So, what are we talking about here, encapsulation in the context of SS7-to-SIP gateways? Let’s have an analogy; it’s always good to have an analogy and it’s certainly better than an apology.
Let’s imagine you are the Martian delegate at a plenary conference between the third and fourth planets, and your speech is delivered in Martian. An interpreter – an electronic device – is used to simultaneously translate your Martian to Mandarin (the language spoken in the other planet’s future). And, except for leaving out the ‘uhmns’ and ‘aahs’ in your delivery, the converter delivers your Red Planet speech in perfectly understandable Mandarin.
You grok what that ‘gateway’ is doing – it gives your audience the essential data, leaving out only the protocol-level messages; those ‘uhmns’ and ‘aahs’ that mean, “Hang on, I’ve got more to say.” That’s a very useful gateway if the receiving endpoint wants to hear only Mandarin.
Here’s a further analogy. If the gateway were to capture the speaker’s voice as a .ufs (Universal file system) attachment and route that through to its destination, what would happen? The receiver would have to perform its own translation, before delivering your Martian speech in Mandarin. Surely, that’s a less useful gateway.