S72 1130 Telecommunication Systems Public Switched Telephone Network

S-72. 1130 Telecommunication Systems Public Switched Telephone Network (PSTN) HUT Comms. Lab, Timo O. Korhonen

Topics in PSTN n n Medium sharing n FDMA/TDMA/CSMA n Circuit/packet switching n Connection-oriented/connectionless switching Digital hierarchies Exchanges n Technologies: development, modern local exchange n Interfaces: ISDN and line interface circuit (LIC) n Signaling n Services n Operation and maintenance (O&M) Terminals in access network: phones, modems, faxes HUT Comms. Lab, Timo O. Korhonen 2

S-72. 1130 Telecommunication Systems Public Switched Telephone Network (PSTN) - Medium Sharing HUT Comms. Lab, Timo O. Korhonen

Medium sharing techniques Medium Sharing Techniques Static Channelization TDMA FDMA Cellular communications Satellites n n Dynamic Medium Access Control Scheduling CDMA IEEE 802. 5 (token ring) Random Access Aloha (contention) IEEE 802. 11 (PCF) (polling) WLAN IEEE 802. 11 (DCF) CSMA/CD WLAN IEEE 802. 3 CSMA/CD Ethernet HDLC (ISO 3309, 4335) (polling) Internet Statistic: Tight coordination, may waste resources with statistical sources, can be expensive (ISDN). For high bulk data rates. Dynamic: Loose coordination, supports statistical multiplexing, flexible networking, cost effective; cheap terminals (Ethernet). For networking with statistical sources. PCF: Point Coordination Function HUT Comms. Lab, Timo O. Korhonen DCF: Distributed Coordination Function HDLC: High-Level Data Link Control CSMA/CD: Carrier Sense Multiple Access/Collision Detection 4

Basic channelization techniques HUT Comms. Lab, Timo O. Korhonen 5

HUT Comms. Lab, Timo O. Korhonen 6

Code Division Multiplexing modulation spreading Channel User code User data n n n DS-spread spectrum system despreading User code demodulation Detected bits Despreader correlates the received signal with the build-in code (=code-matched-filtering) Code division multiplexing comes in two flavors n Direct Sequence (DS) as in UMTS n Frequency Hopping (FH) (Bluetooth) In DS, spreading is done by multiplying signal to be transmitted by the user specific spreading code. In FH transmitted signal hops to the frequencies determined by the spreading code Figure of merit: Code gain Gp = Tb/TSc where b and c refer to bit and chip (=code bit) times, respectively. The larger the code gain, the more resistant system is for interference Applied earlier in military communications, nowadays popular also in wireless multiple access applications as in cellular systems (WDMA) HUT Comms. Lab, Timo O. Korhonen 7

Dynamic medium access Carrier Sense Multiple Access (CSMA) Aloha: • If you have data to send, send the data • If the message collides with another transmission, try resending later HUT Comms. Lab, Timo O. Korhonen CSMA/CD (Listen before sent): • If you have data to send, send the data provided no collision has occurred (sensed by listening CD signal) • If the message collides with another transmission, send CD to inform others, then try resending Ref: A. Leon-Garcia: Communication Networks 2 th ed slide set 8

HUT Comms. Lab, Timo O. Korhonen 9

HUT Comms. Lab, Timo O. Korhonen RTS: Request to send; CTS: Clear to send 10

HUT Comms. Lab, Timo O. Korhonen 11

S-72. 1130 Telecommunication Systems Public Switched Telephone Network (PSTN) - Digital Hierarchies HUT Comms. Lab, Timo O. Korhonen

Digital hierarchies HUT Comms. Lab, Timo O. Korhonen 13

(Also 8+24 x 64 kbps = 1. 544 Mbps) HUT Comms. Lab, Timo O. Korhonen 14

HUT Comms. Lab, Timo O. Korhonen 15

HUT Comms. Lab, Timo O. Korhonen 16

S-72. 1130 Telecommunication Systems Public Switched Telephone Network (PSTN) - Circuit and Packet Switching HUT Comms. Lab, Timo O. Korhonen

