SS7 Presentation

Huawei Confidential. All Rights Reserved

Wireless Curriculum Development Section

OMA003001 GSM Signaling SystemOMA003001 GSM Signaling System

Internal Use

ObjectivesObjectives

Upon completion this course, you will be able to:

Describe the concepts of SS7

Describe the function structure in SS7

Describe the basic format of signaling message unit

State the MTP function & principle

Internal Use

Section 3 Signaling Procedure

Section 2 Signaling Units

Course ArrangementCourse Arrangement

Section 1 Basic Concepts in CCS7

Part 1 Signaling

Part 3 Hierarchy of CCS7 Signaling System

Part 2 CCS7 Signaling Network

Internal Use

Basic Concepts of SignalingBasic Concepts of Signaling

All the control signals used within or between the communication

equipments, whose function are to set up communication level.

Definition of SignalingDefinition of Signaling

Internal Use

Classification of signalingClassification of signaling

Subscriber line signaling: between subscriber and the

communication equipment

Interoffice signaling: between the communication equipments

According to working locationAccording to working location

subscriber line signaling

interoffice signaling

exchange exchangesubscriber line signaling

Internal Use


CAS (Channel Associated Signaling): is composed of line signal

and register signal. For CAS, the signaling channel is combined

with the bear information channel (refers to register signal) or the

two have fixed correspondence (refers to line signal).

According to signaling channelsAccording to signaling channels

switching equipment

signaling equipment

signaling information carried by speech channel

switching equipment

signaling equipment

Internal Use


CAS is limited by the amount of the bandwidth dedicated

from the voice band for signaling, and today’s system need

a high bandwidth for the signaling.

Insufficient use of the bandwidth, as if the out-of-band

signaling is used this means that for each voice channel a

dedicated bandwidth is located continuously for the

signaling, and if the in-band signaling is used this means

that the signaling is limited to the setup and release during

the call and there will be no signaling during a call.

Limitation of CAS

Internal Use


CCS (Common Channel Signaling): the signaling of a group of

voice channels are transmitted on a common high speed data

link in the form of time multiplexing.

According to signaling channelsAccording to signaling channels

switching equipment

signaling equipment

signaling equipment

dedicated channel for carrying signaling

speech channel

switching equipment

Internal Use





part 1 Signaling

part 3 Hierarchy of CCS7 Signaling System

part 2 CCS7 Signaling Network

Internal Use

Definition of CCS7 signaling networkDefinition of CCS7 signaling network

In CCS7, signaling links are independent of voice channels. These signaling links form a network dedicated to the transmission of signaling, the so called CCS7 signaling network.

It is a multifunctional supporting network, can be used in telephone network, circuit-switch data network, ISDN network, and intelligent network, etc..

Internal Use

Signaling Network CompositionSignaling Network CompositionSignaling Network CompositionSignaling Network Composition

SP

STP

SP

Link Link

SP (Signaling Point): SP is the node

generating and receiving signaling

messages on the signaling network. It is

both the source point and the

destination point of the signaling

message STP (Signaling Transfer Point): STP is

neither a signaling source point nor a destination point, and it only functions to forward the message received from a signaling link to another signaling link

A CCS7 signaling network is composed of three elements: Signaling Point (SP), Signaling Transfer Point (STP) and Signaling Link.

Internal Use

Signaling Network CompositionSignaling Network Composition

Signaling Link

It is a physical link that connects respective SPs

and STPs and transfers signaling messages.

Signaling Linkset (LKSET)

It is the collection of a group of signaling links

with the same attributes, that is, the collection of

links between the local SP and an adjacent SP.

Internal Use

1) SP (Signaling Point)SP is the originating or destination point of a CCS7 message. In each signaling network, a SP has an exclusive signaling point code: SPC (14 bits). Since the four signaling networks assign the SPCs independently, only NI+SPC can uniquely locate a SP.Notes: In data setting we usually describe SPC in Hex. Following are some examples of SPC shown in binary and hex.

