Transcript Transport and Signaling in the PSTN

Structure of the PSTN
•
Transport or transmission (PDH, SDH)
•
Switching (see previous lecture)
•
Subscriber signalling (analog or digital)
•
Network-internal signalling (SS7)
•
Intelligent Network (IN) concept
•
Basic components also for circuit-switched
core of mobile networks (PLMN)
Basic functional parts of the PSTN
PSTN
Switching in
exchanges
Subscriber signalling
(analog or ISDN=DSS1)
Transmission
(PDH, SDH)
Networkinternal
signalling
(SS7)
Databases in
the network
(HLR)
PSTN  Circuit-switched technology
Circuit-switched network
Packet-switched network
Based on 64 kbit/s channels
(TDM time slots)
No fixed channel concept
(bit rate is not constant)
Time Division Multiplexing
(TDM)
Statistical multiplexing
(greater flexibility)
Connection-oriented
operation (setup & release
connection => call)
Connectionless operation
(independent routing of
packets) as default
Charging is based on time
duration of connection
More flexible charging
solutions
Optimized for delaysensitive services (speech)
QoS solutions required for
delay-sensitive services
IP network as alternative to PSTN
Voice traffic can
naturally also be
carried over Packetswitched (IP)
networks.
This topic is covered
in a future lecture.
Switching in
exchanges
Subscriber signalling (analog
or ISDN=DSS1)
PSTN
Networkinternal
signalling
(SS7)
IP network
Quality-of-Service (QoS) support needed!
Transmission
(PDH, SDH)
Databases in
the network
(HLR)
Transmission: PDH or SDH systems
PSTN
Switching in
exchanges
Subscriber signalling
(analog or ISDN=DSS1)
Transmission
(PDH, SDH)
Networkinternal
signalling
(SS7)
Databases in
the network
(HLR)
64 kbit/s channel (or TDM time slot)
This is the basic transport unit in both PDH and SDH transport
systems. Note that switching in exchanges in the PSTN is also
based on 64 kbit/s TDM time slots.
When used for voice transport, a 64 kbit/s channel contains PCM
(Pulse Code Modulation) speech, generated according to ITU-T
specification G.711.
Analog speech signal (300…3400 Hz)
Sampling produces 8000 samples/s
Each sample is encoded into
an 8-bit PCM code word
(e.g. 01100101)
time
=> 8000 x 8 bit/s
PDH and SDH transmission bit rates
PDH (Plesiochronous Digital Hierarchy)
Japan
J1
J2
J3
J4
1.5 Mbit/s
6
32
98
USA
T1
T2
T3
T4
1.5 Mbit/s
6
45
140
SONET (North Am.)
STS-1
STS-3
STS-12
STS-48
51.84 Mbit/s
155.52
622.08
2.488 Gbit/s
Europe
E1
E2
E3
E4
2 Mbit/s
8
34
140
SDH
STM-1
STM-4
STM-16
Structure of E1 frame (2.048 Mbit/s)
012
16
31
32 TDM time slots (with 8 bits each / frame)
Time slots 1-31 carry digital signals (usually PCM speech) with
a bitrate of 64 kbit/s.
Time slot 0 is used for frame synchronization:
received bit stream ... where does a new frame begin?
...
...
Time slot 16 usually contains SS7 signalling information.
Structure of STM-1 frame in SDH
9
3
SOH
1
5 SOH
261 bytes
STM-1 payload (contains the
actual information)
AU pointer indicates where
the virtual container starts
in the payload field
STM = Synchronous
transport module
SOH = Section overhead
AU = Administrative unit
Higher-order STM-4 signal is generated using synchronous
byte interleaving:
byte from first STM-1 signal
…
byte from second STM-1 signal
byte from third STM-1 signal
…
byte from fourth STM-1 signal
Bitrate of STM-1 signal
9
3
SOH
261 bytes
STM-1 payload
1
5 SOH
Basic idea: bytes
from a 64 kbit/s
channel are carried
in successive STM-1
frames (exactly one
byte per frame).
