1、 AMERICAN NATIONAL STANDARD FOR TELECOMMUNICATIONS ATIS-0100027.2010(R2015) Availability A Guide to Consistent Definitions As a leading technology and solutions development organization, ATIS brings together the top global ICT companies to advance the industrys most-pressing business priorities. Thr
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11、dered functional across all 10 time periods. Once a non-functional period is declared, each subsequent atomic time window is non-functional until 10 consecutive atomic time periods below the threshold are encountered. In this way, the determination of functionality at any moment depends not only on
12、measurements within that moments atomic time period, but also on those atomic time periods near it in time. Quality of Service (QOS) may affect how the parameters associated with the criteria for functioning vs. non-functioning are set. The availability definition may be configured to express differ
13、ences of availability for different levels of QOS. Generally this is done within the functionality definition. One way for QOS to be expressed is to have different thresholds for different QOS levels. In both the Y.1540 and Y.1561 examples above, services with a lower QOS could find higher lost pack
14、et rates more acceptable. Therefore, such low QOS services could have higher thresholds to express this difference in functionality for lower QOS. However, other means could also be used to configure QOS differences. In the Y.1561 example, a definition in which only 5 atomic level period ratios are
15、required to declare non-functionality places a more stringent requirement on functionality. So, using a consecutive time period requirement less than 10 could be used for availability definitions of higher QOS while a consecutive time period requirement greater than 10 could be used for lower QOS. I
16、n general (particularly in a point-to-point situation), availability definitions are based on a binary state: the subject is either functional or not functional. However, it can be difficult to make such a binary assessment for complex systems or services (that are not necessarily point-to-point). I
17、n such cases, the binary state non-functional (0) or functional (1) can be extended to the range of values from 0 to 1 inclusive. This range can be interpreted as expressing partial outage of the subject or as a probabilistic assessment of the functionality of the subject. The measurements can be ma
18、de in a number of ways packet loss, severely-errored seconds, line card availability, completed connections, etc. Some examples of methodologies associated with multipoint services follow: Example1: ATIS Technical Report ATIS-0100020, ATIS-0100020,“Quantifying the Impact on IP Service Availability f
19、rom Network Element Outages” defines availability in terms of the weighted fraction of time that the service is not in an outage state. The weights (fraction of service lost during an outage) express functionality in terms of partial outage. Example 2: The ATIS Technical Report “A Methodology for Es
20、timating the Availability of Access IP Routers in Terms of Customer Facing Line Card Availability” provides a methodology for estimating network availability for any service by examining a key hardware components (line cards) “up time”ATIS-0100025. By examining the hardware “up-time” as a surrogate
21、of the service availability, one essentially considers a fraction of the service unavailable when an end user is isolated from the network by a line card failure. Example 3: The ATIS Technical Report “DPM Metric for Transactional Services such as VoIP” ATIS-0100008 provides a metric that takes advan
22、tage of customer billing record (CBR) information to estimate defects per million (DPM) that is directly related to availability. 6 UNIVERSAL METHOD During 2008, ITU-T was finalizing the contents of Y.1563, “Ethernet frame transfer and availability performance“, which was issued in January 2009. The
23、 following table shows comparisons of particular aspects of the methods described in ITU-T Y.1540, Y.1561, Y.1563 and the Metro-Ethernet Forum (MEF) 10.2. Additionally, a generic “Universal” method is illustrated that is compatible with all of the methods. The table comes from a presentation made to
24、 the MEF Aligning Availability ad hoc group and ATIS-0100027.2010 7 is included in this Technical Report to illustrate the similarities of the various standardized methods of measuring availability. The “Universal” method is an estimation technique (type aspect) that is looking at a particular servi
25、ce (subject aspect). It describes parameters to be used for the time aspect and the functioning definition aspect. The left hand column of the following table provides a description for the estimation parameters for the four standards documents listed and the Universal Method. In the body of the tab
26、le, the parameter names are shown (if applicable) and if a value is recommended, it is included. If the value is not shown in the table, it is still under study or to be determined by the customer and service provider. More detailed descriptions of the methods and parameters are in the sections foll
27、owing the table. Table 1 - Comparison of Availability Calculation Methods Y.1540 Y.1561 MEF 10.2 Y.1563 Universal Loss threshold for the transition: available unavailable - 0.75 S10.15 CuS10.5 S1Loss threshold for the transition: unavailable available - 0.75 S10.15 CaS10.5 S2Length of the atomic mea
28、surement interval, the loss threshold refers to Tav5 min Tlb1 s delta t Tav1 s TlbMinimal number of samples (packets) for a decision Mav1000 Mlb- - MlbNumber of atomic measurement intervals the decision is based on 1 10 n 10 n Is the window sliding? (no:0 yes:1) 0 1 0 1 Optional, SW = 0 or 1 Availab
29、ility performance objective requirement - - A - - ATIS-0100027.2010 8 Y.1540 Y.1561 MEF 10.2 Y.1563 Universal Measurement interval, the requirement refers to - - T - - Time interval the decision is made for 1*Tav1*Tlbn*delta t 1*Tav1 s (1-SW)*(n-1)*Tlb+ TlbTime interval the transition is based on 1*
30、Tav10*Tlbn*delta t 10*Tav 10 s n*TlbIs there a hysteresis? May current availability status affect the next status? no yes yes yes yes 6.1 Y.1540 ITU-T Rec. Y.1540 defines the IP packet transfer and availability performance parameters. IP service availability function: The IP service is available on
31、an end-to-end basis if the packet loss ratio for that end-to-end case is smaller than a given threshold. If IPLR c1 unavailable state If IPLR = c1 available state The minimum number of packets that should be used in evaluating the IP service availability function is Mav The minimum duration of an in
32、terval of time during which the IP service availability function is to be evaluated is Tav Percent IP service unavailability (PIU): The percentage of total scheduled IP service time (the percentage of Tavintervals) that is (are) categorized as unavailable using the IP service availability function.
