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VoIP and SS7
Chapter 7
2Internet Telephony
Basic functional parts of the PSTN
PSTN
Switching in exchanges
Subscriber signalling (analog or ISDN=DSS1) Network-
internal signalling (SS7)
Transmission (PDH, SDH)
Databases in the network (HLR)
3Internet Telephony
PDH and SDH transmission bit rates
PDH (Plesiochronous Digital Hierarchy)
SONET (North Am.) SDH
STS-1 51.84 Mbit/sSTS-3 155.52 STM-1STS-12 622.08 STM-4STS-48 2.488 Gbit/s STM-16
Japan USA Europe
J1 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
4Internet Telephony
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.
5Internet Telephony
Subscriber signalling
PSTN
Switching in exchanges
Subscriber signalling (analog or ISDN=DSS1) Network-
internal signalling (SS7)
Transmission (PDH, SDH)
Databases in the network (HLR)
6Internet Telephony
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.
1
2
3
4
7Internet Telephony
Analog subscriber signalling in action
LE AUser A User B
Ringing signal
Off-hook (user B
answers)
Off-hook SS7 signalling
(ISUP)Dial tone
B number
Ringback tone (or
busy tone)
LE B
Connection established
LE = local exchange
8Internet Telephony
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
Conn
Alert
Conn
DSS1 signalling messages
Connection established
9Internet Telephony
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.
10Internet Telephony
End-to-end digital signalling
ISUPISUPQ.931Q.931
Q.921Q.921
I.430I.430
Q.931Q.931
Q.921Q.921
I.430I.430
MTP 3MTP 3
MTP 2MTP 2
MTP 1MTP 1
Q.931Q.931
Q.921Q.921
I.430I.430
Q.931Q.931
Q.921Q.921
I.430I.430
ISUPISUP
MTP 3MTP 3
MTP 2MTP 2
MTP 1MTP 1
contains the signalling messages for call control
User interface User interfacePSTN Network
DSS1
SS7
DSS1
11Internet Telephony
Introduction
Channel Associated Signaling Still widely deployed today Considered as old telephony
Common Channel Signaling Separation of signaling and call
paths Signaling System 7 (SS7)
To enable a wide range of services to be provided to the end-user
Caller ID, toll-free calling, call screening, number portability, etc.
SS7 is the foundation for Intelligent Network (IN) services.
12Internet Telephony
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).
ExchangeExchange ExchangeExchange
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.•
•
13Internet Telephony
Common channel signalling (CCS)
In practice, CCS = SS7.
ExchangeExchange ExchangeExchange
Signalling is possible anywhere anytime
DatabaseDatabase
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).
14Internet Telephony
Class 5 End Office Switch
The Telephone Network [1/2]
Circuit Switched Network
Intelligent Peripheral
Signal Transfer Point
Service Control Point
Class 4 Tandem Switch
Service Data Point
+
Transport Layer
Control Layer
SS7 Signaling
ISUP Messages
INAP/TCAP Messages
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The Telephone Network [2/2]
5 Basic Components in Intelligent Networks SSP/Service Switching Point
switching, service invocation STP/Service Transfer Point
signal routing SCP/Service Control Point
service logic execution SDP/Service Data Point
subscriber data storage, access IP/Intelligent Peripheral
resources such as customized voice announcement, voice recognition, DTMF digit collection
SSPSSP
SCPSCP SDPSDP
STPSTPIPIP
SSPSSP
STPSTP
TCAP messages
ISUP messages
Voice
16Internet Telephony
Signalling example
ExchExchExchExchUser A
(calling user)
User A (calling user)
DatabaseDatabase
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)
User B (called user)
ExchExch
17Internet Telephony
SS7 Protocol Suite
ISUPTCAP
SCCP
MAP
MTP Level 3
MTP Level 2
MTP Level 1
OSI Layers
Application
Presentation
Session
Transport
Network
Data Link
Physical
INAP
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MTP Levels 1 & 2
Message Transfer Part Level 1
Handling the issues related to the signals on the physical links between one signaling node and another
Closely to layer 1 of the OSI stack Level 2
Dealing with the transfer of messages on a given link from one node to another
Providing error detection/correction and sequenced delivery of the SS7 messages
signalling network supervision and maintenance functions
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MTP Level 3
Signaling message handling Providing message routing between signaling
points in the SS7 network May pass a number of intermediate nodes
(STP, Signal Transfer Point) MTP level 3 ”users” are ISUP and SCCP
Signaling network management Rerouting traffic to other SS7 signaling links
