structure of the pstn
DESCRIPTION
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). - PowerPoint PPT PresentationTRANSCRIPT
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)
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Basic functional parts of the PSTN
PSTNSwitching in exchanges
Subscriber signalling (analog or ISDN=DSS1) Network-
internal signalling (SS7)
Transmission (PDH, SDH)
Databases in the network (HLR)
PSTN Circuit-switched technology
Based on 64 kbit/s channels (TDM time slots)Time Division Multiplexing (TDM)Connection-oriented operation (setup & release connection => call)Charging is based on time duration of connectionOptimized for delay-sensitive services (speech)
No fixed channel concept (bit rate is not constant)Statistical multiplexing (greater flexibility)Connectionless operation (independent routing of packets) as defaultMore flexible charging solutionsQoS solutions required for delay-sensitive services
Circuit-switched network Packet-switched network
IP network as alternative to PSTN
Switching in exchanges
Subscriber signalling (analog or ISDN=DSS1) Network-
internal signalling (SS7)
Transmission (PDH, SDH)
Databases in the network (HLR)
PSTN
IP network
Voice traffic can naturally also be carried over Packet-switched (IP) networks.This topic is covered in a future lecture.
Quality-of-Service (QoS) support needed!
Transmission: PDH or SDH systems
PSTNSwitching in exchanges
Subscriber signalling (analog or ISDN=DSS1) Network-
internal signalling (SS7)
Transmission (PDH, SDH)
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.
time
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)=> 8000 x 8 bit/s
PDH and SDH transmission bit rates
PDH (Plesiochronous Digital Hierarchy)
SONET (North Am.) SDHSTS-1 51.84 Mbit/sSTS-3 155.52 STM-1STS-12 622.08 STM-4STS-48 2.488 Gbit/s STM-16
Japan USA EuropeJ1 1.5 Mbit/s T1 1.5 Mbit/s E1 2 Mbit/s J2 6 T2 6 E2 8J3 32 T3 45 E3 34J4 98 T4 140 E4 140
Structure of E1 frame (2.048 Mbit/s)
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:
0 1 2 3116
... ...received bit stream ... where does a new frame begin?
Time slot 16 usually contains SS7 signalling information.
Structure of STM-1 frame in SDH
SOH
SOH
AU pointer indicates where the virtual container starts in the payload field
3
5
9 261 bytes
1
STM-1 payload (contains the actual information)
STM = Synchronous transport moduleSOH = Section overheadAU = Administrative unit
Higher-order STM-4 signal is generated using synchronous byte interleaving:
byte from first STM-1 signalbyte from second STM-1 signalbyte from third STM-1 signalbyte from fourth STM-1 signal
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Bitrate of STM-1 signal
SOH
SOH
3
5
9 261 bytes
1
STM-1 payload 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
SOH
VC-4 (Virtual container) POH
AU-4 pointer points to first byte of VC
1 260 bytes
Virtual container “floats” within the payload of STM-1
frames
9
POH = Path overhead
Filling of STM-1 payload in practice
P
In reality, the STM-1 payload is filled like this:
Beginning of next virtual container
Beginning of virtual container
P
Path overhead bytes
STM-1 frame N
STM-1 frame N+1
SDH pointer adjustment (1)
SOH
SOH
VC-4 (Virtual container)
When VC-4 clock rate is larger than STM-1 clock rate=> pointer value is shifted forward three bytes
Three “empty” bytes are
inserted here
oldnewPointer
value updated
SDH pointer adjustment (2)
SOH STM-1 payload
VC-4 (Virtual container)
When VC-4 clock rate is smaller than STM-1 clock rate => pointer value is shifted back three bytes
Three VC bytes are stored here
AU-4 pointer
oldnewPointer
value updated
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)
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Subscriber signalling
PSTNSwitching in exchanges
Subscriber signalling (analog or ISDN=DSS1) Network-
internal signalling (SS7)
Transmission (PDH, SDH)
Databases in the network (HLR)
Analog subscriber signalling
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.
