Signaling System No.7 Protocol Architecture And Sevices part 26 ppsx

Summary
ISUP provides a rich network interface to call processing at an SSP. The increased
bandwidth and protocol standardization allow a greater range of services that are
able to interwork both within a network and across network boundaries. ISUP was
designed to interface well with ISDN access signaling by providing event mapping
and facilitating end-to-end user signaling. The protocol's use of optional message
p
arameters achieves flexibility and extensibility.
ISUP uses a CIC identifier in each message to correlate the signaling with the
correct circuit. The CIC is the key to associating signaling with bearer circuits.
ISUP also provides a set of maintenance messages for diagnostics and maintenance
of ISUP facilities. These messages allow for blocking, testing, and resetting
circuits and inquiring about circuit status.

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Chapter 9. Signaling Connection Control Part (SCCP)
The Signaling Connection Control Part (SCCP) is defined in ITU-T
Recommendations Q.711-Q.716 [58
–63] and for North American markets in ANSI
T1.112 [2
]. SCCP sits on top of Message Transfer Part 3 (MTP3) in the SS7
p
rotocol stack. The SCCP provides additional network layer functions to provide
transfer of noncircuit-related (NCR) signaling information, application
management procedures and alternative and more flexible methods of routing.
N
OTE
Technically, SCCP can also transfer circuit-related signaling information;

however, this is an exception.

As shown in Figure 9-1
, the combination of the MTP, and the SCCP is termed the
N
etwork Service Part (NSP). The NSP follows the principles of the OSI reference
model, as defined in Recommendation X.200 [99
]; as such, it provides a subset of
the Layer 3 services, which are defined in Recommendation X.213 [100].
Figure 9-1. SS7 Stack with the Network Service Part (NSP) Highlighted


SCCP was developed after the MTP, and together with the MTP3, it provides the
capabilities corresponding to Layer 3 of the OSI reference model.
Because SCCP is OSI Layer 3 compliant, in theory it can be transmitted over any
OSI-compliant network.
Because the MTP was originally designed to transfer call-control messages coming
from the Telephony User Part (TUP), it was, therefore, designed to transfer only
circuit-related signaling. In combination with the MTP, the SCCP can transfer
messages that are not circuit-related. These messages are used to support services
such as toll-free calling, Local Number Portability (LNP) and Completion of Calls
to Busy Subscribers (CCBS) in Intelligent Networks and mobility, roaming, and
SMS in cellular networks.
SCCP provides the following additional capabilities over the MTP:
• Enhances MTP to meet OSI Layer 3
• Powerful and flexible routing mechanisms
• Enhanced transfer capability, including segmentation/reassembly when
message is too large to fit into one Message Signal Unit (MSU)
• Connectionless and connection-oriented data transfer services
• Management and addressing of subsystems (primarily database-driven

applications)
SCCP is used extensively in cellular networks. Base Station Subsystem Mobile
Application Part (BSSMAP) and Direct Transfer Application Part (DTAP) use it to
transfer radio-related messages in Global System for Mobile communication
(GSM). In conjunction with Transfer Capabilities Application Part (TCAP), SCCP
is also used throughout the GSM Network Switching Subsystem (NSS) to transport
Mobile Application Part (MAP) signaling between the core GSM components to
enable subscriber mobility and text messaging (SMS), among other items. For
example, when the Visitor Location Register (VLR) queries the Home Location
Register (HLR) to obtain the subscriber's profile, SCCP is responsible for
transferring both the query and the response back to the VLR. For more
information about GSM, see Chapter 13
, "GSM and ANSI-41 Mobile Application
Part (MAP)."
Cellular intelligent network protocols, Wireless Intelligent Network (WIN), and
Customizable Applications for Mobile Enhanced Logic (CAMEL) also use SCCP
with TCAP (see Chapter 10
, "Transaction Capabilities Application Part [TCAP]")
to provide intelligent network functionality in a cellular environment. Figure 9-2

shows a typical cellular protocol stack, as found at a GSM-MSC.
Figure 9-2. Typical SS7 Stack Used in GSM Networks


