1、 International Telecommunication Union ITU-T G.114TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU Amendment 2(11/2009) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS International telephone connections and circuits General Recommendations on the transmission quality for an en
2、tire international telephone connection One-way transmission time Amendment 2: New Appendix III Delay variation on unshared access lines Recommendation ITU-T G.114 (2003) Amendment 2 ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS INTERNATIONAL TELEPHONE C
3、ONNECTIONS AND CIRCUITS G.100G.199 Transmission planning and the E-model G.100G.109 General Recommendations on the transmission quality for an entire international telephone connection G.110G.119General characteristics of national systems forming part of international connections G.120G.129 General
4、characteristics of the 4-wire chain formed by the international circuits and national extension circuits G.130G.139 General characteristics of the 4-wire chain of international circuits; international transit G.140G.149 General characteristics of international telephone circuits and national extensi
5、on circuits G.150G.159 Apparatus associated with long-distance telephone circuits G.160G.169 Transmission plan aspects of special circuits and connections using the international telephone connection network G.170G.179 Protection and restoration of transmission systems G.180G.189 Software tools for
6、transmission systems G.190G.199 GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS G.200G.299 INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES G.300G.399 GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY
7、OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES G.400G.449 COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY G.450G.499 TRANSMISSION MEDIA AND OPTICAL SYSTEMS CHARACTERISTICS G.600G.699 DIGITAL TERMINAL EQUIPMENTS G.700G.799 DIGITAL NETWORKS G.800G.899 DIGITAL SECTIONS AND DIGITAL LINE SY
8、STEM G.900G.999 MULTIMEDIA QUALITY OF SERVICE AND PERFORMANCE GENERIC AND USER-RELATED ASPECTS G.1000G.1999 TRANSMISSION MEDIA CHARACTERISTICS G.6000G.6999 DATA OVER TRANSPORT GENERIC ASPECTS G.7000G.7999 PACKET OVER TRANSPORT ASPECTS G.8000G.8999 ACCESS NETWORKS G.9000G.9999 For further details, pl
9、ease refer to the list of ITU-T Recommendations. Rec. ITU-T G.114 (2003)/Amd.2 (11/2009) i Recommendation ITU-T G.114 One-way transmission time Amendment 2 New Appendix III Delay variation on unshared access lines Summary Appendix III of Recommendation ITU-T G.114 provides an introduction on the eff
10、ect of different IP services on lines with limited bandwidth (e.g., DSL). It explains the mechanism of serialization delay, gives a (very general) overview over prioritization and shows how the maximum delay variation due to concurrent traffic can be calculated. The calculations shown in this append
11、ix are valid for unshared lines only, shared lines are excluded. The intention of this appendix is to make standards developers aware of this behaviour. Source Amendment 2 to Recommendation ITU-T G.114 (2003) was agreed on 12 November 2009 by ITU-T Study Group 12 (2009-2012). ii Rec. ITU-T G.114 (20
12、03)/Amd.2 (11/2009) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications, information and communication technologies (ICTs). The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is
13、responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by
14、the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collab
15、orative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certai
16、n mandatory provisions (to ensure e.g. interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirement
17、s. The use of such words does not suggest that compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU
18、 takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual p
19、roperty, protected by patents, which may be required to implement this Recommendation. However, implementers are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2010 All rights reser
20、ved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T G.114 (2003)/Amd.2 (11/2009) iii CONTENTS Page Appendix III Delay variation on unshared access lines 1 III.1 Introduction 1 III.2 Serialization delay 1 III.3 Prioritiz
