A. Ethernet operations, administration, and maintenance (OAM) is a general term for the management capabilities associated with Ethernet technology. In the context of Ethernet networks, it refers to the tools and utilities available to install, monitor, and troubleshoot the network and allow service providers to offer improved levels of service assurance.
Q. Why is Ethernet OAM important to service providers?
A. Simply put, Ethernet OAM allows carriers to deploy Carrier Ethernet networks. Service providers can use Ethernet OAM to reach out from the central office (CO) all the way to the customer premises providing the "eyes, ears, and hands" with which network operations can be performed. Ethernet OAM provides the eyes and ears of the network, raising warnings and alarms whenever a failure or degradation is detected. Lastly, Ethernet OAM provides the hands for the operator, with diagnostic and troubleshooting utilities to fix problems when they occur.
Q. What is the detection aspect of service assurance?
A. The detection aspect refers to the tools and utilities available to monitor, and troubleshoot the network.
Q. What is the impact of not having Ethernet OAM available?
A. Without Ethernet OAM, the operator has limited visibility from the network operations center (NOC). Oftentimes the only way to diagnose a problem is through a site visit, which contributes significantly to the operating expenses of an access network. Not only are site visits expensive, but they also require a great deal of time.
Q. What types of transport infrastructures can Ethernet OAM support?
A. Carrier networks are typically hierarchical networks that consist of a wide variety of technologies and can span large geographical areas. Therefore, delivery of an Ethernet service could potentially span native Ethernet, Ethernet over SONET, Ethernet over Multiprotocol Label Switching (MPLS), as well as IP/MPLS transport infrastructure.
Q. What network and service layers should Ethernet OAM monitor in support of Ethernet services?
A. Ethernet OAM must provide capabilities on a link level and an end-to-end service level. The Link Layer OAM provides management and troubleshooting of a single link between two Ethernet interfaces, which may be interconnected through Ethernet or an emulated Ethernet connection such as Ethernet over SONET or Ethernet over MPLS. The Service Layer OAM provides management and troubleshooting in a multi-hop Ethernet network for individual customer service instances.
Q. What is the relationship between the OAM layers?
A. It is important to differentiate between different network client and service layers. Each layer needs to have its own OAM mechanism. Furthermore, each layer Fault Management mechanism shall operate independently from the layer below and above it. It is important to adhere to this independent layering protocol in design and development of Fault Management mechanisms for Ethernet services in order to ensure ease of operation over different transport layers.
Q. What is the Cisco® strategy for Ethernet OAM?
A. Cisco Systems® is committed to the creation, development, and deployment of standards-based OAM capabilities for Ethernet which are transparent to the underlying transport technology. Cisco continues to take an active role in the standards bodies (ITU, IEEE, IETF, and MEF) responsible for defining these standards and has developed a comprehensive set of Ethernet OAM capabilities across the Cisco Carrier Ethernet portfolio.
Q. What are the various standard bodies focusing on Ethernet OAM?
A. A number of standard bodies are engaged in Ethernet OAM efforts:
• ITU-T Study Group 13 and Study Group 15
– Requirements for OAM functionality in Ethernet-based networks (Y.1731 - SG 13)
• IEEE
– 802.3ah - Ethernet in the First Mile (Physical OAM)
– 802.1ag - Connectivity Fault Management
• MEF (Metro Ethernet Forum)
– Tracking the standards work of the other bodies, engaged in Service OAM development
– Ethernet Local Management Interface (E-LMI) Recommendation (MEF 16)
• IETF
– Tracking the standards work of the other bodies, engaged in SNMP MIB development
TRANSPORT LAYER OAM
Q. What is Transport Layer OAM and why is it important to service providers?
A. Transport Layer OAM provides useful link layer mechanisms for monitoring link operation such as remote fault indication and remote loopback control. In general, Transport Layer OAM gives network operators the ability to monitor the health of the network and quickly determine the location of failing links or fault conditions. These OAM mechanisms encompass the following areas:
• Provider Edge OAM, as defined by IEEE 802.3ah
• Underlying layers of OAM, such as MPLS OAM, SONET OAM, or ATM OAM, used on various emulated Ethernet links
Given the breadth of this subject, this Q&A focuses only on the provider edge OAM.
Q. What is IEEE 802.3ah OAM?
A. IEEE 802.3ah Link Layer OAM at the provider edge covers the OAM frames used across a physical IEEE 802.3 medium between a provider and a customer, between two provider ports, or even potentially between two customer ports. The major objectives of 802.3ah are as follows:
• Remote failure indication
– A mechanism is provided to indicate to a peer that the receive path of the local DTE is non-operational (this requires physical layer devices that support unidirectional operation).
