by Brian Hall
31. August 2011 15:16
Time-division multiplexing (TDM) has been around a long time and is considered to be a legacy protocol. Most public branch exchanges (PBX) will support TDM trunks provisioned as either channel associated signaling (CAS) or
common channel signaling (CCS). CCS is often referred to as ISDN PRI. ISDN PRI is normally how most TDM PBX's are now provisioned, as there is a significant amount of intelligence that can be carried across these trunks as compared to a CAS trunk.
Here are my two cents as to why SIP is better than TDM.
Easier Integration: By using a SIP PBX it is so much easier integrating other applications with the PBX. There are a multitude of applications that support SIP, which will easily integrate and interoperate with a SIP PBX.
Utilizing a SIP IP PBX also allows you to integrate with other SIP applications such as Voice Mail running on a SIP Voice Mail Server.
More Security: SIP can be so much more secure than TDM. Users and trunks can be provisioned with authentication so no one can just plug into the SIP PBX and start making calls. Users and trunks must be provisioned and can include passwords along with user names to authenticate against. TDM must also be provisioned, but is generally not password protected. If you want to get really secure you can even send your signaling and audio path encrypted by using TLS and SRTP. This does require more processing overhead and is typically done in government installations.
No Limitations: TDM has hardware limitations limiting you to the amount of simultaneous calls you can make. Here, in the USA, most PBX's support T1 cards which limit the PBX to 23 calls per T1 card. If you need more simultaneous calls you will need to purchase additional cards.
With SIP you do not have these hardware limitations. You do not need T1 cards to handle your calls. You are basically limited only by your system configuration such as your CPU speed and memory amount. Your limitations only start after your processor has exceeded what it can handle in call volume, which can be in the hundreds and thousands of calls.
You are also less likely to have call issues due to hardware failure since there are no T1 cards required for SIP.
More Flexibility: SIP is an industry standard, which most vendors will deploy with. Staying with TDM often requires you to purchase only from your vendor thus limiting you to your vendors pricing. With SIP you can purchase from any vendor, which also supports SIP.
I could go on and on but for the sake of brevity let me conclude by saying, "I think SIP ROCKS!"
What are your reasons to using SIP over TDM?