nk (MCML) PPP-The idea with MCML PPP is that single-packet latency is unacceptable on slow links. A 1,500-byte message transfer unit takes 215 milliseconds to transmit on a 56Kbps link, which is significantly more than the recommended delay. MCML PPP allows multi- ple fragment streams on a multilink PPP session. It sets the fragment size to match the delay budget of the link (e.g., 128 bytes equals 18 milliseconds). Small voice packets are interleaved between the fragments of big packets by applying WFQ at the fragment level. VolP Call-Signaling Protocols Three major VoIP protocols are used in call signaling. The first standard applied to support interoperability between VoIP systems was the ITU-T's H.323. While it has the advantage of being a mature standard, there are some negatives associated with it, including its complexity, its tuning for PC conferencing rather than telephony, its very bad scaling properties, and its difficulty working over firewalls. The second protocol that came into play is MGCP, which advocates a centralized control archi- tecture. It is fairly simple: It models current PSTN call control architecture requires high-reliability and high-availability call servers. The third protocol is SIP, which advocates a decentralized control architecture. SIP is transaction based, which means it is a good match for what's called the stupid network paradigm, where it is relatively easy to build in scalability and reliability and there are built-in security mechanisms. H.323 H.323 is a protocol suite defined by the ITU-T for voice transmission over the Internet. First published in 1996, the latest version of H.323, Version 5, was com- pleted in 2003. In addition to voice applications, H.323 provides mechanisms for video communication and data collaboration. H.323 defines a centralized architec- ture for creating multimedia applications that use VoIP, and it is an umbrella speci- fication that includes various other ITU standards. The H.323 architecture includes the following components (see Figure 9.4):