📄 rfc3094.txt
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- In-Sequence Delivery
- Flow Control
- Error Detection/Correction
- Keep Alive
- Local Data Retrieval
- Connection Control
- Protocol Error Detection and Recovery
Signaling Transfer Point (STP):
Packet switches that provide CCS message routing and transport. They
are stored programmed switches that use information contained in the
message in conjunction with information stored in memory to route the
message to the appropriate destination signaling point.
2. Overview of the TALI Protocol
2.1 Traditional PSTN SS7 Networks
The traditional PSTN SS7 network consists of 3 types of devices
connected via dedicated SS7 signaling links.
The 3 primary device types for PSTN networks are:
* SSP: Signaling Service Point. These nodes act as endpoints in
the SS7 network, originating SS7 messages as users attempt to
place phone calls. These nodes contain interfaces into the SS7
data network and the SS7 voice network.
Sprague, et al. Informational [Page 6]
RFC 3094 Tekelec's Transport Adapter Layer Interface April 2001
* STP: Signaling Transfer Point. These nodes act primarily as
switches, switching SS7 traffic from node to node throughout the
network until it reaches another endpoint. An important feature
of each STP is to provide SS7 network management functionality
that allows messages to be delivered even when links and devices
fail. STPs also sometimes provide database type services, such as
Global Title Translations and Local Number Portability.
* SCP: Signaling Control Point. These nodes act as databases.
These nodes contain stored data that is used to turn SS7 Queries
into SS7 Replies.
There are 3 primary types of dedicated SS7 signaling links:
* 56Kbps SS7 (DS0, V35, OCU) links. These links implement the MTP-1
and MTP-2 protocols as defined in [1].
* DS1 High Speed Links. These links use the SAAL protocol to
provide an alternative to 56Kbps SS7 links that is based on newer,
faster technology. These links implement the SS7 protocol as
defined in [8].
* E1 Links.
Figure 1 provides an overview of the traditional PSTN network. In
this network, any of the links can be implemented via either 56
Kbps, DS1, or E1 links.
Sprague, et al. Informational [Page 7]
RFC 3094 Tekelec's Transport Adapter Layer Interface April 2001
^
/ \
/SCP\
/-----\
/ \
/ \
/ \
/ \
/---\ +---+ +---+ /---\
| SSP |-----|STP|----|STP|-----| SSP |
\---/ \ /+-+-+\ /+-+-+ \ / \---/
\/ | \/ | \/
/\ | /\ | /\
/---\ / \+-+-+/ \+-+-+ / \ /---\
| SSP |/----|STP|----|STP|/----| SSP |
\---/ +---+ +---+ \---/
\ /
\ /
\ /
\ ^ /
\/ \/
/SCP\
/-----\
Figure 1: The Traditional PSTN Network
2.2 Converged SS7 Networks
In the converged SS7 network, SS7 devices will reside on both the
traditional PSTN network (with dedicated 56 Kbps and DS1 links) and
on the IP network (with Ethernet links based on IP protocol). The
services of SSPs, STPs, and SCPs can be provided by new types of
devices that reside on IP networks. The IP network is not intended
to completely replace the PSTN, rather devices on the 2 types of
networks must be able to communicate with one another and convert
from 1 lower layer protocol to the other.
Signaling Gateways are new devices that may also function as an STP
in the converged network. SGs provide interfaces to:
* devices on the SCN (traditional SSPs, STPs, and SCPs)
* other SGs
* new devices on the IP network
SGs also continue to perform STP functions such as SS7 network
management and some database services (such as GTT and LNP).
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New devices on the IP network include:
* Media Gateway Controllers. In addition to other functions, these
devices control Media Gateways and perform call processing.
* Media Gateways. In addition to other functions, these devices
control voice circuits that are used to carry telephone calls.
MGs + MGCs combine to provide the functionality of traditional
SSPs.
