Calculate signaling capacity of an SS7 link based on link speed and message size.
SS7 link throughput in Message Signal Units per second is the raw link bandwidth divided by the average MSU size in bits — but standards bodies (ITU-T, ANSI) recommend operating signaling links well below their theoretical maximum, commonly around 40% fill factor, so that if a link or linkset fails, traffic rerouted onto surviving links doesn't overload them. This headroom is a deliberate resilience design choice specific to signaling networks, where a congested or failed signaling path can disrupt call setup for an entire trunk group even if the voice/bearer channels themselves are fine.
MSU throughput
MSU_per_sec = link_speed_bps / (avg_MSU_bytes × 8) × fill_factor
SS7 signaling links form linksets where traffic automatically reroutes to surviving links if one fails — running normal traffic at 40% means the remaining links can absorb a failed link's traffic (roughly doubling load on survivors in an A-B linkset) without exceeding capacity, since signaling congestion can disrupt call setup network-wide, not just for the failed link's calls.
MSU size varies by message type — ISUP call setup/teardown messages, TCAP transaction messages (e.g. for SCCP/database queries), and SCCP management messages all differ in payload size — a network with a heavier proportion of large database-query-style transactions will have a higher average MSU size and correspondingly lower MSU-per-second capacity for the same link speed.
SIGTRAN (M3UA/M2PA over SCTP/IP) decouples signaling from the traditional 64 kbps TDM link, allowing much higher-speed IP links to carry SS7 signaling — the same MSU-per-second math still applies, but with link speed potentially in the Mbps range rather than 64 kbps, vastly increasing capacity while retaining the SS7 message-level protocol semantics.