Signaling System for GSM Networks Rabindranath Nandi & Rahul Ghosh August 11, 2006 ComSoc Lecture Meet

Acknowledgement Mr. Suvadip Basu Deputy Manager, Switch Operations Hutchison Telecom East Ltd. Kolkata – 700 017 August 11, 2006 ComSoc Lecture Meet

Introduction to Signaling Systems Signaling : Signaling is defined as a mechanism by virtue of which network entities exchange information amongst themselves , which are required to set up a communication path. Signaling System : Signaling system is defined as a set of methods or rules followed by network entities to exchange information required for communication set up. Examples of Signaling Systems : SS#7 or CCS#7 (Common Channel Signaling #7) CAS (Channel associated Signaling) DTMF (Dual Tone – Multi frequency) August 11, 2006 ComSoc Lecture Meet

between Subscriber and exchange Areas of Signaling There are mainly three areas of signaling during a telephone call : between subscribers and exchanges. within exchanges. between exchanges. between exchanges Exchange A Exchange B between Subscriber and exchange within exchange August 11, 2006 ComSoc Lecture Meet

Channel Associated Signaling (CAS) In CAS each and every speech channel is associated with a signaling channel. This means for each speech channel a separate signaling channel is required. Speech Speech Signaling Signaling Exchange A Exchange B Speech Speech Signaling Signaling August 11, 2006 ComSoc Lecture Meet

Common Channel Signaling (CCS) In CCS there is a common signaling channel which takes care of all the signaling information to be exchanged during communication. All other channels can be used for speech or data as required. Speech Exchange A Exchange B Speech Common Signaling Channel August 11, 2006 ComSoc Lecture Meet

Advantages of CCS Higher signaling capacity. More number of speech/data channels as there is only one signaling channel. Central offices can exchange information , not related to speech/data between themselves e.g. subscriber data. Various high end features like roaming are possible by using CCS7. August 11, 2006 ComSoc Lecture Meet

CCS7 Network Components The CCS7 Network consists of the following Components : SP -Signaling Point : The SP is the source or originating entity of the signaling message. STP-Signaling Transit Point : These entities transfer the signaling message to another SP or STP without processing the signaling message. SRP – Signaling Relay Point : These are STPs with ability tp process the signaling message. SEP – Signaling End Point : The SEP is defined to be the terminating point of the signaling message August 11, 2006 ComSoc Lecture Meet

CCS7 Network Architecture A typical SS7 network is a mixture of various SPs STPs SRPs and SEPs, As shown below : STP (SPC=200) SP (SPC=100) SEP (SPC=500) SRP (SPC=400) STP (SPC=300) August 11, 2006 ComSoc Lecture Meet

CCS7 Terminologies SPC : SPC is defined as signaling point code which uniquely identifies each element within the SS7 network. Signaling Links : The signaling links transmit signaling messages between the communicating entities. Signaling Link Set : A signaling link set is a collection of signaling links between two signaling entities. Signaling Route : A signaling route is defined to be the path through which signaling messages are exchanged between two entities. There can be multiple signaling routes between two SPCs, wherein there will be one primary route and several alternate routes. August 11, 2006 ComSoc Lecture Meet

SS7 Routing Modes Associated Mode : In the associated mode, the signaling link is routed together with the circuit group belonging to the link. In other words, the signaling link is directly connected to signaling points which are also the terminal points of the circuit group. This mode of signaling is recommended when the capacity of the traffic relation between the signaling points A and B is heavily utilized. Associated Mode of Signaling August 11, 2006 ComSoc Lecture Meet

SS7 Routing Modes Quasi associated mode : In the quasi-associated mode of signaling, the signaling link and the circuit group run along different routes, the circuit group connecting the signaling point. A is directly connected with the signaling point B. For this mode the signaling, the circuit group is carried out via one or more defined signaling transfer points. This signaling mode is favorable for traffic relations with low capacity utilization, as the various signaling links can be used for several destinations. August 11, 2006 ComSoc Lecture Meet

