8733 Ethernet SLA App Guide

8/2/2019 8733 Ethernet SLA App Guide

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The Access Company

Application Guide

Carrier EthernetService Level Agreement Support Tools


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AbstractThe growth in popularity of Business Ethernet services is closely

linked to the level of maturity Ethernet technology has reached,

enabling carriers to deliver and audit hard service level agreement

(SLA) guarantees that satisfy exacting requirements fromenterprise users. Carriers and service providers deploying Business

Ethernet VPNs must be prepared to ensure measurable and

enforceable SLAs that detail commitments for user traffic handling,

availability and performance guarantees, among others.

Focusing on a Layer 2 VPN use case, this application guide reviews

the various service delivery and service assurance support

mechanisms that carriers and telecom providers can utilize to

ensure service reliability, measurable KPIs (key performance

indicators) and SLA commitments.


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Application Guide:Ethernet SLA Support Tools

2009 RAD Data Communications Ltd 1

Contents1 Business Ethernet Services and the Evolution of Carrier SLAs .................................................. 2

1.1 Business Ethernet SLA support tools ............................................................................... 4

2 SLA and Service Description .................................................................................................... 5

2.1 Layer 2 VPN use case ...................................................................................................... 5

2.2 Service description ......................................................................................................... 6

2.3 Traffic Mapping .............................................................................................................. 8

2.4 Bandwidth Commitments .............................................................................................. 10

2.4.1 Effective Throughput ............................................................................................ 13

2.5 Performance Guarantees .............................................................................................. 15

2.6 Layer 2 Control Protocol Processing .............................................................................. 17

2.7 Service Availability, Response and Repair Time .............................................................. 17

3 Service Delivery .................................................................................................................... 19

3.1 Classification ................................................................................................................ 20

3.2 Metering and Policing ................................................................................................... 21

3.3 Hierarchical Scheduling Level 0 ..................................................................................... 23

3.4 Shaping ........................................................................................................................ 26

3.5 Hierarchical Scheduling Level 1 ..................................................................................... 27

3.6 Packet Editing and Marking ........................................................................................... 29

4 Service Assurance ................................................................................................................. 31

4.1 Critical Service Test Points ............................................................................................ 31

4.2 Service Validation Tests ................................................................................................ 33

4.2.1 Connectivity verification ........................................................................................ 36

4.2.2 Fault detection and diagnostic loopbacks .............................................................. 36

4.2.3 Performance monitoring ....................................................................................... 394.2.4 Throughput measurements (RFC 2544) ................................................................. 40

Conclusion ................................................................................................................................... 41


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Application Guide: Ethernet SLA Support Tools

2 2009 RAD Data Communications Ltd

1 Business Ethernet Services and the Evolution of CarrierSLAs

Ethernet services for enterprises are booming: The global business Ethernet services market is

forecasted to reach nearly US$ 39 Billion by 2013, while 2008 saw a 43% increase in demand forbusiness Ethernet service ports in the United States alone1. Enterprise customers are embracing

Layer 2 VPN services as these offer higher bandwidth rates, competitive pricing, flexibility, and

scalability. From their end, carriers and service providers see an opportunity to enhance their

business networking solutions portfolio with next-generation services that are cheaper to operate

and which combine added user value with quick return on investment and sustainable revenue

potential. Even in the midst of the current global economic turmoil, more and more business

Ethernet services are becoming available from incumbents, as well as from Tier 2 and 3 providers,

covering an expending footprint not only on a metro level, but also on a national and international

scale.

The growth in popularity of business Ethernet services is closely linked to the level of maturity

Ethernet technology has reached, enabling carriers to deliver and audit hard service level agreement

(SLA) guarantees that satisfy exacting requirements from enterprise users, as illustrated in Figure 1.

Figure 1: Evolution of SLA parameters for Ethernet services

1 Source: Vertical Systems Group, 2009


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Naturally, service quality and assurance are pre-requisites for the enterprise market a fact that is

clearly recognized by telecom providers, who consider service level agreements important to their

strategy for winning corporate business2. Before they migrate all their corporate traffic to new

Ethernet services, organizations need to be assured that theyll receive appropriate quality of

service (QoS) and performance guarantees to support critical applications. In a Heavy Reading 2008survey, over 87% of polled enterprise users indicated that service reliability was a key factor in

choosing their provider. Enterprise users are expecting the same service consistency and reach that

have been offered by legacy TDM, ATM and Frame Relay a requirement that best effort Ethernet

services were unable to fulfill. They also demand service differentiation to facilitate efficient

operations and to meet their particular business needs, both current and future. Table 1

summarizes the must-have carrier-class service attributes of business Ethernet offerings.

Reliable Resilient, always-on connections featuring four nines or five nines availability Automatic fault isolation and quick troubleshooting; 24x7x365 support Minimal service disruptions due to link failures Quality of service priority guarantees per class of service (CoS) VPN and data security

Economical Low expenditures on customer located equipment and multi-site connectivity High throughput without heavy investments in infrastructure and equipment Scalable data rates, provisioned remotely, for pay as you grow flexibility Minimal down-time for servicing and repair

Accountable Differentiated SLA-based performance commitments for voice, video and data Clear network visibility, proactive service monitoring Real-time, on-demand reporting linked to OSS and billing systems SLA-defined penalties and credits based on performance targets

Limitless Data rates from 1 Mbps to 1 Gbps and beyond Consistent service over any infrastructure (fiber, PDH, SDH/SONET, xDSL) Versatile connectivity options (point-to-point, any-to-any)

Table 1: Business-grade attributes of Carrier Ethernet services

2 Source: The 2007 IBM Institute for Business Value and Economist Intelligence Telecom Industry ExecutiveSurvey


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1.1 Business Ethernet SLA support toolsCarriers and service providers deploying business Ethernet VPNs must also be prepared to deliver

measurable and enforceable SLAs that detail commitments for user traffic handling, bandwidth and

performance guarantees, user control protocols processing and availability, as well as for response

and repair times. This requires the installation of intelligent demarcation devices, or network

termination units (NTUs), at the customer premises, to ensure end-to-end service control and

efficient service provisioning from the service hand-off points. Such Ethernet demarcation devices

are ideally equipped with Ethernet SLA support tools, including advanced service delivery and

service assurance capabilities, as shown in Figure 2.

Figure 2: Ethernet demarcation with SLA support tools


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2 SLA and Service Description2.1 Layer 2 VPN use caseThe following chapters explain the various functionalities and support mechanisms available to

telecom providers for delivering business Ethernet SLAs, using a specific service scenario as an

example. In this scenario, a service provider is delivering a managed Layer 2 VPN service to its

business customer over a native Ethernet network with fiber, PDH and DSL access. The enterprise

uses Ethernet virtual connections (EVCs) to transport various types of traffic between remote

branches and company headquarters, as illustrated in Figure 3 below. Table 2 provides examples of

services and applications matching the different traffic types. The L2 VPN conforms to a service

level agreement, which specifies performance commitments for different QoS levels, depending on

traffic type and application.

