VoIP and SS7

1
VoIP and SS7

• Chapter 7

2
Basic functional parts of the PSTN
PSTN
Switching in exchanges
Transmission (PDH, SDH)
Databases in the network (HLR)
Subscriber signalling (analog or ISDNDSS1)
Network-internal signalling (SS7)
3
PDH and SDH transmission bit rates
PDH (Plesiochronous Digital Hierarchy)
Japan
USA
Europe
J1 1.5 Mbit/s T1 1.5 Mbit/s E1 2 Mbit/s
J2 6 T2 6 E2 8 J3 32 T3 45
E3 34 J4 98 T4 140 E4 140
SONET (North Am.) SDH
STS-1 51.84 Mbit/s STS-3
155.52 STM-1 STS-12 622.08
STM-4 STS-48 2.488 Gbit/s
STM-16
4
Structure of E1 frame (2.048 Mbit/s)
0 1 2
31
16
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.
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Subscriber signalling
PSTN
Switching in exchanges
Transmission (PDH, SDH)
Databases in the network (HLR)
Subscriber signalling (analog or ISDNDSS1)
Network-internal signalling (SS7)
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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
gt 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
LE A
User A
User B
LE 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
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ISDN subscriber signalling in action
LE A
User A
User B
LE 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
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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) gt
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
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End-to-end digital signalling
User interface
User interface
PSTN Network
ISUP
Q.931
Q.931
Q.931
Q.931
ISUP
SS7
MTP 3
MTP 3
DSS1
DSS1
Q.921
Q.921
MTP 2
Q.921
Q.921
MTP 2
I.430
I.430
MTP 1
I.430
I.430
MTP 1
contains the signalling messages for call control
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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.

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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).
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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).
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The Telephone Network 1/2
Service Control Point
Service Data Point

Signal Transfer Point
Control Layer
Intelligent Peripheral
Transport Layer
Class 4 Tandem Switch
Class 5 End Office Switch
Circuit Switched Network
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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

SCP
SDP
TCAP messages
IP
STP
STP
SSP
SSP
ISUP messages
Voice
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Signalling example
Oulu
Tokyo
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.
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SS7 Protocol Suite
OSI Layers
MAP
INAP
Application
Presentation Session
ISUP
TCAP
SCCP
Transport
Network
MTP Level 3
MTP Level 2
Data Link
MTP Level 1
Physical
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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)

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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

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Message Signal Units (MSUs)

• The messages sent in the SS7 network

• Backward Sequence Number
• BSN Indicator Bit
• Forward Sequence Number
• Length Indicator

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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

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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.
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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 (gt 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
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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

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• 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

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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
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ISUP Call Establishment and Release

• A given circuit between two switches is
  identified by OPC, DPC and CIC.

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Signalling using IAM message
STP
STP
SL 4
SL 7
Exchange
Exchange
Exchange
SPC 82
SPC 22
SPC 60
Circuit 20
Circuit 14
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).
Outgoing message OPC 82 CIC 14 DPC 22
SLS 4
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Setup of a call using ISUP
LE A
LE B
Transit exchange
User A
User B
Setup
IAM
IAM
Setup
DSS1 signalling assumed
Number analysis
Alert
ACM
ACM
Alert
Connect
ANM
ANM
Connect
Charging of call starts now
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ISUP message format
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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
STP
Database
Exchange
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 gt DPC) and the SCCP message
can now be routed to the database.
2.
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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
SCCP
SPC 32
MSC located in Espoo
HLR located in Oslo
SPC 99
SPC 82
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
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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 ISDNDSS1)
Network-internal signalling (SS7)
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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
Time switch
LIC
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

Control system

• OM functions

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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
Time switch
LIC
ETC
1. Off hook
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
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Setup of a call (2)
Phase 2. Exchange receives and analyzes user B
number.
Local exchange of user A
Switching system
Time switch
LIC
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
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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
Time switch
LIC
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
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Setup of a call (4)
Phase 4. ACM received gt ringback or busy tone
generated. ANM received gt charging starts.
Local exchange of user A
Switching system
Time switch
LIC
ETC
Group switch
ETC
LIC
ACM, ANM
Tone generator
2. Ringback or busy tone is locally generated 4.
Call continues
Sign.
1. ISUP ACM message indicates free or busy
user B
3. Charging starts when ISUP ANM message is
received
Control system
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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
• lt 110-7 messages should be lost
• lt 110-10 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

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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

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

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(No Transcript)
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.

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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

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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.

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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)

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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

• 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.

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

72
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

73
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

74

• 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

75

• 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

76

• 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

77
INIT Chunk
78
Association Establishment
INIT I-TagTag_A
INIT ACK V-TagTag_A, I-TagTag_Z, Cookie_Z
Z
A
COOKIE Cookie_Z
COOKIE ACK
allocating resources
79
User Data Transfer
User Messages
SCTP user
SCTP Control Chunks
SCTP DATA Chunks
SCTP packets
SCTP
Connectionless Packet Transfer Service (e.g. IP)
80
DATA Chunk
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
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
• 2

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
• The offset of the first segment from the unbroken
  sequence
• 8 (8-4)
• a_rwnd
• The updated buffer space of the sender

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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

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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

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

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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

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

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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

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

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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 -gt SIP -gt 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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