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GSM PROTOCOL
skgochhayat@gmail.com
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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
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Example Of Some Functions Code conversion to facilitate
understanding of the meaning.
Routing of the messages throughNetwork.
Error control to counteract effect of
disturbances. Transmission of Electrical Signals.
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OSI Layers
The OSI model is built of seven ordered
layers:
• Layer-7: Application• Layer-6: Presentation
• Layer-5: Session
•Layer-4: Transport
• Layer-3: Network
• Layer-2: Data Link
• Layer-1: Physical
ApplicationPresentation
Session
Transport
Network
Data Link
Physical
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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 softwaresystems
• Layer-4 ensures end to end reliable data
transmission
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OSI Layers
End System
Network Network Network Network
Data Link Data Link Data Link Data Link
Application
Presentation
Session
Transport
Physical PhysicalPhysical Physical
PSPDN PSTN Dedicated CSPDN
User Support
Layers Network Support Layers
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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 adjacentlayers
The messages exchanged between the
adjacent layers, to obtain the required
services, are called Interface Control
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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 thatcommunicates at a given layer are called peer-
to-peer processes
At the physical layer, communication is direct
At higher layers, communication moves down
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Hierarchical & Peer-to-Peer Communications
Peer-to-Peer
PCI
Interface Control Information (ICI)
Protocol Control Information (PCI)
Interface
N+1-Layer
N-Layer
Interface
N+1-Layer
N-Layer
Hierarchical
ICI
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(N+1)-Layer
(N)-Layer
Data Units in the OSI Model
(N+1)-SDU(N+1)-PCI
(N+1)-PDU
(N+1)-SDU(N+1)-PCI (N)-ICI
(N)-IDU
(N)-PCI
(N)-PDU
(N+1)-SDU(N+1)-PCI
(N)-PCI
(N)-ICI
(N+1)-SDU(N+1)-PCI(N)-SDU
(N)-ICI
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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 betweenthe 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
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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
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OSI Layers
6-5 Interface
5-4 Interface
4-3 Interface
3-2 Interface
2-1 Interface
7-6 Interface
7-Application
6-Presentation
5-Session
4-Transport
3-Network
2-Data Link
1-Physical
6-5 Interface
5-4 Interface
4-3 Interface
3-2 Interface
2-1 Interface
7-6 Interface
7-Application
6-Presentation
5-Session
4-Transport
3-Network
2-Data Link
1-Physical
3-2 Interface
2-1 Interface
3-Network
2-Data Link
1-Physical
Intermediate
Node
Link Link
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Summary of OSI Layers
Functions
6-5 Interface
5-4 Interface
4-3 Interface
3-2 Interface
2-1 Interface
7-6 Interface
Application
Presentation
Session
Transport
Network
Data Link
Physical
Allow access to network
resources
Translate, encrypt and
compress data
Establish, manage and
terminate sessions
Reliable end to end delivery
& error recovery
Movement of packets;
Provide internetworking
Transmit bits; Mechanical
and electrical specifications
Organise bits into streams; Node to node delivery
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Summary of OSI Layers
Functions
6-5 Interface
5-4 Interface
4-3 Interface
3-2 Interface
2-1 Interface
7-6 Interface
Application
Presentation
Session
Transport
Network
Data Link
Physical
Allow access to network
resources
Translate, encrypt and
compress data
Establish, manage and
terminate sessions
Reliable end to end delivery
& error recovery
Movement of packets;
Provide internetworking
Transmit bits; Mechanical
and electrical specifications
Organise bits into streams; Node to node delivery
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GSM System Architecture
BTS
BSC
BSS NSS
TRAU
HLR
VLR
GSTN
ISDN
MSC
SS7
NSS
R
A
DI
O
16Kb/s
64Kb/s
MS
BSS : Base Station Sub-system
BSC : Base Station Controller
BTS : Base Transceiver Station
TRAU : Transcoder / Rate Adapter Unit
NSS : Network and Switching Sub-system
MSC: Mobile service Switching Center
HLR : Home Location Register
VLR : Visitors Location Register
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GSM protocol layers for
signaling
CM
MM
RR
MM
LAPDm
radio
LAPDm
radio
LAPD
PCM
RR’ BTSM
CM
LAPD
PCM
RR’
BTSM
16/64 kbit/s
Um Abis A
SS7
PCM
SS7
PCM
64 kbit/s /
2.048 Mbit/s
