SS7 over IP offers dramatic cost savings for carriers.

BY DAVID BARRY

Snapshot: Like other technologies that have moved from TDM to packet networks, SS7 over IP (SS7oIP) is gaining momentum. Dramatic advances across several fronts, from standards work in the Internet Engineering Task Force (IETF) to much higher levels of reliability and availability, are laying the groundwork. With the exceptional price decreases on port costs that wireless operators are enjoying with early implementations of SS7oIP, this market is poised for growth. Keywords: Cisco IP Transfer Point (ITP), SS7over IP, Stream Control Transmission Protocol (SCTP), TDM

FURTHER READING IETF SIGTRAN standards Next-Generation SS7 Networks with the Cisco IP Transfer Point white paper Cisco ITP

Signaling System 7 (SS7) -- long a staple of legacy time-division multiplexing (TDM) networks -- provides the call setup and other important signaling functions required by mobile wireless networks and the public switched telephone network (PSTN). Like other technologies that have moved from TDM to IP packet networks, SS7 over IP (SS7oIP) is quickly gaining momentum. Dramatic advances across several fronts, from standards work in the Internet Engineering Task Force (IETF) to much higher levels of reliability and availability, are laying the groundwork. With the exceptional price decreases on port costs that wireless operators are enjoying with early implementations of SS7oIP, it's clear that the market is poised for growth.

SS7 is the ubiquitous standard used by phone companies worldwide to enable call setup, control, routing, billing, and information exchange. Defined by the International Telecommunications Union (ITU) Telecommunications Standardization Sector (ITU-T) and the American National Standards Institute (ANSI), SS7 defines the protocols and procedures by which elements of the PSTN exchange information over a digital signaling network. Traditionally, the SS7 network has comprised an out-of-band series of dedicated links that are bidirectional 56- or 64-kbps. These links have typically run over leased-line TDM networks.

In recent years, however, SS7 has increasingly taken on the burden of handling more than call control signaling. New features built into the SS7 protocols have allowed other capabilities such as subscriber authentication, text messaging, and number portability (the ability of consumers to switch service providers and keep the same phone number).

FIGURE 1: As service providers evolve their SS7 signaling transport to IP, they greatly reduce operating expenses. Here, Cisco ITPs are decentralized and collocated at the SEP locations. This approach eliminates leased lines between ITPs and HLRs throughout the network. PowerPoint version General networking icons

One of the most widely used features in wireless networks is that of the Short Message Service (SMS). Delivered to subscribers as "overlay" features in the SS7 transport network, SMSs are the short text messages that subscribers send and receive over their cell phones, personal digital assistants (PDAs), or laptops. Due to its growing popularity, particularly in Europe and Asia, many wireless service providers are realizing an increasingly significant percentage of their total revenues from SMS. The Global System for Mobile Communications (GSM) Association estimates that SMS messages have increased from a few million a day in the mid-1990s to more than 1 billion per day in 2002.

The Operator's Choice

Wireless operators confronted with the need to accommodate escalating data growth while reducing operational and capital costs on their SS7 overlay networks have two options. They can add new, very costly links based on legacy TDM platforms from the traditional telco vendors. Or they can move the SS7 traffic to their IP networks using an SS7oIP solution such as the Cisco IP Transfer Point (ITP) gateway.

"For years, wireless operators have been content to add legacy platforms because it was what they had always done -- it was standard practice -- and SS7oIP was still being developed," says Larry Lang, vice president and general manager in Cisco's Mobile Wireless Group. "Today, that is no longer the case. SS7oIP is established and being used in the field. It is reliable and offers high availability." Many of Cisco's largest wireless operators are deploying Cisco ITP today. The most compelling reason? Cost. "Adding a new link using a legacy platform costs around [US]$7500," explains Lang. Using a Cisco ITP 7500 platform and Cisco ITP, wireless operators are now adding new links for $500 to $1500 per link."

Wind Telecomunicazioni S.p.A. is one of Italy's first integrated service providers and now offers fixed line, mobile, and Internet services. Wind recently completed the first phase of installing Cisco ITP platforms in nine cities throughout Italy. In the first phase, the Cisco ITP platforms are performing as traditional SS7 signaling transfer points (STPs) but are providing Wind "with an immediate 25 percent capital expense savings," according to Luca Monti, mobile and SS7 network engineering manager at Wind.

"We needed to add capacity and the option to continue with legacy platforms no longer made sense," says Monti. "The legacy vendors were trying to add IP capabilities to their old SS7 platforms, but it wasn't a scalable solution because it was based on old technology. The Cisco ITP is the only product that combines SS7 routing capabilities and full-featured IP routing in the same platform. This means we didn't need to add additional routers on the edge, which would have been necessary with a legacy platform."

"What's more," says Monti, "the integration of IP with SS7 in one platform means space savings, which is extremely valuable as our real estate costs are very high. We estimate this gave us an additional 10 percent savings on top of our greatly reduced capital expenses. Also important, the Cisco ITP essentially Ôfuture proofs' our network because it is designed to evolve into a full participant in our Cisco end-to-end IP network."

From a transport standpoint, the Cisco ITP provides unparalleled options by allowing the use of traditional TDM, higher-speed ATM, and very high-speed and efficient advanced IP transport media. Functionally, the Cisco ITP can play many roles in an operator's network. Initially, the Cisco ITP can be deployed as a traditional TDM STP. At any time, the operator can decide to enable ATM high-speed links (HSLs) or IETF Signaling Transport (SIGTRAN) Working Group-based IP transport in the network core or to signaling end-point sites. As the operator adds IP-enabled applications to the network, the same ITP can be used to provide the SIGTRAN Signaling Gateway functionality to interconnect the TDM and IP networks. Together, these important attributes combine to provide a highly flexible signaling solution.