in the case of link failure, congestion or node failure
Load-sharing
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Provides a number of services to the protocol layer above it
The transfer of messages Indicating availability of resources MTP-Transfer request, MTP-Transfer indication,
MTP_Pause indication, MTP-Resume indication, and MTP-Status indication
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ISUP
ISDN User Part Used as the protocol for setting up and
tearing down phone calls between switches
Initial Address Message (IAM) To initiate a call between two switches
Answer Message (ANM) To indicate that a call has been accepted by
the called party Release Message (REL)
To initiate call disconnection
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A connection-oriented protocol Related to the establishment of connections
between users The path of messages and the path of the bearer
might be different
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SCCP
Signaling Connection Control Part Used as the transport layer for TCAP-
based services freephone (800/888), calling card, wireless
roaming Both connection-oriented and
connectionless Mostly connectionless signaling
Global title translation (GTT) capabilities The destination signaling point and subsystem
number is determined from the global title
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TCAP, MAP and INAP
TCAP (Transaction Capabilities Applications Part) Supporting the exchange of non-circuit
related information between signaling points
Queries and responses sent between SSPs and SCPs are carried in TCAP messages
Provides services to INAP (IN Application Part) MAP (Mobile Application Part)
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SS7 Network Architecture
Figure 7-4 depicts a typical SS7 network arrangement.
This configuration serves several purposes.
No direct signaling links A fully meshed signaling network is not
required. The quad arrangement ensures great
robustness.
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Signaling Point (SP)
Each node in an SS7 network is an SP. The signaling address of the SP is
known as a signaling point code (SPC). Linkset
Group of signaling links directly connecting two SPCs
For capability and security reasons Service Switching Point (SSP)
27Internet Telephony
Signal Transfer Point (STP)
To transfer messages from one SPC to another
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Service Control Point (SCP)
A network entity that contains additional logic and that can be used to offer advanced services
The switch sends a message to the SCP asking for instructions. The SCP, based upon data and service logic
that is available, will tell the switch which actions need to be taken.
An good example – toll-free 800 number
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An example A subscriber dials a toll-free 800 number The SSP knows that it needs to query the SCP The SCP contains the translation information The SCP responds to the SSP with a routable
number The SSP routes the call Connectionless signaling The application use the services of TCAP, which in
turn uses the services of SCCP
30Internet Telephony
Message Signal Units (MSUs) The messages sent in the SS7 network
•Backward Sequence Number
•BSN Indicator Bit
•Forward Sequence Number
•Length Indicator
31Internet Telephony
Message Signal Units (MSUs)
The messages sent in the SS7 network The format of an MSU
SIO – Service Information Octet Indicate the upper-level protocol (e.g., SCCP or
ISUP) A sub-service field indicating the signaling
numbering plan SIF – Signaling Information Field
The actual user information The ANSI version and the ITU-T version The routing label
The Destination Point Code (DPC) The Originating Point Code (OPC)
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Signaling Link Selection (SLS) The particular signaling link to be used
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SS7 addressing The ANSI version, 24 bits
Member, cluster, network codes An operator has a network code
The ITU-T version, 14 bits International Signaling Gateway
Use sub-service field National, Nation Spare, International, International
Spare An international gateway has one national point
code and one international code
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International Signaling
35Internet Telephony
Same SPCs can be reused at different network levels
SPC = 277SPC = 277
SPC = 277SPC = 277
International
National
SPC = 277 means different signalling points (network elements) at different network levels.
