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Communication Systems9th lecture
Chair of Communication SystemsDepartment of Applied Sciences
University of Freiburg2008
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Communication Systemsadministrative stuff (lectures and exercises)
Please hand in the theoretical exercise sheets #4 And grab the new sheet (#5) due for the 24th of June 16th and 20th are lectures (all held here in this lecture room), next
practical course is 24th (again at the computer center, exact start time will be announced the lecture before)
You might refer to the electures.informatik.uni-freiburg.de for the lecture recordings and
slides in different formats
www.ks.uni-freiburg.de for details on upcoming lectures and material like the slides of the practical course (containing the “master” solutions to the exercise questions and some hints for the actual exercise sheets, the requirements on this lecture available from the first practical courses slide set)
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Communication Systemslast lectures
Lecture 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
future IPv6 will solve any address scarcities
Very popular traditional service is telephony mostly 1:1 voice communication
with the more and more widely introduced “Voice-over-IP” we could observe a merge of both networks
and we will see a role change in the sphere of network providers (telephony companies vs. IP services providers, (TV) cable networks)
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Communication SystemsUpcoming 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
Components of (digital) telephony networks Digitization of voice - PCM Then introduction to ISDN – a completely digitalized
communication infrastructure call setup and global routing in telephony networks
network definitions and standards
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Communication Systemsplan for this lecture
History of telephony networks and wireless information networks Line switching DTMF – dual tone multi frequency as traditional signalling
source Telephony protocol Standards in telecommunication Digital telephony networks – from analogous source to digitized
data streams - PCM ISDN – Integrated Services Digital Network
D channel protocol
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 Europe and America
1866: Durable transatlantic cable
1876: Bell patents the “phone” (Reiss in Germany – you will find even more inventors of the telephone, for each nation one :))
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 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
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Communication Systemsdevelopment 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 (power) sources (which have some advantages in cases of power failures)
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Communication Systemsdevelopment 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 :-))
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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 the pictures
of old films with young ladies called operators plugging wires to connect subscribers :-))
around 1975
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Communication Systemsline 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
there is an impressive collection of old (mechanical) switchboards in the Nuremberg Communication Museum
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Communication Systemsline 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
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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 switching network
the (long distance) line was already occupied during call setup (=expensive for the telephone companies, especially if no-one answered the call)
Next step was introduction of indirectly operated switching networks middle of the fifties
before routing setup the dial information was collected and then processed
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Communication Systemsline 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 (same amount of time spent for each digit transmitted)
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Communication SystemsDTMF
voice frequency band to the call switching center – frequencies selected in a way that no clash with “normal” voice
multifrequency shift keying (MFSK)
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Communication SystemsDTMF signalling
still in use on analogous lines and for signaling e.g. on voice menu systems – digital equipment uses out-of-band
special codes for signaling other data (e.g. Pay card identification) and for cost signaling between switching centers
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 1970s was to setup connections/routes around the globe to call someone other in the same city (and enjoy the delay because of the huge distances)
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Communication Systemstelephony protocol
Key dials were introduced to telephones – special optimized layout (in contradiction to keyboard, counters layout used today)
So we have a well known “protocol of analogous telephony connection”
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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” 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 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
much less information publically available then for IP and other open protocols
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Communication Systemsstandards 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
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Communication Systemsstandards 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
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Communication Systemsdigital telephony networks - PCM
Voice/speech analogous signal continuous in time and value
domain
characterized by amplitude (signal strength) and frequency
bandwidth in traditional telephony networks 300Hz - 3,4kHz
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Communication Systemsdigital telephony networks - PCM
Sampling of a signal rate at least twice the max
frequency of analogous signal (Nyquest theorem)
2* fmaxb
= 2*3,4kHz = 6,8kHz
internationally the sample frequency was agreed on fSample
=8kHz=8000Hz=8000/s
we get a sample period of T=1/f=1/8000=125µs
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Communication Systemsdigital telephony networks - PCM
Analogous signal Continuous in value domain
Has to be translated into discrete values
A/D converter quantizes the signal
Splitting the value domain into equal intervals
Every measured value is approximated and assigned to one of the defined intervals
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Communication Systemsdigital 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 ...
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Communication SystemsISDN – 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
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Communication SystemsISDN – 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)
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Communication SystemsISDN – 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
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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/s each
1 D channel (data channel for out-of-band signaling) of 16kbit/s
synchronization of 16kbit/s
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Communication SystemsISDN – 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
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Communication SystemsISDN – 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
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Communication SystemsISDN – Basic Rate Interface
Next state (S1 - S4) to be transmitted is indicated in column labeled Go
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Communication SystemsISDN – 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
Bits Voltage00 -2.5001 -0.8310 2.5011 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 steps each
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 onto the S0 bus
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Communication SystemsISDN – Basic Rate Interface
Instead of the traditional wall socket a NTBA (network terminal base adapter) is needed at end users site
NTBA provides the S0 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
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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 the same type)
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Communication Systemsdata 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: 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 three fields 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 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
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Communication Systemsdata 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
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Communication Systemsdata 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
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Communication Systemsdata 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+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 and called 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 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
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Communication SystemsDSS1 layer 3 protocol
Message Type single byte (octet) that indicates what type of message 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
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Communication SystemsDSS1 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 (identifies type and number of B-Channel(s) requested)
progress Indicator (indicates status of outgoing call)
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Communication SystemsDSS1 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 sub address
Called Party Number (destination number, type of number[unknown], numbering plan)
Called Party Number sub address
When Information Elements (IE) consist of multiple octets, the following octet describes how many bytes (octets) are in the Information Element
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Communication Systemsliterature on telephony networks
E. Pehl, Digitale und analoge Datenübertragung See previous season lectures literature hints:
http://www.ks.uni-freiburg.de/php_termindetails.php?id=180
Next lectures: 16th , 20th June (here), next exercise 24th