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1 | 54 Communication Systems 8 th lecture Chair of Communication Systems Department of Applied Sciences University of Freiburg 2006.

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Presentation on theme: "1 | 54 Communication Systems 8 th lecture Chair of Communication Systems Department of Applied Sciences University of Freiburg 2006."— Presentation transcript:

1 1 | 54 Communication Systems 8 th lecture Chair of Communication Systems Department of Applied Sciences University of Freiburg 2006

2 2 | 54 Communication Systems Administrative stuff Lecture on might be called off. Please check the webpage before the lecture , 08.06, are holidays, no lecture, no practical. Next practical course is on 22.06, in RZ basement -101.

3 3 | 54 Communication Systems Last lectures We started with rather modern communication technologies and introduced the Internet Protocol as a global orientated packet switching network technology – IP can be run over very different physical media and intermediate protocols – And IP is used for more and more networked services – Very popular traditional service is telephony mostly 1:1 voice communication – With the upcoming Voice-over-IP we could observe a merge of both networks

4 4 | 54 Communication Systems Upcoming lectures To get an idea how traditional and modern wireless telephony networks work, we give an introduction to ISDN, GSM and UMTS First traditional telephony networks its history and their concepts in general Digitization of voice - PCM Then introduction to ISDN – a completely digitalized communication infrastructure – call setup and global routing in telephony networks

5 5 | 54 Communication Systems plan for this lecture History of telephony networks and wireless information networks Line switching DTMF – dual tone multi frequency Telephony protocol Standards in telecommunication Digital telephony networks – PCM ISDN – Integrated Services Digital Network D channel DSS1 layer 3 protocol

6 6 | 54 Communication Systems History of telephony networks Traditional analogous telephony networks – 1848: State Telegraphy System in Prussia (Siemens) – 1851: First trans-sea cable between Dover and Calais – 1858: Transatlantic line-based telegraphy between Europe and America – 1866: Durable transatlantic cable – 1876: Bell patents the phone (Reiss in Germany) – 1880: participants in US phone network – 1881: Berlin opens the first Fernsprechamt

7 7 | 54 Communication Systems History of wireless information networks Wireless signal transmission – Morse codes transmitted by radio (Marconi) – 1901: Radio-based telegraphy between Europe and the US – 1914: Introducing the teletype/telex system – 1915: Wireless telephony NY – San Francisco – 1920: First public radio transmission in Königs-Wusterhausen – 1923: Start of entertaining radio in Berlin – 1929: First radio-based TV transmission (Funkausstellung in Berlin) – 1935: First regular public TV transmissions in Berlin

8 8 | 54 Communication Systems development of telephony equipment Traditional analogous telephony networks provides most of the standards (partly) in use up to now – Bi-directional voice channel – Bandwidth to carry voice around 300Hz - 3,4kHz – just the characteristics of the end user devices and their microphones and earpieces – You could hook up the old mid-thirties or sixties telephone set to your wall socket of your telephony provider or your private telephone installation – End devices are power supplied by the telephone exchange, so the devices independent of local sources

9 9 | 54 Communication Systems development of telephony equipment Local loop – connection of the end uses device to the telephony exchange – Device is without power when hook on cradle – Call information is signalled with 65V alternating current – When off-hook power supplied at around 60V by a current of 20 – 40mA – Dial plate cuts the local loop for well defined periods to indicate dial information (~60ms cut, ~40ms closed in between – try to dial via cradle – system is rather robust in detection :-))

10 10 | 54 Communication Systems line switching End systems has to be connected somehow to each other – In the early beginnings manual switch boards (you know the pictures of old films with young ladies called operators plugging wires to connect subscribers :-)) – around 1974

11 11 | 54 Communication Systems line switching Switch boards – first direct-dial switch boards appeared around 1900 used in local area nets first and from around 1920 for long distance calls – dial plates (digits 1 – 9, 0) where added to the telephone device – using special relay boards with contacts for each dialed digit – system operated directly controlled until around 1960s

12 12 | 54 Communication Systems line switching and signalling Early phones used a hand generator to signal assistance by the operator at the switch board Now: Identification of each end device through numerical ID composed of digits from decade system – dial plates (digits 1 – 9, 0) where added to the telephone device

