gsm protocol

1. GSM PROTOCOL
skgochhayat

2. Functions Of Protocols
• Session establishment and termination between
users.
• Orderly exchange of Data messages.
• Coding of the information.
• Routing and Sequencing
• Flow control and Congestion control.
• Error checking and recovery.
• Efficient network resource utilization

3. Example Of Some Functions
• Code conversion to facilitate understanding
of the meaning.
• Routing of the messages through Network.
• Error control to counteract effect of
disturbances.
• Transmission of Electrical Signals.

4. OSI Layers
• The OSI model is built of seven ordered layers:
– Layer-7: Application
– Layer-6: Presentation Application
– Layer-5: Session Presentation
– Layer-4: Transport Session
Transport
– Layer-3: Network Network
– Layer-2: Data Link Data Link
Physical
– Layer-1: Physical

5. OSI Layers
• The seven layers can be thought of as
belonging to three sub groups
– Network Support Layers (Layers 1-3)
• Deal with the physical aspects of moving data from one
device to another
– User Support Layers (Layers 5-7)
• Allow interoperability among unrelated software
systems
– Layer-4 ensures end to end reliable data
transmission

6. OSI Layers
End System
Application
User Support Presentation
Layers Session Network Support Layers
Transport
Network Network Network Network
Data Link Data Link Data Link Data Link
Physical Physical Physical Physical
PSPDN PSTN Dedicated CSPDN

7. Hierarchical communication.
• Within a single machine, each layer calls upon
the services of the layer just below it
• The passing of data and network information
is made possible by an interface between each
pair of adjacent layers
• The messages exchanged between the
adjacent layers, to obtain the required
services, are called Interface Control
Information (ICI)

8. Peer-to-Peer communication
• Between machines, layer-n on one machine
communicates with layer-n on another machine
• This communication is governed by an agreed-upon
series of rules and conventions called protocols
• The processes on each machine that communicates at
a given layer are called peer-to-peer processes
• At the physical layer, communication is direct
• At higher layers, communication moves down through
the layers on the transmitting machine and back up
through the layers at receiving machine

9. Hierarchical & Peer-to-Peer Communications
Hierarchical
N+1-Layer PCI N+1-Layer
Interface Interface
N-Layer Peer-to-Peer N-Layer
ICI
InterfaceControl Information (ICI)
Protocol Control Information (PCI)

10. Data Units in the OSI Model
(N+1)-PDU
(N+1)-PCI (N+1)-SDU
(N)-ICI
(N+1)-Layer (N)-IDU
(N+1)-PCI (N+1)-SDU (N)-ICI
(N)-Layer
(N)-ICI
(N)-SDU
(N+1)-PCI (N+1)-SDU
(N)-PCI
(N)-PDU
(N)-PCI (N+1)-PCI (N+1)-SDU

11. Data Units
• Protocol Control Information (PCI)
– (N)-PCI is the protocol control information
exchanged between the (N)-entities to
coordinate their functions
• Service Data Units (SDU)
– (N)-SDU is the data unit transferred between
the ends of a (N)-connection whose identity
is preserved during the transfer
• Protocol Data Unit (PDU)
– (N)-PDU is the combination of (N)-PCI and
(N)-SDU

12. Data Units
• Interface Control Information (ICI)
– (N)-ICI is the information exchanged between
(N+1)-entity and (N)-entity to coordinate their
functions
• Interface Data Unit (IDU)
– (N)-IDU is the total data unit transferred across
the SAP between (N+1)-entity and (N)-entity

13. OSI Layers
7-Application 7-Application
Link Intermediate Link
7-6 Interface 7-6 Interface
Node
6-Presentation 6-Presentation
6-5 Interface 6-5 Interface
5-Session 5-Session
5-4 Interface 5-4 Interface
4-Transport 4-Transport
4-3 Interface 4-3 Interface
3-Network 3-Network 3-Network
3-2 Interface 3-2 Interface 3-2 Interface
2-Data Link 2-Data Link 2-Data Link
2-1 Interface 2-1 Interface 2-1 Interface
1-Physical 1-Physical 1-Physical

14. Summary of OSI Layers Functions
Allow access to network
Application resources
7-6 Interface
Translate, encrypt and
Presentation
compress data
6-5 Interface
Establish, manage and
Session
terminate sessions
5-4 Interface
Reliable end to end delivery Transport
& error recovery 4-3 Interface
Movement of packets;
Network
Provide internetworking
3-2 Interface
Organise bits into streams;
Data Link
Node to node delivery
2-1 Interface
Transmit bits; Mechanical
Physical
and electrical specifications

15. Summary of OSI Layers Functions
Allow access to network
Application resources
7-6 Interface
Translate, encrypt and
Presentation
compress data
6-5 Interface
Establish, manage and
Session
terminate sessions
5-4 Interface
Reliable end to end delivery Transport
& error recovery 4-3 Interface
Movement of packets;
Network
Provide internetworking
3-2 Interface
Organise bits into streams;
Data Link
Node to node delivery
2-1 Interface
Transmit bits; Mechanical
Physical
and electrical specifications

