telecom signalling basics- cas&ccs(ss7)
TRANSCRIPT
Telecom Signaling Basics-CAS andCCS
Common Channel Signaling System No. 7
Contents
• Signaling and its relevance
• Channel associated signaling & its limitations
• Common Channel Signaling/SS#7
• CCS/SS#7 Network Architecture
• Types of Links
• SS7 Protocol Stack
• SS7 Protocol Units
• Basic ISUP Call Set up
• Advantages of SS#7
What signaling is and why it is relevant
The ITU-T defines signaling as, "The exchange of information (other than by speech) specifically concerned with the establishment, release and other control of calls, and network management, in automatic telecommunications operation.“
In telecommunications, the network's components must indicate (that is, signal) certain information to each other to coordinate themselves for providing services. As such, the signaling network can be considered the telecommunications network's nervous system.
Signaling is of two types:
1.Subscriber Signaling or Access Signaling
Address signals , Supervisory signals, Tones & announcements, Ringing
2.Network Signaling. CAS & SS7.
Subscriber Signalling
Address Signals:
Address signals represent the called party number's dialed digits. Address signaling occurs when the telephone is off-hook. For analog lines, address signaling is either conveyed by the dial pulse or Dual-Tone Multiple Frequency (DTMF) methods. Local switches can typically handle both types of address signaling, but the vast majority of subscribers now use Dual-Tone Multi Frequency (DTMF), also known as touch-tone.
Supervisory Signals:
A telephone has two possible supervision states: on-hook or off-hook. On-hook is the condition in which the telephone is not in use, which is signaled when the telephone handset depresses the cradle switch. The term on-hook comes from the days when the receiver part of the telephone rested on a hook. The telephone enters the off-hook condition when the handset is lifted from its cradle, thereby releasing the cradle switch and signaling to the exchange that the subscriber wishes to place an outgoing call.
Subscriber Signalling
• Tones and announcements are audible backward signals, such as dial tone, ring back, and busy-tone, that are sent by a switch to the calling party to indicate a call's progress.
• Ringing is a forward signal sent by the switch to the called subscriber to indicate the arrival of a call. It is known more specifically as power ringing to distinguish it from audible ringing
Network Signaling
• Network signaling takes place between nodes in the core network. This is generally from the local switch, through the core network, and to the destination local switch—in other words, between the calling and the called party switch.
• Network signaling was previously implemented using Channel Associated Signaling (CAS) techniques and systems. However, for the past two decades, it has been replaced with Common Channel Signaling (CCS) systems. System No. 7 (SS7) is almost the exclusive CSS system; thus, CCS can almost be taken to refer exclusively to the use of SS7.
Channel Associated Signaling
• Relationship between call control signals and voice circuits
• Fixed Signaling capacity is set aside for each and every trunk.
• In band MF or SF signals sent to convey signaling Information.
CAS on digital/TDM Trunks.
Supervisory signaling can be performed on digital TDM trunks. On an E1 facility, timeslot 16 is set aside for supervisory signaling bits (TS16). These bits are arranged in a multiframe structure so that specific bits in the multiframe's specific frames represent the signaling information for a given TDM audio channel.
Common Channel Signaling (CCS)/SS7
• Using CAS line information is transported in Channel 16 in following way using a 16 frame multiframe.
• Channel 16 of frame 0 contains multi-frame synchronization.
• Channel 16 of frame 1 contains 4 bits of Channel 1 and 4 bits of Channel 17.
• Channel 16 of frame 2 contains 4 bits of Channel 2 and 4 bits of Channel 18.
• Channel 16 of frame 16 contains 4 bits of Channel 15 and 4 bits of Channel 31. (Please refer to Figure on next Slide)
Channel Associated Signalling
Channel Associated Signalling
• Examples of 4 bit codes used are:
• 1001:IDLE
• 0001:SEIZURE
• 1101:ACK
Limitations of CAS
CAS has a number of limitations, including:
• Susceptibility to fraud
• Limited signaling states
• Poor resource usage/allocation
Common Channel Signaling (CCS)/SS7
• CCS refers to the situation in which the signaling capacity is provided in a common pool, with the capacity being used as and when necessary. The signaling channel can usually carry signaling information for thousands of traffic circuits.
• CCS systems are packet-based, transferring over 200 bytes in a single SS7 packet, as opposed to a few bits allocated to act as indicators in digital CAS.
• CCS supports circuit related and non-circuit related signalling.
Common Channel Signaling (CCS)/SS7
• Circuit-Related Signaling
Circuit-related signaling refers to the original functionality of signaling, which is to establish, supervise, and release trunks. In other words, it is used to set up, manage, and clear down basic telephone service calls.
