I was reading some Cisco documentation about H.323, but there is some still unclear to me.
It would apperar that gatekeepers are optional, therefore as far as I understand H.323 IP telephony should be possible even without gatekeepers, only using H.323 terminals. Is this correct? In this case, who is going to provide address translation (from H.323 aliases to IP) and admission control?
Thank you in advance,
Address translation is handled in the dial-peers on Cisco platforms. On other manufacturers' devices they use similar local static mechanisms - as Paolo points out. Admission control really isn't addressed very well without a GK though. You can attempt to use dial-peer commends like max-connections but it becomes an ugly patchwork.
In small environments where admisssion control is not needed routing via dial-peers is perfectly acceptable, but it does not scale well. The company I work for built an international VoFR toll bypass network in the mid 1990s using 3810 routers without gatekeepers (not supported for VoFR) or a central authority for resolution. Each router had hundreds of DPs to route the calls, and everytime a new site was added all the routers had to be updated - it was not pretty at all.
Once you understand the basics and debugs (debug gatekeeper main 10 being critical) for Cisco gatekeepers they really aren't that bad to implement in simple situations, but they can grow to be complex if you add in via-zones, CAC, CUBE, transcoding at the CUBE gateway etc.. In many situations they can also be run on the same platform as one of the gateways - as long as you are not loading them with significant tasks.