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TRANSCRIPT
1. MS establishes a connection to the network, as in the case of normal
call setup. (This step is not performed if the MS is in busy mode, since there already exists a connection)
2. If the authentication was successful MS sends the short message to the SMS – C via MSC/VLR. The SMS – C in turn forward the short message to its destination. This could be MS or a terminal in the fixed network.
Section 5 – Call Handling
Handover
Changing to a new traffic channel during call setup or busy state is called Handover. The network makes the decision about the change. After receiving the information about the signal strength and quality the BSC ranks the neighboring BTSs using the information.
After a evaluation of the situation and the decision to start the handover procedure, the network is responsible for the setup of a link to the new BTS.
Section 5 – Call Handling
Intra MSC handover: Handover within the same MSC/VLR service
area but different BSCs.
• The BSC request for a handover from MSC/VLR
• New link (MSC/VLR to new BSC to new BTS) is setup and if a free TCH is available, it must be reserved.
• MS receives the order to change to the new frequency and the new TCH.
• If the BTS change has also change of location area, the MS sends a request for location update after the call.
Section 5 – Call Handling
Flowchart
BSC2 MSC BSC1 MS
H.O. Request H.O. Required
H.O. Request Ack
H.O. Command H.O. Command
Measurement Report
H.O. Completed
H.O. Completed Clear Command
Clear Completed
Section 5 – Call Handling
Inter MSC handover: handover between the two BSCs controlled by two
different MScs. Lot of signals exchanges are required before the handover can take place.
• The serving exchange sends a handover request to the target exchange
• Target exchange will take over the responsibility for preparing the connection to the new BTS.
• After the setup of a link between the two exchanges, the serving exchange will send a handover command to the MS.
Section 5 – Call Handling
Flowchart
VLR (MSC-B)
BSC2 MSC-B MSC-A BSC1 MS Measurement
Report H.O. Required
Perform H.O. Allocate H.O.Number
Send H.O. Report
H.O. Request
H.O. Request Ack
RAD CH Ack
I AM (ISUP)
ACM (ISUP) H.O. Command H.O. Command
H.O. Complete
H.O. Complete
Send End Signal
Clear Command
Clear Complete ANS (ISUP)
Section 5 – Call Handling
Exercise
Q1. Name the types of the location updates exists in the mobile network?
Q2. Describe the different kinds of paging messages?
Q3. Calculate the paging capacity (mobiles paged per second) for the following operator setting:
paging message type = 3
blocks reserved for CCCH and AGCH = 3
Section 5 – Call Handling
Q4. Calculate the PCH requirement for following:
Number of subscriber = 75,000
Busy hour calls = 40%
Assume on average 2 pages required per call
Safety margin for peak variation in number of calls =1.2
Paging message of type 2
Q5. Which part of the network allocates the MSRN to the call?
Section 5 – Call Handling
CONTENTS
• Introduction
• Signaling Modes
• CCS 7 Vs. CAS
• CCS 7 Link Types
• CCS 7 Signaling Network
• Signaling Network Components
• CCS 7 Architecture
• CCS 7 Functional Blocks • MTP
• Signaling Data link (Level 1) • Signaling Link Functions (Level 2) • Signaling Network Functions (Level 3)
• MTP User Functions (Level 4)
Section 6 – SS7
• Functions of Signaling Link (Level 2)
• Organization of signaling Information
• Signal Units
• Signal Unit Delimitation/ Flag Imitation Prevention
• Error Detection
• Error Correction
• Basic Method • Positive Ack • Negative Ack
• Preventive Cyclic Re-Transmission
• Error Rate Monitoring
• Signal Unit Error Rate Monitor
• Alignment Error Rate Monitor
Section 6 – SS7
• Signaling Network Functions (level 3)
Service Information Octet • Routing Label
• Signaling Message Handling
• Message Discrimination
• Message Distribution
• Message Routing
• Signaling link Management
• Link activation
• Link restoration
• Flow Control
Section 6 – SS7
Introduction
Common Channel Signaling System No. 7 (i.e., SS7 or C7 ) is a global standard for telecommunications defined by the International Telecommunication Union (ITU)Telecommunication Standardization Sector (ITU-T). The standard defines the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signaling network to effect wireless (cellular) and wire line call setup, routing and control.
Section 6 – SS7
The SS7 network and protocol are used for:
• basic call setup, management, and tear down
• wireless services such as personal communications services (PCS), wireless roaming, and mobile subscriber authentication
• local number portability (LNP)
• toll-free (800/888) and toll (900) wireline services
• enhanced call features such as call forwarding, calling party name/number display, and three-way calling
• efficient and secure worldwide telecommunications
Section 6 – SS7
Signaling Types
There are two types of Signaling :
1.Channel Associated Signaling (CAS)
2.Common Channel Signaling (CCS7)
Channel Associated Signaling: signaling is always sent on the same connection as that of speech.The Signaling is associated with speech.