Figure 3:Managed Layer 2 VPN service over fiber, PDH and DSL access


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Traffic Type Typical Application ExamplesReal Time IP telephony (VoIP), IP video

Priority Data Critical data applications, storage and LAN-to-LANconnectivity between local enterprise routers

Best Effort Business Internet access

Table 2: Common applications and services for various traffic classes

2.2 Service descriptionThe managed Layer 2 VPN in this example is delivered between corporate headquarters and two

branches, which are not only located in remote sites, but are also connected to the service

providers network by different technologies. Network access for Headquarters is fiber-based,

whereas Branch A is connected over multiple bonded copper PDH circuits and Branch B over

SHDSL.bis lines. To meet the particular networking needs of the enterprise, the L2 VPN service is

deployed in a point-to-point EVPL (Ethernet Virtual Private Line) topology between Headquarters

and the branches, using a different EVC for each branch-to-HQ connection. The service provider

installs intelligent Ethernet NTUs at the customer premises. These demarcation devices feature the

service hand-off points (UNI: User-Network Interfaces) and support the particular capabilities

required at each location, as well as the available access:

At Headquarters: A RAD ETX-202A Ethernet over fiber demarcation device provides a servicemultiplexed UNI, whereby all the EVCs share the same UNI for efficient utilization of available

interfaces. The network connection rate is 100 Mbps via two redundant Fast Ethernet/Gigabit

Ethernet ports, enabling future upgrades up to 1 Gbps to accommodate an anticipated increase in

traffic volumes to and from this location.

At Branch A: A RICi Ethernet over bonded PDH demarcation device with a non-multiplexed UNI thatis dedicated to a single EVC and supporting a network access rate of 32 Mbps.

At Branch B: An LA-210 Ethernet over DSL demarcation device with a non-multiplexed UNI,supporting a line rate of up to 22.8 Mbps over four bonded pairs of SHDSL.bis links.


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The NTUs perform traffic processing and SLA management to ensure consistent user experience

and to maintain SLA metrics end-to-end, despite the difference in transport technologies and

devices.

Tables 3 and 4 summarize the different service parameters of the various UNIs and EVCs.

UNI Service Attributes Headquarters BranchesSpeed (Mbps) 10-1,000 A B

1-32 1-22.8

Transmission Mode Full Duplex Full Duplex

MAC Layer IEEE 802.3 IEEE 802.3

Service Multiplexing (Max number of

EVCs/UNI)

Yes (2) No (1)

Table 3: UNI service attributes

EVC Service Attributes ValuesEVC Type Point-to-point

CE-VLAN ID Preservation IEEE

802.1Q3

EVC1 EVC2

Yes No

CE-CoS Preservation IEEE 802.1p No

Unicast Frame Delivery Unconditionally

Multicast Frame Delivery Unconditionally

Broadcast Frame Delivery Unconditionally

Max Frame Size (bytes)4 1,580

Table 4: EVC service attributes

3 See section 2.3: Traffic Mapping4 Maximum frame size should correspond with the relevant burst size values (CBS and EBS). For further details,

see section 2.4: Bandwidth Commitments


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2.3 Traffic MappingThere are two EVCs connecting HQ to the branches: EVC1 links Headquarters to Branch A, while EVC2

connects it to Branch B. Within the network, these EVCs are identified by service provider VLAN tags

(SP-VLANs), which are added to customer frames by the local demarcation device upon entering the

network and then stripped off at network egress (push and pop operations). Inband management

traffic is allocated a dedicated SP-VLAN to separate it from user traffic.

The EVCs deliver real-time (RT), priority data (PD) and best effort (BE) traffic between locations,

with each traffic type representing a different class of service within the EVCs (EVC.CoS). As each

class of service requires its own QoS guarantees, it is marked differently so it can be distinguished

by the Enterprises equipment and, more importantly, by the network: In EVC1, this is done by the

three-bit priority field (P-bits) of a customer-assigned VLAN tag (CE-VLAN), while in EVC2 traffic

classes are identified by different customer VLAN IDs (CE-VID). Since both EVCs are associated with

multiple traffic types, a mapping plan of CE-VLANs and CE-P bits to EVCs is defined in advance to

ensure efficient traffic delivery.

Tables 5 and 6 detail the correlation between VLAN IDs (VIDs) and EVCs.

Service Point UNI H (Headquarters) UNI A (Branch A) Network

EVC EVC1ID Tags CE-VLAN CE-P bit CE-VLAN CE-P bit SP-VLAN SP-P bitRT Traffic

17

6

17

6

2,000

6

PD Traffic 4 4 5BE Traffic 1 1 2Management Traffic N/A 5 7

Table 5: Mapping CE-VLANs to EVC1, services are separated by priority bits


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Service Point UNI H (Headquarters) UNI B (Branch B) Network

EVC EVC2ID Tags CE-VLAN CE-P bit CE-VLAN CE-P bit SP-VLAN SP-P bitRT Traffic 42 x 2 x

2,001

6

PD Traffic 43 y 3 y 4BE Traffic 44 z 4 z 2Management Traffic N/A 6 7

Table 6: Mapping CE-VLANs to EVC2, services are separated by customer VLAN tags

Because the customers equipment in Branch A is capable of traffic differentiation based on P-bit

values, all traffic is assigned a single VID with a separate P-bit per service. User equipment and IT

considerations in Branch B, however, require that each class of service receive its own CE-VLAN tag.

In this case, packets carrying the same CE-VID will be treated similarly by the network, regardless of

their specific CE-P bit value. All traffic assigned to EVC1 carry an outer SP-VID 2,000, while traffic

associated with EVC2 is double-tagged with SP-VID 2,001. The different classes of service within

each EVC are marked with different SP-P bit values.

As can be seen in Tables 5 and 6, the classes of service in each EVC are tagged differently at the

associated UNIs. In EVC1, both locations use CE-VLAN 17.6 (CE-VID 17, CE-P bit 6) to mark RT, 17.4

for PD and 17.1 for BE and therefore ingress/egress CE-VLAN ID preservation is required between

locations. This is not in the case in EVC2, where the various service types are assigned different CE-

VIDs at each location and the local demarcation devices must swap CE-VLAN tags in egress frames

when the SP-VLAN tags are popped, for example, replacing CE-VID 42 with CE-VID 2 for RT traffic

arriving at Branch B from Headquarters.


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2.4 Bandwidth CommitmentsThe EVPL SLA contains throughput commitments, divided into the following bandwidth profile

categories:

Committed Information Rate (CIR): The bandwidth that the service provider guarantees theenterprise, regardless of network conditions.