FF CKCK SIF SIF SIOSIO LILI ControlControl FF
The Service Information Octet (SIO) indicates whether the DPC and OPC are international or national signalling point codes.
36Internet Telephony
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)SIO (one octet)
Routing label (four octets)Routing label (four octets)
CIC (two octets)CIC (two octets)
Message type (one octet) Mandatory fixed part
Message type (one octet) Mandatory fixed part
Mandatory variable partMandatory variable part
Optional partOptional part
Must always be included in ISUP message
E.g., IAM message
E.g., contains called (user B) number in IAM message
37Internet Telephony
The ISDN User Part (ISUP)
ISUP The most-used SS7 application The establishment and release of telephone
calls IAM
Called number, calling number, transmission requirement, type of caller, …
ACM The call is through-connected to the destination A one-way-audio path is opened for ring-back tone Optional
If not returned, no ring-back tone at all
38Internet Telephony
CPG, Call Progress Optional; provide information to the calling switch
ANM, Answer Message Open the transmission path in both directions Instigate charging for the call
REL, Release RLC, Release Complete
CIC, circuit identification Code Indicates the specific trunk between two
switches OPC, DPC, and CIC
39Internet Telephony
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.
ExchangeExchange
STPSTP
ExchangeExchange
Circuit
Signalling link
ISUP Call Establishment and Release
• A given circuit between two switches is identified by OPC, DPC and CIC.
41Internet Telephony
Signalling using IAM message
ExchangeExchange ExchangeExchangeExchangeExchange
SPC = 82
Circuit 14
SPC = 22 SPC = 60Circuit 20
STPSTPSL 4
SL 7
STPSTP
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).
42Internet Telephony
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
Connect
Charging of call starts now
Number analysisDSS1
signalling assumed
43Internet Telephony
ISUP message format
44Internet Telephony
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):
STPSTP DatabaseDatabaseExchangeExchange
STP 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.
1.
SCCP performs global title translation in the STP (0800 or IMSI number => DPC) and the SCCP message can now be routed to the database.
2.
45Internet Telephony
Example: SCCP usage in mobile call
SCCPSCCP
MSC located in EspooMSC located in Espoo HLR located in OsloHLR located in Oslo
STPSTP
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.
46Internet Telephony
To sum it up with an example…
PSTN
Typical 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”
47Internet Telephony
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
LIC
ToneRx
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
48Internet Telephony
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
LIC
ToneRx
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
49Internet Telephony
Time switch
3. Number analysis
Switching system
4. IN triggering actions? Should an external database (e.g. SCP, HLR) be contacted?
LIC
LIC
ToneRx
Group switch
Sign.
ETC
ETC
Control system
Phase 2. Exchange receives and analyzes user B number.
2. Number (DTMF signal) received
1. User A dials user B number
Setup of a call (2)
Local exchange of user A
50Internet Telephony
Time switch
2. Outgoing circuit is reserved
Switching system
LIC
LIC
ToneRx
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)
51Internet Telephony
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…
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Performance Requirements for SS7
Bellcore spec. GR-246-Core MTP
A given route set should not be out of service for more than 10 minutes per year
< 1*10-7 messages should be lost < 1*10-10 messages should be delivered out of
sequence ISUP
Numerous timing requirements
A VoIP network that uses SS7 Must meet the stringent requirements Signaling Transport (Sigtran) group of the IETF
53Internet Telephony
Performance Requirements for SS7
Long-distance VoIP network A given route set should not be
out of service for more than 10 minutes per year.
No more than 1x10-7 messages should be lost.
No more than 1x10-10 messages should be delivered out of sequence.
In ISUP, numerous timing requirements must be met.
How to make sure that VoIP networks can emulate the signaling performance of SS7.