16. GSM System Architecture
BSS NSS
16Kb/s GSTN
R TRAU MSC
64Kb/s
A BSC ISDN
D VLR
BTS SS7
I
O HLR NSS
MS
BSS : Base Station Sub-system
BSC : Base Station Controller NSS : Network and Switching Sub-system
BTS : Base Transceiver Station MSC: Mobile service Switching Center
TRAU : Transcoder / Rate Adapter Unit HLR : Home Location Register
VLR : Visitors Location Register

18. GSM protocol layers for signaling
Um Abis A
MS BTS BSC MSC
CM CM
MM MM
BSSAP BSSAP
RR RR’
RR’ BTSM BTSM
SS7 SS7
LAPDm LAPDm LAPD LAPD
radio radio PCM PCM PCM PCM
16/64 kbit/s 64 kbit/s /
2.048 Mbit/s

20. ISDN Protocol
• Two types of ISDN Interfaces : Basic Rate Interface (BRI), and Primary rate
interface (PRI), provide multiple digital bearer channels over which
temporary connections can be made and data can be sent.
• The result is digital dial access to multiple site concurrently.
Type of Interface Number of Bearer Number of Signaling
Channels (B Channels (D
channels) Channels)
BRI 2 1 (16 Kbps)
PRI(T/1) 23 1 (64 Kbps)
PRI(E/1) 30 1 (64 Kbps)

21. ISDN Channels
• B Channels : Bearer channels (B channels) are used to
transport data. B Channels are called bearer channels
because they bear the burden of transporting the
data. B channels operate up to 64 Kbps, although the
speed might be lower depending on the service
provider.
• D Channels are used for signaling. LAPD is used to
deliver signaling message to the ISDN switch

22. LAPD and PPP on D and B Channels
BRI BRI
B0 SS7 B0
B1 Call Setup B1
D LAPD Flows D LAPD
Call Setup Flows Call Setup Flows
ISDN Network
BRI PPP BRI
B0 B0
B1 B1
D LAPD D LAPD
ISDN Network

23. LAPD and PPP on D and B Channels
• The call is established through the service provider network; PPP
is used as the data link protocol on the B channel from end to
end. LAPD is used between the router and the ISDN switch at
each local central office (CO) and remains up so that new
signaling messages can be sent and received. Because the
signals are sent outside the channel used for data, this is called
out-of-band signaling.
• The BRI encodes bits at 192 kbps, out of which 144 Kbps is used
by B and D Channels rest is used for framing.

24. Use of ISDN as WAN Protocol
• Dial on Demand Routing – Logic is configured
in the routers to trigger the dial when that
traffic needs to get to another site is sent by
user.
• Telecommuting Environment
• Backup to leased lines – When leased line
fails, an ISDN call is established between two
routers.

25. ISDN as WAN Link
Dial on
Demand
Routing ISDN Network
Telecommuting
NT1
ISDN Network
Computer with
ISDN Inerface
Leased Line Backup
Leased Line
BRI BRI
ISDN Network

26. Layer 2 LAPDM Protocol
• Establishment and release of signalling layer 2 connections.
• Multiplexing and de multiplexing of several signalling layer 2
connection on a dedicated control channel and discrimination
between them by including different Service Access Point Identifiers
(SAPI).
• Mapping of signalling layer 2 service data units on protocol data
unit (in case of acknowledged operation service data units may be
segmented and reassembled at destination).
• Detection and recovery of errors due to loss, duplication, and
disorder.
• Flow control.

27. LAPDM Protocol
• The establishment and release of layer 2 connection coincides with
the allocation release, and change of dedication radio channels.
Signaling layer 2 connections are frequently established and
released, and thus an average lifetime of a connection is short.
Multiplexing and demultiplexing deals with arranging different user
(eight channels per frame) in a frame format.
• LAPDm uses on the two modes of operation for the transmission of
layer 3 message; unacknowledged operation of multiple frame
operation.

28. LAPDM Protocol
• On the DCCH both unacknowledged and acknowledged
operations are used,
DCCHs (SDCCH, SACCH and FACCH)
• whereas on the CCCHs only unacknowledged operation
is applied. Thus, both modes are applicable for
transmission over on of the in contrast top information
transfer over CCCHs (BCCH, PCH and AGCH)

29. LAPDM Protocol
• For an unacknowledged information transfer,
the use of layer 3 service implies that the
information transfer is not acknowledged by
the data link layer, and thus error check
facilities are not provided. The transmission
and reception of messages here use data link
service primitives, that is, DL-DATA-REQUEST
and DL-DATA-INDICATION.

30. LAPD and LAPDm
• The main distinction between LAPD and LAPDm is the
absence of address and control fields. Thus, the
protocol is only used for the unacknowledged mode of
operation, which applies to BCCHs and CCCHs only.
Both FCCH and SCH under BCCH do not require
unacknowledged. Similarity, no acknowledgement is
needed for PCH and AGCH.
• The LAPD frame is used internal to BSS, namely,
between BTS and BSC.