Switching in wire-line public networks MPLS X. 21 Cell switching - works with cells (packets) having a fixed size : offers bounded Qo. S guarantees (Qo. S compatible, long packets won’t stuck cells) * Used by European Telecom’s that use X. 21 in circuit switched nets ** Used by British Telecom’s Packet-switched Service (PSS), Data Pac (Canada). . . HUT Comms. Lab, Timo O. Korhonen CSPDN: Circuit switched public data net* PSPDN: Packet switched public data net** MPLS: Multi-protocol label switching DQDB: Distributed queue dual bus Cell switching: Statistical multiplexing of fixed-length packets 18

Circuit Switching Networks n n n End-to-end dedicated circuits between clients n Client can be a person or equipment (router or switch) Generally, circuit can take different forms: n Dedicated path for the transfer of electrical current n Dedicated time slots for transfer of voice samples n Dedicated frames for transfer of Nx 51. 84 Mbps signals n Dedicated wavelengths for transfer of optical signals Circuit switching networks require: n Multiplexing & switching of circuits n Signaling & control for establishing circuits HUT Comms. Lab, Timo O. Korhonen 19

Circuit switching - TDM Circuit switching - dedicated path - constant delay/bandwidth - voice/data - paid by time - examples: PSTN, GSM Time switch - Makes switching between time slots - In the figure incoming slot 3 is switched to outgoing slot 1 for one voice direction - Each coming timeslot stored in Speech Store (SS) - Control store (CS) determines the order the slots are read from SS - The info in CS is determined during setup phase of the call Space switch - makes switching between PCM lines - works with electronic gates controlled by CS HUT Comms. Lab, Timo O. Korhonen Cross-point controlled by CS TDM 20

Packet switching example Packet structure Seq: sequence number Op code: message/control identifier CRC: Cyclic Redundancy Code Node structure Note: - source address required for retransmission in ARQ - byte count could be also an end flag HUT Comms. Lab, Timo O. Korhonen 21

Example: IP 4 packet header in Internet n IPv 4 packet header (to be further discussed in Internet lecture) HUT Comms. Lab, Timo O. Korhonen 22

Example of cell switching: Distributed queue dual buss (DQDB, 802. 6) Function - transport units have a constant length - access units access known subscribers in access unit’s subnets and route packets for them - access protocol applies token ring Properties - decentralized (distributed switching as in FDDI*) - for ATM compatible MANs (Metropolitan Area Networks) - rates: up to 155 Mbps - geographical limit up to 200 km Traffic in opposite directions FDDI: Fiber Distributed Data Interface. See supplementary for more details on DQDB and FDDI HUT Comms. Lab, Timo O. Korhonen Transport Unit of DQDB (same as in ATM) 23

Packet switching - summary n n General characteristics n can generally use packets of varying lengths n packet is assigned an address and the necessary control information n packets are placed in frames Each sent frame stored in a buffer (store & forward) in a receiving node and its information is checked before resending -> delays but errorless transmission possible In summary: packet handing by nodes consists of n checking the packet format n checking for errors (link level - OSI 2) n waiting (buffering) for available outgoing path capacity Nodes have routing tables (network level - OSI 3) HUT Comms. Lab, Timo O. Korhonen 24

Packet Switching: Connection-oriented and connectionless switching Connection oriented - Applies same route - Qo. S well defined - Phases - Connection setup - Data transmission - Release - Packets received in same order - Example: ATM, frame relay, MPLS HUT Comms. Lab, Timo O. Korhonen Connectionless - Use of different routes for each packet possible - Each packet has address fields - Qo. S not guaranteed - Packets may come in different order - Example: IP (Internet Protocol), TCP takes care of cleaning the mess 25

Transfer modes & connections summarized Transfer modes PSTN Circuit switching for voice ISDN -- developed nowadays also for data PCM - well-specified delays - echo problems Packet switching - developed for data - nowadays also for voice - statistical multiplexing - traditionally variable delays IP, Frame-relay ATM HUT Comms. Lab, Timo O. Korhonen Connection types Connection oriented ATM MPLS - hand-shaking - strict error requirements - for fast data transfer X. 25 TCP Connectionless - especially for broadcasting/ streaming - modest error rates often accepted - for fast data in good channels IP, UDP* *User Datagram Protocol 26