SS7 Network Structure SPC=100 SPC=700 NAT 1 NAT 1 SPC=300 SPC=200 INAT 0 SPC=500 SPC=200 August 11, 2006 ComSoc Lecture Meet

SS7 Protocol Structure SS7 protocol stack can be broadly divided into two main categories : A ) MTP – Message Transfer Part : The message transfer part has three levels , namely MTP Layer 1, Layer 2 and Layer 3. The message transfer part (MTP) represents a user-neutral means of transport for messages between the users. B ) User Parts (UP) : Each user part (UP) encompasses the functions, protocols and coding for the signaling via SS7 for a specific user type (e.g. data service, ISDN). In this way, the user parts control the set-up and release of circuit connections, the processing of facilities as well as administration and maintenance functions for the circuits. August 11, 2006 ComSoc Lecture Meet

SS7 Protocol Structure SS7 Users M U P SCCP I S U P B S A P TCAP Users M U P Other User Parts TCAP SCCP MTP Layer 3 (Network Management) MTP Layer 2 (Signaling Link) MTP Layer 1 (Signaling Data Link) August 11, 2006 ComSoc Lecture Meet

The Message Transfer Part MTP is used in SS7 by all user parts as a transport system or message exchange. Messages to be transferred from one user part to another are given to the message transfer part The message transfer part ensures that the messages reach the addressed user part in the correct order without information loss, duplication or sequence alteration and without any bit errors. August 11, 2006 ComSoc Lecture Meet

The Message Transfer Part Signal units : The message transfer part transports messages in signal units of varying length. A signal unit is formed by the functions of level 2. In addition to the message it also contains control information for the message exchange. There are three different types of signal units Message Signal Units (MSU) With message signal units, the message transfer part transfers user messages, i.e., messages from user parts (level 4) and messages from the signaling network management (level 3). Link Status Signal units (LSSU) LSSU contains information for the operation of the signaling link (e.g. for the alignment). Fill-In Signal Units (FISU) FISU are used to maintain the acknowledgement cycle and quality control when no user messages are to be sent in one of the two directions of the signaling. August 11, 2006 ComSoc Lecture Meet

The Message Transfer Part Signal Unit Formats August 11, 2006 ComSoc Lecture Meet

The Message Transfer Part Forward indicator bit (FIB) The forward indicator bit (FIB) is needed during general error correction. It indicates whether a signal unit is being sent for the first time or whether it is being retransmitted. Length indicator (LI) The length indicator (LI) gives the number of octets (one octet = 8 bits) between the length indicator field and the check-bit field, and is used to differentiate between the three signal units. The length indicator field contains different values according to the type of signal unit: 0 = fill-in signal unit 1 or 2 = link status signal unit greater than 2 = message signal unit. The maximum value in the length indicator field is 63 even if the signal information field contains more than 62 octets. (The message signal unit is invalid for longer message signal units. However, it is possible to calculate the exact length of the message signal unit). August 11, 2006 ComSoc Lecture Meet

The Message Transfer Part Service information octet (SIO): The service information octet (SIO) only exists in message signal units. It contains the service indicator and the network indicator. A service indicator is assigned to each user of the message transfer part. It informs the message transfer part which user part has sent the message and which user part is to receive it. The network indicator indicates whether the traffic is national or international. The message transfer part evaluates both items of information. Signal information field (SIF): The signal information field (SIF) only exists in message signal units. It contains the actual user message as well as the addresses. The maximum length of the signaling information field is 272 octets. Check bits (CK) : The check bits (CK) are formed on the transmission side from the contents of the signal unit and are added to the signal unit as redundancy. On the receive side, the message transfer part can determine with the check bits whether the signal unit was transferred without any errors. The signal unit is acknowledged as either positive or faulty on the basis of the check. Status field (SF) : The status field (SF) only exists in link status signal units. It contains status indications for the signaling links for the alignment of the transmit and receive directions. August 11, 2006 ComSoc Lecture Meet