MS BTS BSC MSC
BSSAP
BSSAP
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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
Channels (B
channels)
Number of Signaling
Channels (D
Channels)
BRI 2 1 (16 Kbps)
PRI(T/1) 23 1 (64 Kbps)
PRI(E/1) 30 1 (64 Kbps)
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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 usedto deliver signaling message to the ISDN switch
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LAPD and PPP on D and B
Channels
B0
B1D LAPD
BRI
B0
B1D LAPD
BRI
ISDN Network
PPP
B0
B1D
Call Setup Flows
LAPD
BRI
B0
B1D
Call Setup Flows
LAPD
BRI
Call Setup
Flows
ISDN Network
SS7
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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 Bchannel from end to end. LAPD is used between the router
and the ISDN switch at each local central office (CO) andremains up so that new signaling messages can be sentand received. Because the signals are sent outside thechannel 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.
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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 toanother site is sent by user.
Telecommuting Environment
Backup to leased lines – When leasedline fails, an ISDN call is established
between two routers.
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ISDN as WAN Link
BRI BRI
Leased Line
ISDN Network
Dial on
Demand
Routing
Telecommuting
Leased Line Backup
ISDN Network
ISDN Network
Computer with
ISDN Inerface
NT1
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Layer 2 LAPDM Protocol
Establishment and release of signalling layer 2connections.
Multiplexing and de multiplexing of several signallinglayer 2 connection on a dedicated control channel and
discrimination between them by including differentService Access Point Identifiers (SAPI).
Mapping of signalling layer 2 service data units onprotocol data unit (in case of acknowledged operationservice data units may be segmented and reassembled atdestination).
Detection and recovery of errors due to loss, duplication,and disorder.
Flow control.
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LAPDM Protocol
The establishment and release of layer 2 connectioncoincides with the allocation release, and change of dedication radio channels. Signaling layer 2 connectionsare frequently established and released, and thus an
average lifetime of a connection is short. Multiplexing anddemultiplexing deals with arranging different user (eightchannels per frame) in a frame format.
LAPDm uses on the two modes of operation for thetransmission of layer 3 message; unacknowledged
operation of multiple frame operation.
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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)
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LAPDM Protocol
For an unacknowledged informationtransfer, the use of layer 3 service impliesthat the information transfer is notacknowledged by the data link layer, andthus error check facilities are not provided.The transmission and reception of
messages here use data link serviceprimitives, that is, DL-DATA-REQUESTand DL-DATA-INDICATION.
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LAPD and LAPDm
The main distinction between LAPD andLAPDm is the absence of address and controlfields. Thus, the protocol is only used for theunacknowledged mode of operation, whichapplies to BCCHs and CCCHs only. BothFCCH and SCH under BCCH do not requireunacknowledged. Similarity, noacknowledgement is needed for PCH and
AGCH. The LAPD frame is used internal to BSS,
namely, between BTS and BSC.
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MTP3, SCCP, and TCAPProtocols.
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Class 5
End Office Switch
The Telephone Network [1/2]
Circuit Switched Network
Intelligent
Peripheral
Signal
Transfer
Point
Service
Control
Point
Class 4
Tandem Switch
Service
Data
Point
+
Transport Layer
Control Layer
SS7 Signaling
ISUP Messages
INAP/TCAP Messages
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The Telephone Network [2/2] 5 Basic Components in Intelligent
Networks
• SSP/Service Switching Point
• switching, service invocation
• STP/Service Transfer Point
• signal routing• SCP/Service Control Point
• service logic execution
• SDP/Service Data Point
• subscriber data storage, access
•IP/Intelligent Peripheral
• resources such as customized
voice announcement, voice
recognition, DTMF digit collection
SSP
SCP SDP
STPIP
SSP
STP
TCAP messages
ISUP messages
Voice
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Signalling example
ExchExchUser A
(callinguser)
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 mobileuser is located. The call is then routed to this exchange.
