Notes

Outline

Public Switched Telephone Network (PSTN II/II)

Topics today in PSTN A: Switching types Connectionless/ connection oriented Packet/circuit B: PSNT exchanges and interfaces interface Q.512 using access and trunk networks signaling network management internetworking (telecommunications between networks)

Switching in public networks

Circuit switching

Packet switching

Connection-oriented and connectionless switching

Transfer modes & connections summarized

Example of cell switching: Distributed queue dual buss (DQDB)

Connecting into PSTN exchange: Equipment in the access network

Local exchange

Subscriber stage

Exchange control functions Maintenance functions supervision of subscriber lines and trunk circuits Operational functions administrative data as subscriber database routing database statistical data as from where and whom subscribers call holding times for different equipment types utilization of IN services User services

Exchange user services (examples) Absent-subscriber services as the answering machine Call booking: connection at the desired time Person-to-person call: ensures that call goes to a right person Serial call: setting up several calls Telephone conferencing: several persons participate to call in real-time (compare: tele-conferencing) Directory inquiries: also speech recognition, recorded messages (many of these nowadays available in terminals)

The space-switch (used as a cross-switch and concentrator)

The time-switch One of the time slots of any full-duplex lines is connected to some other line (at a time) Thus two switches / time slot connect a line For 100 full-duplex lines at 19.6 kbps a 1.92 Mbps bus is thus required for no blocking If no fixed assignment of input lines to time slot but on demand allocation -> blocking switch that reduces number of switches and switch clock frequency. For instance 200 lines of 19.6 kbps with bus of 1.92 Mbps -> about half of the devices can connect at any time, eg concentration is 2:1

The time-space-time (TST) switch Works in local exchange and subscriber stage Performs PCM concentration, usually 10:1 … 3:1 Connects subscribers also to information tones and test equipment Time switch contains one bus for incoming and outgoing calls (full-duplex)

PSTN ISDN exchange interfaces (Q.512)

Exchange interfaces and tasks, V1 Purpose of exchange is to organizes connection between exchange terminators! V1: Access to basic ISDN (This is user’s ISDN-u interface that can be used to connect small PBX also) Basic ISDN V1-functions: 2 B + D (2x64 kbps + 16 kbps) channeling structure timing and frame synchronization activate and deactivate terminator operation and maintenance feeding power supply ISDN basic access parameters defined in G.961

Exchange interfaces and tasks, V2-V4 V2: Interface serves typically concentrators 2048 kbit/s eg 30 B + D Electrical standard G.704 V3: Resembles V2 but intended for interface other exchanges (PABX) Electrical standard G.703 30 B + D at 2048 kb/s also 23 B +D at 1544 kb/s (I.431) V4:Interface to private networks (not ITU-T specified), for instance DSLAM (ADSL-interface)

Exchange interfaces and tasks, V5 Between access network and exchange 2048 kbit/s Specifies basic interfaces for Analog access ISDN-access Electrical interface G.703 Channel control and signaling V5 supports interface rates 2048 kbit/s … 8448 kbit/s

Connecting the local loop: Line interface circuit (LIC)

Line interface circuit components Over-voltage protection Test equipment to connect to monitor the line condition faults Voltage feed ringing telephone current supply Detection of hook stage, pulse generated, or dual-tone receiver The hybrid junction (2 wire - 4 wire interface) An A/D converter (uses PCM techniques at 64 kbps)

The hybrid circuit 4-wire connection is used between exchanges and 2-wire connections from exchange to subscribers

The hybrid-circuit

The hybrid circuit summarized The hybrid circuit transforms two-wire connection into 4-wire connection. If the hybrid is unbalanced echo will result Hybrid is balanced when no own voice is leaked into own loudspeaker Hybrid unbalance can result from line impedance changes due to weather conditions Unbalance results echo Echo cancellation circuits are harmful in data connections Nowadays realized by operational amplifier based circuitry that automatically monitors line impedance changes

Network echo suppressor (NES) R: transmission gate, A: attenuator, L: logic circuit When the signal is present on the receiving line the transmitting line is cut-off A kind of semi-duplex approach to solve the echo problem

Network echo canceller (NEC) Signal echo is extracted and subtracted from the received signal More effective than echo suppressor. Often NEC and NES are however both used

PSTN operation and maintenance (Q.513) Different alarm classes Vital functions and circuits   (as SS7 and group switch) use secured paths and backups Procedures provided for: troubleshooting fault diagnostics hardware faults can be isolated Supervision is realized also by connecting maintenance units to the network Important switches have extensive backup equipment

Modern PSTN hierarchy

PSTN Hierarchy cont. Local (example, within a city) Subscriber connections Switching within the local exchange Switching to other exchanges Transit (county level, say between Tampere and Helsinki) Switching traffic between different geographical areas within one country International Gateway-type traffic between different countries DWDM (Dense Wavelength Division Multiplexing) routes Rates can follow SONET or SDH standard

Subscriber signaling for local calls

Inter-exchange signaling Channel associated signaling (CAS) as No.5, R1, R2 analog and digital connections Modern ISDN exchanges apply SS7(digital), that is a common channel signaling method (CSS) that is discussed later in its own lecture CAS is divided into line and register signaling: Line signaling: line state between the trunk-links as answer, clear-forward*, clear-back Register signaling: routing information as B-number, A-category, B-status

Inter exchange signaling (cont.) Three categories of information is transmitted: setup, supervision clearing service related information as forwarding, callback, charging status change information transmission network congestion neighborhood exchange congestion

Example of inter-exchange signaling

Inter-exchange signaling (cont.)

A case study: DX 200 Exchange Various control units apply common busses to control the exchange

A case study: DX 200 Exchange SSU: Subscriber Signaling Unit: controls access network CCSU:Common Channel Signaling Unit (SS7). CCMU: Common Channel Signaling Management Unit: (as MTP, SCCP) PAU: Primary Rate Access Unit: controls basic (64 kbit/s) system interfaces  LSU: Line Signaling Unit: takes care of signaling between transit exchanges and access networks MFSU: MULti-frequency Service Unit: Takes care of signaling when multiple frequency signals are used

A case study: DX 200 Exchange (cont.) BCDU:Basic Data Communication Unit: Serves various data services to OMU as access to X.25 and LANs M: Marker Unit: Controls concentrators / space switches CM: Central Memory: Contains user database, charging, signalling, routing and exchange ensemble. STU: Statistical Unit: Collects statistical information on traffic and charging. CHU:Charging Unit: Maintains charging database obtained from signalling units. OMU:Operation and Maintenance Unit: Allows personnel access to exchange memory, perform tests an traffic measurements.

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