08 1 Telephone Networks

8/2/2019 08 1 Telephone Networks

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CommunicationSystems8th lecture

Chair of Communication SystemsDepartment of Applied Sciences

University of Freiburg2006


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Communication SystemsAdministrative stuff

Lecture on 13.06 might be called off. Please check the webpagebefore the lecture.

06.06, 08.06, 15.06 are holidays, no lecture, no practical.

Next practical course is on 22.06, in RZ basement -101.


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Communication SystemsLast lectures

We started with rather modern communication technologiesand introduced the Internet Protocol as a global orientatedpacket switching network technology

IP can be run over very different physical media andintermediate protocols

And IP is used for more and more networked services

Very popular traditional service is telephony mostly 1:1 voicecommunication

With the upcoming Voice-over-IP we could observe a mergeof both networks


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Communication SystemsUpcoming lectures

To get an idea how traditional and modern wirelesstelephony networks work, we give an introduction to ISDN,GSM and UMTS

First traditional telephony networks its history and theirconcepts in general

Digitization of voice - PCM

Then introduction to ISDN a completely digitalizedcommunication infrastructure

call setup and global routing in telephony networks


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Communication Systemsplan 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


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Communication SystemsHistory 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 Europeand America

1866: Durable transatlantic cable

1876: Bell patents the phone (Reiss in Germany)

1880: 50.000 participants in US phone network 1881: Berlin opens the first Fernsprechamt


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Communication SystemsHistory 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 Knigs-Wusterhausen

1923: Start of entertaining radio in Berlin

1929: First radio-based TV transmission (Funkausstellung inBerlin)

1935: First regular public TV transmissions in Berlin


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Communication Systemsdevelopment of telephony equipment

Traditional analogous telephony networks provides most ofthe standards (partly) in use up to now

Bi-directional voice channel

Bandwidth to carry voice around 300Hz - 3,4kHz just thecharacteristics of the end user devices and their microphonesand earpieces

You could hook up the old mid-thirties or sixties telephone setto 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


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Communication Systemsdevelopment of telephony equipment

Local loop connection of the end uses device to thetelephony 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 toindicate dial information (~60ms cut, ~40ms closed in between

try to dial via cradle system is rather robust in detection :-))


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Communication Systemsline switching

End systems has to be connected somehow to each other

In the early beginnings manual switch boards (you know thepictures of old films with young ladies called operatorsplugging wires to connect subscribers :-))

around 1974


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Switch boards

first direct-dial switch boardsappeared around 1900 used inlocal area nets first and from

around 1920 for long distancecalls dial plates (digits 1 9,0) where added to the telephonedevice

using special relay boards with

contacts for each dialed digit system operated directly

controlled until around 1960s


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Communication Systemsline switching and signalling

Early phones used a hand generator to signal assistance bythe operator at the switch board

Now: Identification of each end device through numerical IDcomposed of digits from decade system

dial plates (digits 1 9, 0) where added to the telephone device


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Communication Systemsautomated 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 switchingnetwork

the (long distance) line was already occupied during callsetup

Next step was introduction of indirectly operated switching

networks middle of the fifties

Before routing setup the dial information was collected andthen processed


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Analogous electronic switching networks appeared with thebeginning of the 1970s

allowed new type of dial indication

DTMF dual tone multi frequency was introduced for dialinformation

Inband signalling

pulse dial information has to be transported via copper wiresand require rather high currents

puls dialing impossible over very long distances (resistorcapacity of wire) and wireless transmission

major speedup for dialing


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Communication SystemsDTMF

voice frequency band to the call switching centerfrequencies selected in a way that no clash with normalvoice

multifrequency shift keying (MFSK)


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Communication SystemsDTMF signalling

Still in use on analogous lines and for signalling e.g. on voicemenu systems digital equipment uses out-of-band

Special codes for signalling other data (e.g. Pay cardidentification) and for cost signalling between switching

centres

Some people were able to produce the needed frequencies toswitch off payment or setup special connections (no cost,used by Telcos for maintenance)