CUTTACKBBSR
KOLKOTA
User B(calleduser)
Exch
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SS7 Protocol Suite
ISUP
TCAP
SCCP
MAP
MTP Level 3 MTP Level 2 MTP Level 1
OSI Layers Application
Presentation
Session
Transport Network Data Link Physical
INAP
Signaling
Connection
Control Part
Transaction
Capabilities
Applications
PartISDN User
Part
IN Application
Part
Mobile
Application
Part
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MTP Levels 1 & 2
Message Transfer Part
Level 1
• Handling the issues related to the signals on thephysical 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 linkfrom one node to another
• Providing error detection/correction and sequenceddelivery of the SS7 messages
• signalling network supervision and maintenancefunctions
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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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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
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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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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
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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
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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 atypical SS7 networkarrangement.
This configurationserves severalpurposes.• No direct signaling links
• A fully meshed signalingnetwork is not required.
• The quad arrangementensures 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 additionallogic and that can be used to offer advanced services
The switch sends a message to the SCPasking for instructions.• The SCP, based upon data and service logic
that is available, will tell the switch whichactions 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•
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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
Same SPCs can be reused at different
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Same SPCs can be reused at differentnetwork levels
SPC = 277
SPC = 277
International
National
SPC = 277 means different signalling points (network elements)at different network levels.
F CK SIF SIO LI Control F
The Service Information Octet (SIO) indicates whether the DPCand 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 beused 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)
Message type (one octet)Mandatory fixed part
Mandatory variable part
Optional part
Must always be included in ISUP message
E.g., IAM message
E.g., contains called (user B) number inIAM message
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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 routinglabel defines the signalling link which is used for transfer of thesignalling information.
The 16-bit circuit identification code (CIC) contained in theISUP message defines the TDM time slot or circuit with which
the ISUP message is associated.
Exchange
STP
Exchange
Circuit
Signalling link
ISUP Call Establishment and
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Release
• A given circuit between two
switches is identified by OPC,DPC and CIC.
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Signalling using IAM message
Exchange ExchangeExchange
SPC = 82
Circuit14
SPC = 22 SPC = 60 Circuit20
STP
SL 4
SL 7
STP
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).
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Setup of a call using ISUP
LE A LE BTransit exchangeUser A User B
Setup IAMIAM
Setup
Alert
Connect
ACM
ANM
ACM
ANM
Alert
Connect
Charging of call starts now
Number analysisDSS1
signalling
assumed
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ISUP message format
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Signalling Connection Control Part (SCCP)
SCCP is required when signalling information is carried betweenexchanges and databases in the network.
An important task of SCCP is global title translation (GTT):
STP DatabaseExchangeSTP with GTT capability
Exchange knows the global title (e.g. 0800 number or IMSInumber 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 orIMSI number => DPC) and the SCCP message can now berouted to the database.
2.
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Example: SCCP usage in mobile call
SCCPSCCP
MSC located in Espoo HLR located in Oslo
STP
SPC = 82 SPC = 99
SPC = 32
SCCP/GTT functionality
Outgoing message:OPC = 82 DPC = 32SCCP: IMSI global title
Processing in STP:Received message is given to SCCP for GTT.SCCP finds the DPC of the HLR: DPC = 99
Mobile switching center (MSC) needs to contact the home locationregister (HLR) of a mobile user identified by his/her InternationalMobile Subscriber Identity (IMSI) number.