Section 6 – SS7
Common Channel Signaling: signaling network is separated from the speech network.Every signaling information will have a label which indicates to which speech connection this signaling information belongs to.The signaling channel has no specific position (timeslot).The same signaling channel carries information for all speech circuits as and when required basis.
Section 6 – SS7
Advantage Of CCS7 Over CAS
• A dedicated signaling link required for each speech channel in CAS e.g. 3 channels in 3 PCMs : CCS 7 uses only 1 channel for a number of PCMs
• CAS is slow, so longer call setup times : CCS 7 - 64kbps fast & efficient.
• In CAS, no possibility of signaling during the “talking phase” : CCS 7 signaling is independent of speech.
• CAS supports limited set of signals : CCS 7 supports signal units of variable length max. 279 octets - so much more signaling info can be exchanged than is possible with CAS.
Section 6 – SS7
• Usage of messages instead of pre-defined bit patterns enables to transfer call related signaling info (call establishment) as well as non call related call info ( location update , handover , short messages etc.)
• CCS 7 - modular ; easy introduction of new & advanced services.
Section 6 – SS7
C7 Signaling Network
SP (SPC)
SP (SPC)
1 2 3 4 5 16
31
30
STP
SL(SLC)
SL(SLC) SLS CIC •SP: Signaling Point
•SPC: Signaling Point Code
•STP: Signaling Transfer Point
•SL: Signaling Link
•SLC: Signaling Link Code
•SLS: Signaling Link Set
•CIC: Circuit Identity code
Section 6 – SS7
signaling Network Components
• Signaling Points
• logically separate entities from a signaling network point of view.
• Origination Point Code
• A signaling point at which a message is generated, i.e. the location of the source User Part function, is the originating point of that message.
Section 6 – SS7
• Destination Point Code
• A signaling point to which a message is destined, i.e. the location of the receiving User Part function, is the destination point of that message.
• Signal Transfer Point
• A signaling point at which a message is received on one signaling link and is transferred to another link, i.e. neither the location of the source nor the receiving User Part function, is a Signal Transfer Point (STP).
For a particular signaling relation, the two signaling points thus function as
originating and destination points for the messages exchanged in the two directions between them.
Section 6 – SS7
• Signaling Links
• The common channel carrying signaling information is called Signaling link.
• Link Set
• A number of signaling links that directly interconnect two signaling points constitute a signaling link-set.
• Signaling Routes
• The pre-determined path, consisting of a succession of SPs/STPs and the interconnecting signaling links, that a message takes through the signaling network between the origination point and the destination point is the signaling route for that signaling relation
Section 6 – SS7
• Signaling Modes
• The term “signaling mode” refers to the association between the path taken by a signaling message and the signaling relation to which the message refers.
Section 6 – SS7
TCAP ISUP TUP
SCCP
Signaling Network
Signaling link
Signaling data link
MTP
Level 1
Level 2
Level 3
Level 4 : User Parts
Layer 3
Layer 1
Layer 2
Layers 4 to 7
CCS 7 Architecture 1
Section 6 – SS7
Message Transfer Part (MTP)
• Function:
• to provide a reliable transfer and delivery of signaling information across the signaling network and to have the ability to react and take necessary actions in response to system and network failures to ensure that reliable transfer is maintained.
• Includes the functions of layers 1 to 3 of the OSI reference model.
• User functions in CCS 7 MTP terms are: – the ISDN User Part (ISUP) – the Telephone User Part (TUP)
Section 6 – SS7
the signaling Connection Control Part (SCCP) – the Data User Part (DUP)
• The SCCP also has Users. These are: – the ISDN User Part (ISUP) – Transaction Capabilities (TC) – Operations Maintenance and Administration Part (OMAP)
Section 6 – SS7
Functions of MTP
Message handling
signaling link
signaling data link
Network management
Level 1
Level 2
Level 3
f
Section 6 – SS7
Signaling Data Link (MTP Level 1 ) • Defines the physical, electrical and functional characteristics and the
physical interface towards the transmission medium (PCM30)
• signaling Data Link is a bi-directional transmission path for signaling consisting of two data channels operating together in opposite directions at the same data rate.
• Digital : 64 kbps channels. For PCM30 HDB3 coding is used
- Minimum allowed bit rate for telephone call control application : 4.8kbps
Section 6 – SS7
Signaling Link Functions (MTP Level 2)
• Together with signaling data link, the signaling link functions provide a signaling link for the reliable transfer of signaling messages between two adjacent signaling points.
• Messages are transferred over signaling link in variable length messages called signal Units which contain additional information to guarantee a secure transmission.
Section 6 – SS7
Functions:
• Delimitation of signaling units by means of Flags.
• Flag limitation prevention by bit stuffing.
• Error detection by means of Check bits included in each signaling unit.
• Error control by re-transmission and signaling unit sequence control by means of sequence numbers and continuous ACKs
• Signaling link failure detection by signaling unit error rate monitoring and signaling link recovery by special procedures.
Section 6 – SS7