Everything worth knowing, nothing that isn't.
How SIGTRAN works, layer by layer
SIGTRAN keeps the SS7 application stack — MAP over TCAP over SCCP — completely unchanged and swaps out only the bottom transport. Where classic SS7 used MTP1/MTP2/MTP3 over TDM timeslots, SIGTRAN runs SCTP over IP and inserts an adaptation layer that emulates the MTP service the upper layers expect. An SMSC, HLR, or firewall speaks exactly the same MAP it always did; it just reaches its peer over an IP association instead of a 64 kbit/s link. See /answers/what-is-sigtran and /answers/sigtran-adaptation-layers.
SCTP: the transport underneath
SIGTRAN does not use TCP. It uses SCTP (Stream Control Transmission Protocol, RFC 4960) because signaling needs three things TCP lacks: it is message-oriented rather than byte-stream, it multiplexes several independent streams to avoid head-of-line blocking, and it is multi-homed — an association can span two IP paths and fail over instantly if one drops. Those are exactly the resilience properties a carrier control plane requires. See /answers/what-is-sctp.
The adaptation layers: M3UA, M2PA, M2UA, SUA
The adaptation layer decides which MTP boundary is emulated. M3UA presents the MTP3 service to SCCP users and is what most operators run to reach SS7/MAP peers today. M2PA links two signaling points as if by an MTP2 link (for IP-based STPs). M2UA backhauls raw MTP2 from a media gateway to a signaling gateway. SUA carries SCCP users directly over SCTP. See /answers/what-is-m3ua and /answers/what-is-sccp.
Benefits over TDM SS7
Far higher bandwidth per association than a 56/64 kbit/s TDM link, cheaper commodity IP transport, built-in multi-homed failover from SCTP, and simpler scaling — you add capacity by adding associations, not by cabling more E1/T1 links. SIGTRAN is why an SMSC gateway can front thousands of SMPP binds and still reach the SS7 world at volume. See /operators/smsc-gateway.
Limitations and the security implication
Moving SS7 onto IP inherits IP's exposure: a signaling endpoint reachable over IP is reachable by anyone who can route to it, so SIGTRAN interconnects widened the SS7 attack surface that GSMA FS.07/FS.11 were written to close. SCTP is also less widely supported by commodity firewalls and NAT than TCP, so associations need careful network design. The mitigation is a signaling firewall inspecting the MAP riding on top. See /operators/ss7-firewall and /answers/gsma-fs07-fs11-signaling-firewall.
Is SIGTRAN the same as SS7?
No — SIGTRAN is not a replacement for SS7, it is a transport for it. The SS7 application protocols (MAP, CAP, ISUP, TCAP, SCCP) are identical; SIGTRAN only carries them over IP instead of TDM. So an SMS delivered over a SIGTRAN link is still an SS7 MAP MT-ForwardSM — which is why an SS7 firewall inspects SIGTRAN and TDM traffic the same way. BroadNet runs SIGTRAN-based interconnects behind 1,000+ operator connections. See /answers/ss7-map-explained.