Compared to H.323 the ITU recommendation H.323 entitled, "Packet-Based Multimedia Communication."

1 Definition

H.323 is an umbrella recommendation that covers all aspects of multimedia communication over packet networks. It is part of the H.32x series1 of protocols that describes multimedia communication over ISDN, broadband (ATM),telephone (PSTN), and packet (IP) networks. Originally developed for video conferencing over a single LAN segment, the protocol has been extended to cover the general problem of telephony over the Internet. The first version was approved by the ITU in 1996 and was adopted by early IP telephony networks because there were no other standards. Version 2 was adopted in 1998 to fix some of the problems and limitations in version 1. Version 3 was adopted in 1999 and includes modifications and extensions to enable communications over a larger network. Version 4 was adopted in 2000 with some major changes to the protocol. Version 5 is currently under revision at the ITU-T. Currently, most deployed systems use Version 2. H.323 has been designed to be backward compatible, so, for example, a version 1 compliant end point can communicate with a version 3 gatekeeper and a version 4 end point.

H.323 is an umbrella recommendation that covers all aspects of multimedia communication over packet networks. It is part of the H.32x series1 of protocols that describes multimedia communication over ISDN, broadband (ATM),telephone (PSTN), and packet (IP) networks. Originally developed for video conferencing over a single LAN segment, the protocol has been extended to cover the general problem of telephony over the Internet. The first version was approved by the ITU in 1996 and was adopted by early IP telephony networks because there were no other standards. Version 2 was adopted in 1998 to fix some of the problems and limitations in version 1. Version 3 was adopted in 1999 and includes modifications and extensions to enable communications over a larger network. Version 4 was adopted in 2000 with some major changes to the protocol. Version 5 is currently under revision at the ITU-T. Currently, most deployed systems use Version 2. H.323 has been designed to be backward compatible, so, for example, a version 1 compliant end point can communicate with a version 3 gatekeeper and a version 4 end point.

2 Composition

Elements of an H.323 network

MCU : Multipoint Channel Unit ⓐ A endpoint on the LAN, commonly used to bridge videoconferencing connections ⓑ Consist of a mandatory Multipoint Controller(MC) + Multipoint Processors Gateway PBN 내에 있는 H.323 터미널과 SCN(Switched Circuit Network) 에 있는 다른 ITU 터미널 (ISDN, PSTN 등에 있는 터미널 ) 과 실시간 양방향 통신을 할 수 있도록 해주는 구성 요소 Gatekeeper H.323 터미널에 대한 주소 변환 (address translation), 접근제어 (access control) 를 하며, Gateway 의 위치를 터미널에게 알려주거나, 대역폭 관리 등을 할 수 있는 구성 요소이다. Gatekeeper 는 H.323 시스템에서 반드시 필요한 구성 요소는 아니지만 위와 같은 관리를 효율적으로 하기 때문에 대부분의 경우에 존재하게 된다. Terminal 1:1 통신이나 회의를 할 때에 audio( 필수 ) 와 video( 선택 ), data( 선택 ) 로 통신 (communication) 을 할 수 있는 능력을 가지고 있는 것 (endpoint) 을 말한다.

* RAS(Registration, Admission, Status) : RAS 는 Gatekeeper 와 Terminal 간의 메시지. Gatekeeper 가 존재하는 H.323 시스템의 경우에, 터미널은 터미널로서 역할을 하기 위해서는 ① Gatekeeper 에 등록 (Registration) 을 해야 하고, ② 다른 터미널과 통신을 하기 위해서는 입장 (Admission) 을 해야 한다. ③ 필요한 경우에 터미널은 Gatekeeper 에게 자신의 상태 (Status) 를 주기적으로 알려주어야 한다.

* H.245 ITU 에서 정한 표준으로서, Call 에 대한 제어 내용을 담고 있다. 이중에서 대표적인 것으로는 Capability Exchange, Open Logical Channel ① Capability Exchange : 터미널이 자신의 능력 ( 어떤 코덱으로 압축된 데이터를 복원할 수 있는가 ) 을 교환함으로써, 보낼 데이터의 압축 형식을 정하게 된다, ② Open Logical Channel : 데이터를 보낼 채널을 연다. 음성 데이터의 경우에는 두 개의 채널이 열리며, H.225 에서 정의된 RTP/RTCP 를 이용하여 데이터를 보내게 된다.

* Q.931 ISDN 에서 Call Signaling 을 할 때 사용되는 ITU 표준이다. 전화를 걸 때 상대 터미널에 Setup 을 보내고, 상대 터미널은 Alert 를 보낸다. 이 때에 전화벨이 울리게 된다. 수화기를 들 때에 상대 터미널에서 Connect 를 보내게 된다. 통화를 끝내기 위해서는 Release Complete 를 주고 받는다.

* Audio/Video Codec VoIP 는 음성 ( 화상 ) 데이터를 전송할 때에 대역폭을 줄이기 위해서 데이터를 압축한다. Codec 은 음성이나 화상 데이터를 압축하고 복원하는 데에 사용된다. 음성 코덱은 G.711, G.723, G.728, G.729 등이 사용되고, 화상 코덱은 H.261, H.263 등을 사용하고 있다.

