1、Report No. 31 A Technical ReportonApril 1994 Delay and EchoPerformance Guidelinesfor SONET Self-HealingRings During DiversityRoutingPrepared byT1A1.1 Working Groupon NetworkPerformance forVoice and VoicebandDataCommittee T1 is sponsored by the Alliance for Telecommunications Industry SolutionsAccred
2、ited by American National Standards InstituteCopyright 1994 by Alliance for Telecommunications IndustrySolutions All rights reserved.No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior written permission of the publisher.A Tech
3、nical Report onDelay and Echo Performance GuidelinesforSONET Self-Healing RingsDuring Diversity RoutingAbstractThis Technical Report addresses echo path delay and echo performance of largeSONET fiber self-healing interoffice rings during their failed state in which diversityprotection routing is use
4、d. Guidelines regarding the delay during ring diversityprotection mode have been included in the recent addendum to ANSI Standard T1.508-1992, Loss Plan for Evolving Digital Networks. The Technical Report is intended toprovide further guidance regarding this emerging technology. The Report includes
5、abrief general description of the operation of large self-healing rings and associatedissues followed by a technical analysis of listener echo, talker echo, and their effect onecho canceler performance in large rings. Finally, based on the above analysis, theReport provides delay and echo performanc
6、e guidelines for rings. For normal operationof a ring, it is assumed that delay performance guidelines in ANSI T1.508-1992 will bemaintained. For operation during a temporary diversity protection mode, which isexpected to be brief and infrequent, it is recommended that higher-than-normal delaysbe al
7、lowed, but that maximum round-trip delay be limited, based on echo performanceconsiderations. In particular, it should be noted that during the diversity protectionmode, round-trip delay may increase to a maximum of 35 ms for intraLATA calls with 6-dB design loss, to a maximum of about 20 ms for int
8、raLATA calls with 0-dB or 3-dBdesign loss, and to a maximum of 20 ms for the access portion of interLATA calls.Because the guidelines presented in this report are based only on voice transmissionquality criteria, it should be noted that other considerations, such as the interworking ofnetwork equipm
9、ent, may present more restrictive delay limitations.Document T1A1/93-059Prepared byT1A1.1Working Group on Network Performance for Voice and Voiceband DataiDelay and Echo Performance Guidelines forSONET Self-Healing Rings During Diversity RoutingFOREWORDThis T1 Technical Report was initiated in the T
10、1A1.1 Subworking Group onLoss/Level Planning. The Technical Report provides network planners anddesigners guidelines concerning the transmission delays and echo performancethat can be anticipated during temporary diversity rerouting of large SONETrings. The intent of the Report is to provide, to the
11、 telecommunications industry,technical information on delay and performance aspects of SONET rings.Accordingly, the use of “should“ or “recommend“ throughout the document is notto be construed as implying any requirement.Suggestions for improvement of the Technical Report are welcome and shouldbe se
12、nt to: Alliance for Telecommunications Industry Solutions - Committee T1Secretariat, 1200 G Street N.W., Suite 500, Washington, D.C. 20005,iiCONTENTS1 - Introduction 11.1 - Scope and Purpose. 11.2 - Overview of Rings, Delay, and Echo Performance Issues 12 - Definitions and Classifications. 32.1 - Ri
13、ng and Ring-Protection-Switching Definitions 32.2 - Ring Classifications . 32.2.1 - Ring. 42.2.2 - Line Switched Ring . 42.2.3 - Path Switched Ring. 42.2.4 - Unidirectional Ring 42.2.5 - Bi-directional Ring . 43 - Performance Issues. 44 - Criteria for Acceptable Degradation of Echo Performance Durin
14、g DiversityProtection 55 - Listener Echo 66 - Talker Echo . 77 - Echo Canceler Tail Delay 88 - Conclusions and Guidelines 9References . 19Abbreviations and Acronyms 20FiguresFigure 1 - Guidelines for NI-to-POT Delay (from ANSI T1.508-1992). 11Figure 2 - Typical Fiber Ring Architecture. 12Figure 2a -
15、 Diversity Protection Example 1. 12Figure 2b - Diversity Protection Example 2. 13Figure 2c - Diversity Protection Example 3. 13Figure 2d - Diversity Protection Example 4. 14Figure 2e - Diversity Protection Example 5. 14Figure 3 - Various Echo Paths 15Figure 4 - Effect of Delay on Listener Echo Perfo
16、rmance. 16Figure 5 - Effect of Delay on Talker Echo Performance (Design Loss = 6 dB) . 17Figure 6 - Effect of Delay on Talker Echo Performance (Design Loss = 3 dB) . 1811 Introduction1.1 Scope and PurposeThis Technical Report addresses the effect that protection diversity routing inSynchronous Optic
17、al NETwork (SONET) rings may have on voice transmission quality.Rural rings can be several hundred miles in circumference and, under diversity routingconditions, additional network nodes and path length can cause a significant increasein transmission and signal processing delay. This could affect ec