Excess Information Rate (EIR): The bandwidth allowance for best effort delivery, for which serviceperformance is notguaranteed and traffic may be dropped if the network is congested.The combination of CIR and EIR rates is typically referred to as PIR, or Peak Information Rate, whichrepresents the total burstable bandwidth sold to the enterprise.Committed Burst Size (CBS): The maximum size, expressed in bytes, of a burst of back-to-backEthernet frames for guaranteed delivery.

Excess Burst Size (EBS): The maximum size of a burst of back-to-back Ethernet frames permittedinto the network without performance guarantees. EBS frames may be queued or discarded if

bandwidth is not available.

According to MEF (Metro Ethernet Forum) specifications, the bandwidth profile service attribute,

which includes some or all of the above categories, can be defined per UNI, per EVC or per CoS

identifier (CoS ID; EVC.CoS). For any given frame, however, only one such model can apply. The

service provider meets the bandwidth guarantees by reserving appropriate network resources andemploying a two-rate/three-color (trTCM) rate-limitation methodology as part of its traffic

engineering policy to ensure compliance by user traffic. For the service discussed in this paper, the

policing function is performed by EVC.CoS granularity, as described in further detail in Chapter 3:

Service Delivery.


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Figure 4: CIR and EIR bandwidth profiles

EIR offerings enable carriers to generate more revenues from a given network capacity

without compromising the quality of premium or real-time CIR services. As bandwidth consumption

fluctuates throughout the day and the week, carriers and service providers can oversubscribe the

network and monetize unused portions of it by selling best effort services, provided that the

customer-located demarcation devices are equipped with reliable traffic management capabilities. This

allows total bandwidth charges to exceed actual infrastructure rates. However, because EIR bandwidth

is shared among users and applications, not all users are able to take advantage of the entire excess

bandwidth simultaneously.

Tip: EIR as a Revenue Generator


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Table 7 lists the bandwidth commitments for each class of service within EVC1 and EVC2, which are

applicable to all UNIs even though these support different access rates. To avoid delays in traffic

delivery, the bandwidth profiles in each EVC should not exceed the lowest UNI speed in the service

points connected by that EVC i.e., 32 Mbps for EVC1 (UNI A) and 22.8 Mbps for EVC2 (UNI B). As can

be seen in Table 7, the total CIR allowance for all classes of service in EVC 1 is 25 Mbps, permitting amaximum of 7 Mbps EIR to meet UNI As access connection speed limit. To better serve corporate

operations, the enterprise purchases higher EIR rates for PD and BE traffic, allowing up to 10 Mbps

for each of these classes of service if no other traffic is transmitted at the time. In EVC2, the total

PIR bandwidth is 30 Mbps, of which 20 Mbps are CIR and 10 Mbps of EIR are divided between PD

and BE traffic, allowing up to 5 Mbps for each, provided that no other traffic is transmitted

simultaneously. RT applications are typically allocated CIR bandwidth only, BE EIR only and PDs

bandwidth profiles are divided between CIR and EIR commitments.

EVC EVC.CoS Bandwidth ProfileCIR (Mbps) EIR (Mbps) CBS (Bytes) EBS (Bytes)

1Real-Time 5 0 150 0Priority Data 20 10 5,000 5,000Best Effort 0 10 0 2,500

Total 25 20 -- --

2Real-Time 5 0 150 0Priority Data 15 5 3,500 3,000Best Effort 0 5 0 2,500

Total 20 10 -- --Table 7: Effective bandwidth commitments per EVC.CoS

The CBS and EBS values should correspond with the frame sizes that typically make up each class of

service, as well as with the maximum frame size allowed at the UNI. Here, for example, a CBS value

of 5,000 bytes for PD traffic in EVC1 permits up to three frames of 1,522 bytes in each burst. A

general rule of thumb correlates between CBS value, frame size and their effect on network delay:

Large frames transmitted in a service that receives a low CBS value are more prone to delays, since

the burst allowance is exhausted quickly by a relatively low number of frames. In such cases, new

frames must await subsequent bursts.


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Figures 5 and 6 illustrate CIR and EIR values in UNI A and UNI B, respectively.

2.4.1 Effective ThroughputWhenplanningbandwidth commitments, the difference between line rate and effective throughputshould be taken into account, as frame header VLAN tags and other overhead may potentially

consume significant portions of available bandwidth. For SLA rate measurement purposes, user data

usually includes IEEE 802.3 Ethernet frames from the destination MAC address (DA) to the Frame

Check Sequence/Cyclic Redundancy Check (FCS/CRC) field, including the CE-VLAN tag. Service

provider data in IEEE 802.1ad (Q-in-Q) networks include four bytes of the SP-VLAN tag, while the

Ethernet protocol itself typically adds the following overhead5:

Preamble + Start Frame Delimiter (7 bytes + 1 byte): Synchronizes receiving networkelements with incoming signals and indicates the start of the frame

IFG Inter-frame Gap (12 bytes): Provides a brief recovery time between frames for thereceiving element (96 bit times/8 bits = 12 bytes)

5 Also exists in LAN traffic on the user side

Figure 5: CIR and EIR values in UNI A Figure 6: CIR and EIR values in UNI B


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Figure 7: User, network and protocol data in 802.3 Ethernet framesObviously, the effective throughput is directly impacted by the frame size. For large Ethernet

frames carrying, for example, 1,500 bytes of data payload at a line rate of 10 Mbps, the calculation

will be as follows:

1. Total frame size = 8B (Preamble + SFD) + 6B (DA) + 6B (SA) + 4B (SP-VLAN) + 4B (CE-VLAN)+ 2B (T/L) + 1,500B (data payload) + 4B (FCS/CRC) + 12B (IFG) = 1,546 bytes

2. User data = 6B (DA) + 6B (SA) + 4B (CE-VLAN) + 2B (T/L) + 1,500B (data payload) + 4B(FCS/CRC) = 1,522 bytes

3. Ethernet overhead = {[8B (Preamble + SFD) + 4B (SP-VLAN) + 12B (IFG)] / 1,546 bytes (totalframe size)} x 100% = 1.55%

4. Effective throughput = [1,522 bytes (user data) / 1,546 bytes (total frame size)] x 10 Mbps(line rate) = 9.84 Mbps

However, smaller frames using the same line rate are characterized by a lower effective throughput

due to higher overhead relative to their size, as demonstrated by the following calculation for a 46-

byte payload data frame:

1. Total frame size = 8B (Preamble + SFD) + 6B (DA) + 6B (SA) + 4B (SP-VLAN) + 4B (CE-VLAN)+ 2B (T/L) + 46B (data payload) + 4B (FCS/CRC) + 12B (IFG) = 92 bytes

2. User data = 6B (DA) + 6B (SA) + 4B (CE-VLAN) + 2B (T/L) + 46B (data payload) + 4B(FCS/CRC) = 68 bytes


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3. Ethernet overhead = {[8B (Preamble + SFD) + 4B (SP-VLAN) + 12B (IFG)] / 92 bytes (totalframe size)} x 100% = 26%

