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

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

The PSTN circa 1900 PSTN Review 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

Multiplexing f t 1900: 25% of telephony revenues went to copper mines PSTN Review Multiplexing 1900: 25% of telephony revenues went to copper mines 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) 5 conversations on single trunk later extended to 12 (group) still later supergroups, master groups, supermaster groups 1963: T-carrier system (TDM) T1 = 24 conversations per trunk later T3 = 28 T1s still later SDH rates with 1000s of conversations per trunk f channels t timeslots

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

Basic TDM CO DXC Subscriber Lines SLIC Hybrid TDM port TDM port Tone & Ann 2w 4w PCM MUX SLIC Hybrid E1/T1 E1/T1 TDM port TDM port DTMF decode Processor

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

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

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

Signaling PSTN with automatic switching requires signaling 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

Signaling Methods Signaling can be performed in many ways PSTN Review Signaling Methods Signaling can be performed in many ways Analog voltage signaling loop-start, ground-start, E&M In-band signaling DTMF, MFR1, MFR2 Channel associated signaling (CAS) AB bits, ABCD bits Common channel signaling (CCS) SS7, QSIG Trunk Associated CCS Separate signaling network CCS

Subscriber - Exchange Signaling* On/off hook DC current flow Dial-tone 350+440 continuous Pulse Dialing DTMF L1 697 L2 770 L3 852 L4 941 H1 1209 H2 1336 H3 1477 H4 1633 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 Invalid (nonworking, unobtainable) number announcement, SIT-tone * US cadences - national differences make break >700ms >100ms (60:40) off-hook H1 H2 H3 H4 L1 1 2 3 A L2 4 5 6 B L3 7 8 9 C L4 * 0 # D

Subscriber - Subscriber Subscriber - Exchange Signaling 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 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 1st and 2nd rings RING CLI >300 ms > 475 ms

Supervision procedures 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. When connecting PBX to CO, PBX is FXO, CO is FXS Analog loop start Digital loop start Analog ground start Digital ground start E&M (wink)

Interexchange Signaling CAS - R2 CAS - R1 CCS - SS7

Optimized Telephony Routing 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

The PSTN circa 1960 PSTN Review 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)

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

Timing In addition to voice, the digital PSTN transports timing PSTN Review Timing In addition to voice, the digital PSTN transports timing This timing information is essential because of the universal use of TDM the requirement of accurate playback (especially for fax/modem) Receiving switches can recover the clock of the transmitting switch 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

The Present PSTN PSTN Network PSTN Review core backbone PSTN Network subscriber line 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 T1/E1, PDH and SONET/SDH “synchronous” protocols Signaling can be channel/trunk associated or via separate network (SS7) Automatic routing Circuit switching (route is maintained for duration of call) Complex routing optimization algorithms (LP, Karmarkar, etc)

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

Digital Loop Carrier Pushes the digital PSTN closer to customer PSTN Review Digital Loop Carrier Pushes the digital PSTN closer to customer AT&T SLC-40, SLC-96, Nortel DMS P-phone, “pair-gain” Access Network TR-08 Mode 1 pair-gain: Replace 96 pairs with 5 T1s (one spare for “span protection”) 96 – 10 = 86 TR-08 Mode 2 pair-gain: Replace 96 pairs with 2 T1s (without “span protection”) 96 – 4 = 92 CLASS 5 UTP/coax/fiber Street cabinet CPE FTTB/FTTC pedestal UTP TR-08 multiplex 96 lines on: Mode 1: 4 T1s Mode 2: 2 T1s (2:1 concentration) GR303/V5.1/V5.2 multiplex up to 2048 lines