STM-1 frame contains 9 x 270 bytes
=> bitrate of STM-1 signal:
9 x 270 x 64 kbit/s = 155.52 Mbit/s
Mapping into STM-1 frames
SOH
AU-4 pointer points to
first byte of VC
SOH
Virtual container
“floats” within the
payload of STM-1
frames
P
O
H
VC-4 (Virtual container)
9
POH = Path overhead
1
260 bytes
Filling of STM-1 payload in practice
In reality, the
STM-1 payload
is filled like this:
P
Beginning of
virtual container
STM-1 frame N
P
STM-1 frame N+1
Path overhead
bytes
Beginning of
next virtual container
SDH pointer adjustment (1)
When VC-4 clock rate is larger than STM-1 clock rate
=> pointer value is shifted forward three bytes
SOH
Pointer
value
updated
SOH
Three “empty”
bytes are
inserted here
old
new
VC-4 (Virtual container)
SDH pointer adjustment (2)
When VC-4 clock rate is smaller than STM-1 clock rate
=> pointer value is shifted back three bytes
SOH
STM-1 payload
Pointer
value
updated
VC-4 (Virtual container)
AU-4 pointer
Three VC bytes
are stored here
old
new
Payload mapping
STM-1 can carry 63 E1 signals.
SDH systems nowadays also
carry ATM and IP traffic.
STM-1
More about SDH…
• SDH pocket guide (there is a link to this material on the
course home page)
• www.iec.org/online/tutorials/sdh
• Section 4.4.1 in ”Understanding Telecommunications 1”
by Ericsson Telecom, Telia and Studentlitteratur 1998
(the corresponding online course is sometimes available
at www.ericsson.com)
Subscriber signalling
PSTN
Switching in
exchanges
Subscriber signalling
(analog or ISDN=DSS1)
Transmission
(PDH, SDH)
Networkinternal
signalling
(SS7)
Databases in
the network
(HLR)
Analog subscriber signalling
1
The calling party (user A) tells the local exchange to set
up (disconnect) a call by generating a short (open) circuit
in the terminal => off-hook (on-hook) operation.
2
The dialled called party (user B) number is sent to the
local exchange in form of Dual Tone Multi-Frequency
(DTMF) signal bursts.
3
Alerting (ringing) means that the local exchange sends a
strong sinusoid to the terminal of user B.
4
In-channel information in form of audio signals (dial tone,
ringback tone, busy tone) is sent from local exchange to
user. User can send DTMF information to network.
Analog subscriber signalling in action
User A
LE A
Off-hook
Dial tone
B number
Ringback
tone (or
busy tone)
Connection established
LE B
SS7
signalling
(ISUP)
User B
LE = local exchange
Ringing
signal
Off-hook
(user B
answers)
ISDN subscriber signalling in action
User A
LE A
Off-hook
B number
Setup
Call proc
Tones
generated
in terminal
LE B
SS7
signalling
(ISUP)
User B
DSS1 signalling
messages
Setup
Alert
Ringing
Conn
Off-hook
(user B
answers)
Alert
Conn
Connection established
What does ISDN originally mean?
1. End-to-end digital connectivity
2. Enhanced subscriber signaling
Idea originated
in the 1980’s
3. A wide variety of new services (due to 1 and 2)
4. Standardized access interfaces and terminals
ISDN is not a “new” network separated from the PSTN.
Interworking with “normal” PSTN equipment is very
important.
ISDN
terminal
interaction is
possible
PSTN
terminal
PSTN vs. ISDN user access
PSTN
300 … 3400 Hz analog transmission band
Basic
Rate
Access
ISDN
2 x 64 kbit/s digital channels (B channels)
Primary
Rate
Access
ISDN
30 x 64 kbit/s digital channels (B channels)
“Poor-performance” subscriber signaling
16 kbit/s channel for signaling (D channel)
=> Digital Subscriber Signalling system nr. 1
(DSS1)
64 kbit/s channel for signaling (D channel)
Mainly used for connecting private branch
exchanges (PBX) to the PSTN.
End-to-end digital signalling
User interface
Q.931
Q.931
DSS1
PSTN Network
ISUP
SS7
ISUP
MTP 3
MTP 3
User interface
Q.931
Q.931
DSS1
Q.921
Q.921
MTP 2
MTP 2
Q.921
Q.921
I.430
I.430
MTP 1
MTP 1
I.430
I.430
contains the signalling messages for call control
Signalling System nr. 7 (SS7)
PSTN
Switching in
exchanges
Subscriber signalling
(analog or ISDN=DSS1)
Transmission
(PDH, SDH)
Networkinternal
signalling
(SS7)
Databases in
the network
(HLR)
History of inter-exchange signalling
CAS
Before 1970, only channel-associated signalling (CAS)
was used. In CAS systems, the signalling is carried inband along with the user traffic.
CCIS
SS6 = CCIS (common channel interoffice signaling) was
deployed in North America as an interim solution, but not
in Europe. CCIS is not the same thing as SS7.
SS7
Starting from 1980 (mainly in Europe), CAS was being
replaced by SS7. The use of stored program control
(SPC) exchanges made this possible. Like CCIS,
signalling messages are transmitted over separate
signalling channels. Unlike CCIS, SS7 technology is not
monolithic, but based on protocol stacks.