Fixed-line networks primarily use SCCP for intelligent network applications and
advanced supplementary services. Fixed-line intelligent networks use Advanced
Intelligent Network (AIN) within North America and Intelligent Network
Application Protocol (INAP) outside of North America (see Chapter 11
,
"Intelligent Networks [IN]

"). AIN/INAP both use SCCP's transport, application
management, and enhanced routing functionalities. Two example supplementary
services that require the use of SCCP include CCBS and Completion of Calls on
N
o Reply (CCNR).
This chapter looks at the functions of SCCP in some detail, beginning with an
outline of the SCCP architecture and then moving onto protocol classes,
connectionless and connection-oriented procedures, SCCP management functions,
and most importantly, SCCP routing, including the use of global titles.

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SCCP Architecture
As shown in Figure 9-3
, SCCP is composed of the following four functional areas:
• SCCP connection-oriented control (SCOC)— Responsible for setting up and
releasing a virtual connection between two SCCP users. SCOC can offer
features including sequencing, flow control, and segmentation and can
override congestion procedures by assigning data priority. The section,
"SCCP Connection-Oriented Control (SCOC)
" describes SCOC in more
detail
• SCCP connectionless control (SCLC)— Responsible for transferring data
between SCCP users without creating a virtual connection. SCLC is
described in the "SCCP Connectionless Control (SCLC)
" Section. In
addition to segmentation, it can perform limited sequencing.
• SCCP routing control (SCRC)— Provides additional routing beyond that

offered by MTP3, through the use of global titles. The "Global Title
Routing" section fully explains global titles.
• SCCP management (SCMG)— Responsible for tracking application status
and informing SCMG at other SCCP nodes, as necessary. It is described
later in this chapter in the section, "SCCP Management (SCMG)
."
Figure 9-3. The SCCP Architecture


The term SCCP Users refers to the applications that use SCCP's services. These are
p
rimarily database-driven applications. Such applications use the services of TCAP
described in Chapter 10
for peer application layer communication and the services
of SCCP for managing the transport of messages between those applications.
Applications that use the services of SCCP are known as subsystems.

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SCCP Message Transfer Services
The SCCP provides two categories of service for data transfer: connection-oriented
services and connectionless services. Within each service category, two classes of
service are defined as follows:
• Class 0— Basic connectionless class
• Class 1— In-sequence delivery connectionless class
• Class 2— Basic connection-oriented class
• Class 3— Flow control connection-oriented class
Connection-oriented Versus Connectionless Services

The analogy of sending letters and postcards best explains the difference between
the connection-oriented and the connectionless services. The postal service carries
out the physical transfer and is therefore analogous to MTP. Connection-oriented
service is much like the exchange of formal letters. When you send a formal letter,
you might assign a reference to it—"Our Reference X." When the receiving party
responds, they might also assign their own reference to the letter and also copy the
sender's reference—"Your Reference X." From that point on, both parties state
their own and each other's assigned reference. SCCP connection-oriented service
uses the same principles; the "Our Reference" is known as the Source Local
Reference (SLR), and the "Your Reference" is known as the Destination Local
Reference (DLR). This is similar in principle to Transmission Control Protocol
(TCP): data is sent only when a virtual connection has
b
een established through the
initial exchange of identifiers. Figure 9-4
illustrates this principle.
Figure 9-4. Analogy of Connection-oriented Service with Official Mail
Correspondence


Connectionless service is like sending postcards, where the sender and recipient do
not establish references. In principle, it is similar to User Datagram Protocol
(UDP): data is sent without first establishing a virtual connection using identifiers.
N
OTE
SCCP transfers the data using the signaling network for transport. Trunks are not
involved.