13 13 | 54 Communication Systems automated line switching Switch boards - routers in the telephony world – major drawbacks of this concept – route of the call is fixed every dialed digit switches the next relay in the switching network the (long distance) line was already occupied during call setup Next step was introduction of indirectly operated switching networks middle of the fifties – Before routing setup the dial information was collected and then processed

14 14 | 54 Communication Systems line switching Analogous electronic switching networks appeared with the beginning of the 1970s – allowed new type of dial indication DTMF – dual tone multi frequency was introduced for dial information – Inband signalling – pulse dial information has to be transported via copper wires and require rather high currents – puls dialing impossible over very long distances (resistor capacity of wire) and wireless transmission – major speedup for dialing

15 15 | 54 Communication Systems DTMF – voice frequency band to the call switching center – frequencies selected in a way that no clash with normal voice – multifrequency shift keying (MFSK)

16 16 | 54 Communication Systems DTMF signalling – Still in use on analogous lines and for signalling e.g. on voice menu systems – digital equipment uses out-of-band – Special codes for signalling other data (e.g. Pay card identification) and for cost signalling between switching centres – Some people were able to produce the needed frequencies to switch off payment or setup special connections (no cost, used by Telcos for maintenance) – Hacking/Cracking started not with computer networks but with automated telephony equipment – challenge of the 70s was to setup routes around the globe to call someone other in the same city (and enjoy the delay because of the huge distances)

17 17 | 54 Communication Systems telephony protocol Key dials were introduced to telephones – special optimized layout (in contradiction to keyboard layout used today) So we have a well known protocol of analogous telephony connection

18 18 | 54 Communication Systems Protocol of analogous telephony connection

19 19 | 54 Communication Systems standards in telecommunication But in telephony world mostly not talked on protocols but interfaces Interfaces are well-defined connection points where different parts of the infrastructure/equipment talk to each other in a certain way International standardization body is ITU (International Telecommunication Union Process of standardization completely different to the workflows in Internet bodies – No bottom up, but top down decisions – Exclusive club of the big (state monopoly) Telcos – High annual fees

20 20 | 54 Communication Systems standards in telecommunication Because of the old (nation state) monopolies there are many differences within the several networks – Numbering schemes – Acoustical indication of dial states (busy, line-free,...) – Different use, assignment of the (wireless) frequency spectrum – Not really compatible equipment (branch exchanges,...) - every firm tries to use their own subset of standards With the introduction of digital networks (ISDN and mobile) agreement on global standards started

21 21 | 54 Communication Systems standards in telecommunication Inter connecting of voice streams has lots of technical problems Up to 1980s computerized switching centers but analogous voice connections – Fault-prone to jamming and noise – Regeneration means amplification of noise too Allow data connections over telephony networks Next step: Fully computerized switching centers – out of band signaling of call setup – digital voice streams allow better/perfect regeneration

22 22 | 54 Communication Systems digital telephony networks - PCM Analogous signal – Continuous in time and value domain – Characterized by amplitude (signal strength) and frequency – Bandwidth in telephony networks 300Hz - 3,4kHz

23 23 | 54 Communication Systems digital telephony networks - PCM Sampling of a signal – Rate at least twice the max frequency of analogous signal (Nyquest theorem) – 2* f maxb = 2*3,4kHz = 6,8kHz – Internationally the sample frequency was agreed on f Sample =8kHz=8000Hz=8000/s – We get a sample period of T=1/f=1/8000=125µs

24 24 | 54 Communication Systems digital telephony networks - PCM Analogous signal – Continuous in value domain – Has to be translated into discrete values A/D convertor quantizes the signal – Splitting the value domain into equal intervals – Every measured value is approximated and assigned to one of the defined intervals

25 25 | 54 Communication Systems digital telephony networks - PCM PCM defines 128 different levels for positive and 128 negative amplitude of the signal thus resolution is 256 bit Sample rate is 8000 per second so we get 8000 Byte per second and a bit stream of 64kbit/s So we have the B channel bandwidth for ISDN...