SPC (binary) SPC (Hex)

00 0000 1011 0101 00b5

10 0011 1101 0111 23d7


Internal Use

Signaling Network CompositionSignaling Network Composition

OPC and DPCOPC and DPC

DPC: Destination Point CodeOPC: Originating Point Code

A message going from one SP to another SP should bearing the SPC of the originating SP and the terminating SP, which are called OPC and DPC respectively. If we compare sending a message to sending a letter, OPC and DPC are similar to the sender's address and receiver's address.

Internal Use

STP is the network node which transfers CCS7 messages.

Link is the data channel which connects the nodes (SPs and STPs) in CCS7 signaling network.Digital link, 64 kb/s


3) Link

2) STP (Signaling Transfer Point)

Internal Use

As shown in the above figure, there are multiple circuits (e.g. one time slot is one circuit) connecting two adjacent SPs. To distinguish them, we should assign each of them a code, CIC (Circuit Identification Code). So CIC is unique in each direction, and is coded in 12 bits. Therefore, in each direction we can have as many as 212=4096 circuits, and the value range of CIC is [0~4095].

CIC and SLCCIC and SLC

link

speech channel

switching equipment

signaling equipment

switching equipment

signaling equipment

CIC: Circuit Identification Code (12 bits)SLC: Signaling Link Code (4 bits)

Internal Use

Correspondence of CIC

and E1:

For E1, the lowest 5 bits of

CIC represent PCM time slot

number, and the higher 7

bits represent PCM system

number.

7 bits 5 bits CICPCM system number PCM time slot number

CIC E1#1 E1#0 CIC

TS0 0000000 00000TS1 0000000 00001

TS31 0000000 11111

0000001 00000 TS0 0000001 00001 TS1

0000001 11111 TS31


Internal Use

Similarly, if there are multiple links connecting two adjacent SPs, to distinguish them, we should assign each of them a code, SLC (Signaling Link Code). So SLC is unique in each direction, and is coded in 4 bits. Therefore, in each direction we can have as many as 24=16 links, and the value range of SLC is [0~15].

......

... ...

CIC=0,CIC=1,

CIC=m1

CIC=0,CIC=1,

CIC=m2

SLC=0,SLC=1,

SLC=n1

SLC=0,SLC=1,

SLC=n2

local SP one direction

another direction


Internal Use

In CCS7 bi-directional trunk circuits are used. The possibility of selecting the same circuit from the two offices at the same time is known as “dual seizure” problem. To prevent this, for each circuit, a "master" control right is assigned to one office, and a "slave" control right over this circuit is assigned to the office at the other end. Usually we assign the master control rights according to the circuit's CIC number : if the CIC number is even then its master control right is given to the office which has larger SPC (Signaling point code), and if the CIC number is odd then its master control right is given to the office which has smaller SPC.Following is an example of CIC and master right distribution.

Dual seizureDual seizure

Internal Use


0 master slave 01 slave master 12 master slave 2

SP ASPC=0002

SP BSPC=0001

2n master slave 2n

2n+1 slave master 2n+1

Internal Use

The principle to avoid dual seizure:When the local office wants to make an outgoing call, it first tries those circuits on which it has master control right; only when all these circuits are busy, will the local SP try the circuits it has slave control right. So when the slave circuits are to be selected, the possibility of "dual seizure" rises again. Hence different "circuit selection modes" are used: for the circuits on which the local office has master control right, "FIFO" mode is adopted, which means the circuit bearing the longest idle period will be selected; while for the circuits on which the local office has slave control right, "LIFO" mode is adopted, which means the circuit bearing the shortest idle period will be selected. In this way, most of the danger of "Dual seizure" can be avoided.


To be continued...

Internal Use

The principle to avoid dual seizure:But still there is an extreme occasion when there is only one idle circuit left for selection from both sides. Then if both sides try to use this circuit at the same time, the possibility of dual seizure rises again. In this case, the principle is the "slave" side should give way to the "master" side.


Internal Use

Working mode refers to the relationship between the signaling link and the voice channels the link serves.Presently two modes are in use:

1) Associated modeThe messages between two adjacent points are conveyed over a link-set directly interconnecting those signaling points, i.e., the link is parallel to the voice path.