4. Effective throughput = [68 bytes (user data) / 92 bytes (total frame size)] x 10 Mbps (linerate) = 7.39 Mbps

The actual throughput experienced by the enterprise is therefore dependent on the relative

proportions of various applications in the traffic mix. A higher share of 68-byte user data packets,

such as being used for most VoIP traffic, will result in lower throughput efficiency. In addition to the

Ethernet-related bandwidth penalties, the physical media used for transmission may require further

overhead for framing and encapsulation. For example, Ethernet over DSL throughput is affected by

the particular transport protocol being used: The traditional DSL protocol stack includes an ATM

sub-layer, which presents heavy bandwidth fines (cell tax) of up to 20%-50%; the more recent

EFM (Ethernet in the First Mile) encoding, such as used by the LA-210 demarcation device at UNI B,

enables improved line utilization and a 5% overhead. Likewise, multi-circuit copper access that is

powered by Ethernet over NG-PDH capabilities, as is the case for the RICi demarcation device at UNI

A, can rely on constant, predictable and lower overhead with GFP (generic framing protocol), VCAT

(virtual concatenation) and LCAS (link capacity adjustment scheme) encapsulation and bonding

tools, compared to the less-efficient HDLC, MLPPP and IMA methods.

2.5 Performance GuaranteesA key element in the SLA defines the performance and QoS guarantees that the service provider

commits to the enterprise, specifically, frame delay, delay variation and frame loss.

Frame Delay (Latency) is the time a transmitted frame travels across the network until it isdelivered. VoIP and real-time services require extremely low latency, as even the smallest delay has

a dramatic effect on service quality. TCP applications are also impacted from increased network

delay, taxing the network resources with re-transmissions when session timeouts occur.

Frame Delay Variation(Jitter) is the difference in delay between consecutive frames, causingthem to arrive at their destination at inconsistent intervals. Jitter is a critical performance parameter

for real-time services.


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Frame Loss Ratio is the percentage of undelivered frames out of all the frames that weretransmitted within a certain time interval. Packet loss might lead to service degradation and can

have a negative effect on throughput when dropped frames are re-transmitted, as is the case with

TCP/IP applications.

The nominal values for the above performance commitments are specified in the SLA, together with

qualifying parameters, such as the service direction (one-way or round-trip), the percentage of

traffic and the time interval for which these commitments are valid. Table 8 details the performance

metrics guaranteed by the service provider for the enterprise. These are presented per class of

service and refer to both EVCs, in all locations.

Performance Attribute Real-Time(VoIP)

Priority(LAN-to-LAN)

Best Effort(Internet

Access)Frame Delay

Value (ms)


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Table 8 also specifies the service providers commitment for service restoration in the event of

network or equipment failure, a parameter that also affects the Service Availability performance

attribute discussed in section 2.7.

2.6 Layer 2 Control Protocol ProcessingAnother aspect of the service that must be defined in advance refers to the handling of user

Ethernet control protocols (L2CP Layer 2 Control Protocols), to avoid duplication of user and

provider bridged protocol data units (BDPUs). Table 9 lists the processing instructions for the

enterprises L2CP, according to MEF recommendations for an EVPL service.

Layer 2 Control ProtocolSTP Spanning Tree Protocol Discard

RSTP Rapid Spanning Tree Protocol Discard

MSTP Multiple Spanning Tree Protocol Discard

Pause IEEE 802.3 x Discard

LACP Link Aggregation Control Protocol Discard

Authentication IEEE 802.1 x Discard

GARP Generic Attribute RegistrationProtocol

Discard

Table 9: Layer 2 control protocol processing instructions

2.7 Service Availability, Response and Repair TimeFinally, the service provider offers various SLA packages that differ in the service support, time to

repair (TTR) and service availability commitments that they offer. In this case, the enterprise selects

the Gold package as it reflects the level of support and reliability best suited for its needs. The

relevant SLA metrics are listed in Table 10.


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Service Level Service CenterHours

ResponseTime

Repair Time AverageAvailability

Standard Mon-Fri08:00-17:00

4 Hours 12 Hours 99.0%

Silver Mon-Sat08:00-20:00


Gold Mon-Sat08:00-20:00


Platinum Mon-Sun00:00-24:00


Table 10: Service package parameters for availability, response and repair time

Service availability, or uptime, is typically calculated on a monthly basis, after measuring the number

of minutes and seconds that the network or service were unavailable to the enterprise.

To determine customer remedies for SLA breaches, unavailability instances include service outages

and network downtimes associated with unscheduled maintenance events. This means that, in a

30-day month with no scheduled down-time, the enterprise should not experience serviceunavailability for more than 4 minutes and 19 seconds throughout the entire month [60 minutes x

24 hours x 30 days x (1-0.9999) unavailability threshold]. According to the terms of the Gold

service package, the service provider assures a maximum TTR of 8 hours from the moment the

customer opens a Trouble Ticket.


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3 Service DeliveryAs per the enterprises SLA terms, VoIP traffic requires different service quality than email

communications and therefore must be handled separately by the network. By delivering multiple

services from each UNI with differentiated, per-service QoS parameters the service provider caters

to the enterprises needs, while lowering its own operational costs and improving its profit margins.

To satisfy the SLA guarantees that are listed in Chapter 2, the Carrier Ethernet demarcation devices

must support multi-priority, multi-flow traffic and ensure latency, jitter and packet delivery

performance for each flow. These devices are therefore equipped with capabilities such as

metering, policing and shaping of user traffic, as well as with a two-stage queuing mechanism that

ensures predictable performance and creates scheduling fairness with better load distribution in

the network.

The EVPL service is defined as CoS-aware, with both bursty and real time traffic and VLAN-based

EVCs. Accordingly, upstream user traffic undergoes the following processing steps by the ETX-202A,

RICi, and LA-210 demarcation devices, to ensure that QoS and SLA commitments are met:

Classification Metering and policing Hierarchical scheduling (Level 0) Shaping Hierarchical scheduling (Level 1)

Marking and editing

The following sections describe in detail ingress traffic processing as performed at Headquarters

(UNI H).

In some cases, downstream traffic also requires rate-limiting in the form of metering,

policing and shaping, to ensure that egress traffic does not exceed user equipment port limits. This

is required when UNIs receive traffic from several sources simultaneously, such as in E-LANservices involving any-to-any connectivity between numerous remote branches and a company

headquarters. In these cases, the aggregate traffic arriving from multiple sites may exceed the

bandwidth limit of the customers local equipment at a particular location. Asymmetric rate-

limiting, i.e., different policies implemented for upstream and downstream traffic, is therefore

often tasked to the local demarcation device.