The dialled called party (user B) number is sent to the local exchange in form of Dual Tone Multi-Frequency (DTMF) signal bursts.
Alerting (ringing) means that the local exchange sends a strong sinusoid to the terminal of user B.
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.
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Analog subscriber signalling in action
LE AUser A User B
Ringing signal
Off-hook (user B
answers)
Off-hook SS7 signalling
(ISUP)Dial toneB number
Ringback tone (or
busy tone)
LE B
Connection established
LE = local exchange
ISDN subscriber signalling in action
LE AUser A User B
Ringing
Off-hook (user B
answers)
Off-hook SS7 signalling
(ISUP)B number
Tones generated in terminal
LE B
Setup
Call proc Setup
Alert
ConnAlert
Conn
DSS1 signalling messages
Connection established
What does ISDN originally mean?
1. End-to-end digital connectivity2. Enhanced subscriber signaling3. 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
PSTN terminal
interaction is possible
Idea originated in the 1980’s
PSTN vs. ISDN user access
300 … 3400 Hz analog transmission band“Poor-performance” subscriber signaling
2 x 64 kbit/s digital channels (B channels)16 kbit/s channel for signaling (D channel) => Digital Subscriber Signalling system nr. 1 (DSS1)
PSTN
Basic Rate
Access ISDN
Primary Rate
Access ISDN
30 x 64 kbit/s digital channels (B channels)64 kbit/s channel for signaling (D channel)Mainly used for connecting private branch exchanges (PBX) to the PSTN.
End-to-end digital signalling
ISUPQ.931
Q.921I.430
Q.931
Q.921I.430
MTP 3MTP 2MTP 1
Q.931
Q.921I.430
Q.931
Q.921I.430
ISUPMTP 3MTP 2MTP 1
contains the signalling messages for call control
User interface User interfacePSTN Network
DSS1
SS7
DSS1
Signalling System nr. 7 (SS7)
PSTNSwitching in exchanges
Subscriber signalling (analog or ISDN=DSS1) Network-
internal signalling (SS7)
Transmission (PDH, SDH)
Databases in the network (HLR)
History of inter-exchange signalling
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.
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.
Before 1970, only channel-associated signalling (CAS) was used. In CAS systems, the signalling is carried in-band along with the user traffic.
CAS
CCIS
SS7
Channel-associated signalling (CAS)CAS means in-band signalling over the same physical channels as the circuit-switched user traffic (e.g. voice).
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).
Exchange ExchangeExchange
Circuit switched connection
Signalling is possible
Signalling is not possible before previous circuit-
switched link is established
CAS has two serious draw-backs:Setting up a circuit switched connection is very slow.•
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Common channel signalling (CCS)In practice, CCS = SS7.
Exchange Exchange
Signalling is possible anywhere anytime
Database
End-to-end signalling is possible before call setup and also during the conversation phase of a call.
The packet-switched signalling network is totally separated from the circuit-switched connections. Consequently:
Signalling to/from databases is possible anytime.• •
There is one drawback: It is difficult to check if the circuit-switched connections are really working (= continuity check).
Signalling example
ExchExchUser A
(calling 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.
OuluTokyo
London
User B (called user)
Exch
ISDN User Part
(ISUP)
Protocol layers (“levels”) of SS7
MTP level 3 (routing in the signalling network) MTP level 2 (link-layer protocol)
MTP level 1 (64 kbit/s PCM time slot)
Signalling Connection Control Part (SCCP)
Transaction Capabilities Application Part (TCAP)
Application protocols (e.g. Mobile Application Part, MAP)
MTP
MTP user
SS7 application protocol for managing
circuit-switched connections
MAP
ISUPTCAP
SCCP
SS7 protocols vs. OSI model
MTP level 3MTP level 2MTP level 1
…Application
PresentationSession
Transport
Network
Data linkPhysical
SS7 protocol stack OSI protocol layer model
OSI protocol layer model
Multiplexing & transport of bits, time slots in PDH or SDH systems
Switching & routing through the communications network Link-layer flow & error control
End-to-end flow & error control
User application (in this case, the actual signalling messages) Data compression & coding
Dialogue control
Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer
Message Trasfer Part (MTP) functions
MTP level 1 (signalling data link level): Digital transmission channel (64 kbit/s TDM time slot)
Frame-based protocol for flow control, error control (using Automatic Repeat reQuest, ARQ), and signalling network supervision and maintenance functions.
