Pipeline Publishing, Volume 7, Issue 7 - Taking Advantage of the Opportunities and Overcoming the Challenges of Carrier Ethernet Networks

Pipeline Publishing, Volume 7, Issue 7

	This Month's Issue:
	Carrier Ethernet Emerges

Taking Advantage of the Opportunities and Overcoming the Challenges of Carrier Ethernet Networks

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The Ethernet First Mile (EFM) 802.3ah provides link layer OAM functionality in the first and last mile. It is media independent and operates at a slow rate of 10 frames per second. Ethernet OAM packet data units (OAMPDU) only work in point-to-point full-duplex networks and are not forwarded by peer devices. They require minimal configuration and deliver following functions:

Device discovery
Remote failure indication
Remote loopback
Link monitoring

During the network initialization, adjacent devices exchange identification information and OAM capabilities. Network devices can notify peer devices in the event of failures. The remote loopback is a link-layer mechanism that operates at the frame level. Link monitoring delivers event notifications, such as status and diagnostics information.

The new standards will provide improved OAM capability to the customer premises demarcation point, potentially reducing OPEX by more than half.

System (UMTS) retrieve their reference frequency from the network. To avoid interference and hand-over problems, they require a frequency stability of 50 to 250 parts per billon. TDM networks such as SONET/SDH are based on technologies that can natively carry a frequency reference at the physical layer. But packet switched networks such as Ethernet are nondeterministic so they impose delay and delay variation on packets.

PTP, also known as IEEE standard 1588v2, provides one solution to this problem by accurately distributing time over packet-based networks. PTP uses a Grandmaster clock at the server that ports timing information to isolated slave clocks that in turn deliver traditional TDM T1 timing to the

The IEEE 802.1ag Connectivity Fault Management (CFM) standard specifies protocols and protocol entities within the architecture of VLAN-aware bridges that enable the detection, verification, and isolation of connectivity failures in virtual bridged LANs (VBLANs). These capabilities can be used in networks operated by multiple independent organizations, each with restricted management access to each other’s equipment. This standard specifies protocols, procedures and managed objects in support of connectivity fault management. It allows verification of the path through bridges and LANs taken from frames addressed to and from specified network users and enable detection and isolation of a connectivity fault to a specific bridge or LAN. The standard defines maintenance domains, maintenance associations, their constituent maintenance points and the managed objects required to create and administer them. It also describes the protocols and procedures that maintenance points use to detect and diagnose connectivity faults within a maintenance domain (MD).

Timing and synchronization functionality

TDM services require various timing and synchronization functions. For example, wireless end nodes of Global System for Mobile Communications (GSM)/ Universal Mobile Telecommunications

installed equipment. Network delays and latency are greatly reduced by measuring the round-trip delay between the master and slave clock, using a technique where the master and slave communicate with short messages to each other in order to measure and cancel out delay and latency inaccuracies. Alternatively, expensive GPS-based clocks are used at each cell site to obtain the timing synchronization required for CDMA services when using IP backhaul. PTP, on the other hand, requires only a central Grandmaster clock at the mobile switching center (MSC) and low-cost PTP slave clocks at the cell sites, which greatly lowers both capital and operating costs.

Synchronous Ethernet provides a mechanism for transferring frequency over the Ethernet physical layer, which is then traceable to an external source such as a network clock. Synchronous Ethernet interfaces can operate in sync or non-sync mode. In sync mode, the transmitter is locked to 4.6 parts per million. The receiver recovers it and passes it to the system/transmitter clock. In this mode, the interface does not work over native Ethernet interfaces. To enable communications between various nodes, Synchronous Ethernet provides an Ethernet Synchronization Status Messaging channel, similar to SONET/SDH Synchronization Status Messaging (SSM) bytes, that allows nodes to deliver their synchronization status to downstream nodes.

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