Basic Telephone Subscriber Line Telephone T dial switch

Basic Telephone Subscriber Line Telephone T dial switch cradle switch off-hook mouth ear R DTMF CO (Local Exchange) tip (+) on-hook C ringer SLIC ring (-) 1

Subscriber Line Interface Circuit ring switch Telephone T tip (+) Subscriber Line ring (-) hybrid current detector crossconnect switch control channel ring generator (100 Vrms 25 Hz) ~ -48 VDC Processor Call States idle dialing calling ringing called on hook dialing in progress after dialing incoming call in progress 2

PSTN Review The PSTN circa 1900 pair of copper wires “local loop” manual routing at local exchange office (CO) • Analog voltage travels over copper wire end-to-end • Voice signal arrives at destination severely attenuated and distorted • Routing performed manually at exchanges office(s) • Routing is expensive and lengthy operation • Route is maintained for duration of call 3

PSTN Review Multiplexing 1900: 25% of telephony revenues went to copper mines n n n standard was 18 gauge, long distance even heavier two wires per loop to combat cross-talk needed method to place multiple conversations on a single trunk 1918: “Carrier system” (FDM) n n n 5 conversations on single trunk later extended to 12 (group) still later supergroups, master groups, supermaster groups 1963: T-carrier system (TDM) n n n channels f timeslots T 1 = 24 conversations per trunk later T 3 = 28 T 1 s still later SDH rates with 1000 s of conversations per trunk t 4

Cross-connect switch Analog Crossbar switch 1 2 3 4 5 6 7 1 2 3 4 8 Digital Cross-connect (DXC) 1 2 3 4 5 t processor 5 6 7 2 1 5 4 3 t Complexity increases rapidly with size 5

Basic TDM CO DXC Subscriber Lines SLIC Hybrid 4 w 2 w SLIC Hybrid PCM MUX TDM port Tone & Ann TDM port E 1/T 1 TDM port DTMF decode Processor 6

PSTN Review PSTN Topology local loop Local Exchange Long distance network subscriber line trunk circuit Local Exchange Many local telephone exchanges had sprung up Bell Telephone acquired them and interconnected them for long distance 7

PSTN Review Old US PSTN Regional centers Class 1 Class 2 Class 3 Class 4 Sectional centers Class 2 Class 4 Class 3 Class 4 Primary centers Toll (tandem) offices circuits, trunks Class 5 local loop Class 5 subscriber lines Class 5 Central (end) offices last mile Class 5 switch is the sole interface to the subscriber lines 8

Numbering Plans An E. 164 International Number has the format : Country-Code Area-Code Exchange-Code Line-Number MAXIMUM 15 DIGITS EX: 972 2 588 9159 Country-Codes 1 2 3 4 5 6 7 8 9 : : : : : N America (US, CA, Caribbean) 1 digit Africa 2 or 3 digits 20 Europe 2 or 3 digits 31 Europe 2 or 3 digits 44 S America 2 or 3 digits 54 Australia & S Pacific 2 or 3 digits 61 Russia 1 digit China & N Pacific 2 or 3 digits 86 Middle East 2 or 3 digits 90 Egypt 27 South Africa 235 Chad Netherlands 354 Iceland UK 49 Germany 420 Czech Republic Argentina 595 Paraguay Australia 675 Papua PR China 855 Cambodia Turkey 972 Israel 9

PSTN Review Signaling PSTN with automatic switching requires signaling The present PSTN has thousands of features and all require signaling support Examples: On-hook / off-hook Pulse / Tone dialing Receiver off-hook Call waiting Caller number identification Call forwarding Hook-flash Fax transmission detect Inter-CO messaging Echo cancellation Voice mail Conference calls Coin-drop Billing 10

PSTN Review Signaling Methods Signaling can be performed in many ways n Analog voltage signaling loop-start, ground-start, E&M n In-band signaling DTMF, MFR 1, MFR 2 n Channel associated signaling (CAS) AB bits, ABCD bits n Common channel signaling (CCS) SS 7, QSIG – Trunk Associated CCS – Separate signaling network CCS 11

Subscriber - Exchange Signaling* n n On/off hook DC current flow Dial-tone 350+440 continuous Pulse Dialing DTMF L 1 697 H 1 1209 L 2 770 H 2 1336 L 3 852 H 3 1477 L 4 941 H 4 1633 make off-hook >100 ms (60: 40) >700 ms break H 1 L 1 1 L 2 4 L 3 7 L 4 * H 2 2 5 8 0 H 3 3 6 9 # n Ring AC with cadence Ring-back 440+480 with 2: 4 cadence Busy 480+620 with 1/2: 1/2 cadence Trunk busy 480+620 but 0. 2: 0. 3 cadence Receiver off-hook 1400+2060+2450+2600 with ). 1: 0. 1 cadence n Invalid (nonworking, unobtainable) number n n H 4 A B C D announcement, SIT-tone * US cadences - national differences 12

