H.323 or SIP?

Despite SIP’s limitations, there are several often-cited reasons demonstrating why SIP is superior to H.323 in supporting IP telephony processes and applications.

Emerging Dominance of IP

H.323 is designed to be interoperable with other ITU-T standards in support of ISDN and ATM networks and includes all of the necessary mechanisms to support interoperability across multiple networks. SIP is designed primarily for IP networks, and the continuing growth of the Internet and its associated protocol makes interoperability a less important issue. Although the Internet has become a ubiquitous presence, the dominance of PSTN services for voice communications will continue for some time, making interoperability an issue for years to come. PSTN signaling interoperability and the more mature nature of H.323 are advantages over SIP at present. H.323 products currently far outnumber SIP offerings, although this is expected to change over time.

Signaling Reliability Mechanism

SIP provides its own reliability mechanism, is independent of the packet layer, and only requires an unreliable datagram service. H.323 requires RTP/RTCP for reliability but provides a better QoS level (see below).

Client/Server Design

SIP messages are exchanged between a client and a server in the same way as HTTP messages. H.323 has a more complex call control protocol. It takes more time to establish an H.323 call than a SIP call. SIP’s client/server operation mode allows security and management features to be implemented easily in SIP, when compared to H.323 calls. SIP uses distributed multicasting signaling support, a more flexible method than the H.323 centralized, unicasting signal method.

Addresses

SIP addresses are like e-mail addresses. Each user is identified through a hierarchical URL that is built around the elements such as a station user’s telephone number or host names. A SIP URL can be easily associated with a user’s e-mail address, greatly simplifying dialing plans. SIP’s use of any URL address for station user identification is less cumbersome than H.323’s requirement for host addressing. Using a single identifier for voice and text communications has its benefits, but only if everyone is using SIP. Private network calls may use the SIP address, but dialing off-network stations will still require the use of the traditional multiple-digit dialing codes.

Complexity and Cost

SIP is a much smaller and less complicated standard that is based on the architecture of existing popular protocols such HTTP and FTP, whereas H.323 is large and complicated. As a result, H.323 products and services are more expensive to develop, and license fees will also be more expensive. Although SIP cost savings are not apparent in the first generation of products, particularly telephones, this appears to be a longer-term advantage for SIP versus H.323.

Command/Message Format

Compared with H.323, SIP uses a simple command format, and the text strings are easier to decode and debug. The entire set of messages is also much smaller than in H.323. This gives SIP an advantage for future SIP software development efforts, particularly development of new features and application services.

QoS Management

SIP supports loop detection, unlike H.323. SIP’s algorithm using “via header” is somewhat better than H.323’s PathValue approach. In other areas, H.323 has the upper hand. Regarding fault tolerance, H.323 supports redundant gatekeepers and endpoints, unlike SIP. H.323 support of Call Admission Control is better than SIP’s reliance on other protocols for bandwidth management, call management, and bandwidth control. H.323 also has limited support of Differentiated Services (Diffserv), but SIP does not. Overall, H.323 wins in this arena.

Firewall/Proxy Design and Configuration

SIP commands can easily be proxied and firewalls can be designed to allow/disallow SIP communications. Getting H.323 through firewalls and proxies is much more complicated.

Extendible and Scalable

SIP is more scalable than H.323 because it is based on a distributed client/server architecture. H.323 often requires peer-to-peer communication, making it more difficult to expand networks. Extending SIP is also easier because of its simpler message format and greater experience with similar protocols such as HTTP. SIP’s use of hierarchical feature names and error codes, which can be IANA registered, is more flexible than H.323’s vendor-specific single extension field.

H.323 offers the benefits of superior network interoperability, better QoS management, and redundancy. SIP is a less complex protocol that is more easily adapted to expansive and growing networks, supports a faster call set-up time, and uses an addressing scheme that leverages the existing DNS system instead of recreating a separate hierarchy of telephony name servers. The two protocols will likely exist side-by-side for many years, until they either merge or are supplanted by something newer and better.