Working modes of CCS7 signaling networkWorking modes of CCS7 signaling network

SPA BSP

Internal Use

2) Quasi-associated mode

In the quasi-associated mode

the message which is going to

arrive at a SP goes through a

path which is predetermined

and via one or more STPs.

Working modes of CCS7 signaling networkWorking modes of CCS7 signaling network

SPA B

STP

SP

Internal Use

When the links between any two offices are transferred by a STP, how will be the signaling path?


Remember, the sole purpose of using signaling is to serve the voice path.

SPA

SPB

TandemC

STP

Internal Use



Note: the arrows show the paths that signaling messages go through. Remember, a signaling path always starts from one end of the voice path it is going to serve, and ends up at the other end of the voice path.

SPA

SPB

TandemC

STP

Internal Use

QuizQuiz

MSU1OPC=?DPC=?

: voice path: signaling link

SPB

SPA SPC SPA SPC

STPB

MSU2OPC=?DPC=?

MSU1OPC=?DPC=?

MSU2OPC=?DPC=?

Tandem Office

In the following two figures, what's the DPC and OPC of the messages?

(a) (b)

Internal Use

AnswerAnswer

MSU1OPC=ADPC=B

: voice path: signaling link

SPB

SPA SPC SPA SPC

STPB

MSU2OPC=BDPC=C

MSUOPC=ADPC=C

MSUOPC=ADPC=C

Tandem Office

Answer:

(a) (b)

Internal Use

BREAK!!!!

Internal Use





part 1 Signaling



Internal Use

The hierarchy of CCS7 system can be functionally divided into two main parts, i.e. Message Transfer Part (MTP) and User Part (UP).

1) MTP (Message Transfer Part)The Message Transfer Part (MTP) provides the functions that enable User Part's significant information to be transferred across the signaling network to the required destination. In addition, functions are included in the MTP to overcome network and system failures that would affect the transfer of signaling information.

Hierarchy of CCS7 signaling systemHierarchy of CCS7 signaling system

Internal Use

2) UP (User Part)

The User Part (UP) is the "user" of

MTP. It includes TUP (Telephone

User Part), ISUP (ISDN User Part),

BSSAP (BSS Application Part).

The figure on the right describes the

relation between UP and MTP.


UP

MTP

TUP

Internal Use


MACMTP1

MTP2

MTP3

SCCP

TCAP

TUP

ISUP

M3UA

IP

INAP

User part

Message Transfer Part

M2UASCTP

Internal Use

Signaling unit (SU) is the minimum unit used to carry

the various signaling messages in CCS7 signaling

system.

Definition of CCS7 signaling unitDefinition of CCS7 signaling unit

Internal Use

MTP could be further divided into 3 levels, namely, level 1-- signaling data link, level 2 -- signaling link, level 3 -- signaling network.

Three levels of MTPThree levels of MTP

A signaling data link is a bi-directional transmission path for signaling, comprising two data channels operating together in opposite directions at the same data rate. It constitutes the lowest functional level (MTP1) in the Signaling System No. 7 functional hierarchy. The rate of digital signaling data link is 64K bps or 2M bps.

1) Level 1: Signaling Data Link

Internal Use

The second level provides such functions as signal unit

delimitation, error detection, error correction, initial

alignment, signaling link error monitoring and flow control. It,

working together with level 1, ensures the reliable

transmission of messages between two adjacent SPs.

MTP2MTP2

2) Level 2: Signaling Link

Internal Use

MTP2

Function of MTP-2: SU DelimitationFunction of MTP-2: SU Delimitation

An SU starts and ends with a flag code (F: 0111 1110).

All the SU start with this 8-bit flag as a mark to the end of the

previous SU and the start of a new SU.

What will happen if the SU itself contains the flag 01111110?

Internal Use

MTP2

Answer

To avoid this case, the 0-bit insertion technique is used. So, the SU is scanned at the transmitting end and 0-bit is inserted after each sequence of five 1’s.

If the SU is “00111111100100….”