The Message Transfer Part Addressing of the signal units The routing label of a signal unit is transported in the signal information field (SIF). It consists of the following Destination Point Code (DPC) Originating Point Code (OPC) Signaling Link Selection (SLS) A code is assigned to each signaling point in the signaling network according to a numbering plan. The message transfer part uses the code for message routing. The destination point code in a message signal unit identifies the signaling point to which this message is to be transferred. The originating point code specifies the signaling point from which the message originates. The contents of the signaling link selection determine the signaling route along which the message is to be transmitted. In this way, the signaling link selection is used for load sharing on the signaling links between two signaling points. The service information octet (SIO) contains additional address information. Using the service indicator, the destination message transfer part identifies the user part for which the message is intended. August 11, 2006 ComSoc Lecture Meet

The Message Transfer Part Routing label of a message signal unit August 11, 2006 ComSoc Lecture Meet

The Message Transfer Part Functions of the MTP Layers Level 1 (signaling data link) defines the physical, electrical and functional characteristics of a signaling data link and the access units. Level 1 represents the bearer for a signaling link. In a digital network, 64-kbit/s channels are generally used as signaling data links. In addition, analog channels (preferably with a bit rate of 4.8 kbit/s) can also be used via modems as a signaling data link. Level 2 (signaling link) defines the functions and procedures for a correct exchange of user messages via a signaling link. The following functions must be carried out in level 2: – delimitation of the signal units by flags. – elimination of superfluous flags. – error detection using check bits. – error correction by re transmitting signal units. – error rate monitoring on the signaling data link. – restoration of fault-free operation, for example, after disruption of the signaling data link August 11, 2006 ComSoc Lecture Meet

The Message Transfer Part Level 3 (signaling network) defines the internetworking of the individual signaling links. A distinction is made between the two following functional areas: – message handling, i.e. directing the messages to the desired signaling link, or to the correct user part. – signaling network management, i.e. control of the message traffic, for example, by means of changeover of signaling links if a fault is detected and change back to normal operation after the fault is corrected The various functions of level 3 operate with one another, with functions of other levels and with corresponding functions of other signaling points. August 11, 2006 ComSoc Lecture Meet

The Message Transfer Part MTP Functions August 11, 2006 ComSoc Lecture Meet

The User Parts - ISUP ISDN user part (ISUP) The ISDN user part (ISUP) covers the signaling functions for the control of calls, for the processing of services and features and for the administration of circuits in ISDN. The ISUP has interfaces to the message transfer part and the signaling connection control part (SCCP) for the transport of message signal units. The ISUP can use SCCP functions for end-to-end signaling. The structure of the ISUP Message is shown Below : August 11, 2006 ComSoc Lecture Meet

ISDN User Part The routing label comprises the destination point code, the originating point code and the signaling link selection. The circuit identification code (CIC) assigns the message to a specific circuit. A circuit identification code is permanently assigned to each circuit. The message type defines the function and the format of an ISUP message. There are different message types for the call set-up, the call release and the administration of circuits. Message types for the call set-up Initial Address Message (IAM) The IAM is the first message sent to the next network node during call set-up. It is used for seizing a circuit and contains all information necessary for routing to the terminating network node. August 11, 2006 ComSoc Lecture Meet

ISDN User Part Subsequent Address Message (SAM) The SAM transports the digits which were not yet contained in the IAM. Address Complete Message (ACM) The calling network node is informed with the ACM that the terminating network node was reached. Answer Message (ANM) The ANM informs the calling network node that the called party has answered. The call charge registration normally begins with the ANM. Message types for call release Release Message (REL) The REL initiates the release of a circuit connection. Any unsuccessful circuit connection set-up is likewise released with REL. It also includes the cause of the failure of the call set-up. August 11, 2006 ComSoc Lecture Meet