* RTP/RTCP IETF 에서 권고하는 프로토콜 (RFC1889) 로서, 음성 데이터를 실시간으로 전송할 때 이 프로토콜을 사용한다.

Protocol Description H.225 Registration, Admission, Status (RAS) and call signaling H.245 Control signaling(media control), media negotiation and capability exchanges T.120 Multipoint graphic communication G.7xx Audio codecs H.26x Video codecs RTP Real-Time transport Protocol (RFC 3550) RTCP RTP Control protocol (RFC 3550) H.235 Privacy and encryption H.450 Supplementary services for call setup signaling Terminal to gatekeeper Summary

3 Process

Release Complete Q.931 메시지로서 전체 통화를 끝낼 때, 양 터미널이 주고 받는다. End Session H.245 를 이용하는 Control Channel 을 닫는다. Close Logical Channel Logical Channel 을 닫는다. H.245 에 정의되어있는 메시지이다. Open Logical Channel 실제 음성 데이터를 보낼 채널을 연다 Capability Exchange Capability Exchange 하여 코덱의 종류를 결정 Setup / Alert / Connect 상대 터미널과 Q.931 메시지를 주고 받아서 연결 ARQ(Admission Request) / LRQ(Location Request) 통화시작을 위해 Gatekeeper 에 ARQ 보냄. 상대터미널을 찾을 필요가 있을 경우에는 LRQ 도 보냄 Register 터미널을 Gatekeeper 에 등록

Assumed that both terminals have already registered with the gatekeeper using RRQ message. RAS, Call Signaling Connection negotiation

Assumed that both terminals have already registered with the gatekeeper using RRQ message. RAS, Call Signaling Connection negotiation Open logical channels

RAS, Call Signaling Connection negotiation Open logical channels RTP Media Session

H.245 control signaling RAS

4 Version

“Visual Telephone Systems over Networks with Non-Guaranteed Quality of Service.” Very slow, 7 round trips to establish a call Version1 To speed up the call setup. 2 schemes were added to H.323 FastStart : Setup message contains the TerminalCapabilitySet information H.245 tunneling : H.245 messages are encapsulated in Q.931 messages in the call signaling channel Version2 Added more features to H.323. Of interest to Internet devices is the support for H.323 URLs, full UDP support instead of TCP Version3,4 Currently being worked on by the ITU-T and is scheduled to be finished in late Version5

5 Comparison

一長一短

SIPH.323 Text (less complexity)Binary-encoded ASN.1 msg(smaller size) Presence, IM capabilityOnly signaling protocol PC, telecommunication (key players), but callsignaling + IM protocol for 3G Simple phone calls (major players) Robust security mechanisms- Development cycleDeployed System

6 Conclusion

Coexist SIP and H.323 have actually become more similar in some functionality. SIP: ① DNS and URL support, “focus” ② UDP, “TCP”, use extensions H.323: ① MCU, H.323 address ② TCP, “UDP”

SIP Some key signaling components needed to do video conferencing are still not fully standardized in the IETF. As a result, a fully functional, standards-based SIP videoconferencing system is not currently available. As bandwidth and cost factors, the deployment of videoconferencing systems has been disappointing. Now the bandwidth and cam costs is falling. SIP will play a bigger role in these future software-based video systems than the current dedicated hardware videoconferencing systems

7 Appendix ASN.1

1. 선언은 ::= <---- 요걸로 한다. 2. 시작은 모듈이름 ::= BEGIN ~ 끝날 땐, END 모듈이름 DEFINITION ::= BEGIN ( 내용 ) END 3. 웬만하면 소문자로 시작한다. 4. IMPORT 를 통해 데이터 타입을 불러오게 되고, 이것을 사용할 수 있다. int 형은 Integer32, Counter 등의 데이터 타입으로 쓸 수 있고, string 형은 DisplayString

TEST-MIB DEFINITIONS ::= BEGIN IMPORTS OBJECT-TYPE, MODULE-IDENTITY, Integer32, enterprises FROM SNMPv2-SMI DisplayString FROM SNMPv2-TC; myTest MODULE-IDENTITY LAST-UPDATED " Z" ORGANIZATION "woorie.net" CONTACT-INFO "woorie.net" DESCRIPTION "Just Test!!!!" REVISION " Z" DESCRIPTION "This file defines the test." ::= { enterprises 4444 } myTable OBJECT-TYPE SYNTAX SEQUENCE OF MyTableEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "table obj" ::= { myTest 1} myTableEntry OBJECT-TYPE SYNTAX MyTableEntry MAX-ACCESS not-accessible STATUS current DESCRIPTION "An entry contains test msg" INDEX{ idx } ::={ myTable 1} MyTableEntry ::= SEQUENCE { idx Integer32, text DisplayString } idx OBJECT-TYPE SYNTAX Integer32 ( ) MAX-ACCESS read-create STATUS current DESCRIPTION "Reference Index for myTableEntry" ::= {myTableEntry 1} text OBJECT-TYPE SYNTAX DisplayString MAX-ACCESS read-create STATUS current DESCRIPTION "log message" ::= { myTableEntry 2} END

-myTest(4444) | +--myTable(1) | +--myTableEntry(1) | Index: idx | +-- CR-- Integer32 idx(1) | Range: CR-- String text(2) Textual Convention: DisplayString Size: