuing. • Extended Erlang B: No alternate route assumption. No queuing. • Poisson: Assumes even load over a group of circuits. Not very sensitive

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