Evan RoggenkampVoIP/IP Telephony

H.323

Designed for ISDN networks originally

Tuned to work over TCP/IP Protocol Suite Built With: (some of

them) H. 225 – Handles call setup/teardown and

Q. 931 Operations H. 225 RAS (registration & admission

status) – Handles gatekeeper signaling H. 245 – Feature Negotiation H. 261/H. 263 – Video Conferencing H. 450 – Supplementary services (Hold,

transfer) T. 120 – Data Transfer, Application Sharing

H. 323 Defined

H.323 Protocol Suite

Reasons to use H.323 Widely supported – interoperable (Cisco

Default) Many call routing and manipulation

options Supports, voice, video, data conferencing Supports fractional PRI support Supports Caller-ID support on FXO or T1

Cards PRI Call Preservations Non-facility associated signaling support

(allows you to pull one signaling channel for multiple channels of data)

H. 323 Gatekeeper

Reasons to use H.323

Call Flow Process: Call comes in from the PSTN. Enters the H. 323 Gateway.

Send it to call manager. CCM & H. 323 Gateway exchange H. 225 setup messages CCM & H. 323 Gateway exchange H. 245 feature

negotiation Once the call has been set up and the features negotiated,

the Call Manager rings the phone using SCCP. RTP Streams sent between the PSTN, H. 323, & to the

phone. It does not go through the call manager anymore. When the line hangs up a SCCP message is sent from the

phone to CCM, and a H. 225 teardown is sent from there to the H .323 Gateway effectively ending the process.

See VISIO *Fast start combines H. 225 and H. 245 in one process. This

is the default on cisco routers.

Call Flow Process

H.323 Defaults & Notes

VOIP G. 729 Codec (6729R8) VAD Enabled (bandwidth preservation) –

generally disabled: hardware intensive) DTMF Relay Disabled Preference 0 (Failover, duplicate dial-peers

pointing to same destination) Audio = DSCP EF Signaling/DSCP AF31

(QoS) Huntstop Disabled (keeps hunting) RSVP = Best Effort Fax Relay Disabled Playout Delay = 40ms (de-jitter buffer)

POTS DID is disabled (external internal) Preference 0 Digit strip enabled (strips explicitly

defined numbers i.e. outside line) Register w/gatekeeper Huntstop Disabled

Gatekeepers

H. 323 Components H. 323 Terminal – must run a full version of the

H. 323 protocol suite Cisco IP Phone is not a true H. 323 Terminal “H.

323 compliant” H. 323 Gateway – transition and communicate

from to a H. 323 network to a non H. 323 network or a different H. 323 network.

Gatekeeper has multiple functions Are not required for smaller establishments (2 –

3 sites) Bandwidth control PSTN re-direction (failover/quality of service) Management of your entire network Address translation

Multipoint Control Unit Device that handles primarily conference calls. Mixes multiple signals and send them out a

single stream

H. 323 Components

Without a Gatekeeper, a call is made and H. 225 negotiates the setup process over the WAN or the PSTN. Once the call is negotiated, H. 245 negotiates the call features. Once successful, RTP becomes active and voice travels between the phones.

Gatekeeper can be a Cisco router, a Microsoft IIS Server, a free open-source PBX application, or many other devices.

Before any calls are made, the Gateway is preconfigured to register with the Gatekeeper: Registration Request (RRQ) The gatekeeper will reply with Registration Confirmed (RCF) Errors will produce a RRJ.

A call goes into the Gateway from extension 4401 and the Dial Peer in the gateway says to consult the Gatekeeper. This is an Admission Request message (ARQ). If the Gatekeeper ok’s the Request, it sends a Admission Confirm message (ACF) back to the Gateway. All of these things (RRQ/RCF; ARQ/ACF) is sent using H.225 + RAS. (This is what is used to communicate with a Gatekeeper) The gateway will then send the Gateway the IP address of the router it needs to set up the requested call.

Now that the Gateway of the originating call knows the IP address of the remote Router it needs to query to set up a call to extension 4402, H. 225 comes into play again and asks the remote Gateway permission to set up a call. The Gateway does not know, so it asks the Gatekeeper (ARQ) if it is okay to set up the call. If it does (ACF) the remote Gateway accepts the request for connect. The call is considered set up and H. 245 comes into play between the two Gateways to negotiate features. The Gatekeeper keeps track of the bandwidth that this call has consumed in the WAN cloud.

Once the call has ended the Gateways send a Disconnect Request (DRQ) to the Gatekeeper, who will then reply with a Disconnect Confirmed (DCF). The call is disconnected and the Gatekeeper recalculates available WAN link bandwidth.

H. 323 with Gateway

http://hive1.hive.packetizer.com/users/h323forum/papers/h.323_white_paper.pdf

http://hive1.hive.packetizer.com/users/packetizer/papers/h323/h323_protocol_overview.pdf

http://idkf.bogor.net/idkf/fisik/voip/tutorials/h323.pdf

http://www.cbtnuggets.com/it-training-videos

Sources