Gatekeepers PT 1

Its been a while since I updated. We completed some work on MGCP, H323 and SIP, but I felt they were incomplete for use on the blog. I will get those up as soon as I get the rest of the info. I may also do it in a part 2, but for now lets talk about gatekeepers.


What are they?

In MMA they are refered to guys who can beat any other fighter in there weight class on any given night, but for some reason or another they can't muster up enough win's to get the title shot. O


Any other definition.

Well in the world of Voice/Video they can be used for a few task.

Address resolution
Admission Control
Bandwidth management
Zone Management
Call Authorization


Address Resolution

the number you dial has to be turned into a IP address so we can reach the distant endpoint, so it does "Resolution" kinda similar to DNS.

Admission Control.

You can't just make a call when a gatekeeper is in town. Gateways have to go out to the gatekeeper and request for a call to be placed

Bandwidth Management.

When we are using multiple clusters the location function doesn't work as we have different call managers around so the gatekeeper can provide CAC, by keeping track of the calls being made. This gets pretty funky with how G711 under uses the bandwidth and G729 over uses the bandwidth and we will cover this later on.

Zone Management.

Gatekeepers keep track of all the gateways in there zone through a registration process. Gateways just can't come online with talking to the gatekeeper 1st.

Call Authorization

Gatekeepers can control access to certain gateways. You can use a AAA server with Radius to get this to work.



Gatekeeper signalling

H323 uses a H225 setup messages before it send the H245 messages for capabilities. When using gatekeepers for signalling we still use H225, but we use a form called Ras. So H225 Ras is used for the signalling between gateways and gatekeepers. and there are several sub messages when using them. Stay tuned as we break down the gatekeeper signalling and call flow in the next post.

What is H323

H323 is a suite of protocols which are used to control Voice and Video over IP based networks. Under H323 we have the following

H225- Set's up the call setup and Teardown between Dvices.
H225(RAS) Protocol between the endpoints and the gatekeer
H235- Specified Security for messages between the gateway and gatekeeper
H245- Controls Traffic flow, Performs DTMF, limits the media, Capability exchange.
H261/263- Video Conferencing
H.450 Supplementary services between H323 devices.

H323 components
Gateways, Gatekeepers, Terminals, and MCU's(multipoint control unit)


Gateways

These contain the connection between a IP based network, and the TDM network(PSTN) We configuring our dial-peers here, these can register with the Gatekeepers, These are usually are routers in most deployments. They can also handle phones being registered with them VS the call manager.

Gatekeepers.

Centralized point in a VOIP deployment. Gatekeepers can perform CAC(call admission control), and perform address Translation. H323 Can get messy with Dial-Peers configured to point to all devices. With a Gatekeeper(s) you can configure your entire dial-plan here and save on T/S hours as gateways will all have the same info.

Terminals

IP phones, Video phones and so on. They perform 2 way communication in the IP network. They have to support H245 for channel and capability control. They sent RTP streams from there devices to the other terminal.

MCU

Allow several participants in a conference.


How does H323 Work

The gateways exchange H225 Call setup Massages bewteen each other, then a H245 message is sent with Capability exchange. The IP address of the devices are sent and if it all works RTP packets start to flow.


H323 Fast Start/Fast Connect

When you are using this one of the setup messages will have fast start encluded which has the list of capabilities and list of codec options. The other end will reply with a fast start of its own with the same info. This allows the call to start right away without the delay that we hear sometimes when a call is setting up.

So how do we configure all of this.

Stay tuned its coming in future Post as we get closer to the written and need to start getting ready for labbing.