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Ong, et al. Informational [Page 6]
RFC 2719 Framework Architecture for Signaling Transport October 1999
For interworking with SS7-controlled SCN networks, the SG terminates
the SS7 link and transfers the signaling information to the MGC using
signaling transport. The MG terminates the interswitch trunk and
controls the trunk based on the control signaling it receives from
the MGC. As shown below in case (a), the SG, MGC and MG may be
implemented in separate physical units, or as in case (b), the MGC
and MG may be implemented in a single physical unit.
In alternative case (c), a facility-associated SS7 link is terminated
by the same device (i.e., the MGU) that terminates the interswitch
trunk. In this case, the SG function is co-located with the MG
function, as shown below, and signaling transport is used to
"backhaul" control signaling to the MGCU.
Note: SS7 links may also be terminated directly on the MGCU by
cross-connecting at the physical level before or at the MGU.
SGU
+--------+
SS7<------>[SG] |
(ISUP) | | |
+---|----+
ST | SGU MGCU
+---|----+ +--------+ +--------+
| [MGC] | SS7---->[SG] | | [MGC] |
| | | | | | | | | |
+---|----+ +---|----+ +--|-|---+
MGCU | ST | | |
| | ST | |
Media +---|----+ Media +---|----+ +--|-|---+
------->[MG] | ----->[MG/MGC]| SS7 link-->[SG]| |
stream | | stream | | Media------> [MG] |
+--------+ +--------+ stream +--------+
MGU MGU MGU
(a) (b) (c)
Notes: ST = Signaling Transport used to carry SCN signaling
Figure 2: Example Implementations
Ong, et al. Informational [Page 7]
RFC 2719 Framework Architecture for Signaling Transport October 1999
In some implementations, the function of the SG may be divided into
multiple physical entities to support scaling, signaling network
management and addressing concerns. Thus, Signaling Transport can be
used between SGs as well as from SG to MGC. This is shown in Figure 3
below.
SGU MGCU
+---------+ +---------+
| | ST | |
| [SG2]------------------------------>[MGC] |
| ^ ^ | | |
+---|-|---+ +---------+
| |
| | ST
ST| +--------------------------------+
| |
| |
SS7 +---|----------+ SS7 +----|---------+
-----------> [SG1] | -----------> [SG1] |
media | | media | |
------------------->[MG] | ------------------->[MG] |
stream +--------------+ stream +--------------+
MGU MGU
Figure 3: Multiple SG Case
In this configuration, there may be more than one MGU handling
facility associated signaling (i.e. more than one containing it's own
SG function), and only a single SGU. It will therefore be possible to
transport one SS7 layer between SG1 and SG2, and another SS7 layer
between SG2 and MGC. For example, SG1 could transport MTP3 to SG2,
and SG2 could transport ISUP to MGC.
2.3 ISDN Interworking for Connection Control
In ISDN access signaling, the signaling channel is carried along with
data channels, so that the SG function for handling Q.931 signaling
is co-located with the MG function for handling the data stream.
Where Q.931 is then transported to the MGC for call processing,
signaling transport would be used between the SG function and MGC.
This is shown in Figure 3 below.
Ong, et al. Informational [Page 8]
RFC 2719 Framework Architecture for Signaling Transport October 1999
MGCU
+-------------+
| [MGC] |
| | | |
+-----|-|-----+
| |
| O device control
| |
Q.931/ST O |
| |
+-----|-|-----+
| | | |
Q.931---->[SG]| |
signals| | |
| | |
Media---->[MG] |
stream | |
+-------------+
MGU
Figure 4: Q.931 transport model
2.4 Architecture for Database Access
Transaction Capabilities (TCAP) is the application part within SS7
that is used for non-circuit-related signaling.
TCAP signaling within IP networks may be used for cross-access
between entities in the SS7 domain and the IP domain, such as, for
example:
- access from an SS7 network to a Service Control Point (SCP) in IP.
- access from an SS7 network to an MGC.
- access from an MGC to an SS7 network element.
- access from an IP SCP to an SS7 network element.
A basic functional model for TCAP over IP is shown in Figure 5.
