Cisco - Cisco Signaling Link Terminal (SLT) Data Sheet

Most of the new telephony services deployed today depend on the power and reliability of Signaling System 7 (SS7). The global deregulation and privatization of the telecommunications industry, the proliferation of Intelligent Networks and Advanced Intelligent Networks (AINs), and the convergence of voice and data networks have spurred several trends in SS7 networking:

Clearly, SS7 plays a critical role in the advancement of service-provider network architectures. The Cisco Signaling Link Terminal (SLT) enables service providers to reliably transport or backhaul SS7 protocols across IP-based packet networks.

The Cisco SLT is an integral part of the Cisco PGW 2200 PSTN Gateway architecture, and acts as the interface between the SS7 network and the Cisco PGW 2200.

Why Interconnect Using SS7 Signaling?

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Reduced recurring charges—In general, tariffs favor interconnection using SS7 signaling. Inter-machine trunks (IMTs) are less expensive than ISDN-based facilities, both on a one-time (installation and provisioning) and recurring (monthly charge) basis. This equates to lower monthly expenses, reduced cost of goods sold, and higher margins for service providers.

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Opportunity to enter new markets— Many countries will not permit alternative carriers such as competitive local exchange carriers (CLECs) and Internet telephony service providers (ITSPs) to interconnect to the Public Switched Telephone Network (PSTN) with signaling methods other than SS7.

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Network scalability—SS7 signaling offers superior scaling over other signaling methods such as R2, Primary Rate Interface (PRI), and channel associated signaling (CAS).

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Compatibility with other Cisco signaling methods—Cisco solutions enabled by SS7 are compatible with most signaling methods supported by Cisco Systems. Customers can rest assured that revenue-generating services supported by their VoIP network will continue to operate and be fully supported with the addition of the SS7 signaling option.

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Faster customer response time— Telecommunications operations tend to be optimized around IMT provisioning. Therefore, IMTs can generally be provisioned more quickly than PRI, CAS, or R2 trunks. SS7 enables service providers to contract for IMTs as opposed to PRIs, CAS, or R2 trunks.

Cisco PGW 2200 Node and Signaling Backhaul Architecture

Figure 1 depicts the architecture of a Cisco PGW 2200 node. The node consists of the Cisco SLTs and redundant UNIX hosts interconnected via an IP signaling control network.

In Figure 1, access ("A") links or fully associated ("F") links from the SS7 network are physically connected on the Cisco SLT via one of several supported interface cards. The Cisco SLT terminates Message Transfer Part (MTP) Layers 1 and 2 of the SS7 protocol stack. Because MTP 2 is a message- and processor-intensive layer of SS7 signaling, terminating it on the Cisco SLT frees the Cisco PGW 2200 from wasting cycles on lower-layer functions.

Using the Cisco Reliable User Datagram Protocol (RUDP), the Cisco SLT backhauls the upper-layer SS7 protocols across an IP signaling control network to the Cisco PGW 2200. Cisco RUDP is a simple, connection-oriented, packet-based, transport protocol based on RFC 908 (Reliable Data Protocol) and RFC 1151 (Version 2 of the Reliable Data Protocol).

The Cisco SLTs use the Cisco IOS^Ž Software SS7 Signaling Link Terminal feature set, providing reliable interoperability within the Cisco PGW 2200 node.

Cisco Session Manager software manages the communication sessions with the Cisco PGW 2200. When the Cisco SLT feature is used with a redundant pair of controllers, the Cisco Session Manager software maintains separate communication sessions with each controller in the pair. The session between the Cisco SLT and the active controller transports the SS7 traffic, while the session between the Cisco SLT and the standby controller provides backup. Cisco Session Manager software uses Cisco RUDP to communicate between the Cisco SLT and the Cisco PGW 2200. Figure 2 illustrates this concept.

Standalone and Integrated SLT Options

Designed to meet the individual needs of customers, the Cisco SLT is available in both standalone and integrated configurations. The standalone product runs a special-purpose Cisco IOS Software image on the Cisco 2611 and Cisco 2651 multiservice platforms specifically designed to terminate SS7 signaling links. When used with one of the many supported WAN interface cards (WICs) or voice WAN interface cards (VWICs), the standalone Cisco SLT provides a high-performance, economical product with which to terminate SS7 facilities. Figure 3 illustrates the standalone Cisco SLT architecture.

The Cisco AS5350, Cisco AS5400, and Cisco AS5400HPX universal gateways offer, as an option, an integrated version of the Cisco SLT. In this case, the universal gateways function as network access servers or voice gateways, or both, and SLTs. SS7 signaling links are terminated directly on the universal gateway as well as with IMTs from the PSTN. Figure 4 illustrates the Cisco integrated SLT architecture.