21、ation . 2 III.4 Measurement examples 3 III.5 Theoretical considerations 5 III.6 Conclusions 6 III.7 Abbreviations and acronyms 6 Rec. ITU-T G.114 (2003)/Amd.2 (11/2009) 1 Recommendation ITU-T G.114 One-way transmission time Amendment 2 New Appendix III Delay variation on unshared access lines (This
22、appendix does not form an integral part of this Recommendation) III.1 Introduction For some time now, the coverage of broadband access for customers has been growing higher and higher. This broadband access is used for different services, starting with Internet, and nowadays, increasingly TV and voi
23、ce services. Packet-based networks offer high flexibility to deliver all of these services over the same network. If more than one service is used at the same time or if one service uses more than one session at a time, there is the possibility of interference. One very real effect will be the influ
24、ence of other services/sessions on VoIP-media traffic. III.2 Serialization delay Serialization delay of a packet is the time it takes to clock every bit of a packet onto the line. A packet ready to be sent will normally be put in a playout buffer, from where it will be clocked onto the line at the p
25、hysical line speed (see Figure III.1). G.114(03)Amd.2(09)_FIII.11 0 0 1 00 1 0 0 00 1 0 1 11 1 1 1 11 0 0 0 01 1 0 0 01 1 0 1 11 1 1 0 110 01101 1 110000LinespeedPlayout bufferLineFigure III.1 Serialization delay The equation to calculate the serialization delay tserialisationis as follows: bit/sbit
26、LinespeedPacketsizestionserialisat= With Packetsize = size of a packet on the physical layer Linespeed = line speed on the physical layer Table III.1 shows some example calculations. 2 Rec. ITU-T G.114 (2003)/Amd.2 (11/2009) NOTE It is recommended to do this calculation on the physical layer even if
27、 it could be done on any other layer, as long as the packet size and the line speed are calculated for the same layer. It has to be taken into account that the packet size needs to represent the size of a packet, including all headers and trailers (for further calculations it may also be necessary t
28、o include the minimal distance between two packets). Table III.1 Example of serialization delays with different line speeds and packet sizes Line speed Packet size Serialization delay 1 Gbit/s 1500 bytes 0.012 ms 1 Gbit/s 200 bytes 0.0016 ms 100 Mbit/s 1500 bytes 0.12 ms 100 Mbit/s 200 bytes 0.016 m
29、s 10 Mbit/s 1500 bytes 1.2 ms 10 Mbit/s 200 bytes 0.16 ms 1 Mbit/s 1500 bytes 12 ms 1 Mbit/s 200 bytes 1.6 ms 100 kbit/s 1500 bytes 120 ms 100 kbit/s 200 bytes 16 ms III.3 Prioritization The general concept of prioritization is that traffic with higher priority is favoured against traffic with lower
30、 priority on the same line. Prioritization is important in the case where a bandwidth limitation exists (more input capacity than output capacity) or in case of congestion (these two effects can be related). Prioritization is a strong method for queue management (strict priority), which means that t
31、his traffic is prioritized in any case; or a weaker bandwidth allocation (like a weighted fair queuing) in order to allow lower “prioritized“ traffic to pass even in the case of congestion. Normally, VoIP media traffic is in the highest priority class which uses strict priority queues (in fact, inte
32、rnal network control traffic is prioritized even higher). For the remainder of the traffic, there are several priority classes possible, which normally use fair queuing. Figure III.2 shows a functional diagram of a typical prioritization algorithm in network equipment. G.114(03)Amd.2(09)_FIII.2W1W2W
33、3Strict priorityClass Xe.g., AFxReal-timetrafficLinePlayout bufferClass Ye.g., AFyClass Ze.g., BEFigure III.2 Prioritization principle Rec. ITU-T G.114 (2003)/Amd.2 (11/2009) 3 Classes X, Y and Z are three differently prioritized classes (in Figure III.2, classes X and Y are two different assured fo
34、rwarding (AF) classes, while class Z is a best effort (BE) class). In this case, the prioritization is according to different weights (W1, W2 and W3) of the classes, but every class will get its time slot to send (based on the weighting). This means that it is possible that a lowest-prioritized pack
35、et from the queue of class Z can be sent to the playout buffer, even if there are packets in the higher prioritized queues of class X and Y. Since the real-time class is allocated strict priority, no packet out of queues X, Y and Z can be sent to the playout buffer if there is a packet in the real-t