SIGTRAN (Signaling Transport) group of IETF
54Internet Telephony
Softswitch Architecture
Signaling(SS7)
Gateway
TrunkingGateway
CallAgent
SCP
ResidentialGateway
InternetSS7 Network
STP
TrunkingGatewayTrunking
GatewayCO
Switch
ResidentialGatewayResidential
Gateway
MGCP/MEGACO
MGCP/MEGACO
RTP
SIGTRAN
55Internet Telephony
Signaling Transport (SIGTRAN)
Addressing the issues regarding the transport of signaling within IP networks The issues related to signaling performance
within IP networks and the interworking with PSTN
SIP/MEGACO/ISUP Interworking Translating the MTP-based SS7 message
(e.g., IAM) to IP-based message (e.g., IP IAM) Just a simple translation from point code to
IP address ???
57Internet Telephony
SIGTRAN
Issues discussed in SIGTRAN Address translation How can we deploy an SS7 application (e.g.,
ISUP) that expects certain services from lower layers such as MTP when lower layers do not exist in the IP network?
For transport layer, the ISUP message must be carried in the IP network with the same speed and reliability as in the SS7.
UDP x TCP x
RFC 2719, “Framework Architecture for Signaling Transport”
58Internet Telephony
SIGTRAN Architecture
Signaling over standard IP uses a common transport protocol that ensures reliable signaling delivery.
Error-free and in-sequence Stream Control Transmission Protocol (SCTP)
An adaptation layer is used to support specific primitives as required by a particular signaling application.
The standard SS7 applications (e.g., ISUP) do not realize that the underlying transport is IP.
59Internet Telephony
ISUP Transport to MGC
NIF (Nodal Interworking Function) is responsible for interworking between the SS7 and IP networks
60Internet Telephony
SIGTRAN Protocol Stack
SCTP: fast delivery of messages (error-free, in sequence delivery), network-level fault tolerance
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Adaptation Layer [1/3] M2UA (MTP-2 User Adaptation Layer)
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Adaptation Layer [2/3]
M2PA (MTP-2 Peer-to-Peer Adaptation Layer) An SG that utilizes M2PA is a signaling node for the
MGC. It is effectively an IP-based STP.
SG can processing higher-layer signaling functions, such as SCCP GTT.
Adaptation Layer [3/3] M3UA (MTP3-User Adaptation Layer) SUA (SCCP-User Adaptation Layer)
Applications such as TCAP use the services of SUA. IUA (ISDN Q.921-User Adaptation Layer) V5UA (V5.2-User Adaptation Layer)
64Internet Telephony
SCTP
To offer the fast transmission and reliability required for signaling carrying.
SCTP provides a number of functions that are critical for telephony signaling transport. It can potentially benefit other applications needing
transport with additional performance and reliability. SCTP must meet the Functional Requirements
of SIGTRAN.
65Internet Telephony
Why not use TCP?
TCP provides both reliable data transfer and strict order-of-transmission, but SS7 may not need ordering. TCP will cause delay for supporting order-of-
transmission. The limited scope of TCP sockets complicates
the task of data transmission using multi-homed hosts.
TCP is relatively vulnerable to DoS attack, such as SYN attacks.
66Internet Telephony
What Supported By Using SCTP?
To ensure reliable, error-free, in-sequence delivery of user messages (optional).
To support fast delivery of messages and avoid head-of-line blocking.
To support network-level fault tolerance that is critical for carrier-grade network performance by using multi-home hosts.
To provide protection against DoS attack by using 4-way handshake and cookie.
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SCTP Endpoint & Association
Endpoint The logical sender/receiver of SCTP packets. Transport address = IP address + SCTP port
number An endpoint may have multiple transport addresses
(for multi-homed host, all transport addresses must use the same port number.)
Association A protocol relationship between SCTP
endpoints. Two SCTP endpoints MUST NOT have more
than one SCTP association.
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Multi-Homed Host
Host A
SCTP User
Host B
One IP address
One SCTP association with multi-homed redundant
SCTP
SCTP User
SCTP
One IP address One IP address
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SCTP Streams
A stream is a one-way logical channel between SCTP endpoints. The number of streams supported in an
association is specified during the establishment of the association.
To avoid head-of-line blocking and to ensure in-sequence delivery In-sequence delivery is ensured within a
single stream.
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SCTP Functional View
SCTP User Application
Acknowledgement and Congestion Avoidance
Chunk Bundling
Packet Validation
Path Management
Association startup and takedown
Sequenced delivery within streams
User Data Fragmentation
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SCTP Packets & Chunks
A SCTP packet can comprise several chunks.
Chunk Data or control
Source Port Number Destination Port Number
Verification Tag
Checksum
Chunk Type Chunk Flags Chunk Length
Chunk Value
. . .
Common Header
Chunk 1
Chunk N
0 16 3115. . . . . .
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Chunk Type ID Value Chunk Type -------- --------------- 0 - Payload Data (DATA) 1 - Initiation (INIT) 2 - Initiation Acknowledgement (INIT ACK) 3 - Selective Acknowledgement (SACK) 4 - Heartbeat Request (HEARTBEAT) 5 - Heartbeat Acknowledgement (HEARTBEAT ACK) 6 - Abort (ABORT) 7 - Shutdown (SHUTDOWN) 8 - Shutdown Acknowledgement (SHUTDOWN ACK) 9 - Operation Error (ERROR) 10 - State Cookie (COOKIE ECHO) 11 - Cookie Acknowledgement (COOKIE ACK) 12 - Reserved for Explicit Congestion Notification Echo (ECNE) 13 - Reserved for Congestion Window Reduced (CWR) 14 - Shutdown Complete (SHUTDOWN COMPLETE) … - Reserved for IETF
73Internet Telephony
SCTP control chunks
INIT chunk Initiate an SCTP association between two
endpoints Cannot share an SCTP packet with any other
chunk INIT ACK
Acknowledge the initiation Must not share a packet with any other chunk
SACK Acknowledge the receipt of Data chunks Inform the sender of any gaps
Only the gaps need to be resent
74Internet Telephony
HEARTBEAT When no chunks need to be sent Send periodic HEARTBEAT messages Contain sender-specific information
HEARTBEAT ACK Containing heartbeat information copied form
HEARTBEAT ABORT
End an association abruptly Cause information Can be multiplexed with other SCTP control
chunks Should be the last chunk, or …
75Internet Telephony
SHUTDOWN A graceful termination of an association Stop sending any new data Wait until all data sent has been
acknowledged Send a SHUTDOWN to the far end
Indicate the chunk received Upon receipt of a SHUTDOWN
Retransmit data that are not acknowledged Send a SHUTDOWN ACK
SHUTDOWN ACK SHUTDOWN COMPLETE
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ERROR Some error condition detected
E.g., a chunk for a non-existent stream
COOKIE ECHO Used only during the initiation of an association An INIT ACK includes a cookie parameter Information specific to the endpoint, a timestamp, a
cookie lifetime Upon receipt of an INIT ACK
Return the cookie information in COOKIE ECHO Can be multiplexed; must be the first chunk
COOKIE ACK Can be multiplexed; must be the first chunk
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INIT Chunk
Advertised Receiver Window Credit (a_rwnd)
Number of Outbound Streams
Type = 1 Chunk Flags Chunk Length
Initial TSN (Transmission Sequence Number)
. . .
0 16 3115. . . . . .
Initial Tag
Number of Inbound Streams
Optional / Variable-Length Parameter
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Association Establishment
A Z
INIT [I-Tag=Tag_A]
INIT ACK [V-Tag=Tag_A, I-Tag=Tag_Z, Cookie_Z]
COOKIE [Cookie_Z]
COOKIE ACK
allocating resources
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User Data Transfer
SCTP user
SCTP Control Chunks
SCTP
SCTP DATA Chunks
User Messages
SCTP packets
Connectionless Packet Transfer Service (e.g. IP)
80Internet Telephony
DATA Chunk
Stream ID = S
Type = 0 Reserved Chunk Length
Payload Protocol ID
. . .