Subscriber - Exchange Signaling n n Hook-flash on-hook for 0. 1 - 1 sec Echo suppressor disabler 2100 continuous – FAX CED 2100 2. 6 - 4 sec – Modem ANS 2100 with phase reversals every 450 ms for 3. 3 s n n – ANSAM 2100 with phase reversals and 15 Hz AM modulation CNG – FAX 1100 with 0. 5: 3 cadence – Modem 1300 with 0. 5: 2 cadence Caller Line Identification (CLI/CND) – 1200 bps FSK (V. 23) data signal 1300 = 1 2100 = 0 – Between 1 st and 2 nd rings RING >300 ms CLI > 475 ms RING 13

Supervision procedures n FXO/FXS Foreign Exchange Office / Foreign Exchange Subscriber FXS is like exchange - provides voltage, ringing, dialtone FXO is like a phone - requires voltage, detect ring, etc. n When connecting PBX to CO, PBX is FXO, CO is FXS n Analog loop start n Digital loop start n Analog ground start n Digital ground start n E&M (wink) 14

Interexchange Signaling n CAS - R 2 n CAS - R 1 n CCS - SS 7 15

PSTN Review Optimized Telephony Routing Circuit switching (route is maintained for duration of call) Route “set-up” is an expensive operation, just as it was for manual switching Today, complex least cost routing algorithms are used Call duration consists of set-up, voice and tear-down phases 16

PSTN Review The PSTN circa 1960 trunks circuits local loop subscriber line automatic routing through universal telephone network • Analog voltages used throughout, but extensive Frequency Division Multiplexing • Voice signal arrives at destination after amplification and filtering to 4 KHz • Automatic routing • Universal dial-tone • Voltage and tone signaling • Circuit switching (route is maintained for duration of call) 17

PSTN Review The Digitalization of the PSTN Shannon (Bell Labs) proved Digital Communications Analog is better than Communications and the PSTN became digital Better means n More efficient use of resources (e. g. more channels on trunks) n Higher voice quality (less noise, less distortion) n Added features 18

PSTN Review Timing In addition to voice, the digital PSTN transports timing n This timing information is essential because of – the universal use of TDM – the requirement of accurate playback (especially for fax/modem) n Receiving switches can recover the clock of the transmitting switch n Every telephony network has an accurate clock called “stratum 1” Clocks synchronized to it are called “stratum 2” Clocks synchronized to them are called “stratum 3” and so on 19

PSTN Review The Present PSTN core backbone subscriber line PSTN Network • Analog voltages and copper wire used only in “last mile”, but core designed to mimic original situation • Voice signal filtered to 4 KHz at input to digital network • Time Division Multiplexing of digital signals in the network • Extensive use of fiber optic and wireless physical links • T 1/E 1, PDH and SONET/SDH “synchronous” protocols • Signaling can be channel/trunk associated or via separate network (SS 7) • Automatic routing • Circuit switching (route is maintained for duration of call) • Complex routing optimization algorithms (LP, Karmarkar, etc) 20

PSTN Review Nonvoice services The PSTN can even be used to transport non-voice signals such as FAX or Vo. P course DATA PSTN n These services disguise themselves as voice by using a modem n Proper timing is essential n Special signaling is required – turn off LEC – turn off call waiting – service recognition – capabilities negotiation – mutual identification – end of page/document – modem recognition – modem training – data compression 21

PSTN Review Digital Loop Carrier Pushes the digital PSTN closer to customer AT&T SLC-40, SLC-96, Nortel DMS P-phone, TR-08 Mode 1 pair-gain: Replace 96 pairs with 5 T 1 s Access Network CLASS 5 “pair-gain” (one spare for “span protection”) UTP/coax/fiber Street FTTB/FTTC cabinet CPE 96 – 10 = 86 TR-08 Mode 2 pair-gain: Replace 96 pairs with 2 T 1 s pedestal UTP (without “span protection”) 96 – 4 = 92 TR-08 multiplex 96 lines on: Mode 1: 4 T 1 s Mode 2: 2 T 1 s (2: 1 concentration) GR 303/V 5. 1/V 5. 2 multiplex up to 2048 lines 22