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Introduction |
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review of early
exchanges |
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PSTN Standards |
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User services & terminals |
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Modern exchange technology |
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interface standards |
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access and trunk networks |
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signaling |
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network management |
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internetworking (telecommunications between
networks) |
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PSTN switching is based on circuit switching by duplex*
connections |
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Temporary bidirectional connections |
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Originally for speech (voice) only at 300-3400
Hz |
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Earlier two subscribers connected by purely
physical connection (physical switch contacts) |
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Nowadays by time slots ~ ISDN is integrated to
PSDN |
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PCM is the TDMA standard for the digital
transmission |
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PCM time slots consist of 8 bit samples |
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For voice digital exchange sets up 64 kbit/s
connections |
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Data connections by (1) modems, (2) ISDN
interface
(3) leased lines via X.25 / Frame relay, or (4) ADSL |
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1876 A. G. Bell telephone patent |
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1878 The first exchange constructed in La Porte,
the US |
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could connect any two of the 21 subscribers |
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manual switching (!) |
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1891 first automatic exchange: Strowger Switch
by Almon B. Strowger: an undertaker in Kansas City |
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A 100 line
Strowger switch: |
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each user has its
own selector |
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no concentrators |
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expensive |
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One of the 100 subscribers lifts his handset
-> Call finder is activated to search the line. |
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After the line is located other relays connect
the dial-tone generator. |
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The subscriber selects two digits. |
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The first digit selects the subscriber group by
using the group selector. |
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The second digit selects the line selector. |
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Selection is done by sending pulses that move
the selectors stepwise. |
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When connection is established a ringing tone is
sent. |
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Note that only 10 subscribers of 100 can call at
the same time to different numbers! (why?) (concentration is 1:10) |
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Register-controlled setup (1920 -) |
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B-subscriber number receiver by a register |
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register controls all the remaining call setup
stages |
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Distributed control |
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Markers indicate idle switches |
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Thus markers control path routing |
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Stored program control, 1960s and 1970s (SPC) |
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New services |
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supervision (operation & maintenance
O&M) |
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integrated charging |
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gathering statistics |
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IN services |
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Easier updating and maintenance |
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Coil loading was used to enhance higher
frequency range |
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Frequency division multiplexing (FDM) with
single sideband (SSB) modulation was used in trunk networks |
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Network intelligence and value-added services |
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not supported as such |
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operators were anyhow intelligent :) |
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value added services by tracking what happens in
the area! |
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Inter-exchange signaling |
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call setup took about 15 seconds |
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channel-associated signaling (CAS: No.5, R1,R2*) |
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about 10% of trunk line capacity was taken by
signaling |
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Operation and maintenance |
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using local info-bases and local workforce |
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network maintenance was based on on-field
check-ups |
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Fixed-line phones (analog, ISDN) |
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Cordless phones (PBX-RF interface: DECT2) |
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Fax |
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Pay phones |
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Private Branch Exchange (PBX) |
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Gateways to Public Land Mobile Networks (PLMN): |
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GSM |
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wireless local area networks (WLAN) |
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Local loop data extensions |
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modems |
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ADSL technology |
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(leased lines) |
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Basic service |
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Bearer service (local loop access): analog
(/ISDN) |
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Value-added services (telephonist-originated)
services as |
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directory inquiry (118) |
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weather, stock exchange, ticket reservation ... |
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Supplementary services (Intelligent Terminal
(IN) implementation) |
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distributed supplementary as ‘call forwarding
unconditional’, ‘call waiting’, ‘queuing’ ... |
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centralized supplementary services (IN) use
specialized routing & charging as VPN, credit card calls, free phone
(receiver pays), universal access number (connected automatically to the
nearest office), ... |
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Gets still more subscribers! |
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ISDN very popular in switches (in Finland
all-digital exchanges) |
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ISDN getting popular also for local loop access |
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Versatile access part |
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Conventional local loop technology develops fast |
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Remote controlled O&M |
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IN services fully-developed - Intelligence moves
to terminals |
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Fiber-optical DWDM links connect exchanges |
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Common channel signaling (SS7) |
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SDH-based (Synchronous Digital Hierarchy)
trunk-networking |
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A basic phone can be
made by using just
four units |
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The bell |
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The hook switch |
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The keypad |
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The speech circuit |
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Modern keypads use dual-tone dialing |
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The speech circuit adapts
voice levels and isolates
mic and speaker |
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Dual-tone dialing is used in subscriber loop to
transmit the selected B-subscriber number |
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Earlier pulse selection was applied (very rare
nowadays) |
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Diagnostic unit |
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Checks faults and controls the modem |
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Interface and line units |
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Adapt the modem and terminal |
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Modem performs A/D and D/A conversion and select
rate such that transmission quality criteria (error rate) can be meet |
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ITU-T specifies several modem standards as |
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V.26 (11/88) - 2400 bits per second modem for
use on 4-wire leased lines |
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V.27 (11/88) - 4800 bits per second modem for
use on leased lines |
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V.27ter (11/88) - 4800/2400 bits per second
modem for use in the general switched telephone |
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V.29 (11/88) - 9600 bits per second modem for
use on point-to-point 4-wire leased lines |
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V.90 (09/98) - 56 000 bit/s downstream and up to
33 600 bit/s upstream modem for use in the general switched telephone |
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Faxes follow standard PSTN modem communications
recommendations or IEEE recommendations, as V.17 (02/91) (- Wire modem for
facsimile applications with rates up to 14 400 bit/s) |
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Faxes are divided into groups: |
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Group 1 (´68): Analog scanning, 2400 bits/s |
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Group 2 (´76): Analog scanning, 4800 bits/s |
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Group 3 (´80): Digital scanning, 14400 bits/s |
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Group 4 (´84): Digital scanning, 64 kbit/s
(ISDN) |
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Example of modules in group 3 transmitting fax: |
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Series D Recommendations - General tariff
principles |
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Series E Recommendations - Overall network operation,
telephone service, service operation and human factors |
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Series G Recommendations - Transmission systems
and media, digital systems and networks |
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Series I Recommendations - Integrated services
digital network (ISDN) |
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Series M Recommendations - Network maintenance:
international transmission systems, telephone circuits, telegraphy,
facsimile, and leased circuits |
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Series O Recommendations - Specifications of measuring
equipment |
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Series P Recommendations - Telephone transmission
quality, telephone installations, local line networks |
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Series Q Recommendations - Switching and signaling
(Signaling Systems no:4,5,6, and 7, Register Signaling no: R1, R2, IN -
Service) |
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Series V Recommendations - Data communication
over the telephone lines |
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Recommendation E.134 (03/93) - Human factors
aspects of public terminals: Generic operating procedures |
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Notes