SIP vs H323
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H.323 is the more mature of the two, but problems may arise due to lack of flexibility. SIP is currently less defined, but has greater scalability which could ease internet application integration. Which protocol will win out in the end? It is still too early to tell, but our unbiassed analysis will help you decide which protocol best suites your application.
| H.323 | SIP | |
|---|---|---|
| Architecture | H.323 covers almost every service, such as capability exchange, conference control, basic signaling,QoS, registration, service discovery, and so on. | SIP is modular because it covers basic call signaling, user location, and registration. Other features are in other separate orthogonal protocols |
| Components | Terminal/Gateway | UA |
| Gatekeeper | Servers | |
| Protocols | RAS/Q.931 | SI |
| H.245 | SDP | |
| Call control Functionality | ||
| Call Transfer | Yes | Yes |
| Call Forwarding | Yes | Yes |
| Call Holding | Yes | Yes |
| Call Parking/Pickup | Yes | Yes |
| Call Waiting | Yes | Yes |
| Message Waiting Indication | Yes | No |
| Name Identification | Yes | No |
| Call Completion on Busy Subscriber | Yes | Yes |
| Call Offer | Yes | No |
| Call Intrusion | Yes | No |
| H.323 splits them across H.450, RAS, H.245 and Q.931 | ||
| Advanced features | ||
| Multicast Yesgnaling | Yes, location requests (LRQ) and auto gatekeeper discovery (GRQ). | Yes, e.g., through group INVITEs. |
| Third-party Call Contro | Yes, through third-party pause and re-routing which is defined within H.323. More sophisticated control is defined by the related H.450.x series of standards. | Yes, through SIP as described in separate Internet Drafts. |
| Conference | Yes | Yes |
| Click for Dial | Yes | Yes |
| Scalability | ||
| Large Number of Domains | The initial intent of H.323 was for the support of LANs, so it was not inherently designed for wide area addressing. The concept of a zone was added to accommodate wide area addressing. Procedures are defined for “user location” across zones for email names. Annex G defines communication between administrative domains, describing methods to allow for address resolution, access authorization and usage reporting between administrative domains. In multi-domain searches, there is no easy way to perform loop detection. Performing the loop detection can be done (using the PathValue field), but introduces other issues related to scalability. | SIP inherently supports wide area addressing. When multiple servers are involved in setting up a call, SIP uses a loop detection algorithm similar to the one used in BGP, which can be done in a stateless manner, thus avoiding scalability issues. The SIP Registrar and redirect servers were designed to support user location. |
| Large Number of Calls | H.323 call control can be implemented in a stateless manner. A gateway can use messages defined in H.225 to assist the gatekeeper in performing load balancing across gateways. | Call control can be implemented in a call stateless manner. SIP supports n to n scaling between UAs and servers. SIP takes less CPU cycles to generate signaling messages; therefore a server could theoretically handle more transactions. SIP has specified a method of load balancing based upon the DNS SRV record translation mechanisms. |
| Connection state | Stateful or Stateless | Stateful or Stateless. A SIP call is independent of the existence of a transport-layer connection, but instead signals call termination explicitly. |
| Internationalization | Yes, H.323 uses Unicode (BMPString within ASN.1) for some textual information (h323-id), but generally has few textual parameters. | Yes, SIP uses Unicode (ISO 10646-1), encoded as UTF-8, for all text strings, affording full character set neutrality for names, messages and parameters. SIP provides for the indication of languages and language preferences. |
