1. VoIP and SS7
Chapter 7

2. Basic functional parts of the PSTN
Switching in exchanges
Transmission (PDH, SDH)
Databases in the network (HLR)
Subscriber signalling (analog or ISDN=DSS1)
Network-internal signalling (SS7)

3. PDH and SDH transmission bit rates
PDH (Plesiochronous Digital Hierarchy)
Japan
USA
Europe
J Mbit/s T Mbit/s E1 2 Mbit/s
J T E2 8
J T E3 34
J T E4 140
SONET (North Am.) SDH
STS Mbit/s
STS STM-1
STS STM-4
STS Gbit/s STM-16

4. Structure of E1 frame (2.048 Mbit/s)
0 1 2 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.

5. Subscriber signalling
PSTN
Switching in exchanges
Transmission (PDH, SDH)
Databases in the network (HLR)
Subscriber signalling (analog or ISDN=DSS1)
Network-internal signalling (SS7)

6. 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

7. Analog subscriber signalling in action
User A
LE A
LE B
User B
SS7 signalling (ISUP)
LE = local exchange
Off-hook
Dial tone
B number
Ringing signal
Ringback tone (or busy tone)
Off-hook (user B answers)
Connection established

8. ISDN subscriber signalling in action
User A
LE A
LE B
User B
SS7 signalling (ISUP)
Off-hook
DSS1 signalling messages
Setup
B number
Setup
Call proc
Ringing
Alert
Tones generated in terminal
Alert
Off-hook (user B answers)
Conn
Conn
Connection established

9. PSTN vs. ISDN user access
300 … 3400 Hz analog transmission band
“Poor-performance” subscriber signaling
PSTN
2 x 64 kbit/s digital channels (B channels)
16 kbit/s channel for signaling (D channel) => Digital Subscriber Signalling system nr. 1 (DSS1)
Basic 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.
Primary Rate Access ISDN

10. End-to-end digital signalling
User interface
PSTN Network
User interface
Q.931
Q.931
ISUP
ISUP
Q.931
Q.931
SS7
DSS1
MTP 3
MTP 3
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

11. Introduction Channel Associated Signaling Common Channel 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.

12. 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
Signalling is not possible before previous circuit-switched link is established
Circuit switched connection
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).

13. 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).

14. The Telephone Network [1/2]
SS7 Signaling
ISUP Messages
INAP/TCAP Messages
Service
Control
Point
Service
Data
Point +
Signal
Transfer
Point
Control Layer
Intelligent
Peripheral
Transport Layer
傳統電信網路中電話建立的過程為：語音(Voice)部份是經由局用交換 機(CO Switches)之間的Trunk介面傳輸；信令(Signaling)部份則是透 過SS7網路傳遞。如果只是一般的通話，負責通話控制(Call Control) 的SS7 ISUP信令，僅會透過SS7網路的信號轉運點(Signal Transfer Point；STP)，在發話端交換機(Originating Switch)與受話端交換機 (Terminating Switch)之間傳送；然而，當通話需要智慧型網路服務 (IN Services)時，則要發送SS7 TCAP信令，經由SS7網路轉送至提供 該服務的服務控制點(Service Control Point；SCP)，要求提供該項服 務；若是該項服務包括語音播放(Announcement) 、互動語音回應 (Interactive Voice Response；IVR)，此時SCP就會要求智慧型設備 (Intelligent Peripheral；IP)播放語音給發話端，同時偵測發話端的按 鍵，並將按鍵結果之DTMF回傳給SCP，用來決定該項服務的進行。
Class 4
Tandem Switch
Class 5
End Office Switch
Circuit Switched Network

15. 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
SCP
SDP
TCAP messages IP
STP
STP
SSP
SSP
ISUP messages
Voice

16. Signalling example Tokyo Oulu Exch User A (calling user)
User B (called user)
Exch
Exch
London
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.

17. SS7 Protocol Suite TCAP ISUP SCCP MTP Level 3 MTP Level 2 MTP Level 1
OSI Layers
Application
INAP
MAP
Presentation
Session
TCAP
ISUP
Transport
SCCP
Network
MTP Level 3
MTP Level 2
Data Link
Physical
MTP Level 1

18. MTP Levels 1 & 2 Message Transfer Part Level 1 Level 2
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

19. MTP Level 3 Signaling message handling Signaling network management
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

20. 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

21. 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

22. 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

23. 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

24. 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)

25. 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.