It should be changed to “001111101100100”

Example

Internal Use

MTP2MTP2

Function of MTP-2: Error ControlFunction of MTP-2: Error Control

Error detection

Implemented with 16-bit Check bits (CK) field that is calculated at the

transmitting end and recalculated at the receiver end.

CK employs the 16-digit CRC (Cyclic Redundancy Code) check

algorithm.

Error correction

Two methods for error correction, both of these methods are depend on the

retransmission of the MSU affected with error.

Basic error correction (BEC) method (unidirectional transmission delay

<15ms).

Preventive cyclic retransmission (PCR) correction mode (unidirectional

transmission delay >=15ms) : only used for satellite circuits.

Internal Use

MTP2MTP2

Function of MTP-2: Error Rate MonitoringFunction of MTP-2: Error Rate Monitoring

In order to ensure QoS of the signaling link, the "error degree" of

SU on the signaling link is monitored.

When the signaling link transfers SU with errors to a certain

degree, the signaling link should be judged as faulty, and Level 3

should be noticed for appropriate processing.

Internal Use

Functions on this level include signaling message handlingsignaling network management.

MTP3MTP3

3) Level 3: Signaling Network

Internal Use

MTP3

Signaling Message Handling (SMH)

Message distribution

If the DPC indicated in the message is the same as the SPC of the

local node, the local node should delivers the message to the

concerning user part according to the UP indicator in the message.

Message routing (According to the DPC)

If the DPC indicated is not of that local node, so a routing should be

done on the received message.

Signaling Network Management (SNM)

The signaling message can be reliably transferred in the signaling

network even if some points or transmission links are faulty in the

signaling network, like choosing another route.

Internal Use

MTP3

Function of MTP-3:Function of MTP-3: Message processingMessage processing

Internal Use

The 4-level structure of CCS7 systemThe 4-level structure of CCS7 system

Logicalchannel

Logicalchannel

Logicalchannel

Physicalchannel

Message Discrimination

Message Distribution

Message Routing

User Part

L4

SP A

Signaling NetworkL3

L2 Signaling Link

L1

Signaling Data Link

SP B

User Part

Signaling Network

Signaling Link

Signaling Data Link

Internal Use

BREAK!!!!

Internal Use





part 1 Signaling



Internal Use

Signaling unit (SU) is the minimum unit used to carry

the various signaling messages in CCS7 signaling

system.

Definition of CCS7 signaling unitDefinition of CCS7 signaling unit

Internal Use

Types of CCS7 signaling unitTypes of CCS7 signaling unit

There are three types of SU1. Fill-in Signaling Unit (FISU)

2. Link Status Signaling Unit (LSSU)

3. Message Signaling Unit (MSU)

An active signaling link should carry a continuous SU stream in both of its directions.

Internal Use

Fill-in Signaling Unit (FISU)Fill-in Signaling Unit (FISU)

It is used when there is no MSU or LSSU being sent on the link, in order to keep the link active and to test the quality of the link using a CRC check.

It is sent between any two adjacent signaling point on level 2 (MTP2).

F CK LI FIB FSN BIB BSN F

8 16 2 6 1 7 1 7 8Sending Directing

Fill-in signal unit (FISU)

F: 8-bit Flag, used to indicates the end/start of SU.

CK: 16-bit check sum, used to detect the errors.

LI: it is the number of octet (8-bit) between itself and the CK field, LI = 0 for FISUs, LI = 1 or 2 for LSSUs, and LI >2 for MSUs.

Internal Use

Link Status Signaling Unit (LSSU)Link Status Signaling Unit (LSSU)

Originating and terminating at the 3rd level, it carries no detailed signaling message, used to indicate the link status when the link is put into use or malfunctions, so as to set up or recover the signaling links.