31. MTP3, SCCP, and TCAP
Protocols.

32. The Telephone Network [1/2]
SS7 Signaling
Service Service
ISUP Messages +
Control Data
INAP/TCAP Messages Point Point
Signal
Transfer Control Layer
Point
Intelligent Transport Layer
Peripheral
Class 4
Class 5
Tandem Switch
End Office Switch
Circuit Switched Network

33. 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 SDP
– SCP/Service Control Point TCAP messages
• service logic execution
IP
– SDP/Service Data Point STP STP
• subscriber data storage, access
– IP/Intelligent Peripheral SSP
ISUP messages
SSP
• resources such as customized voice
announcement, voice recognition, Voice
DTMF digit collection

34. Signalling example
BBSR Exch CUTTACK
User A Exch User B
(calling Exch (called
user) user)
KOLKOTA
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.

35. SS7 Protocol Suite
OSI Layers
IN Application
Application INAP MAP Part
Transaction Mobile
Capabilities Presentation Application
Applications Session TCAP ISUP Part
Part
ISDN User
Signaling Transport SCCP Part
Connection
Control Part Network MTP Level 3
Data Link MTP Level 2
Physical MTP Level 1

36. 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

37. 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

38. Services
– 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

39. 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

40. Connection-Oriented Protocol
– 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

41. SCCP
• Signaling Connection Control Part
• Used as the transport layer for TCAP-based
services
– Free phone (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

42. 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)

43. 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.

44. 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)

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

46. 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

47. – 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

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

49. 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)

50. • Signaling Link Selection (SLS)
– The particular signaling link to be used

51. • 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

52. • International Signaling

53. 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 F

54. 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) E.g., IAM message
Mandatory fixed part
Mandatory variable part E.g., contains called (user B) number in
Optional part IAM message

55. 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

56. – 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

57. 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

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

59. Signalling using IAM message
STP
STP
SL 4
SL 7
SPC = 82 SPC = 22 Circuit SPC = 60
20
Circuit
14
Exchange Exchange Exchange
Outgoing message: Processing in (transit) exchange(s):
OPC = 82 CIC = 14 Received IAM message contains B-number.
DPC = 22 SLS = 4 Exchange performs number analysis (not part of
ISUP) and selects new DPC (60) and CIC (20).

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

61. ISUP message format

62. 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):
Exchange STP Database
STP with GTT capability
1. Exchange knows the global title (e.g. 0800 number or IMSI
number in a mobile network) but does not know the DPC of
the database related to this global title.
2. SCCP performs global title translation in the STP (0800 or
IMSI number => DPC) and the SCCP message can now be
routed to the database.

63. 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: Processing in STP:
OPC = 82 DPC = 32 Received message is given to SCCP for GTT.
SCCP: IMSI global title SCCP finds the DPC of the HLR: DPC = 99

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

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

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

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

68. 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 ETC
Group
switch Tone switch
LIC Rx ETC
1. Tone receiver IAM
is disconnected Sign. (contains
information
CIC = 24)
2. Outgoing circuit is reserved
3. Outgoing signalling message (ISUP IAM)
contains user B number
Control system

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

70. 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

71. 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

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

73. 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 ???

75. 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”

76. 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.

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

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

79. •
Adaptation Layer Layer)
M2UA (MTP-2 User Adaptation
[1/3]

80. •
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.

81. 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)

82. 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.

83. 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.

84. 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.

85. 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.

86. 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

87. 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.

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

89. SCTP Packets & Chunks
• A SCTP packet can comprise several chunks.
• Chunk
– Data or control
0 . . . 15 16 . . . 31
Source Port Number Destination Port Number
Common
Header Verification Tag
Checksum
Chunk Type Chunk Flags Chunk Length
Chunk 1
Chunk Value
Chunk N . . .

90. 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

91. 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

92. • 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 …

93. • 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

94. • 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

95. INIT Chunk
0 . . . 15 16 . . . 31
Type = 1 Chunk Flags Chunk Length
Initial Tag
Advertised Receiver Window Credit (a_rwnd)
Number of Outbound Streams Number of Inbound Streams
Initial TSN (Transmission Sequence Number)
Optional / Variable-Length Parameter
. . .

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

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

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

99. • 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

100. – 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

101. SACK Chunk
0 . . . 15 16 . . . 31
Type = 3 Chunk Flags Chunk Length
Cumulative TSN Ack
Advertised Receiver Window Credit (a_rwnd)
Number of Gap Ack Blocks = n Number of Duplicate TSNs = x
Gap Ack Block #1 Start Gap Ack Block #1 End
. . .
Duplicate TSN #1
. . .

102. • 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

103. • 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

104. 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

105. 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

106. • 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

107. 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

108. • Robust Signaling Architecture

109. 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

110. • 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

111. • 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

112. 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

113. • 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

114. • 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

115. 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

116. • 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

117. • 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

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

119. 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

120. • 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