The CCITT Communication Protocol for Videophone Teleconferencing Equipment

Transcription

1 The CCITT Commuicatio Protocol for Videophoe Telecoferecig Equipmet Ralf Hiz Daimler-Bez AG Istitut ffir Iformatiostechik Tcl / Fax / Wilhelm-R.uge-Str Ulm Abstract Durig the last years, the CCITT has defied the commuicatio protocol for videophoe telccofercclg equipmet. This paper gives a summary ad a survey of the cocepts ad properties of this protocol. 1 Summary I the last years, some effort has bee made to defie a protocol stack which ca be used by differet termial equipmet for audio~ video, ad data trasmissio. This paper gives a summary ad a survey of the recommedatios of the CCITT commuicatio protocol for videophoe telecoferecig equipmet. The described protocol has bee defied durig the last years by CCITT Study Group XV with cooperatio of atioal workig groups. I oe of these workig groups the author represeted the Daimler Bez Research Istitute. To get experiece i the field of videophoe telecoferecig, this istitute has developed ad produced fuctioal models of ISDN videophoes [MAY89]. 2 Goal The quality of trasmitted pictures i a videophoe sessio is strogly depedet o the bitrate which is available for the trasmissio of video iformatio. Sice the pictures become better with higher bitrates, the protocol has to be able to cope with

2 359 bitrates up to the rage of some Mbit/s. O the other had, it must be assumed that i the ear future oly 1 or 2 * 64 kbit/s coectios (by meas of arrowbad ISDN) will be available for most of the edusers. For that reaso, a protocol desiged for both usig 64 kbit/s coectios, ad for workig o higher bitrates, too, is desirable. The goal of the workig groups was to defie a protocol for audiovisual services (e.g. videophoe), which is uiversal usable, i.e. ot specialized for specific etworks. Existig recommedatios, such as G.704, X.30/I.461, etc. should be take ito accout, existig hard- ad software should be usable, ad the realizatio of the protocol should be possible o simple microprocessors. 3 Result The result of the activities is a series of recommedatios. To sum up, some of the more importat properties are listed below. I cotrast to e.g. the ISDN protocol (D chael), this protocol uses ibad sigallig, i.e. user data ad protocol data are trasmitted via oe commo chael The protocol makes it possible to multiplex data (merge data from differet sources ad trasmit them over oe chael), as well as to split data (sed data from oe source over more tha oe chael). It is suitable for applicatio both i etworks sychroized cetrally ad decetrally, ad it is desiged to work i poit-to-poit ad i poit-to-multipoit coectios. The "basic set" of recommedatios for this protocol is represeted i the followig list. 9 H "Framework For Recommedatios For Audiovisual Services". This is a survey of 45 recommedatios, draft recommedatios ad titles for plaed recommedatios, which are defied or have to be defied for audivisual services [CCITTg0f]. 9 tl320 - "Narrowbad Visual Telephoe Systems Ad Termial Equipmet". This recommedatio describes the techical requiremets of termial equipmet to be used for the arrowbad videophoe service. Narrowbad meas: The trasmissio rate is up to 1920 kbit/s [CCITTg0a]. 9 H "Frame Structure For A 64 To 1920 kbit/s Chael I Audiovisual Teleservices". This recommedatio defies the frame structure (sytax ad sematics) used by the protocol [CCITTg0b]. 9 H "System For Establishig Commuicatio Betwee Audiovisual Termials Usig Digital Chaels Up To 2 Mbit/s". This recommedatio defies the procedural aspects of the protocol [CCITTg0c]. 9 H "Frame-Sychroous Cotrol Ad Idicatio Sigals For Audiovisual Systems". This recommedatio describes the exchage of cotrol ad idicatio