26 26 | 54 Communication Systems ISDN – Integrated Services Digital Network The development of digital switching networks led to standardization and integration of additional services into the same network – three virtual multiplex channels over the same two wire infrastructure – digital telephony (two independent lines on basic rate interface) – fax, telex – video telephony (H.323 devices may use ISDN as transport layer for their applications) – data communication of 64 or 128kbit/s

27 27 | 54 Communication Systems ISDN – Integrated Services Digital Network Prerequisite for ISDN was digitalized infrastructure The ISDN standard was defined in the early 1980s by the ITU – several national standards evolved, 1TR6 in Germany, NI-1/2 in United States, DACS in UK,... – DSS1 is the EURO-ISDN used in many other countries too available from 1993 – EURO ISDN was defined by the new founded ETSI (European Telecommunication Standards Institute in 1988)

28 28 | 54 Communication Systems ISDN – Integrated Services Digital Network ISDN is commonly used in all European countries since 2000 – all switching centers use ISDN backends – so called analogous telephony devices (POTS – plain old telephony service) are converted to digital service at the local switching center – 50% of the European BRI connections are in Germany – Germany has a 30% worldwide share

29 29 | 54 Communication Systems ISDN – and the OSI protocol stack (mostly D channel)

30 30 | 54 Communication Systems ISDN – Basic Rate Interface BRI provides a total data rate of 160kbit/s – standard end user connection – 2 B channels (bearer - for data, digitized voice,...) of 64kbit/s each – 1 D channel (data channel for out-of-band signaling) of 16kbit/s – synchronization of 16kbit/s

31 31 | 54 Communication Systems ISDN – Basic Rate Interface Physical layer specifications of the Uk0 – Operates over two-wire cable up to 5 km (depending on cable diameter and quality) – Switching center provides a 90V current to power the NTBA and one device (emergency function – to be independent on local power supply for at least one telephone) – Other physical layer specifications for alternate U interfaces

32 32 | 54 Communication Systems ISDN – Basic Rate Interface BRI network termination is defined by the Uk0 interface – a special encoding (4B3T) is used: 4 bit digital to 3 baud ternary – 4B3T is a "block code" that uses Return-to-Zero states – allows reduction of symbol rate to 120 kBaud (¾th) and thus distances up to 8km – reduction of low frequencies in the signal spectrum – better detection of code errors – three states: negative pulse, no pulse, positive pulse

33 33 | 54 Communication Systems ISDN – Basic Rate Interface next state (S1 - S4) to be transmitted is indicated in column labled Go

34 34 | 54 Communication Systems ISDN – Basic Rate Interface Alternate encoding: 2B1Q – 2 bit digital to 1 baud quaternary representation 2B1Q transmission can be simply described as an amplitude modulation scheme for DC pulses Ordering of data blocks depends on the encoding used

35 35 | 54 Communication Systems Uk0 – bit streams from switching center to NTBA Each frame consists of 120 ternary steps – 2*B+1*D takes 108 steps in 4 ternary blocks (tb) with 27 steps each – Sync channel occupies 11 steps and a maintenance channel (mc) 1 step

36 36 | 54 Communication Systems Uk0 – bit streams from NTBA to switching center Connection is full-duplex over the two wires – Echo compensation and terminating set is needed – NTBA splits the data streams to separate up and down onto the S0 bus

37 37 | 54 Communication Systems ISDN – Basic Rate Interface Instead of the traditional wall socket a NTBA (network terminal base adapter) is needed at end users site NTBA provides the S 0 bus to which end user devices are connected – Unidirectional – on pair of wires for each direction – Allows up to 12 wall sockets, 8 ISDN devices (or analogous devices via a/b converter) – Provides device power up to 4,5W

38 38 | 54 Communication Systems ISDN – S0 Provides the same B and D channels as Uk0 – Maintains the step and octet frequency – Handles the device plugging and device activation, deactivation – Has to be terminates with resistors of 110 Ohm – Uses modified AMI code with currents of -0,75 and 0,75V

39 39 | 54 Communication Systems S0 – AMI code Modified AMI code (avoid long sequences of symbols of the same type)

40 40 | 54 Communication Systems data link layer for the D channel No distinct layering for B channels – PCM or data directly put into frames as shown on previous slides LAPD – Link Access Procedure on D channel – Derived from High-Level Data Link Control Protokoll (HDLC) – Broadcasts only for network termination device D2 frame margin – octet of binary pattern: Keeping of frame sequence Error discovery Multiplexing of more than one logical D2 connections Flow control

