Signaling Network Functions (MTP Level 3)

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1 Signaling Network Functions (MTP Level 3) Level 3 in principle defines those transport functions and procedures that are common to and independent of the operation of individual signaling links. These functions fall into two major categories: Signaling message handling functions These transfer the message to the proper signaling link or User Part.The main functions are:- Message discrimination function Message distribution function Message routing function

2 signaling network management functions These control the current message routing and configuration of the signaling network facilities and in the case of signaling network failures, control the reconfigurations and other actions to preserve or restore the normal message transfer capability. Contains signaling link management, traffic management and route management.the main functions are:- Signaling link management Signaling traffic management Signaling route management

3 MTP User functions (Level 4) User Parts defines the functions and procedures of the signaling system that are particular to a certain type of user of the system. The following entities are defined as User Parts in CCS 7. Telephone User Part (TUP) The TUP Recommendations define the international telephone call control signaling functions for use over CCS 7. Data User Part (DUP) The Data User Part defines the protocol to control interexchange circuits used on data calls, and data call facility registration and cancellation.

4 ISDN User Part (ISUP) The ISUP encompasses signaling functions required to provide switched services and user facilities for voice and non-voice applications in the ISDN. Signaling Connection Control Part (SCCP) The SCCP provides additional functions to the Message Transfer Part to provide connectionless and connectionoriented network services to transfer circuit-related, and noncircuit-related signaling information. Key Enhancements by SCCP

5 Enhanced Addressing Capability upto 255 users can be addressed by the use of Subsystem Numbers (SSN) SCCP provides a routing function which allows signaling messages to be routed to a signaling point based on, for example, dialled digits. This capability involves a translation function which translates the global title (e.g. dialled digits) into a signaling point code and a sub-system number. Connectionless and Connection-Oriented Services Class 0 : basic connectionless service Class 1 : sequenced connectionless service Class 2 : basic connection-oriented service Class 3 : flow control connection-oriented service

TCAP TCAP provides services for non-circuit related services.tcap receives messages from SCCP and routes it to the user.tcap makes it possible to have several transactions running simultaneously.

6 TCAP TCAP provides services for non-circuit related services.tcap receives messages from SCCP and routes it to the user.tcap makes it possible to have several transactions running simultaneously. TCAP consists of component sub-layer and the transaction sublayer.the component layer provides information exchange between two layers by the means of dialogues. A dialogue will contain several components like action, response etc.the transaction identifier gives each transaction a unique identity which is also known as transaction identifier.

7 TCAP acts as a secretary to a manager who has several engineers reporting to it. The secretary handles all the transactions from the manager and sends it across the appropriate engineer and also keeps track of each transactions by having identified files for each engineers transaction. Section 6 SS7

Global Title Global title is the address of the Signaling Point which does not clearly mention the destination address for routing. It is translated by SCCP to get the destination address.e.g. the dialled digits.

8 Global Title Global title is the address of the Signaling Point which does not clearly mention the destination address for routing. It is translated by SCCP to get the destination address.e.g. the dialled digits.on an incoming call,gmsc uses the Global title to determine the destination. A MAP message entering or originating from an exchange must either be a terminating message or a message to be routed to another exchange.

9 By analyzing the global title(gt) of the called address,the SCCP will either route the message to another node with the help of global title routing case (GTRC) or terminate the message in the node. In the terminating node the message will be distributed to the correct user with the help of the subsystem number (SSN).

10 Organization of Signaling Information Signal Unit : - A group of bits forming a separately transferable entity used to convey information on a signaling link. Are of variable length; maximum length : 280 bytes (including 272 signaling information bytes) Three types of signal units, differentiated by the length indicator field contained in each.

11 {length limitation is imposed to control the delays one message can cause to others due to their emission time} Fill-in signal unit (FISU) ; LI = 0 Link status signal unit (LSSU) ; LI = 1or 2 Message signal unit (MSU) ; LI = 3 to 63

12 MSU: Signal Units convey the signaling information between the user parts (level 4) of the adjacent signaling points. E.g. IAM, ACM, REL. LSSU: a signal unit which contains status information about the signaling link. FISU : a signal unit containing only error control and delimitation information which is transmitted when there are no MSUs or LSSUs to be transmitted. This is done to allow for a consistent error monitoring so that faulty links can be quickly detected and removed from service even when traffic is low.

13 1 Section 6 SS7 Signal Units F 8 CK 16 SIF 8n,n>=2 SIO 8 2 LI 6 FIB 1 FSN 7 BIB 1 BSN 7 F 8 MSU F 8 CK 16 SF 8 or 16 2 LI 6 FIB 1 FSN 7 BIB 1 BSN 7 F 8 LSSU F 8 CK 16 2 LI 6 FIB 1 FSN 7 BIB 1 BSN 7 F 8 FISU

14 SU Delimitation / Flag imitation Prevention Signal Unit Delimitation : A unique pattern on the signaling data link is used to delimit a signal unit : Main part of Message Flag imitation Prevention : >> to ensure that no false flags are produced in the signal units, only five consecutive one s are allowed inside the signal unit. If more than five one s occur consecutively, a zero is inserted after the fifth one and is removed again in the receiving signal terminal. This is called bit stuffing.