26. 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)

27. Signal Transfer Point (STP)
To transfer messages from one SPC to another

28. 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

29. 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

30. Message Signal Units (MSUs)
The messages sent in the SS7 network
Backward Sequence Number
BSN Indicator Bit
Forward Sequence Number
Length Indicator

31. 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)

32. Signaling Link Selection (SLS)
The particular signaling link to be used

33. International Signaling Gateway
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

34. International Signaling

35. Same SPCs can be reused at different network levels
International
SPC = 277
National
SPC = 277
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

36. 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)
Must always be included in ISUP message
Message type (one octet)
Mandatory fixed part
E.g., IAM message
Mandatory variable part
E.g., contains called (user B) number in IAM message
Optional part

37. The ISDN User Part (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

38. CIC, circuit identification Code
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

39. 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

40. ISUP Call Establishment and Release
A given circuit between two switches is identified by OPC, DPC and CIC.

41. Signalling using IAM message
STP
STP
SL 4
SL 7
Exchange
SPC = 82
Exchange
SPC = 22
Exchange
SPC = 60
Circuit 20
Circuit 14
Outgoing message:
OPC = 82 CIC = 14
DPC = 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).

42. Setup of a call using ISUP
User A
LE A
Transit exchange
LE B
User B
Setup
IAM
IAM
Setup
DSS1 signalling assumed
Number analysis
Alert
ACM
ACM
Alert
Connect
ANM
ANM
Connect
Charging of call starts now

43. ISUP message format

44. 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 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.

45. 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
SPC = 32
SCCP
MSC located in Espoo
HLR located in Oslo
SPC = 82
SPC = 99
Outgoing message:
OPC = 82 DPC = 32
SCCP: IMSI global title
Processing in STP:
Received message is given to SCCP for GTT. SCCP finds the DPC of the HLR: DPC = 99

46. To sum it up with an example…
Part B, Section 3.3 in ”Understanding Telecommunications 2”
PSTN
Typical operation of a local exchange
Transmission (PDH, SDH)
Databases in the network (HLR)
Subscriber signalling (analog or ISDN=DSS1)
Network-internal signalling (SS7)

47. Basic local exchange (LE) architecture
Modern trend: Switching and control functions are separated into different network elements (separation of user and control plane).
Switching system
Subscriber stage
TDM links to other network elements
LIC
Time switch
ETC
Group switch
Tone Rx
ETC
LIC
Tone generator
Sign.
Exchange terminal circuit
Line interface circuit
Switch control
E.164 number analysis
SS7 Signalling equipment
Charging
User databases
O&M functions
Control system

48. Local exchange of user A
Setup of a call (1)
Phase 1. User A lifts handset and receives dial tone.
Local exchange of user A
Switching system
4. Tone Rx is connected
1. Off hook
LIC
Time switch
ETC
Group switch
Tone Rx
ETC
LIC
5. Dial tone is sent (indicating “network is alive”)
Tone generator
Sign.
2. Check user database. For instance, is
user A barred for outgoing calls?
3. Reserve memory for user B number
Control system

49. Local exchange of user A
Setup of a call (2)
Phase 2. Exchange receives and analyzes user B number.
Local exchange of user A
Switching system
LIC
Time switch
ETC
Group switch
Tone Rx
ETC
LIC
2. Number (DTMF
signal) received
1. User A dials user B number
Sign.
3. Number analysis
4. IN triggering actions? Should an external
database (e.g. SCP, HLR) be contacted?
Control system

50. Local exchange of user A
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
E.g.,
CIC = 24
LIC
Time switch
ETC
Group switch
Tone Rx
ETC
LIC
IAM (contains information CIC = 24)
1. Tone receiver
is disconnected
Sign.
2. Outgoing circuit is reserved
3. Outgoing signalling message (ISUP IAM)
contains user B number
Control system

51. Local exchange of user A
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
ETC
Group switch
ETC
LIC
ACM, ANM
2. Ringback or busy tone is locally generated
4. Call continues…
Tone generator
Sign.
1. ISUP ACM message indicates free or busy
user B
3. Charging starts when ISUP ANM message
is received
Control system

52. 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

53. 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

54. Softswitch Architecture
SS7 Network
Internet
SCP
Signaling
(SS7)
Gateway
MGCP/ MEGACO
SIGTRAN
Call
Agent
STP
MGCP/ MEGACO
Trunking
Gateway
Residential
Gateway
Trunking
Gateway
Residential
Gateway CO
Switch
Trunking
Gateway
Residential
Gateway
RTP

55. 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 ???

