1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 ( TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU,).%G0G042!.3-)33)/./ amended at Melbourne, 1988)The CCITT,considering(a) that there is growing evidence of a customer demand for a videoconference service;(b) that circuits to meet this demand can, at presen
2、t, be provided effectively by digital transmission using theprimary digital group;(c) that switched digital transmission networks known as the Integrated Digital Network (IDN) and IntegratedServices Digital Network (ISDN) are under study, but the methods of exploiting these networks for the transmis
3、sion ofprimary digital groups will not become clear until the studies have progressed further;(d) that the existence of different digital hierarchies and different television standards in different countriescomplicates the problems of defining hypothetical reference connections;(e) that a hypothetic
4、al reference connection may be used as a guide to simplify the problems of connectionsbetween countries with different television standards and digital hierarchies,appreciatingthat rapid advances are being made in research and development of video coding and bit-rate reductiontechniques which may le
5、ad to further Recommendations being proposed for hypothetical reference connections forvideoconferencing at bit rates which are multiples or sub-multiples of 384 kbit/s during subsequent study periods, sothat this may be considered as the first of an evolving series of Recommendations,noting(a) that
6、 a hypothetical reference connection is a model in which studies relating to overall performance may bemade, thereby allowing comparisons with standards and objectives; on this basis, limits for various impairments can beallocated to the elements of the connection;(b) that such a model may be used:
7、by an Administration to examine the effects on transmission quality of changes of impairmentallocations in national networks, by the CCITT for studying the allocation of impairments to component parts of international networks, to test national rules for prima facie compliance with any impairment cr
8、iteria which may berecommended by the CCITT for national systems;(c) that hypothetical reference connections are not to be regarded as recommending particular values ofimpairments allocated to constituent parts of the connection, and they are not intended to be used for the design oftransmission sys
9、tems,and recognizingthat the planning of the necessary transmission networks for a videoconference service will be facilitated ifrecommended hypothetical reference connections are available, even if only in a preliminary form without details of alltransmission and switching arrangements,2 Fascicle I
10、II.6 - Rec. H.110recommends(1) that the hypothetical reference connection and means for digital transmission illustrated in Figures 1/H.110and 2/H.110 shall be used as the model for studies of the overall performance of international videoconferenceconnections, both intra-regional1)and inter-regiona
11、l1), which are provided using minimum numbers of encoding anddecoding equipments;(2) that hypothetical reference connections of a more complex type, as, for example, those illustrated inFigure 3/H.110, being representative of many connections that may be employed in practice, should be studied furth
12、er.Note 1 The hypothetical reference connection shown in Figure 1/H.110 contains the basic transmissionelements, but is incomplete because switching has been excluded and the local ends and parts of the national network ateach end of the connection have been left unspecified.Note 2 Because the arran
13、gements of transmission systems interconnecting regions using different digitalhierarchies have not yet been standardized, and because videoconferencing is likely to be a minority service in suchtransmission systems, it seems prudent to consider videoconference connections both where the primary hie
14、rarchicallevel on the inter-regional link is 1.5 Mbit/s and where it is 2 Mbit/s. In Figure 2b/H.110, the change between2048 kbit/s and 1544 kbit/s transmission is placed at the 2048 kbit/s end of the long international network. The longdistance part of the connection is thus operated at the lower b
15、it rate. Where the international network is provided on asystem which uses the 2048 kbit/s hierarchy, Figure 2c/H.110 maintains the efficiencies offered by the arrangementshown in Figure 2b/H.110, by making available the six vacated time slots for other use. Figure 2d/H.110 offers thepossibility of
16、improved picture quality compared with Figures 2b/H.110 and 2c/H.110 by making full use of theavailable 2048 kbit/s for the videoconferencing signal. This arrangement would require a 2048 kbit/s codec compatiblewith 525-line Video Standards, or the use of an external standards converter. This is for
17、 further study.Note 3 The lengths which have been assigned to the parts of the connections have been arbitrarily chosen, buthave some consistency with existing CCITT and CCIR Recommendations. They are intended to be representative oflong international connections, but not the longest possible. The l
18、engths will likely require revision when studies on theerror rates of digital paths have progressed to the stage when the error rates of the paths used in the connections can bepredicted.Note 4 The propagation delay is one of the main factors to be studied based upon the structures and lengths ofthe
19、 connections in Figures 1/H.110, 2/H.110 and 3/H.110. However, in the absence of subjective test results, thespecification of requirements for videoconferencing connections must await further study. This study and particularlyoperational experience are required to determine the extent to which Recom
20、mendation G.114, which applies totelephone connections, relates to videoconferencing connections.Note 5 In Figures 1/H.110 and 3/H.110, the codecs may be located anywhere within the international ornational networks including the international gateway or the customers premises.Note 6 The extensions
21、beyond the codec shown as A or D in Figures 1/H.110 and 3/H.110 may includewideband analogue or high-speed digital transmission systems on terrestrial bearers. It is not expected that thesetransmission systems will have any significant influence on the quality of the picture or sound, or, on the pro
22、pagationdelay, other than that due to their length.Note 7 For inter-regional operation, television standards conversion between 525-line and 625-line videosignals may be required. This conversion may be performed by the codecs themselves, or provided by externalequipment.Note 8 The arrangements show
23、n in Figure 2/H.110 provide for the simplest means of transmission. Morecomplex means are possible and are not precluded.Note 9 The hypothetical reference connection shown in Figure 3/H.110 is of a more complex type than theconnection shown in Figure 1/H.110, in that it includes codecs in cascade, a
24、nd, possibly an external TelevisionStandards Converter. The picture quality attainable with these more complex connections may be degraded withrespect to that attainable using the connection illustrated in Figure 1/H.110. This and other aspects of the more complexconnection must be studied further._
25、1)The term “intra-regional” is used here to describe connections within a group of countries which share a common televisionscanning standard and a common digital hierarchy, and may or may not be in geographical proximity. The term “inter-regional”is used here to describe connections between groups of countries which have different television scanning standards and/ordifferent digital hierarchies.Fascicle III.6 - Rec. H.110 3 4Fascicle III.6 - Rec. H.110Fascicle III.6 - Rec. H.110 5 6 Fascicle III.6 - Rec. H.110
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