Tip: Rate Limitation of Downstream Traffic


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3.1 ClassificationTraffic arriving from customer equipment is first classified according to its type. The demarcation

devices QoS engine associates incoming traffic by flows, which represent the various classes of

service within a particular EVC (EVC.CoS), i.e., Real-Time, Premium Data and Best-Effort. The

demarcation devices sort traffic by the user port through which it arrives, together with different

CoS ID selectors for each EVC. These are the customers VLAN tag priority fields for EVC 1 and user-

assigned VLAN tags for EVC2, as per the mapping charts in Tables 5 and 6. Consequently, Flows 1, 2

and 3 make up EVC1, while Flows 4, 5 and 6 are delivered in the network as EVC2.

Figure 8: Traffic classification into flows

High flexibility in traffic classification, manifested by the ability to support a wide variety of

sorting criteria, allows service providers to identify various traffic types at fine granularity and ensure

appropriate quality of service for each flow. In addition, it eliminates the limitation of by-VLAN-only

classification, which is restricted to 4,096 unique IDs. Ideally, criteria alternatives will include such CoS ID

selectors as VLAN ID, 802.1p, DSCP, IP precedence, EtherType, MAC address, IP address, and many others,as well as their combinations, depending on the capabilities of the demarcation devices.

Tip: QoS Classification Criteria


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3.2 Metering and PolicingOnce the flows are established, a metering and policing function is applied for each flow to regulate

traffic according to the contracted CIR, EIR, CBS, and EBS bandwidth profiles. Rate limitation is

performed according to the Dual Token Bucket mechanism, using a trTCM algorithm, as seen in

Figure 9.

Figure 9: Rate limitation using two-rate-three-color metering

Let us take, for example, a flow containing Priority Data traffic in EVC1, for which the QoS

parameters defined in the SLA are as follows: CIR = 20 Mbps, EIR = 10 Mbps, CBS = 5,000 bytes,and EBS = 5,000 bytes. Three Ethernet frames are sent by the user 2 microseconds apart; all three

are 1,522 bytes in sizeand all are mapped to flow number 2, marked as 17.4 (CE-VLAN ID 17, P-bit

4). The first frame drains 1,522 bytes of the 5,000 CBS bytes, leaving 3,478 bytes remaining. Since

the frame size is smaller than the CBS limit, it is marked as Green and admitted forward. The

token bytes are refilled at the CIR rate of 20 Mbps, or 2.5 Megabyte per second, resulting in 5

additional bytes in the bucket when the second frame arrives (2.5 Megabyte per second x 2

microseconds). Together, there are now 3,483 bytes available for the second frame (3,478 + 5).

The second drains another 1,522 bytes, leaving an allowance of 1,961 bytes in the bucket, to which

5 bytes are added by the time the third frame arrives. The total of 1,966 bytes (1,961 + 5) is still

enough for the 1,522 bytes of the third frame, but not for the one following it 2 microseconds

later.

Green: Frames admitted tonetwork

Yellow

: Frames admitted to

network on a best effort basis

Red: Discarded frames


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The fourth frame is also 1,522 bytes in size, however, at the time of its arrival there are only 449

bytes available, after the previous frame drained 1,522 bytes and 5 bytes were added between

frames (1,966 1,522 + 5 = 449). The fourth frame is therefore examined by the excess bandwidth

threshold, and, as it is within the EBS limit of 5,000 bytes, it is marked as Yellow and passed

forward on a best effort basis.

Since non-conformant packets are discarded, rather than queued or buffered, the metering

function is accompanied by a policing function. Another method for traffic engineering shaping

is used at a later stage to ensure that transmission is performed in a way that best utilizes network

resources.

Figure 10: Traffic metering and policing


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3.3 Hierarchical Scheduling Level 0The next traffic processing phase defines the order in which the various flows are forwarded, using

a two-step scheduling mechanism so that each flow receives the desired scheduling priority.

In level 0, different flows are assigned separate output queue blocks, each containing schedulingslots corresponding with CoS delivery priorities. Technically, each of the six flows in our EVPL service

can be assigned a dedicated queue block; however, this is not necessary as the flows are already

sorted by class of service. Instead, the three flows associated with EVC1 (17.6, 17.4, and 17.1) are

assigned one queue block while the flows associated with EVC2 (CE-VLAN IDs 42-44) are assigned

another, resulting in a total of two blocks. In the latter case, each of the three VLANs is

permanently mapped to its designated CoS queue, regardless of the CE-P bit it carries.

Figure 11: Traffic scheduling, level 0


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As can be seen in Figure 11, each queue cluster contains up to eight slots, whereby CoS 7 is

mapped to the highest priority queue, normally reserved for the service providers management

traffic, and CoS 0 the lowest. The ETX-202A supports a combination of traffic scheduling

techniques, whereby applications requiring low latency and jitter are mapped to Strict Priority

queues, while other services are mapped to the remaining slots using weighted fair queuing (WFQ):

The Strict Priority queues ensure minimal latency and jitter for the RT traffic, even when alarge amount of bursty data traffic is sent over the same uplink. Strict Priority traffic will

always be processed first, while flows mapped to the WFQ slots are buffered until the Strict

Priority queues are empty.

The WFQ technique avoids scheduling starvation of lower priority queues and ensuresrelatively fair allocation of bandwidth by sharing it among all flows. In this manner, packets

belonging to lower classes of service are not penalized when higher priority queues are not

empty and may still receive transmission time. QoS-conformant scheduling is handled by

assigning different weights to the various queues instead of equally dividing overall

bandwidth among all active flows.

To ensure adherence to SLA bandwidth guarantees, it is important to correlate weight

distribution among the queues with the committed rates allocated for each service. In EVC1, for

example, with network access rate of 32 Mbps, the real-time traffic is mapped to one of the

Strict Priority queues to ensure expedited delivery. According to the enterprises SLA bandwidth

commitments listed in Table 7, the RT flow requires a committed rate of 5 Mbps, leaving 27

Mbps to be divided between the other two classes of service that are mapped to the WFQ slots

CoS 4 (PD traffic) and CoS 1 (best effort traffic). The recommended weight ratio between

these queues is 22:5. This allows for 22 Mbps for CoS 4 to ensure it receives its 20 Mbps CIR

value and some of the EIR bandwidth, leaving 5 Mbps of EIR for CoS 1.

The same principal can be applied to EVC2, for which the access rate is 22.8 Mbps. After

securing 5 Mbps of CIR for the RT traffic with Strict Priority queuing, the remaining 17.8 Mbps

are divided at a ratio of 8:1 between the PD and BE flows, respectively. This ensures 15 Mbps

CIR and 1 Mbps EIR for CoS 4 and almost 2 Mbps EIR for CoS 1.

Tip: Using Scheduling Queues to Deliver SLA Bandwidth Guarantees


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While most of the management traffic is mapped to the highest SP queue, OAM (operations,

administration and maintenance) traffic and performance measurement messages should be

assigned the same queue slot as that of the data they test. In other words, OAM messages testing

CE-VLAN 17.6 are mapped to the Strict Priority queue assigned to CoS 6.

