Table Of Contents
White Paper—Summer 2004
Cisco IP Transfer Point as the Signaling Gateway for the Cisco PGW 2200 Softswitch
The Cisco PGW 2200 Softswitch is a carrier-class call agent that performs the signaling and call-control tasks—such as digit analysis, routing, and circuit selection—within the public switched telephone network (PSTN) gateway infrastructure. The Cisco PGW 2200 gives service providers the ability to smoothly route voice and data calls between the PSTN and newer packet networks.
Until recently, the Cisco PGW 2200 Softswitch interfaced with the Signaling System 7 (SS7) network through a pair of SS7 link termination devices (SLTs) on Cisco 2600 Series chassis. Although serviceable, these SLTs had limited scalability, flexibility, features, and support for open standards. These limitations have been addressed with the addition of the Cisco IP Transfer Point (ITP) as the signaling gateway for the Cisco PGW 2200. The Cisco ITP solves these limitations with proven technology and a carrier-class system.
The Cisco ITP was introduced to the service provider market in 2001 and is deployed globally. It supports capabilities that include signaling transfer point (STP), signaling transport over IP and ATM, and signaling gateway for next-generation end nodes such as service switching point (SSP) and service control point (SCP). Cisco ITP is certified by Telcordia as an ANSI STP, is completely based on open standards, and provides STP-class availability. Cisco ITP is offered on multiple platforms and can scale from 4 to 800 links.
This document describes the use of the Cisco ITP as a signaling gateway (ITP-SG) specifically for the Cisco PGW 2200. It discusses the platforms, relevant signaling standards and features, and suggested system configurations.
Additional information about Cisco ITP can be found at:
Additional information about the Cisco PGW 2200 Softswitch can be found at:
Cisco ITP-SG with the Cisco PGW 2200 Softswitch
The relationship between the Cisco PGW 2200 Softswitch and the Cisco ITP-SG can be accomplished several ways, adding flexibility to the benefits of uniting these two systems. The ITP can be designed to act as both an STP and as the signaling gateway to the Cisco PGW 2200. Another way to configure this system is to use the ITP as only a signaling gateway. By using distributed routing technology, the Cisco PGW 2200 and mated-paired ITP-SG(s) can be configured to appear to the rest of the signaling network as a single SSP; all three devices operate under a single point code (Figure 1).
This "virtual SSP" capability greatly simplifies the implementation and management of the Cisco PGW 2200 into the signaling network. Rather than requiring three point codes to be provisioned (one for each Cisco ITP-SG, plus one for the Cisco PGW 2200), only one point is configured in the SS7 network. A-links are connected between the STPs and the ITP-SG mated pairs. The routing tables in the STPs treat this point code as if terminating at a single SSP (end node), hence the "virtual" SSP. The Cisco ITP-SG distributed architecture reliably enables this capability.
Cisco PGW 2200 and ITP-SG Share Point Code Using Distributed MTP3 Feature
Cisco ITP-SG Distributed Architecture
Cisco ITP-SG software allows link-sets to span two ITPs, enabling a single point code to be shared among them. The software modules that enable a pair of ITP-SGs to operate as a single device include:
•Cisco ITP group feature
–Distributed Message Transfer Part Level 3 (MTP3)
–Distributed MTP3 User Adaptation (M3UA)
•Distributed Signaling Gateway Mate Protocol (DSGMP)
These modules work together to support load sharing of links within a linkset between two physical ITP-SGs, and yet function as a single ITP-SG device. The distributed architecture of the paired ITP-SGs is controlled by one of the ITP-SGs. When the two ITP-SGs are deployed, they negotiate which will become the "manager" and which will become the "alternate." The manager handles the synchronization and management messaging. If the manager unit fails, or if connectivity between the two units fails, the alternate immediately takes over its own tasks.
This distributed architecture allows customers to achieve increased availability, non-distributive hardware swaps, and reduced point code use. If work is being performed on one of the Cisco ITP-SG units, the other is capable of handling the traffic load. Neither the Cisco PGW 2200 nor the SS7 network is affected, which greatly enhances both availability and serviceability. Cisco 2651XM, 7200VXR, and 7301 routers support this distributed MTP3 feature.
SS7 and Sigtran Open Standards
Cisco ITP uses open industry standards for traditional SS7 over time-division multiplexing (TDM) links, as well as for next-generation signaling over ATM (MTP3b) and IP (Signaling Transport [Sigtran]). In addition, Cisco ITP has been successfully tested to interoperate with equipment from 18 other vendors using Sigtran protocols.
Cisco ITP supports the signaling standards shown in Table 1.
The IETF created the Sigtran working group to develop a set of standard protocols for transporting legacy SS7 signaling over IP networks (SS7oIP). The standards have been developed to address lower-layer functions first, providing SS7-equivalent redundancy and availability. Building on this foundation, mechanisms for transporting (or backhaul of) SS7 signaling to IP-based endpoints were added.
