Even though the framework and parameters for an SLA are defined, you also need to have a standard way to measure these parameters so that real comparisons and verifications can be made. FRF.19 provides the capability to test, diagnose, and measure the quality of a Frame Relay service in a comparable and consistent fashion with Operations, Administra-tion, and Maintenance (OA&M) information that can be passed within an OA&M Frame Relay message type. Used to define a reference model for typical Frame Relay network con-figurations, and indicating monitoring points for VCs that span different network sections and administrative boundaries, this information can consistently measure conformance to an SLA that may be in place for a connection. Figure 7-28 shows the reference model for the monitoring points that could be used to verify the three different types of SLAs defined in FRF.13, along with additional network diagnostic information.
FRF.19 is designed to either supplement or replace the ITU I.620 Frame Relay opera-tions and maintenance principles and funcopera-tions; it does not claim interoperability with these functions of the ITU I.610 specification. Note that the indicated monitoring points marked “MP” in Figure 7-28 are supported with FRF.19 and are defined as Frame Relay OA&M monitoring points (FROMP), while the monitoring point marked “AMP” is an ATM end-point device included to illustrate an FR/ATM interworking scenario. How-ever, OA&M monitoring of the Frame Relay DLCI while performing service inter-working to an ATM endpoint is not yet standardized.
The complete network monitoring reference model is laid out by describing different virtual circuits that cross the reference network, three individual circuit models, and an
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ATM & MPLS Theory & Application: Foundations of Multi-Service Networkingadministrative domain reference model. Figure 7-28 indicates these three models.
The reference network shows a Frame Relay network that also interoperates with an ATM network. There are several virtual connections that can be made across this net-work. A VC may cross a single Frame Relay access network section, two Frame Relay ac-cess network sections connected via an NNI, two Frame Relay acac-cess network sections connected via a Frame Relay transit network, two Frame Relay access network sections connected by an ATM transit network section using network interworking, or a single Frame Relay VC connection spanning a single access network section terminated as an ATM VC with service interworking. Each VC has different monitoring points in order to verify performance with the SLA type, either an end-to-end, to-edge, or edge-to-edge queue type. A virtual connection may also cross administrative boundaries
Figure 7-28. OA&M monitoring points
The OA&M monitoring and reporting functionality is set up automatically with sig-naling procedures for both PVCs and SVCs. A Frame Relay MP may initiate an MP dis-covery process in order to engage devices that are advertising their presence and are willing to participate in OA&M. This process is initiated with a HELLO message that each MP sends out periodically. Upon receipt in return of a valid HELLO message an MP peer, which indicates the OA&M capabilities that are available for the duration of the Frame Relay connection, other OA&M messages (other than the HELLO) can be trans-mitted toward the MP closest to the VC endpoints. HELLO messages apply only within a single domain; and when a VC crosses administrative boundaries, the MP may generate separate HELLO messages for every administrative domain to which it belongs.
A Frame Relay MP may belong to multiple domains and may be allowed to generate separate HELLO messages for each of these domains, and may advertise different OA&M capabilities as well. An MP at an administrative boundary can also not be allowed to partic-ipate in exchanging information with other domains. The administrative domains provide for well-defined zones of OA&M processing. All OA&M messages contain a domain iden-tification that is used to identify the intended administrative domain. A Frame Relay MP at a domain boundary will not forward its domain’s HELLO messages beyond the domain boundary, it also will, by comparing the origin location of the MP message, detect and dis-card counterfeit messages destined toward points inside the MP domain that enter from outside its domain. Messages destined for other domains will be passed through in either direction without interpretation or discard.
Three service measurements are supported by the OA&M service verification mes-sage: frame transfer delay (FTD), frame delivery ratio (FDR), and data delivery ratio (DDR). The measurements are independent of each other and use individual information fields, but they could be combined in a single message.
The FTD is measured as a round-tripdelay, and this is divided in half to obtain the one-way FTD value, as defined in FRF.13. The OA&M message initiates the measurement by sending an “initiator” TX time stamp representing the time the opening bit of the frame begins transmission. The responding Frame Relay MP copies this value and then adds a “responder” RX timestamp, representing the arrival time of the closing bit of the frame, as well as the opening bit of the frame that will begin transmission. A pad field is used to assure that the received and sent messages are exactly the same length. Once the initiator receives this message back and records the timestampof the arrival time of the closing bit of the received frame, the FTD is determined by
FTD = ((Initiator_RX – Initiator_TX) – (Responder_TX – Responder_RX)) / 2 The FDR is measured by completing several exchanges between the initiator and responder MPs. The beginning of a measurement requires a synchronization of the respective MP frame counters. Attempted frame transmissions are referred to as Frames Offered. Successfully delivered frames are referred to as Frames Delivered. These
measures are further categorized as being within the committed information rate (Frames OfferedCor Frames DeliveredC) or as burst excess ( Frames Offeredeor Frames Deliverede), as follows:
Frames OfferedC= Frames OfferedC2 – Frames OfferedC1 Frames Offerede= Frames Offerede2 – Frames Offerede1 Frames DeliveredC= Frames ReceivedC2 – Frames ReceivedC1 Frames Deliverede= Frames Receivede2 – Frames Receivede1 Frames LostC= Frames OfferedC– Frames DeliveredC Frames Loste= Frames Offerede– Frames Deliverede FDRC= Frames DeliveredC/ Frames OfferedC FDRe= Frames Deliverede/ Frames Offerede
The DDR measurement requires synchronization, recording, and storage of the coun-ters for data offered and data received in order to establish the ratio of octets delivered and octets offered in the same way as the FDR. The values for DDR are calculated by us-ing the same methodology as the FDR, substitutus-ing data octets for frames to arrive at DDRC, DDRe, and DDR:
DDRC= Data DeliveredC(committed) / Data OfferedC(committed) DDRe= Data Deliverede(excess) / Data Offerede(excess)
Frame Relay diagnostics may also be performed on the VC between two MP points within the same domain. Two diagnostics are supported: latching and nonlatching loopback. The latching form is a service maintenance action that removes the VC from service. The nonlatching one is used to echo an individual OA&M frame without taking the VC out of service so that frames are still forwarded. Latching causes all the arriving frames of a specific VC to be looped back toward the transmitter, while other VCs passing through the same device will not be affected. While in latching loopback, only OA&M cells directly addressed to the loopback MP points will be processed, thus enabling the FTD, FDR, and DDT to still be measured and additional diagnostic information to be pro-cessed. Diagnostic information may be indications of whether latching loopback is en-abled or not, and whether the physical layer is up or down.