As the queues are filling up, new packets face a growing risk of being discarded due to lack of

buffer space. When packets arriving to overrun queues are dropped indiscriminately, such as in a

Tail Drop mechanism, differentiated QoS cannot be maintained and network performance is

hindered by intermittent periods of flooding and underutilization. The ETX-202As QoS engine

solves such issues by employing a weighted random early detect (WRED) mechanism for intelligent

queue management and congestion avoidance. The WRED algorithm monitors the state and size of

each queue and determines whether an incoming packet should be buffered or dropped, based on

statistical probabilities: Green-marked packets are directed to their respective queues, while

yellow-marked packets are admitted forward in accordance with their WRED profile.

Near-empty queues accept all incoming packets but as they begin to fill, the drop probability for

new packets increases. The different queues are allocated different occupancy thresholds, above

which incoming packets are discarded at random at a growing rate as the queue fills, until the

queue has reached a maximum threshold and all incoming packets are dropped. As can be seen in

Figure 12, various classes of service are assigned drop values that reflect their priorities. This way,

packets of lower classes of service with lower QoS commitments will be dropped earlier and at a

greater rate than those of a higher CoS.

Figure 12: WRED profiles for different classes of service


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The blue curve in Figure 12 represents the lowest priority queues for classes of service 0 and 1.

Their random packet discarding begins early, for example, when the queues are only 20% full. As

the queue reaches 40% occupancy it hits the 60% drop probability mark, after which packet

dropping picks up pace until the WRED mechanism stops the random discard and drops all packets.

By contrast, the highest priority queues for classes of service 6 and 7, represented by the purplecurve, do not drop packets until they are almost full.

3.4 ShapingTraffic coming out of the level 0 queue blocks is shaped to smooth out bursts and avoid buffer

overruns in subsequent network elements. At this stage, output packets from each buffer block

undergo a shaping function so that the overall traffic volume from each block does not exceed a

preset bandwidth value. Shaping is performed according to a Token Bucket algorithm, with a single

rate bandwidth profile that is based on the accumulated CIR values of all the flows mapped to the

relevant queue block and a certain allowance of excess rate that the service provider assigns to the

enterprise to avoid congestion:

Shaper rate queue block 0 = 5 Mbps (CIR Flow 1) + 20 Mbps (CIR Flow 2) + 0 (CIR Flow 3) =25 Mbps + excess allowance

Shaper rate queue block 1 = 5 Mbps (CIR Flow 4) + 15 Mbps (CIR Flow 5) + 0 (CIR Flow 6) =20 Mbps + excess allowance

The shaping function also compensates for the network data packet overhead that is added at later

stages, as well as for service provider OAM traffic. Packets exceeding the shaper value are delayed

in the buffer until they can be transmitted to the network. The multiple shaping rates mechanism is

an important tool to ensure that outgoing traffic volume is in line with the access connection of the

remote service point: The shaper for queue block 0 matches the bonded PDH bandwidth capacity of

UNI A, while the shaper for queue block 1 is set to meet UNI Bs xDSL access rate.

The shaping phase is illustrated in Figure 13.


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Figure 13: Shaping queue block buffer traffic

3.5 Hierarchical Scheduling Level 1In the second step of the scheduling process, each queue cluster in level 0 receives a queue slot in

level 1, with each slot corresponding with a different EVC. Allocation of different scheduling

priorities to the queues effectively sets the precedence each EVC receives at the network ingress,

as it defines the priority in which the EVC data is transmitted. In this case, the scheduling is

performed by WFQ with weights assigned at a ratio of 5:4 to queue 1, in effect giving precedence

to EVC1 traffic at the network entrance.


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The 1 Gbps rate of the physical network interface can easily accommodate the total bandwidth

consumption of both EVCs, however, the queue management and buffering system influences

traffic delay and must therefore take into account the enterprises SLA commitments.

Figure 14: A two-level hierarchical scheduler


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By managing bandwidth consumption and transmission priorities with CoS granularity, multi-level

hierarchical scheduling enables predictable, per-SLA latency and jitter performance across the

network. In addition, it provides fair distribution of bandwidth among traffic classes and users over

shared connections, by allocating excess bandwidth not required for critical applications to lower-

priority traffic. Implementation of such capabilities at the service hand-off point reduces the risk ofcongestion at the network core, further facilitating the service providers ability to meet delay and

loss guarantees.

3.6 Packet Editing and MarkingThe final stage in preparing user traffic to network transmission involves packet editing and

marking. This includes adding service provider VLAN tags (packet editing) according to the EVC

mapping attributes listed in tables 5 and 6. In this manner, packets belonging to Flows 1, 2 and 3

are stacked with an SP-VLAN tag whose ID value is 2000, while Flows 4, 5 and 6 are added SP-VLAN

tag 2001. In addition, the packets are marked with service provider priority bits in the outer SP-

VLAN tag to denote the priority each EVC.CoS receives while in the network.

Figure 15 illustrates all the various stages included in service delivery processing as describedabove.

As the packets are already marked by their level of CIR/EIR conformance (green and

yellow), metering continuity in the network can be achieved by using the P-bit field to signal a

packets color so that it has a greater chance of maintaining its status and priority throughout

the transmission. This is especially useful in color-blind networks, as well as in 802.1Q color-

aware networks with no discard eligible (yellow) marking.

Tip: Using P-bits for Color Marking


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Figure 15: Ethernet service delivery process


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4 Service AssuranceA third, crucial piece of the L2 VPN service puzzle relates to the providers ability to verify that the

actual service performance and network availability experienced by the enterprise matches SLA

guarantees. This is done by performing remote, end-to-end OAM tests, preferably without affecting

the service and in line with actual user traffic. In addition to meeting customer service expectations

and optimizing network operations, service monitoring procedures and remote loopback testing

contribute to the service providers profit margins by minimizing the risk of penalties associated

with SLA breaches.

4.1 Critical Service Test PointsService validation and testing is required at the following points throughout the service lifecycle:

At initial service turn-up: Prior to handing off the L2 VPN service to the enterprise, theservice provider must perform acceptance tests to verify that the service is running

smoothly according to the SLA, per the pre-defined classes of service. Testing at this point

also serves for generating a baseline for performance parameters, to which future test

results will be compared. Specifically, KPI (key performance indicators) metrics for end-to-

end throughput, packet delivery ratio, latency, and jitter are established. These results are

recorded and archived for customer reporting, SLA comparison and future use as needed. It

is advisable to perform burn-in stability tests over an interval of 24 hours, at minimum, to

accurately establish service behavior.

Ongoing monitoring: KPI measurements are also performed on an on-going basis, tomonitor network health and ensure that QoS is maintained per class-of-service and in

accordance with the contracted SLA. Continuous monitoring is required to detect service

degradation and network congestion, prompting relevant alerts and advising when an

increase in bandwidth is required. When service outages or connectivity faults are identified,

Trouble Tickets are initiated and appropriate remedial actions taken. The collected data is

used for billing purposes, while reports of network and service conditions are available to

the enterprise periodically and on-demand. OAM tests are performed at a frequency that

balances between the need to quickly detect and repair problems before they escalate, and

the service providers desire to limit the toll such tests take on network and bandwidth

resources.