FF CK SF LI FIB FSN BIB BSN

8 16 8 2 6 1 7 1 7 8Sending Directing

Link status signal unit (LSSU)

Status Field (SF): Main part of LSSU, indicating status of the local terminal

link,8bit or 16bit

H G F E D C B A

0 0 0 "0" loss locating (Trying to alignment)

0 0 1 "N" Normal locating (Link is aligned)

0 1 0 "E" Emergency locating (Link is aligned)

0 1 1 "OS" Service interruption (terminal out-of-service)

1 1 0 "PO" Processor fault (MTP2 can not reach MTP3)

1 0 1 "B" Link congestion (MTP2 congestion)

Internal Use

Message Signaling Unit (MSU)Message Signaling Unit (MSU)

Used to transfer the signaling message from the 4th level or the signaling network management message from the 3rd level. So, the actual signaling message is being inserted in this type of message.

A typical contents of the message is shown in the figure below. As it can be seen there are 2 additional fields over these fields of FISU and LSSU; SIF (Signaling Information Field) and SIO (Service Information Octet).

FF CK LI FIB FSN BIB BSN

8 16 2 6 1 7 1 7 8

SIF SIO

8n(n= 2) 8 Sending directing

Message signal unit (MSU)

Internal Use

Message Signaling Unit (MSU)

SIOSIF

SISSF

MeaningDCBA

International networkmessageReserved(international)National networkmessageReserved (national)

0 0 0 0

0 1 0 0

1 0 0 0

1 1 0 0

CKF

º ¬Ò åDCBA

Signaling network management message

Signaling network test and maintenance message

Reserved

SCCP

TUP

ISUP

DUP

Reserved

0 0 0 0

0 0 0 1

0 0 1 0

0 0 1 1

0 1 0 0

0 1 0 1

0 1 1 0

0 1 1 1

1 0 0 0

1 1 1 1

...

...

meaning

Meaning of SU Field (MSU-SIO)Meaning of SU Field (MSU-SIO)

Internal Use

Message Signaling Unit (MSU)

Management message SLC OPC DPC

Signaling message CIC OPC DPC

SLS

Signaling message CIC SLS OPC DPC

SCCP user data SLS OPC DPC

Type A: MTP management message

Type C: ISUP message

Type B: TUP message

Type D: SCCP message

Meaning of SU Field (MSU-SIF)Meaning of SU Field (MSU-SIF)

Internal Use





part 1 Signaling



Internal Use

Basic signaling procedureBasic signaling procedure

IAM: initial address

message, the part or all of

the called number are sent

in this message

ACM: address complete

message

ANC: answer signal, charge

CLF: clear forward message

RLG: release guard signal

After hearing the busy tone, the called party hooks on.

the called phone rings

the caller listen to the ring back tone

the called partypick up the phone

the caller hooks on first

IAM (6660008)

ACM

ANC

communication

CLF

RLG

SPBSPA

Internal Use

Call connection through tandem officeCall connection through tandem office

SAM: Subsequent address message, following IAM, used to send the subsequent digits of the called number which haven't been sent by IAM. SAO: Subsequent address message with one signal. Similar to SAM, but can send only one digit.CBK: Clear backward signal

the called party hooks on first

LE TM LE

...

IAM* (66)

SAM ( or SAO ) (60)

SAO (8)

ACM

ANC

CBKCLF

RLG

IAM** (6660008)ACM

ring back tone

ANC

conversation CBK

CLFRLG

Internal Use

Send caller number initiativelySend caller number initiatively

the called phone rings

the called partypick up the phone


IAI : Initial address message with add- tional information. If the caller number is to be sent initiatively, IAI is used.

the originating the destinationLE toll office toll office

...

IAI(66, 8880003)

SAM( or SAO) (60)

SAO (8)

ACM

ANC

CLF

RLG

CLF

RLG

IAM**(6660008)

ACM

ANCring back tone

conversation

Internal Use

The opposite office ask for caller number (e.g.,Malicious call tracing, CLID)

The opposite office ask for caller number (e.g.,Malicious call tracing, CLID)

the called phone ringsthe called party

pick up the phone


GRQ: General request message. It is used to ask for caller number and some other information.GSM: General forward setup information message. It is used to send caller number and some other information.

LE

ACM

ANC

CLF

GRQ

CLF

RLG

IAM(6660008)

GRQ

ANC

ring back tone

conversation

LETM

GSM(8880003)GSM(8880003)

ACM

IAM(6660008)

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SS7 Presentation