H.323• An International Telecommunications Union (ITU) standard.• Architecture consisting of several protocolso G.711: Encoding and decoding of speech (other protocols are supported as well)o H.245: Negotiates what type of compression to use as well as the bitrateo RTP/RTCP: (Real Time Transport Protocol) Handles the actual voice communicationo Q.931: Standard telephony functions (connection establishment, dial tones, ringing)o H.225: PC to Gatekeeper function that utilizes the Registration/Admission/Status (RAS) channel

Voice over IP (3)

Logical channels between the caller and callee during a call.

Voice over IP (2)

The H323 protocol stack.

Voice over IP

The H323 architectural model for Internet telephony.

Making a call with H.3231. Locate the Gatekeeper by broadcasting a UDP discover packet to port 1718. Gatekeeper responds with its IP address.

2. Register with the Gatekeeper via RAS.

3. Request bandwidth to be allotted for the call via RAS

(call setup begins – since telephone communication is connection oriented, TCP is used instead of UDP)

4. Client sends a SETUP request with the number being called

5. Gatekeeper sends a CALL PROCEEDING signal back to the client

6. Gatekeeper forwards the SETUP request to the Gateway

7. Gateway makes an ordinary telephone call to the desired number

8. The end office of the destination number sends back an ALERT signal to signify that the call was successful (phone is ringing)

9. When the call is answered the end office sends a CONNECT signal

10. At this point, the gatekeeper is bypassed and all packets are sent directly the gateway. H.245 determines the capabilities of both sides and communication commences via RTP.

SIP – The Session Initiation Protocol

• Designed by the IETF• Text-based protocol modeled on HTTP• Works well with existing Internet Applications• SIP URL:

• sip:rob@cs.wku.edu• Handles the setup of a call while actual data transfer is

handled by a protocol like RTP.

SIP – The Session Initiation Protocol

The SIP methods defined in the core specification.

SIP (2)

Use a proxy and redirection servers with SIP.

Comparison of H.323 and SIP