57. 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”

58. 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.

59. ISUP Transport to MGC
NIF (Nodal Interworking Function) is responsible for interworking between the SS7 and IP networks

60. SIGTRAN Protocol Stack
SCTP: fast delivery of messages (error-free, in sequence delivery), network-level fault tolerance

61. Adaptation Layer [1/3]
M2UA (MTP-2 User Adaptation Layer)

62. 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.

63. 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)

64. 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.

65. 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.

66. 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.

67. SCTP Endpoint & Association
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.

68. Multi-Homed Host Host A Host B SCTP User SCTP User SCTP SCTP
One IP address
One IP address
One IP address
One SCTP association with multi-homed redundant

69. 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.

70. SCTP Functional View SCTP User Application Sequenced delivery
within streams
Association
startup
and
takedown
User Data Fragmentation
Acknowledgement
and
Congestion Avoidance
Chunk Bundling
Packet Validation
Path Management

71. 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 16 31 15

72. Chunk Type ID Value Chunk Type -------- ---------------
Payload Data (DATA)
Initiation (INIT)
Initiation Acknowledgement (INIT ACK)
Selective Acknowledgement (SACK)
Heartbeat Request (HEARTBEAT)
Heartbeat Acknowledgement (HEARTBEAT ACK)
Abort (ABORT)
Shutdown (SHUTDOWN)
Shutdown Acknowledgement (SHUTDOWN ACK)
Operation Error (ERROR)
State Cookie (COOKIE ECHO)
Cookie Acknowledgement (COOKIE ACK)
Reserved for Explicit Congestion Notification Echo (ECNE)
Reserved for Congestion Window Reduced (CWR)
Shutdown Complete (SHUTDOWN COMPLETE)
… Reserved for IETF

73. SCTP control chunks INIT chunk INIT ACK SACK
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

74. HEARTBEAT HEARTBEAT ACK ABORT 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 …

75. SHUTDOWN SHUTDOWN ACK SHUTDOWN COMPLETE
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

76. ERROR COOKIE ECHO COOKIE ACK 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

77. INIT Chunk Advertised Receiver Window Credit (a_rwnd)
Number of Outbound Streams
Type = 1
Chunk Flags
Chunk Length
Initial TSN (Transmission Sequence Number) 16 31 15
Initial Tag
Number of Inbound Streams
Optional / Variable-Length Parameter

78. Association Establishment
INIT [I-Tag=Tag_A]
INIT ACK [V-Tag=Tag_A, I-Tag=Tag_Z, Cookie_Z] Z A
COOKIE [Cookie_Z]
COOKIE ACK
allocating resources

79. User Data Transfer User Messages SCTP user SCTP DATA Chunks
SCTP Control Chunks
SCTP packets
SCTP
Connectionless Packet Transfer Service (e.g. IP)

80. DATA Chunk . . . 16 31 15 Stream ID = S Type = 0 Reserved Chunk Length
Payload Protocol ID 16 31 15
TSN
Stream Sequence Number = n
User Data (Sequence n of Stream S) U B E
U : unordered
B : begin
E : end

81. 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

82. 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

83. SACK Chunk 16 31 15 . . . Advertised Receiver Window Credit (a_rwnd)
Number of Gap Ack Blocks = n
Type = 3
Chunk Flags
Chunk Length 16 31 15
Cumulative TSN Ack
Number of Duplicate TSNs = x
Gap Ack Block #1 Start
Gap Ack Block #1 End
Duplicate TSN #1

84. 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

85. 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
8 (8-4)
a_rwnd
The updated buffer space of the sender

86. 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

87. M3UA Operation M3UA over SCTP Application Server ASP
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

88. Routing Key Network Appearance
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

89. 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

90. Robust Signaling Architecture

91. 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

92. 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

93. 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

94. 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

95. 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

96. 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
One or more SS7 destinations are restricted from
The M3UA may use a different SG

97. ASP management ASPUP – ASP Up ASPDN – ASP Down UP ACK – ASP Up Ack
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

98. ACTIVE ACK – ASP Active Ack INACTIVE ACK – ASP Inactive Ack
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

99. BEAT – Heartbeat ERR – Error message NFTY – Notify 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
To communicate the occurrence of certain events

100. Routing Key Management Messages
Registration Request (REG REQ)
An ASP = a DPC/OPC/CIC range
Registration Response
Deregistration Request
Deregistration Response

101. 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

102. 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

103. During link changeover
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

104. 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

105. 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

107. 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

109. 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