3 360 iformatio betwee commuicatio parters usig the frame structure defied i H.221 [CCITT90d]. 9 H "Video Codec For Audiovisual Services At p * 64 kbit/s". This recommedatio describes the codig ad decodig algorithm for movig video. The factor p is i the scope of 1 to 30 [CCITT90e]. 9 G "System Aspects for the Use of the 7 khz Audio Codec withi 64 kbit/s" [CCITT88]. Some defiitios i G.725 form a subset of H.221 ad H.242. For the time beig, some of these recommedatios are beig take over by ETSI (Europea Telecommuicatio Stadards Istitute) ad are adapted to Europea coditios. 4 Videophoe Termial Equipmet The recommedatio H.320 "Narrowbad Visual Telephoe Systems Ad Termial Equipmet" describes the techical requiremets of termial equipmet for the videophoe service with a data trasmissio rate of up to 1920 kbit/s. This recommedatio defies the adequate termial equipmet ad divides it ito several logical blocks. These blocks are defied i their fuctioal behaviour, ad relatios to appropiate CCITT recommedatios (e.g.h.221, H.242, H.230, H.261 ad the series) are give. Other recommedatios which describe termial equipmet suitable for commuicatio with the protocol described i this article, are e.g (System Aspects for the Use of the 7 khz Audio Codec withi 64 kbit/s) ad H.261 (Video Codec For Audiovisual Services At p * 64 kbit/s). 5 Sytax of Protocol Data Uits Recommedatio H.221 "Frame Structure For A 64 To 1920 kbit/s Chael I Audiovisual Teleservices" describes the sytax of protocol data uits. The structure defied i this recommedatio is ot the same as it is used i commo protocols like HDLC. Istead, the octet structure of the trasmissio chaels is used for formig eight subchaels (see fig. 1). Each bit of a octet is part of a differet subchaeh All bits #1 together build the first subchael,... all bits #7 together build the seveth subchael. The eighth subchael is called "service chael". It has a more extesive fuctioality tha the other seve subchaels, ad it has a special iteral structure. Usig a ISDN B chael with a trasmissio rate of 64 kbit/s, each subchael

4 361 provides a trasmissio rate of 8 kbit/s. Besides this "vertical" divisio, there is a "horizotal" divisio: 80 octets (i.e. 80 bits per subchael) form a "frame", 16 succeedig frames form a "multiframe". A multiframe cosists of eight "suhmultiframes" (SMF), each cosistig of two cosecutive frames. The begiig ad the ed of this frame structure withi the octet stream (the "frame aligmet" ad the "multiframe aligmet") are itroduced by the structure of the service chael. The aligmet is ecessary for the correct iterpretatio of received data. The iformatio cocerig the frame limits are coded withi the service chael. For that reaso, the bits cotaiig this iformatio are called "Frame Aligmet Sigal" (FAS, fig. 2). The FAS is costituted by the first eight bits of a frame withi the service chael. For it is also ecessary to kow, whether or ot the commuicatio parter has yet foud the frame- ad multiframe aligmet, each parter uses oe bit (the 'A'-bit) to sigal his state of sychroizatio o the frame limits. Aother fuctioality provided by the FAS is the sychroizatio of multiple chaels. Sice a coectio ca be formed by up to six (ISDN B) chaels, some more sychroizatio steps are ecessary: The delay betwee the chaels has to be hadled, ad the chaels have to be treated i the correct order. For the first, a multiframe couter is used. By meas of this couter, a relative delay of up to +/ secods betwee two 64 kbit/s chaels ca be egalized. For the latter, a chael umberig has bee itroduced. Followig the FAS, the ext bits of the service chael form the "Bit-rate Allocatio Sigal" (BAS). The BAS is used for the trasmissio of iformatio cocerig the capabilities of a termial, ad it is used for sigallig the mode i which the local trasmitter is workig. Furthermore, the service chael cotais the "Ecryptio Cotrol Sigal" (ECS). These bits may, i future, trasmit iformatio regardig ecryptio. If o iformatio about ecryptio has to be trasmitted, these bits cotai user data. User data are also cotaied i bits Procedures of the Protocol Recommedatio H.242 "System For Establishig Commuicatio Betwee Audiovisual Termials Usig Digital Chaels Up To 2 Mbit/s" describes a ibad sigallig protocol. This protocol is used to establish, maitai, ad discoect a coectio ad to react o errors durig these phases. Sice a coectio ca cosist of up to six chaels, there has bee made a distictio betwee the chael which has bee established first ("Iitial Chael") ad all other chaels ("Additioal Chael(s)").

