S38.115 Signaling Protocols Modeling of signaling systems Subscriber signaling

S38.115 Signaling Protocols Modeling of signaling systems Subscriber signaling
S38.115 Signaling Protocols
ÿ Modeling of signaling systems
ÿ Signaling flow charts
ÿ (Extended) Finite state machines
ÿ Subscriber signaling
PSTN
ÿ Impulse code
ÿ Multifrequency code (DTMF - dual tone multifrequency)
ÿ Trunk signaling
ÿ Register signaling
ÿ Line signaling
ÿ R2 signaling system
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Signaling Protocols
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Summary of course scope
SIP or
ISUP
H.323 or
SIP
IP
CAS, R2
PABX
ISDN
D
IP
Control Part
of an Exchange
Or
Call Processing
Server
V5
ISUP
INAP
Megaco/MGCP/…
circuit
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HLR/
HSS
MAP
CCS7
AN
r
ete
m
a
i
Media Gateway
or Switching Fabric
Signaling Protocols
SCP
packets
2-2
Signaling Flow Chart illustrates the main events
Calling Subscriber (A)
Originating Exchange
Local loop
on-hook
Terminating Exchange
Called Subscriber (B)
Local loop
trunk
on-hook
off-hook
Dial tone
1. digit
.
:
last. nr
Seizure
Start dialing
1. Address signal
.
: Last address signal
Alerting or ringing tone
Ringing
off-hook
Answer
Through Connection
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Signaling Protocols
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Signaling Flow Chart example cont...
Calling subscriber (A)
Originating Exchange
Local loop
trunk
Terminating Exchange
Called party (B)
Local loop
Information/voice transfer
on-hook
Release forward
on-hook
Clear-back
• NB: Exchanges must signal both in forward and backward direction
on incoming and on outgoing side simultaneously.
• Incoming and Outgoing signaling can be separated, so can
• Incoming Call Control and Outgoing Call control.
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Extended Finite State Machine is very suitable
for modeling signaling senders and receivers.
Algebraic representation
< s0, I, O, U, S, fs, fo , fu >
Is ⊂ I - set of possible incoming
signals in state s
s0 - initial state
I - set of incoming signals
O - set of outgoing signals
S - Set of States
U - Set of values of state variables
fs : (S×I×U) → S - next state
fo : (S×S) → O - outgoing signal
fu : (S×S) → us ⊂ U - new values of state
variables
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i ∈ Is can be unique in the
signaling system or context
dependent.
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Graphical representation of an FSM
i2/o3
s1
s2
5
i5 /o
i 6/o
2
i 1/o 1
s0
s4
i3/o2
i 6/o 2
s5
i5/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 Systems are FSMs or sets of FSMs.
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SDL representation of an FSM
SDL - Spesification and Description
Language
State a
Received
signal
Received
signal
Task
Received
signal
Condition
Sent
signal
Condition
Sent
signal
Task
State x
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State z
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A subscriber as an SDL -state machine
On-hook
Wish to call
Off-hook
WF-dial tone
Dial tone
Ringing
Off-hook
Talk
A
Push a digit button
Call State
Push digit
on-hook
WF_ ringing tone
Ringing tone
on-hook
Wish to disconnect
On-hook
WF Answer
B-Answer
Talk
A
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No-Answer
What is missing in the figure??
on-hook
On-hook
Signaling Protocols
2-8
You can model the world like this
Model of
the
Environment
System
under
Development
Model of the
Environment
• Can use verification tools to debug your design.
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Execution models of FSM programs
Initialisation
Do Forever
Receive Message
A <- Branch (State, (Secondary state,) Message)
Execute Transition (A)
Od
• 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.
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Table representation of an FSM
Current State
i0
s1
s1
s0
s1
s2
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Next State
Incoming signal
i1
i2
s0
s0
s2
s1
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Signaling is used to allocate network resources for
the call
ÿ
ÿ
ÿ
ÿ
ÿ
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…).
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A Signaling System
ÿ
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:
ÿ
ÿ
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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Subscriber or loop signaling
ÿ
ÿ
ÿ
The terminal (a phone) sends information to the network
in either rotary impulses or in Dual-Tone-Multifrequency (DTMF-) signals.
A DTMF-signal has two frequencies out of six.
Such Frequencies are used that they have no harmonic
components with the other frequencies:
°
°
°
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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)
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DTMF-signals are created with a push
button phone
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1209Hz
1336Hz
1447Hz
1633Hz
697Hz
1
2
3
A
770Hz
4
5
6
B
852Hz
7
8
9
C
941Hz
*
0
#
D
Signaling Protocols
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Impulse signals are created by the rotary
disk
ÿ
ÿ
ÿ
ÿ
ÿ
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
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Signaling Protocols
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Responses to the terminal
ÿ
Terminal receives the following indications as responses
to the signals it has sent:
Semantics
Frequency
Timing
Dial tone
425 Hz
continuous
Ringing tone
425 Hz
1s on, 4s silence
Engaged/Busy
425 Hz
300 ms on, 300 ms off
Queueing
950 Hz
950 Hz
1400 Hz
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
exchanges usually do not see them!
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Call establishment procedure or signaling sets
up the call between two parties across the network
ÿ
ÿ
ÿ
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.
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.
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Line signaling takes care of call
supervision and tear-down (release)
ÿ
ÿ
ÿ
ÿ
ÿ
ÿ
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.
