Telephone Switching Systems

1. Telephone Switching Systems

2. Analog systems • History- first they used operators then--- • The dial phone was invented in the 1880s by Almond Brown Stroger, who was a Kansas City, MO undertaker and was convinced that the Bell Telephone operator was sending calls for his funeral home to the operator's brother-in-law... Stroger invented the dial telephone and installed automatic exchanges in the US and Europe. In 1924, the Bell Telephone System decided that using operators was NOT the way to go, and they licensed Stroger's technology. The step by step switch used to receive the dial pulses is/was called a Stroger Switch, after its inventor. • Systems improved

3. Prior to the breakup of AT&T5 classes of switches existed the in Bell system • Class 1exchanges were international gateways - handing off and receiving traffic from outside the USA and Canadian networks. • Class 2exchanges were tandem exchanges which interconnected whole regions of the AT&T network. • Class 3exchanges were tandem exchanges connecting major population centers within particular region of the AT&T network.

4. Switch classes • Class 4 or Tandem switch is used for long distance communications in the Public Switched Telephone Network (PSTN) to interconnect Class 5 switches. • Class 5 switches are located at the local telephone company or Central office and this switch directly serves the customer. Class 5 switch services include basic dial-tone, and feature sets which came at an additional monthly charge. The local branch office could provide additional digital and data services to subscribers.

5. STANDARDS • Signaling System 7 brief review • Prior to SS7 there was SS6 • with the AT&T breakup • Switching systems could now be manufactured by others

6. History of PBX systems • Initially AT&T had a lock on switching systems and they were located at the COs • Deregulation allowed for competition • Open to competition • the AT&T hierarchy had 5 levels of switching called classes from beginning with the incoming call to relaying across country. • The beginning and end point of a call occurred at the class 5 switch called the central office or “Branch Exchange”

7. Introduction of Private Branch Exchanges • PABX - Private Automatic Branch eXchange • EPABX - Electronic Private Automatic Branch eXchange • CPBX • AT&T • Siemens • Nortel

8. Why buy a PBX • Initially, the primary advantage of PBXs was cost savings on internal phone calls: handling the circuit switching locally reduced charges for local phone service. As PBXs gained popularity, they started offering services that were not available in the operator network, such as hunt groups, call forwarding, and extension dialing. • Standard phone lines ran about $30/mo (page 117) • 200 Centrex lines X $30/mo = $6000/monthly charges for phone lines

9. Costs • Prior to owning a PBX • 200 Centrex lines X $30/mo = $6000/monthly charges for phone lines • After cutover • 20 DID (Direct inward Dial) lines ~ $25/mo = $500/mo • 10 CO ~ $30/mo =$300/month • Monthly savings $5,200 • HOWEVER • System costs capital and recurring (maintenance) • Growth • Plans were based on a 10 life cycle

10. Centrex • Centrex was a service provided to customers who wanted additional features rather than a basic phone line. • Features like: • Speed dial, last number redial, call hold, call transfer, 3 way calling... • At the time FVTC purchased its PBX, Centrex could not offer the full feature set that the PBX could and there were costs associated with adding features to a Centrex line.

11. Case history • ~1980 FVTC purchased internal lines for $1 • ~1981 phones were turned over to the college • Bid went out in ~1984 • System 75 by AT&T was selected • Based on feature set • Price • Support • Reputation

12. Initial Configuration • 10 CO trunks • 20 DID trunks • 200 internal Lines • Chose long distance carrier • STS under the UW contract • Still used today • System Administration • Programming new lines • Cross connecting from the PBX on out • Install phones and test • Run monthly bills • check for abuse

13. Cutover • AT&T implemented a riser cable from the PBX to the MDF • 200 lines dropped from Ameritech • 200 lines cross connected from MDF to IDFs via riser cables • Station lines connected to the IDF • Each line was pre-tested • Made our own test sets • Checked each pair on the risers • Tested for dial tone and communications to the PBX • dial tone • Break dial tone • President’s line

14. System still in place today • Upgraded through the years 75-85 • Now it is a Definity Class switch • capable of VoIP • _1500__ of internal lines • __5_ PRIs (23 lines used per PRI) • DIDs ( within PRIs ~ 1000 numbers) • _12__COs backup • _0_Tie • _0_T1 • _0_DS1 • _100__Mb trunk

15. Analog and Digital Systems • Two significant developments during the 1990s led to new types of PBX systems. One was the massive growth of data networks and increased public understanding of packet switching. Companies needed packet switched networks for data, so using them for telephone calls was tempting, and the availability of the Internet as a global delivery system made packet switched communications even more attractive. These factors led to the development of the VoIP PBX. (Technically, nothing was being "exchanged" anymore, but the acronym PBX was so widely understood that it remained in use.)

16. Hybrid and Digital systems • Even the System 75 was capable of hybrid and digital phones. • Hybrid phones used both analog (POTS) lines for voice communications but also has a strong feature set with programmable buttons for speed dialing, automatic callback, hold, multi line appearance indicators... • Digital phones converted analog signals to digital and that is what was transmitted to the PBX. • The PBX converted it back to analog when dialing out to the PTSN or to another analog/hybrid phone. • Plus again a strong feature set of programmable buttons

17. Key systems • PBXs are different from Key systems in that users of key systems manually select their own outgoing lines, while PBXs select the outgoing line automatically. Key systems are much smaller in scope ~ 5 – 20 lines • A key system or key telephone system is a multi-line telephone set typically used in small office environments. Usually the numbers of incoming lines were exceeded by the number of extensions on the system. Thus, cost savings from monthly recurring costs could be realized. • Key systems are noted for their expandability and having individual line selection buttons for each connected phone line. Intercom capability was a standard feature allowing one or two button pushes to dial another office.

18. Trunking used to connect systems • CO • Tie • T1/DS1 • PRI

19. Hosted VoIP systems – States RFP • PBX services is hard for small companies to invest in. To maintain their own system internal telecom expertise is needed for planning and support. These considerations gave rise to the concept of hosted PBX. In a hosted setup, the PBX is located at and managed by the telephone service provider, and features and calls are delivered via the Internet. The customer just signs up for a service, rather than buying and maintaining expensive hardware. This essentially removes the branch from the private premises, moving it to a central location.

20. Review of Lab • RJ 11 is used in ___________ • RJ 45 is used in ___________ • What is the difference between the two connectors? • 66 Blocks • 110 Blocks • IDF • MDF

21. Cable Specifications • Number of pairs of wire • Gauge of wire • Diameter • Temperature range • Min. Bend Radius • Jacket material • Nominal capacitance • Nominal Velocity of Propagation • Impedance at 10 MHz • Plenum or non plenum rated

22. Trunking • Type of link • POTS line • DS0 • T1 • DS1 • DS3 • OC1 • OC3 • OC12

23. Basic continuity test • Matching up wire for wire • Resistance test for continuity • Not a complete picture of the cable’s required capabilities

24. reading assignment • Chapter 6 • Modulation and Multiplexing