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Figure 16: RADs Ethernet OAM Resource Utilization Calculator

RAD has developed a modeling tool to determine the network resources

required for OAM procedures. In the enterprises service topology, for example, periodic

multicast OAM messages for connectivity fault management may require a bandwidth

rate of 23,000 bytes per second and 34 FPS (frames per second) per demarcation device,

if sent every 1,000 milliseconds. Under the same network conditions, bandwidth

consumption per demarcation device climbs to over 40,000 bytes per second and 53 FPS,

if the service provider increases testing frequency to 100 millisecond intervals.

To receive a copy of RADs OAM Calculator, please contact us at [email protected].

Tip: OAM Bandwidth Calculation


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On-demand monitoring and troubleshooting: When a service outage is reported, a suite oftests is performed to remotely localize the fault prior to a technician dispatch. This reduces

MTTR (mean time to repair) and minimizes the effect on users, while lowering operating

expenses by eliminating unnecessary (and expensive) truck rolls and ensuring that

technicians are sent to the right location.

4.2 Service Validation TestsThe OAM tests performed by the ETX-202A, RICi and LA-210 at the various locations conform to the

relevant industry standards:

IEEE 802.3-2005 (formerly 802.3ah): Ethernet Link OAM is part of the Ethernet in the FirstMile, or EFM, set of standards. It relates to a single Ethernet link, typically the access

connection between the customer premises and the network edge. Specific link monitoring

procedures include auto-discovery, heartbeat, and fault notification messages; link

statistics; MIB variable retrieval; and remote loopbacks.

IEEE 802.1ag: Ethernet Service OAM, also termed Connectivity Fault Management (CFM),enables Ethernet service monitoring over any path, whether a single link or end-to-end,

allowing the service provider to manage each EVC separately regardless of the underlying

transport. CFM partitions a network into maintenance domains and hierarchy levels that are

allocated between users, service providers and third-party operators. It assigns maintenance

end points, or MEPs, to the edges of each domain and maintenance intermediate points, or

MIPs, to ports within domains. This helps define the relationships between all entities from

a maintenance perspective and permits each entity to monitor the layers under its

responsibility to easily localize problems. Service monitoring procedures include continuity

check, link trace, loopback, and alarm indication signal. As can be seen in Figure 17, the

EVPL service provided to the enterprise involves a single maintenance domain with one level.


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Figure 17: Ethernet Service OAM maintenance domain levels ITU-T Y.1731: The OAM Functions and Mechanisms for Ethernet-based Networks standard

is used for Ethernet service performance monitoring, enabling the service provider to

measure frame delay, delay variation and frame loss SLA parameters. It also includes fault

management functionalities similar to CFMs, such as continuity check, loopbacks, and link

trace.

Figure 18 displays the various network sections to which different OAM procedures apply.

Figure 18: Ethernet Link, Connectivity and Service layer OAM over different network segments


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Table 11 summarizes the different OAM tools available to the service provider for SLA verification.

These are performed either directly between sites or via an external test set probe:

Function Tools

Connectivity Verification& Fault Detection

IEEE 802.3ah heartbeat

Y.1731/IEEE 802.1ag Continuity Check (Unicast/Multicast)

Y.1731/IEEE 802.1ag Loopback (MAC Ping, Unicast/Multicast) on

demand

Fault IsolationY.1731/IEEE 802.1ag Link Trace (MAC Trace-route)

Y.1731/IEEE 802.1ag Loopback (MAC Ping, Unicast/Multicast)

L3 Ping and Trace-route

L1 IEEE 802.3ah Loopback, MIB variable retrieval

Fault Propagation Subscriber port shutdownITU-T Y.1731 Alarm Indication Signal

Fault Notification ITU-T Y.1731 Remote Defect IndicationIEEE 802.3ah Dying Gasp, SNMP Trap

Diagnostic LoopbacksL1 physical interface Loopback

L1 IEEE 802.3ah Loopback

L2/3 in-service and out-of-service Loopback at line-rate or lower, with

MAC/IP swap, per EVC/VLAN, EVC.CoS, or MAC address flows

PerformanceManagement

ITU-T Y.1731 Packet Loss, Packet Delay, Packet Delay Variation with

statistics collection per EVC.CoS (Unicast/Multicast)

RFC2544 Throughput measurements

L3 Performance Measurements

BER Testing

Table 11: OAM tools for SLA verification

Further details on selected OAM tests performed by the service provider are described in the

following sections.


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4.2.1 Connectivity verificationThe demarcation devices are configured to send periodic 802.3-2005 (802.3ah) heartbeat

messages and 802.1ag/Y.1731 Continuity Check (CC) frames. These fault detection and discovery

mechanisms are used to check the status of link and service connectivity for service acceptance at

turn-up and on an on-going basis at pre-set intervals for preventive maintenance. For example, the

ETX-202A installed at Headquarters (MEP #1 in EVC1) can be configured to send 802.1ag CC

messages to the RICi unit installed at Branch A (MEP #2 in EVC1) every 1 second, and to check for

incoming CC messages sent at that period. Loss of continuity (LOC) is declared if no CC messages

are received from MEP #2 within 3.5 seconds (3.5 times the transmission period), at which point

MEP #1 activates the RDI flag in the next CC message to MEP #2 to indicate connectivity problem. It

also notifies the user and sends an alarm to the network management system (NMS), so that the

remote MEP can initiate an uplink connection switchover.

4.2.2 Fault detection and diagnostic loopbacksRemote diagnostic loopback tests are performed on-demand at service turn-up and, once the

service is running, to enable quick isolation of problems and minimize false repair calls. This can be

done at port-level (Layer 1) according to 802.3-2005, by testing two physically connected

elements, such as the ETX-202A at Headquarters and the provider edge (PE) device to which it is

connected. When using EFM OAM, an OAM entity (e.g. the PE) can activate an out-of-service

loopback mode in a remote entity (e.g. the ETX 202A), whereby every frame received by the ETX

202A is transmitted back on that same port except for OAM control frames so that the device can

exit the loopback state. EFM OAM loopback messages cannot be forwarded by Ethernet bridges and

are therefore limited to single links.

Loopback protocols can also be executed in-service, to analyze service connectivity across EVCs

without taking the customer link down or affecting untested traffic. For example, a customer

service representative (CSR) working a Trouble Ticket relating to EVC2 can generate an end-to-end

flow loopback test between the ETX-202A at Headquarters and the LA-210 at Branch B. The

procedure is selective and executed per a variety of flow criteria, including VLAN ID, class of service

(P-bit) and source or destination MAC or IP address. This allows the loopback messages to traverse

multiple hops, including intermediary switches or bridges, without disrupting the traffic flows thatare not being tested.