Channel-associated signalling (CAS)
CAS means in-band signalling over the same physical channels as
the circuit-switched user traffic (e.g. voice).
Signalling is possible
Exchange
Exchange
Exchange
Circuit switched connection
Signalling is not possible
before previous circuitswitched link is established
CAS has two serious draw-backs:
• Setting up a circuit switched connection is very slow.
• Signalling to/from databases is not feasible in practice (setting
up a circuit switched connection to the database and then
releasing it would be extremely inconvenient).
Common channel signalling (CCS)
In practice, CCS = SS7.
Signalling is possible anywhere anytime
Exchange
Exchange
Database
The packet-switched signalling network is totally separated from
the circuit-switched connections. Consequently:
• Signalling to/from databases is possible anytime.
• End-to-end signalling is possible before call setup and also
during the conversation phase of a call.
There is one drawback: It is difficult to check if the circuit-switched
connections are really working (= continuity check).
Signalling example
Tokyo
User A
(calling
user)
Oulu
Exch
Exch
Exch
London
User B
(called
user)
Database
A typical scenario:
User A calls mobile user B. The call is routed to a specific
gateway exchange (GMSC) that must contact a database
(HLR) to find out under which exchange (MSC) the mobile
user is located. The call is then routed to this exchange.
Protocol layers (“levels”) of SS7
ISDN User Part
(ISUP)
MTP
user
SS7 application
protocol for
managing circuitswitched connections
Application protocols (e.g.
Mobile Application Part, MAP)
Transaction Capabilities
Application Part (TCAP)
Signalling Connection
Control Part (SCCP)
MTP level 3 (routing in the signalling network)
MTP
MTP level 2 (link-layer protocol)
MTP level 1 (64 kbit/s PCM time slot)
SS7 protocols vs. OSI model
SS7 protocol stack
MAP
OSI protocol
layer model
…
TCAP
Application
Presentation
ISUP
Session
Transport
SCCP
MTP level 3
Network
MTP level 2
Data link
MTP level 1
Physical
OSI protocol layer model
Application layer
Presentation layer
Session layer
User application (in this case,
the actual signalling messages)
Data compression & coding
Dialogue control
Transport layer
End-to-end flow & error control
Network layer
Switching & routing through the
communications network
Data link layer
Link-layer flow & error control
Physical layer
Multiplexing & transport of bits,
time slots in PDH or SDH systems
Message Trasfer Part (MTP) functions
MTP level 1 (signalling data link level):
Digital transmission channel (64 kbit/s TDM time slot)
MTP level 2 (signalling link level):
Frame-based protocol for flow control, error control (using
Automatic Repeat reQuest, ARQ), and signalling network
supervision and maintenance functions.
MTP level 3 (signalling network level):
Routing in the signalling network between signalling points
(using signalling point codes).
MTP level 3 ”users” are ISUP and SCCP (other ”users” such
as TUP or DUP are not widely used any more).
MTP level 2 frame formats
Level 3 user information
MSU (Message Signal Unit)
F
CK
SIF
SIO
LSSU (Link Status Signal Unit)
F
CK
SF
LI
Control
FISU (Fill-In Signal Unit)
F
LSB
CK
LI
Control
F
MSB
F
LI
Control
Network:
• National
• International
User part:
• ISUP
• SCCP
• Signalling
network
management
F
MTP level 2 frames
MSU (Message Signal Unit):
• Contains actual SS7 signalling messages
• The received frame is MSU if LI > 2
(LI = number of octets)
LSSU (Link Status Signal Unit):
• Contains signalling messages for MTP level 2
(signalling link) supervision
• The received frame is LSSU if LI = 1 or 2
FISU (Fill-In Signal Unit):
• Can be used to monitor quality of signalling link
at receiving end
• The received frame is FISU if LI = 0
Signalling points (SP) in SS7
Network elements (relevant from signalling point of view) contain
signalling points identified by unique signalling point codes.
Signalling Transfer Points only relay signalling messages
STP
STP
SP
Signalling Point (in a database,
such as HLR in mobile network)
MAP
STP
SP
ISUP
Exchange
Signalling Point (signalling
termination in an exchange)
Signalling point code (SPC)
SS7 signalling messages contain MTP level 3 routing information in
the form of a routing label:
MSB
International (and most national)
signalling networks (ITU-T):
LSB
SIO octet
14-bit Destination Point Code (DPC)
14-bit Originating Point Code (OPC)
4-bit Signalling Link Selection (SLS)
DPC
DPC
OPC
OPC
OPC
SLS
North American national signalling
network (ANSI):
Signalling message
payload
24-bit DPC and OPC, 5-bit SLS code
Format for international SPC:
Zone
3 bits
Area/Network
8 bits
SP
3 bits
For examples, see:
www.numberingplans.com
Same SPCs can be reused at different
network levels
International
SPC = 277
SPC = 277
National
SPC = 277 means different signalling points (network elements)
at different network levels.
