Oma003001 Gsm Signalling System-mtp Issue2.0

No Spreading without Permission. Security Level: Internal
Course Objectives
Upon completion
Understand the important
Understand the basic signaling
Section 3 Signaling Procedure
Section 2 Signaling Units
Part 1 Signaling
Part 2 CCS7 Signaling Network

Definition of signaling
between communication equipment, whose
Signals except bearer information (voice
information and non-voice information),
exchange.
? Subscriber line signaling: between
subscriber and the exchange;
? Interoffice signaling: between exchanges.

a. CAS(Channel associated signaling):
CAS 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).
Classification of signaling: According to signaling channels
switching
equipment
signaling
equipment
b. 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.
switching
equipment
signaling
equipment
signaling
speech channel
1) CAS:
mutual - control transmission and receiving, slow speed
no signaling channel during conversation
The comparison of CAS and CCS
2) CCS: packet data signal, large information volume
non - mutual - control mechanism, fast speed
independent data link for signaling, signaling could be
transmitted during conversation
? require low error - code - ratio
? require conduction check of the voice channels
Please press button to continue your study.

Course Arrangement
part 1 Signaling
part 2 CCS7 Signaling Network

Definition 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.
in telephone network, circuit-switch data network, ISDN
network, and intelligent network, etc.. Fundamentally,
CCS7 signaling network is a packet switching data
network used for dedicated purpose.

Existing CCS7 signaling networks
networks):
The existing CCS7 signaling networks
International network, International reserved network,
national network, national reserved network.

Three elements in a CCS7 signaling network
A CCS7 signaling network is composed of three elements:
Signaling Point (SP), Signaling Transfer Point (STP)
and Signaling Link.
the right, in a CCS7
signaling network, SP is
represented by a circle,
square, link is represented
is represented by a solid
line.
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)
Three elements in a CCS7 signaling network

OPC and DPC (1)
DPC: Destination Point Code
OPC: 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.
2) STP (Signaling Transfer Point)
STP is the network node which transfers CCS7 messages.
3) Link
Link is the data channel which connects the nodes (SPs
and STPs) in CCS7 signaling network.
Digital link, 64 kb/s

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 mode
i.e., the link is parallel to the voice path.
Working modes of CCS7 signaling network
SP A BSP
2) Quasi-associated mode
message which is going to arrive
at a SP goes through a path
which is predetermined and via
one or more STPs.
SP A B
When the links between any two offices are transferred by a
STP, how will be the link path?

signaling is to serve the voice path.
SP A
SP B
Tandem C
When the links between any two offices are transferred
by a STP, how will be the link path?
Note: the arrows show the paths that
signaling messages go through.
starts from one end of the voice path
it is going to serve, and ends up at
the other end of the voice path.

OPC and DPC (II)
STPB
MSU2
OPC=?
DPC=?
MSU1
OPC=?
DPC=?
MSU2
OPC=?
DPC=?
and OPC of the messages?
(a) (b)
OPC and DPC (II)
STPB
MSU2
Course Arrangement
part 1 Signaling

The hierarchy of CCS7 system can be functionally divided into
two 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.

2) UP (User Part)
MTP. It includes TUP (Telephone
User Part), ISUP (ISDN User Part),
etc.
receives interoffice control signals
during call process.
relation between UP and MTP.
Hierarchy of CCS7 signaling system

MTP could be further divided into 3 levels, namely, level 1-- signaling
data link, level 2 -- signaling link, level 3 -- signaling network.These
three levels are similar to the first three levels of OSI model.
Three levels of MTP
A signaling data link is a bi-directional transmission path for signaling,
comprising two data channels operating together in opposite

The second level provides such functions as signal
unit delimitation, signal unit alignment, 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.
Three levels of MTP
2) Level 2: Signaling
Functions on this level include signaling message handling and
signaling network management.
In the transmission of messages, the function of signaling
message handling sends the messages to the proper link or user
part; while when malfunctions come up, the network management
function realizes the reorganization of the signaling network, and
when congestion appears, the network management function
carries out flow control at the network level, so as to ensure the
reliable transmission of signaling under extreme conditions.
Three levels of MTP
3) Level 3: Signaling
The 4-level structure of CCS7 system
Logical
channel
Logical
channel
Logical
channel
Physical
channel
Message
Discrimination
Message
Distribution
Message
Routing
The distribution of CCS7 system in C&C08
AM/CM
OPT
MPU
LPN7
BNET
DT
OPT
MPU
Course Arrangement
part 1 Signaling

Signaling unit (SU) is the
minimum unit used to carry the
various signaling messages in

1. MSU (Message Signaling Unit)
Used to transfer the signaling message from the 4th level or the
signaling network management message from the 3rd level.
2. LSSU (Link Status Signaling Unit)
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.
3. FISU (Fill-in Signaling Unit)
Originating and terminating at the 2nd level, it is used to fill in the
vacant locations when the link is free or congested, so as to keep the
link in the status of communication, and sometimes it is also used to
confirm the receive of message from the opposite side.
The classification of signaling units--According to the sources

The structure of MSU, LSSU and FISU
FF CK LI FIB FSN BIB BSN
8 16 2 6 1 7 1 7 8
SIF SIO
FF CK SF LI FIB FSN BIB BSN
8 16 8 2 6 1 7 1 7 8 Sending
DirectionStructure of LSSU
8 16 2 6 1 7 1 7 8 Sending
DirectionStructure of FISU
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. 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].
Important fields in TUP MSU-- CIC, SLC
link

Correspondence of CIC
CIC represent PCM time slot
number, and the higher 7 bits
represent PCM system
number.
7 bits 5 bits CIC PCM system number PCM time slot number
CIC E1#1 E1#0 CIC
TS0 0000000 00000
TS1 0000000 00001
TS31 0000000 11111
0000001 00000 TS0
0000001 00001 TS1
0000001 11111 TS31

Similarly, if there are multiple links connecting two adjacent SPs, to
distinguish them, we should assign each of them a code, SLC. 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].
... ...
... ...

Different from CAS, in CCS7 we usually use bi-directional trunk
circuits. This induces the possibility of selecting the same circuit from
the two offices at the same time, which is called "dual seizure".
To prevent this, for each circuit, we assign its "master" control right to
one office, and the "non-master" or "slave" control right over this
circuit 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 seizure
Dual seizure

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.
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.
Dual seizure

Course Arrangement
part 1 Signaling

Basic signaling procedure
IAM: initial address
the called number are sent
in this message
ACM: address complete
Call connection through tandem office
SAM: Subsequent address message,
subsequent digits of the called
number which haven't been sent
by IAM.
CBK: Clear backward signal
Send caller number initiatively
sent initiatively, IAI is
...
The opposite office ask for caller number (e.g.,Malicious
call tracing, CLID)
for caller number and some
other information.
caller number and some
The cooperation of CCS7 and R2
the called
phone rings
SAO (0)
SAO (8)
A1 8 (the last digit)
A3 II-1 B6
ring back tone
Summary
important aspects of CCS7:
signaling network, CCS7 hierarchy
are the most important concepts.
Signaling procedure

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Oma003001 Gsm Signalling System-mtp Issue2.0