Ong, et al. Informational [Page 9]
RFC 2719 Framework Architecture for Signaling Transport October 1999
+--------------+
| IP SCP |
+--|----|------+
| |
SGU | | SGU
+--------------+ | | +--------------+
| | | | | |
SS7<--------->[SG] ---------+ | | [SG]<---------> SS7
(TCAP) | | | | | | |
+------|-------+ | +------|-------+
| | |
O +------------+ O
MGCU | | | MGCU
+-------|----|--+ +-----|--------+
| | | | | | |
| [MGC] | | [MGC] |
| | | | | |
+-------|-------+ +-----|--------+
| |
+-------|-------+ +-----|------+
Media | | | | | | Media
<------+---->[MG] <---+--RTP stream---+--> [MG] <-+-------->
stream| | | | stream
+---------------+ +------------+
MGU MGU
Figure 5: TCAP Signaling over IP
3. Protocol Architecture
This section provides a series of examples of protocol architecture
for the use of Signaling Transport (SIG).
3.1 Signaling Transport Components
Signaling Transport in the protocol architecture figures below is
assumed to consist of three components (see Figure 6):
1) an adaptation sub-layer that supports specific primitives, e.g.,
management indications, required by a particular SCN signaling
application protocol.
2) a Common Signaling Transport Protocol that supports a common set
of reliable transport functions for signaling transport.
3) a standard, unmodified IP transport protocol.
Ong, et al. Informational [Page 10]
RFC 2719 Framework Architecture for Signaling Transport October 1999
+-- +--------------------------------+
| | SCN adaptation module |
| +--------------------------------+
| |
S | +--------------------------------+
I | | Common Signaling Transport |
G | +--------------------------------+
| |
| +--------------------------------+
| | standard IP transport |
+-- +--------------------------------+
Figure 6: Signaling Transport Components
3.2. SS7 access for Media Gateway Control
This section provides a protocol architecture for signaling transport
supporting SS7 access for Media Gateway Control.
****** SS7 ******* SS7 ****** IP *******
*SEP *--------* STP *------* SG *------------* MGC *
****** ******* ****** *******
+----+ +-----+
|ISUP| | ISUP|
+----+ +-----+ +---------+ +-----+
|MTP | |MTP | |MTP | SIG| | SIG |
|L1-3| |L1-3 | |L1-3+----+ +-----+
| | | | | | IP | | IP |
+----+ +-----+ +---------+ +-----+
STP - Signal Transfer Point SEP - Signaling End Point
SG - Signaling Gateway SIG - Signaling Transport
MGC - Media Gateway Controller
Figure 7: SS7 Access to MGC
Ong, et al. Informational [Page 11]
RFC 2719 Framework Architecture for Signaling Transport October 1999
3.3. Q.931 Access to MGC
This section provides a protocol architecture for signaling transport
supporting ISDN point-to-point access (Q.931) for Media Gateway
Control.
****** ISDN ********* IP *******
* EP *--------------* SG/MG *------------* MGC *
****** ********* *******
+----+ +-----+
|Q931| | Q931|
+----+ +---------+ +-----+
|Q921| |Q921| SIG| | SIG |
+ + + +----+ +-----+
| | | | IP | | IP |
+----+ +---------+ +-----+
MG/SG - Media Gateway with SG function for backhaul
EP - ISDN End Point
Figure 8: ISDN Access
3.4. SS7 Access to IP/SCP
This section provides a protocol architecture for database access,
for example providing signaling between two IN nodes or two mobile
network nodes. There are a number of scenarios for the protocol
stacks and the functionality contained in the SIG, depending on the
SS7 application.
In the diagrams, SS7 Application Part (S7AP) is used for generality
to cover all Application Parts (e.g. MAP, IS-41, INAP, etc).
Depending on the protocol being transported, S7AP may or may not
include TCAP. The interface to the SS7 layer below S7AP can be either
the TC-user interface or the SCCP-user interface.
Figure 9a shows the scenario where SCCP is the signaling protocol
being transported between the SG and an IP Signaling Endpoint (ISEP),
that is, an IP destination supporting some SS7 application protocols.
Ong, et al. Informational [Page 12]
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