36、ime queue (this implicitly means that there has to be some sort of bandwidth control for the real-time class to avoid a blockage of all other traffic). In the playout buffer, there is no prioritization. Packets in the playout buffer will be sent to the line in the order they arrived (FIFO, first in,
37、 first out). If the playout buffer is full, not even a packet from the real-time queue can be sent to it. The prioritized packet from the real-time queue has to wait until the packet in the playout buffer is sent to the line. With the equation for the serialization delay, the time taken can be calcu
38、lated. This effect leads to delay variation for real-time traffic. In a usual priority implementation, the playout buffer has the capability to hold two packets; this means that, in the worst case, two low-prioritized packets have to be sent before a high priority packet can be sent. The equation fo
39、r the maximum delay variation, tdelayvariation, due to this effect will be the one for serialization delay multiplied by the number of packets which the playout buffer can hold. NOTE 1 In difference to the equation for serialization delay, where the actual packet size of the packet of interest has t
40、o be taken, in the following equation for the maximum delay variation, the maximum packet size possible on the link has to be taken. bit/sbit*sLinespeedizeMaxPacketstsNbrofPacketiationvardelay= With NbrofPackets = maximum number of packets in the playout buffer MaxPacketsize = maximum size of a pack
41、et on the link (physical layer) Linespeed = line speed on the physical layer NOTE 2 A typical maximum packet size for IP networks is around 1500 bytes at the IP-layer. III.4 Measurement examples Figures III.3 to III.5 show real IP packet delay variation measurements for a VoIP call between two DSL a
42、ccess examples, customer A with 6400/640 kbit/s access speed, and customer B with 4608/576 kbit/s access speed. 4 Rec. ITU-T G.114 (2003)/Amd.2 (11/2009) G.114(03)Amd.2(09)_FIII.30 2 4 6 8 10121416182022242628303234363840424 4602468101214Delayvariation(IPDV)msTime sInternetdownloads during these tim
43、esMaximumdelay variation due to concurrenttrafficFigure III.3 Delay variation (IPDV) of a call A to B, with parallel download at B Figure III.3 shows the delay variation (IPDV) over time measured at the customer B side, with two intermittent Internet downloads also on the B side. According to the eq
44、uation for the serialization delay, one Internet packet (1500 bytes at IP layer approximately 1696 bytes on the physical layer in this case) will have a serialization delay of 2.94 ms (downstream bit rate B: 4608 kbit/s). Since the maximal delay variation measured is much higher (nearly 12 ms, taken
45、 as the difference of the maximum IPDV and the IPDV without Internet download), it can be assumed that the playout buffer of the network equipment involved in the prioritization towards the DSL line holds up to four IP packets. With a better implementation of the prioritization algorithm, IPDV could
46、 be reduced by 9 ms. G.114(03)Amd.2(09)_FIII.4Delay variation (IPDV)msTime s051015202530350 2 4 6 8 10121416182022242628303234363840424446Internet uploads by customer B during these timesMaximumdelay variation due to concurrenttraffic (upload)Customer AInternetdownload starts hereMaximum delay varia
47、tion due to concurrent traffic (download)Figure III.4 Delay variation (IPDV) of a call B to A with parallel download at A and upload at B Figure III.4 shows the delay variation (IPDV according to i1) over time measured at the customer A side, with a continuous download at the A side (starting at 4s)
48、 and two intermittent uploads at the B side. The serialization delay of one Internet packet (1500 bytes at IP layer approximately 1696 bytes on the physical layer in this case) on the A side will be 2.12 ms (downstream bit rate A: 6400 kbit/s). Since the maximal delay variation measured is much high
49、er (about 8 ms, taken as the difference of the maximum IPDV without upload and the IPDV without up- and download), the conclusion from the previous measurement is confirmed: the playout buffer of the network equipment involved in the prioritization towards the DSL line holds up to four IP packets. Rec. ITU-T G.114 (2003)/Amd.2 (11/2009) 5 The serialization delay of one Internet packet (1500 bytes at IP layer approximately 1696 bytes on the physical layer in this case) on the B side will be 23.5