The Service Information Octet (SIO) indicates whether the DPC
and OPC are international or national signalling point codes.
F
CK
SIF
SIO
LI
Control
F
ISDN User Part (ISUP)
ISUP is a signalling application protocol that is used for establishing
and releasing circuit-switched connections (calls).
• Only for signalling between exchanges (ISUP can never be
used between an exchange and a stand-alone database)
• Not only for ISDN (=> ISUP is generally used in the PSTN)
Structure of ISUP message:
SIO (one octet)
Routing label (four octets)
CIC (two octets)
Message type (one octet)
Mandatory fixed part
Mandatory variable part
Optional part
Must always be included in ISUP message
E.g., IAM message
E.g., contains called (user B) number in
IAM message
ISUP signalling messages
Basic ISUP signalling messages:
Call setup:
IAM (Initial address message)
ACM (Address complete message)
From LE A to LE B
From LE B to LE A
ANM (Answer message)
Call release:
REL (Release message)
RLC (Release complete message)
Direction depends
on releasing party
(user A or user B)
Difference between SLS and CIC
The four-bit signalling link selection (SLS) field in the routing
label defines the signalling link which is used for transfer of the
signalling information.
The 16-bit circuit identification code (CIC) contained in the
ISUP message defines the TDM time slot or circuit with which
the ISUP message is associated.
Signalling link
STP
Exchange
Exchange
Circuit
Signalling using IAM message
STP
STP
SL 4
SL 7
SPC = 82
SPC = 22
Circuit
14
Exchange
Outgoing message:
OPC = 82 CIC = 14
DPC = 22 SLS = 4
Exchange
Circuit
20
SPC = 60
Exchange
Processing in (transit) exchange(s):
Received IAM message contains B-number.
Exchange performs number analysis (not part of
ISUP) and selects new DPC (60) and CIC (20).
Setup of a call using ISUP
User A
Setup
DSS1
signalling
assumed
Alert
Connect
LE A
Transit exchange
IAM
LE B
IAM
User B
Setup
Number analysis
ACM
ANM
Charging of call starts now
ACM
ANM
Alert
Connect
Call setup: Signalling sequence 1
User A
Off hook
LE A
TE
LE B
Dial tone
Local exchange detects setup
request and returns dial tone
B number
Local exchange:
• analyzes B number
• determines that call
should be routed via
transit exchange (TE)
User B
Call setup: Signalling sequence 2
User A
LE A
TE
LE B
User B
Initial address message (IAM)
ISUP message IAM is sent to transit exchange (TE).
TE analyzes B number and determines that call should
be routed to local exchange of user B (LE B).
IAM message is sent to LE B.
There now exists a circuit-switched path (the path is
“cut through”) between user A and LE B.
Call setup: Signalling sequence 3
User A
LE A
Ringback
tone
TE
Address complete
message (ACM)
LE B
User B
Ringing signal
or
Ringing signal is sent to user B (=> user B is alerted).
Ringback tone (or busy tone) is sent to user A.
(Ringback/busy tone is generated locally at LE A or is
sent from LE B through circuit switched path.)
Call setup: Signalling sequence 4
User A
Charging
starts now
LE A
TE
LE B
Answer message (ANM)
User B
User B answers
Conversation over this “pipe”
User B answers, connection is cut through at LE B.
Charging of the call starts when ISUP message ANM is
received at LE A (the normal case).
The 64 kbit/s bi-directional circuit switched connection
is now established.
E.164 numbering scheme
In each exchange, the B number is analyzed at call setup
(after the IAM message containing the number has been
received) and a routing program (not part of ISUP)
selects the next exchange to which the call is routed.
00
358
9
1234567
International number
0
9
1234567
National number
1234567
User number
Prefix
Country code
358
Area code
or mobile network code, e.g. 40
9
E.164 number structure
Max. 15 digits
00
358
Prefix
Country code
(1-3 digits)
9
1234567
Subscriber number
National destination code (1-3 digits)
Area code, e.g. 9
Mobile network code, e.g. 40
For examples, see:
www.numberingplans.com
MSISDN number
Signalling sequence for call release
User A
LE A
TE
LE B
User B
Conversation over this “pipe”
On hook
Charging
stops
Release message (REL)
Release complete message (RLC)
The circuits between exchanges are released one by one.