For more information about Sigtran, visit:
Operating as the signaling gateway for the Cisco PGW 2200, the Cisco ITP uses Sigtran M3UA/SCTP/IP between the Cisco ITP and Cisco PGW 2200. To the SS7 network, the ITP uses SS7/TDM, HSL (SS7oATM), or M2PA/SCTP/IP depending on that service provider's requirements.
SCTP was designed by the IETF Sigtran Working Group. Input for this transport layer was provided by engineers from leading network and switching companies, including Cisco Systems. The IETF realized that existing protocols for transport over IP failed to provide the security and availability required for SS7. SCTP provides for these requirements with several new features:
•Multiple streams within a single link association avoid the issue of head-of-line blocking caused by retransmission of messages.
•Messages are delivered in a sequence.
•Supporting selective acknowledgement of message delivery eliminates the retransmission of messages that where successfully delivered, but followed a lost message unit.
•Multi-homing allows for quick retransmission via a different path. There are typically two paths—a primary and a secondary. Normally, the traffic flows via the primary path, and an SCTP "heartbeat" is sent over the secondary path.
•The SCTP heartbeat provides a timely and meaningful reachability check. While several TCP implementations offer a "keepalive," they have a default interval of once every two hours. The TCP implementation is used for state "clean-up," as opposed to the more frequent SCTP "fast override" objective.
•The SCTP security cookie and four-way handshake protect against SYN attacks—the most common form of denial of service attacks.
These combinations of features are not available in TCP or User Datagram Protocol (UDP) transport protocols. Once SCTP was created, the Sigtran working group had to build protocol stacks to operate on top of SCTP to provide the necessary equivalents to the SS7 stack. Since Sigtran supports SS7 in an IP environment the Sigtran working group approached this work with the decentralized model of most IP architectures in mind. They developed four Sigtran protocols that are designed for different functions within the signaling network. The Sigtran protocol for a switching end node that requires handling of MTP3 and ISDN User Part (ISUP) traffic (like a local switch or softswitch) requires M3UA.
M3UA is a Sigtran protocol designed for delivering SS7 MTP3-User Part messages, as well as supporting certain MTP network management functions over SCTP transport to a IP-based application endpoint. The M3UA signaling gateway terminates the SS7 MTP2 and MTP3 protocol layers and delivers ISUP, SCCP or any other MTP3 user protocol messages.
The application server process (ASP) is the Sigtran term for identifying a process or database from which the signaling gateway will send and receive M3UA traffic. In Figure 2, the ASP represents the IP endpoints of the Cisco PGW 2200.
The application server process (ASP) is the signaling gateway's component of a process or database (call agents and HLRs (Home Location Registers), for example) existing on an application endpoint.
In Figure 2, the STP on the right uses MTP1, MTP2, and MTP3 for transporting SCCP and ISUP messages into the network. The Cisco ITP-SG terminates the SS7 links, translates the MTP3 messages into M3UA messages, and transports them to the Sigtran-ready Cisco PGW 2200 over SCTP/IP. M3UA at the Cisco PGW 2200 delivers SCCP and ISUP in a manner equivalent to MTP3.
In this network relationship, the Cisco PGW 2200 Softswitch is referred to as the ASP. While the application server is actually a logical entity that is also located on the Cisco PGW 2200, the Cisco ITP-SG maintains an application server state machine that provides the routing context via a specific set of routing keys. These routing keys are used to examine signaling traffic and to make routing decisions. The ITP-SG (application server) can support one or many ASPs. M3UA routing key parameters include:
•Destination point code (DPC) (minimum required)
•Origination point code (OPC)
•ISUP circuit identifier code (CIC) range*
Note: Routing by CIC range and global title is not currently supported on the Cisco PGW 2200.
In most cases of interworking with the Cisco PGW 2200, only DPC and sometimes OPC will be used as routing keys. As a tandem/toll switch, DPC/OPC are generally sufficient; keeping the routing context simple helps to prevent errors in routing and simplifies operational management.
An ASP is defined as an SCTP endpoint of the Cisco PGW 2200. The ASP is a process instance of an application server—either an active or standby process.
The relevant traffic mode supported in the M3UA ASP is active-standby (override). The Cisco PGW 2200 (ASP) sends the "ASP active" message to the Cisco ITP-SG to indicate that it is ready to process signaling traffic for a particular application server. The ASP indicates its desired traffic mode in the active message. The other Cisco PGW 2200 (ASP) host serves as the standby system.
The override value indicates that the ASP will take over all traffic in an application server, overriding any currently active ASPs in the application server.
The call setup and teardown process is accomplished with a series of signaling messages sent between the Cisco PGW 2200 and the signaling network. Once such a message string is initiated, it is important that the rest of the string continue across the same route and equipment. To help ensure this process, the Cisco ITP-SG uses ASP bindings, which manage the signaling traffic and any associated management messages to follow a set path. In addition to maintaining local ASP bindings, the ITP-SG manager will maintain the state of the alternate's ASP bindings. The ITP-SG manager will make all binding establishment decisions for both ITP-SGs. Binding teardown decisions are made unilaterally. Received binding update checkpoint messages for newly active bindings are treated as request or response, based on the ITP's status. That is, a binding update (active) received by the ITP-SG manager is always a request and an update (active/inactive) received by the alternate is always a response.