41 41 | 54 Communication Systems higher layer protocols for the D channel ITU Recommendation Q.921

42 42 | 54 Communication Systems layer 2 for the D channel Flag – character is part of the Header information, hexadecimal 7E Address is two bytes (octets) long, and consists of three fields – Service Access Point Identifier (SAPI) – Command/Response (C/R) bit – Terminal Endpoint Identifier (TEI)

43 43 | 54 Communication Systems layer 2 for the D channel Control one or two octets (bytes) in length, indicates one of three frame formats – Information – Supervisory – Unnumbered Information carries Layer 3 Call Control (Q.931) data – It may carry Unnumbered Information data (TEI assignment) or XID (Connection Management/parameter negotiation) information

44 44 | 54 Communication Systems data link layer for the D channel Protocol handles the TEI (Terminal Endpoint Identifier) allocation – All devices on S0 using the same bus and have to be addressable – TEI assignment is started by the connected devices after successful initialization of physical layer synchronization – Non automatic assignment uses ID0 – 63, automatic 64 – 126 – There is a special group TEI 127 Protocol elements – information lowermost bit is set to 0

45 45 | 54 Communication Systems data link layer for the D channel Protocol elements – Receive Ready - (01) – Set Asyncronous Balance Mode Extended - (6F/7F) – Unnumbered Information - (03) – Disconnect - (43/53) – Unnumbered Acknowledgement – (63/73) Flow control uses sequence numbers for sending and receiving 00:E1:04:00:... Octets #4 for sending and #5 for receiving in the information frame

46 46 | 54 Communication Systems data link layer for the D channel error detection D channel protocol uses rather sophisticated error detection protocol Generates frame checksums Generator polynom g(x) = (x +1)(x15+x14+x13+x12+x4+x2+x +1) g(x) = x16+x12+x bit frame checksum Inverted residue of binary division p1(x) = xk (x15+x x2+x +1) p2(x) = x16d(x)

47 47 | 54 Communication Systems data link layer for the D channel error detection Checking for added or lost binary zeros Thus cyclic Hamming codes implemented Error detection for one, two and three bit error

48 48 | 54 Communication Systems network layer for the D channel DSS1 protocol handels the call setup of the calling and called site Call destruction after finishing the session Restaring and parking if required Error handling

49 49 | 54 Communication Systems DSS1 layer 3 protocol Protocol Discriminator – part of the Layer 3 header information – single byte (octet) that is usually set to a value of (hexadecimal "08") - meaning Q.931 call maintenance Reference Value consists of either two or three bytes (octets) – BRI systems have a 7-bit Call Reference value (127 references) – no particular end-to-end significance – Either end can assign an arbitrary value – used to associate messages with a particulary channel connection

50 50 | 54 Communication Systems DSS1 layer 3 protocol Message Type single byte (octet) that indicates what type of message is being sent/received

51 51 | 54 Communication Systems DSS1 layer 3 protocol – message types Message Type – four categories – Call Establishment – Call Information – Call Clearing – Miscellaneous

52 52 | 54 Communication Systems DSS1 layer 3 protocol – information elements Each type of message has Mandatory and Optional Information Elements, identified with single byte (octet) – Bearer Capability (identifies transport requirements of the requested B-Channel) – Cause (identifies reasons for disconnect or incomplete calls) – Channel Identification (indentifies type and number of B- Channel(s) requested) – Progress Indicator (Indicates status of outgoing call)

53 53 | 54 Communication Systems DSS1 layer 3 protocol – information elements – Network Specific Facilities (Useful for North American PRI calls - identifies network type, Carrier ID, Carrier Service Type[WATS/SDN/ASDS,etc.]) – Calling Party Number (caller ID) – Calling Party Number subaddress – Called Party Number (destination number, type of number[unknown], numbering plan) – Called Party Number subaddress When Information Elements consist of multiple octets, the following octet describes how many bytes (octets) are in the Information Element

54 54 | 54 Communication Systems literature on telephony networks Have a nice holiday week! E. Pehl, Digitale und analoge Datenübertragung freiburg.de/php_termindetails.php?id=180 freiburg.de/php_termindetails.php?id=180...


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