(The generation of “hanging circuits” should be avoided,
since these are blocked from further use.)
Signalling Connection Control Part (SCCP)
SCCP is required when signalling information is carried between
exchanges and databases in the network.
An important task of SCCP is global title translation (GTT):
STP with GTT capability
Exchange
STP
Database
1. Exchange knows the global title (e.g. 0800 number or IMSI
number in a mobile network) but does not know the DPC of
the database related to this global title.
2. SCCP performs global title translation in the STP (0800 or
IMSI number => DPC) and the SCCP message can now be
routed to the database.
Why GTT in STP network node?
Global title translation (GTT) is usually done in an STP.
Advantage: Advanced routing functionality (= GTT) needed
only in a few STPs with large packet handling capacity,
instead of many exchanges.
Exchange
Exchange
Database
Database
STP
Exchange
Exchange
Exchange
Exchange
Example: SCCP usage in mobile call
Mobile switching center (MSC) needs to contact the home location
register (HLR) of a mobile user identified by his/her International
Mobile Subscriber Identity (IMSI) number.
SCCP/GTT functionality
STP
SCCP
MSC located in Espoo
SPC = 82
Outgoing message:
OPC = 82 DPC = 32
SCCP: IMSI global title
SPC = 32
SCCP
HLR located in Oslo
SPC = 99
Processing in STP:
Received message is given to SCCP for GTT.
SCCP finds the DPC of the HLR: DPC = 99
More about SS7…
• Chapter 4 in ”Engineering Networks for Synchronization,
CCS7, and ISDN” by P.K.Bhatnagar 1997 (this belongs to
the distributed course material)
• www.iec.org/online/tutorials/ss7
• Part E in ”Understanding Telecommunications 2” by
Ericsson Telecom, Telia and Studentlitteratur 1998
(the corresponding online course is sometimes available
at www.ericsson.com)
To sum it up with an example…
Part B, Section 3.3 in ”Understanding
Telecommunications 2”
PSTN
Typical operation of
a local exchange
Subscriber signalling
(analog or ISDN=DSS1)
Transmission
(PDH, SDH)
Networkinternal
signalling
(SS7)
Databases in
the network
(HLR)
Basic local exchange (LE) architecture
Modern trend: Switching and control functions are separated into
different network elements (separation of user and control plane).
Subscriber stage
LIC
Time
switch
LIC
Tone
Rx
Tone generator
Line
interface
circuit
Switching system
Group
switch
TDM links
to other
network
elements
ETC
ETC
Sign.
Exchange
terminal
circuit
• Switch control
• E.164 number analysis
• Charging
• User databases
• O&M functions
SS7 Signalling
equipment
Control system
Setup of a call (1)
Phase 1. User A lifts handset and receives dial tone.
Local exchange of user A
4. Tone Rx is connected
1. Off hook
LIC
LIC
5. Dial tone
is sent
(indicating
“network is
alive”)
Time
switch
Tone
Rx
Tone generator
Switching system
ETC
Group
switch
ETC
Sign.
2. Check user database. For instance, is
user A barred for outgoing calls?
3. Reserve memory for user B number
Control system
Setup of a call (2)
Phase 2. Exchange receives and analyzes user B number.
Local exchange of user A
Switching system
LIC
LIC
1. User A
dials user B
number
Time
switch
Tone
Rx
2. Number (DTMF
signal) received
Group
switch
ETC
ETC
Sign.
3. Number analysis
4. IN triggering actions? Should an external
database (e.g. SCP, HLR) be contacted?
Control system
Setup of a call (3)
Phase 3. Outgoing circuit is reserved. ISUP Initial address message
(IAM) is sent to next exchange.
Local exchange of user A
Switching system
LIC
Time
switch
LIC
Tone
Rx
Group
switch
1. Tone receiver
is disconnected
ETC
E.g.,
CIC = 24
ETC
Sign.
2. Outgoing circuit is reserved
3. Outgoing signalling message (ISUP IAM)
contains user B number
Control system
IAM
(contains
information
CIC = 24)
Setup of a call (4)
Phase 4. ACM received => ringback or busy tone generated. ANM
received => charging starts.
Local exchange of user A
Switching system
LIC
Time
switch
LIC
2. Ringback
or busy tone
is locally
generated
4. Call
continues…
Tone generator
Group
switch
ETC
ETC
Sign.
1. ISUP ACM message indicates free or busy
user B
3. Charging starts when ISUP ANM message
is received
Control system
ACM,
ANM