Title: Signaling System for GSM Networks
1Signaling System for GSM Networks
- Rabindranath Nandi
-
- Rahul Ghosh
2Acknowledgement
- Mr. Suvadip Basu
- Deputy Manager,
- Switch Operations
- Hutchison Telecom East Ltd.
- Kolkata 700 017
3Introduction 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
- SS7 or CCS7 (Common Channel Signaling 7)
- CAS (Channel associated Signaling)
- DTMF (Dual Tone Multi frequency)
4Areas 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
5Channel 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
6Common 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
7Advantages 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.
8CCS7 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
9CCS7 Network Architecture
- A typical SS7 network is a mixture of various SPs
STPs SRPs and SEPs, As shown below
STP (SPC200)
SP (SPC100)
SEP (SPC500)
SRP (SPC400)
STP (SPC300)
10CCS7 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.
11SS7 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
12SS7 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.
13SS7 Network Structure
SPC100
SPC700
NAT 1
SPC300
NAT 1
SPC200
SPC800
SPC900
SPC800
SPC100
NAT 0
SPC100
SPC400
SPC400
SPC100
SPC600
NAT 1
SPC300
INAT 0
SPC500
SPC200
14SS7 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.
15SS7 Protocol Structure
SS7 Users
I S U P
B S 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)
16The 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.
17The 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.
18The Message Transfer Part
Signal Unit Formats
19The 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).
20The 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.
21The 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.
22The Message Transfer Part
Routing label of a message signal unit
23The 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
24The 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.
25The Message Transfer Part
MTP Functions
26The 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
-
27ISDN 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.
28ISDN 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.
29ISDN 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.
30ISDN 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
31ISDN User Part Call Setup Procedures
32ISDN User Part Call Release Procedures
33SCCP 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
34SCCP 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
35SCCP 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
36SCCP 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).
37SCCP 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.
38SCCP 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
39SCCP 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
40SCCP Protocol Classes
41SCCP 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.
42SCCP Procedures - Connectionless
43SCCP Functional Blocks
44SCCP Subsystems
- Important Subsystems
- MAPHLR 6 MAPVLR 7 MAPMSC 8 MAPEIR9 BSSAP
254 CAP 146 SINAP - 247 GPRS -149
0
1
254
255
Subsystems
MAP
SCCP
45SCCP 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
46Few 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
47Questions/ Queries
- mailto rahulghosh_at_ieee.org
48