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The Reliable Transport of IP-Based SS7 over Satellite

Tue, 07/08/2003 - 9:10am

By Jason Lawlor, Performance Technologies

Emerging IP technologies are making it possible for carriers and service providers to lower transport costs, increase robustness and broaden geographic reach by reliably sending IP-based SS7 messages over satellite. The Stream Control Transmission Protocol (SCTP) from the Internet Engineering Task Force's Signaling Transport Group (IETF SIGTRAN) is one such standard making this possible.

SCTP was implemented specifically to provide a reliable method of transport for SS7 traffic over IP. Transport and transmission utilizing the SCTP protocol provides sequence numbering, message acknowledgement, message retransmission, multi-homing, multi-streaming and fast-failure detection. All of these new capabilities work to alleviate the quality of service and latency issues that can occur with SCTP's predecessor, TCP.
SCTP Answers the Call

Prior to the introduction of SCTP, there was no reliable way to transport SS7 traffic over IP. While TCP provides both reliable data transfer and strict order of transmission delivery of data, there are several quality of service and latency issues involved with TCP transport. Also, some applications require reliable transfer without sequence maintenance and others require only partial data ordering. Both of these cases cause unnecessary delays because of the head-of-line blocking of TCP. SCTP helps to specifically alleviate such problems.

Because the TCP protocol is stream-oriented, applications using this protocol must create their own record marking to delineate messages. There is also a limited scope of TCP sockets, which can make it difficult to provide highly available data transfer using multi-homed hosts. Each of these limitations can cause significant problems when transporting PSTN signaling across the IP network. The multi-streaming capabilities of SCTP provide the ability to multiplex data from a number of upper layer applications onto one channel called an association. Congestion control is then applied to the association rather than the individual data streams. To help avoid head-of-line blocking, data sequencing is performed within a stream, so that if a packet belonging to a particular stream is lost, subsequent packets from the same stream will be stored in the receiver's stream buffer until the lost packet is retransmitted from the source. And unlike with TCP, data from other streams can still be passed to upper layer applications.
oving Reliability and Costs

concept of multi-homing allows separate physical paths to carry the same data. Two physical IP networks can be used to provide redundancy while the protocol ensures the data arrives at the correct destination via the available path. This can improve the reliability of satellite transmission by providing network layer redundancy over multiple satellite links. In the past, with standard TDM circuits, the inherent delays associated with satellite transmission were addressed with a feature called preventive cyclic retransmission (PCR). As a satellite moves relative to the earth, timers that are established on a message transmission can be exceeded regardless of the health of the link. PCR is used to retransmit all unacknowledged messages during idle periods.

SS7 over IP over satellite transmission is now becoming increasingly popular due to the ever-expanding geographic reach of wireless services and the need to deploy these services in remote regions around the globe in a cost effective manner. Since SCTP can help alleviate the latency issues associated with satellite transmission, it enables satellite transmission to be a cost-effective alternative for providers looking to improve legacy solutions. SS7 traffic can be packetized without the need to replace legacy, TDM-based SS7 network components. Devices exist that can encapsulate the traditional SS7 traffic into a packet and transport it via an IP environment to its end destination. There, it can be turned back into standard SS7 traffic so legacy SS7 equipment can process the message.

Conclusion

of the same problems exist with sending SS7 messages over IP over satellite as with sending SS7 over TDM over satellite. The SCTP protocol can actually help increase the robustness of SS7 over IP on satellite links. SCTP's multi-streaming features provide an effective and reliable solution for dealing with the inherent delays of satellite transmission that aren't typically present or well-tolerated in a standard terrestrial SS7 TDM-based network. This new technology capability can also help with packet loss issues by providing guaranteed delivery, thus dramatically improving the reliability of SS7 links over satellite. The IETF's SIGTRAN working group is continually working on developing new standards that will make SS7 over IP an even more beneficial proposition in the months and years to come.

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