0 16 3115. . . . . .
TSN
Stream Sequence Number = n
User Data (Sequence n of Stream S)
U B E
U : unorderedB : beginE : end
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Payload data chunk Carry information to and from the ULP U: unordered bit
The information should be passed to the ULP without regard to sequencing
B and E: beginning and end bits Segment a given user message
TSN: Transmission Sequence Number (32-bit) Independent of any streams Assigned by SCTP An INIT has the same TSN as the first DATA chunk TSN ++ for each new DATA chunk
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S: Stream Identifier (16-bit) n: stream sequence number (16-bit)
Begins at zero Increments for each new message
Payload protocol identifier For the users to pass further information about
the chunk but is not examined by the SCTP
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SACK Chunk
Advertised Receiver Window Credit (a_rwnd)
Number of Gap Ack Blocks = n
Type = 3 Chunk Flags Chunk Length
0 16 3115. . . . . .
Cumulative TSN Ack
Number of Duplicate TSNs = x
Gap Ack Block #1 Start Gap Ack Block #1 End
. . .
. . .
Duplicate TSN #1
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Transferring data Reliable transfer SACK chunk
Cumulative TSN The highest TSN value received without any gaps 4
The number of Gap Ack Blocks The number of fragments received after the
unbroken sequence 2
The number of duplicate TSNs 2
85Internet Telephony
Gap Ack Block number 1 start The offset of the first segment from the unbroken
sequence 3 (7-4)
Gap Ack Block number 1 end The offset of the first segment from the unbroken
sequence 8 (8-4)
a_rwnd The updated buffer space of the sender
86Internet Telephony
SCTP Robustness
Robustness is a key characteristic of any carrier-grade network.
To handle a certain amount of failure in the network without a significant reduction in quality
INIT and INIT ACK chunks may optionally include one or more IP addresses (a primary address + several secondary addresses).
Multi-homes hosts SCTP ensures that endpoint is aware of the
reachability of another endpoint through the following mechanisms.
SACK chunks if DATA chunk have been sent HEARTBEAT chunks if an association is idle
87Internet Telephony
M3UA Operation
M3UA over SCTP Application Server
A logical entity handling signaling for a scope
A CA handles ISUP signaling for a SS7 DPC/OPC/CIC-range
An AS contains a set of Application Server Processes (ASPs)
ASP A process instance of an AS Can be spread across multiple IP addresses Active ASPs and standby ASPs
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Routing Key A set of SS7 parameters that identifies the
signaling for a given AS OPC/DPC/CIC-range
Network Appearance A mechanism for separating signaling traffic
between an SG and an ASP E.g., international signaling gateway
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Signaling Network Architecture
No single point of failure SGs should be set up at least in pairs ASPs
A redundant or load-sharing configuration Spread over different hosts
Point code All ASPs and the connected SG share the
same PC A single SS7 signaling endpoint
All ASPs share a PC != that of the SG ASPs: a signaling endpoint; SG: an STP
A group of ASPs share a PC
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Robust Signaling Architecture
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Services Provided by M3UA
Offer the same primitives as offered by MTP3 MTP-Transfer request MTP-Transfer indication MTP-Pause indication
Signaling to a particular destination should be suspended
MTP-Resume indication Signaling to a particular destination can resume
MTP-Status indication Some change in the SS7 network E.g., network congestion or a destination user part
becoming unavailable
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Transferring application message A CA sends an ISUP message MTP-Transfer request A SCTP DATA chunk Transmitted to a SG M3UA – MTP3 To the SS7 network
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M3UA Messages Messages between peer M3UA entities A header + the M3UA message content The entities can communicate information
regarding the SS7 network If a remote destination becomes unavailable The SG becomes aware of this through SS7
signaling-network management messages The SG pass M3UA messages to the CA The ISUP application at the CA is made aware
MTP-Pause indication
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Signaling Network Management MSGs
S7ISO – SS7 Network Isolation When all links to the SS7 network have been lost