15 Error Detection Error Detection : -each signal unit has standard CCITT 16 bit cyclic redundancy check (CRC) checksum to enable the receiving terminal to check that all bits have been received correctly. CK generated by transmitting SP on all fields except the Flag. Receiving SP calculates CK and compares with CK in the signal unit. Mismatch interpreted as error in received signal unit & error correction procedures are invoked.

16 Error Correction Two forms of error correction methods are used : Basic method Preventive cyclic re-transmission (PCR) Basic Method: re-transmission occurs only when transmitting SP is informed by receiving SP about the signal units received in error is a positive / negative ACK re-transmission error correction system

17 For sequence control, each signaling unit is assigned forward & backward sequence numbers and forward & backward indicator bits. Sequence Numbering is performed independently at the two SPs interconnecting the link. The sequence numbers are 7 bits long, meaning that at most 127 messages can be transmitted without receiving a positive ACK.

18 1 Section 6 SS7 Positive Acknowledgment MSU saved in RTB MSU saved in RTB Both MSU deleted fm RTB FSN=125,FIB=BIB=1 FSN=126,FIB=BIB=1 BSN=126,FIB=BIB=1 FSN=35,FIB=BIB=1 Correctly received Correctly received MSU with positive ack,fsn=34 MSU,BSN remains 126

19 Negative Acknowledgment Errored MSU is discarded and not delivered to level 3 of MTP SP sends a negative ack in the next SU BSN retains the FSN of last correctly received MSU BIB is inverted All messages with FSN > received BSN sent one by one by fetching from RTB FIB value inverted in all retransmitted messages Until all messages in the RTB are retransmitted, no fresh MSUs are sent.

20 Preventive Cyclic Re-transmission Preventive Cyclic Retransmission: Retransmission takes place for signal units whose correct reception is not confirmed by the receiving SP is a positive ACK cyclic re-transmission forward error correction system. A copy of the transmitted MSU is retained at the transmitting terminal unit until a positive ACK for that MSU is received.

21 Re transmission Rules : when there are no new MSUs to be sent, all MSUs not positively acknowledged are retransmitted cyclically. If new signal units are available, the retransmission cycle (if any) is interrupted and the signal units transmitted with first priority. Under normal conditions, with no MSUs to be transmitted or cyclically re-transmitted, FISUs are sent continuously.

22 Basic Versus PCR In both methods, only errored MSUs and LSSUs are corrected. Errors in FISUs are detected but not corrected Both methods are designed to avoid out of sequence and duplicated messages when error correction takes place. PCR method is used when the propagation delay is large (satellite transmission).

23 With large propagation delays, the basic method becomes inappropriate because NACK system causes message delays to be too long for the erroneous MSUs CCITT recommendations : PCR should be used when one way propagation delay exceeds 15ms. Drawback of PCR : inefficient bandwidth utilization I.e. the maximum load level a link can be engineered for is much less with PCR.

24 Error Rate Monitoring Level 2 functions detect a failure in the following circumstances: High error rate on the signaling units. Excessive re-alignment period. Excessive ACK delay. Signaling terminal failure. Reception of continuous FISUs. Two types of signaling error rate monitor is provided signaling Unit Error Rate Monitor (SUERM). Alignment Error Rate Monitor(AERM).

25 Signaling Unit Error Rate Monitor Is used while a signaling link is In Service. It provides the criteria for taking a signaling link OOS due to excessive error rate. Is based on a signaling unit error count (including FISUs), incremented & decremented using the leaky bucket algorithm.

26 For each errored signaling unit, the count is incremented by one and for each 256 signaling units received (whether errored or not), a positive count is decremented by one (a zero count is left at zero). When the count reaches 64, an excessive error rate indication is sent to Level 3 and the signaling link is put OOS. The error rate on signaling units should not exceed 64 consecutive erroneous signaling units or 1 erroneous signaling unit out of every 256 on an average.

27 Alignment Error Rate Monitor Is used while a signaling link is in the proving state of the initial alignment procedure. Provides a criteria for rejecting a signaling link for service during the initial alignment due to an excessive error rate.

28 The Alignment error rate monitor is a linear counter which is started at zero at the start of the proving period and the count is incremented by one for each LSSU unit received in error. A proving period is aborted if the threshold for the alignment error rate monitor count is exceeded before the proving period timer expires. Parameter Value Tin 5 Tie 1 M 5

29 Message Label types (SIF) MTP management messages: Label type A Management information SLC Originating point code Destination point code TUP messages: Label type B Signalling information Circuit ID code SLS Originating point code Destination point code ISUP mess ages: Label type C Signalling information Circuit ID code SLS Originating point code Destination point code SCCP messages: Label type D Signalling information SLS Originating point code Destination point code Routing label T /d06 FIGURE 7/Q.700 SS No. 7 message label types

30 CIC SLS SLC Message Label identity of the physical circuit that carries the call for which the signaling information is meant. signaling link selection is used for load sharing between signaling links. signaling link code identifies the signaling link connecting the origination and destination SPs For implementation of level 3 functions, the required fields are : Service Information Octet (SIO) Routing Label

Course 5 The SS7 signaling systems.

Course 5 The SS7 signaling systems. Zsolt Polgar Communications Department Faculty of Electronics and Telecommunications, Technical University of Cluj-Napoca General aspects; The SS7 architecture; Node