LSlides2
Summary of course scope
S38.3115 Signaling Protocols
SIP or
ISUP
H.323 or
SIP
IP
9 Modeling of signaling systems
9 Signaling flow charts
9 (Extended) Finite state machines
9 Classification of Legacy Signaling Systems
9Subscriber signaling
PABX
ISDN
9 Impulse code
9 Multifrequency code (DTMF - dual tone
multifrequency)
PSTN
…
AN
9 Register signaling
9 Line signaling
Signaling Protocols
2-1
Signaling Flow Chart illustrates the main events
Originating Exchange
Local loop
on-hook
trunk
Media Gateway
or Switching Fabric
© Rka/ML -k2008
on-hook
2-2
Called party (B)
Local loop
Information/voice transfer
on-hook
Release forward
1. digit
last. nr
SCP
packets
Terminating Exchange
Originating Exchange
Local loop
trunk
Dial tone
.
:
INAP
Signaling Protocols
Calling subscriber (A)
Local loop
off-hook
ISUP
Signaling Flow Chart example cont...
Called Subscriber (B)
Terminating Exchange
MAP
Megaco/MGCP/…
circuit
9 An Example of Analogue signaling: R2
Calling Subscriber (A)
HLR/
HSS
CCS7
9Trunk signaling
© Rka/ML -k2008
IP
Control Part
of an Exchange
Or
Call Processing
Server
CAS, R2
r
ete
am
i
D
on-hook
Clear-back
Seizure
Start dialing
The purpose of this slide
is to illustrate the method!
1. Address signal
.
: Last address signal
Alerting or ringing tone
Answer
• NB: Exchanges must signal both in forward and backward direction
on incoming and on outgoing side simultaneously.
Ringing
• Incoming and Outgoing signaling can be separated, so can
off-hook
• Incoming Call Control and Outgoing Call control.
Through Connection
Signaling Protocols
2-3
Extended Finite State Machine is very suitable
for modeling signaling senders and receivers.
Signaling Protocols
2
i ∈ Is can be unique in the
signaling system or context
dependent.
s0
s5
The resulting overall model is one
of communicating FSMs.
This is different from e.g. the
client-server model or even
client-agent-server model.
Signaling Protocols
s4
i 6/o 2
EFSA – Exteded Finate State
Automaton
s2
5
ss0 --initial
initialstate
state
0
II -- set
setof
ofincoming
incomingsignals
signals
OO -- set
of
outgoing
set of outgoingsignals
signals
SS -- Set
of
States
Set of States
UU --Set
of
values
of
state
Set of values of statevariables
variables
fsf ::(S×I×U)
→ S - next state
s (S×I×U) → S - next state
fof ::(S×S)
→ O - outgoing signal
o (S×S) → O - outgoing signal
→ u ⊂ U - new values of state
fuf ::(S×S)
u (S×S) → us s ⊂ U - new values of state
variables
variables
i2/o3
s1
i 1/o 1
i 6/o
< s0, I, O, U, S, fs, fo , fu >
2-4
Graphical representation of an FSM
Is ⊂ I - set of possible incoming
signals in state s
Algebraic representation
© Rka/ML -k2008
© Rka/ML -k2008
i5 /o
© Rka/ML -k2008
i5/o2
i3/o2
s3
FSM - Finite State Machine
The use of FSMs is well known also in computer languages e.g.
for lexical analysis. In this course it turns out that all most import
real time programs in a Switching System are FSMs or sets of FSMs.
2-5
© Rka/ML -k2008
Signaling Protocols
2-6
A subscriber as an SDL -state machine
SDL representation of an FSM
On-hook
SDL - Spesification and Description
Language
State a
Received
signal
Received
signal
Task
Call State
on-hook
on-hook
WF Answer
No-Answer
On-hook
A
2-7
You can model the world like this
What is missing in the figure??
on-hook
Talk
State z
Wish to disconnect
On-hook
B-Answer
Signaling Protocols
Talk
A
WF_ ringing tone
Ringing tone
Task
State x
© Rka/ML -k2008
Off-hook
Push digit
Condition
Sent
signal
Ringing
Off-hook
WF-dial tone
Dial tone
Push a digit button
Received
signal
Condition
Sent
signal
Wish to call
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Signaling Protocols
2-8
How are these methods used in
implementing signaling systems?