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The receiving device LA-210 swaps the source and destination MAC addresses of incoming

packets prior to looping them back, so as not to create a conflict in the switches or bridges along

the path. End-to-end loopback tests can also be performed for Layer 3 services, in which case the

receiving device swaps the IP addresses.

Figure 19: OAM loopback tests in Layers 1, 2 and 3

The test setup procedure includes such user-defined parameters as the MAC address of the tested

device, preferred testing standard, test run-time, and other relevant metrics. If a connectivity error

is detected, i.e., the LA-210 does not respond within a specified period of time, the CSR would

locate the failure point by attempting to loop another device in the path, or by sending a Link Trace

request for hop-by-hop path tracking, to identify non-responsive maintenance intermediate points

(MIPs).

The test results detail loss and error rates (BER) for the loopback frames, as well as round trip

duration and delays. These metrics help determine the service performance and connection quality,

as described in further detail in section 4.2.3. The Link Trace test results display the responsive

nodes, enabling the CSR to map the service path, pinpoint problematic MIPs and dispatch a

technician to the right location for a quick repair. Alternatively, the CSR can use the 802.1ag

Loopback test to isolate faulty MIPs, by looping successive intermediary points until the fault is

identified.

Table 12 summarizes the various Loopback methodologies and their main capabilities.


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Method L1 PHY IEEE802.3-2005(802.3ah)

IEEE802.1ag ITU-TY.1731 L2 LB withMAC Swap L3 LB withIP AddressSwap

In-Service/Out-of-Service(OOS)

OOS OOS In-Service In-Service In-Service

+ OOS

In-Service

+ OOS

Performedat Line Rate X X

Performedon ActualData X X

Per Flow(Incl. CoS) X X X X

TraversesL2 BridgedNetworksX X

TraversesL3 RoutedNetworks

X N/A N/A N/A N/A

Mechanism ViaManagement

Console

802.3ah LB 802.1ag

LB

Y.1731 LB Automaticper MACAddress

Automaticper MAC, IP

Address

Standard X X

Table 12: Loopback Tests Comparison


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4.2.3 Performance monitoringThe demarcation devices at the various enterprise locations perform proactive performance

measurements for the EVPL service on an on-going basis. By analyzing the measured data, the

devices calculate and report standard performance metrics per class of service flow, including

Y.1731-defined latency, jitter and packet loss.

Frame Delay (latency): Tests the travel time across the network for delivered frames andmeasured end-to-end as the customer views it, i.e., between ingress UNI-N (network side

of the UNI) and egress UNI-N. The test measures the elapsed time between the start of

transmission of the first bit of a time-stamped Delay Measurement (DM) frame at a source

MEP (e.g. ETX-202A at Headquarters), and the arrival of the last bit of that same frame at

the destination (e.g. RICi at Branch A). The receiving MEP then compares the time-stamp to

its own reference clock and calculates end-to-end transmission delay. Unidirectional delay

measurements require that both MEPs are synchronized. Alternatively, frame delay can be

measured on a round-trip basis, by analyzing the difference between the transmit time

stamp in a DM message and the receive time stamp of the DM reply that was returned to

the originating MEP.

Frame Delay Variation (jitter): Measures the variation in frame delay by comparing the timeinterval between consecutive frames belonging to the same CoS flow at the ingress UNI to

the delay in arrival of the same frames at the egress UNI. In round-trip delay variation

calculations, FDV is defined as the difference between two consecutive frame delay

measurements at the same MEP.

Frame Loss: Uni-directional (dual-ended) frame loss ratio is determined by analyzing thecounters for sent and received CC messages at the service end points and measuring the

number of lost/discarded frames out of all frames that should have been delivered within a

specified time interval. Bi-directional (single-ended) frame loss ratio measurements refer

only to the initiating device and involve the exchange of Loss Measurement (LM) messages

and LM replies.

The ETX-202A, RICi and LA-210 record minimum, maximum and average values for delay and delayvariation, together with the rate of frame loss and the number of seconds during which the service

was unavailable all for a pre-set interval. Performance statistics are collected and sent periodically

to the service providers NMS to deliver an up-to-date account of service quality, as well as an

historical view of network and service behavior, without over-taxing the network with excessive


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management traffic. This enables both provider and customer to easily evaluate actual performance

over time and compare it to SLA guarantees.

The continuous monitoring of KPI for multiple MEPs and flows simultaneously allows the service

provider to detect degradation in service quality and to take remedial actions to quickly restore

appropriate performance levels.

When the counters of any of the tested parameters rise above or drop below pre-set thresholds

within the specified sampling period, the demarcation devices send SNMP traps to notify the

associated management station,and update the event log for future reference.

4.2.4 Throughput measurements (RFC 2544)The Internet Engineering Task Force (IETF) standard RFC 2544, Benchmarking Methodology for

Network-Interconnect Devices, defines out-of-service testing procedures for evaluating the

performance of network devices, among which are network throughput measurements. These allow

the service provider to baseline service performance and determine the available bandwidth for

each EVC, by establishing the maximum transmission rate at which no packets are dropped.

Throughput measurements can be conducted from the providers NOC (network operations center)

using a test head device, or initiated by the ETX-202A at Headquarters, starting at the service

hand-off point and testing every network element along the service path. RFC 2544 outlines a set

of frame sizes for which throughput measurements are conducted, including 64, 128, 256, 512,

1024, 1280, and 1518 bytes, as well as Jumbo Frames. This helps pinpointing the processing delays

caused by short frames and identifying network equipment that is having trouble handling larger

packets. Based on packet receive-rate results, the service provider can also determine the maximum

packet length that can be offered to the enterprise with SLA guarantees, after calculating frame

extension allowance for headers, SP-VLAN tags and other overhead.


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ConclusionAs Ethernet technology progresses, it presents telecom providers with opportunities to tap into a

rapidly growing market and improve their competitive advantage by offering business clients new

customized services at a higher speed and lower cost. Key enablers for achieving these goals are

carrier-grade demarcation devices equipped with service delivery and service assurance capabilities.

This application guide reviews the various tools carriers and service providers can utilize to ensure

business-grade performance for Ethernet services and to meet enterprise expectations for service

reliability, measurable KPIs and SLA guarantees. By executing sophisticated traffic management

schemes and standardized testing procedures right off the user premises, carriers can also manage

their network resources smartly and lower their spending on equipment and operations.


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North America HeadquartersRAD Data Communications Inc.900 Corporate DriveMahwah, NJ 07430 USATel: (201) 529-1100,Toll free: 1-800-444-7234Fax: (201) 529-5777

il k d

International HeadquartersRAD Data Communications Ltd.24 Raoul Wallenberg St.Tel Aviv 69719 IsraelTel: 972-3-6458181Fax: 972-3-6498250E-mail: [email protected]

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8733 Ethernet SLA App Guide

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