Communication
Systems
8th lecture
Chair of Communication Systems
Department of Applied Sciences
University of Freiburg
2006
1 | 54
Communication Systems
Administrative stuff
●
Lecture on 13.06 might be called off. Please check the webpage
before 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.
2 | 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
3 | 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
4 | 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
5 | 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: 50.000 participants in US phone network
–
1881: Berlin opens the first “Fernsprechamt”
6 | 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
7 | 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
8 | 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 :-))
9 | 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
10 | 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
11 | 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
12 | 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
13 | 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
14 | 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)
15 | 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)
16 | 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”
17 | 54
Communication Systems
Protocol of analogous telephony connection
18 | 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 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
19 | 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
20 | 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
21 | 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
22 | 54
Communication Systems
digital 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
23 | 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
24 | 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 ...
25 | 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
26 | 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)
27 | 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
28 | 54
Communication Systems
ISDN – and the OSI protocol stack (mostly D channel)
29 | 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
30 | 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
31 | 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
32 | 54
Communication Systems
ISDN – Basic Rate Interface
●
next state (S1 - S4) to be transmitted is indicated in column
labled Go
33 | 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
Bits
00
01
10
11
Voltage
-2,50
-0,83
2,50
0,83
34 | 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
35 | 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
36 | 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 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
37 | 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
38 | 54
Communication Systems
S0 – AMI code
●
Modified AMI code (avoid long sequences of symbols of the
same type)
39 | 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: 01111110
Keeping of frame sequence
Error discovery
Multiplexing of more than one logical D2 connections
Flow control
40 | 54
Communication Systems
higher layer protocols for the D channel
●
ITU Recommendation Q.921
41 | 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)
42 | 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
43 | 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
44 | 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
45 | 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+x5+1
●
●
16 bit frame checksum
Inverted residue of binary division
p1(x) = xk (x15+x14+...+x2+x +1)
p2(x) = x16d(x)
46 | 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
47 | 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
48 | 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 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
49 | 54
Communication Systems
DSS1 layer 3 protocol
●
Message Type single byte (octet) that indicates what type of
message is being sent/received
50 | 54
Communication Systems
DSS1 layer 3 protocol – message types
●
Message Type – four categories
– Call Establishment
–
Call Information
–
Call Clearing
–
Miscellaneous
51 | 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 BChannel(s) requested)
–
Progress Indicator (Indicates status of outgoing call)
52 | 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
53 | 54
Communication Systems
literature on telephony networks
●
●
●
●
Have a nice holiday week!
E. Pehl, Digitale und analoge Datenübertragung
http://www.ks.unifreiburg.de/php_termindetails.php?id=180
...
54 | 54