18、ho performance andecho canceler operation on intraLATA calls, interLATA calls or both.Transmission delay for exchange access is treated in ANSI T1.506a1and ANSIT1.508.2Figure 1 graphically depicts the typical range of round-trip delay in exchangeaccess networks for normal routing. Additional delay g
19、uidelines for exchange accessprovided on SONET rings are given in ANSI T1.508a.3This Technical Reportsupplements the information in these standards, and includes material on intraLATAconnections as well as exchange access connections.This Technical Report presents the results of analysis of the voic
20、e transmissionperformance of large SONET rings during temporary diversity rerouting caused by anoutage on the normal route, and provides round-trip delay guidelines for networkproviders, end-users, and network equipment designers and manufacturers.4, 5, 6, 7, 8, 9The effects of listener and talker e
21、cho on end-user perception of voice transmissionquality and the effects of increased delay on echo canceler performance are discussed.Guidelines for maximum delay are given for exchange access and intraLATAconnections during diversity rerouting.1.2 Overview of Rings, Delay, and Echo Performance Issu
22、esNetwork survivability is a major concern for network providers. To enhance networksurvivability, some transport facility networks are being arranged geographically in theform of fiber rings.4, 5, 10, 11, 12, 13Unidirectional and bi-directional, and path switched andline switched, types of rings ma
23、y be used. The delay during a failure depends on thetype of ring because each type uses a different method for routing the protectionchannels. The details of these routing methods are outside the scope of thisdocument. In general, line switched rings switch the working channels to and from theprotec
24、tion channels at the two nodes adjacent to a failure, effectively replacing thefailed span with the long arc of the ring between these two nodes. Path switched ringsswitch a failed working channel to the protection channel along the other arc of the ringthat is not used by the working channel. Trans
25、mission in bi-directional rings is normallyvia the short arc of the ring but, for diversity routing, is switched temporarily to theprotection channels which follow the complementary long arc around the ring.Transmission in unidirectional rings operating in the normal working condition generallyflows
26、 in one direction, thereby using the entire circumference for duplex operationbetween two nodes, and is switched to the other direction for diversity routing.However, for unidirectional path switched rings, transmission in the normal workingcondition may be arranged so that both duplex directions of
27、 a working channel flowalong the short arc of the ring and transmission over the long arc occurs only when aprotection switch is invoked; this arrangement would, by definition, no longer be aTECHNICAL REPORT NO. 312unidirectional ring (see Section 2.2.4). More information on these ring types is give
28、n inSection 2.Diversity routing events are expected to be brief and infrequent. Usually, diversityrouting will be invoked when a failure such as a fiber cut or a fiber-optic equipment cardfailure has occurred. At times, operations activities (e.g., adding a node to a ring) willrequire the initiation
29、 of diversity routing. However, these events are not common andthe duration and time of occurrence are controlled.This transmission diversity feature of SONET rings is an advantage because itmaintains service for the end user during what would otherwise be an outage. SONETrings will survive any sing
30、le-point failure by restoring 100% of the protected traffic.They will also survive any multiple-point failure (such as a failure in which both spansadjacent to a node fail) with a high degree of restored traffic, depending on the trafficpattern and the location of the failures. However, the increase
31、d delay of the long partof the ring could degrade voice echo performance and end-user perception of theservice during the diversity protection period.SONET rings are being deployed in both metropolitan and rural areas. Metropolitanrings will have small circumferences, and therefore increased delay i
32、s usually not aconcern.However, delay is of concern for rings in rural areas. The choice of fiber routes forthese rings is usually limited and rural rings will often have long circumferences, up toseveral hundred miles. Providing additional fiber routes to reduce the circumference ofsuch rings may n
33、ot be practical.Interoffice fiber rings in rural areas could have a circumference of several hundredmiles. A typical interoffice rural ring could interconnect host end offices (HEOs), theirsubtending remote switching units (RSUs), and other stand-alone end offices (EOs).The ring typically can carry