User Data and Segmentation
The data (from subsystems) is sent in information elements called Network Service

Data Units (NSDUs). SCCP provides the capability to segment or reassemble an
N
SDU that is too large to fit in a single MTP message (MSU) so that it can be
transmitted over a number of MSUs (16 maximum). When using the
connectionless classes, if an NSDU is greater than 255 octets when using a UDT
message or 254 when using a XUDT message, the originating node splits the
N
SDU into a number of XUDT messages. For a description of UDT and XUDT
messages, see section "Message Types
" and refer to Appendix C, "SCCP Messages
(ANSI/ETSI/ITU-T)." If an NSDU is greater than 255 octets when using the
connection-oriented classes, the originating node splits the NSDU into several DT
messages. The receiving node reassembles the NSDU. For a description of the DT
message, see the section on "Message Types
" and refer to Appendix C.
Theoretically, the maximum amount of user data is 3952
[1]
octets in ITU-T SCCP
[58
-61] and [2] 3904 octets in ANSI SCCP. This excludes optional parameters and
global titles, which will appear to be repeated in each message. The ITU-T
recommends using 2560 as the maximum NSDU size as a safe implementation
value [16
] because it allows for the largest global title and numerous optional
p
arameters. The section on "SCCP Routing Control (SCRC)" covers global titles.
[1]
3952 = (254 - 7) * 16, where 254 is the user data length fitting in one XUDT, 16
is the maximal number of segments and 7 is the length of the optional parameter:
"segmentation" is followed by the end of the optional parameters octet [16

].
The parameter Protocol Class within each SCCP message specifies the protocol
class. Before giving a further explanation of connectionless and connection-
orientated procedures the following sections discuss the four classes of data
transfer that SCCP provides.
Connectionless Protocol Classes
Class 0 provides a basic connectionless service and has no sequencing control. It
does not impose any conditions on the Signaling Link Selection (SLS) values that
MTP3 inserts; therefore, SCCP messages can be delivered out of sequence. Class 0
can be considered a pure connectionless service. See Chapter 7, "Message Transfer
Part 3 (MTP3)," for information about the SLS field.
Class 1 service adds sequence control to the Class 0 service by requiring the SCCP
to insert the same SLS field for all NSDUs that have the same Sequence Control
p
aramete
r
. The higher layers indicate to SCCP whether or not a stream of NSDUs
should be delivered in sequence. Therefore Class 1 is an enhanced connectionless
service that provides basic in sequence delivery of NSDUs. Failures at the MTP
level can still result in messages being delivered out of sequence.
TCAP is the typical user of SCCP connectionless services. The other user is Base
Station Subsystem Application Part (BSSAP), which is used solely for GSM
cellular radio related signaling. See Chapter 3
, "The Role of SS7," for a brief
description of BSSAP. Although the applications (subsystems) use TCAP directly,
they are considered SCCP users because TCAP is considered transparent. See
Chapter 10
for more information about TCAP.
N
OTE

Common subsystems include Local Number Portability (LNP), Customizable
Application Part (CAP), MAP, INAP, and AIN.

Table 9-1
shows the connectionless service protocol classes and features.
Table 9-1. Connectionless Service Protocol Classes
Protocol Class and
Name
Features Example Use
Protocol Class 0: Basic
Connectionless
Independent message
transport, no sequencing
Some BSSMAP
messages (Paging),
TCAP
Protocol Class 1:
Connectionless Service
Independent message
transport, limited
sequencing
TCAP

Connection-oriented Protocol Classes
Class 2 provides a basic connection-oriented service by assigning local reference
numbers to create a logical connection. Messages that belong to the same
connection are assigned the same SLS value to ensure sequencing. Class 2 does not
p
rovide flow control, loss, or missequence detection.
Class 3 is an enhanced connection-oriented service that offers detection of both

message loss and mis-sequencing (for each connection section). Class 3 also offers
flow control using an expedited data transfer function. The ETSI European SCCP
standard, ETS 300-009 [10
], offers support for Class 3 only from V1.4.2
(November 1999) onwards.
The ITU-T had specified a Class 4, but this was never implemented on live
networks and was later removed in White Book editions.
Table 9-2
shows the connection-oriented service protocol classes and features.
Table 9-2. Connection-oriented Service Protocol Classes
Protocol Class and
Name
Features Example Use
Protocol Class 2: Basic
Connection-oriented
Service
Logical signaling connection used
for message transport
Some BSSMAP
messages
(Setup)
Protocol Class 3:
Connection-oriented
Service
Logical signaling connection used
for message transport, and flow
control (expedited data transfer)
No known
current use