– Subscriber-access physical layer devices that support unidirectional operation in the direction from Optical Line Termination and Optical Network Unit (OLT to ONU) allow OAM remote failure indication from OLT during fault conditions.
– Physical layer devices other than those listed above do not support unidirectional operation allowing OAM remote failure indication during fault conditions.
• Remote loopback
– A mechanism is provided to support a data link layer frame-level loopback mode.
• Link monitoring
– A mechanism is provided to support event notification that permits the inclusion of diagnostic information.
– A mechanism is provided to support polling of the diagnostic data.
• Miscellaneous
– Implementation and activation of OAM is optional.
– A mechanism is provided that performs OAM capability discovery.
– An extension mechanism is provided and made available for higher-layer management applications.
SERVICE LAYER OAM
Q. What is Service Layer OAM and why is it important to service providers?
A. Service Layer OAM focuses on the "end-to-end" connectivity of an Ethernet virtual circuit. Because this circuit can be virtual, the capabilities to debug Ethernet wires such as those provided by the transport layer OAM mechanisms do not necessarily extend end-to-end. Connectivity Management uses standard Ethernet frames that are distinguished from ordinary data frames only by destination MAC address or EtherType, which are seen an either relayed or terminated by provider bridges. In effect, Service OAM provides end-to-end manageability and visibility of Ethernet connections over any transport infrastructure (such as MPLS, Virtual Private LAN Services [VPLS], and provider bridges (IEEE 802.1ad). Service Layer OAM also provides mechanisms to support the performance monitoring of a service and enable the customer-edge device to request and receive status from the Ethernet network using E-LMI. This means it can configure itself to access Carrier Ethernet services. The latter is an important aspect of ensuring the end-to-end service-level agreement (SLA) as well as deploying services more rapidly.
Q. What is IEEE 802.1ag Connectivity Fault Management (CFM)?
A. IEEE 802.1ag Connectivity Fault Management allows service providers to manage each customer service instance individually. A customer service instance, or Ethernet Virtual Connection (EVC), is the service that is sold to a customer and is designated by the Service-VLAN tag. Hence, 802.1ag operates on a per-Service-VLAN (or per-EVC) basis. It enables the service provider to know if an EVC has failed, and if so, provides the tools to enable rapid isolation of the failure.
Q. Does IEEE 802.1ag Service Layer OAM require all the devices in an EVC to support the protocol?
A. Yes. IEEE 802.1ag uses special, reserved MAC addresses which will not be recognized by Ethernet interfaces that do not support Service Layer OAM.
Q. What are the categories of messages in 802.1ag Ethernet OAM?
A. Connectivity Check Messages, Traceroute Messages, Loopback Messages, and Alarm Indication Signal (AIS) Messages.
• Continuity Check Messages: These are "heart-beat" messages exchanged periodically between maintenance endpoint. They allow maintenance endpoints to discover other maintenance endpoints within a domain, and allow maintenance intermediate points to discover maintenance endpoint.
• Traceroute Messages: These are transmitted by a maintenance endpoint on the request by the administrator to track the path (hop-by-hop) to a destination maintenance endpoint. They allow the transmitting node to discover vital connectivity data about the path. They are similar in concept to UDP Traceroute.
• Loopback Messages: These are transmitted by a maintenance endpoints on the request by the administrator to verify connectivity to a particular maintenance endpoint. Loopback indicates whether the destination is reachable or not; it does not allow hop-by-hop discovery of the path. It is similar in concept to ICMP Echo (Ping).
• AIS Messages: These messages are generated by a maintenance endpoint or maintenance intermediate point that discovers a connectivity fault, and they notify other devices of this fault.
Q. What impact does Ethernet OAM have on the network manager's OSS?
A. Ethernet OAM enables service providers to easily troubleshoot and provision services for their customers and deliver improved SLAs. Adding IEEE 802.3ah implementation in a network manager's OSS can achieve the following benefits:
• For troubleshooting, IEEE 802.3ah OAM enables network managers to monitor and troubleshoot a single Ethernet link. It is particularly valuable in the first-mile connection to the customer demarcation, where most link issues typically occur.
• For provisioning, 802.3ah enables network managers to monitor a link for critical events, and then if necessary, put the remote device into "loopback" mode to do testing on the link. It also discovers unidirectional links, which occur when only one direction of transmission fails. Current management protocols for Ethernet do not provide the physical, link-level management enabled by 802.3ah.