Nice to know: MPLS versus ATM n n n n Both MPLS and ATM provide a connection-oriented service Signaling protocols: RSVP/LDP for MPLS and PNNI for ATM MPLS is able to work with variable length packets. ATM transports fixed-length (53 byte) cells ATM: Segmentation, transport and re-assembly using an adaptation layer -> complexity and overhead MPLS: Adds a label to the header of each packet and transmits it on the network MPLS connection (LSP) is uni-directional, bi-directionality requires two LSP connections that may take different paths. ATM point-to-point connections (Virtual Circuits), are bidirectional using same path MPLS was designed from the start to be complimentary to IP HUT Comms. Lab, Timo O. Korhonen 27

S-72. 1130 Telecommunication Systems Public Switched Telephone Network (PSTN) - Exchange Technology, Interfaces and Services HUT Comms. Lab, Timo O. Korhonen

Early exchanges n n n Topology of the first network 1876 A. G. Bell telephone patent using Strowger switch 1878 The first exchange constructed in La Porte, the US n could connect any two of the 21 subscribers n manual switching 1891 first automatic exchange: Strowger Switch by Almon B. Strowger: an undertaker in Kansas City A 100 line Strowger switch: n each user has its own selector n no concentrators n expensive See further info also at: http: //www. seg. co. uk/telecomm/ HUT Comms. Lab, Timo O. Korhonen via selectors n cross-bar switch 29

An early analog PBX: 100 subscriber exchange (Step-by-step subscriber controlled call set-up) A-subs. MAIN PARTS: - Call finders (CF) - Group selectors (GS) - Line selectors (LS) HUT Comms. Lab, Timo O. Korhonen B-subs. Call setup: 1. A-sub. picks up handset (CF detects) - exchange sends line available -tone 2. A-sub. sends pulses (GS, LS activated) - exchange sends ringing tone PBX: Private Branch Exchange 30

Modern local PSTN exchange Signaling (SS 7) with users and other exchanges Switch Recorded announcements: faults/subscriber services conference calls, call waiting, broadcasting. . . HUT Comms. Lab, Timo O. Korhonen O&M HW Control Traffic concentration - Operation & maintenance support (Q. 513) - Charging - Supplementary (IN) services (as credit card call) - Subscriber data, switch control ETC: Exchange terminal circuit IN: Intelligent network 31

Concentrator n Non-concentrating circuit switch: all inputs can be connected to all outputs simultaneously. Requires Nx. N = N 2 switches N inputs cross-bar switch N outputs n n Note that at the same time only (1/N)*100 % of switches in use -> inefficient utilization of switches It is possible to reduce number of switches by using some multistage switching strategy. However, an importan point is to know if the switch is still non-blocking (connection requests are never denied due to lack of connectivity resources) HUT Comms. Lab, Timo O. Korhonen 32

Multistage Space Switch n n Large switch built from multiple stages of small switches The n inputs to a first-stage switch share k paths through intermediate crossbar switches Larger k (more intermediate switches) means more paths to output In 1950 s, Clos asked, “How many intermediate switches required to make switch non-blocking? ” - Answer: k>=2 n-1, number of switches ~N 1/2 HUT Comms. Lab, Timo O. Korhonen Ref: A. Leon-Garcia: Communication Networks 2 th ed slide set 33

Subscriber stage Connects to: digit receivers, info tones, test equipment internet access (DSLAM) To ETC MUX Concentrator Centrex* service Control system: subscriber authentication, routing, billing, O & M, . . . ETC: Exchange terminal circuit Speech store: shift registers storing bits for time switching Control store: gates guiding speech store switches HUT Comms. Lab, Timo O. Korhonen * Leased PBX function from local exchange 34