S
CCP Connectionless Control (SCLC)
SCLC is used to provide the capabilities that are necessary to transfer one NSDU
in the "data" field of a UDT, Long Unit Data (LUDT), and XUDT message. For a
description of SCCP messages, see section "Message Types
" and Appendix C. The
SCLC routes the message without regard to the route that the messages follow
through the network. These services are provided without setting up a logical
connection.
SCLC formats the user data into a message of the appropriate protocol class (0 or 1
in the case of connectionless) and transfers it to SCRC for routing. The section on
"SCCP Routing Control (SCRC)
" describes SCRC. On receiving a message, SCLC
is responsible for decoding and distributing the message to the appropriate
subsystem. Figure 9-5
shows data transfer using SCLC: data is simply sent without
the prior establishment of references at each side.
Figure 9-5. The Transfer of Connectionless Messages from One SCCP User to
Another


S
CCP Connection-Oriented Control (SCOC)
SCOC is used to route messages through a specific, fixed logical network path. To
establish a dedicated logical connection between an originating SCCP user
(subsystem) and a terminating SCCP user (subsystem), the SCCP users residing at
different nodes throughout the network communicate with each other.
A signaling connection between the SCCP users is established, making both SCCP
users aware of the transaction by using the DLR and SLR parameters. The
signaling connection is released at the end of the transaction (information transfer).

This is similar to SS7 protocol TUP/ISUP, which is used to control telephony calls,
in that a connection is setup and released at a later time. However, the connection
is virtual; there is not a trunk with user traffic being set up and released—rather,
there is a virtual connection over the signaling network for the purpose of data
transfer between applications (subsystems).
N
OTE
SCCP connection-oriented services (Class 2 and Class 3) are virtual connections
between users of the signaling system and bear no relation to connections between
subscribers (trunks).

Connection-oriented procedures can be split into three phases:
• Connection Establishment Phase— The SCCP users set up a logical, fixed
path that the data packets will follow. The path might involve only two or
three nodes with SCCP capability or, depending on how many intermediate
nodes exist between the originator and terminator, it might involve a much
larger number.
• Data Transfer Phase— After the connection is established, the data that is to
be transferred is converted into an NSDU and sent in a DT1 or DT2
message. For a description of SCCP messages, see the section on "Message
Types" and Appendix C. Each NSDU is uniquely identified as belonging to
a specific signaling connection. In this way, it is possible for the SCCP to
simultaneously handle independent signaling connections.
• Connection Release Phase— After all NSDUs have been transmitted and
confirmed, either or both of the user applications that initiated the process
release the logical path. A release can also occur if the connection fails.
An example of a connection-oriented data transfer is carried out in Figure 9-6
. At
the request of the SCCP user, SCCP A establishes a logical connection by sending
a Connection Request (CR) message to SCCP B and assigning a SLR to the

request. The remote node confirms the connection by sending a Connection
Confirm (CC) message and includes its own SLR and a DLR that is equal to SCCP
A's SLR. This gives both sides a reference for the connection.
Figure 9-6. The Transfer of Connection-oriented Messages from One SCCP User
to Another Using a Temporary Connection


The CR message contains the address of the destination SCCP node and user. The
subsequent data message DT1 only needs to send the DLR because the logical
connection has been established through the proceeding exchange of SLR and
DLR. The clear-down messages contain both SLR and DLR. If intermediate nodes
are involved, they make associations between pairs of SLR/DLRs to establish the
logical connection. Upon release, the SLR/DLR references are available for further
use on other transactions. SCCP nodes can establish multiple simultaneous logical
connections through the use of the SLR and DLR.
In Figure 9-5
, if SCCP B received a CR message and either the SCCP B or the
SCCP A could not establish the connection, a Connection Refused (CREF)
message would have been returned.
Classes 2 and 3 (discussed previously) can either establish temporary connections
(that is, on demand by SCCP user), as shown in Figure 9-5
, or permanent signaling
connections that are established by management action. Temporary connections
are analogous to dialup connections, and permanent connections are analogous to
leased lines. The connection establishment and release services are not required on
p
ermanent connections.
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