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New approaches to Ethernet over SONET/SDH

Posted: 16 Jun 2003 ?? ?Print Version ?Bookmark and Share

Keywords:ethernet? sonet? sdh? virtual concatenation? generic framing procedure?

Today's communications landscape is dominated by two networking technologies: Ethernet LAN and SONET/SDH in public telecommunication companies' (PTT) WAN infrastructure. Connecting remote sites via a single high-speed LAN for employee access to corporate servers, remote storage sites and Web hosts represents a critical reason for Ethernet LANs to be extended over metropolitan or international distances.

Interworking functions

A number of technologies have been employed to transport LAN traffic over the PTT network - frame relay, ATM, packet over SONET/SDH, multilink-PPP and others - each requiring interworking the native Ethernet traffic to the transport protocol prior to transmission. In some service models, customers are required to interwork their traffic prior to handing it off to the public network; in others, the carrier takes complete responsibility of the function. Both approaches call for specialized equipment for interworking and create network management issues.

The interworking function must generally terminate the Ethernet and map the underlying Internet Protocol (IP) traffic into a new Layer 2 (L2) or, alternatively, encapsulate the Ethernet within another L2 technology.

One of the challenges of Ethernet over SONET/SDH is the differing rates between the two technologies. Ethernet rates are typically 10Mbps, 100Mbps and 1Gbps, always increasing in factors of 10. On the other hand, SONET/SDH rates are optimized for voice traffic and do not match the optimal rates for transporting the Ethernet data stream. These rate mismatches make carrying a single Ethernet connection over a SONET pipe bandwidth inefficient.

To help optimize the transport of Ethernet over SONET/SDH links, two new technologies have been standardized. The first, virtual concatenation (VC), allows for non-standard SONET/SDH multiplexing to address bandwidth mismatch. The second, generic framing procedure (GFP), provides deterministic encapsulation efficiency and eliminates interworking.

Virtual concatenation: VC is a technique that allows SONET/SDH channels to be multiplexed in arbitrary arrangements. Using this, the SONET/SDH transport pipes may be "right-sized" for Ethernet transport. For SONET, VC rates are designated by STS-m-nv for high-order concatenation, where the nv indicates a multiple n of the STS-m base rate. Similarly, low-order VC is designated by VT-m-nv (virtual tributary). For SDH, the rates are designated by VC-m-nv.

All the intelligence to handle VC is located at the endpoints of the connections, so each SONET/SDH channel may be routed independently through the network. Equipment at the center of the network need not be aware of the VC. This allows for deployment over existing SONET/SDH networks.

VC provides flexibility in choosing the transport size to better match the desired bandwidth requirements. In addition to sizing the transport paths to handle the anticipated peak bandwidth, it may be used to create an arbitrary-sized transport pipe.

Generic framing procedure: GFP, based on ITU recommendation G.7041, is a protocol for mapping packet data into an octet-synchronous transport such as SONET/SDH. Unlike high-level data link control (HDLC)-based protocols, GFP does not use any special characters for frame delineation. Instead, it has adopted the cell delineation protocol used by ATM to encapsulate variable length packets. A fixed amount of overhead is required by the GFP encapsulation that is independent of the contents of the packets. In contrast to HDLC, whose overhead is data dependent, the fixed amount of overhead per packet allows deterministic matching of bandwidth between the Ethernet stream and the virtually concatenated SONET/SDH stream.

The GFP overhead can consist of up to three headers: a core header containing the packet length and a cyclic redundancy check (CRC) that is used for packet delineation; a type header identifying the payload type; and an optional extension header. Frame delineation is performed on the core header, which contains the 2B packet length and a CRC. The receiver would hunt for a correct CRC and then use the received packet length to predict the location of the start of the next packet.

- Mimi Dannhardt

Technical Advisor, Product Research Group

PMC-Sierra Inc.

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