33、Percent IP service availability (PIA) The percentage of total scheduled IP service time (the percentage of Tavintervals) that is (are) categorized as available using the IP service availability function. 6.2 Y.1561 ITU-T Rec. Y.1561 defines the performance and availability parameters for MPLS networ
34、ks. It is noted, that connection-oriented services require a more continuous packet transfer than other packet services. A severe loss block (SLB) outcome occurs for a block of packets observed during time interval Tlbat ingress MP0when the ratio of lost packets at egress MPito total packets in the
35、block exceeds s1. ATIS-0100027.2010 9 Evaluation of successive blocks (time intervals) should be non-overlapping. The minimum number of packets that should be used in evaluating the severe loss block outcome is Mlb. MPLS service availability function The onset of unavailability begins with the occur
36、rence of ten consecutive SLBs. These ten seconds are part of unavailable time. A period of unavailability ends with the occurrence of ten consecutive seconds, none of which are SLB. These ten seconds are part of available time. The ten-second criteria are supported using a sliding window with one-se
37、cond granularity. Percent MPLS service unavailability (PIU): The percentage of total scheduled service time that is categorized as unavailable using the MPLS service availability function. Percent MPLS service availability (PIA): The percentage of total scheduled service time that is categorized as
38、available using the MPLS service availability function. 6.3 Y1563 ITU-T Rec. Y.1563 defines the performance and availability parameters for Ethernet service. The Ethernet service availability definition is based on a model which uses two states corresponding to the ability or inability of the networ
39、k to sustain the service in the available state. Transitions between the states of the model are governed by the occurrence of patterns of severe errored seconds in the Ethernet layer (SESETH). This Recommendation views availability from the network perspective, where availability performance is cha
40、racterized independently of user behaviour. A severe errored second (SESETH) outcome occurs for a block of frames observed during a one-second interval at ingress MP0 when the corresponding FLR (i.e., the ratio of lost frames tototal frames in the block) at egress MPi exceeds s1. A provisional value
41、 s1 of 0.5 is proposed, and different values may also be chosen depending on the class of service (CoS). Evaluation of successive one-second intervals is non-overlapping.A period of unavailable time begins at the onset of 10 consecutive SESETH outcomes. The corresponding period of time is considered
42、 to be part of unavailable time. During the unavailable time period, the Ethernet network is in unavailable state. A new period of available time begins at the onset of 10 consecutive non-SESETH outcomes. The corresponding period of time is considered to be part of available time. During the availab
43、le time period, the Ethernet network is in available state. All 10-second intervals evaluated for state determination must be entirely composed of scheduled service time (the time interval when the service is intended to be operational, and is usually specified in a service agreement). This means th
44、at all seconds of scheduled service time are evaluated at least once for state determination purposes. Because an Ethernet service is bidirectional, an Ethernet network is in the unavailable state if either one, or both directions, are in the unavailable state. The unidirectional availability can be
45、 measured by the criteria mentioned above. 6.4 MEF 10.2 The Metro Ethernet Forum document 10.2 MEF 10.2 (Ethernet Services Attributes Phase 2) defines the performance parameters for Ethernet Virtual Connections. The MEF is currently revising this document to align the definition with the definitions
46、 currently existing in ITU documentation. Since changes may be made in future versions of the MEF document, the following paragraphs reflect the contents of the current (10.2) version. ATIS-0100027.2010 10 Availability Performance is the percentage of time within a specified time interval during whi
47、ch the Frame Loss Ratio Performance is small Informally, Availability Performance is based on Service Frame loss during a sequence (of length n) of consecutive small time intervals (delta t). If the previous sequence was defined as available, and if the frame loss is high (Cu) for each small time in
48、terval in the current sequence, then the current sequence is defined as unavailable. Otherwise the current sequence is defined as available. On the other hand, if the previous sequence was defined as unavailable, and if frame loss is low (Ca) for each small time interval in the current sequence, the
49、n the current sequence is defined as available. Otherwise, the current sequence is defined as unavailable. The values of Cuand Cacould be different; however, alignment with other definitions would make them equal. For bi-directional services, both directions must be available for the service to be available. For multi-point services, a specified subset of pairs of end-points must be available for the service to be considered available. 6.5 Universal Method A universal Ethernet (packet technology) availabili