• Non-Circuit-Related Signaling
Non-circuit-related signaling refers to signaling that is not related to the establishment, supervision, and release of trunks. Non-circuit-related signaling allows the transfer of information that is not related to a particular circuit, typically for the purpose of transmitting both the query and response to and from telecommunication databases.
SS7 Architecture
SS7 Architecture
• A telecommunications network consists of a number of switches and application processors interconnected by transmission circuits. The SS7 network exists within the telecommunications network and controls it. SS7 achieves this control by creating and transferring call processing, network management, and maintenance to the network's various components.
• An SS7 network has three distinct components: Service Switching Points, Signal Transfer Points, and Service Control Points. These components may be generically referred to as “nodes” or “signaling points” and are connected to each other via “data links”.
CCS/SS7 Network Architecture
CCS Nodes
SSP-Service Switching Point
STP-SignallingTransfer Point
SCP-Service Control Point
SSP
SCP AIN SCP
STP
SLK
SS7 Architecture
• Service Switching Point
• A Service Switching Point (SSP) is a voice switch that incorporates SS7 functionality. It processes voice-band traffic (voice, fax, modem, and so forth) and performs SS7 signaling.
• Signal Transfer Point
• A Signal Transfer Point (STP) is responsible for the transfer of SS7 messages between other SS7 nodes, acting somewhat like a router in an IP network.
• Service Control Point
• A Service Control Point (SCP) acts as an interface between telecommunications databases and the SS7 network. Telephone companies and other telecommunication service providers employ a number of databases that can be queried for service data for the provision of services.
Signalling Point Code,Links, Linksets
• Point Code
SS7 network nodes are called signaling points (SPs). Each SP is addressed by an integer called a point code (PC). The international network uses a 14-bit PC. The national networks also use a 14-bit PC—except North America and China, which use an incompatible 24-bit PC, and Japan, which uses a 16-bit PC
• Signaling Links and Linksets
SPs are connected to each other by signaling links over which signaling takes place. The bandwidth of a signaling link is normally 64 kilobits per second (kbps). Links are typically engineered to carry only 25 to 40 percent of their capacity so that in case of a failure, one link can carry the load of two.To provide more bandwidth and/or for redundancy, up to 16 links between two SPs can be used. Links between two SPs are logically grouped for administrative and load-sharing reasons. A logical group of links between two SP is called a linkset.
Route and Routesets
• SS7 routes are statically provisioned at each SP. There are no mechanisms for route discovery. A route is defined as a preprovisioned path between source and destination for a particular relation.All the preprovisioned routes to a particular SP destination are called the routeset.
SS7 Links
• Access Links (A Links): They connect "outer" SPs (SSPs or SCPs) to the STP backbone.
• Cross Links (C Links) :Cross links (C links), are used to connect two STPs to form a mated pair
• Bridge Links (B Links) :Bridge links (B links) are used to connect mated pairs of STPs to each other across different regions within a network at the same hierarchical level.
• Diagonal Links (D Links) :They connect mated STP pairs that belong to different hierarchical levels or to different networks altogether
• Extended Links (E Links) :Extended links (E links), shown in connect SSPs and SCPs to an STP pair, as with A links, except that the pair they connect to is not the normal home pair.
• Fully-Associated Links (F Links) :Fully-associated links (F links), are used to connect network SSPs and/or SCPs directly to each other without using STPs.
Types of CCS/SS7 Links
• Access Link
SSP
STP
STP
A
A
A
A
SSP
SCP
A
A
Types of CCS Links
• Bridge Link
STP
STP STP
STP
B
B
B
B
Local STPs
Local STPs
Types of CCS Links
• Cross Link
C C
STP
STP STP
STP
Types of CCS/SS7 Links
• Diagonal Link
D
D
STP
STP STP
STPLocal STPsMated Pair
Regional STPsMated Pair
D
D
Types of CCS Links
• Extended Links
STP
STP STP
STP
SSP SSP
Local Area 1 Local Area 2
E E
Types of CCS Links
• Fully associated Links
STP
SSP SSP
STP
STP
SSP SSP
STP
F F
Signalling Links
• Signalling Links(Figure shows the Typical Network)
SSP
SSP
SSPSTP STP
STPSSP
SCPSCP
STP
SS7 Protocol is
• Software providing language for CCS network
• Responsible for passing signaling information in CCS network
• Layered architecture
• The SS7 physical layer is called MTP level 1 (MTP1), the data link layer is called MTP level 2 (MTP2), and the network layer is called MTP level 3 (MTP3). Collectively they are called the Message Transfer Part (MTP). The MTP protocol is SS7's native means of packet transport.
• With reference to the Open System Interconnection (OSI) seven-layer reference model, SS7 uses a four-level protocol stack. SS7 uses the term levels when referring to its architecture.