Exchange control functions n n Maintenance functions n supervision of subscriber lines and trunk circuits Operational functions n administrative data as n n n statistical data as n n subscriber database routing database from where and whom subscribers call holding times for different equipment types utilization of IN services User services Sample of Intelligent network (IN) services HUT Comms. Lab, Timo O. Korhonen 35

Connecting the local loop: Line interface circuit (LIC) Used for signaling in certain coin-operated pay-phones and PBX HUT Comms. Lab, Timo O. Korhonen 36

Line interface circuit components n n n Over-voltage protection Test equipment to connect to monitor the line condition faults Voltage feed n ringing n telephone current supply Detection of n 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) HUT Comms. Lab, Timo O. Korhonen 37

The hybrid-circuit n 4 -wire connection is used between exchanges and 2 -wire connections from exchange to subscribers Two-wire HUT Comms. Lab, Timo O. Korhonen 38

The hybrid-circuit (a directional coupler) If the impedance Zb equals the line impedance no incoming voice (down right) leaks to outgoing voice (up right) but the signal goes via the two wire connection on the left To exchange Local loop From exchange Nowadays realized by operational amplifiers HUT Comms. Lab, Timo O. Korhonen 39

The hybrid-circuit summarized n n n The hybrid circuit transforms two-wire connection into 4 wire connection. If the hybrid is unbalanced echo will result n 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 HUT Comms. Lab, Timo O. Korhonen 40

Network echo suppressor (NES) n R: transmission gate, A: attenuator, L: logic circuit When the signal is present on the receiving line the transmitting line is cut-off n A semi-duplex approach to solve the echo problem n HUT Comms. Lab, Timo O. Korhonen 41

Network echo canceller (NEC) n n Signal echo is extracted and subtracted from the received signal More effective than echo suppressor. Often NEC and NES are however both used HUT Comms. Lab, Timo O. Korhonen 42

Hierarchy of PSTN n n n Local (example, within a city) n Subscriber connections (local loop) n Switching within the local exchange n Switching link to other exchanges Transit (county level, say between Tampere and Helsinki) n Switching traffic between different geographical areas within one country International n Gateway-type traffic between n Between different countries DWDM (Dense Wavelength Division Multiplexing) routes Rates follow SONET, PDH or SDH standards SDH - transport of 1. 5/2/6/34/45/140 Mbps within a transmission rate of 155. 52 Mbps - carries for instance ATM and IP within rates that are integer multiples of 155. 52 Mbps HUT Comms. Lab, Timo O. Korhonen 43

Operation and maintenance of PSTN (Q. 513) n n Different alarm classes Vital functions and circuits (as SS 7 and group switch) use secured paths and backups Procedures provided for: n troubleshooting n fault diagnostics n n n A supervision plan by network levels: hardware faults can be isolated Supervision is realized also by connecting maintenance units to the network Important switches have extensive backup equipment HUT Comms. Lab, Timo O. Korhonen 44

Signaling n n n Telecom nets require more and more processor capacity: n More subscribers n Setting up connection is getting increasingly complex n Number of supplementary (IN-based) services increasing Signaling in PSTN divided to user signaling in local loop and to inter-exchange signaling Three categories of information is transmitted in signaling: n setup: supervision, setting/clearing of connections n service related information as n forwarding, callback, charging n status change information n transmission network congestion n neighborhood exchange congestion HUT Comms. Lab, Timo O. Korhonen 45

Channel associated and common channel signaling n n n Channel associated signaling (CAS) as No. 5, R 1, R 2 n analog and digital connections Modern ISDN exchanges apply SS 7(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: n Line signaling: n n line state between the trunk-links as n answer, clear-forward*, clear-back Register signaling: n routing information as n B-number, A-category, B-status *A-subscriber’s on-hook message transmitted to B exchange HUT Comms. Lab, Timo O. Korhonen 46

Connecting into PSTN exchange: Equipment in the access network Distribution point On-line subscriber with several telephones ADSL modem Cross connection point Twisted pair - connections DSLAM ISDN 2 B+D 144 kb/s Q. 512 specifies exchange interfaces Wireless access (or radio access point) ISDN connection example: 30 B+D (2. 048 Mb/s) Business subscriber HUT Comms. Lab, Timo O. Korhonen Multiplexer Private Branch Exchange 47