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To sum it up with an example…
PSTN
Typical operation of a local exchange
Subscriber signalling(analog or ISDN=DSS1)
Network-
internalsignalling(SS7)
Transmission(PDH, SDH)
Databases inthe network(HLR)
Part B, Section 3.3 in ”UnderstandingTelecommunications 2”
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Basic local exchange (LE) architecture
Timeswitch
TDM linksto othernetworkelements
• Switch control
Switching system
• E.164 number analysis• Charging
• User databases
LIC
LIC
Tone
Rx
Groupswitch
Sign.
ETC
ETC
Exchangeterminalcircuit
Lineinterfacecircuit
SS7 Signallingequipment
Control system• O&M functions
Subscriber stage
Modern trend: Switching and control functions are separated into
different network elements (separation of user and control plane).
Tone generator
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Setup of a call (1)
Timeswitch
2. Check user database. For instance, is
user A barred for outgoing calls?
Switching system
3. Reserve memory for user B number
LIC
LIC
Tone
Rx
Groupswitch
Sign.
ETC
ETC
Control system
Phase 1. User A lifts handset and receives dial tone.
1. Off hook
Local exchange of user A
4. Tone Rx is connected
5. Dial toneis sent(indicating “network is
alive”)
Tone generator
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Timeswitch
3. Number analysis
Switching system
4. IN triggering actions? Should an externaldatabase (e.g. SCP, HLR) be contacted?
LIC
LIC
Tone
Rx
Groupswitch
Sign.
ETC
ETC
Control system
Phase 2. Exchange receives and analyzes user B number.
2. Number (DTMFsignal) received
1. User Adials user Bnumber
Setup of a call (2)
Local exchange of user A
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Timeswitch
2. Outgoing circuit is reserved
Switching system
LIC
LIC
Tone
Rx
Groupswitch
Sign.
ETC
ETC
Control system
3. Outgoing signalling message (ISUP IAM)contains user B number
Phase 3. Outgoing circuit is reserved. ISUP Initial address message
(IAM) is sent to next exchange.
Setup of a call (3)
1. Tone receiveris disconnected
Local exchange of user A
E.g.,CIC = 24
IAM(containsinformationCIC = 24)
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Timeswitch
1. ISUP ACM message indicates free or busy
user B
Switching system
LIC
LICGroupswitch
Sign.
ETC
ETC
Control system
3. Charging starts when ISUP ANM messageis received
Phase 4. ACM received => ringback or busy tone generated. ANM
received => charging starts.
Setup of a call (4)
Local exchange of user A
ACM,ANMTone generator2. Ringback
or busy toneis locallygenerated
4. Callcontinues…
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Performance Requirements for
SS7
Bellcore spec. GR-246-Core
• MTP
• A given route set should not be out of service for
more than 10 minutes per year
• < 1*10-7 messages should be lost
• < 1*10-10 messages should be delivered out of
sequence
• ISUP
• Numerous timing requirements
A VoIP network that uses SS7
• Must meet the stringent requirements
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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
S ft it h A hit t
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Softswitch Architecture
Signaling(SS7)
Gateway
Trunking
Gateway
Call
Agent
SCP
Residential
Gateway
InternetSS7 Network
STP
TrunkingGatewayTrunking
Gateway
CO
Switch
ResidentialGatewayResidential
Gateway
MGCP/
MEGACO
MGCP/
MEGACO
RTP
SIGTRAN
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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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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 notexist in the IP network?
• For transport layer, the ISUP message mustbe carried in the IP network with the samespeed and reliability as in the SS7.
• UDP x
• TCP x
RFC 2719, “Framework Architecture for
Signaling Transport”
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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 notrealize 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
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SIGTRAN Protocol Stack SCTP: fast delivery of messages (error-free, in sequencedelivery), network-level fault tolerance
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Adaptation Layer [1/3] M2UA (MTP-2 User Adaptation
Layer)
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Adaptation Layer [2/3] M2PA (MTP-2 Peer-to-Peer Adaptation Layer)
• An SG that utilizes M2PA is a signaling node for the
MGC.