5 362 Each chael has to sed at least sychroizatio iformatio by meas of the FAS, ad user data. The iitial chael has some more work to do: Additioal tasks before ad durig a commuicatio are the exchage of capability iformatio, sedig of commad codes ad maagemet of additioal chaels. While a coectio is established, data for cotrol ad idicatio as described i H.320 ("Frame-Sychroous Cotrol Ad Idicatio Sigals For Audiovisual Systems") ca be exchaged. 6.1 Procedures There are three basic sequeces defied for the protocol which are used as buildig blocks for more complex procedures. All of them make use of the BAS. These basic procedures are: 9 The capability exchage sequece for forcig the commuicatio parters ito a defied state, ad for iformig the commuicatio parter about the ow capabilities (fig. 3). 9 The mode switchig sequece for switchig the receiver of the commuicatio parter to a mode coformig to the ow trasmitter's mode (fig. 4). s The frame reistatemet sequece for chagig the trasmissio mode from a "uframed" format (i.e. without the FAS, BAS ad ECS) back to a framed format (fig. 5). Whe a commuicatio betwee two or more parters starts, the parters have to fid a commo mode for operatio. This commo mode should make use of as much of the termial's capabilities as possible, sice oly i this case all of their facilities ca be used. For this, the mode iitializatio procedure has bee defied. If, durig a coectio, oe of the commuicatio parters wats to chage his trasmissio mode (e.g. from voice to picture), he ca do so by usig the procedure for dyamic mode switchig. It eables him to make use of differet capabilities of the coected termials. Prior to discoectig or to usig special etwork services (e.g. call trasfer), it is ecessary for the coected termials to work i a mode where 3.1 khz PCMO) audio sigals ca be decoded. For this case, the mode 0 forcig procedure has bee defied. I case of errors due to mode mismatch~ the mode mismatch recoverig procedure forces a mode reiltializatio. Also, a set of possbile error coditios ad the reactios o them has bee icluded ito recommedatio H.242. (1) pulse code modulatio

6 Relatio to Network Sigallig The establishmet ad discoectio of chaels caot be achieved by meas of the described ibad sigallig protocol. Istead, etwork sigallig protocols have to be used. I case of a ISDN, this is the D chael protocol. Prior to discoectig a chael, the termials are forced to work i the simplest mode ("mode 0") for beig able to e.g. decode audio sigals comig from a PABX (2). 7 Literature ICCITT88] CCITT SGXVIII; Recommedatio G.725: System Aspects for the Use of the 7 khz Audio Codec withi 64 kbit/s; Melboure: July 1988 [CCITTg0a] CCITT SGXV; Recommedatio H.320: Narrowbad Visual Telephoe Systems Ad Termial Equipmet; COM XV-R 37-E, August 1990 [CCITTg0b] CCITT SGXV; Recommedatio H.221: Frame Structure For A 64 To 1920 kbit/s Chael I Audiovisual Teleservices; COM XV-R 37-E, August 1990 [CCITTg0c] CCITT SGXV; Recommedatio H.242: System For Establishig Commuicatio Betwee Audiovisual Termials Usig Digital Chaels Up To 2 Mbit/s; COM XV-R 37-E, August 1990 [CCITTg0d] CCITT SGXV; Recommedatio ti.230: Frame Sychroous Cotrol Ad Idicatio Sigals For Audiovisual Systems; COM XV-R 37-E, August 1990 [CCITT90e] CCITT SGXV; Recommedatio H.261: Video Codec For Audiovisual Services At p * 64 kbit/s; COM XV-R 37-E, August 1990 [CCITT90f] CCITT SGXV; Framework For Recommedatios For Audiovisual Services; Geeva, July 1990 [MAY89] May, Fraz: Algorithme ud Realisierug eies ISDN-Bildtelefo-Codecs; tz Bd. 42 (1989) Heft3, S (2) private automatic brach exchage

7 I El... I ] I I I ~ ~8bit=l~ I I 1 ~ I 1 I I I I I ] I I I I _A J F I i ] H-- [ I... J ~it 1 ]bit 2 ]bit 3 ]bit 4 ]bit 5 ]bit 6 ]bit 7 ]bit 8_J subchaels 1 7 service chael Fig. 1:H.221 subchaels S U b R e 1 S I Jl S UlUlU S C C h ilil h e I e e 1 III m I 8 S ulu bib h & a i e e I,I I subchael #8 i = service chael 1 octet umber s FAS u 8 b --9- c BAS h a 16 ]-7- ECS 24 e 1 i 80 Fig. 2:H.221 frame structure

8 365 termial X (iitiates sequece) termial Y ay kid of data (e.g. uframed) start timer start ~equece sed data with H.221 framig, se.._.~, c..d capability set sed data with H.221 framig, sed capability set detect capabilities A-bit ;= 0 gai multiframe aligmet gai multi frame aligmet ~ t := 0 complete set of capabilities received time ~time Fig. 3: Successful capability exchage sequece

9 _ 366 termial X (iitiates sequece) ay kid of data termial Y.-.11 Submultiframe~ Submultiframe\ / +l V data with H.221 format adba comm format as specified i ~-vio-us- c-omm validate BAS commad switch to appropiate mode time time Fig. 4: Mode switchig sequece

10 367 termial X (iitiates sequece) termial Y ay kid of dat; ay kid of data X seds i a uframed mode X..._...d.~amdata with H.221 format time time Fig. 5: Frame reistatemet sequece