PSTN ISDN exchange interfaces Q. 512 Basic ISDN access Private Network Interface Concentrator service CN: Concentrator ET: Exchange T. LT: Line T. AN: Access network NT: Network T. (ISDN) T : Terminal PABX Interface Peek to Q-recommendations HUT Comms. Lab, Timo O. Korhonen V 4 Access Network interface NT: Network T. (in ISDN) T: Terminal 48

Exchange interfaces and tasks, V 1 n n n Purpose of exchange is to organizes connection between exchange terminators! V 1: Access to basic ISDN (This is user’s ISDN-u interface that can be used to connect small PBX also) Basic ISDN V 1 -functions: n 2 B + D (2 x 64 kbps + 16 kbps) channeling structure n timing and frame synchronization n activate and deactivate terminator n operation and maintenance n feeding power supply n ISDN basic access parameters defined in G. 961 HUT Comms. Lab, Timo O. Korhonen 49

Exchange interfaces and tasks, V 2 -V 4 n n n V 2: Interface serves typically concentrators n 2048 kbit/s eg n 30 B + D n Electrical standard G. 704 (frames, signaling. . . ) V 3: Resembles V 2 but intended for interface other exchanges (PABX) n Electrical standard G. 703 n 30 B + D at 2048 kb/s (SDH E-1, Europe) n also 23 B +D at 1544 kb/s (I. 431) (SDH T-1, US) V 4: Interface to private networks (as such not ITU-T specified), for instance DSLAM (ADSL-interface specified by ADSL-forum - ANSI T 1. 413 , ITU-T: G. 992) HUT Comms. Lab, Timo O. Korhonen 50

Exchange interfaces and tasks, V 5 n n n Between access network and exchange 2048 kbit/s basic rate Specifies basic interfaces for n Analog access n ISDN-access Electrical interface G. 703 Channel control and signaling V 5 supports interface rates 2048 kbit/s … 8448 kbit/s HUT Comms. Lab, Timo O. Korhonen 51

S-72. 1130 Telecommunication Systems Public Switched Telephone Network (PSTN) - Basic Technology in Local Loop HUT Comms. Lab, Timo O. Korhonen

PSTN user services Basic Value Added Supplementary n n Basic service n bearer service (local loop access): analog (/ISDN) Value-added services (telephonist-originated) services as n n n directory inquiry (118) weather, stock exchange, ticket reservation. . . Supplementary services (Intelligent Terminal (IN) implementation) n distributed supplementary as ‘call forwarding unconditional’ (Q. 82. 2), ‘call waiting’, ‘queuing’. . . n 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), . . . HUT Comms. Lab, Timo O. Korhonen 53

Analog telephone terminal n n n A basic phone can be made by using just four units n The bell n The hook switch n The keypad n The speech circuit Modern keypads use dual-tone dialing The speech circuit adapts voice levels and isolates mic and speaker HUT Comms. Lab, Timo O. Korhonen 54

Analog local loop interface Loop current used for signaling & message in analog local loop Digital-lines to ISDN central office Analog-line HUT Comms. Lab, Timo O. Korhonen per trunk signaling in local loop: - long setup time - hacking easy - voice grade circuits - interference & cross-talk sensitive - expensive 55

Dual-tone dialing n n Dual-tone dialing is used in subscriber loop to transmit the selected B-subscriber number [Earlier: Pulse dial (very rare nowadays)] HUT Comms. Lab, Timo O. Korhonen 56

Extending analog PSTN to digital transmission: Modems n n Computer n Modem performs A/D and D/A conversion and selects rate such that transmission quality criteria (error rate) can be meet Interface and line units n Adapt the modem and terminal Diagnostic unit n Checks faults and controls the modem HUT Comms. Lab, Timo O. Korhonen Interface and check Demod. Mod. Diagnostics Line unit Line 57