9
Signaling Flows may be provided in the protocol
specification for all main sequences of events
° if not, they are drawn by the implementor for example by measuring an
existing historic system with no valid documentation
System
under
Development
9
Model of the
Environment
9
SDLs are drafted for the signaling system independent of
the implementation environment
The system independent SDLs are used as a starting
point for the implementation specification for the target
implementation (computer or system) environment
° details are added gradually. E.g. the execution model is taken into
account.
• Can use verification tools to debug your design.
© Rka/ML -k2008
Signaling Protocols
This approach is one example. More ad hoc approach is also
possible but I do not recommend it.
2-9
Execution models of FSM programs
Current State
• Execution model 1: Complete the current Transition always before
starting anything else (non-pre-emptive scheduling)
• Execution model 2: A Transition can be interrupted at any time if there
is a new task with higher priority (pre-emptive scheduling)
• Depending on implementation a Transition may or may not contain a new
(secondary) Receive Message Statement.
Signaling Protocols
Signaling Protocols
2 - 10
Table representation of an FSM
Initialisation
Do Forever
Receive Message
A <- Branch (State, (Secondary state,) Message)
Execute Transition (A)
Od
© Rka/ML -k2008
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2 - 11
s0
s1
s2
© Rka/ML -k2008
i0
s1
s1
Next State
Incoming signal
i1
i2
s0
s0
s2
s1
Signaling Protocols
2 - 12
Signaling is used to allocate network resources for
the call in a CSN
Signaling carries control information from the end user and
another exchange. The info implies that certain circuits and
devices in the exchange need to change state.
Call state includes records on all resources allocated for the
call (time slots, signal receivers and senders, memory,
processes, records etc). It is vital that all resources are released
when the call is released.
Signals can be decadic impulses, voice band tones or binary
signals or messages transported in a packet network.
Signals transferred on a local loop between a terminal and the
local exchange form subscriber signaling.
When two exchanges send and receive signals we talk about
trunk signaling (inter-exchange signaling, inter-carrier
signaling etc…).
9
9
9
9
9
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Signaling Protocols
9
9
9
2 - 13
The terminal (an analogue phone) sends information to
the network in either rotary impulses or in Dual-ToneMulti-frequency (DTMF-) signals.
A DTMF-signal has two frequencies out of eight!! Not 6!
Such Frequencies are used that they have no harmonic
components with the other frequencies:
°
°
°
A signaling system is a given < s0, I, O, U, S, fs, fo , fu >.
One of the key structural properties of a signaling
system is, how signaling information is associated with the
voice path.
In the PSTN, depending on penetration of digital
exchanges, the following types of signaling are used:
9
9
Subscriber or loop signaling in PSTN
9
A Signaling System
Network
Loop signaling
Trunk signaling
Analogue
Pulse- and multi-frequency
Channel Associated
Digital
Pulse- and multi-frequency
Common Channel
ISDN
DSS1 (Q.920…Q.931)
(digital sign systems nr 1)
Common Channel Signaling
(CCS #7)
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Signaling Protocols
2 - 14
DTMF-signals are created with a push
button phone
1209Hz
Good immunity to voice signals (incl. whistling) is achieved
No interference between dial tone and the first digit
Impact of local loop is minimized (attenuation is proportional to square
root of frequency)
1336Hz
1447Hz
1
2
3
A
770Hz
4
5
6
B
852Hz
7
8
9
C
941Hz
*
0
#
D
697Hz
1633Hz
Pushing a button creates a continuous signal with 2 frequences
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Signaling Protocols
2 - 15
Impulse signals are created by the rotary
disk
9
9
9
9
9
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Signaling Protocols
2 - 16
Response tones to the terminal
9
Impulses are created by cutting and reconnecting the
local loop (current on and off).
On/off states in an impulse are 40 and 60 ms.
The number of such impulses is a telephony signal, e.g.
digit 3.
Between two signals an interval of 400-800 ms is used to
separate signals.
Signals are created on the backward rotation of the disk
Terminal receives the following indications as responses
to the signals it has sent:
Semantics
Frequency
Timing
Dial tone
Ringing tone
425 Hz
425 Hz
continuous
1s on, 4s silence
Engaged/Busy
Queueing
425 Hz
950 Hz
950 Hz
1400 Hz
300 ms on, 300 ms off
650 ms
325 ms
1300 ms on, 2600 ms off
In terms of modelling the signaling flow, tones are like signals.
However, tones are transported in the voice band and intermediate
nodes usually do not process them in any way!