* IP based SCPs. The database services that are related to SS7 can
be moved onto devices on the IP network.
Figure 2 provides an overview of the converged SS7 network.
----- +----+
/\ / \-------------| SG |
/ \----| SCN | +----+ +----+
/SCP \ \ /------| SG | |
------ ----- +----+ |
| | | |
| | | |
| | -----
| | / \ /\
| | | IP |----/ \
| /---\ \ / /SCP \
| | SSP | ----- ------
| \---/ / \
| | / \
/---\ | / \
| SSP | | +---+ +---+
\---/ +----+ |MGC| |MGC|
| | MG | +---+ +---+
| +----+\ \ /
| \ \ /
| \ -----
| \ / \
+----+ | IP |
| MG |-----------\ /
+----+ -----
Figure 2: The Converged SS7 Network
In theory, the TALI protocol can be used between 2 nodes to carry SS7
traffic across TCP/IP. Some of the areas that TALI could be used
include:
Sprague, et al. Informational [Page 9]
RFC 3094 Tekelec's Transport Adapter Layer Interface April 2001
- For SG to SG communication across IP
- For SG to MGC communication across IP
- For SG to IP based SCP communication across IP
- For communication between multiple IP based SCPs
- For communication between multiple MGCs
- For communication between MGCs and MGs
- For other IP devices such as DNS, Policy Servers, etc.
In reality, the communication between MGCs, or between MGC and MG is
probably better suited to using other protocols. With respect to the
Signaling Gateway implementation, the TALI protocol is used to carry
SS7 traffic:
- For SG to SG communication
- For SG to MGC communication
- For SG to IP based SCP communication
2.3 TALI Protocol Stack Overview
The Transport Adapter Layer Interface is the proposed interface that
provides SCCP, ISUP, and MTP messaging encapsulation within a TCP/IP
packet between two switching elements. In addition, TALI provides
SCCP Management (SCMG), MTP Primitives, dynamic registration of
circuits, and routing of call control messages based on circuit
location.
The major purpose of the TALI protocol is to provide a bridge between
the SS7 Signaling Network and applications that reside within an IP
network. Figure 3 provides a simple illustration that highlights the
protocol stacks used for transport of SS7 MSUs on both the SS7 side
and the IP side of the SG.
Sprague, et al. Informational [Page 10]
RFC 3094 Tekelec's Transport Adapter Layer Interface April 2001
SS7 traffic SS7 traffic
via 56Kbps links via TALI
+-----------+ +----+ +--------+
|Traditional| | SG | | IP |
|SS7 Devices|<------>| |<-------->| Devices|
+-----------+ +----+ +--------+
SS7 SS7, TALI, TCP/IP
protocol stack protocol stack
+---------------+ +---------------+
|SS7 application| |SS7 application|
|layer | |layer |
+-------+-------+ +-------+-------+
| TCAP | ISUP | | TCAP | ISUP |
+-------+ | +-------+ |
| SCCP | | | SCCP | |
+-------+-------+ +-------+-------+
| MTP3 | | MTP3 |
+---------------+ +---------------+
| MTP2 | | TALI |
+---------------+ +---------------+
| MTP1 | | TCP |
| (& phy. | +---------------+
| layer) | | IP |
+---------------+ +---------------+
| MAC |
| (& phy. |
| layer) |
+---------------+
Figure 3: TALI Protocol to carry SS7 over TCP/IP
From Figure 3, several observations can be made:
* The TALI layer is used when transferring SS7 over IP.
* When SS7 traffic is carried over a IP network, the MTP2 and MTP1
layers of a traditional 56 Kbps link are replaced by the TALI,
TCP, IP, and MAC layers
* The TALI layer sits on top of the TCP layer.
* The TALI layer sits below the various SS7 layers (MTP3, SCCP/TCAP,
ISUP, and applications). The data from these SS7 layers is
carried as the data portion of TALI service data packets.
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