The integrated SLT option is appropriate for any VoIP network deployment, but is well suited for smaller, highly distributed networks requiring many small points of presence (POPs) or local interconnect points. Combined with Sun Netra 20s as the Cisco PGW 2200 active and standby hosts and a Cisco AS5350 acting as the universal gateway and SLT, it is possible to build a "micro-POP" that can be deployed in as little as three rack units (3 RUs).

Supported SS7 Network Interconnect Methods Using the Cisco SLT

When the Cisco PGW 2200 is used in Cisco VoIP networks, the solution will support common signaling interconnect methods as well as features that exploit the power and savings of transporting SS7 signaling over IP-based networks.

"A" Links

Access or "A" links are used between the service switching point (SSP), served by the Cisco PGW 2200, and the signaling transfer point (STP) to connect the VoIP network to the PSTN. These links are dedicated to signaling, meaning that no bearer traffic is provisioned on the facility, even if there are available time slots not being used by signaling. For reliability, there are generally, at a minimum, two "A" links provisioned between the SLTs and the home STPs. While most commonly seen in North America, "A" links are used in other geographic areas such as Asia and South America, depending on availability of facilities.

"F" Links

Fully associated or "F" links are generally deployed when a large volume of traffic exists between two SSPs (between the Cisco PGW 2200 and a PSTN switch, for example) or when it is not feasible to connect directly to an STP. "F" links are not commonly used in North America, but they are widely deployed in Europe, Asia, and South America. While "F" links can be provisioned with only signaling on the facility, the most common configuration includes the provisioning of bearer channels on time slots not used by SS7 signaling.

Drop-and-Insert Option

The "drop-and-insert" application feature, also known as time-division multiplexing (TDM) cross-connect, allows customers to deploy "F" links with both signaling and bearer traffic on the same facility. The drop-and-insert feature grooms the SS7 signaling channels from the facility and backhauls the signaling to a Cisco PGW 2200 for processing. Bearer channels are "hairpinned" on the Cisco SLT interface card and sent to a voice gateway by connecting a cable between the egress port on the Cisco SLT voice WAN interface card (VWIC) and an available T1 or E1 port on a gateway.

The integrated SLT offers internal drop-and-insert, which requires no external cabling. All bearer channels are internally hairpinned on the gateway and sent to the VoIP network. Figure 5 depicts both the Cisco SLT and integrated SLT drop-and-insert architectures.

Using the VWIC "Drop-and-Insert" Cards to Terminate Four "F" Links

The Cisco SLT that is based on the Cisco 2651 Multiservice Platform can terminate up to four SS7 "A" or "F" signaling links. The exact configuration used to support four links is determined by the WICs chosen to terminate the physical facilities and the SS7 signaling links within those facilities.

While configuring four links on most of the available WICs and VWICs is intuitive, terminating four "F" links on the Cisco SLT based on the Cisco 2651 requires careful consideration. Generally, "F" links are provisioned with bearers on the available channels not being used by SS7 signaling. Separating the bearer channels from the SS7 signaling channels requires a de-multiplexing function, either internal or external to the Cisco SLT. While external de-multiplexing is possible, it is generally more cost-effective to use the drop-and-insert function available on the interface cards with part numbers VWIC-2MFT-TI-DI and VWIC-2MFT-E1-DI.

To terminate four "F" links with bearers on the Cisco SLT based on the Cisco 2651 requires installing two of the WICs enabled with the drop-and-insert function (the VWIC-2MFT-TI-DI or VWIC-2MFT-E1-DI) in the two available WIC slots (card slots W0 and W1) on the Cisco SLT. Two "F" links, configured with two signaling channels per facility, are terminated on the SLT on each of the WICs. Figure 6 illustrates this concept.

In situations where interconnect regulations permit the provisioning of only one signaling channel per facility on an "F" link, the Cisco SLT based on the Cisco 2651 is restricted to supporting a maximum of two "F" links.

Remote SS7 Signaling Backhaul

The Cisco SLT supports both colocated and remote termination of SS7 signaling links. With colocated configurations, the Cisco SLTs are physically located with the Cisco PGW 2200, and are connected to the host via a LAN. Remote configurations are defined as deployments where the SS7 signaling is backhauled over an IP-based WAN from an SSP or STP to the Cisco PGW 2200. Figure 7 illustrates this concept.