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Signaling Protocols
2 - 17
© Rka/ML -k2008
Signaling Protocols
2 - 18
Summary of analogue subscriber signaling
1
2
3
A
4
5
6
7
8
9
C
*
0
#
D
Call establishment procedure or signaling sets
up the call between two parties across the network
LE
B
On-hook, off-hook
Digits: pulses or DTMF
Polarity reversal
9
9
Ringing
Current or voltage
9
Dial tone
Busy tone
In band tones
Ringing tone
Incoming and outgoing registers were used in crossbar and relay exchanges. In digital exchanges the
same functions are performed by programs. Allocating Register phase call processing and signaling
to separate programs may save memory, but will make call control more difficult during the call. When
computer memory became plentiful and ISDN emerged, the separating of register and line signaling
phases lost its importance.
Other tones
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Signaling Protocols
2 - 19
Line signaling takes care of call
supervision and tear-down (release)
9
9
9
9
9
9
Signaling Protocols
2 - 21
An example of route descriptions
Primary routeing
alternative
© Rka/ML -k2008
9
° Dialed digits ( from call set up signaling)
° Incoming circuit group,
° Origin or subscriber category (e.g. operator in R2 group II)
9
Analysis may return
° a set of routeing alternatives
° an instruction to perform number translation (e.g. 0800-numbers):
In this case, the analysis may need to be repeated
9
Analysis trees are built by MML-commands issued by
the operator based on a route plan
© Rka/ML -k2008
Line/Set-up
Second routeing
alternative
Last alternative
Route 3
Nodes of this tree may contain information
that is needed in signaling, for example:
When to start end-to-end signaling etc...
© Rka/ML -k2008
2 - 23
Direction
--> (forward)
L
seizing-acknowledgement
<-- (backward)
S
request for an address signal
<--
S
address signal
-->
S
congestion signals
<--
S
address complete signals
<--
S
subscriber free (charge)
<--
S
subscriber free (no charge)
<--
S
subscriber line busy
<--
answer signal
<--
L
charging pulse
<--
L
clear-back signal
<--
L
release-guard signal
<--
L
L
© Rka/ML -k2008
Signal
seizing signal
L
Different algorithms exist for seizure.
Circuit groups can be either unidirectional
or bi-directional (as cmp. to call set-up)
Signaling Protocols
2 - 22
L
L
seizure = search and reservation of a free circuit or trunk
Outgoing circuits or trunks
Signaling Protocols
Some Signals used in trunk signaling
Route 1
Trunk
group
2 - 20
Analysis result is determined by
The tree is traversed according to some
algorithm until and idle outgoing
circuit is found or the tree ends, in which
case the call is blocked.
Route 2
Signaling Protocols
Number Analysis links the information received
from signaling to call routeing
Line signaling is used to control the state of line or
channel specific equipment.
Line signaling starts when the call has been set up and call
routeing has been performed.
Line signaling supervises call tear-down and may also
send charging information to a charging point (Finland).
Call signaling ends with the release commands to
exchange devices and circuits that the call was using.
Another name: supervisory signaling.
Often physically line signals look quite different from
register signals.
© Rka/ML -k2008
Trunk signaling can be divided into two phases: call set-up
control or inter-register signaling and line signaling.
In setting up a call, devices called incoming and outgoing
register were used in earlier exchange types, thus register
signaling.
Call set up (register phase) ends in the ringing state, and devices
seized for the call (such as registers) are released for use by other
calls.
clear-forward
blocking
remove blocking
Signaling Protocols
-->
<-<-2 - 24
R2 and N2 are Channel Associated trunk
signaling systems
Channel Associated Signaling (CAS)
9
9
9
A category of trunk signaling between exchanges
Is originally based on properties of electrical circuits
typical in crossbar and relay exchanges.
In Channel Associated signaling the association of the
voice path with the signal path is 1:1 and may be based on
space or frequency or time division multiplexing.
°
°
9
9
Space division: each voice copper pair is associated with a signaling
copper pair. Wastes a lot of copper, therefore, different multiplexing
schemes have been developed.
In frequency and time division multiplexing (TDM), the location of
the signaling channel determines the associated voice channel. PCM
(pulse code multiplexing) is an example of a TDM system, that uses
time-slot 16 to carry signaling of the voice channels. A multi-frame
structure is used to establish the association between the voice and
the signaling channels.
© Rka/ML -k2008
Signaling Protocols
9
9
2 - 25
R2 is a call establisment signaling system
establishment signals are sent in-band. In-band
means using the voice path for signaling
(subscribers can not talk at the same time!)