Among the options that operators can choose as they evolve their SS7 signaling are four possible paths:

• Lowering capital expenses by supplementing the existing SS7 transport plane with the Cisco ITP

• Introducing high-speed links into the SS7 core and edge

• Introducing IP links into the SS7 network core

• Maximizing operations efficiencies via IP-enabled service end points (SEPs)

Lowering Capital Expenses

For those operators that are solely interested in cost-efficiently expanding their SS7 transport capacity, the first option is to deploy the Cisco ITP as a traditional STP. In this scenario, the Cisco ITP (running on the Cisco 7500 Series Router) is installed rather than continuing to install legacy platforms that will soon be obsolete with 3G. The Cisco ITP supports all traditional STP functions. Cisco ITP runs a standard and full-functioned SS7 STP stack and provides carrier-class reliability while delivering significant performance and cost efficiencies -- without forcing architectural changes. The operator can continue to use legacy TDM circuits such as T1s or E1s, but its capital expenses are slashed; it is now able to install new links for under US$1000 per link.

Introducing High-Speed Links

While swapping a legacy platform with a Cisco ITP slashes capital costs, the operator is still reliant on slow-speed TDM links. This is due to the 16-link per linkset limitation of the SS7 protocol, which might result in possible bandwidth bottlenecks between ITP platforms or between ITPs and SEPs such as home location registers (HLRs) and SMS centers.

FIGURE 2: Wireless providers can evolve their SS7 signaling to IP as they choose -- from standard SS7 to higher-speed ATM to very high-speed IP. The process is efficient because Layer 3, the MTP3 protocol, which handles routing and contains high-availability intelligence, remains unchanged. PowerPoint version General networking icons

With the Cisco ITP platforms, operators can replace low-speed TDM links in the core and the access layer with high-speed links without requiring SS7 network architectural changes. To do so requires a swapping of the traditional SS7 Layer 2 protocol with a higher-speed Layer 2 ATM protocol, the HSL-ATM (AAL5, SCCOP, and SSCF-NNI), as shown in Figure 2. This can be accomplished because Layer 2 in the SS7 protocol architecture mainly provides reliable and sequenced message service unit (MSU) delivery. All routing and high-availability intelligence is located at the SS7 Message Transfer Part Layer 3 (MTP3) layer, which remains largely unchanged.

Figure 2 shows the traditional SS7 protocol stack (MTP1 and MTP2) compared with the protocol stack used by a Cisco ITP to deliver higher-speed links (running HSL-ATM or the SIGTRAN IP protocols). Changing Layer 1 and 2 requires replacing MTP2 and MTP1 with the reliable MSU delivery layers of HSL-ATM. The MTP3 and SCCP layers are unchanged. The result is the ability to add high-speed links without making network architectural changes while increasing linkset bandwidth capacity.

IP Links in the SS7 Network Core

The next step for operators is to deploy IP in the SS7 network core. Typically, several drivers are identified for deploying IP links in the signaling core. One is that after an IP core is built, it is extensible, flexible, and reusable. Another key driver is that IP core transport is more efficient than TDM links. IP bandwidth can be shared by all traffic, whereas TDM bandwidth can be used only by two adjacent nodes. IP links can also be used to carry any type of signaling traffic through the IP core. Some operators have seen up to 50 percent significant savings in leased line costs by offloading SS7 traffic to IP links versus transporting over traditional TDM bandwidth.

The protocol stack used for IP communications was developed by the IETF SIGTRAN Working Group (see sidebar) and includes, most prominently, Stream Control Transmission Protocol (SCTP) and an adaptation layer on top of SCTP -- MTP-User-Peer-to-Peer Adaptation Layer (M2PA). M2PA and SCTP provide MTP3 with reliable transport-layer services equivalent to MTP2. M2PA/SCTP/IP is simply an SS7 Layer 1 and 2 substitution. IP links may run over traditional T1 or E1 facilities, as well as other IP LAN or wide-area transport types, including Fast Ethernet and Gigabit Ethernet. The introduction of IP for signaling transport can be a compelling solution for operators that want to reduce wide-are costs such as leased lines. For operators that already have an IP network in place -- using their General Packet Radio Service (GPRS) IP network, for example -- the cost efficiencies can be realized even faster.

Maximize Operations Efficiency via IP-Enabled SEPs

In the final phase, operators can extend the IP network to SEP sites to eliminate TDM leased lines. In this distributed design, the Cisco ITP platforms reside at the SEP sites. Costly leased lines are no longer required to connect the once-remote Cisco ITPs to SEP devices such as the HLR or the SCPs; a short cable is all that is required.

In this design, all classical SS7 architecture principles continue to apply, such as mated-pair transfer points, load balancing over "costed" routes, and combined/alternate link sets. For operators without current STPs, this design is a clean entry point into deploying a next-generation signaling architecture and allows these operators to immediately realize the benefits of IP.

As wireless operators confront ever-increasing bandwidth demands on their SS7 signaling overlay network, they must add capacity. But continuing on the legacy platform path is no longer advisable because of cost and obsolescence. With the Cisco ITP, operators can realize dramatic benefits -- an estimated ten-to-one improvement in SS7 port cost over traditional legacy solutions, according to Cisco estimates. They'll also see operational efficiencies by taking advantage of the shared bandwidth of IP core networks. And yet they will be able to protect their existing investment in legacy signaling while also deploying the IP infrastructure for tomorrow's applications.

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