The ability to backhaul SS7 signaling over a wide-area, IP-based network offers cost-savings benefits, including:

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Lowering the overall, upfront cost of deploying VoIP networks by reducing equipment costs, especially for rollouts of multiple or distributed POPs over a wide geographic area

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Dramatically reducing recurring telecommunications costs by eliminating dedicated, point-to-point SS7 signaling links

In many countries, the cost of dedicated signaling links can be prohibitive. Dedicated signaling links may also be unavailable due to facilities shortages. With the Cisco SLT, customers can now take advantage of IP-based WANs to backhaul SS7 signaling from STPs and SSPs to the Cisco PGW 2200.

Table 1 Cisco SLT Features
Feature	Description
Physical layer interfaces (MTP Layer 1)	•Cisco SLT (2611-based) –Terminate up to two 64-Kbps or 56-Kbps SS7 signaling links –T1, E1, V.35, RS-449, or RS-530 physical interfaces to the SS7 network –Up to two SS7 signaling links can be supported per T1 or E1 port •Cisco SLT (2651-based) –Terminate up to four 64-Kbps or 56-Kbps SS7 signaling links –T1, E1, V.35, RS-449, or RS-530 physical interfaces to the SS7 network –Up to two SS7 signaling links can be supported per T1 or E1 port •Integrated SLT (Cisco AS5350, Cisco AS5400, and Cisco AS5400HPX) –Terminate up to four 64-Kbps or 56-Kbps SS7 signaling links –CT-3, T1, E1, and V.35 physical interfaces to the network –Up to four SS7 signaling links can be supported per CT-3, T1, or E1 port
Terminates processor-intensive MTP Layer 2 functions	•Link-state control (LSC) provides the overall coordination of the session •Initial alignment control (IAC) provides the link alignment processing •Transmit control provides transmit flow control and processing •Receive control provides receive flow control and processing •Congestion control provides congestion onset and abatement processing •Signal unit error rate monitor (SUERM) provides monitoring of signal unit events •Signal unit delimitation detects individual signal units •Signal unit alignment enforces signal unit encoding rules and bit patterns •Error detection detects bit errors in signal units by using the cyclic redundancy check (CRC) field •Error correction uses positive and negative acknowledgments and retransmits errored signal units •Alignment error rate monitor (AERM) monitors link alignment errors
Standalone Cisco SLT supports multiple serial interface cards (multiflex serial interface cards)	•1-port T1 multiflex trunk interface (VWIC-1MFT-T1) •1-port E1 multiflex trunk interface (VWIC-1MFT-E1) •2-port T1 multiflex trunk interface (VWIC-2MFT-T1) •2-port E1 multiflex trunk interface (VWIC-2MFT-E1) •2-port T1 multiflex trunk interface with drop-and-insert (VWIC-2MFT-T1-DI) •2-port E1 multiflex trunk interface with drop-and-insert (VWIC-2MFT-E1-DI) •1-port high-speed serial interface (WIC-1T) •2-port high-speed serial interface (WIC-2T) •1-port serial with 4-wire 56/64-Kbps data service unit/channel service unit (DSU/CSU) interface card (WIC-1DSU-56K4)
Feature-rich multiflex interface cards	•Single or dual port, T1 or E1 capability •E1 versions support both balanced and unbalanced modes •Physical-layer alarm-forwarding feature between the two T1/E1 ports on dual-port cards •Drop-and-insert (also called TDM cross-connect) between the T1/E1 ports on dual-port cards, used to hairpin bearer channels to a media gateway device and allow the interchange of TDM slots between the ports on a 2-port card •Shared between Cisco 2600 and 3600 series for common inventory sparing for various network applications
Integrated SLT	•Cisco AS5350 Universal Gateway –2-port T1 or E1 PRI card –4-port T1 or E1 PRI card –8-port T1 or E1 PRI card –CT-3 card •Cisco AS5400 and AS5400HPX universal gateways –2-port T1 or E1 PRI card –4-port T1 or E1 PRI card –8-port T1 or E1 PRI card –CT-3 card
Platform support	•Standalone SLT –Cisco 2611 and 2651 multiservice platforms •Integrated SLT –Cisco AS5350, Cisco AS5400, and Cisco AS5400HPX universal gateways
Deployment configurations	•Colocated—The Cisco SLT is physically located with the Cisco PGW 2200 host controller and is connected to the node via a LAN •Remote—The Cisco SLT is physically located somewhere other than with the Cisco PGW 2200 (generally with the STP or SSP), and SS7 signaling is backhauled to the node via an IP-based WAN
Remote SLT general deployment guidelines	•End-to-end delay (one way) must be less than 150 ms •Packet loss must not exceed one percent (preferably below 0.5 percent) •Less than one E-6 line error •Cisco SLT remote guidelines available at: http://dial.cisco.com/design-guides/index.html •The CiscoWorks Internetwork Performance Monitor (IPM) is highly recommended to measure the quality of IP-based, wide-area backhaul networks
Cisco IOS Software Release	•Standalone SLT –12.2(11)T or later release –Available at www.cisco.com •Integrated SLT –12.2(11)T or later release –Available at www.cisco.com
Memory requirements	•Standalone SLT –Minimum required DRAM and Flash to support the standalone Cisco SLT feature set are 48-MB DRAM and 16-MB Flash •Integrated SLT –Minimum required DRAM and Flash to support the integrated Cisco SLT feature set are 256-MB DRAM and 32-MB Flash
Certification	•Standalone SLT –Network Equipment Building Systems (NEBS) Level 3 and European Telecommunication Standards Institute (ETSI) compliance kit including 23- or 24-in. rack mounts, grounding lug kit, shielded LAN cables, and bezel removal kit (for additional unit depth reduction) •Integrated SLT –NEBS Level 3 –European Telecommunication Standards Institute (ETSI)