SS7 Protocol Stack
OSI/SS7
OSI Reference Model SS7 Protocol Model
Physical
Data Link
Network
Transport
Session
Presentation
Application
Physical
Data Link
Network
SCCP
TCAPI
S
U
P
T U P }
MTP Level 1
MTP Level 2
MTP Level 3
Layer 1
Layer 2
Layer 3
Layer 4
Layer 5
Layer 6
Layer 7
Message Transfer Part(MTP)
• MTP is comprised of three levels in the SS7 protocol model. The MTP functional area of the SS7 protocol provides reliable transfer of SS7 signaling messages.
• MTP Level 3 ( Network Layer)
– Message discrimination - determines who the message is addressed to
– Message distribution - directs a locally significant message to the appropriate internal user (i.e., SCCP module)
– Message routing - reads the originating point code (OPC) / destination point code (DPC) to determine which signaling node to send message to
• MTP Level 2 (Data Link Layer)
– Provides for transfer of signaling data over individual signaling links
– Responsible for error free transmission of data
– Ensures error free transfer of data in proper sequence between signaling nodes
• MTP Level 1 ( Physical Layer)
– Provides transport for signaling link
– Defines physical, electrical and functional characteristics of a signaling link
– Provides the means to access the signaling link
– Typically the transmission path is configured as 56 kbps or 64 kbps
ISUP
• TUP and ISUP
TUP and ISUP sit on top of MTP to provide circuit-related signaling to set up, maintain, and tear down calls. TUP has been replaced in most countries because it supports only POTS calls. Its successor, ISUP, supports both POTS and ISDN calls as well as a host of other features and added flexibility.Both TUP and ISUP are used to perform interswitch call signaling. ISUP also has inherent support for supplementary services, such as automatic callback, calling line identification, and so on. The level 4 ISUP message consists of a Circuit Identification Code (CIC), message type, ie: Initial Address Message (IAM), Address Complete Message (ACM), Answer Message (ANM), Release Message (REL) and Release Complete Message (RLC) .
SCCP
• SCCP
The combination of the MTP and the SCCP is called the Network Service Part (NSP) in the specifications .The addition of the SCCP provides a more flexible means of routing and provides mechanisms to transfer data over the SS7 network. Such additional features are used to support noncircuit-related signaling, which is mostly used to interact with databases (SCPs). SCCP controls global title (GT) routing. A GT is a directory number that serves as an alias for a physical network address. A physical address consists of a point code and an application reference called a subsystem number (SSN). Centralized (Signal Transfer Points) are then used to convert the GT address into a physical address; this process is called Global Title Translation (GTT).
TCAP
TCAP
TCAP allows applications (called subsystems) to communicate with each other (over the SS7 network) using agreed-upon data elements. These data elements are called components. Components can be viewed as instructions sent between applications. TCAP also provides transaction management, allowing multiple messages to be associated with a particular communications exchange, known as a transaction. Most TCAP services can be viewed as a dialogue of questions and answers. A switch needs additional information that is associated with call processing, or with a particular service that causes it to send a TCAP query that requests the needed information.
SS7- PROTOCOL UNIT-FISU
Fill-in Signal Unit
Flag BSNBIBFSNFIBLISpareCheck Sum
First Last
Fill-in Signal Unit (FISU)
16 2 6 1 7 1 7 8 Length in bits
Level 2
SS7 PROTOCOL UNIT-FISU
• Flag - acts as a delimiter for SUs. A flag marks the end of one SU and the start of the next SU. Always looks like this 01111110.
• BSN - used to confirm receipt of SUs and to ensure they are received in the order they were transmitted. The BSN value will match that of the last successfully received SUs FSN. Value can be 0-127, or 128 possible SUs in receive buffer
• BIB - Indicates a negative acknowledgement if the bit does not match the FIB bit value, either 0 or 1
• FSN - Contains the sequence number of a transmitted SU. Can be between 0-127, or 128 possible SUs in transmit buffer
SS7 PROTOCOL UNIT-FISU
• FIB - Used in error recovery like the BIB.When messages are transmitted in error, and the BIB has changed from 1 to 0, or 0 to 1, the FIB will change to match the BIB when the corrupted messages have been successfully retransmitted. If there are no erred messages, the BIB and FIB will have the same value, 0 or 1.
• LI - Indicates length of the SU. FISU LI = 0, LSSU LI = 1 or 2, MSU LI = 3 to 63. If the MSU is larger than 63 octets (bytes) it will not be indicated. Max 273 octets.
• Spare - Used as a filler since the LI only uses 6 of the possible 8 bits
• Check Sum - is calculated from transmitted message by transmitting SP, and inserted in the message. On receipt it is recalculated by receiving SP. If not the same, the message is corrupt and retransmission is requested
SS7 PROTOCOL UNIT-LSSU
• Link Status Signal Unit
LI FlagBSNBIBFSNFIBSpareStatusCheck Sum
First Last
Link Status Signal Unit (LSSU)
16 8 or 16 2 6 1 7 1 7 8
Level 2 Level 2Level 3
SS7 PROTOCOL UNIT-LSSU
• LSSU has one unique field, the status filed (SF). The SF can have any one of six different status indications .