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Number Analysis links the information received
from signaling to call routeing
ÿ
Analysis result is determined by
° Dialed digits ( from call set up signaling)
° Incoming circuit group,
° Origin or subscriber category (e.g. operator - see R2 group II)
ÿ
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
ÿ
Analysis trees are built by MML-commands issued by
the operator based on a route plan
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An example of route descriptions
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.
Primary routeing
alternative
Second routeing
alternative
Route 1
Last alternative
Route 2
Route 3
Trunk
group
Nodes of this tree may contain information
that is needed in signaling, for example:
When to start end-to-end signaling etc...
seizure = search and reservation of a free circuit or trunk
Outgoing circuits or trunks
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Different algorithms exist for seizure.
Circuit groups can be either unidirectional
or bi-directional (as cmp. to call set-up)
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Some Signals used in trunk signaling
Line/Set-up
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Signal
Direction
L
L
seizing signal
seizing-acknowledgement
S
S
request for an address signal
address signal
<--->
S
S
congestion signals
address complete signals
<-<--
S
S
subscriber free (charge)
subscriber free (no charge)
<-<--
S
L
subscriber line busy
answer signal
<-<--
L
L
charging pulse
clear-back signal
<-<--
L
L
release-guard signal
clear-forward
<--->
L
L
blocking
remove blocking
<-<--
Signaling Protocols
--> (forward)
<-- (backward)
2 - 22
Channel Associated Signaling (CAS)
ÿ
ÿ
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 may be based on space or
frequency or time division multiplexing.
°
°
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.
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R2 and N2
ÿ
ÿ
ÿ
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. Sender continues sending a signal until it sees an
acknowledgement signal from the the other end. This
ensures reliable and fast operation.
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Compelled signaling method
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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R2 and carriage of signals
ÿ
ÿ
ÿ
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.
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.
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Signaling Protocols
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‘Forward’-signals
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Signal
1
Group I
1
Group II
Ordinary subscriber
2
3
2
3
Subscriber with priority
Test call
4
5
4
5
Coin box
Operator
6
7
6
7
Data transmission call
Ordinary subscriber
8
9
8
9
Data transmission call
Priority extension
10
11
0
Special serv operator
Operator
Forwarded call
12
13
Negative ack
Test equipment
National signal
National signal
14
15
Network Operator specific
End of pulsing
National signal
National signal
Signaling Protocols
2 - 27
‘Backward’-signals
Signal
1
Group A
Send next digit
Group B
subscriber line free
2
3
Repeat last but one address signal
Hop to receiving Group B signals
Send special info tone
subscriber line busy
4
5
Congestion in national network
Send A-subscriber category
Congestion
unallocated number
6
7
Connect to voice path
Repeat number n - 2
subscriber line free, charge
subscriber line free, no charge
8
9
Repeat number n - 3
Send country code of A-subs
subscriber line out of order
reroute to operator
Network Operator Specific
subscriber number changed
10
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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PCM-frame structure
1 ylikehys = =
1616
kehyst
1 multi-frame
frames
K0
K1
K2
K3
K4
K5
K6
K7
K8
K9 K10 K11 K12 K13 K14 K15
1 frame
(odd frame)
1 kehys==32
32time
aikavslots
li (pariton
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
KL
puhekanavat
1-151 - 15
Voice
channels
MA
Frame alignement
kehyslukitustime slot T0
aikav li T0
Signaling
merkinantotime
slot
T16
aikav
li T16
B1 B2 B3 B4 B5 B6 B7 B8
C 1 A D D D D D
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
puhekanavat
16-3016 - 30
Voice
channels
kanavan 1
Channel
1
merkinantobitit
signaling
bits
kaukop
h lytys
Far
endn alarm
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kanavan 16
merkinantoChannel 16
bitit
signaling
bits
Signaling Protocols
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Even numbered PCM 30 -frame
1 ylikehys ==16
1 multi-frame
16kehyst
frames
K0
K1
K2
K3
K4
K5
K6
K7
K8
K9 K10 K11 K12 K13 K14 K15
frame==3232aikav
time lislots
(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
KL
puhekanavat
Voice
channels1-15
1 - 15
Frame alignement
kehyslukitustime slot
T0
aikav li 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
kehyksess
in even frames
CRC-bitti
CRC
-bit
MA
Signaling
merkinantotime slot
aikav
li T16T16
B1 B2 B3 B4 B5 B6 B7 B8
0 0 0 0 1 A 1 1
ylikehysMulti-frame
lukitusmerkki
alignement
kehyksess 0
in frame 0
puhekanavat
Voice channels
16 - 16-30
30
puhekanava 26
aikaväli T27
B1 B2 B3 B4 B5 B6 B7 B8
näytteen
amplitudin
Voice Sample
suuruus
amplitude value
Multi-frame
ylikehyslukitush
alarmlytys
polariteetti
polarity
Applies only to K0, other
even numbered, look at the previous slide
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Signaling Protocols
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R2 - line signals
ÿ
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:
NB first abcd
are forward bits
second abcd are
backward bits
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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
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Signaling after set-up of the call
ÿ
ÿ
ÿ
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”.
° It must be possible to detect DTMF -signals among voice.
° Polarity reversal can cause seizure of a register during a call.
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Signaling Protocols
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Limitations of analogue signaling systems
ÿ
ÿ
ÿ
ÿ
ÿ
ÿ
ÿ
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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.
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