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 3 (signalling network level):
MTP level 2 (signalling link level):
MTP level 2 frame formats
F CK SIF SIO LI Control F
F CK SF LI Control F
F CK LI Control F
MSU (Message Signal Unit)
LSSU (Link Status Signal Unit)
FISU (Fill-In Signal Unit)
Level 3 user information
Network: • National• International
User part: • ISUP• SCCP• Signalling network management MSBLSB
MTP level 2 frames
MSU (Message Signal Unit):Contains actual SS7 signalling messagesThe 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) supervisionThe 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 endThe received frame is FISU if LI = 0
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Signalling points (SP) in SS7
Network elements (relevant from signalling point of view) contain signalling points identified by unique signalling point codes.
Exchange
STP SP
SPSTP
Signalling Point (in a database, such as HLR in mobile network)
Signalling Transfer Points only relay signalling messages
Signalling Point (signalling termination in an exchange)
STP
MAP
ISUP
Signalling point code (SPC)SS7 signalling messages contain MTP level 3 routing information in the form of a routing label:
SIO octetDPC
DPC
LSBMSB
OPCOPC
OPC SLSSignalling message
payload
International (and most national) signalling networks (ITU-T):14-bit Destination Point Code (DPC)14-bit Originating Point Code (OPC)4-bit Signalling Link Selection (SLS)
North American national signalling network (ANSI):24-bit DPC and OPC, 5-bit SLS code
Format for international SPC: Zone Area/Network SP3 bits 3 bits8 bits
For examples, see: www.numberingplans.com
Same SPCs can be reused at different network levels
SPC = 277
SPC = 277
International
National
SPC = 277 means different signalling points (network elements) at different network levels.
F CK SIF SIO LI Control F
The Service Information Octet (SIO) indicates whether the DPC and OPC are international or national signalling point codes.
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)
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Structure of ISUP message:SIO (one octet)
Routing label (four octets)CIC (two octets)
Message type (one octet) Mandatory fixed part
Mandatory variable partOptional part
Must always be included in ISUP messageE.g., IAM message
E.g., contains called (user B) number in IAM message
ISUP signalling messagesBasic ISUP signalling messages:
Call setup:
IAM (Initial address message)ACM (Address complete message)ANM (Answer message)
From LE A to LE B
From LE B to LE A
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.
Exchange
STP
ExchangeCircuit
Signalling link
Signalling using IAM message
Exchange ExchangeExchange
SPC = 82
Circuit 14
SPC = 22 SPC = 60Circuit 20
STPSL 4
SL 7
STP
Outgoing message:OPC = 82 CIC = 14DPC = 22 SLS = 4
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
LE A LE BTransit exchange User A User B
Setup IAMIAM
Setup
Alert
Connect
ACM
ANM
ACM
ANM
Alert
ConnectCharging of call starts now
Number analysisDSS1 signalling assumed
Call setup: Signalling sequence 1
User A User B
Off hook
Dial tone
B number
Local exchange detects setup request and returns dial tone
Local exchange: analyzes B number determines that call should be routed via transit exchange (TE)
LE A LE BTE
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Call setup: Signalling sequence 2
User A User BLE A LE BTE
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 User BLE A LE BTE
Address complete message (ACM)
Ringing signalRingback tone
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.)
or
Call setup: Signalling sequence 4
User A User BLE A LE BTE
Answer message (ANM)User B answers
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.