ISDN User Part Release Complete Message (RLC) With the RLC, the disconnection of the set-up of a circuit is indicated and the reception of the RLC is acknowledged. After the transmission or reception of the RLC the circuit is released and becomes available for a new call set-up. Message types for the administration of circuits Blocking message (BLO) The BLO is used for blocking a circuit. Unblocking message (UBL) The UBL is used for canceling a block on a circuit. August 11, 2006 ComSoc Lecture Meet

ISDN User Part The fixed mandatory part of the ISUP message contains parameters which must be present for a certain message type and which have a fixed length. For the IAM these are, for example, parameters for- the type of connection (e.g. connection via a satellite link) the requirements for the transmission link (e.g. 64 kbit/s end-to-end) the requirements for the signaling system (e.g. ISUP end-to-end) the type of the calling party (ISDN subscriber = normal subscriber) The variable mandatory part of the ISUP message contains parameters of variable length. An example of one such parameter for the IAM is: the directory number or at least part of the number which is required for routing to the terminating network node. If a message has an optional part, the parameters that can be transmitted in the optional part are specified for the message. These may be parameters of fixed or variable length. Examples for the IAM are: Directory number of the calling party parameters for the message type (e.g. closed user group) user information August 11, 2006 ComSoc Lecture Meet

ISDN User Part – Call Setup Procedures August 11, 2006 ComSoc Lecture Meet

ISDN User Part – Call Release Procedures August 11, 2006 ComSoc Lecture Meet

SCCP – Signaling Connection Control Part Introduction : The signaling connection control part (SCCP) is used as a supplement to the message transfer part. It provides additional functions for the transfer of messages between network nodes and between network nodes and other signaling points. From the point of view of the message transfer part, the SCCP is a user with its own service indicator. The combination of the SCCP and the message transfer part is called the network service part (NSP). Two Varieties of SCCP : Connection Oriented Connectionless August 11, 2006 ComSoc Lecture Meet

SCCP – Signaling Connection Control Part Connectionless SCCP : Connectionless SCCP does not require logical connection between SCCP users . Without logical signaling connection an SCCP user can send single messages to other SCCP users. Connection Oriented SCCP :With logical signaling connection an exchange of messages between two SCCP users is possible. A logical signaling connection arises through the mutual network node of the originating point codes between the SCCPs in the signaling points of the signaling relation. The messages to the other SCCP users can thus be directly addressed. The SCCP can send messages via the MTP network. The SCCP possesses its own routing function. The SCCP can use the following as address parameters: the destination point code (DPC) a global title (GT Address) a subsystem number (SSN No) which identifies the SCCP User August 11, 2006 ComSoc Lecture Meet

SCCP – Message Structure Structure of an SCCP message An SCCP message consists of: a routing label a message type fixed mandatory part variable mandatory part optional part August 11, 2006 ComSoc Lecture Meet

SCCP – Message Structure Connectionless SCCP : Unidata (UDT) : SCCP messages are sent to a destination with the UDT message. It is used for the protocol classes 0 and 1. Unidata service (UDTS) A transmitting SCCP is informed with the UDTS message that a UDT message could not be conveyed to the destination. It is used for the protocol classes 0 and 1 Extended unidata (XUDT) :Signaling information is sent in a connectionless mode, whereby optional parameters are allowed (for segmentation). Extended unidata service (XUDTS) Signaling information received from an XUDT message is sent back to its originating point if the XUDTS was not able to reach the destination. The user must already have requested this feature (Return option). August 11, 2006 ComSoc Lecture Meet

SCCP – Message Structure Connection Oriented SCCP : Connection Request (CR) The far-end signaling point of a signaling relation is informed with the CR message that a logical signaling connection is to be set up. The CR message can be sent as either a message on its own or together with another message, depending on the protocol class used. Connection Confirm (CC) The set-up of a logical signaling connection is confirmed by the distant side with the CC message. Message types for the release of a logical signaling connection: Released (RLSD) The RLSD message initiates the release of a logical signaling connection. It can be sent from either end of the connection. Release complete (RLC) The release of a logical signaling connection is confirmed with the RLC message. August 11, 2006 ComSoc Lecture Meet