Cisco SLT Platform Information

Cisco 2600 Series Multiservice Platforms

In combination with this application-specific version of the Cisco IOS Software, the Cisco SLT hardware component uses the widely deployed Cisco 2600 Series multiservice platforms. The Cisco 2600 Series, driven by a powerful Reduced Instruction Set Computer (RISC) processor, provides the high-performance routing required in today's networking infrastructures.

The Cisco 2600 Series meets service providers' critical physical requirements for equipment depth that fits next to transmission equipment on standard 12-inch-deep rack with a one-rack-unit (1 RU) height. NEBS compliance is assured by using the NEBS/ETSI kit included with the Cisco SLT. Common Language Equipment Identifier (CLEI) coding is provided for easy identification and tracking of central-office (CO) equipment. Options for either AC or DC power are available.

It is important to note that the standalone Cisco SLT supports only the SS7 MTP 2 Serial Protocol. Therefore, the serial interfaces cannot be configured for other protocols. It is also important to note that the Cisco SLT is not an SS7-over-IP router. It can only be used as a part of the Cisco PGW 2200 node to backhaul higher-layer SS7 protocols over the node IP signaling control network.

For comprehensive details about the Cisco 2600 Series multiservice platforms, please visit:

Cisco AS5350, AS5400, and AS5400 HPX Platform Specifications

The Cisco AS5350 Universal Gateway is the only one-RU gateway supporting two, four, and eight T1/E1 configurations providing universal port access for voice, data, and fax services on any port at any time. The Cisco AS5350 takes advantage of the full-feature richness and strong routing capabilities of Cisco IOS Software. The Cisco AS5350 can be deployed in a variety of network architectures from corporate to service-provider networks.

The Cisco AS5350 offers hot-swap capability on all cards and the fan tray, allowing hardware maintenance to be performed on active access servers with little or no service interruption. During a hot swap, any card may be removed, inserted, or replaced, affecting only calls on the card being removed.

The Cisco AS5350 complies with NEBS Level 3 requirements, as defined by Telcordia SR-3580, and also complies with European requirements as defined by the ETSI.

The Cisco AS5400 and Cisco AS5400HPX universal gateways are the highest-density, carrier-class access servers on the market, offering unparalleled capacity in only two RUs. Their high density (up to one CT3 or 14,112 ports in a seven-foot rack), low power consumption (6.6A at 48 volts DC per CT3), and Universal Port readiness make them ideal for many network deployment architectures, especially colocation environments and "mega-POPs."

As carrier-class access servers, the Cisco AS5400 and Cisco AS5400HPX have hot-swappable cards, an internal redundant power supply, and environmental monitoring. Their rich set of Cisco IOS Software features enables Internet service providers (ISPs) and enterprise network managers to meet traditional dial-in needs, while supporting the migration to newer technologies.

The Cisco AS5400 and Cisco AS5400HPX comply with NEBS Level 3 requirements, as defined by Telcordia SR-3580, and also comply with European requirements as defined by the ETSI.

For comprehensive details about the Cisco AS5400 and Cisco AS5400HPX, please visit:

When used with the Integrated SLT option, the Cisco AS5350, Cisco AS5400, and Cisco AS5400HPX universal gateways function as both an SS7 signaling link termination point and universal gateway. The SLT option on these gateways is not an SS7-over-IP router. It can only be used as a part of the Cisco PGW 2200 node to backhaul higher layer SS7 protocols over the node IP signaling control network.

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Data Sheet

Cisco Signaling Link Terminal