Hacking/Cracking started not with computer networks butwith automated telephony equipment challenge of the 70swas to setup routes around the globe to call someone other inthe same city (and enjoy the delay because of the hugedistances)


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Communication Systemstelephony protocol

Key dials were introduced to telephones special optimizedlayout (in contradiction to keyboard layout used today)

So we have a well known protocol of analogous telephonyconnection


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Communication SystemsProtocol of analogous telephony connection


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Communication Systemsstandards in telecommunication

But in telephony world mostly not talked on protocols butinterfaces

Interfaces are well-defined connection points where differentparts of the infrastructure/equipment talk to each other in acertain way

International standardization body is ITU (InternationalTelecommunication Union www.itu.int)

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


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Because of the old (nation state) monopolies there are manydifferences 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


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Inter connecting of voice streams has lots of technicalproblems

Up to 1980s computerized switching centers but analogousvoice 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


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Communication Systemsdigital telephony networks - PCM

Analogous signal

Continuous in time and valuedomain

Characterized by amplitude(signal strength) andfrequency

Bandwidth in telephonynetworks 300Hz - 3,4kHz


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Sampling of a signal

Rate at least twice the maxfrequency of analogous signal(Nyquest theorem)

2* fmaxb = 2*3,4kHz = 6,8kHz

Internationally the samplefrequency was agreed onfSample=8kHz=8000Hz=8000/s

We get a sample period ofT=1/f=1/8000=125s


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Analogous signal

Continuous in value domain

Has to be translated intodiscrete values

A/D convertor quantizes thesignal

Splitting the value domain intoequal intervals

Every measured value isapproximated and assigned toone of the defined intervals


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PCM defines 128 differentlevels for positive and 128negative amplitude of thesignal

thus resolution is 256 bit

Sample rate is 8000 persecond

so we get 8000 Byte per

second and a bit stream of64kbit/s

So we have the B channelbandwidth for ISDN ...


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Communication SystemsISDN Integrated Services Digital Network

The development of digital switching networks led tostandardization and integration of additional services intothe same network

three virtual multiplex channels over the same two wire

infrastructure

digital telephony (two independent lines on basic rateinterface)

fax, telex

video telephony (H.323 devices may use ISDN as transportlayer for their applications)

data communication of 64 or 128kbit/s


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Prerequisite for ISDN was digitalized infrastructure

The ISDN standard was defined in the early 1980s by theITU

several national standards evolved, 1TR6 in Germany, NI-1/2in United States, DACS in UK, ...

DSS1 is the EURO-ISDN used in many other countries tooavailable from 1993

EURO ISDN was defined by the new founded ETSI (European

Telecommunication Standards Institute in 1988)


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ISDN is commonly used in all European countries since2000

all switching centers use ISDN backends

so called analogous telephony devices (POTS plain oldtelephony 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


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Communication SystemsISDN and the OSI protocol stack (mostly D channel)


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Communication SystemsISDN 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/seach

1 D channel (data channel for out-of-band signaling) of16kbit/s

synchronization of 16kbit/s


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Physical layer specifications of the Uk0

Operates over two-wire cable up to 5 km (depending on cablediameter and quality)

Switching center provides a 90V current to power the NTBAand one device (emergency function to be independent onlocal power supply for at least one telephone)

Other physical layer specifications for alternate U interfaces


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BRI network termination is defined by the Uk0 interface

a special encoding (4B3T) is used: 4 bit digital to 3 baudternary

4B3T is a "block code" that uses Return-to-Zero states

allows reduction of symbol rate to 120 kBaud (th) and thusdistances up to 8km

reduction of low frequencies in the signal spectrum

better detection of code errors

three states: negative pulse, no pulse, positive pulse


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next state (S1 - S4) to be transmitted is indicated in columnlabled Go


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Alternate encoding: 2B1Q 2 bit digital to 1 baudquaternary representation

2B1Q transmission can be simply described as an amplitudemodulation scheme for DC pulses

Ordering of data blocks depends on the encoding used

Bits Voltage

00 -2,50

01 -0,83

10 2,50

11 0,83


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Communication SystemsUk0 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 stepseach

