Functionality, differences, etc. of Softswitch, Session Border Controller, and Access Router

Please summarize the core functionalities of a Softswitch, Session Border Patroller, and Access Router.

I'd also like to know the differences between each of these entities and what roles they play in the overall carrier picture.  Specifically: Which one does the actual routing (LCR, ASR)? Which one handles the SIP/H323/etc registration? Which handles the codec translations? And can any of these function without the other?

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Reid PalmeiraTelecom EngineerCommented:
okay so this is pretty long and involved and will vary depending upon the network implementation. There's also usually additional pieces here like core routers and possibly MPLS (again depends on the network implementation) that aren't in the original questions.

Generally speaking..

your softswitch is where the magic happens. This is ultimately where your SIP endpoint devices will be registering to. BUT the registrations may be coming through some kind of SBC or other device. in most cases they will for two reasons the first is security and the second is NAT translation. Your softswitch is usally the box with the PSTN connections. In smaller enterprises you might have a softswitch and some PRI lines or whatever. In the carrier space this box will have your SS7 links, FGD trunks, whatever else depending on whether it's a class 5 switch, class 4, etc.. As far as SIP goes though this is your registerr server because it's got the connectiosn to the PSTN.

let me backtrack at this point. Some background reading
SIP Connect from the SIP forum will explain in more detail than I care to go into here about how the end user PBX would connect to the provider/carrier and provide some basic technical requirements. This is NOT an RFC or a standard mind you, just one common set of guidelines for implementation. And even at that it's pretty open to interpretation. If your turning up SIP services as a carrier the best advice I can give you is to plan well and MAKE SURE you spend time on Interop testing the FULL end to end solution.

Okay so SIP in general you have a user agent client and a user agent server. Ultimately your softswitch is the client is maybe a SIP phone or an IP PBX with a SIP trunk back to your softswitch. In between that you may have an SBC which acts as a proxy. For example say your network is setup so all of your VoIP customers are cuoming from public IP addresses. Public = Internet = any kinds of crazy stuff can happen and you need someway to protect your expensive new softswitch. This is one of the main functions of the SBC. It sits with an interface on the public side to proxy customer traffic for your nice, safe, private softswitch.  That may mean that endpoint devices like IP phone will register to the SBC (so the SBC acts as a server to endpoints but acts as a client to your switch) or it may simply inspect the traffic and pass it through (proxy) to your softswitch.

The second reason for an SBC is related to NAT - Network Address Translation. There's a bunch of ways to deal with NAT issues. SIP is connectionless so you don't necessarily keep the session going as you would with say a telnet or SSH session. This is problematic for most setups where your customer office has a single public IP and a bunch of private IP's behind it. Your SBC deals with the proxying of information and keeps track of what devices (SIP users) on what IP on what TCP/UDP port, allowing the NAT to function more smoothly. I say more smoothly because implementations will vary. Again interop is key. There are other alternatives as well, STUN, TURN, ICE blah blah woof woof, not going to go through it all because I'm writing off the cuff.

As far as your access routers, include here the idea of not just your provider edge routers but also your core routers and any MPLS LSR's you might have to go through. These are all essentially just IP transport for your SIP signalling and RTP voice traffic. You want to make sure the IP routing is setup with solid QOS tagging and that you have end to end QOS through your entire network from the provide edge (down to the customer edge I suppose if you're providing those routers) all the way through to your softswitch. This might be done via a number of different QOS methods. DSCP/TOSBIT, IP Precedence, whatever you want that's supported by your equipment vendors and that you most appropriately know how to setup. MPLS-TE can be highly useful but in larger MPLS networks can be kind of a pain to setup

I'm staying off the H323 stuff here because of that functions differently. H323 architecture is a little different. As far as SIP architecture, cisco has a decent overview So your codec negotiation, etc. That's handled by SDP (Session Description Protocol) and it's up to all the devices involved, proxy servers, the User Agents themselves, all the pieces from the sip client to the sip server to negotiate things like codec, setup things like SIP encryption (TLS, whatever). Consider the overall architecture and you realize that yes certain peices can function in and of themselves but it comes down to what you consider to be "functioning." Your provider edge routers (access routers) will do just fine if your softswitch loses an SS7 link. Your SBC will work just fine without QOS. Your SIP client will work (in some implementations) without a proxy (SBC). It just won't work very nicely without everything lined up and tested end to end.

Let's cut it there, see what kind of thinking path that leads you down. Post back with any questions or things you need clarification on. I'm not really proofreading this as I go so there may be some errors in speeling ;) or grammar

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