• Busy (B) - level 2 is busy, or congested at transmitting signaling point. A SP will suspend MSUs when it receives a busy LSSU. If the condition lasts 3-6 sec. Level 3 will be informed of a link failure and begin alignment procedure.
• Processor Outage (PO) - transmitting signaling point cannot communicate with levels 3 and 4, possibly due to a CPU or total nodal failure, or being manually taken out of service with the canc-slk command.
• Out of Service (OS) - signaling point cannot transmit or receive any MSUs for reasons other than processor outage. Upon receipt of an SIOS the receiving SP stops the transmission of MSUs and begins transmitting FISUs. It is also sent at the beginning of the alignment procedure.
SS7 PROTOCOL UNIT-LSSU
• Out of Alignment(O) - sent when a link has failed, is restored and alignment procedure has been initiated, but proving period parameters have not been met.
• Normal Alignment(N) - procedure used when there is more than one slk in the affected linkset. During the alignment process, the slk is looking for 4 successful normal alignment LSSUs in a 2.3 sec. proving period.If it fails it will go out of alignment again.
• Emergency Alignment (E) - procedure used when there is there is only one slk in a linkset. During the alignment process, the slk is looking for 1 successful emergency alignment LSSU in a .6 sec. proving period.
SS7- PROTOCOL UNIT-MSU
• Message Signal Unit
FlagBSNBIBFSNFIBLISpareSIOSIFCheck Sum
First LastMessage Signal Unit (MSU)
16 8-272 8 2 6 1 7 1 7 8
Level 2Level 2 Level 3
(octets)
Message Signal Unit
• MSU has two unique fields:
• Service Information Octet (SIO) which tells level 4 what type of message it is about to receive and the type of network message, and its priority
• The service indicator field tells the type message:
– value - 0 indicates a signaling network management message
– value - 1 indicates a signaling network test and maintenance message
– value - 3 indicates a SCCP message
– value - 5 indicates an ISUP message
• Two bits indicate the network type:
– value - 0 indicates an international network
– value - 2 indicates a national network
• Two bits indicate the message priority:
– value - 0 indicates lowest priority messages
– value - 3 indicates highest priority message
• Signaling Information Field (SIF) has:
– the routing label of the message (OPC/DPC)
– signaling link selector used to distribute traffic over multiple signaling links if available & level 4 user data
Service Information Octet
SIO-Service Indicator Values
SIF-Service Information Field
Originating Point Code (OPC)— Identifies the node originating the messageDestination Point Code (DPC)— Identifies the destination nodeSignaling Link Selector (SLS)— An identifier used for load sharing across linksets and links
The SIF contains the actual user data being transported by MTP, such as telephone numbers, control signals, or maintenance messages.
Routing Labels – ANSI & ITU
CCS/SS7 CALL SETUP
• EX (2) SS7 Voice Circuit SW1
SW2
(1)
(6)
On-hook On-hook
Off-hook
Off-hook
(8) (8)
(5b)
(5a)(3) IAM
(4) ACM
(5a) primary ring voltage sent to called party
(5b) ring-back tone is sent to calling party
(6) ANM
(7)
(8) REL
(9) RLC
ISUP Call Steps.
1.The IAM contains the information needed to set up a call. For a basic call, it is the first . The party served by the Sw-1 has gone off-hook and has dialed the digits of a party served by Sw-2.
2.The Sw-1 selects an idle voice trunk between the Sw-1 and the Sw-2.
3.The Sw-1 sends an initial address message (IAM) containing the dialed digits, and the idle voice trunk data to the Sw-2.
4.The Sw-2 performs a test on the called party’s line to ensure it is operational, and not busy. If the call can be placed, the Sw-2 sends an address complete message (ACM) to the Sw-1.
ISUP Call Steps.
5.When the called party phone is answered, an answer message (ANM) is sent to the Sw-1, and then the voice trunk reserved for the call in step 2 is cut-through so the conversation may begin, and the billing process is begun at the Sw-1 where the call originated.
6.The two parties have a conversation.
7.Following the conversation, when either of the parties goes on-hook, a release message (REL) is transmitted across the voice trunk to the far end office.
8. When the on-hook is repeated in the opposite direction, a release complete message (RLC) is sent to the far end that indicates the end of the call. The voice trunk is then released so it can be used for another call.
Advantages of SS7
• Reliable digital format
• Faster call set up.
• Higher speed data transport
• Robust Network Management
• Reducing fraud
• Access to centralized databases
• Revenue generating services
• Better utilization of facilities