Q. What are the functions of the Ethernet Local Management Interface (E-LMI) protocol?
A. The E-LMI protocol has local significance on the User-Network Interface (UNI) between the Metro Ethernet network and the customer edge (CE) as defined the Metro Ethernet Forum recommendation (MEF 16). It delivers Ethernet service-level status and availability all the way to the customer premises. Cisco has implemented this Ethernet OAM innovation on customer premise equipment as that is employed at the customer edge, as well as on User Provider Edge (U-PE) devices that are deployed within in the Metro Ethernet Network (MEN). Figure 1 illustrates the scope of the E-LMI protocol.
Figure 1. Ethernet Local Management Interface (E-LMI) in a Metro Ethernet Network
E-LMI defines the protocol and procedures that convey the information that allows auto-configuration of the CPE device. The E-LMI protocol also provides the means for notification of the status of an Ethernet Virtual Connection. In particular, the E-LMI protocol includes the following procedures:
• Notification to the CPE of the addition of an EVC
• Notification to the CPE of the deletion of an EVC
• Notification to the CPE of the availability (active) or unavailability (inactive) state of a configured EVC
• Link integrity verification
• Communication of UNI and EVC attributes to the CPE
ETHERNET FTTH - IMPROVING THE CONSUMER EXPERIENCE
Q. What are the primary Fiber to the Home (FTTH) technologies and architectures available to deliver broadband services to consumers and businesses?
A. Broadband services may be delivered to both consumers and business customers using standard IEEE 802.3 Ethernet technology in point-to-point, star, and ring architectures using a single fiber connection to each subscriber. Rather than use the earlier terminology of "Active Ethernet," a more appropriate term for these types of service deployments is simply Ethernet FTTH (E-FTTH). Passive Optical Network (PON) technology may also be used to deploy FTTH broadband services in which a single fiber connection is shared (split) among multiple subscribers. Fiber to the home, building, or campus (FTTx) encompasses a wide spectrum of deployment and technology options, such as Fiber to the Basement (FTTB) for residential and business customers in multi-dwelling units (MDUs) and multi-tenant units (MTUs) respectively. Fiber to the Curb (FTTC) brings fiber from the service provider further into a neighborhood or business park than Fiber to the Node (FTTN), but both methods use remote terminal equipment (xDSL) that employs copper connections to each subscriber. xDSL technology provides service extensibility beyond the fiber reach; however, the potential data rates are a function of the length of the copper loop to the subscriber. (Rates decline over distances greater than 1000 feet.)
Q. Why is FTTH such a popular topic right now?
A. Triple-play services to consumers with high-definition video, video on demand (VoD), and interactive broadband media applications are accelerating the need for bandwidth, and FTTH provides a number of solutions to reduce congestion in the last mile.
Q. What are the relevant interface standards for Ethernet FTTH and PON FTTH?
A. The relevant interface standards are as follows:
• IEEE 802.3 Ethernet - Interfaces with bidirectional data rates from 10 Mbps to 10 Gbps
Q. What are the primary reasons that service providers have chosen to deploy Cisco Ethernet FTTH?
A. Cisco Ethernet FTTH provides a lower incremental subscriber cost, greater service flexibility, and a more robust, resilient solution with more bandwidth per subscriber readily available for future services.
Q. What are the cost considerations for the deployment of FTTH?
A. Civil engineering (construction) costs account for up to 70 percent of the FTTH deployment costs. Fiber cabling for PON FTTH and Ethernet FTTH typically account for less than 10 percent of the overall deployment costs.
Q. Does FTTH PON interoperate with Ethernet FTTH?
A. None of the PON FTTH interfaces interoperate with each other because of specialized physical layer, optics, and MAC layer differences. As a result the PON FTTH interfaces do not interoperate with standard IEEE 802.3 Ethernet FTTH interfaces.
Q. Can Ethernet FTTH be supported on a single bidirectional fiber?
A. Yes, Ethernet FTTH supports data rates from 100 Mbps to 1 Gbps on a single bidirectional fiber.
Q. What is the range PON FTTH and Ethernet FTTH subscribers may be connected to the service provider location?
A. PON FTTH subscribers may be connected up to 20 km from the service provider location depending on how many subscribers are sharing a connection to the Optical Line Terminal (OLT). Ethernet FTTH subscribers may be connected through ring, star, or direct connections up to 80 km from the service provider location using longer-range Small Form-Factor Pluggable (SFP) optics.
Q. Which Cisco platforms provide Ethernet FTTH solutions?
A. The Cisco Catalyst® 4500 Series Switch is the primary Ethernet FTTH aggregation platform, typically deployed in the CO as OLT equipment. The Cisco ME 3400 and 3750 Series switches are optimized to support Ethernet FTTH multi-dwelling unit (MDU) applications for both business and consumer services. The new Cisco ME 4924 Ethernet Access Switch provides FTTX access aggregation switching for Etherent FTTH, FTTN (xDSL) and PON FTTH applications.