34、exchange access traffic for interLATA calls as well as localtraffic. The basic architecture of an interoffice ring, connecting HEOs, RSUs, and otherEOs, is depicted in Figure 2.During diversity routing events, when the ring is in the protection state, the length of thetransmission path can increase,
35、 thereby affecting performance. As the circumferenceincreases, the echo path length and delay may also increase. Thus, service is restoredbut at risk of a reduced level of performance rather than the risk of loss of service thatwould occur when using traditional transmission systems.While delay is t
36、ypically small on the normal route, by design and geography, it can belarge on the long-way-around diversity protection route. Examples of normal anddiversity protection routes are illustrated in Figures 2a through 2e. (These figuresdepict various connection cases on the ring architecture shown in F
37、igure 2. For theseexamples, a bi-directional, line switched ring is assumed.) A long rural ring of 420 routemiles circumference (one-way) has a facility round-trip propagation delay of about 7 msin addition to processing delay at the various network elements. The delay duringdiversity protection rer
38、outing could be more than the delay of one trip around the ringand up to twice or three-times the long-way-around delay, because of ring-designcharacteristics such as tandeming, backhauling, and limited connectivity. BackhaulingTECHNICAL REPORT NO. 313is the use of the same route more than once. An
39、example of backhauling on theprotection route between a host and a remote is shown in Figure 2a, while an exampleof backhauling on the normal route between the same host and remote is shown infigure 2b. Limited connectivity refers to cases in which some nodes have connectivityonly through a certain
40、intermediate node. An example of multiple trips around a ringcaused by limited connectivity is shown in Figure 2c. In this case, connectivity betweentwo remotes is provided only via their host, with the result that each host-remote linkuses the diversity protection route.2 Definitions and Classifica
41、tions2.1 Ring and Ring-Protection-Switching DefinitionsThe following definitions are based on those in ANSI T1.105.01-1993.14bi-directional. See Section 2.2.line. A transmission medium, together with the associated equipment, required toprovide the means of transporting information between two conse
42、cutive NetworkElements (NEs) one of which originates the line signal while the other terminates theline signal (see Figures 2 and 3 in ANSI T1.105).line switched. See Section 2.2.path. A logical connection between the point at which a standard frame format for thesignal at the given rate is assemble
43、d, and the point at which the standard frame formatfor the signal is disassembled (see Figures 2 and 3 of ANSI T1.105).path switched. See Section 2.2.protection channels. The channels allocated to transport the working traffic duringdiversity routing. When there is diversity routing, traffic on the
44、affected workingchannels is bridged onto the protection channels.restoration. The process of re-establishing traffic continuity in the event of a failure, orfailures, affecting that traffic.working channels. The channels over which working traffic is normally transported.working traffic. Traffic tra
45、versing a ring normally carried in working channels, except inthe event of a ring or span diversity routing, in which case it is restored on theprotection channels.unidirectional. See Section 2.2.2.2 Ring ClassificationsThe following ring classifications are based on those in ANSI T1.105.01-199314Fu
46、rtherdescription of these ring types is given there.TECHNICAL REPORT NO. 3142.2.1 RingA ring is a collection of nodes forming a closed loop whereby each node is connectedto two adjacent nodes via a duplex communications facility. A ring provides redundantbandwidth or redundant network equipment, or
47、both, so that distributed servicesautomatically can be restored following a failure or degradation in the network. Suchrings are self-healing.SONET rings can be divided into two general categories, line switched and pathswitched, according to the SONET layer used to trigger automatic diversity routi
48、ngaction. Furthermore, these categories can be divided into unidirectional and bi-directional subcategories, according to the direction of the traffic flow under normalworking conditions.2.2.2 Line Switched RingA line switched architecture uses SONET line layer indications to trigger the diversityro
49、uting action. Switching action to recover from failures is performed at only the linelayer, and does not involve path layer indications. Line layer indications include linelayer failure conditions and signaling messages that are sent between nodes to effect acoordinated line diversity routing.In the event of a failure, line switched rings establish ring switches at the two nodesadjacent to the failure. In addition, four-fiber bi-directional line switched rings may alsohave the capability of using span switching.2.2.3 Path Switched RingPath switched rings con