9 Originally, R2 was specified for trunk signaling =
I.e. between public network exchanges in analogue
PSTN
9 Later digital R2 appeared (analogue signals are
represented in a digital form but the signals are
basically the same)
9 Later R2 was adopted for PABXs. Direct Dialling
In (DDI) can be implemented for PABX subscribers
using R2. This use has survived the longest.
Signaling Protocols
2 - 27
R2 and carriage of signals
9
9
9
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Signaling Protocols
2 - 26
Compelled signaling method
9 Call
© Rka/ML -k2008
Among CAS systems, in Finland, the most widely spread is
probably R2. A CAS system called N2, developed by Siemens
was also widely used especially by the Helsinki Telephone
Company.
R2 is the most powerful among anologue CAS systems and was
originally specified by ITU-T and elaborated by national
standardization.
R2 is a forward and backward compelled signaling system for
call establishment. Sender continues sending a signal until it sees
an acknowledgement signal from the the other end. This ensures
reliable and fast operation.
Each R2 signal is a continuous signal of two voice band
frequencies on the voice path. R2 frequencies are not the same as
DTMF that are used in the local loop.
Beginning of a signal
Signal is detected.
Acknowledgement sending is
started
Acknowledgement is
detected. Signal is stopped
Signal end is detected.
Acknowledgement is stopped.
End of Acknowledgement
is detected. New signal
begins.
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Signaling Protocols
2 - 28
‘Forward’-signals
R2 - system is based on end-to-end signaling. Intermediate
exchanges just pick the information they need for routeing
the call, then they through connect the voice path and the rest
of the signals can travel transparently onwards.
R2 uses MF -coding, in which a signal is a combination of
two voice band frequencies. Both forward and backward
directions have their own set of six frequencies producing 15
possible signals in both directions.
Signal
Group I
Group II
1
1
Ordinary subscriber
2
2
Subscriber with priority
3
3
Test call
4
4
Coin box
5
5
Operator
6
6
Data transmission call
7
7
Ordinary subscriber
8
8
Data transmission call
° R2 is not the same as DTMF: different frequencies and different semantics
of signals. Similar physical representation of signals.
9
9
Priority extension
10
0
Operator
These signals are grouped into two subgroups (I.e. each
physical signal is used twice!) the use of which is controlled
by the receiving end.
11
Special serv operator
Forwarded call
12
Negative ack
National signal
13
Test equipment
National signal
14
Network Operator specific
National signal
15
End of pulsing
National signal
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Signaling Protocols
2 - 29
© Rka/ML -k2008
Signaling Protocols
2 - 30
‘Backward’-signals
Signal
PCM-frame structure has place for CAS
Group A
Group B
1
Send next digit
subscriber line free
2
Repeat last but one address signal
Send special info tone
3
Hop to receiving Group B signals
subscriber line busy
4
Congestion in national network
Congestion
5
Send A-subscriber category
unallocated number
6
Connect to voice path
subscriber line free, charge
7
Repeat number n - 2
subscriber line free, no charge
8
Repeat number n - 3
subscriber line out of order
9
Send country code of A-subs
reroute to operator
Network Operator Specific
subscriber number changed
10
1 ylikehys = =
16 16
kehystä
1 multi-frame
frames
K0
K1
K2
K3
K4
K5
K6
K7
K8
K9 K10 K11 K12 K13 K14 K15
1 frame
slots(pariton
(odd frame)
1 kehys==32
32time
aikaväliä
kehys)
256 bits
T0 T1
T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T18 T19 T20 T21 T22 T23 T24 T25 T26 T27 T28 T29 T30 T31
puhekanavat
1-151 - 15
Voice
channels
KL
Frame alignement
kehyslukitustime slot T0
aikaväli T0
B1 B2 B3 B4 B5 B6 B7 B8
C 1 A D D D D D
puhekanavat
16-3016 - 30
Voice
channels
B1 B2 B3 B4 B5 B6 B7 B8
a b c d a b c d
databitit
Data
bits for mgt
CRC
-bit
CRC-bitti
MA
Signaling
merkinantotime
slotT16
T16
aikaväli
kanavan 1
Channel
1
merkinantobitit
signaling
bits
kaukopään
hälytys
Far
end alarm
kanavan 16
merkinantoChannel 16
bitit
signaling
bits
NB: Because of many variants, the exact signals may be different in
different implementations. Naturally, both ends need to follow exactly
the same implementation!
