Early Cisco SS7/IP Solution

Cisco was working on a SLT device before the SS7/IP IETF standardization efforts began. The Cisco SLT is a modular access router (Cisco 2611 or 2651) that terminates SS7 signaling links and backhauls MTP Level 3 and above to a PGW 2200 (formerly SC 2200 and VSC 3000) MGC. Figure 14-26 shows an example configuration of two Cisco SLTs providing SS7 termination and backhaul for the Cisco PGW 2200 Softswitch.

Figure 14-26. Cisco SLT Example

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The SLT supports either SS7 A-link or F-link configurations. As noted previously, some SS7 links are deployed with bearer channels that are provisioned on the time slots that are not used by signaling channels. The SLT supports a drop-and-insert feature, which allows the signaling channels to be groomed from the facility. The bearer channels are hair pinnned on the interface card that is to be sent to a MG. Figure 14-27 shows an example of the drop-and-insert feature.

Figure 14-27. Example of SLT Drop-and-Insert Feature

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Each 2611 SLT can terminate up to two SS7 links, and the 2651 SLT can terminate up to four links. Both have support for ANSI, ITU, TTC, and NTT variants. Several physical layer interfaces are supported on the SLT, including V.35, T1, and E1.

The SLT function can also be integrated into the MG, as is done on some of the Cisco universal gateways. The following Web site contains more information about the Cisco SLT:

http://www.cisco.com/en/US/products/hw/vcallcon/ps2152/products_data_sheet09186a0080091b58.html

To deliver the backhauled messages to the PGW2200 reliably, the SLT makes use of Reliable UDP (RUDP) and Session Manager (SM) protocols. A generic backhaul protocol layer is used to provide adaptation between MTP Level 2 and MTP Level 3. Figure 14-28 shows the protocol stacks used by the SLT and PGW2200.

RUDP is a simple packet-based transport protocol that is based on Reliable Data Protocol (RFC 1151 [148] and RFC 908 [149]). RUDP has the following features:

• Connection-oriented

• Guarantees packet delivery with retransmission

• Maintains session connectivity using keepalive messages

• Provides notification of session failure

The SLT maintains up to two RUDP sessions to each PGW2200 host. The use of two sessions provides for additional reliability because they provide for two different network paths between the SLT and the PGW2200.

The SM layer manages the RUDP sessions under control of the PGW2200. A single RUDP session is used to pass messages between the SLT and PGW2200 based on RUDP session availability and the PGW2200 hosts' Active/Standby state. The Active PGW2200 selects one or two possible RUDP sessions and indicates its selection to the SLT via the SM protocol.

The generic backhaul protocol layer is very similar to M2UA; it provides the same basic functionality for backhauling MTP Level 3 and above over IP to the PGW2200.