Analog and digital interfaces of modems Analog modem V. 34 V. 24 Analog exchange Interface Analog local loop Digital modem V. 34 G. 711 decoder G. 711 encoder V. 24 G. 711 (11/88) - Pulse code modulation (PCM) of voice frequencies V. 34 (02/98) - A modem operating (up to 33 600 bit/s) for use in 2 -wire analog PSTN HUT Comms. Lab, Timo O. Korhonen V. 34 Digital exchange Interface G. 711 Digital modems: Generate G. 711 signals and receive V. 34 signals passed through G. 711 encoder. Connected to a digital switched network through a digital interface Analog modems: Generate V. 34 signals and receive G. 711 signals that have been passed through G. 711 decoder in an analog PSTN local loop 58

What is specified in modem recommendation? n n n n n Data signaling rates, symbol rates, carrier frequencies pre-emphasis, scrambler, framing Encoder (for instance TCM (Trellis coding) in V. 90) Interface circuits (terminal-modem interface: V. 24) Rate adaptation (real-time, at steps of 2. 4 kb/s) Data compression (V. 42 bis, MNP 5) Error correction (V. 42, MNP 10) PCM quantization curve ; m-law (US) or A-law (Europe) Start-up signals and sequences Operating procedures Testing conditions HUT Comms. Lab, Timo O. Korhonen 59

Examples of modem recommendations 300 Hz - 3. 6 k. Hz n ITU-T specifies several modem standards for different rates as n V. 26 (11/88) - 2400 bits per second modem for use on 4 -wire leased lines n V. 27 (11/88) - 4800 bits per second modem for use on leased lines n V. 29 (11/88) - 9600 bits per second modem for use on point-to-point 4 -wire leased lines n 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 n V. 36, V. 37 - 48 kbit/s & up at 60 -108 k. Hz HUT Comms. Lab, Timo O. Korhonen 60

Fax communications over PSTN n 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) n Faxes are divided into groups: n n n Group 1 2 3 4 (´ 68): (´ 76): (´ 80): (´ 84): Analog scanning, 2400 bits/s Analog scanning, 4800 bits/s Digital scanning, 14400 bits/s Digital scanning, 64 kbit/s (ISDN) Example of modules in group 3 transmitting fax: A 4/US letter, 1144 lines Gray scales by dithering Scanning Coding HUT Comms. Lab, Timo O. Korhonen Modified Huffman QAM, V. 27 ter/ V. 29 Compression Modem (D/A) 61

PSTN in ITU-T standards n n n (www. itu. org) Series D Recommendations - General tariff principles Series E Recommendations - Overall network operation, telephone service, service operation and human factors Series G Recommendations - Transmission systems and media, digital systems and networks Series I Recommendations - Integrated services digital network (ISDN) Series M Recommendations - Network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile, and leased circuits ITU: International Telecommunications Union HUT Comms. Lab, Timo O. Korhonen 62

More PSTN standards n n (www. itu. org) Series O Recommendations - Specifications of measuring equipment Series P Recommendations - Telephone transmission quality, telephone installations, local line networks Series Q Recommendations - Switching and signaling (Signaling Systems no: 4, 5, 6, and 7, Register Signaling no: R 1, R 2, IN - Service) Series V Recommendations - Data communication over the telephone lines HUT Comms. Lab, Timo O. Korhonen 63

Example: Q-recommendations: Switching and signalling* (Illustrative examples denoted by arrows) HUT Comms. Lab, Timo O. Korhonen *http: //www. itu. int/recommendation. asp? type=products&lang=e&parent=T-REC-Q 64

Switching and signalling (cont. ) HUT Comms. Lab, Timo O. Korhonen 65

Review questions n n n n What are the major properties of dynamic and static medium sharing ? Give some examples of both. Sketch a block diagram of direct sequence spread spectrum-system Briefly explain the basic principle of packet and circuit switching. Describe connection orientated and connectionless switching and mention an example of both. Discuss operation and maintenance functions of PSTN. Sketch the basic modules of modern local PSTN exchange What is a hybrid circuit? Describe its basic properties. Describe functions of line interface circuit. HUT Comms. Lab, Timo O. Korhonen 66