I will mention here though that these challenges
are partly what drove the 3GPP to begin development on IMS.
But more than just voice services rely on the backbone network. Signaling is also
transported through the backbone along with the voice traffic. Since signaling (SS7
specifically) is already packet data, there is no need to convert the signaling data to
packet. However, the SS7 protocol relies on transport protocols that were developed
for use on TDM facilities. The transport layers of SS7 (the Message Transfer Part) will
not support IP facilities and therefore must be replaced with a new transport protocol
before SS7 can be transported via IP.
Moving from Legacy to Convergence 31
This was the work of the IETF, which developed the SIGnaling TRANsport (SIGTRAN)
protocol. SIGTRAN replaces the MTP layers of the SS7 protocol with transport protocols
developed specifically for IP networks.
These protocols emulate the MTP services, over IP networks. This is necessary for a
number of reasons, QoS within the signaling networks being one of the primary drivers.
There are many procedures provided by level 2 and level 3 of the SS7 protocols to ensure
the availability of signaling links, as well as the integrity of the signaling data itself. One
of the simplest of these procedures is the transmission of a fill-in signal unit (FISU).
The FISU is really just a protocol flag carrying no real information.
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