DUNA – Destination Unavailable Sent from the SG to all connected ASPs Destination(s) within the SS7 network is not
available Allocate 24 bits for each DPC
DUNA is generated at the SG It determines from MTP3 network management
message The M3UA of the ASP
Create MTP-Pause indication
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DAVA - Destination Available Sent from SG to all concerned ASPs Mapped to the MTP-Resume indication
DAUD – Destination State Audit Sent from an ASP to an SG To query the status of one or more
destination The SG responds with DAVA, DUNA, or SCON
SCON – SS7 Network Congestion Sent from the SG to ASPs The route to an SS7 destination is congested Mapped to the MTP-Status indication
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DUPU – Destination User Part Unavailable Sent from the SG to ASPs A given user part at a destination is not
available The DPC and the user part in question Mapped to MTP-Status indication Cause codes
DRST – Destination Restricted Sent from the SG to ASPs One or more SS7 destinations are restricted
from The M3UA may use a different SG
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ASP management
ASPUP – ASP Up Used between M3UA peers The adaptation layer is ready to receive
traffic or maintenance messages ASPDN – ASP Down
An ASP is not ready UP ACK – ASP Up Ack DOWN ACK – ASP Down Ack
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ASPAC – ASP Active Sent by an ASP Indicate that it is ready to be used To receive all messages or in a load-sharing
mode Routing context
Indicate the scope is applicable to the ASP DPC/OPC/CIC-range
ASPIA – ASP Inactive ACTIVE ACK – ASP Active Ack INACTIVE ACK – ASP Inactive Ack
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BEAT – Heartbeat Between M3UA peers Still available to each other When M3UA use the services of SCTP
The BEAT message is not required at the M3UA level
SCTP includes functions for reachability information
ERR – Error message A received message with invalid contents
NFTY – Notify Between M3UA peers To communicate the occurrence of certain
events
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Routing Key Management Messages
Registration Request (REG REQ) An ASP = a DPC/OPC/CIC range
Registration Response Deregistration Request Deregistration Response
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M2UA Operation
MTP3/M2UA/SCTP The CA has more visibility of the SS7
network More tightly coupled to the SG
MTP3 Routing and distribution capabilities
M2UA uses similar concepts to those used by M3UA ASPUP, ASPDN, ASPAC, ASPIA and ERR Exactly the same functions In M2UA, the ASP is an instance of MTP3
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M2UA-specific messages DATA
Carry an MTP2-user Protocol Data Unit ESTABLISH REQUEST
To establish a link to the SG ESTABLISH CONFIRMATION RELEASE REQUEST
Request the SG to take a particular signaling link out of service
RELEASE CONFIRM RELEASE INDICATION
The SG autonomously take a link out of service
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STATE REQUEST Sent from a CA to the SG to cause the SG to
perform some action on a signaling link Link alignment, or flushing transmit buffers
STATE CONFIRM STATE INDICATION
The SG can autonomously send During link changeover
The CA must retrieve certain information from the SG
DATA RETRIEVAL REQUEST DATA RETRIEVAL CONFIRM DATA RETRIEVAL INDICATION
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M2PA Operation
IP-based SS7 links No FISUs sent; only LSSUs and MSUs Establish SCTP associations between
M2PA peers Two streams One for MSU The other for LSSU
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Interworking SS7 with VoIP Arch
Interworking softswitch and SS7 At least two SGs Use SCTP as MEGACO Transport
An Internet draft Reliable and quick transport
Use SCTP to transport SIP message? Might not be easy No semi-permanent relationship
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ISUP encapsulation in SIP When the softswitch network provides a
transit function Interworking often leads to a lowest
common denominator result Retry-after header ISUP -> SIP -> ISUP
Are the ISUP messages the same? The reliable delivery of provisional response
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Interworking H.323 and SS7
The H.323 gateway Terminate SS7 links and voice trunks A close relationship exits between ISUP and
Q.931 IAM and Setup ANM and Connect
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