• It is effectively an IP-based STP.
• SG can processing higher-layer signaling functions,
such as SCCP GTT.
Adaptation Layer [3/3]
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p y [ ]
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 andreliability 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.
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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.
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What Supported By Using SCTP?
To ensure reliable, error-free, in-sequence delivery of user messages
(optional).
To support fast delivery of messages and
avoid head-of-line blocking.
To support network-level fault tolerance
that is critical for carrier-grade network
performance by using multi-home hosts.
To provide protection against DoS attack
by using 4-way handshake and cookie.
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SCTP Endpoint & Association
Endpoint
• The logical sender/receiver of SCTP packets.
• Transport address = IP address + SCTP port number
• An endpoint may have multiple transport addresses (for multi-homed host, all transport addresses must use the
same port number.)
Association
• A protocol relationship between SCTP endpoints.• Two SCTP endpoints MUST NOT have more than one
SCTP association.
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Multi-Homed Host
Host A
SCTP User
Host B
One IP address
One SCTP association with
multi-homed redundant
SCTP
SCTP User
SCTP
One IP address One IP address
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SCTP Streams
A stream is a one-way logical channel
between SCTP endpoints.
• The number of streams supported in an
association is specified during theestablishment of the association.
To avoid head-of-line blocking and to
ensure in-sequence delivery• In-sequence delivery is ensured within a single
stream.
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SCTP Functional View
SCTP User Application
Acknowledgementand
Congestion Avoidance
Chunk Bundling
Packet Validation
Path Management
Associationstartup
and
takedown
Sequenced deliverywithin streams
User Data Fragmentation
SCTP P k t & Ch k
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SCTP Packets & Chunks
A SCTP packet can comprise several chunks.
Chunk
• Data or control
Source Port Number Destination Port Number
Verification Tag
Checksum
Chunk Type Chunk Flags Chunk Length
Chunk Value
. . .
Common
Header
Chunk 1
Chunk N
0 16 3115. . . . . .
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Chunk Type• ID Value Chunk Type
• -------- ---------------• 0 - Payload Data (DATA)
• 1 - Initiation (INIT)
• 2 - Initiation Acknowledgement (INIT ACK)
• 3 - Selective Acknowledgement (SACK)
• 4 - Heartbeat Request (HEARTBEAT)• 5 - Heartbeat Acknowledgement (HEARTBEAT ACK)
• 6 - Abort (ABORT)
• 7 - Shutdown (SHUTDOWN)
• 8 - Shutdown Acknowledgement (SHUTDOWN ACK)
• 9 - Operation Error (ERROR)
• 10 - State Cookie (COOKIE ECHO)
• 11 - Cookie Acknowledgement (COOKIE ACK)
• 12 - Reserved for Explicit Congestion Notification Echo (ECNE)
• 13 - Reserved for Congestion Window Reduced (CWR)
• 14 - Shutdown Complete (SHUTDOWN COMPLETE)
• … - Reserved for IETF
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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
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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 controlchunks
•
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SHUTDOWN• A graceful termination of an association
• Stop sending any new data
• Wait until all data sent has been
acknowledged
• Send a SHUTDOWN to the far end
• Indicate the chunk received
•Upon receipt of a SHUTDOWN
• Retransmit data that are not acknowledged
• Send a SHUTDOWN ACK
SHUTDOWN ACK
SHUTDOWN COMPLETE
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ERROR
• Some error condition detected
• E.g., a chunk for a non-existent stream
COOKIE ECHO• Used only during the initiation of an association
• An INIT ACK includes a cookie parameter
• Information specific to the endpoint, a timestamp, a
cookie lifetime• Upon receipt of an INIT ACK
• Return the cookie information in COOKIE ECHO
• Can be multiplexed; must be the first chunk
COOKIE ACK
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INIT Chunk
Advertised Receiver Window Credit (a_rwnd)
Number of Outbound Streams
Type = 1 Chunk Flags Chunk Length
Initial TSN (Transmission Sequence Number)
. . .