A packet received on either Cisco ITP-SG that matches an ASP binding that is locally inactive but active on the other ITP-SG is rerouted to that ITP-SG. Such rerouting takes place regardless of the state of any other ASPs in the application server—rerouting is invoked even if the application server is locally active. This maintains the binding state.
MTP/M3UA Management Messages
MTP management messages are maintained between the M3UA and SS7 environment. Table 2 lists the corresponding messages.
Choice of Flexible, Scalable Platforms
Another benefit of migrating from the SLT to the Cisco ITP-SG is flexibility and scalability. The Cisco ITP-SG is available in several platforms, including the Cisco 2651XM, 7200VXR, 7301, 7507, and 7513 routers (Table 3). Service providers already using the Cisco 2651XM-based SLT can upgrade to Cisco ITP on the same platform by purchasing and loading the appropriate software license. The distributed technology allowing mated-paired ITP-SGs and the Cisco PGW 2200 to share a single point code is supported in all ITP platforms except the Cisco 7500 series. The Cisco 7500 Series platforms incorporate the ability to be fully hardware redundant.
ITP-SG platforms are typically deployed using mated-pair configurations which greatly enhance the system and overall availability.
The Cisco ITP can also be configured as an SUA signaling gateway, for signaling over IP (SoIP) and signaling over ATM (SoATM) transport, as a fully functional STP, SS7 probe, and a MAP (Mobile Application Part) proxy for RADIUS/HLR authentication. The Cisco ITP can simultaneously support signaling gateway and STP.
For more information about these features, visit:
Examples of Cisco ITP-SG Hardware/Software Configurations
Tables 4-6 are sample parts lists of Cisco ITP-SG configurations. Chassis, software, and memory are required. The port cards listed are options available for this product; choose whichever interfaces are required (for example, T1 ports). Additional information about the Cisco ITP can be found at:
Note: The Cisco 2651-ITP supports a maximum of four SS7 links and a maximum of two WICs or virtual WICs (VWICs). Cards supporting SS7 may be T1/E1 or serial. M3UA/SCTP/IP to Cisco PGW 2200 occurs via internal 10/100 Ethernet ports. While dual AC power is shown, Cisco ITPs are available in dual DC power as well.
Note: The Cisco 7204VXR-ITP and 7206VXR-ITP support a maximum of 24 SS7 links. The 7204 and 7206 can support four or six cards and a maximum of 48 T1 ports. Cards supporting SS7 may be T1/E1 or serial. M3UA/SCTP/IP to Cisco PGW 2200 occurs via internal 10/100 Ethernet ports. While dual AC power is shown, Cisco ITPs are available in dual DC power as well.
Note: Cisco 7301-ITP supports a maximum of 48 SS7 links, and a maximum of one 8-port adapter card. Cards supporting SS7 may be T1/E1 or serial. M3UA/SCTP/IP to Cisco PGW 2200 occurs via internal 10/100 Ethernet ports. While dual AC power is shown, Cisco ITPs are available in dual DC power as well.
Examples of Cisco PGW 2200/ITP-SG Configurations
This example uses A-links to terminate between the Cisco ITP-SG and STP. The redundant Cisco PGW 2200 and the mated-paired ITP-SGs all share one point code (Figure 3). The Cisco PGW 2200 operates in active/standby (override) mode. The ITP-SG operates in Interdevice Redundancy mode. Routing is by DPC only. In this case, the DPC is the point code shared by the ITP-SG and the Cisco PGW 2200. A-links are used because the SS7 links are between a virtual SSP and an STP.
"A-Link" Termination between ITP-SG and STP—Cisco PGW2200 and ITP-SG Share Point Code Using Distributed MTP3
This example uses F-links to terminate between the Cisco ITP-SG and SSP (Figure 4). The redundant Cisco PGW 2200 and the mated-paired ITP-SG all share one point code. The Cisco PGW 2200 operates in active/standby (override) mode. The ITP-SG operates in Interdevice Redundancy mode. Routing is by DPC only. In this case, the DPC is the point code shared by the ITP-SG and the Cisco PGW 2200. F-links are used because the SS7 links are between a virtual SSP and an "other" SSP.
"F-Link" Termination between ITP-SG and SSP—Cisco PGW2200 and ITP-SG Share Point Code Using Distributed MTP3 Feature
This example illustrates how the Cisco ITP can simultaneously function as an STP and a signaling gateway (Figure 5). The Cisco ITPs are configured to operate as both STPs and signaling gateways, and as such, each ITP has a unique point code. The redundant Cisco PGW 2200 shares a point code that is different from the ITP. The Cisco PGW 2200 operates in active/standby (override) mode. Routing is by the Cisco PGW 2200 DPC only. Links to the "other" SSP are F-links, and links to the STPs are B/D links.
ITP Functioning as both STP and Signaling Gateway—"B/D-Link" Termination between ITP and STP, "F-Link" Termination between ITP and SSP