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Signaling Protocols
2 - 31
Even numbered PCM 30 -frame
© Rka/ML -k2008
1 ylikehys ==16
1 multi-frame
16kehystä
frames
K1
K2
K3
K4
K5
K6
K7
K8
K9 K10 K11 K12 K13 K14 K15
There are a number of variants of Line signaling for R2. A typical
variant in Finland was (is) PCM -line signals. PCM -line signals are
sent in timeslot 16 of the PCM -frame, so that the four bits (a, b, c,
d) in the multi-frame dedicated to the corresponding voice channel
are used as follows:
frame==3232aikaväliä
time slots
(even frame)
11kehys
(parillinen
kehys)
T0 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17 T18 T19 T20 T21 T22 T23 T24 T25 T26 T27 T28 T29 T30 T31
puhekanavat
Voice
channels1-15
1 - 15
KL
Frame alignement
kehyslukitustime slot
T0
aikaväli T0
B1 B2 B3 B4 B5 B6 B7 B8
C 0 0 1 1 0 1 1
7 bitin lukitusmerkki
7 bits
for alignement
joka
toisessa
kehyksessä
in even frames
CRC
-bit
CRC-bitti
MA
puhekanavat
Voice channels
16 - 16-30
30
Signaling
merkinantotime slot
aikaväli
T16T16
puhekanava 26
aikaväli T27
B1 B2 B3 B4 B5 B6 B7 B8
0 0 0 0 1 A 1 1
ylikehysMulti-frame
lukitusmerkki
alignement
kehyksessä 0
in frame 0
B1 B2 B3 B4 B5 B6 B7 B8
NB first abcd
are forward bits
second abcd are
backward bits
näytteen
amplitudin
Voice Sample
suuruus
amplitude value
Multi-frame
ylikehyslukitushälytys
alarm
polarity
polariteetti
Applies only to K0, other
even numbered, look at the previous slide
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Signaling Protocols
2 - 33
© Rka/ML -k2008
9
9
9
It is typical in CAS systems that after setting up the call,
terminals can not control the network in any way except
initiate release.
This is due to closing the signaling “connection” between
the phone and the local exchange.
Workaround methods have been developed. An LE can
supervise the voice channel traffic and possible DTMF
signals on the voice path or the line card can detect
“polarity reversal”.
9
9
9
9
° It must be possible to detect DTMF -signals among voice.
° Polarity reversal can cause seizure of a register during a call. The register
can reserve other signaling resources as needed.
9
9
© Rka/ML -k2008
Signaling Protocols
Signal
Idle
Seizure
Seizing ack
B-answer
Charging
B off-hook
Clear-back
Clear-forward
Clear forward
Clear forward
Blocking
forward-transfer
a
1
0
0
0
0
0
0
1
1
1
1
0
b
0
0
0
0
0
0
0
0
0
0
0
1
c
0
0
0
0
0
0
0
0
0
0
0
0
d
1
1
1
1
1
1
1
1
1
1
1
1
a
1
1
1
0
1
1
0
1
0
0
1
1
b
0
0
1
1
0
1
0
1
1
0
1
1
c
0
0
0
0
0
0
0
0
0
0
0
0
d
1
1
1
1
1
1
1
1
1
1
1
1
Signaling Protocols
2 - 34
Limitations of analogue signaling systems
Signaling after set-up of the call
9
2 - 32
R2 - line signals
9
K0
Signaling Protocols
2 - 35
Only a small set of signals -> difficult to add new
services.
Context dependent semantics of signals -->
modularization of programs is difficult.
Signaling FSM controls the state of Exchange
resources on a micro -level --> complex call control..
Separate, e.g. DSPs are needed for signal detection
and translation of R2 and DTMF signals.
Voice channel and signaling channel have a fixed
mapping. No signaling unless voice channel has been
seized.
Difficult to control the call after the setup.
A lot of national and vendor specific variants.
© Rka/ML -k2008
Signaling Protocols
2 - 36
A Classification of Signaling
Outside voice band
Out of band
Common Channel
DSS1, ISUP
Set up
DSS 1, ISUP
Supervisory signaling
Line Signaling
CAS
Polarity reversal
on subscriber lines
In band
Register signaling
R2, DTMF
Rotary
During a call
and Release
CAS is used for this
segment as well
In voice band
© Rka/ML -k2008
Signaling Protocols
2 - 37
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