SCCP – Message Structure Connection Oriented SCCP Message types for message transfer: Data form 1 (DT1) SCCP messages can be transferred in both directions with the DT1 message after the set-up of a logical signaling connection. It is only used in protocol class 2. Data form 2 (DT2) With the DT2 message, SCCP messages can be transferred in both directions after the set-up of a logical signaling connection, and the reception of SCCP messages can be confirmed by the opposite side. It is only used in protocol class 3. The fixed mandatory part of the SCCP message contains parameters which must be present for a certain message type and which have a fixed length. For the CR message these are, for example: – the local reference – the protocol class used for the message transfer August 11, 2006 ComSoc Lecture Meet

SCCP – Message Structure Connection Oriented SCCP The variable mandatory part of the SCCP message contains parameters of variable length. For the CR message these are, for example: – the directory number of the called party – the identifier of the SCCP user (e.g. ISUP, TCAP) The optional part of the SCCP message contains parameters which can occur in every message type. The parameters in question can be of either fixed or variable length. For the CR message these are, for example: – the directory number of the calling party – user messages to be transferred August 11, 2006 ComSoc Lecture Meet

SCCP – Protocol Classes August 11, 2006 ComSoc Lecture Meet

SCCP – Protocol Classes For the transfer of connectionless messages, the SCCP provides the protocol classes 0 and 1: Protocol class 0 For the protocol class 0 the SCCP messages are sent singly and independently of one another by the message transfer part. Protocol class 1 For the protocol class 1 the SCCP messages are sent in the order defined by the user. Protocol class 2 For the setting up of a logical signaling connection (Connection Oriented), the SCCP s of the signaling points of the signaling relation concerned send their own originating point codes to one another. In addition, they assign local references to the process for which they set up a logical signaling connection (e.g. for using a feature during an existing connection), and likewise inform one another. Messages can then be exchanged. Each SCCP can assign incoming messages to the process concerned by means of the local reference. This protocol class guarantees for a correct message order. August 11, 2006 ComSoc Lecture Meet

SCCP Procedures - Connectionless August 11, 2006 ComSoc Lecture Meet

SCCP Functional Blocks August 11, 2006 ComSoc Lecture Meet

SCCP Subsystems Important Subsystems MAPHLR – 6 MAPVLR – 7 MAPMSC – 8 MAPEIR–9 BSSAP – 254 CAP – 146 SINAP - 247 GPRS -149 1 254 255 Subsystems MAP SCCP August 11, 2006 ComSoc Lecture Meet

SCCP Examples Connectionless SCCP : Connectionless SCCP is used for a number of GSM features like Roaming (GTT), SMS. Other user parts like TCAP use connectionless SCCP for transactions. Connection Oriented SCCP : Connection Oriented SCCP is used for DATA/FAX feature in GSM. Other User parts like BSSAP use Connection Oriented SCCP for signaling messages exchanged between BSC and MSC, and also for messages exchanged between MS (Mobile Station) and MSC August 11, 2006 ComSoc Lecture Meet

Few Notes There are other user parts like : BSSSAP, TCAP etc. The TCAP is an application signaling protocol (OSI layer 7, application layer) The A – Interface is defined to be the interface between Base Station Subsystem (BSS) and the Switching Subsystem (SSS). This interface connects the BSC (Base station Controller) with The MSC (Mobile Switching Center). At the A – Interface the SCCP supports the subsystem known as BSSAP or Base Station Subsystem Application Part. How to check the status of different bits in a signaling message? Use Network Analyzer with Protocol Tester August 11, 2006 ComSoc Lecture Meet

Questions/ Queries mailto : rahulghosh@ieee.org August 11, 2006 ComSoc Lecture Meet

Thank You August 11, 2006 ComSoc Lecture Meet