Charging starts now
Conversation over this “pipe”
E.164 numbering scheme
00
0
358 9
9
1234567
1234567
1234567
International number
National number
User numberPrefixCountry codeArea code
3589
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.
or mobile network code, e.g. 40
E.164 number structure
00 358 9 1234567
Prefix
For examples, see: www.numberingplans.com
Country code (1-3 digits)
National destination code (1-3 digits)
Max. 15 digits
Subscriber number
Area code, e.g. 9Mobile network code, e.g. 40
MSISDN number
Signalling sequence for call release
User A User BLE A LE BTE
On hook 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.)
Charging stops
Conversation over this “pipe”
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 DatabaseExchangeSTP with GTT capability
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.
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SCCP performs global title translation in the STP (0800 or IMSI number => DPC) and the SCCP message can now be routed to the database.
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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
STP
Database
Exchange
Exchange
ExchangeExchange
Database
Exchange
Example: SCCP usage in mobile call
SCCPSCCP
MSC located in Espoo HLR located in Oslo
STP
SPC = 82 SPC = 99
SPC = 32
SCCP/GTT functionality
Outgoing message:OPC = 82 DPC = 32SCCP: IMSI global title
Processing in STP:Received message is given to SCCP for GTT. SCCP finds the DPC of the HLR: DPC = 99
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.
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)
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To sum it up with an example…
PSTNTypical operation of a local exchange
Subscriber signalling (analog or ISDN=DSS1) Network-
internal signalling (SS7)
Transmission (PDH, SDH)
Databases in the network (HLR)
Part B, Section 3.3 in ”Understanding Telecommunications 2”
Basic local exchange (LE) architecture
Time switch
TDM links to other network elements
• Switch control
Switching system
• E.164 number analysis• Charging• User databases
LIC
LICToneRx
Group switch
Sign.
ETC
ETC
Exchange terminal circuit
Line interface circuit
SS7 Signalling equipment
Control system• O&M functions
Subscriber stage
Modern trend: Switching and control functions are separated into different network elements (separation of user and control plane).
Tone generator
Setup of a call (1)
Time switch
2. Check user database. For instance, is user A barred for outgoing calls?
Switching system
3. Reserve memory for user B number
LIC
LICToneRx
Group switch
Sign.
ETC
ETC
Control system
Phase 1. User A lifts handset and receives dial tone.
1. Off hook
Local exchange of user A
4. Tone Rx is connected
5. Dial tone is sent (indicating “network is alive”)
Tone generator
Time switch
3. Number analysis
Switching system
4. IN triggering actions? Should an external database (e.g. SCP, HLR) be contacted?
LIC
LICToneRx
Group switch
Sign.
ETC
ETC
Control system
Phase 2. Exchange receives and analyzes user B number.
2. Number (DTMF signal) received1. User A
dials user B number
Setup of a call (2)
Local exchange of user A
Time switch
2. Outgoing circuit is reserved
Switching system
LIC
LICToneRx
Group switch
Sign.
ETC
ETC
Control system
3. Outgoing signalling message (ISUP IAM) contains user B number
Phase 3. Outgoing circuit is reserved. ISUP Initial address message (IAM) is sent to next exchange.
Setup of a call (3)
1. Tone receiver is disconnected
Local exchange of user A
E.g., CIC = 24
IAM (contains information CIC = 24)
Time switch
1. ISUP ACM message indicates free or busy user B
Switching system
LIC
LIC Group switch
Sign.
ETC
ETC
Control system
3. Charging starts when ISUP ANM message is received
Phase 4. ACM received => ringback or busy tone generated. ANM received => charging starts.
Setup of a call (4)
Local exchange of user A
ACM, ANMTone generator2. Ringback
or busy tone is locally generated
4. Call continues…