0 16 3115. . . . . .
Initial Tag
Number of Inbound Streams
Optional / Variable-Length Parameter
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Association Establishment
A Z
INIT [I-Tag=Tag_A]
INIT ACK [V-Tag=Tag_A, I-Tag=Tag_Z, Cookie_Z]
COOKIE [Cookie_Z]
COOKIE ACK
allocatingresources
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User Data Transfer
SCTP user
SCTP Control Chunks
SCTP
SCTP DATA Chunks
User Messages
SCTP packets
Connectionless Packet Transfer Service (e.g. IP)
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DATA Chunk
Stream ID = S
Type = 0 Reserved Chunk Length
Payload Protocol ID
. . .
0 16 3115. . . . . .
TSN
Stream Sequence Number = n
User Data (Sequence n of Stream S)
U B E
U : unorderedB : beginE : end
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Payload data chunk
• Carry information to and from the ULP
• U: unordered bit
• The information should be passed to the ULP
without regard to sequencing
• B and E: beginning and end bits
•Segment a given user message
• TSN: Transmission Sequence Number (32-bit)
• Independent of any streams
• Assigned by SCTP
• An INIT has the same TSN as the first DATA chunk
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• S: Stream Identifier (16-bit)
• n: stream sequence number (16-bit)
• Begins at zero
• Increments for each new message
• Payload protocol identifier
• For the users to pass further information about the
chunk but is not examined by the SCTP
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SACK Chunk
Advertised Receiver Window Credit (a_rwnd)
Number of Gap Ack Blocks = n
Type = 3 Chunk Flags Chunk Length
0 16 3115. . . . . .
Cumulative TSN Ack
Number of Duplicate TSNs = xGap Ack Block #1 Start Gap Ack Block #1 End
. . .
. . .
Duplicate TSN #1
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Transferring data
• Reliable transfer
• SACK chunk
• Cumulative TSN
• The highest TSN value received without any gaps
• 4
• The number of Gap Ack Blocks
• The number of fragments received after the
unbroken sequence
• 2
• The number of duplicate TSNs
• 2
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• Gap Ack Block number 1 start
• The offset of the first segment from the unbrokensequence
• 3 (7-4)
• Gap Ack Block number 1 end
• The offset of the first segment from the unbrokensequence
• 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 primaryaddress + several secondary addresses).• Multi-homes hosts
SCTP ensures that endpoint is aware of the
reachability of another endpoint through thefollowing 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 SS7DPC/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 thesame PC
• A single SS7 signaling endpoint
• All ASPs share a PC != that of the SG
• ASPs: a si nalin end oint SG: an STP
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Robust Signaling Architecture
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Services Provided by M3UA
Offer the same primitives as offered by
MTP3
• MTP-Transfer request
• MTP-Transfer indication
• MTP-Pause indication
• Signaling to a particular destination should be
suspended• MTP-Resume indication
• Signaling to a particular destination can resume
• MTP-Status indication
• Some change in the SS7 network
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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 SS7signaling-network management messages
• The SG pass M3UA messages to the CA
• The ISUP application at the CA is made aware
• MTP-Pause indication
gna ng e wor anagemenMSGs
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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
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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
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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
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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
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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
•
Routing Key Management
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Routing Key Management
Messages
Registration Request (REG REQ)
• An ASP = a DPC/OPC/CIC range
Registration Response Deregistration Request
Deregistration Response
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M2UA Operation
MTP3/M2UA/SCTP
The CA has more visibility of the SS7
network• More tightly coupled to the SG
MTP3
• Routing and distribution capabilities
M2UA uses similar concepts to those
used by M3UA
• ASPUP, ASPDN, ASPAC, ASPIA and ERR
•
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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 linkout of service
• RELEASE CONFIRM
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