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EoS: Ethernet over SONET/SDH

EoS represents a group of industry standard specifications for optimal transport of Ethernet through SONET/SDH. Ethernet and SONET/SDH are the two primary link technologies used in the network communications while Ethernet in the LAN and SONET/SDH in the Telco/PTT WAN. However, Ethernet rates do not match SONET/SDH rates and Ethernet traffic can not be effectively carried over the SONET/SDH network directly. Ethernet over SONET/SDH (EoS) technologies address this problem and turn the Sonet/SDH MAN/WAN infrastructure backbone into a transparent Ethernet segment for attached servers and clients. Packet over SONET/SDH (PoS), the traditional transmission of IP data over Sonet frames via PPP, is gradually replaced by EoS in many cases.

EoS specifications account for the mapping, aligning, bandwidth management, sequencing and delay compensation of the individual channels. The core technologies in the EoS architectures are the encapsulation schemes to match Ethernet and SONET/SDH rates effectively and manage bandwidth usage. Currently, there are a few encapsulation techniques used: virtual concatenation (VC) and the link capacity adjustment scheme (LCAS) techniques, the generic framing procedure (GFP) and link access procedure for SDH (LAPS) techniques.

Virtual Concatenation (VC): VC allows for non-standard SONET/SDH multiplexing in order to address the bandwidth mismatch problem between Ethernet and SONET/SDH. Using virtual concatenation, the SONET/SDH transport pipes may be “right-sized” for Ethernet transport. Virtual Concatenation allows SONET channels to be multiplexed together in arbitrary arrangements, which permits custom-sized SONET pipes to be created that are any multiple of the basic rates. Virtual concatenation is valid for STS-1 rates as well as for Virtual Tributary (VT) rates. All the intelligence to handle virtual concatenation is located at the endpoints of the connections, so each SONET channel may be routed independently through the network without it requiring any knowledge of the virtual concatenation. In this manner, virtually concatenated channels may be deployed on the existing SONET/SDH network with a simple endpoint upgrade. All the equipment currently in the center of the network need not be aware of the virtual concatenation.

Link Capacity Adjustment Scheme (LCAS): a supporting technology to the Virtual Concatenation, LCAS dynamically changes the amount a bandwidth used for a virtual concatenated channel and provides “tuning” of the allocated bandwidth depends on customer needs. LCAS is also useful for fault tolerance and protection since the protocol has the ability to remove failed links from the Virtually Concatenated Group (VCG). Using Link Capacity Adjustment Scheme (LCAS), signaling messages are exchanged within the SONET overhead in order to change the number of tributaries being used by a VCG. The number of tributaries may be either reduced or increased, and the resulting bandwidth change may be applied without loss of data in the absence of network errors.

Link Access Procedure for SDH (LAPS): a type of high-level data link controller (HDLC), LAPS includes data link service and protocol specification used in transporting IP packets over SDH networks. LAPS provides a point-to-point unacknowledged connectionless service over SONET/SDH. LAPS enables the encapsulation of IPv6, IPv4, PPP, and other higher-layer protocols.

Generic Framing Procedure (GFP): another key encapsulation scheme in EoS and more robust technology than LAPS, GFP maps Ethernet packet data into an octet-synchronous transport such as SONET. GFP has adapted 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 stream. Within GFP, there are two different mapping modes defined: frame based mapping and transparent mapping. Each mode is optimized for providing different services.


Protocol Structure - EoS: Ethernet over SONET/SDH

 

Related Protocols

POS: Packet Over SONET/SDH, Ethernet, SONET/SDH, Generic Framing Procedure (GFP), Virtual Concatenation, LAPS, LCAS

Sponsor Source

EoS protocols are mainly defined by ITU-T.


Reference

  1. ITU-T G.707/Y.1322, October 2000: Network Node Interface for the Synchronous Digital Hierarchy ([G707])
  2. ITU-T G.783, October 2000: Characteristics of SDH Equipment Functional Blocks ([G783])
  3. ITU-T G.803, March 2000: Architecture of Transport Networks Based on SDH. ([G803])
  4. T G.805, March 2000: Generic Functional Architecture of Transport Networks ([G805])
  5. T G.7041/Y1303, January 2002: Generic Framing Procedure ([G7041])
  6. T G.7042/Y1305, November 2001: LCAS for Virtually Concatenated Signals ([G7042])
  7. T Recommendation X.85/Y.1321, March 2001: IP over SDH Using LAPS
  8. T Recommendation X.86, February 2001: Ethernet over LAPS