Wireless Systems Design

Satellites Promise To Reliably Transport IP-Based SS7

Increasingly, cellular carriers and service providers are using IP networks to transport and route Signaling System 7 (SS7) traffic. The SS7 protocol boasts a mechanism for offloading the traditional public-switched-telephone-network (PSTN) data-traffic congestion onto a wireless or wireline digital-broadband network. Compared to transportation over dedicated circuit-switched networks, this alternative appears reliable and inexpensive. The ubiquity of IP has even helped carriers find ways to lower operating expenses. In addition, they can leverage existing data networks that may be currently underutilized.

By using satellites to transport their SS7 signaling messages, carriers and service providers hope to access new markets. A robust method for transporting IP-based SS7 over satellite can be found in the IP protocols that hail from the Internet Engineering Task Force's Signaling Transport Group (IETF SIGTRAN). The Stream Control Transmission Protocol (SCTP), for example, defines IP-based, SS7 satellite transmission.

Transport and transmission that utilize the SCTP protocol provide sequence numbering, message acknowledgement, message retransmission, multi-homing, multi-streaming, and fast-failure detection. By using this protocol in conjunction with satellites, providers can optimize their existing IP connections. They may then alleviate the quality-of-service and latency issues that occur with SCTP's predecessor, TCP.

Due to the ever-expanding geographic reach of wireless services, SS7 over IP over satellite transmission is now becoming increasingly popular. Another driver is the need to cost effectively deploy these services in remote regions around the globe. SCTP also can help alleviate latency issues. As a result, satellite transmission is now attracting providers that are looking to improve upon legacy systems.

Devices now exist that can packetize SS7 traffic and transport it via an IP environment to its end destination. There, it can be turned back into standard SS7 traffic. Legacy SS7 equipment can then process the message. All of these steps can be completed without having to replace legacy, TDM-based SS7 network components.

Prior to SCTP's introduction, there was no reliable way to transport SS7 traffic over IP. Although standard TCP was good for most data, it has some shortcomings when it's used as the primary method of transporting signaling messages from the PSTN. TCP, on the one hand, provides reliable data transfer and strict order-of-transmission data delivery. Unfortunately, it also has several quality-of-service and latency issues. Some applications require reliable transfer without sequence maintenance. Others require only partial data ordering. Furthermore, TCP sockets have a limited scope. TCP cannot efficiently support highly available data transfer using multi-homed hosts.

In contrast, SCTP supports multi-streaming—a feature that provides multiplexed data from a number of upper-layer applications onto one channel. That channel is called an "association." Congestion control is applied to the association rather than the individual data streams. One of the main concerns with TCP is head-of-line blocking. To avoid it, data sequencing is performed within a stream. If a packet belonging to a particular stream is lost, subsequent packets from the same stream will be stored until the lost packet is retransmitted. But data from other streams can still be passed to upper-layer applications.

The concept of multi-homing is a notable feature of SCTP. This technique allows separate physical paths to carry the same data. For instance, two physical IP networks can be used to provide redundancy while the protocol ensures that the data arrives at the correct destination via the available path. Thus, the single network interface is prevented from becoming a single point of failure in the communications path. In terms of SS7, this technique can improve the reliability of IP transmission. It provides network-layer redundancy over separate IP links.

As the march toward IP continues, the protocols and mechanisms that are used to transport SS7 data will continue to advance. Today, many of the same problems arise when either SS7 messages are sent over IP over satellite or SSM is sent over TDM over satellite. Fortunately, the SCTP protocol can help increase the robustness of SS7 over IP on satellite links. Its multi-streaming capability provides an effective and reliable solution for dealing with the inherent delays of satellite transmission. Typically, those delays aren't present or well tolerated in a standard, terrestrial, SS7 TDM-based network.

SCTP's multi-streaming capability also can increase the tolerance of packet-loss issues. It provides tools to help guarantee delivery and deal with the typical delays of satellite communication. This aspect, in turn, improves the reliability of SS7 links over satellite. Additionally, the IETF's SIGTRAN working group continues to develop new standards. Those standards will make SS7 over IP an even more beneficial proposition in the months and years to come.

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