Sync channel occupies 11 steps and a maintenance channel(mc) 1 step


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Communication SystemsUk0 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 ontothe S0 bus


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Instead of the traditional wall socket a NTBA (networkterminal base adapter) is needed at end users site

NTBA provides the S0

bus to which end user devices areconnected

Unidirectional on pair of wires for each direction

Allows up to 12 wall sockets, 8 ISDN devices (or analogousdevices via a/b converter)

Provides device power up to 4,5W


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Communication SystemsISDN 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


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Communication SystemsS0 AMI code

Modified AMI code (avoid long sequences of symbols of thesame type)


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Communication Systemsdata link layer for the D channel

No distinct layering for B channels PCM or data directlyput 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: 01111110

Keeping of frame sequence

Error discovery

Multiplexing of more than one logical D2 connections

Flow control


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Communication Systemshigher layer protocols for the D channel

ITU Recommendation Q.921


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Communication Systemslayer 2 for the D channel

Flag

character is part of the Header information, hexadecimal 7E

Address is two bytes (octets) long, and consists of threefields

Service Access Point Identifier (SAPI)

Command/Response (C/R) bit

Terminal Endpoint Identifier (TEI)


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Communication Systemslayer 2 for the D channel

Control one or two octets (bytes) in length, indicates one ofthree 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


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Protocol handles the TEI (Terminal Endpoint Identifier)allocation

All devices on S0 using the same bus and have to beaddressable

TEI assignment is started by the connected devices aftersuccessful 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


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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 andreceiving

00:E1:04:00:...

Octets #4 for sending and #5 for receiving in theinformation frame


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Communication Systemsdata link layer for the D channel error detection

D channel protocol uses rather sophisticated errordetection protocol

Generates frame checksums

Generator polynom

g(x) = (x +1)(x15+x14+x13+x12+x4+x2+x +1)

g(x) = x16+x12+x5+1

16 bit frame checksum

Inverted residue of binary division

p1(x) = xk (x15+x14+...+x2+x +1)

p2(x) = x16d(x)


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Communication Systemsdata 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


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Communication Systemsnetwork layer for the D channel

DSS1 protocol handels the call setup of the calling andcalled site

Call destruction after finishing the session

Restaring and parking if required

Error handling


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Communication SystemsDSS1 layer 3 protocol

Protocol Discriminator

part of the Layer 3 header information

single byte (octet) that is usually set to a value of 00001000(hexadecimal "08") - meaning Q.931 call maintenance

Reference Valueconsists of either two or three bytes(octets)

BRI systems have a 7-bit Call Reference value (127references)

no particular end-to-end significance

Either end can assign an arbitrary value

used to associate messages with a particulary channelconnection


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Communication SystemsDSS1 layer 3 protocol

Message Type single byte (octet) that indicates what type ofmessage is being sent/received


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Communication SystemsDSS1 layer 3 protocol message types

Message Type four categories

Call Establishment

Call Information

Call Clearing

Miscellaneous

C S


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Communication SystemsDSS1 layer 3 protocol information elements

Each type of message has Mandatory and OptionalInformation Elements, identified with single byte (octet)

Bearer Capability (identifies transport requirements of therequested 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)

C i i S


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Communication SystemsDSS1 layer 3 protocol information elements

Network Specific Facilities (Useful for North American PRIcalls - identifies network type, Carrier ID, Carrier ServiceType[WATS/SDN/ASDS,etc.])

Calling Party Number (caller ID)

Calling Party Number subaddress

Called Party Number (destination number, type ofnumber[unknown], numbering plan)

Called Party Number subaddress

When Information Elements consist of multiple octets, thefollowing octet describes how many bytes (octets) are in theInformation Element

C i ti S t


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Communication Systemsliterature on telephony networks

Have a nice holiday week!

E. Pehl, Digitale und analoge Datenbertragung

http://www.ks.uni-freiburg.de/php_termindetails.php?id=180

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08 1 Telephone Networks